p38 MAPK is a major regulator of MafA protein stability under oxidative stress.
ABSTRACT Mammalian MafA/RIPE3b1 is an important glucose-responsive transcription factor that regulates function, maturation, and survival of beta-cells. Increased expression of MafA results in improved glucose-stimulated insulin secretion and beta-cell function. Because MafA is a highly phosphorylated protein, we examined whether regulating activity of protein kinases can increase MafA expression by enhancing its stability. We demonstrate that MafA protein stability in MIN6 cells and isolated mouse islets is regulated by both p38 MAPK and glycogen synthase kinase 3. Inhibiting p38 MAPK enhanced MafA stability in cells grown under both low and high concentrations of glucose. We also show that the N-terminal domain of MafA plays a major role in p38 MAPK-mediated degradation; simultaneous mutation of both threonines 57 and 134 into alanines in MafA was sufficient to prevent this degradation. Under oxidative stress, a condition detrimental to beta-cell function, a decrease in MafA stability was associated with a concomitant increase in active p38 MAPK. Interestingly, inhibiting p38 MAPK but not glycogen synthase kinase 3 prevented oxidative stress-dependent degradation of MafA. These results suggest that the p38 MAPK pathway may represent a common mechanism for regulating MafA levels under oxidative stress and basal and stimulatory glucose concentrations. Therefore, preventing p38 MAPK-mediated degradation of MafA represents a novel approach to improve beta-cell function.
- [Show abstract] [Hide abstract]
ABSTRACT: Type 2 diabetes (T2DM) commonly arises from islet β cell failure and insulin resistance. Here, we examined the sensitivity of key islet-enriched transcription factors to oxidative stress, a condition associated with β cell dysfunction in both type 1 diabetes (T1DM) and T2DM. Hydrogen peroxide treatment of β cell lines induced cytoplasmic translocation of MAFA and NKX6.1. In parallel, the ability of nuclear PDX1 to bind endogenous target gene promoters was also dramatically reduced, whereas the activity of other key β cell transcriptional regulators was unaffected. MAFA levels were reduced, followed by a reduction in NKX6.1 upon development of hyperglycemia in db/db mice, a T2DM model. Transgenic expression of the glutathione peroxidase-1 antioxidant enzyme (GPX1) in db/db islet β cells restored nuclear MAFA, nuclear NKX6.1, and β cell function in vivo. Notably, the selective decrease in MAFA, NKX6.1, and PDX1 expression was found in human T2DM islets. MAFB, a MAFA-related transcription factor expressed in human β cells, was also severely compromised. We propose that MAFA, MAFB, NKX6.1, and PDX1 activity provides a gauge of islet β cell function, with loss of MAFA (and/or MAFB) representing an early indicator of β cell inactivity and the subsequent deficit of more impactful NKX6.1 (and/or PDX1) resulting in overt dysfunction associated with T2DM.The Journal of clinical investigation 07/2013; · 15.39 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Dipeptidyl peptidase-4 (DPP-4) inhibitor and exercise have proven to be effective treatments for diabetes. However, the effects of these interventions in compensatory hyperinsulinemia prediabetic period is unknown. The purpose of this study was to determine if these interventions have protective effects on β-cell function and preventive effects on the onset of diabetes in prediabetic kkay mice. After 2 weeks of high-fat diet feeding, we treated 7-week-old mice with a normal diet, high-fat diet, exercise training, or the DPP-4 inhibitor for 8 weeks. C57BL/6J mice served as a normal control. Kkay mice without intervention developed diabetes at week 15, but no diabetic mice were observed in the DPP-4I or exercise groups as well as the normal control group. The DPP-4I and exercise groups showed improved body weight, blood glucose level, glucose tolerance, insulin sensitivity, islet area, and islet morphology. In addition, the proportion of Ki67-positive β-cells in the treatment groups was obviously higher than that in the untreated groups. MafA(V-maf musculoaponeurotic fibrosarcoma oncogene homolog A) expression in the treated groups increased markedly. However PDX-1 (pancreatic and duodenal homeobox-1) expression did not differ significantly among the groups. The results show that exercise and DPP-4I treatment conducted during the hyperinsulinemic prediabetic stage contribute to the maintenance of β-cell function and morphology, enhance β-cell proliferation, extend the compensatory insulin hypersecretion period, and delay disease onset. The expression of PDX-1 was not altered significantly during the early stages of diabetes. However, the reduced expression of the insulin transcription factor MafA may play an important role in the development of prediabetes.Peptides 09/2013; · 2.52 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Specification and maturation of insulin(+) cells accompanies a transition in expression of Maf family of transcription factors. In development, MafA is expressed after specification of insulin(+) cells that are expressing another Maf factor, MafB; after birth, these insulin(+) MafA(+) cells stop MafB expression and gain glucose responsiveness. Current differentiation protocols for deriving insulin-producing β-cells from stem cells result in β-cells lacking both MafA expression and glucose-stimulated insulin secretion. So driving expression of MafA, a β-cell maturation factor in endocrine precursors could potentially generate glucose-responsive MafA(+)β cells. Using inducible transgenic mice, we characterized the final stages of β-cell differentiation and maturation with MafA pause/release experiments. We found that forcing MafA transgene expression, out of its normal developmental context, in Ngn3(+) endocrine progenitors blocked endocrine differentiation and prevented the formation of hormone(+) cells. However, this arrest was reversible such that with stopping the transgene expression, the cells resumed their differentiation to hormone(+) cells, including α-cells, indicating that the block likely occurred after progenitors had committed to a specific hormonal fate. Interestingly, this delayed resumption of endocrine differentiation resulted in a greater proportion of immature insulin(+)MafB(+) cells at P5, demonstrating that during maturation the inhibition of MafB in β-cell transitioning from insulin(+)MafB(+) to insulin(+)MafB(-) stage is regulated by cell-autonomous mechanisms. These results demonstrate the importance of proper context of initiating MafA expression on the endocrine differentiation and suggest that generating mature Insulin(+)MafA(+)β-cells will require the induction of MafA in a narrow temporal window to achieve normal endocrine differentiation.Developmental Biology 10/2013; · 3.87 Impact Factor