Regulation of insulin gene expression by glucose in pancreatic  cells is largely dependent on a cis-regulatory element, termed RIPE3b/C1, in the insulin gene promoter. MafA, a member of the Maf family of basic leucine zipper (bZip) proteins, is a -cell-specific transcriptional activator that binds to the C1 element. Based on increased C1-binding activity, MafA protein levels appear to be up-regulated in response to glucose, but the underlying molecular mechanism for this is not well understood. In this study, we show evidence supporting that the amino-terminal region of MafA is phosphorylated at multiple sites by glycogen synthase kinase 3 (GSK3) in  cells. Mutational analysis of MafA and pharmacological inhibition of GSK3 in MIN6  cells strongly suggest that the rate of MafA protein degradation is regulated by glucose, that MafA is constitutively phosphorylated by GSK3, and that phosphorylation is a prerequisite for rapid degradation of MafA under low-glucose conditions. Our data suggest a new glucose-sensing signaling pathway in islet  cells that regulates insulin gene expression through the regulation of MafA protein stability.
Abstract. Insulin is a critical hormone in the regulation of blood glucose levels. It is produced exclusively by pancreatic islet β-cells. β-cell-enriched transcription factors, such as Pdx1 and Beta2, have dual roles in the activation of the insulin gene promoter establishing β-cell-specific insulin expression, and in the regulation of β-cell differentiation. It was shown that MafA, a β-cell-specific member of the Maf family of transcription factors, binds to the conserved C1/RIPE3b element of the insulin promoter. The Maf family proteins regulate tissue-specific gene expression and cell differentiation in a wide variety of tissues. MafA acts synergistically with Pdx1 and Beta2 to activate the insulin gene promoter, and mice with a targeted deletion of mafA develop age-dependent diabetes. MafA also regulates genes involved in β-cell function such as Glucose transporter 2, Glucagons-like peptide 1 receptor, and Prohormone convertase 1/3. The abundance of MafA in β-cells is regulated at both the transcriptional and post-translational levels by glucose and oxidative stress. This review summarizes recent progress in determining the functions and roles of MafA in the regulation of insulin gene transcription in β-cells.
OBJECTIVETissue-specific self-antigens are ectopically expressed within the thymus and play an important role in the induction of central tolerance. Insulin is expressed in both pancreatic islets and the thymus and is considered to be the primary antigen for type 1 diabetes. Here, we report the role of the insulin transactivator MafA in the expression of insulin in the thymus and susceptibility to type 1 diabetes.RESEARCH DESIGN AND METHODSThe expression profiles of transcriptional factors (Pdx1, NeuroD, Mafa, and Aire) in pancreatic islets and the thymus were examined in nonobese diabetic (NOD) and control mice. Thymic Ins2 expression and serum autoantibodies were examined in Mafa knockout mice. Luciferase reporter assay was performed for newly identified polymorphisms of mouse Mafa and human MAFA. A case-control study was applied for human MAFA polymorphisms.RESULTSMafa, Ins2, and Aire expression was detected in the thymus. Mafa expression was lower in NOD thymus than in the control and was correlated with Ins2 expression. Targeted disruption of MafA reduced thymic Ins2 expression and induced autoantibodies against pancreatic islets. Functional polymorphisms of MafA were newly identified in NOD mice and humans, and polymorphisms of human MAFA were associated with susceptibility to type 1 diabetes but not to autoimmune thyroid disease.CONCLUSIONSThese data indicate that functional polymorphisms of MafA are associated with reduced expression of insulin in the thymus and susceptibility to type 1 diabetes in the NOD mouse as well as human type 1 diabetes.
MAFA is a member of the MAF family of basic leucine zipper transcription factors and is a critical regulator of insulin gene expression and islet b-cell function. To be degraded by the proteasome, MAFA must be phosphorylated by GSK3 and MAP kinases at multiple serine and threonine residues (Ser49, Thr53, Thr57, Ser61, and Ser65) within its amino-terminal domain. In this study, we report that MAFA degradation is stimulated by PA28g (REGg and PSME3), a member of a family of proteasome activators that bind and activate the 20S proteasome. To date, only a few PA28g-proteasome pathway substrates have been identified, including steroid receptor coactivator 3 (SRC3) and the cell cycle inhibitor p21 (CIP1). PA28g binds to MAFA, induces its proteasomal degradation, and thereby attenuates MAFA-driven transcriptional activation of the insulin promoter. Co-expression of GSK3 enhanced the PA28g-mediated degradation of MAFA, but mutants that contained alanine substitutions at the MAFA phosphorylation sites did not bind PA28g and were resistant to degradation. We also found that a PA28g mutant (N151Y) that did not stimulate p21 degradation enhanced MAFA degradation, and another mutant (K188D) that promoted greater p21 degradation did not enhance MAFA degradation. These results suggest that PA28g stimulates MAFA degradation through a novel molecular mechanism that is distinct from that for the degradation of p21.
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