Fondation Rene Touraine Pour la Recherche en Dermatologie


17th SCIENTIFIC MEETING 2009

Friday 13th November 2009, à Paris,

au Musée des Moulages de l’Hôpital Saint-Louis

1 avenue Claude Vellefaux – 75010 Paris
***

THE ADIPOCYTE
9h00–9h40C. DANI Differentiation of embryonic and adult stem cells into adipocytes
9h40–10h20K.N. FRAYN Metabolic functions of adipocytes
  Pause 
10h40–11h20P. VALET Endocrine functions of adipocytes
11h20–12h00D. RICQUIER Adipocytes and thermoregulation
12h00 Winners of fellowships 
12h10 Lunch Pause 
14h00–14h30K. CLEMENT Adipocytes and inflammation
14h30–15hD.B. SAVAGE Genetic lipodystrophies
15h00–15h30J. CAPEAU Acquired lipodystrophies in HIV-infected patients
  Pause 
16h00–16h30B. FEVE Current knowledge in the involvement of steroid receptors in adipocyte biology
16h30–17h00A.J. VIDAL-PUIG Adipose tissue expandability, lipotoxicity and the metabolic syndrome

La Fondation René Touraine est une Fondation reconnue d’utilité publique.

Les Membres Fondateurs sont: ABBOTT, BOOTS HEALTHCARE, CHANEL, FONDATION LEO, GALDERMA INTERNATIONAL, LABORATOIRES BAILLEUL, LABORATOIRES BERGADERM, LABORATOIRES PIERRE FABRE, LABORATOIRES ROCHE, LABORATOIRES SÉROBIOLOGIQUES, LA ROCHE POSAY LABORATOIRE PHARMACEUTIQUE, L’ORÉAL, L.V.M.H., SCHERING-PLOUGH, WYETH.

Registration : Fondation René Touraine, Hôpital Saint-Louis, Pavillon Bazin,

1, Avenue Claude Vellefaux – 75475 – Paris – France.

Tél : 33 1 53 72 20 60 – Fax: 33 1 53 72 20 61 – e-mail:fondation.r.touraine@stlouis.inserm.fr

Differentiation of embryonic and adult stem cells into adipocytes

Christian Dani
Laboratoire «Cellules Souches et Differenciation», UMR 6543 CNRS Université de Nice Sophia-Antipolis, Faculté de Medecine, Nice cedex, France

Hypertrophy and hyperplasia are observed in severe obesity. Hyperplasia results from excessive proliferation of adipose precursor cells prior to their differentiation. While the events controlling late steps of adipocyte differentiation have been largely explored, the mechanisms that direct progenitors towards the adipogenic lineage and molecular events regulating their self-renewal remain unknown. Key information regarding the developmental origins of adipocytes is also lacking.

Dani C, Smith A G, Dessolin S, Leroy P, Staccini L, Villageois P, Darimont C, Ailhaud G. Differentiation of embryonic stem (ES) cells into adipocytes in vitro. J Cell Sci 1997: 110 (Pt 11): 1279–1285.

The capacity of mouse embryonic stem (ES) cells to undergo adipocyte differentiation in vitro provides a powerful model to address the earliest steps of adipogenesis.

Monteiro M C, Wdziekonski B, Villageois P, Vernochet C, Iehle C, Billon N, Dani C. Commitment of mouse embryonic stem cells to the adipocyte lineage requires retinoic acid receptor beta and active GSK3. Stem Cells Dev 2009: 18: 457–463.

It has been shown in this work that activation of retinoic acid (RA) receptor β, by using the selective synthetic retinoid (CD2314), was sufficient to induce differentiation of ES cells into adipocytes.

Billon N, Iannarelli P, Monteiro M C, Glavieux-Pardanaud C, Richardson W D, Kessaris N, Dani C, Dupin E. The generation of adipocytes by the neural crest. Development 2007: 134: 2283–2292.

Transcriptional profiling revealed that clusters of genes modulated by RA in ES cells were enriched in neural crest-associated genes, suggesting that neuroectoderm rather than mesoderm was a source of adipocytes. Furthermore, mapping of neural crest derivatives in vivo using sox10-Cre transgenic mice demonstrated that in the cranial region, adipocytes originated from the neural crest, whereas in the trunk they originated from the mesoderm.

Rodeheffer M S, Birsoy K, Friedman J M. Identification of white adipocyte progenitor cells in vivo. Cell 2008: 135: 240–249.

In this work, the authors identified in the mouse adipose tissue stroma, a cell subpopulation containing adipose progenitors that were able to restore adipose tissue after transplantation into lipodystrophic mice.

Tang W, Zeve D, Suh J M, Bosnakovski D, Kyba M, Hammer R E, Tallquist M D, Graff J M. White fat progenitor cells reside in the adipose vasculature. Science 2008: 322: 583–586.

Tang et al. showed that the precursor cells that give rise to white adipocytes reside within the walls of the blood vessels that supply adipose tissue.

Sengenès C, Lolmède K, Zakaroff-Girard A, Busse R, Bouloumié A. Preadipocytes in the human subcutaneous adipose tissue display distinct features from the adult mesenchymal and hematopoietic stem cells. J Cell Physiol 2005: 205: 114–122.

It is demonstrated that in human adipose tissue, the adipose progenitor pool resides in the CD34+/CD31 cell population.

Zaragosi L E, Ailhaud G, Dani C. Autocrine fibroblast growth factor 2 signaling is critical for self-renewal of human multipotent adipose-derived stem cells. Stem Cells 2006: 24: 2412–2419.

As the maintenance of adipocyte precursor pool is regulated by self-renewal of stem cells, authors have undertaken the identification of molecular events involved in proliferation and differentiation of human Multipotent Adipose-derived Stem cells. They identified the occurrence of autocrine/paracrine FGF loop in the maintenance of self-renewal of these stem cells.

Metabolic functions of adipocytes

Keith N. Frayn
Oxford Centre for Diabete, Endocrinology and Metabolism, University of Oxford, Oxford, UK

The major metabolic functions of the mature white adipocytes are fat deposition and fat mobilization. The bulk of the cell is occupied by a single droplet of triacylglycerol (TG). Fat deposition occurs by the uptake of fatty acids into the cell, and to a smaller extent by the synthesis of fatty acids de novo, e.g. from glucose. These fatty acids are esterified to glycerol 3-phosphate to form TG. Fat mobilization consists of enzymatic hydrolysis of the TG to release fatty acids, which can be delivered into the circulation for transport to other tissues.

These pathways are highly regulated. To put this into perspective, the whole-body store of TG, mostly within white adipocytes, reflects extremely closely the integrated lifetime imbalance of energy intake and energy expenditure. This means that fat cell TG content is highly controlled, presumably by signals that include hormones and the activity of the autonomic nervous system. But although there must be long-term regulation of adipocyte fat content, there is also short-term regulation, with minute-to-minute and hour-to-hour control of fat storage and mobilization, especially in response to meal intake and exercise.

Adipose tissue metabolism depends critically upon perfusion. The major metabolic exchanges with the blood are of hydrophobic molecules, fatty acids and TGs, transported bound to albumin or incorporated into lipoprotein particles, respectively. It is, therefore, not surprising that adipose tissue blood flow (ATBF) is greater than that of resting skeletal muscle, and is extremely sensitive to nutritional and other stimuli. ATBF increases during exercise, for instance, a time when fatty acid delivery is increased; and also after meals, when delivery of dietary fat in the form of TG to the tissue is important.

Although many features of adipose tissue and adipocyte metabolism have been worked out over the past half-century, there have also been some recent developments, including the identification of a major TG lipase expressed in adipocytes, adipose triglyceride lipase (ATGL), and the identification of a novel pathway for stimulation of fat mobilization in human adipose tissue via the natriuretic peptides, atrial natriuretic peptide (ANP) and B- or brain-type natriuretic peptide (BNP).

Historical

Wertheimer E, Shapiro B. The physiology of adipose tissue. Physiol Rev 1948: 28: 451–464.

This review article sets the scene for unravelling the metabolic functions of adipose tissue during the second half of the 20th century.

Gordon R S. Unesterified fatty acid in human blood plasma. II. The transport function of unesterified fatty acid. J Clin Invest 1957: 36: 810–815.

This work was the first to demonstrate the release of non-esterified fatty acids (then called unesterified fatty acids) from subcutaneous adipose tissue.

Metabolic functions of adipocytes and adipose tissue

Arner P, Kriegholm E, Engfeldt P, Bolinder J. Adrenergic regulation of lipolysis in situ at rest and during exercise. J Clin Invest 1990: 85: 893–898.

In an elegant demonstration of the power of the microdialysis technique for studying adipose tissue metabolism in situ, Peter Arner’s group showed that beta-adrenergic stimulation is important for fat mobilization during exercise, but not relevant for lipolysis in the post-absorptive state.

Frayn K N, Shadid S, Hamlani R, Humphreys S M, Clark M L, Fielding B A, Boland O, Coppack S W. Regulation of fatty acid movement in human adipose tissue in the postabsorptive-to-postprandial transition. Am J Physiol 1994: 266: E308–E317.

Studies of human subcutaneous adipose tissue metabolism in vivo show key features of the interaction between the pathways of fat mobilization and fat deposition, and their regulation by insulin.

Evans K, Burdge G C, Wootton S A, Clark M L, Frayn K N. Regulation of dietary fatty acid entrapment in subcutaneous adipose tissue and skeletal muscle. Diabetes 2002: 51: 2684–2690.

Application of stable-isotope techniques to investigate fat storage in subcutaneous adipose tissue in the postprandial state demonstrated that there is a key locus of control that is ‘downstream’ from the action of lipoprotein lipase on circulating TG; namely, regulation of the uptake and storage of fatty acids within the adipocyte.

Strawford A, Antelo F, Christiansen M, Hellerstein M K. Adipose tissue triglyceride turnover, de novo lipogenesis, and cell proliferation in humans measured with 2H2O. Am J Physiol Endocrinol Metab 2004: 286: E577–E588.

This work used a stable-isotope approach to look at the turnover of different aspects of human adipose tissue: cells, TG and the contribution of de novo lipogenesis to net TG storage. The results show surprisingly dynamic cell and TG turnover. The contribution of de novo lipogenesis to net fat deposition was also found to be higher than many had suspected, averaging about 20% of stored non-essential fatty acids in adipocyte TG.

Karpe F, Fielding B A, Coppack S W, Lawrence V, Macdonald I A, Frayn K N. Oscillations of fatty acid and glycerol release from human subcutaneous adipose tissue in vivo. Diabetes 2005: 54: 1297–1303.

This was the first work to show in humans that lipolysis is pulsatile, with bursts of fatty acid release occurring at approximately 12–14 min intervals, not directly related to the pulsatility of insulin secretion. The phenomenon of pulsatile lipolysis had been shown previously in canine adipose tissue by Richard Bergman et al.

Moro C, Crampes F, Sengenes C, De Glisezinski I, Galitzky J, Thalamas C, Lafontan M, Berlan M. Atrial natriuretic peptide contributes to physiological control of lipid mobilization in humans. FASEB J 2004: 18: 908–910.

This was the first demonstration of an unsuspected pathway for stimulation of fat mobilization in humans, brought about by atrial natriuretic peptide. It is probably particularly important during exercise.

Nielsen S, Guo Z, Johnson C M, Hensrud D D, Jensen M D. Splanchnic lipolysis in human obesity. J Clin Invest 2004: 113: 1582–1588.

There has been considerable speculation that visceral fat depots liberate fatty acids at a high rate, and these fatty acids have a direct (adverse) effect on hepatic metabolism. In this study, the authors use hepatic vein catheterization in conjunction with stable-isotope techniques to measure the contribution of visceral fat to hepatic fatty acid delivery. The results show that peripheral rather than visceral fat is the major contributor of fatty acids to the systemic circulation (as Michael Jensen’s group had shown previously), but that the contribution of visceral fat-derived fatty acids to the fatty acid supply to the liver is significant, and becomes more so with increasing visceral fat stores.

Zimmermann R, Strauss J G, Haemmerle G, Schoiswohl G, Birner-Gruenberger R, Riederer M, Lass A, Neuberger G, Eisenhaber F, Hermetter A, Zechner R. Fat mobilization in adipose tissue is promoted by adipose triglyceride lipase. Science 2004: 306: 1383–1386.

Until the early part of the 21st century, the major regulatory enzyme for TG hydrolysis in the adipocyte was considered to be hormone-sensitive lipase (HSL). Around 2000 the first of several descriptions of HSL-null mice appeared, showing that these animals had relatively normal fat cells, and (depending upon strain) that catecholamine-stimulated lipolysis was reduced but not abolished in isolated adipocytes. This led to the identification of adipose triglyceride lipase (ATGL), described in this work. Although the debate is still on-going, it seems likely that ATGL is the major TG lipase of mammalian adipocytes, with HSL acting mainly as a diacylglycerol lipase.

Spalding K L, Arner E, Westermark P O, Bernard S, Buchholz B A, Bergmann O, Blomqvist L, Hoffstedt J, Näslund E, Britton T, Concha H, Hassan M, Rydén M, Frisén J, Arner P. Dynamics of fat cell turnover in humans. Nature 2008: 453: 783–787.

The authors used isotopic labelling of adipose tissue components resulting from atomic bomb testing and consequent atmospheric contamination, to investigate turnover of adipocytes in humans. These studies are combined with data from serial studies of different groups of patients, including those experiencing massive weight loss following bariatric surgery. The data show that the number of fat cells remains essentially constant in adulthood, but that approximately 10% of fat cells are renewed annually.

Ruge T, Hodson L, Cheeseman J, Dennis A L, Fielding B A, Humphreys S M, Frayn K N, Karpe F. Fasted to fed trafficking of fatty acids in human adipose tissue reveals a novel regulatory step for enhanced fat storage. J Clin Endocrinol Metab 2009: 94: 1781–1788.

Studies of human adipose tissue metabolism in vivo over a 24-hour period show very dynamic regulation of fat storage and mobilization, with increasing fat storage during a typical day, following three meals, brought about largely by up-regulation of a step ‘distal to’ lipoprotein lipase action, namely, adipocyte uptake of fatty acids.

Regulation of adipose tissue blood flow

Ardilouze J, Fielding B, Currie J, Frayn K, Karpe F. Nitric oxide and beta-adrenergic stimulation are major regulators of pre- and postprandial subcutaneous adipose tissue blood flow in humans. Circulation 2004: 109: 47–52.

Use of a novel technique (microinfusion) to study the regulation of blood flow in human subcutaneous adipose tissue shows an important role for nitric oxide (NO) in the post-absorptive state, with a major role for beta-adrenergic vasodilatation following meal ingestion.

Endocrine functions of adipocyte

Philippe Valet
Université de Toulouse, UPS, Institut de Médecine Moléculaire de Rangueil, Inserm U858, IFR150, Toulouse cedex 4, France

One of the major features in adipocyte biology is the discovery of its complex secretory potencies. A number of lipids, peptides, hormones and pro-inflammatory cytokines termed adipokines, secreted by the adipocyte and identified in the circulation, exert endocrine effects. These adipokines, usually acting through activation of specific receptors, have been described to possess a number of effects in tissues involved in the nervous, endocrine and metabolic control. They allow the adipocyte to initiate potent actions in the regulation of food intake, energy expenditure, glucose/lipid disposal and inflammation. They also participate in the settlement of insulin resistance by actions on liver, skeletal muscle and pancreas function, and could be involved in the prevention or worsening of atherogenic processes. In addition, some of the factors secreted by the adipocyte exert local autocrine and paracrine actions, mainly affecting adipose tissue metabolism, remodelling, adipogenesis and angiogenesis.

1959: The idea of a circulating factor associated with obesity

Hervey G R. The effects of lesions in the hypothalamus in parabiotic rats. J Physiol 1959: 145: 336–352.

The first experimental results showing the involvement of adipose tissue in the generation of soluble factors has been produced by parabiosis experiments in lean and obese rats. Whereas the obese rats behaviour was unchanged, the lean ones stopped eating, leading Hervey to suggest the existence of a circulating factor released in the blood of obese rats which was inefficient in obese but active in lean rats. The idea of an endocrine function of adipose tissue was born.

After 30 years, adipocytes secrete bioactive factors…

Shillabeer G, Forden J M, Lau D C. Induction of preadipocyte differentiation by mature fat cells in the rat. J Clin Invest 1989: 84: 381–387.

The cellular demonstration of the role of adipocyte in soluble factor release has been carried out in vitro. Conditioned medium of isolated mature adipocytes was able to stimulate pre-adipocyte differentiation in a separate dish.

Six more years and leptin is identified!

Halaas J L, Gajiwala K S, Maffei M, Cohen S L, Chait B T, Rabinowitz D, Lallone R L, Burley S K, Friedman J M. Weight-reducing effects of the plasma protein encoded by the obese gene. Science 1995: 269: 543–546.

The discovery of the ob gene in both mouse and human and the associated protein: leptin has been made by J. Friedman’s group. Leptin is secreted by adipocytes into the bloodstream and leads to the inhibtion of food intake and the rise in energy expenditure by acting on the central nervous system. After such a discovery, several other adipokines were described to be involved in numerous biological effects (for review, see Ronti T et al. Clin. Endocrinol. 2006, 64: 355–365).

A quite original adipocyte production: H2O2

Marti L, Morin N, Enrique-Tarancon G, Prevot D, Lafontan M, Testar X, Zorzano A, Carpéné C. Tyramine and vanadate synergistically stimulate glucose transport in rat adipocytes by amine oxidase-dependent generation of hydrogen peroxide. J Pharmacol Exp Ther 1998: 285: 342–349.

The study of the roles and regulations of amine oxidases, such as the Semi-carbazide Sensitive one (SSAO) led to the identification of this enzyme in the plasma membrane of adipocytes and its ability to generate H2O2. Such a production by the fat cell is able to provoke insulino-mimetic responses, such as the stimulation of glucose transport, the inhibition of lipolysis, adipogenesis, and to improve glucose tolerance.

Apelin is able to mimic insulin in a diabetic status

Boucher J, Masri B, Daviaud D, Gesta S, Guigné C, Mazzucotelli A, Castan-Laurell I, Tack I, Knibiehler B, Carpéné C, Audigier Y, Saulnier-Blache J S, Valet P. Apelin, a newly identified adipokine up-regulated by insulin and obesity. Endocrinology 2005: 146: 1764–1771.

Dray C, Knauf C, Daviaud D, Waget A, Boucher J, Buléon M, Cani P D, Attané C, Guigné C, Carpéné C, Burcelin R, Castan-Laurell I, Valet P. Apelin stimulates glucose utilization in normal and obese insulin-resistant mice. Cell Metab 2008: 8: 437–445.

One of the last adipokines described is named apelin. Apelin is secreted by adipocytes in response to insulin and/or TNFαin vitro and in vivo. It is regulated during obesity-weight loss and is involved in angiogenic process within adipose tissue. Recently, a very interesting effect in glucose metabolism has been described as apelin is able to stimulate glucose utilization in muscle and adipose tissue in diabetic mice models.

LPA, an adipokine interacting with keratinocytes

Valet P, Pagès C, Jeanneton O, Daviaud D, Barbe P, Record M, Saulnier-Blache J S, Lafontan M. Alpha2-adrenergic receptor-mediated release of lysophosphatidic acid by adipocytes. A paracrine signal for preadipocyte growth. J. Clin. Invest 1998: 101: 1431–1438.

Mazereeuw-Hautier J, Gres S, Fanguin M, Cariven C, Fauvel J, Perret B, Chap H, Salles J P, Saulnier-Blache J S. Production of lysophosphatidic acid in blister fluid: involvement of a lysophospholipase D activity. J. Invest. Dermatol 2005: 125: 421–427.

The production of a bioactive phospholipid, the Lyso-Phosphatidic Acid (LPA) by adipocytes has been shown in 1998, and its role in adipogenesis process and its link with obesity-associated diabetes further described. More recently, LPA has been found in blister fluid and, because of its ability to enhance keratinocyte migration, it could play an important role in re-epithelialization occurring after blister rupture.

Adipocytes and thermoregulation

Daniel Ricquier
Faculté de Médecine Paris Descartes, Université Paris Descartes, CNRS BIOTRAM, Paris Cedex 15, France

Body temperature in mammals reflects an equilibrium between thermogenesis and heat loss. Thermogenesis comprises basal heat production released from metabolic pathways in most organs, and regulatory or adapted thermogenesis performed in skeletal muscles and brown adipose tissue. Whereas the main function of adipose tissue is to store energy in large adipocytes referred to as white adipocytes, a morphologically distinct type of adipocytes, the brown adipocytes, have a strong capacity to oxidize fatty acids rapidly and dissipate oxidation energy as heat. Physiologically, these brown adipocytes are present and potentially active in mammalian newborns, in hibernators at arousal and in animals chronically exposed to a cold environment. Upon exposure to cold, sympathetic fibres directly release noradrenaline (norepinephrine) on the surface of brown adipocytes. Free fatty acids resulting from stimulated lipolysis immediately activate a proton pathway present in the inner mitochondrial membrane. Actually, this proton pathway is a unique mitochondrial carrier, named Uncoupling Protein (UCP, then renamed UCP1). UCP1 decreases the mitochondrial membrane potential, activates a proton circuit shunting the ATP-synthase and consequently uncouples respiration from ATP synthesis. In other words, upon UCP1 activation, the energy generated from fatty acid oxidation and coenzyme reoxidation is not used for ADP phosphorylation, but is dissipated as heat. Clearly, these brown adipocytes, comprising brown adipose tissue depots, participate in regulatory thermogenesis in response to cold. Their role in diet-induced thermogenesis was also proposed. A considerable part of the research on brown adipocytes and UCP1 was made to investigate their possible implication or utilization in the stimulation of fat oxidation to counteract fat accumulation in obese patients. Whereas a role for brown adipose tissue in infants is accepted, its function in adult humans was viewed as very minor. Interestingly, a number of recent studies demonstrated that many adults do possess some amount of active brown fat. Very recent data also highlighted the vicinity of brown adipocyte progenitors and myocyte progenitors.

Brown adipose tissue thermogenesis, brown adipocytes, UCP1

Nedergaard J, Cannon B, Lindberg O. Microcalorimetry of isolated mammalian cells. Nature 1977: 267: 518–520.

This was an important paper reporting the direct recording of heat production by isolated brown adipocytes in response to a physiological stimulation by norepinephrine. The data confirmed the original work of two groups which previously showed that the mitochondrial respiration of brown adipocytes was loosely coupled to ATP synthesis and, therefore, was thermogenic.

Nicholls D G, Locke R M. Thermogenic mechanisms in brown fat. Physiol Rev 1984: 64: 1–64.

This is a state-of-the-art review on mechanisms underlying thermogenesis in brown adipocytes. A large part of this work describes the different experiments establishing that brown adipocyte mitochondria exhibit a unique and regulated proton pathway that is responsible for a phyiologically regulated uncoupling of substrate oxidation from ADP phosphorylation leading to energy dissipation as heat.

Bouillaud F, Ricquier D, Thibault J, Weissenbach J. Molecular approach to thermogenesis in brown adipose tissue: cDNA cloning of the mitochondrial uncoupling protein. Proc Natl Acad Sci USA 1985: 82: 445–448.

This paper reports the first cloning of a (complete) cDNA corresponding to the brown adipocyte mitochondrial uncoupling protein (UCP, later renamed UCP1, UCP2 was identified). Use of this cDNA confirmed the tissue specificity of UCP1 expression. The same authors used this cDNA to isolate the corresponding gene and to express UCP1 in yeast, mammalian cells and transgenic mice.

Himms-Hagen J, Ricquier D. Brown Adipose Tissue. In Bray G., Bouchard C. and James WPT, eds. Handbook of Obesity. New York: Marcel Dekker, 1997: 415–441.

A review of several aspects of brown adipose tissue including physiological and biochemical aspects.

Enerbäck S, Jacobsson A, Simpson E M, Guerra C, Yamashita H, Harper M E, Kozak L P. Mice lacking mitochondrial uncoupling protein are cold-sensitive but not obese. Nature 1997: 387: 90–94.

A very important paper describing mice invalidated for ucp1 using homologous recombination. As expected, the cold-sensitivity of the null mice confirmed the main role of UCP (UCP1) in thermogenesis in response to cold exposure. The data also established that, in basal conditions, UCP1 is not a regulator of fat mass and body weight.

Cannon J, Nedergaard J. Brown adipose tissue: function and physiological significance. Physio Rev 2004: 84: 277–359.

A very complete review of hundreds of papers devoted to brown adipose tissue over 40 years of research.

Brown adipocytes in humans revealed by morphological and biochemical analysis of biopsies

Ricquier D, Néchad M, Mory G. Ultrastructural and biochemical characterization of human brown adipose tissue in pheochromocytoma. J Clin Endocrinol Metab 1982: 54: 805–807.

This was a paper of interest regarding human brown fat. As brown adipose tissue activity and development are positively controlled by norepinephrine (noradrenaline) released by the sympathetic nerves that directly innervate brown adipocytes, the authors looked for hypothetical brown adipocytes in patients bearing phaeochromocytoma and presenting elevated circulating concentration of norepinephrine. They identified characteristic brown adipocytes exhibiting mitochondria presenting a regulatable uncoupling of respiration. These data demonstrated that adult humans, at least in this specific pathological situation, do have brown adipocytes.

Lean M E, James W P, Jennings G, Trayhurn P. Brown adipose tissue in patients with phaeochromocytoma. Int J Obesity 1986: 10: 219–227.

Another interesting paper about human brown adipose tissue obtained at laparotomy from three subjects with high circulating noradrenaline concentrations in the presence of phaeochromocytoma. Typical brown adipose tissue was observed. The authors concluded that human adult brown adipose tissue has the biochemical potential for the thermogenic activity required to contribute to the regulation of energy balance.

Garruti G, Ricquier D. Analysis of uncoupling protein and its mRNA in adipose tissue deposits of adult humans. Int J Obesity 1992: 16: 383–390.

This significant study confirmed the presence of brown adipocytes in infants and strongly suggested that adult humans still possess brown adipose tissue. The authors investigated the presence of UCP1 and its mRNA in fat perirenal or periadrenal biopsies taken from infants, or from various adult patients undergoing surgery for pheochromocytoma, other endocrine pathologies and non-endocrine pathologies. All the infant biopsies were positive for UCP1 and 19 out of 30 human biopsies were positive for UCP1 or its mRNA.

Kortelainen M L, Pelletier G, Ricquier D, Bukowiecki L J. Immunohistochemical detection of human brown adipose tissue uncoupling protein in an autopsy series. J Histochem Cytochem. 1993: 4: 759–764.

The authors used an immunohistochemical method for the inner mitochondrial membrane uncoupling protein (UCP1) to examine whether brown adipose tissue could be detected in human fat tissue excised from around the common carotid arteries and in the subscapular region and from around the thoracic aorta obtained at medicolegal autopsies. UCP1 was detected in all the cases examined. These results show that brown adipose tissue is present in infants and also in adult humans.

Zingaretti M C, Crosta F, Vitali A, Guerrieri M, Frontini A, Cannon B, Nedergaard J, Cinti S. The presence of UCP1 demonstrates that metabolically active adipose tissue in the neck of adult humans truly represents brown adipose tissue. FASEB J 2009: May 5 [Epub ahead of print].

An important paper confirming the conclusions of the papers above and establishing the presence of typical brown adipocytes in the neck adipose tissue of 35 patients undergoing surgery for thyroid diseases. The authors were able to identify UCP1 and visualize a rich sympathetic innervation and a dense capillary network characteristic of functional brown adipocytes. These data demonstrated that human adults indeed possess brown adipose tissue.

Brown adipocytes in humans recently revealed using PET-Scan

Nedergaard J, Bengtsson T, Cannon B. Unexpected evidence for active brown adipose tissue in adult humans. Am J Physiol Endocrinol Metab 2007: 293: E444–452.

This is a very important review highlighting important data that went rather unnoticed, because of their publication from 1996 to 2006 in specialized journals such as Am J Roentgenol, Ann Endocrinol, Breast J, Clin Nucl Med, Eur J Nucl Med Mol Imaging, Exp Clin Endocrinol Diabetes, J Comput Assist Tomogr, J Nucl Med, Korean J Radiol, Mol Imaging Biol, Nucl Med Commun, and Pediatr Radiol. A clear conclusion from the well-informed analysis made by Nedergaad, Bengtsson and Cannon of all these papers is that the use of fluorodeoxyglucose positron emission tomography (FDG PET) to trace tumour metastasis can identify characteristic depots of brown adipose tissue in adult humans. The human depots are somewhat differently located from those in rodents, the main depots being found in the supraclavicular and the neck regions with some additional paravertebral, mediastinal, para-aortic and suprarenal (but no interscapular) localizations. Moreover, brown adipose tissue activity in man is acutely cold induced and is stimulated via the sympathetic nervous system. The authors concluded that the prevalence of active brown adipose tissue in the population may be at least in the range of some tens of percent. Importantly, a substantial fraction of adult humans possess active brown adipose tissue that has the potential to be of metabolic significance for normal human physiology as well as to become pharmaceutically activated in efforts to enhance fat oxidation to combat obesity.

van Marken Lichtenbelt W D, Vanhommerig J W, Smulders N M, Drossaerts J M, Kemerink G J, Bouvy N D, Schrauwen P, Teule G J. Cold-activated brown adipose tissue in healthy men. N Engl J Med 2009: 360: 1500–1508.

Cypess A M, Lehman S, Williams G, Tal I, Rodman D, Goldfine A B, Kuo F C, Palmer E L, Tseng Y H, Doria A, Kolodny G M, Kahn C R. Identification and importance of brown adipose tissue in adult humans. N Engl J Med 2009: 360: 1509–1517.

Virtanen K A, Lidell M E, Orava J, Heglind M, Westergren R, Niemi T, Taittonen M, Laine J, Savisto N J, Enerbäck S, Nuutila P. Functional brown adipose tissue in healthy adults. N Engl J Med 2009: 360: 1518–1525.

Saito M, Okamatsu-Ogura Y, Matsushita M, Watanabe K, Yoneshiro T, Nio-KobayashiJ, Iwanaga T, Miyagawa M, Kameya T, Nakada K, Kawai Y, Tsujisaki M. High Incidence of Metabolically Active Brown Adipose Tissue in Healthy Adult Humans: Effects of Cold Exposure and Adiposity. Diabetes 2009: 58: 1526–1531.

This is a series of four recent papers, three joined papers published in the same issue of the NEJM and another concomitant one appearing in Diabetes. These papers were based on the use of in vivo 2-[(18)F ]fluoro-2-deoxyglucose (FDG) uptake into adipose tissue and positon emission tomography (FDG PET) as referred to above. The publication of the data in very well-known journals made the data highly regarded. The conclusions perfectly fit with the data presented in the review paper just above and other papers above. The authors demonstrated the presence of brown adipose tissue in adult man and also reported that this tissue was strongly activated upon exposure to cold, as it is known in rodents. Interestingly, the data were confirmed by both histological examinations and immunodetection of UCP1.

Brown adipocytes progenitors: white adipocyte versus myocyte lineage

Moulin K, Truel N, André M, Arnauld E, Nibbelink M, Cousin B, Dani C, Pénicaud L, Casteilla L. Emergence during development of the white-adipocyte cell phenotype is independent of the brown-adipocyte cell phenotype. Biochem J 2001: 356: 659–664.

This was an interesting paper investigating the question of a putative transformation of brown adipocytes into white adipocytes during post natal development. Using several lines of transgenic mice, the authors concluded that, contrary to what is generally accepted, most white adipocytes do not derive from brown adipocytes during normal development.

Guerra C, Koza R A, Yamashita H, Walsh K, Kozak L P. Emergence of brown adipocytes in white fat in mice is under genetic control. Effects on body weight and adiposity. J Clin Invest 1998: 102: 412–420.

An interesting genetic analysis of brown adipocyte emergence. Looking at brown adipocyte presence in several strains of mice, the authors noticed that large differences occurred in white fat tissues, particularly in retroperitoneal fat. Among the AXB recombinant inbred strains, the Ucp1 mRNA levels varied up to 130-fold. This large induction at the mRNA level was accompanied by a corresponding increase in brown adipocytes. This genetic variation in mice provided an experimental approach to identify genes controlling the induction of brown adipocytes in white fat tissues.

Timmons J A, Wennmalm K, Larsson O, Walden T B, Lassmann T, Petrovic N, Hamilton D L, Gimeno R E, Wahlestedt C, Baar K, Nedergaard J, Cannon B. Myogenic gene expression signature establishes that brown and white adipocytes originate from distinct cell lineages. Proc Natl Acad Sci U S A. 2007: 104: 4401–4406.

A very important paper based on the use of microarray analysis of primary pre-adipocytes, and leading to the rather surprising and striking discovery that brown pre-adipocytes demonstrate a myogenic transcriptional signature, whereas both brown and white primary pre-adipocytes demonstrate signatures distinct from those found in immortalized adipogenic models. These data confirmed the distinct origin for brown versus white adipose tissue, and also represented a plausible explanation as to why brown adipocytes ultimately specialize in lipid catabolism rather than storage, much like oxidative skeletal muscle tissue.

Seale P, Bjork B, Yang W, Kajimura S, Chin S, Kuang S, Scimè A, Devarakonda S, Conroe H M, Erdjument-Bromage H, Tempst P, Rudnicki M A, Beier D R, Spiegelman B M. PRDM16 controls a brown fat/skeletal muscle switch. Nature 2008: 454: 961–967.

This is a very important paper confirming the conclusions presented in the paper just above. Seale et al. demonstrated that brown, but not white, fat cells arise from precursors belonging to the myogenic lineage. They identified PRDM16, a transcriptional regulator controlling a bidirectional cell fate switch between skeletal myoblasts and brown fat cells. Loss of PRDM16 from brown fat precursors causes a loss of brown fat characteristics and promotes muscle differentiation. Conversely, ectopic expression of PRDM16 in myoblasts induces their differentiation into brown fat cells. Very clearly, Spiegelman et al. demonstrated that PRDM16 specifies the brown fat lineage from a progenitor expressing myoblast markers.

Crisan M, Casteilla L, Lehr L, Carmona M, Paoloni-Giacobino A, Yap S, Sun B, Léger B, Logar A, Pénicaud L, Schrauwen P, Cameron-Smith D, Russell A P, Péault B, Giacobino J P. A reservoir of brown adipocyte progenitors in human skeletal muscle. Stem Cells 2008: 26: 2425–2433.

A very important paper claiming that a population of cells present in human skeletal muscle can develop in culture as functional brown adipocytes. In other words, an important conclusion from this study is that a reservoir of brown adipocyte progenitors in skeletal muscle could facilitate increased, fat oxidation and energy loss in obese patients.

Does UCP1 exist in cells other than brown adipocytes? Is UCP1 present in skin?

Ricquier D, Raimbault S, Champigny O, Miroux B, Bouillaud F. Comment to Shinohara et al. (1991) FEBS Letters 293, 173–174. The uncoupling protein is not expressed in rat liver. FEBS Lett 1992: 303: 103–106.

Whereas numerous studies confirmed the specific presence of UCP1 in brown adipocytes. A few teams reported data against this dogma. In this paper the authors refuted data claiming the presence of UCP1 in liver.

Frontini A, Rousset S, Cassard-Doulcier A M, Zingaretti C, Ricquier D, Cinti S. Thymus uncoupling protein 1 is exclusive to typical brown adipocytes and is not found in thymocytes. J Histochem Cytochem 2007: 55: 183–189.

In response to a paper from R. Porter’s laboratory describing UCP1 expression in thymocytes (Carroll et al. J Biol Chem 2005: 280: 15534–15543), this paper, based on histology and immunological detection of UCP1, argues against the presence and a direct role for this mitochondrial transporter in immune cells. It is reported that typical brown adipocytes are adjacent to thymic lobes or may be deeply embedded between these lobes.

Adams A E, Carroll A M, Fallon P G, Porter R K. Mitochondrial uncoupling protein 1 expression in thymocytes. Biochim Biophys Acta 2008: 1777: 772–776.

A surprising paper reporting the presence of UCP1 in thymocytes. The data were confirmed using Ucp1-KO mice. This paper is very intriguing and challenges the accepted idea that UCP1 is uniquely expressed in brown adipocytes.

Mori S, Yoshizuka N, Takizawa M, Takema Y, Murase T, Tokimitsu I, Saito M. Expression of uncoupling proteins in human skin and skin-derived cells. J Invest Dermatol 2008: 128: 1894–1900.

This is another surprising paper challenging the unique expression of UCP1 in brown adipocytes. Using RT-PCR, the authors identified UCP1 mRNA in human skin-derived cells as well as in human epidermis. Using immuno-histochemistry, they also reported the presence of UCP1 at the protein level in the granular layer of the epidermis, in sweat glands and in hair follicles. In addition, in agreement with what is known in brown adipocytes, Mori et al. described an up-regulation of skin UCP1 by adrenergic agonists and retinoic acid. Finally, the authors proposed a role for UCP1 either in skin thermogenesis or in the regulation of reactive oxygen species.

Induction of UCP1 and brown adipocytes in man

Champigny O, Ricquier D, Blondel O, Mayers R M, Briscoe M G and Holloway B R. ß3-adrenoceptor stimulation restores message and expression of brown-fat mitochondrial uncoupling protein in adult dogs. Proc Natl Acad Sci USA 1991: 88: 10774–10777.

A paper supporting the importance of adipocyte ß3-adrenoreceptor activation in promoting brown fat development in large-size adult mammals. Administration of such a compound reinduced active brown adipocytes in dogs and also reduced increase in body fat content. However, further studies conducted in man did not confirm these data as ß3-adrenoreceptor agonists were pooorly active and in humans and also induced ß2-adrenoceptor effects.

Krief S, Lönnqvist F, Raimbault S, Baude B, Van Spronsen A, Arner P, Strosberg A D, Ricquier D, Emorine L J. Tissue distribution of beta 3-adrenergic receptor mRNA in man. J Clin Invest. 1993: 91: 344–349.

This is a significant paper dealing with the question of ß3-adrenoceptor presence in human. This study demonstrated that ß3-adrenoceptor transcripts were abundant in infant perirenal brown adipose tissue, characterized by the presence of uncoupling protein (UCP1) mRNA. This study also demonstrated that in adults, brown fat deposits, which weakly abundant, are the main sites of ß3-adrenoceptor expression, which also occurs in gallbladder and colon.

Champigny O, Ricquier D. Evidence from in vitro differentiating cells that adrenoceptor agonists can increase uncoupling protein mRNA level in adipocytes of adult humans: an RT-PCR study. J Lipid Res 1996: 37: 1907–1914.

This is an interesting paper illustrating the potential of cultured human pre-adipocytes to induce UCP1 gene transcription in response to adrenergic signals. The model described can be used to screen drugs facilitating fat oxidation and thermogenesis.

Ghorbani M, Himms-Hagen J. Appearance of brown adipocytes in white adipose tissue during CL 316,243-induced reversal of obesity and diabetes in Zucker fa/fa rats. Int J Obes Relat Metab Disord 1997: 21: 465–475.

This important paper relates the marked efficiency of CL 316,243, a specific agonist of ß3-adrenoceptors, in inducing brown adipocytes containing UCP1 in rodents. Interestingly, a second effect of this drug was a remodelling of white adipose depots and also a reduction of hyperinsulinaemia and hyperglycaemia of fa/fa rats. Clearly, the CL compound is an effective anti-obesity and anti-diabetic agent in fa/fa rats. Unfortunately, future studies in humans will not confirm these data.

Frayn K N, Langin D, Karpe F. Fatty acid-induced mitochondrial uncoupling in adipocytes is not a promising target for treatment of insulin resistance unless adipocyte oxidative capacity is increased. Diabetologia 2008: 5: 394–397.

This is an interesting paper reminds that activation of lipolysis and activation of respiration uncoupling are not sufficient to decrease body fat content and insulin resistance unless a stimulation of mitochondriogenesis allowing fat oxidation is not obtained.

Rossmeisl M, Barbatelli G, Flachs P, Brauner P, Zingaretti M C, Marelli M, Janovská P, Horáková M, Syrový I, Cinti S, Kopecký J. Expression of the uncoupling protein 1 from the aP2 gene promoter stimulates mitochondrial biogenesis in unilocular adipocytes in vivo. Eur J Biochem 2002: 269: 19–28.

Interesting paper showing that ectopic expression of UCP1 in white adipocytes in transgenic mice mitigates the development of obesity by both increasing energy expenditure and decreasing in situ lipogenesis. Surprisingly, the expression of UCP1 in white adipocyte triggered mitochondriogenesis and participated in an apparent conversion of white into brown adipocytes.

Tiraby C, Tavernier G, Lefort C, Larrouy D, Bouillaud F, Ricquier D, Langin D. Acquirement of brown fat cell features by human white adipocytes. J Biol Chem 2003: 278: 33370–33376.

This is a very interesting paper showing that overexpression of the transcriptional co-activator PGC-1alpha in human white adipocytes induces both mitochondria and UCP1 in a manner that fatty acid oxidation was significantly enhanced. These data support the concept of an alteration in energy balance in man through a conversion of white to brown adipose tissue.

Rodrigue-Way A, Demers A, Ong H, Tremblay A. Endocrinology. A growth hormone-releasing peptide promotes mitochondrial biogenesis and a fat burning-like phenotype through scavenger receptor CD36 in white adipocytes. Endocrinology 2007: 148: 1009–1018.

This is an important paper demonstrating that hexarelin, a partner of CD36 strongly induces mitochondriogenesis, UCP1 and thermogenesis in 3T3-L1 adipocytes.

Adipocyte and inflammation

Karine Clément
INSERM Nutriomique U872 (Eq 7), Université Pierre et Marie Curie-Paris 6 Centre de Recherche des Cordeliers, Paris et Pôle Endocrinologie et Nutrition, CRNH Ile de France, Batiment Husson Mourrier Hopital Pitié-Salpêtrière, Paris, France

The increase in circulating inflammatory factors found in obesity and the recent discovery of macrophage accumulation in human adipose tissue opened a new field of research in obesity. This so-called ‘low grade’ inflammatory state is characterized by the moderate but chronic systemic rise of an increasing panel of molecules, which carry out in addition to pro- or anti-inflammatory action immune and/or metabolic functions. The qualitative and quantitative alterations in the production of these molecules – the adipokines – by the different cell types of the adipose tissue in obesity are considered as important factors which can modify the local biology in the tissue, and may at the same time contribute to obesity complications. Low grade inflammation in obesity associates with inflammatory cell accumulation in adipose tissue (macrophages, lymphocytes, mastocytes and others). Inflammatory cell accumulation modifies the biology of adipocytes (lipolysis, inflammation, insulin-sensitivity) and pre-adipocytes (inflammation, proliferation, diminished differentiation) as well as promotes the development of interstitial fibrosis. These phenomena are the hallmark of the pathological alteration of the adipose tissue in human obesity.

Hotamisligil G S, Shargill N S, Spiegelman B M. Adipose expression of tumor necrosis factor-alpha: direct role in obesity-linked insulin resistance. Science 1993: 259: 87–91.

Fifteen years ago, links between the inflammation of adipose tissue and its potential role in systemic complications of obesity were proposed in a pioneering work by Hotamisligil et al. In this study carried out in mice, this group showed that the white adipose tissue synthesises a pro-inflammatory cytokine – the tumor necrosis factor α (TNFα) – the expression of which was elevated in adipocytes of insulin-resistant and obese mice. Insulin sensitivity in the animals could be improved using TNFα antibodies. These first observations underlined the existence of a link between a pro-inflammatory cytokines, produced and secreted by the adipose tissue, and the development of insulin resistance in rodents. However, the neutralization of TNFα did not appear to be very effective in controlling insulin resistance in humans.

Trayhurn P, Wood I S. Adipokines: inflammation and the pleiotropic role of white adipose tissue. Br J Nutr 2004: 92: 347–355.

This is a review paper discussing the contribution of adipose tissue in secreting active molecules, which might exert systemic actions. These molecules called ‘adipokines’ group a very broad panel of factors participating in the innate, adaptive immunity, growth, haematopoiesis and metabolism amongst others. The most studied ones remain adipocyte-produced leptin and adiponectin. Historically, these adipokines are involved in the organs cross-talking that connect weight homeostasis, insulin sensitivity and many other key biological systems. In contrast to leptin, known as a pro-inflammatory mediator, adiponectin has compelling anti-inflammatory functions and is an insulin-sensitizer molecule. These adipokines are specifically secreted by adipocytes whereas many other adipose tissue-produced adipokines are synthesized by other cell types grouped in the stroma vascular fraction.

Weisberg S P, McCann D, Desai M, Rosenbaum M, Leibel R L, Ferrante A W, Jr. Obesity is associated with macrophage accumulation in adipose tissue. J Clin Invest 2003: 112: 1796–1808.

Xu H, Barnes G T, Yang Q, Tan G, Yang D, Chou C J, Sole J, Nichols A, Ross J S, Tartaglia L A, Chen H. Chronic inflammation in fat plays a crucial role in the development of obesity-related insulin resistance. J Clin Invest 2003: 112: 1821–1830.

These two North American studies have shown that adipose tissues of obese patients or obese rodents are the site of macrophage accumulation in proportion to body mass index and/or fat mass. These macrophages are an important source of inflammatory mediators by the adipose tissue. Based on transplantation studies of bone marrow in irradiated mice, it is established that the accumulation of adipose tissue macrophages derived mostly from bone marrow cells. In rodents, this inflammation in the adipose tissue is associated with the installation of insulin resistance and thus represents a link between obesity and obesity co-morbidities.

Keophiphath M, Achard V, Henegar C, Rouault C, Clement K, Lacasa D. Macrophage-secreted factors promote a profibrotic phenotype in human pre-adipocytes. Mol Endocrinol 2009: 23: 11–24.

Several observations support the hypothesis of the adverse role of macrophage accumulation in adipose tissue biology and diseases associated with obesity. The paracrine dialogues are interaction between cells located in close proximity to each other through mediators. By this paracrine action, the macrophages could contribute to the local control of growth of fat and modify the biology of adipocytes and pre-adipocytes via local production of adipokines. In this study, the presence of pre-adipocytes with secreted media of human macrophages results in a drastic change of pre-adipocytes’ phenotypes, which acquire a pro-inflammatory phenotype by secreting mediators, such as interleukins 6 and 8. These cells also proliferate, acquire migratory properties and differentiate poorly. These ‘inflammatory’ pre-adipocytes also synthesize elements of the extracellular matrix. They could thus contribute to the establishment of interstitial fibrosis in the adipose tissue during human obesity.

Permana P A, Menge C, Reaven P D. Macrophage-secreted factors induce adipocyte inflammation and insulin resistance. Biochem Biophys Res Commun 2006: 341: 507–514.

Suganami T, Tanimoto-Koyama K, Nishida J, Itoh M, Yuan X, Mizuarai S, Kotani H, Yamaoka S, Miyake K, Aoe S, Kamei Y, Ogawa Y. Role of the Toll-like receptor 4/NF-kappaB pathway in saturated fatty acid-induced inflammatory changes in the interaction between adipocytes and macrophages. Arterioscler Thromb Vasc Biol 2007: 27: 84–91.

These two studies show that the mature adipocytes undergo profound changes in their biology when co-cultured in the presence of activated macrophages. Besides a pro-inflammatory state, there is an increased lipolysis and resistance to the effects of insulin. The TNFα is a mediator of these effects. The Nuclear, factor-kappa B (NF-κb) pathway well-known to be involved in the regulation of primary inflammatory responses, is induced in both the pre-adipocyte (Keophiphath M et al., Mol Endocrinol, 2009: 23: 11–24) and the adipocytes, when these cells are in contact with macrophage secretions. The toll-like receptors (TLR), and in particular TLR4, appear to be major players that lead to the induction or suppression of genes that orchestrate the inflammatory response. Fatty acids released by adipocytes after adrenergic stimulation are also potent inducers of TLR4/NF-kappa B. Thus, excess fatty acids released by adipocytes and stimulated by the inflammatory milieu bind TLR4 and induce these inflammatory changes, thereby exacerbating inflammation of macrophages.

Clement K, Viguerie N, Poitou C, Carette C, Pelloux V, Curat C A, Sicard A, Rome S, Benis A, Zucker J D, Vidal H, Laville M, Barsh G S, Basdevant A, Stich V, Cancello R, Langin D. Weight loss regulates inflammation-related genes in white adipose tissue of obese subjects. Faseb J 2004: 18: 1657–1669.

Reducing weight even modestly improves complications related to obesity. Numerous studies have shown that reducing dietary intake (diet reduced in calories) and sometimes increasing physical activity are factors reducing systemic inflammation. By studying gene expression in obese women, it is shown that the improvement of systemic inflammation during a moderate diet-induced weight loss is associated with a change in expression of inflammatory genes in the subcutaneous adipose tissue. These genes belong to functional families including cytokines, interleukins, the cascade of complement factors and acute phase molecules as well as molecules involved in cellular contact and remodelling.

Cancello R, Henegar C, Viguerie N, Taleb S, Poitou C, Rouault C, Coupaye M, Pelloux V, Hugol D, Bouillot J L, Bouloumié A, barbatelli G, Cinti S, Svensson P A, Barsh G S, Zucker J D, Basdevant A, Lanvin D, Clement K. Reduction of macrophage infiltration and chemoattractant gene expression changes in white adipose tissue of morbidly obese subjects after surgery-induced weight loss. Diabetes 2005: 54: 2277–2286.

The improvement in the inflammatory profile during weight loss is not only related to the reduction in expression of pro-inflammatory mediators, but also to the increased expression of anti-inflammatory factors such as IL-10 or IL-1-Ra. Reduction in the expression of inflammatory genes in adipose tissue was confirmed in subjects after drastic weight loss induced by obesity surgery and was associated with a significant reduction of macrophage accumulation in human adipose tissue.

Henegar C, Tordjman J, Achard V, Lacasa D, Cremer I, Guerre-Millo M, Poitou C, Basdevant A, Stich V, Viguerie N, Langin D, Bedossa P, Zucker J D, Clement K. Adipose tissue transcriptomic signature highlights the pathological relevance of extracellular matrix in human obesity. Genome Biol 2008: 9: R14.

In this study, the authors analyzed the transcriptomic signature of the subcutaneous white adipose tissue in obese subjects, in stable weight conditions and after weight loss following bariatric surgery. These analyses highlighted a significant up-regulation of genes and biological themes related to extracellular matrix (ECM) constituents, including members of the integrin family, and suggested that these elements could play a major mediating role in a chain of interactions that connect local inflammatory phenomena to the alteration of WAT metabolic functions in obese subjects. Tissue and cellular investigations, driven by the analysis of transcriptional interactions, revealed an increased amount of interstitial fibrosis in obese WAT, associated with an infiltration of different types of inflammatory cells. This human study opens perspectives in understanding the biology of human WAT and its pathological changes indicative of tissue deterioration associated with the development of obesity.

Lumeng C N, Bodzin J L, Saltiel A R. Obesity induces a phenotypic switch in adipose tissue macrophage polarization. J Clin Invest 2007: 117: 175–184.

This is a paper studying the migration of macrophages to adipose tissue upon high-fat diet, in mice. The authors found a novel population of adipose tissue macrophage in adipose tissue of obese mice. Diet-induced obesity decreased the expression of resident macrophages in ATMs, while increasing expression of genes such as those encoding TNF-alpha that are characteristic of M1 or ‘classically activated’ macrophages. Whereas macrophages from lean mice expressed many genes characteristic of M2 or ‘alternatively activated’ macrophages, including interleukin 10, macrophages from obese mice express pro-inflammatory mediators. The anti-inflammatory cytokine IL-10, which was overexpressed in adipose tissue macrophages from lean mice, protected adipocytes from TNF-alpha-induced insulin resistance. This paper suggests that diet-induced obesity leads to a shift in the activation state of macrophages from an M2-polarized state to an M1 pro-inflammatory state that contributes to insulin resistance.

Genetic lipodystrophies

David B. Savage
Metabolic Research Laboratories, Institute of Metabolic Science, University of Cambridge, Addenbrooke’s Hospital, Cambridge, UK

White adipose tissue (WAT) is essential for efficient energy (lipid) storage and release. It also synthesizes and secretes important metabolic hormones such as leptin and adiponectin. The importance of fat in human metabolism is highlighted by lipodystrophic syndromes, a heterogeneous cluster of disorders characterized by a pathological state of adipose tissue deficiency. Lipodystrophy is distinct from leanness, a state in which ‘empty adipocytes’ can readily adapt to positive energy balance. Instead, in lipodystrophic subjects, positive energy balance leads to ectopic fat deposition in the liver and other organs, insulin resistance and diabetes. Paradoxically, the metabolic consequences of having ‘too little’ fat (lipodystrophy) are remarkably similar to those of having ‘too much’ fat (obesity).

Lipodystrophies result from either the failure of adipocyte development or premature destruction of adipocytes because of genetic or immunological mechanisms. The fat loss is typically partial or generalized. Recent progress in understanding the genetic basis of several forms of familial lipodystrophy has facilitated improved clinical diagnostic workup in patients with lipodystrophy as well as providing novel insights into adipocyte biology. Biallelic loss of function mutations in BSCL2, AGPAT2 or CAV1 account for > 90% of all cases of congenital generalized lipodystrophy, whereas heterozygous mutations in LMNA and PPARG account for > 50% of all inherited cases of partial lipodystrophy. Rare homozygous and compound heterozygous mutations in LMNA and ZMPSTE24 have also been reported and in one family with partial lipodystrophy, a heterozygous mutation in AKT2 was identified. We have also recently identified novel subtypes of inherited partial lipodystrophy; these will be discussed at the meeting.

Frayn K N. Adipose tissue as a buffer for daily lipid flux. Diabetologia 2002: 45: 1201–1210.

This is an excellent short review describing the biological role of adipose tissue.

Garg A. Acquired and inherited lipodystrophies. N Engl J Med 2004: 350: 1220–1234.

Garg A, Agarwal AK. Lipodystrophies: Disorders of adipose tissue biology. Biochim Biophys Acta 2009: 1791: 507–513.

These two recent reviews describe the clinical features and genetic aetiology of human lipodystrophies.

Savage D B, Petersen K F, Shulman GI. Disordered lipid metabolism and the pathogenesis of insulin resistance. Physiol Rev 2007: 87: 507–520.

This is a recent summary of data suggesting that primary abnormalities in lipid metabolism contribute to the pathogenesis of insulin resistance and type 2 diabetes.

Oral E A, Simha V, Ruiz E, Andewelt A, Premkumar A, Snell P, Wagner A J, DePaoli A M, Reitman M L, Taylor S I, Gorden P, Garg A. Leptin-replacement therapy for lipodystrophy. N Engl J Med 2002: 346: 570–578.

This is a landmark study demonstrating the metabolic benefits of leptin replacement therapy in humans with generalized lipodystrophy.

Semple R K, Chatterjee V K and O’Rahilly S. PPAR gamma and human metabolic disease. J Clin Invest 2006: 116: 581–589.

This is a succinct review of the human disorder associated with PPARG mutations. These are of particular interest as PPARg is the receptor for thiazolidinediones (TZDs), which are now widely used in the treatment of type 2 diabetes.

Capell B C, Collins F S. Human laminopathies: nuclei gone genetically awry. Nat Rev Genet 2006: 12: 940–942.

Remarkably, mutations in the LMNA gene have been associated with up to 13 different diseases, of which partial lipodystrophy is one. The genetic and molecular details of these disorders are reviewed in this paper.

Acquired lipodystrophies in HIV-infected patients

Jacqueline Capeau
INSERM, UPMC Univ Paris 06, UMR-S 938, Centre de Recherche Saint-Antoine, AP-HP, Hôpital Tenon, Paris, France

The different classes of antiretroviral drugs used to treat HIV infection allow, at present, the control of the viral infection in most cases. However, at the time when protease inhibitors (PI) were introduced in association with the class of nucleoside reverse transcriptase inhibitors (NRTI), a number of patients underwent lipodystrophy, with severe peripheral lipoatrophy and, in some patients, excessive visceral fat accumulation. These alterations in fat distribution were associated with metabolic disturbances, insulin resistance, diabetes and dyslipidaemia (1).

The pathophysiology of this lipodystrophy is complex and implies the two antiretroviral classes (2,3,4). Moreover, it was recently shown that a chronic treatment of different cell types with some PIs and also with the two thymidine analogues of NRTIs was able to induce cellular premature senescence (5,6) in keeping with the phenotype of premature ageing observed in long-term HIV-infected patients.

More recently, a role for HIV infection has been also postulated, given that HIV can act on adipocytes, in particular through the release by infected resident macrophages inside adipose tissue of viral proteins, which alter adipocyte phenotype (7).

Therefore, taken as a whole, the severity of HIV-related lipodystrophy, observed when patients were treated with first generation antiretrovirals, could result from different factors, all aggressing adipocytes. These factors include the simultaneous use of drugs able to impact negatively on adipose tissue, but also long-term HIV infection, with probably, in most patients, the constitution of virus reservoirs in macrophages inside adipose tissue.

In most patients who switched from first- second-generation antiretrovirals, lipodystrophy improved, but the reversion is slow and sometimes incomplete. Plastic surgery could provide a valuable improvement at least for facial lipoatrophy when severe (8,9).

1. Grinspoon S, Carr A. Cardiovascular risk and body-fat abnormalities in HIV-infected adults N Engl J Med 2005: 352: 48–62.

This is an excellent general review on the clinical aspects of lipodystrophies and related metabolic complications.

2. Gougeon M L, Penicaud L, Fromenty B, Leclercq P, Viard J P, Capeau J. Adipocytes targets and actors in the pathogenesis of HIV-associated lipodystrophy and metabolic alterations. Antivir Ther 2004: 9: 161–177.

This is a general review on the pathophysiology of lipodystrophy and the alterations observed in vivo in patients’ adipose tissue and in vitro in cultured adipocytes treated with different antiretrovirals.

3. Lagathu C, Kim M, Maachi M, Vigouroux C, Cervera P, Capeau J, Caron M, Bastard J P. HIV antiretroviral treatment alters adipokine expression and insulin sensitivity of adipose tissue in vitro and in vivo. Biochimie 2005: 87: 65–71.

This is a review paper on the deleterious impact of first-generation and some second-generation PI on adipocytes, these drugs being able to inhibit differentiation, induce insulin resistance and increase the production by adipocytes of pro-inflammatory cytokines. The deleterious role of the second antiretroviral drug class playing a leading role in lipoatrophy, thymidine analogue NRTI and in particular the two first very active molecules, stavudine and zidovudine is also presented. These drugs were able to alter markedly mitochondrial potential and increase oxidative stress in adipocytes, leading to decreased adiponectin secretion.

4. Kim M J, Leclercq P, Lanoy E, Cervera P, Antuna-Puente B, Maachi M, Dorofeev E, Slama L, Valantin M A, Costagliola D, Lombes A, Bastard J P, Capeau J. A 6-month interruption of antiretroviral therapy improves adipose tissue function in HIV-infected patients: the ANRS EP29 Lipostop Study. Antivir Ther 2007: 12: 1273–1283.

The demonstration of the toxic effects of antiretroviral drugs on adipose tissue was investigated in HIV-infected patients who were able to stop any antiretroviral treatment for at least 6 months. In this ANRS Lipostop study, initial adipose tissue inflammation was markedly decreased, this improvement being related to the interruption of stavudine or zidovudine. However, both PI and thymidine analogues negatively impacted on other different adipocyte functions.

5. Caron M, Auclair M, Donadille B, Bereziat V, Guerci B, Laville M, Narbonne H, Bodemer C, Lascols O, Capeau J, Vigouroux C. Human lipodystrophies linked to mutations in A-type lamins and to HIV protease inhibitor therapy are both associated with prelamin A accumulation, oxidative stress and premature cellular senescence. Cell Death Differ 2007: 14: 1759–1767.

This paper stresses the similarities between lamin-linked and HIV-related lipodystrophies and revealing that a major pathway, which could explain the effect of PIs, results from their ability to inhibit the enzyme ZMP-STE24 involved in prelamin maturation, therefore, leading to the accumulation of farnesylated prelamin A, to increased oxidative stress and to induction of an early cellular senescence. This point is important to consider given the phenotype of premature ageing, which is frequently observed in HIV-infected patients.

6. Caron M, Auclair M, Vissian A, Vigouroux C, Capeau J. Contribution of mitochondrial dysfunction and oxidative stress to cellular premature senescence induced by antiretroviral thymidine analogues. Antivir Ther 2008: 13: 27–38.

This paper indicates that altered mitochondrial function induced by thymidine analogues NRTI results in premature cellular senescence.

7. Giralt M, Domingo P, Villarroya F. HIV-1 Infection and the PPARgamma-Dependent Control of Adipose Tissue Physiology. PPAR Res 2009; 2009: 607902.

This is a review on the effect of the virus on adipose tissue and adipocytes.

8. Pirmohamed M. Clinical management of HIV-associated lipodystrophy. Curr Opin Lipidol 2009: 20: 309–314.

This study demonstrates different options to treat lipodystrophy.

9. Mulligan K, Khatami H, Schwarz J M, Sakkas G K, DePaoli A M, Tai V W, Wen M J, Lee G A, Grunfeld C, Schambelan M. The effects of recombinant human leptin on visceral fat, dyslipidemia, and insulin resistance in patients with human immunodeficiency virus-associated lipoatrophy and hypoleptinemia. J Clin Endocrinol Metab 2009: 94: 1137–1144.

Treatment with recombinant leptin was able to improve dyslipidemia and visceral fat hypertrophy.

Current knowledge in the involvement of steroid receptors in adipocyte biology

Bruno Fève
INSERM U693, Faculté de Médecine Paris-Sud, Université Paris 11, Le Kremlin Bicêtre cedex, France

Besides their role in the control of water and electrolytes homeostasis, recent data clearly indicate that aldosterone and the mineralocorticoid receptor (MR) are involved in adipocyte biology. It has been recently shown that aldosterone promotes white and brown adipocyte differentiation in vitro through specific activation of the MR. In addition, a non-epithelial pro-inflammatory role for MR activation has been recently inferred from studies on mineralocorticoid/salt administration in experimental animal models and from clinical studies. The mineralocorticoid system, could, hence represent a potential target for new therapeutical strategies in obesity and the metabolic syndrome.

Zennaro M C, Caprio M, Fève B. Mineralocorticoid receptors in the metabolic syndrome. Trends Endocrinol Metab 2009, in press.

This review article explains how in cells and tissues devoid of type 2 11-beta hydroxysteroid dehydrogenase glucocorticoids cannot be inactivated. As a consequence, mineralocorticoid receptors are not protected from activation by glucocorticoids. Under these conditions, glucocorticoids very likely represent the main effectors for mineralocorticoid receptors. In pre-adipose and adipose cells, mineralocorticoid receptors mediate both pro-adipogenic and pro-inflammatory effects, and could, thus represent important factors of the metabolic syndrome.

Ronconi V, Turchi F, Bujalska I J, Giacchetti G, Boscaro M. Adipose cell-adrenal interactions: current knowledge and future perspectives. Trends Endocrinol Metab 2008: 19: 100–103.

In light of recent experimental data, cross-talk between adipose tissue and the adrenal gland, particularly via the mineralocorticoid aldosterone, has been proposed. Aldosterone can induce adipogenesis, and human white adipose tissue is reported to release as-yet-uncharacterized factors that stimulate adrenocortical steroidogenesis and aldosterone production. These data could provide new insights in the pathophysiology of obesity-related disorders, including hypertension and aldosterone excess, with further studies necessary for confirming and better defining such adipose–adrenal interactions.

Zennaro M C, Le Menuet D, Viengchareun S, Walker F, Ricquier D, Lombès M. Hibernoma development in transgenic mice identifies brown adipose tissue as a novel target of aldosterone action. J Clin Invest 1998: 101: 1254–1260.

With the aim to identify the regulatory mechanisms controlling tissue-specific expression of the human mineralocorticoid receptor in vivo, Zennaro et al. generated transgenic mouse models expressing the SV40 large T antigen under the control of each of the two promoters (P1 and P2) of the human mineralocorticoid receptor gene. Unexpectedly, mice that overexpress SV40 T antigen under the control of the P1 promoter died prematurely from voluminous liposarcomas, which originated from brown adipose tissue, as evidenced by the strong expression of the uncoupling protein UCP1. Cell lines derived from this hibernoma were established, and retained the ability to differentiate in vitro to in brown adipocytes. These cells allowed to demonstrate that endogenous MR activation promotes brown adipogenesis and mediates down-regulation of thermogenesis.

Caprio B, Fève B, Claës A, Viengchareun S, Lombès M, Zennaro M C. Pivotal role of the mineralocorticoid receptor in corticosteroid-induced adipogenesis. FASEB J 2007: 21: 2185–2194.

This original work demonstrates that mineralocorticoid receptors are expressed in adipocytes in a differentiation-dependent manner. Strikingly, whereas aldosterone promotes adipose conversion when added to steroid-free differentiation medium, mineralocorticoid receptor down-regulation by RNA interference strongly reduces the capacity of pre-adipocytes to differentiate. This effect cannot be mimicked by glucocorticoid receptor down-regulation. These data show that the mineralocorticoid receptor represents an important proadipogenic factor that may mediate both aldosterone and glucocorticoid effects on adipose tissue development. Thus, mineralocorticoid receptors may be of pathophysiological relevance to the development of obesity and the metabolic syndrome.

Corbould A. Effects of spironolactone on glucose transport and interleukin-6 expression in adipose cells of women. Horm Metab Res 2007: 39: 915–918.

The mineralocorticoid receptor antagonist spironolactone is able to increase basal and insulin-stimulated glucose uptake in human adipocytes, without altering insulin sensitivity. This effect was not due to changes in abundance of glucose transporters 1 and 4, or to the degree of adipocyte differentiation. However, the fact that this effect is not reproduced by the active metabolite of spironolactone canrenoic acid, and is independent of androgen, glucocorticoid and overall mineralocorticoid receptor blockade, questions the actual involvement of these receptors in spironolactone effect.

Wada T, Ohshima S, Fujisawa E, Koya D, Tsuneki H, Sasaoka T. Aldosterone inhibits insulin-induced glucose uptake by degradation of Insulin Receptor Substrate (IRS) 1 and IRS2 via reactive oxygen species-mediated pathway in 3T3-L1 adipocytes. Endocrinology 2009: 150: 1662–1669.

In adipocytes, aldosterone alters insulin-stimulated glucose uptake by reducing the amounts of insulin receptor substrate 1 (IRS1) and IRS2, in a time- and dose-dependent manner. Degradation of IRSs was prevented by a glucocorticoid receptor antagonist and antioxidant N-acetylcysteine, but not by a mineralocorticoid receptor antagonist. These results indicate that aldosterone deteriorates metabolic action of insulin by facilitating the degradation of IRSs via glucocorticoid receptor-mediated production of reactive oxygen species.

Guo C, Ricchiuti V, Lian B Q, Yao T M, Coutinho P, Romero J R, Li J, Williams G H, Adler G K. Mineralocorticoid receptor blockade reverses obesity-related changes in expression of adiponectin, peroxisome proliferator-activated receptor-gamma, and proinflammatory cytokines. Circulation 2008: 117: 2253–2261.

The authors show that in db/db mice, mineralocorticoid receptor blockade with eplerenone for 16 weeks prevents the obesity-induced changes in several parameters, including the induction of TNF-alpha, PAI-1, MCP-1 expression, and macrophage infiltration, and the repression of adiponectin and PPAR-gamma. In addition, treatment of undifferentiated pre-adipocytes with aldosterone increases mRNA levels of TNF-alpha and MCP-1, and reduces the transcripts levels of adiponectin and PPAR-gamma, supporting the notion of a direct effect of the hormone on gene expression. Thus, mineralocorticoid receptor blockade reduces expression of pro-inflammatory and prothrombotic factors in adipose tissue, and increases the expression of adiponectin in adipose tissue of obese diabetic mice. These effects on adiponectin and adipokine gene expression may represent a novel mechanism for the cardioprotective effects of mineralocorticoid receptor blockade.

Hirata A, Maeda N, Hiuge A, Hibuse T, Okada T, Kihara S, Funahashi T, Shimomura I. Blockade of mineralocorticoid receptor reverses adipocyte dysfunction and insulin resistance in obese mice. Cardiovasc Res 2009 Jun 27 [Epub ahead of print].

This work shows that in ob/ob or db/db obese mice, treatment with the MR antagonist eplerenone significantly reduces insulin resistance, and suppresses macrophage infiltration and reactive oxygen species (ROS) production in adipose tissues. In 3T3-L1 adipocytes, aldosterone and H2O2 increase intracellular ROS and emergence of a pro-inflammatory pattern, whereas mineralocorticoid blockade inhibits such effects. Thus, mineralocorticoid receptor blockade attenuates obesity-related insulin resistance partly through reduction of fat ROS production, inflammatory process and induction of cytokines.

Liu G, Grifman R, Feily B, Chatterton J E, Staal F W, Li Q X. Mineralocorticoid receptor is involved in the regulation of genes responsible for hepatic glucose production. Biochem Biophys Res Commun 2006: 342: 1291–1296.

This study reveals a novel role of the mineralocorticoid receptor in the regulation of genes related to hepatic glucose production. RNAi-mediated mineralocorticoid receptor silencing decreases the expression of glucose-6-phosphatase, phosphoenolpyruvate carboxykinase and fructose-1,6-bisphosphatase 1, key enzymes of neoglucogenesis in liver. Mineralocorticoid receptor-specific antagonists also down-regulate the expression of glucose-6-phosphatase, whereas a specific agonist enhances glucose-6-phosphatase expression. These data suggest an original role for mineralocorticoid receptor in mediating de novo glucose synthesis in hepatocytes.

Caprio M, Newfell B G, La Sala A, Baur W, Fabbri A, Rosano G, Mendelsohn M E, Jaffe I Z. Functional mineralocorticoid receptors in human vascular endothelial cells regulate Intercellular Adhesion Molecule-1 expression and promote leukocyte adhesion. Circ Res 2008: 102: 1359–1367.

This important article describes, for the first time, the presence of functional mineralocorticoid receptors on human endothelial cells from coronary or aortic origins. These endothelial cells express 11-beta hydroxysteroid dehydrogenase-2, which protects the mineralocorticoid receptor from cortisol activation. In addition, aldosterone stimulates the expression of Intercellular Adhesion Molecule-1, which in turn promotes leukocyte adhesion. These findings demonstrate that aldosterone activates endogenous endothelial cell mineralocorticoid receptors and proatherogenic gene expression in clinically important human endothelial cells. This study describes a novel mechanism by which aldosterone may influence isachemic cardiovascular events and supports a new explanation for the decrease in ischaemic events in patients treated with aldosterone antagonists.

Adipose tissue expandability, lipotoxicity and the metabolic syndrome

Antonio Vidal-Puig
Metabolic Research Laboratories, Level 4, Institute of Metabolic Science, Addenbrooke’s Hospital, Cambridge, UK

Whereas there is a general agreement that the development of obesity requires positive energy balance, it is less intuitively clear why the expansion of the adipose tissue characteristic of obesity should be associated with important metabolic complications. Similarly, there is epidemiological evidence of progressive impairment in insulin sensitivity in parallel with increases in fat mass. However, this concept is challenged by specific clinical paradigms such as lipodistrophy, metabolically healthy morbid obesity and pharmacological strategies that improve insulin sensitivity by promoting adipose tissue expansion. So understanding of the mechanisms on these divergences may provide important insights to design rational therapeutic approaches. Based on our own and others observations, we recently proposed the adipose tissue expandability hypothesis to explain the link between obesity and metabolic complications. In our opinion, the current epidemic of obesity poses an unprecedented challenge to the mechanisms controlling adipose tissue expansion, which determines an individually determined a maximum threshold of efficient expansion. In fact, we believe that the individual capacity for adipose expansion is finite and per se may be a key determinant of the metabolic complications associated with positive energy balance and obesity.

Sethi J K, Vidal-Puig A J. Thematic review series: adipocyte biology. Adipose tissue function and plasticity orchestrate nutritional adaptation. J Lipid Res 2007: 48: 1253–1262.

This review provides background on adipocyte biology to facilitate complete understanding of the concept of adipose tissue expandability and its implications for disease.

Virtue S, Vidal-Puig A. It’s not how fat you are it is what you do with it that counts. PLOS Biol 2008: 6: e237.

This study analyses the formulation of adipose tissue expandability hypothesis to explain the link between obesity and its metabolic complications.

Tan C Y, Vidal-Puig A. Adipose tissue expandability: the metabolic problems of obesity may arise from the inability to become more obese. Biochem Soc Trans 2008: 36 (Pt 5):935–940.

This study is a discussion of the medical implications and limitations of the adipose tissue expandability hypothesis.

Medina-Gomez G, Gray S L, Yetukuri L, Shimomura K, Virtue S, Campbell M, Curtis R K, Jimenez-Linan M, Blount M, Yeo G S, Lopez M, Seppanen-Laakso T, Ashcroft F M, Oresic M, Vidal-Puig A. PPAR gamma 2 prevents lipotoxicity by controlling adipose tissue expandability and peripheral lipid metabolism. PLoS Genet 2007: 3: e64.

This paper shows the relevance of adipose tissue expansion to maintain energy homeostasis. Of relevance, the POKO mouse is a model of positive energy balance and restricted adipose tissue expansion mediated by defective PPARg2, resulting in a mouse that could be considered overweight based on body weight parameters, but that has an inappropriately severe metabolic syndrome for its degree of obesity. This model allows speculations related to failure of adipose tissue in the context of normal body weight.

Gray S L, Nora E D, Grosse J, Manieri M, Stoeger T, Medina-Gomez G, Burling K, Wattler S, Russ A, Yeo G S, Chatterjee V K, O’Rahilly S, Voshol P J, Cinti S, Vidal-Puig A. Leptin deficiency unmasks the deleterious effects of impaired peroxisome proliferator-activated receptor gamma function (P465L PPARgamma) in mice. Diabetes 2006: 55: 2669–2677.

This manuscript provides evidence showing how just a 15% reduction in adipose tissue mass in a mouse model of morbid obesity results in worsening insulin sensitivity even from very early ages when differences in body weight. This is an example illustrating the more importance of the fact that total amount of fat stored is the mismatch between storage capacity and nutrient availability. This paper shows the concept of impaired adipose tissue expansion as a pathogenic mechanism even in the context of morbid obesity.

Kim J Y, van de Wall E, Laplante M, Azzara A, Trujillo M E, Hofmann S M, Schraw T, Durand J L, Li H, Li G, Jelicks L A, Mehler M F, Hui D Y, Deshaies Y, Shulman G I, Schwartz G J, Scherer PE. Obesity-associated improvements in metabolic profile through expansion of adipose tissue. J Clin Invest. 2007: 117: 2621–2637.

This paper presents evidence of the opposite paradigm. Increasing adipose tissue expansion promotes healthy obesity and prevents ectopic deposition of lipid in metabolic relevant organs such as liver or muscle, maintaining insulin sensitivity despite obesity. This paper clearly supports adipose tissue expandability as a key factor linking obesity with metabolic complications.

Tran T T, Yamamoto Y, Gesta S, Kahn C R. Beneficial effects of subcutaneous fat transplantation on metabolism. Cell Metab 2008: 7: 410–420.

This paper provides evidence related to the specific metabolic characteristics of specific adipose tissue depots and lends support to the concept that adipose tissue expansion is fundamental to maintain metabolic homeostasis.

Lazar M A. How obesity causes diabetes: not a tall tale. Science 2005: 307: 373–375

This is an important review that provides background information to integrate the adipose tissue expandability hypothesis in the context of other proposed theories linking obesity with the metabolic syndrome. Interestingly, the information provided in this review, together with the information provided in other references, allows us to integrate the adipose tissue expandability concept with the concept of obesity as a low-grade inflammation state.

Weisberg S P, McCann D, Desai M, Rosenbaum M, Leibel R L, Ferrante A W Jr. Obesity is associated with macrophage accumulation in adipose tissue. J Clin Invest 2003: 112: 1796–1808.

This is a seminal paper identifying macrophage within adipose tissue as important modulators of obesity-associated insulin resistance.

Spalding K L, Arner E, Westermark P O, Bernard S, Buchholz B A, Bergmann O, Blomqvist L, Hoffstedt J, Näslund E, Britton T, Concha H, Hassan M, Rydén M, Frisén J, Arner P. Dynamics of fat cell turnover in humans. Nature 2008: 453: 783–787.

This is a seminal paper showing that a number of adipocytes in adipose tissue are relatively fixed for a specific individual, suggesting important mechanisms controlling adipose tissue expandability and reinforcing the concept that in the presence of positive energy balance, a mismatch with adipose tissue expandability may lead to adverse metabolic conditions.

Karelis A D, Brochu M, Rabasa-Lhoret R. Can we identify metabolically healthy but obese individuals (MHO). Diabetes Metab 2004: 30: 569–572.

This paper addresses the existence of metabolically healthy obesity in humans.

Karelis A D, Faraj M, Bastard J P, St-Pierre D H, Brochu M, Prud(homme D, Rabasa-Lhoret R. The metabolically healthy but obese individual presents a favorable inflammation profile. J Clin Endocrinol Metab 2005: 90: 4145–4150.

This paper characterizes the human healthy obese individual.

Belfort R, Harrison S A, Brown K, Darland C, Finch J, Hardies J, Balas B, Gastaldelli A, Tio F, Pulcini J, Berria R, Ma J Z, Dwivedi S, Havranek R, fincke C, DeFronzo R, Bannayan G A, Schenker S, Cusi K. A placebo-controlled trial of pioglitazone in subjects with nonalcoholic steatohepatitis. N Engl J Med 2006: 355: 2297–2307.

This paper illustrates how strategies aiming to promote expansion of adipose tissue through pharmacological treatments help to improve fat deposition and inflammation in liver by facilitating fat deposition in adipose tissue.

Acknowledgements

Thanks to the help of Abbott, Chanel, Galderma International, Laboratoires Bailleul, Laboratoires Pierre Fabre, La Roche Posay Laboratoire Pharmaceutique, Leo Pharma, and L.V.M.H.

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