Using adipose‐derived mesenchymal stem cells to fight the metabolic complications of obesity: Where do we stand?

Summary Obesity is a critical risk factor for the development of metabolic diseases, and its prevalence is increasing worldwide. Stem cell‐based therapies have become a promising tool for therapeutic intervention. Among them are adipose‐derived mesenchymal stem cells (ADMSCs), secreting numerous bioactive molecules, like growth factors, cytokines, and chemokines. Their unique features, including immunosuppressive and immunomodulatory properties, make them an ideal candidates for clinical applications. Numerous experimental studies have shown that ADMSCs can improve pancreatic islet cell viability and function, ameliorate hyperglycemia, improve insulin sensitivity, restore liver function, counteract dyslipidemia, lower pro‐inflammatory cytokines, and reduce oxidative stress in the animal models. These results prompted scientists to use ADMSCs clinically. However, up to date, there have been few clinical studies or ongoing trails using ADMSCs to treat metabolic disorders such as type 2 diabetes mellitus (T2DM) or liver cirrhosis. Most human studies have implemented autologous ADMSCs with minimal risk of cellular rejection. Because the functionality of ADMSCs is significantly reduced in subjects with obesity and/or metabolic syndrome, their efficacy is questioned. ADMSCs transplantation may offer a potential therapeutic approach for the treatment of metabolic complications of obesity, but randomized controlled trials are required to establish their safety and efficacy in humans prior to routine clinical use.

randomized controlled trials are required to establish their safety and efficacy in humans prior to routine clinical use.

K E Y W O R D S
adipose tissue, ADMSCs, metabolic syndrome, obesity

| INTRODUCTION
Obesity is a serious global public health problem, responsible for about 4.7 million premature deaths each year. 1,2 The incidence of obesity is increasing dramatically, as reported by the World Health Organization (WHO); it affected over 650 million adults worldwide in 2016. 3 Obesity is defined as the excessive accumulation or abnormal distribution of fat tissue. 4 The current most widely used criteria for classifying obesity is the body mass index (BMI), which ranges from class 1 of obesity (BMI ≥ 30.0 kg/m 2 ) to severe or morbid obesity (BMI ≥ 40 kg/m 2 ). Obesity can progressively cause and/or exacerbate a wide spectrum of metabolic comorbidities, including type 2 diabetes mellitus (T2DM), hypertension, dyslipidemia, cardiovascular disease (CVD), nonalcoholic fatty liver disease (NAFLD), and fertility problems. 5,6 The severity and duration of obesity are associated with the metabolic syndrome (MS), which occurs in 4.9% of nonobese patients to 35.3% in patients with obesity. 7 According to the National Institutes of Health, a subject has MS if it satisfies three or more of the following traits: large waist circumference (≥89 cm for women and ≥102 cm for men), hypertriglyceridemia (≥1.7 mmol/L), reduced highdensity lipoprotein cholesterol (HDL-C) (<1.04 mmol/L in men or <1.3 mmol/L in women), hypertension (≥130/≥85 mm Hg), and elevated fasting blood glucose (≥5.6 mmol/L). 8 In light of the alarming data, it is of primary importance to elucidate the mechanism through which obesity leads to the adipose tissue dysfunction followed by metabolic derangements. Nevertheless, many cohort studies have reported that some individuals with obesity remain insulin sensitive and are metabolically "healthy" despite similar total fat mass. [9][10][11][12] Metabolically healthy obese (MHO) subjects exhibit increased subcutaneous adiposity and are characterized by a lower degree of systemic inflammation, but still they are at a higher risk of cardiovascular complications. [13][14][15][16] Longitudinal studies provide convincing evidence that MHO is only a transient condition. [17][18][19] Therefore, it is important to identify individuals with obesity at increased risk of developing obesity-related metabolic diseases that can benefit most from weight loss. Many factors are responsible for the established obesity-related disease complications, which lead to a not effective treatment and management of patients with obesity. Recently, numerous strategies have been proposed to minimize health-related consequences of obesity, including cell-based therapy. Mesenchymal stem cell (MSC) therapies may represent promising adjunctive therapy for patients with obesity, thus reducing the economic burden of treatment throughout the patient's life. [20][21][22][23] Both bone marrow-derived MSC and adiposederived MSCs (ADMSCs) have become the most commonly used stem cells for cellular therapy in a variety of human diseases. ADMSCs seem to be superior to other MSCs in many aspects, including ease of isolation, their abundance, and better immunomodulatory properties.
Patients may be treated with autologous or allogenic ADMSCs with low risk of cellular rejection. These advantages together with the minimal immunogenicity and high immunoregulatory capacity make them attractive for clinical use. In this review, we outline the current understanding of ADMSCs-based therapies in obesity and its associated diseases from the animal model to the preclinical and clinical trials.

| ADIPOSE TISSUE
Adipose tissue was historically considered to be merely an energy store, but this concept was changed after the discovery of leptin in 1990 by Friedman's group. 24 Since this discovery, other cytokines, hormones, and peptides, collectively referred to as "adipokines," have been identified. 25 Adipose tissue develops extensively in homeothermic organisms, and the proportions to body weight vary considerably between species. Averagely, it constitutes about 15%-20% of body mass of men and 20%-25% of women. This connective tissue influences the whole body as it is responsible for energy storage and distribution, fat accumulation, thermoregulation, hormone synthesis, glucose, and insulin homeostasis. 26,27 Fat tissue can be classified into brown adipose tissue (BAT) and white adipose tissue (WAT), which differ in function, distribution and morphology. 28 In adult humans, BAT had long been considered to be absent; however, recent investigations have shown that BAT is found to be distributed throughout the cervical, supraclavicular, mediastinal, suprarenal, and paravertebral regions. 29 The mitochondrial abundance and high vascularization in comparison with the WAT give it a brown color appearance. High expression of uncoupling protein 1 (UCP1) in their inner mitochondrial membrane is responsible for energy dissipation in the process called nonshivering thermogenesis. 30 In turn, white adipocytes not only control energy balance by storing and mobilizing triacylglycerols but also secrete a variable amount of hormones and paracrine factors.
Although white adipocytes are distributed throughout the body, their principal depots are the subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT). 31 SAT is found beneath the skin, some deposits are gluteal, femoral, and abdominal, while visceral fat surrounds internal organs and is concentrated in the abdominal cavity, further subdivided into mesenteric, omental, perirenal, and pertoneal depots. Importantly, WAT depots are functionally distinct, SAT stores excess lipid, and thus preventing ectopic lipid deposition, while VAT protects the visceral organs. Typically, VAT can be identified by a higher number of smaller adipocyte size, whereas SAT by larger adipocytes. In healthy middle-aged adults, only 5%-15% of total body fat is considered VAT; the rest is SAT, the largest body fat deposit.
When the storage capacity of adipocytes exceeds (like in obesity), further caloric overload leads to the expansion of adipose tissue in a given fat compartment through increase in adipocyte size (hypertrophy) and/or proliferation of precursor cells (hyperplasia). 29,32 Simultaneously, the precursor cells of the stromal vascular fraction (SVF) in adipose tissue undergo numerous functional changes, begin to be recruited and committed towards adipocyte lineage. This series of events is called "adipose tissue remodeling." 33 However, in obesity, aberrant adipose tissue remodeling may induce dysregulation of fat tissue in secreted cytokines, hormones, and metabolites. 34 This causes ectopic lipid deposition in the liver, skeletal muscle, heart, pancreas, as well as in the visceral depots and leads to impaired glucose and lipid metabolism, systemic insulin resistance (IR), an increased risk of T2DM and CVD development. 35,36 It is worth nothing that the distribution of adipose tissue appears to be more important than the total amount of the body fat. Indeed, VAT is more metabolically active, has higher free fatty acids (FFAs) and glucose uptake, is less insulin sensitive, and therefore is thought to be more deleterious in the development of obesity-related metabolic complications. 37 39 Three weeks after transplantation, the abundance of adipokine gene transcript (i.e., adiponectin, leptin, visfatin, and resistin) was adjusted to the expression level in the resident (thigh) depot. These observations support the notion that adipose tissue depots have "residence memory" and local factors, such as glucose levels, are involved in the epigenetic regulation of adipokine gene promoters. 39

| MESENCHYMAL STEM CELLS
MSCs also referred to as "mesenchymal stromal cells" are fibroblastlike multipotent cells characterized by the capacity of self-renewal and ability to differentiate into cell types of mesodermal origin, including adipocytes, chondrocytes, and osteoblasts. Stem cell research has advanced considerably since pluripotent cells were first isolated from mouse embryos in 1981 40 ; however, the first report with embryonic stem cell lines derived from human blastocysts was published in 1998. 41 The clinical relevance of MSCs was initially based on harnessing their potential for tissue regeneration and repair, and the discovery of their paracrine properties has greatly expanded the range of therapeutic applications for which they are currently being explored. MSCs are attractive cell therapy agents in the treatment of various diseases, especially in the treatment of conditions involving autoimmune and inflammatory processes. Several characteristics favor their use in a wide range of diseases, such as their autocrine and paracrine activities, immunomodulatory and immunosuppressive properties, with their minimal immunogenicity and ethical restrictions. 42 The cells can be obtained from human's multiple organs and structures like bone marrow, adipose tissue, liver, pancreas, spleen, thymus, skeletal muscle, dental pulp, dermis, and neonatal tissues (umbilical cord, amniotic fluid, fetus, placenta), but the most frequently used sources of MSCs remain bone marrow and adipose tissue. 43 To standardize MSCs, in 2006, the International Society for Cell and Gene Therapy (ISCT) proposed the following minimal criteria: (1) They must be plastic adherent when maintained in standard culture conditions; (2) they must express the surface markers CD73, CD90, and CD105 and lack of expression of hematopoietic and endothelial antigens CD14 (or CD11b), CD19 (or CD79α), CD34, CD45, and HLA-DR surface markers; (3) they must be able to differentiate into adipocytes, chondrocytes, and osteocytes in vitro (trilineage potential). 44
ADMSCs show the typical characteristics of MSCs, after in vitro stimulation can differentiate into mesodermal lineages cell types (adipocytes, osteoblasts, chondrocytes, fibroblasts, and myocytes) as well as no mesodermal cell types, such as neurons, hepatocytes, endothelial cells, and cardiomyocytes. 55 According to the standard criteria, cultured ADMSCs are plastic-adherent, spindle-shape cells characterized by the expression of positive markers: CD13, CD29, CD44, CD73, CD90, and CD105 and the lack of CD45 and CD31 on their surface.
Moreover, the characterization of ADMSCs includes additional positive markers like CD10, CD26, CD36, CD49d, and CD49c and low or negative markers like CD3, CD11b, CD49f, CD106, and podocalyxinlike protein (PODXL). 56 ADMSCs constitute up to 2% of SVF, a heterogeneous mesenchymal population of cells, compared with the low cell yield (0.001%-0.002%) of BM-MSCs. A large amount of ADMSCs is isolated from SAT by liposuction or fat excision with an efficiency up to 500 times greater than from bone marrow isolation. The liposuction procedure provides 100 ml to 3 L of lipoaspirate that is routinely discarded. By processing this material, ADMSCs are isolated from the SVF, yielding up to six billion cells in one passage. 54 are better adapted to oxidative stress, hypoxia-induced apoptosis or have a greater angiogenetic force when exposed to harsh conditions compared with BM-MSCs. 63 Their advantage in immune regulation is due to the secretion of higher levels of pro-inflammatory and antiinflammatory cytokines such as interleukins (IL-6, IL-8), interferon γ (IFN-γ), and transforming growth factor (TGF-β). ADMSCs also release higher amount of growth factors including granulocyte colonystimulating factor (G-CSF), granulocyte macrophage colonystimulating factor (GM-CSF), nerve growth factor (NGF), or insulin-like growth factor 1 (IGF-1) compared with the BM-MSCc. [64][65][66][67] Overall, the superior characteristics of ADMSCs compared with other MSCs along with their abundance and easy cell access to cells encourage scientists to complete researchers among ADMSCs.
In recent years, it has been proven that there are differences between ADMSCs isolated form lean and individuals with obesity.

| RESEARCH AMONG ADMSCS USAGE IN THE TREATMENT OF OBESITY-RELATED MORBIDITIES
Obesity is a complex, multifactorial disease. Usually excessive fat deposition in obesity is closely related to environmental factors such as an increased consumption of saturated fats, carbohydrates, sugars, and decreased physical activity as well as to genetic and epigenetic factors. 70   shown that ADMSCs transplantation did not affect HFD-induced weight gain. 71 Furthermore, no significant changes in body weight were noted in a diabetic and obese mouse model following Sod2 or catalase (CAT)-upregulated ADMSCs therapy. 72 Likewise, in the mouse, NASH model injection of ADMSCs or their small extracellular vesicles (sEVs) did not significantly change body weight and liver-tobody weight ratio. 73 However, these results contradict with other study demonstrating that obese mice treated with brown ADMSCs significantly reduced body weight. 74 Similarly, ADMSCs infusion significantly suppress the increase in body weight in db/db obese mice and DIO mice. 75 On the other hand, Cao et al. confirmed the ADMSCs effectiveness, however, isolated from mice not humans, in reducing body weight in DIO animals. 76 In another study, a single ADMSCs transplantation did not change overall body weight, while a second ADMSCs injection significantly decreased the weight of the obese mice. 77 Interesting results were provided by the work of Shree et al. 78 C57BL/6 HFD-fed mice were administered with human ADMSCs or metformin-preconditioned ADMSCs. It turned out that mice treated with ADMSCs alone did not change body weight, but significant weight reduction was observed in the metformin-preconditioned ADMSCs group. 78 Table 1). The most significant studies are highlighted in this review.

| Promotion of insulin production
Obesity is associated with the incidence of multiple comorbidities such as diabetes mellitus, a metabolic disease generally classified into type 1 diabetes mellitus (T1DM) or T2DM. Decreased insulin sensitivity in peripheral tissues such as adipose tissue, liver, and skeletal muscles coupled with a progressive loss of adequate insulin secretion is hallmark of T2DM, which accounts for approximately 90%-95% of all diabetes cases. The conventional treatment including lifestyle modifications or pharmacological intervention is only a symptomatic treatment that cannot improve insulin sensitivity or β-cell dysfunction. 85 Therefore, a new therapeutic approach is needed to produce significant anti-diabetic effects and overcome long-term diabetic complications. Recently, ADMSCs therapy has attracted great attention as a more promising treatment for diabetes due to, inter alia, its capacity to differentiate into insulin producing cells (IPCs). The potential of ADMSCs to derive IPCs was discovered in 2003, 86 but initially these cells did not secrete insulin. A few years later, Kang et al. developed a method for the differentiation of IPCs to functional endocrine hormone-producing cells. 87 Chandra et al. explored the potential of ADMSCs to generate pancreatic hormone-expressing islet-like cell aggregates (ICAs) from murine epididymal MSCs. 88 What is interesting, mature, differentiated ICAs from ADMSCs exhibited enhanced transcript levels of pancreatic endoderm markers like pancreatic and duodenal homeobox gene 1 (Pdx-1), neurogenin-3 (Ngn3), paired box 4 (Pax4), NK2 homeobox 2 (Nkx2.2), NK6 homeobox 1 (Nkx6.1), islet 1 transcriptional factor (isl-1), and insulin. 86,89,90 Histological analysis showed that ICAs acquire β-cell features like secretory cells with vacuoles and granules. 88 The pancreatic-hormone expressing IPCs transplantation into streptozotocin (STZ)-induced diabetic mice restored the normoglycemia within two weeks. 88 Also Lee et at. confirmed that ADMSCs could differentiate into insulin-producing cells by exogenously expressed the Pdx-1. 91 Although Pdx-1-induced human ADMSCs reduced blood glucose levels, it did not restore normoglycemia in vivo. 91 An ex vivo experiment performed on human ADMSCs and their ability to differentiate into insulin secreting cells was completed by Dave et al. 92 ADMSCs were collected from subcutaneous abdominal AT, cultured in a differentiation medium and analyzed 10 days later. 92 Interestingly, after this time, ADMSCs began to secrete genes necessary for pancreatic development, that is, Pdx-1, Pax-6, and isl-1. 92 All three of these factors are essential for the reprogramming nonpancreatic cells to fully functional β cells, in which glucose stimulation leads to the secretion of insulin and C-peptide.
Moreover, Karaoz et al. compared the ability of MSCs derived from adipose tissue and those isolated from bone marrow to differentiate into pancreatic cells. 67 It turned out that the differentiation potential of ADMSCs into insulin-producing cells was higher compared with BM-MSCs. Therefore, ADMSCs could be considered as a preferred cell of choice than BM-MSCs as their ability to restore metabolic complications of diabetes is better. However, to make stem cell-based therapy an ideal candidate for clinical implementation, resolution of certain impending issues is needed. For example, neither the longterm stability of ADMSC-derived IPCs nor the extent to which in vitro derived IPCs resemble endogenous islets has been identified. What is the minimum number of IPCs to achieve glucose homeostasis to an extent maintained by the endogenous pancreas? Besides, the ADMSCs to IPC differentiation protocol also need to be standardized.
In addition to IPC differentiation, ADMSCs promote insulin production by restoring islet function and increasing pancreatic β-cell mass. TBC1D1. 93 Any impairment of insulin effects on target tissues defines F I G U R E 2 Mechanisms of ADMSCs actions on glucose homeostasis and liver functions. ADMSCs therapy is effective in restoring glycemic status i.e. promotes insulin production and improves insulin sensitivity. Additionally, transplantation of ADMSCs reverses liver steatosis, through reduced inflammation, reduced apoptosis, and improved hepatocyte regeneration. Abbreviations: ADMSCs, adipose-derived mesenchymal stem cells; AKT, serine/threonine kinase 1; ALT, alanine aminotransferase; AST, aspartate aminotransferase; GLUT4, glucose transporter 4; G6Pase, glucose-6-phosphatase; HLCs, hepatocyte-like cells; HO-1, heme oxygenase-1; IL-1β, interleukin 1β; IL-6, interleukin 6; IL-8, interleukin 8; IPCs, insulin producing cells; IRS-1, insulin receptor substrate 1; LDH, lactate dehydrogenase; MCP-1, aka CCL2, monocyte chemoattractant protein 1; NQO1, NAD(P)H quinone oxidoreductase 1; PEPCK, phosphoenolpyruvate carboxykinase; PPAR-γ, peroxisome proliferator-activated receptor gamma; SOD, superoxide dismutase; TBIL, total bilirubin level; TNF-α, tumor necrosis factor α IR. In clinical practice, it identifies the reduced effect of insulin on glucose metabolism. Large-scale population studies have shown that obesity is the most important independent risk factor for the onset and development of IR and further its progression into T2DM. 35,36 Although many individuals with obesity do not progress to diabetes mellitus, it is generally accepted that two defects are required for progression from IR to T2DM. First, peripheral IR is a primary condition in obesity and a precondition for the onset of type 2 diabetes. Second, dysfunction of β cells to secrete adequate level of insulin to maintain glucose homeostasis is responsible for the progression of IR to a diabetic state. 94,95 The pathophysiology of IR is complex; although some disturbances in the insulin signaling pathway are known, the mecha-  A marked enhancement in insulin sensitivity after the ADMSCs multiple infusions.
Pancreatic islet function was markedly restored. The ratio of insulin-positive cells per islet was increased.
Yu et al. 80 Murine ADMSCs Reduction in blood glucose level There were no significant differences in plasma insulin levels. Insulin sensitivity was increased.
Ameliorated the destruction to pancreatic islets and restored β-cell mass.

TNF-α expression was reduced
ADMSCs infusion reduced liver weight, steatosis and expression of IL-6, TNF-a,  (Figure 2). Other researchers demonstrated that ADMSC sheets transplantation into the subcutaneous sites improved glucose intolerance in mice fed with high-fat and high-sucrose diet (HF/HSD). 79

Rat ADMSCs
The level of glucose and HbA1c was significantly lower.
ADMSCs infusion improved insulin sensitivity in diabetic rats. ADMSCs treatment did not change serum insulin and C-peptide levels. tissues. 84 The same phenomenon was observed by Yu et al.; in this study, multiple intravenous infusions of ADMSCs (weekly for 24 weeks), were effective in restoring glucose homeostasis, ameliorating IR, and altering the progression of metabolic complications in a rat model of long-term T2DM complications. 80 It seems that multiple ADMSCs infusions, rather than a single infusion or several infusions have therapeutic potential in the treatment of advanced stage of T2DM. In overall, available studies proved that treatment with ADMSCs has an anti-diabetic effect and alleviates not only the early stages but also the long-term complications of diabetes such as chronic kidney disease, pulmonary fibrosis, liver fibrosis or steatosis, cardiovascular complications, and female infertility.  103 ADMSCs were used also to restore ovarian function in a rat model of premature ovarian insufficiency (POI). Stem cells injection increased the number of antral follicles and the pregnancy rate increased from 50% (control group) to 72.7% (ADMSCs group). 104 Although ADMSCs therapy might be a potential treatment strategy for female infertility, the current research is in preclinical phase or at a very early clinical trial phase.

| Dyslipidemia and atherosclerosis
Dyslipidemia is characterized by elevated levels of serum TGs, total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and low levels of HDL-C. 105 There is strong positive correlation between increased BMI, WAT capacity, and dyslipidemia. Importantly, hyperlipidemia is one of the major risk factors for CVD, the leading cause of morbidity and mortality worldwide, through the development and progression of atherosclerosis. 106 One of the first stages of the atherosclerotic process is the aggregation of atherosclerotic lipoprotein particles in the compliant wall of the coronary artery, mainly small dense LDL and oxidized LDL, followed by a reactive inflammatory process, smooth muscle cell (SMC) proliferation, fibrosis, and calcification. 107 Therefore, the controlling serum lipids profile is a cornerstone in the prevention and treatment of atherosclerotic CVD. Despite lifestyle changes, patients with dyslipidemia and atherosclerosis need to follow complex medication regimens. Statins and fibrates are the most commonly used treatments with the high pharmacological efficiency to reduce TC, TG, LDL, and serum concentrations. 108 However, several adverse effects and limitations are associated with their use. For example, they cannot be taken by women of childbearing age (without contraception), breastfeeding women, with impaired liver function and patients under 10 years of age. 109 Recently, cellular therapy using ADMSCs has offered the potential to normalize hyperlipidemia and associated CVDs, which is related to their ability to improve serum lipid profile, promote angiogenesis, and inhibit cell apoptosis inhibiting the MAPK and NF-κB pathways. 114 Using the cell culture model, murine bone marrow-derived macrophages were incubated with ADMSCs or with conditioned medium from ADMSCs, and interestingly both types of intervention were able to reprogram macrophages to regulatory/M2-like phenotype. 115 To further established regulatory effects of ADMSCs on macrophages, Souza-Moreira et al.
have determined the molecular mechanism of lipid droplet formation in macrophages using conditioned medium from ADMSCs. 116 They revealed that paracrine factors released by ADMSCs promotes lipid droplet biogenesis via mTOR Complex 1 (mTORC1) and 2 (mTORC2) and macrophage polarization as indicated by increased IL-10 secretion and nitric oxide (NO) release. These findings suggest that ADMSC may hold therapeutic potential for dyslipidemia and associated CVDs, but the major obstacle is that successful results come only from animal models or in vitro studies.

| Immunomodulatory and anti-inflammatory effects of ADMSCs in reliving inflammation
Scientists in the early 1990s discovered the relationship between elevated levels of several pro-inflammatory cytokines and excess adiposity. 117 Obesity induces a state of low-grade chronic inflammation, which is a hallmark of the infiltration of immune cells in adipose tissue and the production of pro-inflammatory cytokines and chemokines.
The deregulated inflammatory pathway leads to adipose tissue dys-

| Nonalcoholic fatty liver disease
NAFLD ranges from simple steatosis to more progressive NASH with active hepatocellular necrosis, liver inflammation, and tissue damage, to cirrhosis, and in some cases hepatocellular carcinoma (HCC). 131,132 NAFLD is characterized by presence of steatosis in more than 5% of hepatocytes with or without mild lesions. 56,133 Although NAFLD is highly prevalent in the general population, most of the affected patients mainly suffer from simple, non-life-threatening steatosis, while 5%-10% of NAFLD patients actually develop NASH. The liver failure is highly associated with the incidence of severe obesity, and MS. 131 It affects 75%-90% of patients with obesity or morbid obesity compared to general population. 134 The underlying pathogenesis of NAFLD is multifactorial and complex. However, among the most important NAFLD inducers are accumulation of hepatic lipids, ER/oxidative stress, pro-inflammatory cytokines, mitochondrial dysfunction, and genetic and epigenetic factors. 135 138 In the another study, transplantation of HLCs preserved liver functions (via the secretion of IL-10, IL-6, and TGF-β) and prolonged the survival of mice with carbon tetrachloride (CCl 4 )-induced liver injury. 139 On the other hand, the use of HCLs for liver failure has also been questioned by studies that found that transplanted MSCs, but not HCLs were more effective in regaining liver function. 140,141 Although HLCs can improve liver function in vitro, these immature hepatocytes simply progress to the cell death pathway, whereas ADMSCs are not so sensitive to the damaged microenvironment.
Thus, autologous transplantation of ADMSCs in vivo is recommended for liver regeneration.
There is evidence that secreted chemotactic cytokines and inflammatory factors from injured liver tissue or hepatocytes attract ADMSCs infusion. 72 Although the animals did not restore liver function to the level of a healthy individual, they still showed inhibition of NAFLD development. 72 In another study, serum levels of TBIL and AST decreased significantly 2 weeks after an infusion of ADMSCs, although these changes were not detected 4 weeks after transplantation. The authors speculate that the therapeutic effects of ADMSCs were partially attenuated by continuing to feed HFD during the experiment. 45 Recently, the therapeutic potential of ADMSCs and their sEVs, which possess immunomodulatory activities, were assessed in melanocortin type-4 receptor knockout (Mc4r-KO) NASH mouse model. 73 Interestingly, ADMSCs and sEVs demonstrated antiinflammatory and anti-fibrotic effects to a similar extent. Histological analysis showed that both of treatments had no effect on fat accumulation, but improvement in liver fibrosis was still observed. 73 In addition, ADMSCs transplantation reduced liver fibrosis through reducing type 1 collagen fibers in the analyzed hepatocytes together with a decrease in tissue inhibitor of metalloproteinases-1 (TIMP-1) and matrix metalloproteinase-2 (MMP-2) in T2DM rats with NASH-like features. 80 Similarly, Liao et al. found that ADMSCs could ameliorate liver fibrosis in rats with type 2 diabetes. They noticed that the TGF-β1//mothers against decapentaplegic homolog 3 (SMAD3) signaling pathway, which plays an important role in the progression of liver fibrosis, was downregulated after ADMSCs transplantation in the fibrotic liver tissues. 128 Altogether, the reported data indicate that ADMSCs ameliorate the development of chronic liver disease, including NAFLD, liver fibrosis and cirrhosis in animal models and may account for their broad therapeutic efficacy in the clinical treatment of liver disease.
The safety and efficacy of ADMSCs in the treatment of chronic liver failure have been demonstrated in a phase I clinical trial. Autologous freshly isolated ADMSCs with the maximal number of cells 6.6 Â 10 5 cells/kg were injected into the common hepatic artery of four patients with liver cirrhosis. 151 No serious adverse effects were observed during the 1-month study period. All patients had F I G U R E 4 Potential therapeutic application of ADMSCs in the treatment of obesity and related comorbidities such as diabetes, insulin resistance, vascular disorders, infertility, and NAFLD. Abbreviations: ADMSCs, adipose-derived mesenchymal stem cells; NAFLD, nonalcoholic fatty liver disease upregulated the expression of liver regeneration-related factors (HGF and IL-6) and exhibited improved synthetic liver function (prothrombin time, albumin) at follow-up. 151 This clinical study demonstrates the therapeutic efficacy of ADMSCs in maintaining liver function in patients with liver cirrhosis.

| CONCLUSIONS
ADMSCs are promising candidates for cell-based therapy due to their abundancy, ease of isolation, multilineage potential, self-renewal capacity, anti-apoptotic, anti-oxidative, and antiinflammatory properties. Growing evidence points to the therapeutic potential of ADMSCs in the treatment of obesity-related metabolic complications such as T2DM, IR, hepatic steatosis, fibrosis, infertility, vascular disorders, and systemic inflammation (Figure 4). The mechanisms by which ADMSCs exert beneficial effects include their ability to differentiate into specific cells, that is, HLCs, IPCs, as well as the release of a broad spectrum of biomolecules that can restore liver, pancreatic islet β cell, and endothelial function, improve IR, and subsequently promote the suppression of apoptosis, inflammation and ROS production (Figures 2 and 3). Importantly, no animal deaths or any serious adverse events were reported following ADMSCs injection in the tested models. Success in preclinical research has led to the progression to clinical trials. Majority of the clinical trials conducted so far have used autologous ADMSC, which minimize the risk to recipients. Indeed, very few serious adverse events were reported.
Nevertheless, it is known that ADMSCs acquired from patients with chronic inflammatory diseases like obesity are less effective in immunomodulation compared with lean, metabolically healthy individuals.
Because health characteristics and age of the donor impact on the ADMDCs properties, allogenic ADMSCs may provide more effective cellular therapy. Up to date, only a few randomized, controlled trails have been performed to assess the therapeutic potential of these cells in alleviating metabolic complications of obesity. Some of the major problems that still need to be resolved concern to the standardization of cell processing and culture protocols, ideal transplant route, dosing, and timing of ADMSCs administration. Systemic infusion appears to be more effective, while peripheral intravenous injections are easier to handle and have fewer side effects. Likewise, the dosage varies from study to study; in some cases, a single injection was not sufficient; therefore, multiple injections were administered.

CONFLICT OF INTEREST
The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS
AM created the concept, collected the data, and designed the outline.
AM and BEN prepared a draft of the manuscript that was revised by AC. All authors contributed to the article and approved the submitted version.