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p38 MAPK Is a Major Regulator of MafA Protein Stability under Oxidative Stress

Section of Islet Transplantation and Cell Biology (T.K., I.E.K., W.N., S.S., S.B.-W., G.W., A.S.), Section on Vascular Cell Biology (P.G., G.K.), Joslin Diabetes Center, and Department of Medicine (T.K., I.E.K., W.N., P.G., G.K., S.B.-W., G.W., A.S.), Harvard Medical School, Boston, Massachusetts 02215; and Diabetes and Obesity Research Program (D.R.L.), Garvan Institute of Medical Research, St. Vincent’s Hospital, Sydney, New South Wales 2010, Australia

Address all correspondence and requests for reprints to: Dr. Arun Sharma, Joslin Diabetes Center, One Joslin Place, Boston, Massachusetts 02215. E-mail: arun.sharma{at}joslin.harvard.edu.

Mammalian MafA/RIPE3b1 is an important glucose-responsive transcription factor that regulates function, maturation, and survival of β-cells. Increased expression of MafA results in improved glucose-stimulated insulin secretion and β-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 β-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 β-cell function.