Blood glucose-lowering nuclear receptor agonists only partially normalize hepatic gene expression in db/db mice

DPHC From Alpinia officinarum Ameliorates Oxidative Stress and Insulin Resistance via Activation of Nrf2/ARE Pathway in db/db Mice and High Glucose-Treated HepG2 Cells

(R)-5-hydroxy-1,7-diphenyl-3-heptanone (DPHC) from the natural plant Alpinia officinarum has been reported to have antioxidation and antidiabetic effects. In this study, the therapeutic effect and molecular mechanism of DPHC on type 2 diabetes mellitus (T2DM) were investigated based on the regulation of oxidative stress and insulin resistance (IR) in vivo and in vitro. In vivo, the fasting blood glucose (FBG) level of db/db mice was significantly reduced with improved glucose tolerance and insulin sensitivity after 8 weeks of treatment with DPHC. In vitro, DPHC ameliorated IR because of its increasing glucose consumption and glucose uptake of IR-HepG2 cells induced by high glucose. In addition, in vitro and in vivo experiments showed that DPHC could regulate the antioxidant enzyme levels including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px), thereby reducing the occurrence of oxidative stress and improving insulin resistance. Western blotting and polymerase chain reaction results showed that DPHC could promote the expressions of nuclear factor erythroid 2-related factor 2 (Nrf2), the heme oxygenase-1 (HO-1), protein kinase B (AKT), and glucose transporter type 4 (GLUT4), and reduced the phosphorylation levels of c-Jun N-terminal kinase (JNK) and insulin receptor substrate-1 (IRS-1) on Ser307 both in vivo and in vitro. These findings verified that DPHC has the potential to relieve oxidative stress and IR to cure T2DM by activating Nrf2/ARE signaling pathway in db/db mice and IR-HepG2 cells.

Effects of D-Chiro-Inositol on Glucose Metabolism in db/db Mice and the Associated Underlying Mechanisms

In this study, we observed the effect of D-chiro-inositol (DCI) on glucose consumption in type 2 diabetic db/db mice, and investigated the relevant mechanism. We discovered that the stability of 24-h blood glucose under the nonfasting condition and decreased glucose tolerance were both alleviated after treatment with DCI. Moreover, the content of glycosylated protein and advanced glycation end products in the serum was reduced, the damage in the liver tissue was alleviated, and the synthesis of liver glycogen was significantly promoted. In addition, DCI increased the expression of insulin receptor substrate 2 (IRS2), phosphatidylinositol 3-kinase (PI3K), protein kinase B (AKT), glucose transporters 4 (GLUT4), and phospho-AKT (S473) protein. In contrast, DCI decreased the expression level of glycogen synthase kinase 3β (GSK3β) protein in liver tissue to various degrees, as shown by immunohistochemistry and western blotting. Furthermore, DCI increased the mRNA expression of IRS2, PI3K, AKT, and GLUT4, and reduced that of GSK3β in liver tissue, as demonstrated by polymerase chain reaction. Finally, DCI promoted glucose consumption in high glucose-stimulating HepG2 cells and increased the expression of IRS2 protein in HepG2 cells, as revealed by fluorescence staining and flow cytometry. Our results indicate that DCI can significantly improve glucose metabolism in diabetic mice and HepG2 cells. This effect may be associated with the upregulation of IRS2, PI3K, AKT, and GLUT4 and downregulation of GSK3β.

Protective effects of fish oil and pioglitazone on pancreatic tissue in obese KK mice with type 2 diabetes

n-3 Polyunsaturated fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have protective effects against the pancreatic β-cell dysfunction through several mechanisms. Thiazolidines are insulin sensitizers and are used in treating patients with type 2 diabetes. Our previous study demonstrated that a combination of fish oil, which is rich with EPA and DHA, and pioglitazone exerts beneficial effects on obesity and diabetes through their actions on the liver and adipose tissue. However, it remains largely unknown whether such combination therapy affects the pancreas. To answer this question, KK mice, which serve as a model for obesity and type 2 diabetes, were treated for 8 weeks with fish oil and pioglitazone. The combined regimen suppressed pancreatic islet hypertrophy (mean islet area decreased by an average of 49% vs. control) compared with mice treated with fish oil or pioglitazone alone (decreased by an average of 21% and 32% vs. control, respectively). Compared with the controls, individual or combined treatment significantly increased the percentage of β-cell area in the pancreatic islets, significantly decreased endoplasmic reticulum stress, and reduced the percentage of apoptotic cell death in the pancreatic islets. These findings suggest that fish oil and/or pioglitazone prevents β-cell dysfunction by improving the insulin resistance and decreasing the ER stress.

Liver X receptor agonist downregulates growth hormone signaling in the liver

Liver X receptor (LXR) agonists have been shown to influence the development of hyperlipidemia and atherosclerosis in mouse models. It has also been demonstrated that some LXR agonists can cause hepatic steatosis in experimental animals. Growth hormone (GH) is known to regulate hepatic metabolism and the absence of hepatic GH receptors (GHR) leads to hepatic steatosis. In this study, we analyzed whether the actions of LXR agonists could involve interference with GH signaling. We showed that LXR agonists impair GH signaling in hepatocytes. LXR agonist treatment attenuated GH induction of suppressor of cytokine signaling 2 (SOCS2), SOCS3, and CIS mRNA levels in BRL-4 cells. Likewise, the activity of a luciferase reporter vector driven by the GH response element (GHRE) of the SOCS2 gene was inhibited by simultaneous treatment with an LXR agonist. The inhibitory effect of LXR agonists on GH signals can be mimicked by overexpression of the LXR regulated factors, sterol regulatory element binding protein 1 (SREBP1) and SREBP2, in hepatic cells. In both cases total and phosphorylated signal transducers and activators of transcription 5b (STAT5b) protein levels were significantly reduced. DNA binding assays demonstrated that SREBP1 binds to an E-box within a previously defined GHRE in the SOCS2 gene promoter, but does not compete with STAT5b binding to a nearby site in the same promoter construct. Taken together, our findings indicate that the inhibitory effects of LXR agonists on GH signaling are mediated by SREBP1, through the downregulation of STAT5b gene transcription and stimulation of STAT5b protein degradation. The findings provide a new insight into the understanding of the molecular actions of LXR agonists, which may be of relevance to their pharmacological actions.

Transcriptomic analysis of insulin-sensitive tissues from anti-diabetic drug treated ZDF rats, a T2DM animal model

Gene expression changes have been associated with type 2 diabetes mellitus (T2DM); however, the alterations are not fully understood. We investigated the effects of anti-diabetic drugs on gene expression in Zucker diabetic fatty (ZDF) rats using oligonucleotide microarray technology to identify gene expression changes occurring in T2DM. Global gene expression in the pancreas, adipose tissue, skeletal muscle, and liver was profiled from Zucker lean control (ZLC) and anti-diabetic drug treated ZDF rats compared with those in ZDF rats. We showed that anti-diabetic drugs regulate the expression of a large number of genes. We provided a more integrated view of the diabetic changes by examining the gene expression networks. The resulting sub-networks allowed us to identify several biological processes that were significantly enriched by the anti-diabetic drug treatment, including oxidative phosphorylation (OXPHOS), systemic lupus erythematous, and the chemokine signaling pathway. Among them, we found that white adipose tissue from ZDF rats showed decreased expression of a set of OXPHOS genes that were normalized by rosiglitazone treatment accompanied by rescued blood glucose levels. In conclusion, we suggest that alterations in OXPHOS gene expression in white adipose tissue may play a role in the pathogenesis and drug mediated recovery of T2DM through a comprehensive gene expression network study after multi-drug treatment of ZDF rats.

Liver X receptors, nervous system, and lipid metabolism

Lipids in the nervous system are represented by cholesterol and phospholipids as constituents of cell membranes and, in particular, of myelin. Therefore, lipids are finely regulated to guarantee physiological functions. In the central nervous system, cholesterol is locally synthesized due to the presence of the blood brain barrier. In the peripheral nervous system cholesterol is either up-taken by lipoproteins and/or produced by de novo biosynthesis. Defects in lipid homeostasis in these tissues lead to structural and functional changes that often result in different pathological conditions depending on the affected pathways (i.e. cholesterol biosynthesis, cholesterol efflux, fatty acid biosynthesis etc.). Alterations in cholesterol metabolism in the central nervous system are linked to several disorders such as Alzheimer's disease, Huntington disease, Parkinson disease, Multiple sclerosis, Smith-Lemli-Opitz syndrome, Niemann-Pick type C disease, and glioblastoma. In the peripheral nervous system changes in lipid metabolism are associated with the development of peripheral neuropathy that may be caused by metabolic disorders, injuries, therapeutics, and autoimmune diseases. Transcription factors, such as the Liver X receptors (LXR), regulate both cholesterol and fatty acid metabolism in several tissues including the nervous system. In the last few years several studies elucidated the biology of LXR in the nervous system due to the availability of knock-out mice and the development of synthetic ligands. Here, we review a survey of the literature focused on the central and peripheral nervous system and in physiological and pathological settings with particular attention to the roles played by LXR in both districts.

Rosiglitazone attenuates casein-induced hepatic endoplasmic reticulum stress in Sprague-Dawley rats: a novel model of endoplasmic reticulum stress

The proteins found in cow milk have been reported to cause systemic inflammation. Endoplasmic reticulum (ER) stress is known to be involved in the development of several metabolic disorders including insulin resistance and non-alcoholic fatty liver disease. However, the effect of thiazolidinediones (TZDs) on ER stress is still controversial. This is why we want to investigate in this study whether casein, which is the major protein in cow's milk, induces ER stress in the liver and whether rosiglitazone can attenuate these changes. Nine-week-old male Sprague-Dawley (SD) rats were separated into three groups: (1) vehicle treated; (2) daily subcutaneous injections of 1 mL 10% casein; (3) daily subcutaneous injection of 1 mL 10% casein and rosiglitazone 4 mg/[kg d]. After 6 weeks, body weight, food intake, glucose and lipid parameters, and serum AST/ALT levels were measured after an overnight fast. Real time RT-PCR and immunohistochemical staining for various ER stress markers were performed, and a TUNEL analysis was also performed. After 6 weeks, casein injection induced weight reduction, systemic inflammation, and hepatic dysfunction in SD rats. Casein injection increased both the gene and protein expression of ER stress markers in the liver and also caused hepatocyte apoptosis. Rosiglitazone treatment attenuated casein-induced systemic inflammation, ER stress, deteriorated liver function, and increased apoptosis. In conclusion, our results may provide further insight into the effects of casein on chronic inflammatory diseases, and to have a better understanding of the mechanism of the anti-inflammatory properties of rosiglitazone regardless of its hypoglycemic effect.

n-3 fatty acids ameliorate hepatic steatosis and dysfunction after LXR agonist ingestion in mice

Liver X receptor (LXR) agonists slow atherogenesis, but cause hepatic steatosis and dysfunction in part by increasing expression of sterol regulatory element binding protein 1-c (SREBP1-c), a transcription factor that upregulates fatty acid (FA) synthesis. n-3 FAs decrease hepatic FA synthesis by down-regulating SREBP1-c. To test the hypothesis that n-3 FAs decrease hepatic steatosis in mice given LXR agonist, C57BL/6 mice received daily gavage of an LXR agonist T0901317 (LXR(T)) or vehicle for 4weeks with concomitant intakes chow or high-fat diets enriched in saturated fat (SAT) or n-3 fat (n-3). Mice on LXR(T) and SAT developed hepatomegaly with a large increase in size and number of hepatic lipid droplets; an n-3 diet reduced liver weight/body weight with decreased hepatic steatosis and triglyceride levels. Effects of n-3 diet on hepatic lipogenesis were linked to a blunting of LXR(T) upregulation of hepatic SREBP1-c and FA synthase mRNA. n-3 diets also normalized LXR(T)-mediated increases of plasma ALT and AST levels, whereas SAT diet increased these markers.

CONCLUSION

These studies suggest that n-3 FAs when given together with LXR agonists have the potential to improve both hepatic steatosis and hepatotoxicity in humans that might receive LXR agonists to decrease risk of atherosclerosis.

Metformin inhibits nuclear receptor TR4-mediated hepatic stearoyl-CoA desaturase 1 gene expression with altered insulin sensitivity

OBJECTIVE

TR4 is a nuclear receptor without clear pathophysiological roles. We investigated the roles of hepatic TR4 in the regulation of lipogenesis and insulin sensitivity in vivo and in vitro.

RESEARCH DESIGN AND METHODS

TR4 activity and phosphorylation assays were carried out using hepatocytes and various TR4 wild-type and mutant constructs. Liver tissues from TR4 knockout, C57BL/6, and db/db mice were examined to investigate TR4 target gene stearoyl-CoA desaturase (SCD) 1 regulation.

RESULTS

TR4 transactivation is inhibited via phosphorylation by metformin-induced AMP-activated protein kinase (AMPK) at the amino acid serine 351, which results in the suppression of SCD1 gene expression. Additional mechanistic dissection finds TR4-transactivated SCD1 promoter activity via direct binding to the TR4-responsive element located at -243 to -255 on the promoter region. The pathophysiological consequences of the metformin→AMPK→TR4→SCD1 pathway are examined via TR4 knockout mice and primary hepatocytes with either knockdown or overexpression of TR4. The results show that the suppression of SCD1 via loss of TR4 resulted in reduced fat mass and increased insulin sensitivity with increased β-oxidation and decreased lipogenic gene expression.

CONCLUSIONS

The pathway from metformin→AMPK→TR4→SCD1→insulin sensitivity suggests that TR4 may function as an important modulator to control lipid metabolism, which sheds light on the use of small molecules to modulate TR4 activity as a new alternative approach to battle the metabolic syndrome.

Coordinate Transcriptomic and Metabolomic Effects of the Insulin Sensitizer Rosiglitazone on Fundamental Metabolic Pathways in Liver, Soleus Muscle, and Adipose Tissue in Diabetic db/db Mice

Rosiglitazone (RSG), developed for the treatment of type 2 diabetes mellitus, is known to have potent effects on carbohydrate and lipid metabolism leading to the improvement of insulin sensitivity in target tissues. To further assess the capacity of RSG to normalize gene expression in insulin-sensitive tissues, we compared groups of 18-day-treated db/db mice with increasing oral doses of RSG (10, 30, and 100 mg/kg/d) with untreated non-diabetic littermates (db/+). For this aim, transcriptional changes were measured in liver, inguinal adipose tissue (IAT) and soleus muscle using microarrays and real-time PCR. In parallel, targeted metabolomic assessment of lipids (triglycerides (TGs) and free fatty acids (FFAs)) in plasma and tissues was performed by UPLC-MS methods. Multivariate analyses revealed a relationship between the differential gene expressions in liver and liver trioleate content and between blood glucose levels and a combination of differentially expressed genes measured in liver, IAT, and muscle. In summary, we have integrated gene expression and targeted metabolomic data to present a comprehensive overview of RSG-induced changes in a diabetes mouse model and improved the molecular understanding of how RSG ameliorates diabetes through its effect on the major insulin-sensitive tissues.

Intersection of the unfolded protein response and hepatic lipid metabolism

The liver plays a central role in whole-body lipid metabolism by governing the synthesis, oxidization, transport and excretion of lipids. The unfolded protein response (UPR) was identified as a signal transduction system that is activated by ER stress. Recent studies revealed a critical role of the UPR in hepatic lipid metabolism. The IRE1/XBP1 branch of the UPR is activated by high dietary carbohydrates and controls the expression of genes involved in fatty acid and cholesterol biosynthesis. PERK mediated eIF2alpha phosphorylation is also required for the expression of lipogenic genes and the development of hepatic steatosis, likely by activating C/EBP and PPARgamma transcription factors. Further studies to define the molecular pathways that lead to the activation of the UPR by nutritional cues in the liver, and their contribution to human metabolic disorders such as hepatic steatosis, atherosclerosis and type 2 diabetes that are associated with dysregulation of lipid homeostasis, are warranted.

Gene expression regulated by pioglitazone and exenatide in normal and diabetic rat islets exposed to lipotoxicity

BACKGROUND

Hyperlipidaemia has been suggested to contribute by pro-apoptotic actions to the loss of beta-cell mass, its secretory defects, and thereby impaired beta-cell function in type 2 diabetes. Treatment of genetically diabetic rats and also type 2 diabetic patients with pioglitazone, a PPAR-gamma agonist, lowers fasting levels of plasma glucose and triglycerides, and has been suggested to protect beta-cells against diabetic lipotoxicity in vitro and in vivo. Another recently launched anti-diabetic drug, exenatide, an incretin mimetic, has been shown to stimulate insulin secretion, growth, and proliferation of pancreatic beta-cells and to protect them against apoptosis. We aimed to investigate global alterations in beta-cell gene expression under lipotoxic conditions and the influence of in vitro treatment with pioglitazone and exenatide.

METHODS

Global gene expression profiling was thus performed to characterize genes differently regulated by palmitate, pioglitazone, and exenatide in isolated islets from non-diabetic Wistar rats and type 2 diabetic Goto-Kakizaki (GK) rats.

RESULTS

Gene expression profiling revealed significant changes in islet mRNAs involved in control of several aspects of beta-cell function, e.g. epigenetic regulation of gene expression, cell differentiation and morphogenesis, also metabolism, response to stimulus, transport, and signal transduction. Pioglitazone and exenatide appear to significantly impact epigenetic processes, e.g. stable alterations in gene expression potential, which arise during development and cell proliferation. Bcl2-like 1 (Bcl2l1), an anti-apoptotic protein, and Bcl2 modifying factor (Bmf), a pro-apoptotic protein, were both down-regulated by pioglitazone and exenatide in the presence of palmitate in diabetic GK islets. In contrast, Bmf was downregulated by pioglitazone in the presence of palmitate in non-diabetic Wistar islets. Exposure of non-diabetic Wistar islets to palmitate led to a reduction in the expression of PPAR beta/delta. This suggests that palmitate may increase the accumulation of triglycerides by reducing PPAR signalling. Moreover, treatment with either pioglitazone or exenatide restored and increased the expression of PPAR beta/delta in non-diabetic Wistar islets.

CONCLUSIONS

Taking into account that these drugs target different components of the epigenetic machinery, our findings suggest that they might participate in restoring normal gene activity in dysfunctional islets and that additive benefits may occur. Whether such events contribute to the beta-cell sparing, proliferative, and anti-apoptotic effects of these drugs in diabetes remains to be elucidated.

Pioglitazone reduces ER stress in the liver: direct monitoring of in vivo ER stress using ER stress-activated indicator transgenic mice

It is known that endoplasmic reticulum (ER) stress is provoked under diabetic conditions and is possibly involved in the development of insulin resistance. In this study, using ER stress-activated indicator (ERAI) transgenic mice which express green fluorescent protein under ER stress conditions, we directly evaluated the effects of a diabetic agent pioglitazone on in vivo ER stress under diabetic conditions. In high fat and high sucrose diet-induced diabetic ERAI transgenic mice, 8 weeks of pioglitazone treatment reduced the accumulation of fat droplets in the liver and attenuated the development of insulin resistance. In the liver of the ERAI transgenic mice, ERAI fluorescence activity was clearly reduced as early as after 4 weeks of pioglitazone treatment, preceding the improvement of insulin resistance. In addition, after the pioglitazone treatment, serum free fatty acid and triglyceride levels were decreased, and serum adiponectin levels were increased. These data indicate that pioglitazone treatment suppresses ER stress in the liver which may explain, at least in part, the pharmacological effects of pioglitazone to reduce insulin resistance.

Effect of pioglitazone treatment on endoplasmic reticulum stress response in human adipose and in palmitate-induced stress in human liver and adipose cell lines

Obesity and elevated cytokine secretion result in a chronic inflammatory state and may cause the insulin resistance observed in type 2 diabetes. Recent studies suggest a key role for endoplasmic reticulum stress in hepatocytes and adipocytes from obese mice, resulting in reduced insulin sensitivity. To address the hypothesis that thiazolidinediones, which improve peripheral insulin sensitivity, act in part by reducing the endoplasmic reticulum stress response, we tested subcutaneous adipose tissue from 20 obese volunteers treated with pioglitazone for 10 wk. We also experimentally induced endoplasmic reticulum stress using palmitate, tunicamycin, and thapsigargin in the human HepG2 liver cell line with or without pioglitazone pretreatment. We quantified endoplasmic reticulum stress response by measuring both gene expression and phosphorylation. Pioglitazone significantly improved insulin sensitivity in human volunteers (P = 0.002) but did not alter markers of endoplasmic reticulum stress. Differences in pre- and posttreatment endoplasmic reticulum stress levels were not correlated with changes in insulin sensitivity or body mass index. In vitro, palmitate, thapsigargin, and tunicamycin but not oleate induced endoplasmic reticulum stress in HepG2 cells, including increased transcripts CHOP, ERN1, GADD34, and PERK, and increased XBP1 splicing along with phosphorylation of eukaryotic initiation factor eIF2alpha, JNK1, and c-jun. Although patterns of endoplasmic reticulum stress response differed among palmitate, tunicamycin, and thapsigargin, pioglitazone pretreatment had no significant effect on any measure of endoplasmic reticulum stress, regardless of the inducer. Together, our data suggest that improved insulin sensitivity with pioglitazone is not mediated by a reduction in endoplasmic reticulum stress.

The role for endoplasmic reticulum stress in diabetes mellitus

Accumulating evidence suggests that endoplasmic reticulum (ER) stress plays a role in the pathogenesis of diabetes, contributing to pancreatic beta-cell loss and insulin resistance. Components of the unfolded protein response (UPR) play a dual role in beta-cells, acting as beneficial regulators under physiological conditions or as triggers of beta-cell dysfunction and apoptosis under situations of chronic stress. Novel findings suggest that "what makes a beta-cell a beta-cell", i.e., its enormous capacity to synthesize and secrete insulin, is also its Achilles heel, rendering it vulnerable to chronic high glucose and fatty acid exposure, agents that contribute to beta-cell failure in type 2 diabetes. In this review, we address the transition from physiology to pathology, namely how and why the physiological UPR evolves to a proapoptotic ER stress response and which defenses are triggered by beta-cells against these challenges. ER stress may also link obesity and insulin resistance in type 2 diabetes. High fat feeding and obesity induce ER stress in liver, which suppresses insulin signaling via c-Jun N-terminal kinase activation. In vitro data suggest that ER stress may also contribute to cytokine-induced beta-cell death. Thus, the cytokines IL-1beta and interferon-gamma, putative mediators of beta-cell loss in type 1 diabetes, induce severe ER stress through, respectively, NO-mediated depletion of ER calcium and inhibition of ER chaperones, thus hampering beta-cell defenses and amplifying the proapoptotic pathways. A better understanding of the pathways regulating ER stress in beta-cells may be instrumental for the design of novel therapies to prevent beta-cell loss in diabetes.

The endoplasmic reticulum in pancreatic beta cells of type 2 diabetes patients

AIMS/HYPOTHESIS

Pancreatic beta cells have highly developed endoplasmic reticulum (ER) due to their role in insulin secretion. Since ER stress has been associated with beta cell dysfunction, we studied several features of beta cell ER in human type 2 diabetes.

METHODS

Pancreatic samples and/or isolated islets from non-diabetic controls (ND) and type 2 diabetes patients were evaluated for insulin secretion, apoptosis (electron microscopy and ELISA), morphometric ER assessment (electron microscopy), and expression of ER stress markers in beta cell prepared by laser capture microdissection and in isolated islets.

RESULTS

Insulin release was lower and beta cell apoptosis higher in type 2 diabetes than ND islets. ER density volume was significantly increased in type 2 diabetes beta cells. Expression of alpha-mannosidase (also known as mannosidase, alpha, class 1A, member 1) and UDP-glucose glycoprotein glucosyl transferase like 2 (UGCGL2), assessed by microarray and/or real-time reverse transcriptase polymerase chain reaction (RT-PCR), differed between ND and type 2 diabetes beta cells. Expression of immunoglobulin heavy chain binding protein (BiP, also known as heat shock 70 kDa protein 5 [glucose-regulated protein, 78 kDa] [HSPA5]), X-box binding protein 1 (XBP-1, also known as XBP1) and C/EBP homologous protein (CHOP, also known as damage-inducible transcript 3 [DDIT3]) was not higher in type 2 diabetes beta cell or isolated islets cultured at 5.5 mmol/l glucose (microarray and real-time RT-PCR) than in ND samples. When islets were cultured for 24 h at 11.1 mmol/l glucose, there was induction of BiP and XBP-1 in type 2 diabetes islets but not in ND islets.

CONCLUSIONS/INTERPRETATION

Beta cell in type 2 diabetes showed modest signs of ER stress when studied in pancreatic samples or isolated islets maintained at physiological glucose concentration. However, exposure to increased glucose levels induced ER stress markers in type 2 diabetes islet cells, which therefore may be more susceptible to ER stress induced by metabolic perturbations.

Elk1 and SRF transcription factors convey basal transcription and mediate glucose response via their binding sites in the human LXRB gene promoter

The nuclear receptors LXRα (NR1H3) and LXRβ (NR1H2) are attractive drug targets for the treatment of diabetes and cardiovascular disease due to their established role as regulators of cholesterol and lipid metabolism. A large body of literature has recently indicated their important roles in glucose metabolism and particularly LXRβ is important for proper insulin production in pancreas. In this study, we report that glucose induces transcription via the LXRB gene promoter. The transcription start site of the human LXRB gene was determined and we identified two highly conserved, and functional, ETS and Elk1 binding sites, respectively, in the LXRB gene promoter. The Elk1 binding site also bound the serum responsive factor (SRF). Mutation of these sites abolished binding. Furthermore, mutation of the binding sites or siRNA knockdown of SRF and Elk1 significantly reduced the promoter activity and impaired the glucose response. Our results indicate that the human LXRB gene is controlled by glucose, thereby providing a novel mechanism by which glucose regulates cellular functions via LXRβ.

Application of DNA Microarrays in the Study of Human Obesity and Type 2 Diabetes

DNA microarrays have provided medical researchers with a powerful tool to study the mechanisms of complex diseases, including obesity and type 2 diabetes (T2D). The technology has been used to dissect virtually every aspect of the genetic and molecular basis of these two diseases. Gene expression profiling is the major application of DNA microarrays so far. Subcutaneous fat, visceral fat, adipocyte and preadipocyte, muscle, liver, pancreas and specific nuclei in the hypothalamus under normal and disease conditions are used in addressing the profile of gene expression in obesity and T2D. Comparisons of fat depots in humans and animal models - including ob/ob and db/db mice, diet-induced obese mice, fa/fa Zucker rats, gene knockout (plin (-/-), GLUT4 (-/-)) and transgenic mice (GLUT4-Tg) - have been employed in microarray experiments. The effects of various interventions, such as hormonal and drug treatments, exercise, and surgery, have been studied to determine the expression profile of different developmental stages in cells and the effect of treatment on the two diseases. In this review, the application of microarrays in elucidating the role of retinol binding protein 4 as a link between obesity and T2D is discussed. The possible role in obesity of a common genetic variant near the INSIG2 gene and the discovery of the BBS9 gene are also discussed. The problems and challenges are summarized under eight categories and suggestions for the future direction of research in this area are proposed.

Prevention of high-fat diet-induced adipose tissue remodeling in obese diabetic mice by n-3 polyunsaturated fatty acids

OBJECTIVE

Obesity is associated with reduced insulin sensitivity and extensive reorganization of adipose tissue. As polyunsaturated fatty acids (PUFA) appear to inhibit diabetes development, we investigated PUFA effects on markers of matrix remodeling in white adipose tissue.

METHODS AND PROCEDURE

Male obese diabetic (db/db) mice were treated with either a low-fat standard diet (LF), or high-fat diets rich in saturated and monounsaturated fatty acids (HF/S), n-6 PUFA (HF/6) or the latter including marine n-3 PUFA (HF/3). White adipose tissue was analyzed for gene expression, fatty acid composition and by immunofluorescence.

RESULTS

HF/S treatment increased adipose tissue expression of a number of genes involved in matrix degradation including matrix metalloproteinase (MMP)-12, -14 and cathepsin K, L and S compared with LF. MMP-12 gene was expressed in macrophages and adipocytes, and MMP-12 protein colocalized with both cell types. In addition, mean adipocyte area increased by 1.6-fold in HF/S-treated mice. Genes essential for collagen production, such as procollagen I, III, VI, tenascin C and biglycan were upregulated in HF/S-treated animals as well. N-3 PUFA supplementation resulted in enrichment of these fatty acids in adipose tissue. Moreover, n-3 PUFA inhibited the HF/S-induced upregulation of genes involved in matrix degradation and production I restored mean adipocyte area and prevented MMP-12 expression in macrophages and adipocytes.

CONCLUSION

N-3 PUFA prevent high-fat diet-induced matrix remodeling and adipocyte enlargement in adipose tissue of obese diabetic mice. Such changes could contribute to diabetes prevention by n-3 PUFA in obese patients.