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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information

Obesity is a major cause of insulin resistance and contributes to the development of type 2 diabetes. The altered expression of genes involved in mitochondrial oxidative phosphorylation (OXPHOS) has been regarded as a key change in insulin-sensitive organs of patients with type 2 diabetes. This study explores possible molecular signatures of obesity and examines the clinical significance of OXPHOS gene expression in the livers of patients with type 2 diabetes. We analyzed gene expression in the livers of 21 patients with type 2 diabetes (10 obese and 11 nonobese patients; age, 53.0 ± 2.1 years; BMI, 24.4 ± 0.9 kg/m2; fasting plasma glucose, 143.0 ± 10.6 mg/dl) using a DNA chip. We screened 535 human pathways and extracted those metabolic pathways significantly altered by obesity. Genes involved in the OXPHOS pathway, together with glucose and lipid metabolism pathways, were coordinately upregulated in the liver in association with obesity. The mean centroid of OXPHOS gene expression was significantly correlated with insulin resistance indices and the hepatic expression of genes involved in gluconeogenesis, reactive oxygen species (ROS) generation, and transcriptional factors and nuclear co-activators associated with energy homeostasis. In conclusion, obesity may affect the pathophysiology of type 2 diabetes by upregulating genes involved in OXPHOS in association with insulin resistance markers and the expression of genes involved in hepatic gluconeogenesis and ROS generation.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information

Obesity is a major cause of insulin resistance (1) and contributes to the development of type 2 diabetes. In patients with diabetes, the coexistence of obesity further increases insulin resistance and makes glycemic control difficult (2). Absolute or relative failure of insulin action on the liver causes impaired suppression of glucose production (3), and hepatic insulin resistance leads to impaired suppression of gluconeogenesis, contributing to fasting hyperglycemia in humans (3,4) and in various animal models (5). However, little is known about how obesity causes hepatic insulin resistance in patients with type 2 diabetes.

Given that obesity and type 2 diabetes are both multifactorial disorders (6,7), a comprehensive approach to identifying biological pathways or coregulated gene sets associated with the diseases is appropriate in attempting to clarify the molecular signatures of obesity and type 2 diabetes (8). Recent gene chip technology data suggest that the altered expression of genes involved in mitochondrial oxidative phosphorylation (OXPHOS) is a key change in the insulin-sensitive organs of patients with type 2 diabetes (8,9,10,11). Mitochondrial OXPHOS is a major source of reactive oxygen species (ROS) production in most cells (12), and ROS comprise one of the many factors that have been suggested to play a role in multiple forms of insulin resistance (13,14). ROS include highly reactive moieties, such as superoxide anions, hydrogen peroxide, and hydroxyl radicals, which are formed as by-products of cellular metabolism. Specifically, electrons passing down the OXPHOS pathway “leak” from the main path and reduce oxygen molecules directly. Therefore, as a rule, increased mitochondrial oxygen flux leads to increased formation of ROS (15,16). Genes involved in OXPHOS may be downregulated systemically in the skeletal muscle (9,10), adipose tissue (11), and peripheral blood mononuclear cells (17) of patients with type 2 diabetes, suggesting that the OXPHOS pathway plays a causal role in insulin resistance in type 2 diabetes (18). In contrast, genes involved in OXPHOS are coordinately upregulated in the livers of patients with type 2 diabetes (8). We speculate that acetyl-CoA, a glucose-derived substrate for OXPHOS, fluxes into the liver in response to hyperglycemia and induces the expression of genes for OXPHOS. In this regard, visceral adiposity may further distribute fatty acid-derived acyl-CoA to mitochondrial OXPHOS in the liver via the portal vein and upregulate genes for OXPHOS in patients with obesity. However, human evidence for a molecular signature of obesity in this pathway is lacking. In addition, whether upregulation or downregulation of OXPHOS genes contributes to insulin resistance remains controversial (19,20).

Therefore, we analyzed gene expression in the livers of patients having type 2 diabetes with or without obesity. Obesity coordinately upregulated genes involved in the OXPHOS pathway in the livers of patients with type 2 diabetes. We further investigated the relationship of insulin resistance indices and hepatic gene expression for glucose and lipid metabolism and ROS generation with OXPHOS gene expression.

Methods and Procedures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information

Samples

The patients gave written informed consent for both the study and the histological examination for liver diseases, including nonalcoholic fatty liver disease, which often complicate diabetes. The experimental protocol was approved by the ethics committee at our institution and was carried out in accordance with the Declaration of Helsinki.

Liver biopsy specimens were obtained from 21 patients with type 2 diabetes (15 men, 6 women; mean age, 53.0 ± 2.1 years; mean BMI, 24.4 ± 0.9 kg/m2; mean fasting plasma glucose, 7.94 ± 0.59 mmol/l; mean HbA1c, 7.3 ± 0.3%; mean alanine aminotransferase, 34.4 ± 5.5 IU/l) admitted to Kanazawa University Hospital between 2000 and 2003. Nine patients with diabetes in our previous study (8) were involved in the current study. We then added 12 patients with diabetes and focused on the impact of obesity on hepatic gene expression in a total of 21 patients with type 2 diabetes. The samples were frozen in liquid nitrogen immediately and stored at −80 °C until use. All subjects tested negative for the hepatitis B and C viruses, and all reported drinking <20 g of ethanol per day. The patients were diagnosed based on criteria established by an expert committee on the diagnosis and classification of diabetes mellitus (21). The patients with diabetes were treated with diet therapy alone or with insulin. The insulin regimen for all insulin-treated patients (2 of 10 obese patients and 5 of 11 nonobese patients) was prandial dosing with rapid-acting insulin analogue. None was prescribed any oral hypoglycemic agent or basal insulin replacement. Pharmacological treatments did not include statins, angiotensin-converting enzyme inhibitors, or angiotensin II receptor blockers. Overweight was defined as BMI ≥25 kg/m2, the Japanese criterion for obesity (22), which was also near the median BMI (24.4 kg/m2) of the subjects. The clinical characteristics of the study subjects are shown in Supplementary Table S1 online.

Statistical analysis

All data are expressed as means ± s.e.m. To test the significance of expression ratios for individual genes or pathways, we used a supervised analysis with a permutation-based method with BRB-ArrayTools software (23), developed for the statistical analysis of DNA chip gene expression data by the Biometric Research Branch of the US National Cancer Institute. It is a class comparison tool based on univariate F-tests designed to find genes differentially expressed between predefined clinical groups. The permutation distribution of the F statistic based on 2,000 random permutations was used to confirm statistical significance. We screened 535 human gene sets, 285 BioCarta pathways, 101 KEGG pathways, and 149 gene sets described previously (9). GenMAPP (Gene MicroArray Pathway Profiler, see http:www.genmapp.org) (24) was used for illustrating the pathway with the expression ratios of the involved genes.

To evaluate the correlation between OXPHOS gene expression levels and clinical parameters, we computed the mean centroid of the OXPHOS genes, as reported previously (8,9). We normalized the expression levels of the 144 OXPHOS genes to a mean of 0 and a variance of 1 across the 21 individuals. The OXPHOS mean centroid is the mean of these 144 gene expression levels. For the univariate linear regression analysis to assess the contribution of clinical parameters to the OXPHOS mean centroid, we report squared correlation coefficients and P values. The Statistical Package for the Social Sciences (version 11.0; SPSS, Chicago, IL) was used for the statistical analyses.

Additional details on methods

For details on the laboratory studies, liver pathology, DNA chip analyses, and real-time PCR, see Supplementary Methods and Procedures online.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information

Patient characteristics

The clinical characteristics of the study subjects are shown in Supplementary Table S1 online. No significant differences were observed in age, fasting plasma glucose, HbA1c level, or serum lipid levels between the overweight (n = 10) and normal-weight groups (n = 11). BMI and alanine aminotransferase values were elevated significantly in the overweight group (P < 0.05). In addition, insulin resistance, estimated with homeostasis model assessment of insulin resistance and QUICKI, was also higher in the overweight group and liver steatosis tended to be more severe. BMI was significantly correlated with QUICKI (r = −0.482, P = 0.037), alanine aminotransferase (r = 0.584, P = 0.005), aspartate aminotransferase (r = 0.671, P = 0.001), and liver steatosis scores (r = 0.598, P = 0.004). No significant differences were observed in the serum levels of high-sensitivity C-reactive protein, free fatty acids, or adipocytokines such as tumor necrosis factor-α, adiponectin, and leptin between the overweight and normal-weight groups (Supplementary Table S1 online).

Upregulation of gluconeogenesis and β-oxidation of fatty acids

We screened 535 human pathways (BioCarta, KEGG pathways; Affymetrix, Santa Clara, CA) and extracted those metabolic pathways significantly altered by obesity in the livers of patients with type 2 diabetes using BRB-ArrayTools software (Table 1). Some of these pathways are illustrated in Figure 1. Glucose metabolism pathways, including the tricarboxylic acid cycle (Supplementary Figure S1a online), pentose phosphate pathway (Supplementary Figure S1b online), pyruvate metabolism, and glycolysis pathway, were coordinately upregulated in the obese group compared to the nonobese group (Figure 1).

Table 1.  Pathways that distinguish obese and nonobese patients with type 2 diabetes
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Figure 1. Obesity-associated upregulation of carbohydrate and lipid metabolism pathways in the livers of patients with type 2 diabetes. The genes involved in glycolysis (blue arrow) and gluconeogenesis (red arrow) were upregulated in the livers of obese vs. nonobese patients with type 2 diabetes. The genes involved in the glycolysis and gluconeogenesis pathways, such as the tricarboxylic acid cycle, pentose phosphate pathway, β-oxidation pathway, and cholesterol biosynthesis, are shown in the Supplementary Figure S1 online. Each pathway was adapted from a view in Gene MicroArray Pathway Profiler. The fold changes presented beside the names of the genes are for the obese vs. nonobese patients. The genes significantly (P < 0.05) upregulated in obesity are in red; the genes significantly (P < 0.05) downregulated are in green. Genes analyzed and not significantly altered in obesity are in gray. NADPH, nicotinamide adenine dinucleotide phosphate; PEPCK, phosphoenolpyruvate carboxykinase; TCA, tricarboxylic acid.

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Obesity was associated with significant upregulation of genes encoding the key gluconeogenesis enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK), phosphoglycerate kinase, fructose-1,6-bisphosphatase, glucose phosphate isomerase, and glucose-6-phosphatase (G-6-Pase) (obese/nonobese: 1.50, 1.50, 1.38, and 1.57; P = 0.016, 0.005, 0.016, and 0.042, respectively; Figure 1). Therefore, the expression of genes involved in glucose metabolism was coordinately upregulated with insulin resistance in the livers of obese patients with type 2 diabetes.

Genes encoding enzymes of the fatty acid β-oxidation pathway, including carnitine palmitoyltransferase and acetyl-coenzyme A acyltransferase, were upregulated in obesity (Supplementary Figure S1c online). We focused on the peroxisome proliferator-activated receptor (PPAR)-α and PPAR-γ as master control genes for the β-oxidation of fatty acids (25,26). The PPAR-α and PPAR-γ genes were both upregulated in obesity (obese/nonobese: 1.50 and 2.00; P = 0.0037 and 0.0546, respectively). The PPAR-γ gene expression level correlated with BMI (r = 0.611, P = 0.003), homeostasis model assessment of insulin resistance (r = 0.680, P = 0.001), and quantitative insulin sensitivity check index (QUICKI) (r = −0.805, P < 0.001), as well as the expression levels of its target genes, the CD36 gene (27) (r = 0.864, P < 0.001) and uncoupling protein 2 (UCP2) gene (28,29) (r = 0.532, P = 0.013). Expression of above representative genes was confirmed with a real-time PCR method (Supplementary Figure S2 and Supplementary Table S2 online).

Upregulation of the OXPHOS pathway and its association with insulin resistance

The most significant pathway enzyme genes coordinately altered in obesity were the 144 genes involved in the mitochondrial OXPHOS pathway (Permutation P < 0.00001; Table 1). In addition, the adenosine triphosphate (ATP) synthesis and ubiquinone biosynthesis pathways, which share genes with the OXPHOS pathway, were altered significantly. Scatterplot analyses revealed that obesity was associated with the coordinated upregulation of the OXPHOS pathway genes in the livers of patients with type 2 diabetes (data not shown), although the relevant differences were subtle at the level of individual genes, as reported in skeletal muscle (9). The mean percentage of correct classification using the 144 OXPHOS genes was 81% with the compound covariate predictor (P = 0.004) and 81% with linear discriminant analysis (P = 0.004). Genes involved in the OXPHOS pathway and significantly altered genes are illustrated in Figure 2, Supplementary Figure S2, and Supplementary Table S2 online. The protein complexes of the OXPHOS pathway consist of nicotinamide adenine dinucleotide (NADH)–ubiquinone oxidoreductase (complex I), succinate-ubiquinone oxidoreductase (complex II), ubiquinol-cytochrome c reductase (complex III), cytochrome c oxidase (complex IV), and ATP synthase (complex V). In terms of individual genes, 30 genes involved in complexes I-V of the electron transport chain were upregulated in obesity (P < 0.05), whereas no gene was significantly downregulated in this pathway (Figure 2).

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Figure 2. Obesity is associated with the coordinated upregulation of genes involved in oxidative phosphorylation (OXPHOS) in the livers of patients with type 2 diabetes. Illustration of the OXPHOS pathway genes significantly upregulated in obesity created using Gene MicroArray Pathway Profiler software. The fold changes presented beside the names of the genes are for obese vs. nonobese patients. The genes significantly (P < 0.05) upregulated in obesity are in red; the genes significantly (P < 0.05) downregulated are in green. Genes analyzed and not significantly altered in obesity are in gray. ATP, adenosine triphosphate.

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Next, we analyzed how the upregulation of OXPHOS pathway genes in the liver may be associated with pathology in type 2 diabetes. To evaluate any correlation between OXPHOS pathway gene expression and clinical or biochemical parameters in type 2 diabetes, we computed the mean centroid of the OXPHOS pathway genes, as described previously (8,9). The mean centroid of OXPHOS genes in the liver was significantly higher in obese than nonobese patients (0.279 ± 0.485 vs. −1.941 ± 0.563; P < 0.05, Supplementary Figure S3a online). The mean centroid of the OXPHOS genes in the liver was significantly correlated with insulin resistance indices, such as the fasting serum insulin level, homeostasis model assessment of insulin resistance, and QUICKI, and tended to be correlated with BMI, alanine aminotransferase, and Plasminogen activator inhibitor type-1 (Table 2). Therefore, the upregulation of OXPHOS pathway genes may be associated with insulin resistance and related pathologies such as obesity, fatty liver, and atherosclerosis.

Table 2.  Correlation between the OXPHOS mean centroid and clinical or biochemical parameters of individuals with type 2 diabetes
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Hepatic insulin resistance contributes to impaired suppression of gluconeogenesis in humans (3,4) and in various animal models (5). In addition, gluconeogenic enzymes such as PEPCK require ATP for their catalytic actions. We hypothesized that obesity-associated upregulation of genes in the OXPHOS pathway contributes to hyperglycemia via the supply of ATP for gluconeogenic enzymes. Indeed, the OXPHOS mean centroid was significantly correlated with the expression of gluconeogenic genes such as PEPCK1, PEPCK2, and glucose transporter type 2 (Table 3). Furthermore, the insulin resistance index QUICKI was significantly correlated with the expression of gluconeogenic genes (data not shown).

Table 3.  Correlation of the gene expressions of key transcription factors and gluconeogenesis with the OXPHOS mean centroid
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Regulatory factors controlling OXPHOS pathway gene expression

To examine the regulatory factors that might control the expression of OXPHOS pathway genes in the human liver, we analyzed the expression of transcriptional factors and nuclear co-activators reported to be associated with OXPHOS or energy homeostasis, including PGC-1α, PGC-1β, nuclear res piratory factor-1, PPARs, estrogen-related receptors, thyroid receptors, steroid receptor co-activator-1, and transcriptional intermediary factor-2 (ref. 8). Gene expression for PGC-1β, PPAR-γ, estrogen-related receptors-α, steroid receptor co-activator-1, TR-α, and transcriptional intermediary factor-2 was significantly correlated with the mean centroid of the OXPHOS genes (Table 3). However, the expression of PGC-1α, which has been reported to be correlated with OXPHOS gene expression in skeletal muscle in type 2 diabetes (9,10), was not correlated with the mean centroid of the OXPHOS genes in the liver (Table 3).

Augmented expression of genes involved in ROS generation

Increased mitochondrial oxygen flux through the OXPHOS pathway leads to increased formation of ROS as by-products (15) and may contribute to the development of insulin resistance in patients with obesity. To address the possibility that the liver in obese subjects generated excessive ROS, we examined genes involved in oxidative stress and redox. Genes encoding enzymes associated with peroxisomal β-oxidation of fatty acids (ACOX3), the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex (NOX1, CYBB, CYBA, NCF1, and NCF2), and the stress-responsive cytochromes (CYP2E1 and CYP4A11), all of which are involved in ROS generation in the liver (15,30,31), were upregulated in obesity (Table 4, Supplementary Figure S2, and Supplementary Table S2 online). In addition, genes encoding ROS-scavenging systems, such as glutathione S-transferase, protein disulfide isomerase, catalase, selenoproteins, and glutathione peroxidase, were upregulated in obesity. Most of the above genes involved in oxidative stress and redox were significantly correlated with the OXPHOS mean centroid (Table 4).

Table 4.  Expression of genes involved in oxidative stress and its correlation with the OXPHOS mean centroid in the liver
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In addition, we calculated the mean centroid of 28 ROS-related genes curated from GO (Gene set card for GO_ROS, Molecular Signatures Database, http:www.broad.mit.edugseamsigdbcardsGO_ROS.html). As shown in Supplementary Figure S3b online, the mean centroid of the 28 ROS-related genes was significantly elevated, together with the OXPHOS pathway (Supplementary Figure S3a online), in obese patients compared to nonobese subjects. The mean centroid of the ROS-related genes was positively correlated with OXPHOS-related genes (r = 0.917, P < 0.0001; Supplementary Figure S3c online), suggesting that upregulation of the OXPHOS pathway in patients with obesity is associated with the ROS-related pathway. These findings suggest that obesity results in coordinated upregulation of genes involved in ROS generation in concert with the upregulation of genes involved in OXPHOS in the liver.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information

Previously, we demonstrated that the gene expression profile in the liver of patients with type 2 diabetes is significantly different from that in the normal liver (32) and that genes involved in the OXPHOS pathway and gluconeogenesis are coordinately upregulated in the diabetic liver (8). Here, we demonstrated that obesity further upregulated type 2 diabetes-associated OXPHOS pathway gene expression with insulin resistance. We did not observe a similar effect of obesity on hepatic gene expression involved in the OXPHOS pathway in nondiabetic subjects (data not shown), suggesting that mitochondrial dysfunction has an important role in the pathophysiology of type 2 diabetes, as reported previously (18). However, the causative or compensatory nature of this OXPHOS gene expression in the pathogenesis of insulin resistance remains controversial (19). In fact, the expression profile in the liver looks like a mirror image of that in the skeletal muscle of patients with type 2 diabetes, in which PGC-1α-responsive genes of the OXPHOS pathway are coordinately downregulated and predict total-body aerobic capacity (9,10). Recently, Pospisilik et al. (20) demonstrated that a reduction in the gene expression and function of mitochondrial OXPHOS protects mice against obesity and diabetes. This finding suggests that the moderate deficiency in OXPHOS that is observed in the skeletal muscle of insulin-resistant humans is not a causative factor in diabetes, but may instead be a compensatory response (19,20).

In our comprehensive search for metabolic pathways altered in obesity, we found coordinated upregulation of genes that encode the enzymes of gluconeogenesis, including PEPCK and G-6-Pase, with increases in insulin resistance. Increased fatty acid oxidation promotes the gluconeogenesis pathway, in part by maintaining pyruvate carboxylase in an active state (33). Moreover, the increased fat burden on the liver is usually associated with the upregulation of key gluconeogenic enzymes and sets the scene for inappropriately high rates of hepatic glucose output (34). Our results suggest that obesity might latently enhance the gluconeogenic pathway, even before fasting plasma glucose levels increase, in patients with type 2 diabetes. Therefore, this profile may provide the molecular basis for the observation that, in the presence of type 2 diabetes, obesity has an independent and additive effect, further increasing the proportion of gluconeogenesis to total endogenous glucose production (35).

As branches of glucose metabolism, the pathways for the pentose phosphate and tricarboxylic acid cycles were upregulated in obesity. The pentose phosphate cycle supplies NADPH and H+ required for fatty acid and cholesterol synthesis and can also lead to ROS generation via NADPH oxidase (36). The tricarboxylic acid cycle links glycolysis, gluconeogenesis, fatty acid metabolism, and ATP synthesis. In accordance with the upregulation of these pivotal pathways, the pathways for the biosynthesis and β-oxidation of fatty acids and cholesterol biosynthesis were upregulated in obesity, as reported previously in an experimental animal model fed a high-fat diet (37) and in genetically obese animals (38,39,40). Obesity-associated upregulation of the fatty acid β-oxidation pathway was supported by the upregulation of the genes for PPAR-α and PPAR-γ, the master control genes for fatty acid β-oxidation (25,26). The expression level of UCP-2, a target gene of PPAR-γ (28,29), was significantly correlated with that of the PPAR-γ gene.

These findings suggest that the metabolic pathways identified in our study seem to be “coordinately,” rather than “coincidentally,” regulated. To our knowledge, no previous study has demonstrated obesity-associated alterations in glucose and lipid metabolism in the human diabetic liver. Furthermore, hepatic gene expression in obesity did not seem to be altered by adipocyte-derived factors alone because the serum levels of adipocytokines, such as adiponectin, leptin, and tumor necrosis factor-α, did not change significantly in the obese patients

The activation of the tricarboxylic acid cycle and the β-oxidation of fatty acids distribute acetyl CoA and acyl CoA, respectively, to the mitochondrial OXPHOS pathway. Increased mitochondrial oxygen flux leads to increased formation of ROS (15,16). Moreover, ROS have been suggested to play a causal role in experimental models of insulin resistance in hepatocytes and adipocytes (13,41). An inhibitor of the electron transport chain complex or an uncoupler of the OXPHOS pathway prevented high glucose-induced increases in ROS generation in bovine aortic endothelial cells, thereby preventing NF-κB activation (42). Therefore, obesity-induced activation of the OXPHOS pathway may stimulate ROS formation in the liver, activate NF-κB, and increase inflammation-associated insulin resistance (1). However, a molecular signature indicating increased ROS generation in the human liver complicated by obesity has been lacking. In this study, we showed that obesity resulted in the upregulation of genes for the OXPHOS pathway in the liver of patients with type 2 diabetes. Indeed, the expression of OXPHOS genes was significantly correlated with the expression of gluconeogenic genes and with insulin resistance, suggesting that obesity latently enhances the OXPHOS pathway and insulin resistance in patients with type 2 diabetes.

Note that unlike in the skeletal muscle of patients with type 2 diabetes (9,10), the expression level of PGC-1α was not significantly correlated with the severity of either obesity or insulin resistance, or with the expression levels of genes encoding the enzymes of gluconeogenesis (data not shown) or the OXPHOS pathway (Table 3). Instead, transcriptional factors or nuclear co-activators reportedly associated with the OXPHOS pathway or energy homeostasis, such as PGC-1β, nuclear respiratory factor-1, PPARs, estrogen-related receptors, and thyroid receptors (8), were significantly correlated with the mean centroid of the OXPHOS pathway genes. The identification of the upstream master gene(s) controlling the expression of the OXPHOS pathway genes is an important issue in elucidating the mechanism(s) underlying obesity-induced insulin resistance.

Along with the OXPHOS pathway genes, the genes encoding enzymes associated with ROS generation, such as those involved in the mitochondrial and peroxisomal β-oxidation of fatty acids (15), NADPH oxidase (30), and stress-responsive cytochromes (31) were also upregulated in obesity. Cells possess a variety of defenses to protect against the toxic effects of increased ROS. Accordingly, genes encoding redox and ROS-scavenging systems, such as catalase and glutathione peroxidase, may be upregulated in obesity. These findings suggest that obesity causes oxidative stress in the liver, partly attributable to ROS generated from the OXPHOS pathway together with the peroxisomal β-oxidation of fatty acids, NADPH oxidase, and cytochromes. A limitation of our study was that the analysis was performed at the gene expression level only. The limited liver samples available from patients with type 2 diabetes did not allow functional or structural analyses.

In summary, we provide the first evidence that the increased expression of genes involved in OXPHOS in the human liver links obesity in diabetes with insulin resistance. Further functional study is necessary to establish the causative role of OXPHOS in the pathogenesis of obesity and type 2 diabetes.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information

We thank Dr Yamashita and Dr Arai for performing the liver biopsies, and Yuki Rikimaru and Masahiro Uchikata for technical assistance. This study was supported in part by a grant-in-aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The English in this document has been checked by at least two professional editors, both native speakers of English. For a certificate, see: http:www.textcheck.comcgi-bincertificate.cgiidiHYETX

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods and Procedures
  5. Results
  6. Discussion
  7. SUPPLEMENTARY MATERIAL
  8. Acknowledgment
  9. Disclosure
  10. REFERENCES
  11. Supporting Information

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