Mechanism for antioxidative effects of thiazolidinediones in pancreatic β-cells

Thiazolidinediones (TZDs) are synthetic ligands of peroxisome proliferator-activated receptor-γ (PPARγ), a member of the nuclear receptor superfamily. TZDs are known to increase insulin sensitivity and also to have an antioxidative effect. In this study, we tested whether TZDs protect pancreatic β-cells from oxidative stress, and we investigated the mechanism involved in this process. To generate oxidative stress in pancreatic β-cells (INS-1 and βTC3) or isolated islets, glucose oxidase was added to the media. The extracellular and intracellular reactive oxygen species (ROS) were measured to directly determine the antioxidant effect of TZDs. The phosphorylation of JNK/MAPK after oxidative stress was detected by Western blot analysis, and glucose-stimulated insulin secretion and cell viability were also measured. TZDs significantly reduced the ROS levels that were increased by glucose oxidase, and they effectively prevented β-cell dysfunction. The antioxidative effect of TZDs was abolished in the presence of a PPARγ antagonist, GW9662. Real-time PCR was used to investigate the expression levels of antioxidant genes. The expression of catalase, an antioxidant enzyme, was increased by TZDs in pancreatic β-cells, and the knockdown of catalase significantly inhibited the antioxidant effect of TZDs. These results suggest that TZDs effectively protect pancreatic β-cells from oxidative stress, and this effect is dependent largely on PPARγ. In addition, the expression of catalase is increased by TZDs, and catalase, at least in part, mediates the antioxidant effect of TZDs in pancreatic β-cells.     

Inhibition of the receptor for advanced glycation endproducts (RAGE) protects pancreatic β-cells

Advanced glycation endproducts (AGEs) and the receptor for AGEs (RAGE) have been linked to the pathogenesis of diabetic complications, such as retinopathy, neuropathy, and nephropathy. AGEs may induce β-cell dysfunction and apoptosis, another complication of diabetes. However, the role of AGE-RAGE interaction in AGE-induced pancreatic β-cell failure has not been fully elucidated. In this study, we investigated whether AGE–RAGE interaction could mediate β-cell failure. We explored the potential mechanisms in insulin secreting (INS-1) cells from a pancreatic β-cell line, as well as primary rat islets. We found that glycated serum (GS) induced apoptosis in pancreatic β-cells in a dose- and time-dependent manner. Treatment with GS increased RAGE protein production in cultured INS-1 cells. GS treatment also decreased bcl-2 gene expression, followed by mitochondrial swelling, increased cytochrome c release, and caspase activation. RAGE antibody and knockdown of RAGE reversed the β-cell apoptosis and bcl-2 expression. Inhibition of RAGE prevented AGE-induced pancreatic β-cell apoptosis, but could not restore the function of glucose stimulated insulin secretion (GSIS) in rat islets. In summary, the results of the present study demonstrate that AGEs are integrally involved in RAGE-mediated apoptosis and impaired GSIS dysfunction in pancreatic β-cells. Inhibition of RAGE can effectively protect β-cells against AGE-induced apoptosis, but cannot reverse islet dysfunction in GSIS.     

Chronic palmitic acid-induced lipotoxicity correlates with defective trafficking of ATP sensitive potassium channels in pancreatic β cells

Lipotoxicity is associated with a high level of fatty acid accumulation in pancreatic β-cells. An overload of free fatty acids contributes to pancreatic β-cell apoptosis and dysfunction. Insulin secretion involves sequential ionic events upon glucose stimulation. ATP sensitive potassium (K_ATP) channels serve as glucose sensors and effectively initiate glucose-stimulated insulin secretion. This study investigated the effects of lipotoxicity on the trafficking of K_ATP channels in pancreatic β cells using chronic palmitic acid –injected mice and treated insulinoma cells. The chronic palmitic acid -injected mice displayed type II diabetic characteristics. The pancreatic sections of these mice exhibited a decrease in the expression of K_ATP channels. We then tested the time and dose effects of palmitic acid on the cell viability of INS-1 cells. We observed a significant decrease in the surface expression of K_ATP channels after 72 h of treatment with 0.4 mM palmitic acid. In addition, this treatment induced pancreatic β-cell apoptosis by increasing cleaved caspase 3 protein level. Our results demonstrated cotreatment with glibenclamide, the sulfonylurea compounds for type II diabetes mellitus, in palmitic acid -treated cells reduces cell death and recovers the glucose stimulated insulin secretion through increasing the surface expression of K_ATP channels. Importantly, glibenclamide also improved glucose tolerance, triglyceride concentration, and insulin sensitivity in the palmitic acid-injected mice. In conclusion, an increase in the surface expression of K_ATP channels restores insulin secretion, reduces pancreatic β-cell’s apoptosis, highlighting correct trafficking of K_ATP channels is important in survival of β-cells during lipotoxicity.     

Beta-arrestin-1 protein represses adipogenesis and inflammatory responses through its interaction with peroxisome proliferator-activated receptor-gamma (PPARgamma)

One of the master regulators of adipogenesis and macrophage function is peroxisome proliferator-activated receptor-γ (PPARγ). Here, we report that a deficiency of β-arrestin-1 expression affects PPARγ-mediated expression of lipid metabolic genes and inflammatory genes. Further mechanistic studies revealed that β-arrestin-1 interacts with PPARγ. β-Arrestin-1 suppressed the formation of a complex between PPARγ and 9-cis-retinoic acid receptor-α through its direct interaction with PPARγ. The interaction of β-arrestin-1 with PPARγ repressed PPARγ/9-cis-retinoic acid receptor-α function but promoted PPARγ/nuclear receptor corepressor function in PPARγ-mediated adipogenesis and inflammatory gene expression. Consistent with these results, a deficiency of β-arrestin-1 binding to PPARγ abolished its suppression of PPARγ-dependent adipogenesis and inflammatory responses. These results indicate that the regulation of PPARγ by β-arrestin-1 is critical. Furthermore, in vivo expression of β-arrestin-1 (but not the binding-deficient mutant) significantly repressed adipogenesis, macrophage infiltration, and diet-induced obesity and improved glucose tolerance and systemic insulin sensitivity. Therefore, our findings not only reveal a molecular mechanism for the modulation of obesity by β-arrestin-1 but also suggest a potential tactical approach against obesity and its associated metabolic disorders.     

Dissociation of diabetes and obesity in mice lacking orphan nuclear receptor small heterodimer partner

Mixed background SHP(-/-) mice are resistant to diet-induced obesity due to increased energy expenditure caused by enhanced PGC-1α expression in brown adipocytes. However, congenic SHP(-/-) mice on the C57BL/6 background showed normal expression of PGC-1α and other genes involved in brown adipose tissue thermogenesis. Thus, we reinvestigated the impact of small heterodimer partner (SHP) deletion on diet-induced obesity and insulin resistance using congenic SHP(-/-) mice. Compared with their C57BL/6 wild-type counterparts, SHP(-/-) mice subjected to a 6 month challenge with a Western diet (WestD) were leaner but more glucose intolerant, showed hepatic insulin resistance despite decreased triglyceride accumulation and increased β-oxidation, exhibited alterations in peripheral tissue uptake of dietary lipids, maintained a higher respiratory quotient, which did not decrease even after WestD feeding, and displayed islet dysfunction. Hepatic mRNA expression analysis revealed that many genes expressed higher in SHP(-/-) mice fed WestD were direct peroxisome proliferator-activated receptor alpha (PPARα) targets. Indeed, transient transfection and chromatin immunoprecipitation verified that SHP strongly repressed PPARα-mediated transactivation. SHP is a pivotal metabolic sensor controlling lipid homeostasis in response to an energy-laden diet through regulating PPARα-mediated transactivation. The resultant hepatic fatty acid oxidation enhancement and dietary fat redistribution protect the mice from diet-induced obesity and hepatic steatosis but accelerate development of type 2 diabetes.