Regulation of Insulin and CHOP Gene Expression in Pancreatic Beta Cells

Insulin is a major hormone in maintaining glucose homeostasis. It is essential to understand the mechanisms by which insulin gene expression is regulated in pancreatic beta cells. In addition to examining histone modifications on the insulin gene promoter, I focused on the effect of MafA modification on insulin expression. MafA is a transcriptional activator of the insulin gene via binding to the RIPE3b/C1 (rat insulin promoter element 3b) element. Mutagenesis showed that MafA was post-translationally modified by SUMO-1/2 (small ubiquitin-like modifier) mainly at lysine 32. Low glucose starvation or hydrogen peroxide stimulation increased sumoylation of MafA. Forced sumoylation of MafA reduced its transcriptional activity towards the insulin gene promoter and increased its suppression of the CHOP (C/EBP homologous protein) gene promoter. However, sumoylation of MafA did not alter its nuclear localization, protein stability, or apparently its DNA binding to the insulin promoter in beta cells. These studies suggest that MafA sumoylation modulates gene transcription in beta cells. In type I diabetes, beta-cell apoptosis is the major reason for immune-mediated pancreatic beta-cell death. IL-1beta (interleukin 1beta), a proinflammatory cytokine, induces ER (endoplasmic reticulum) stress and activates proapoptotic networks in beta cells, such as NF-kappaB (nuclear factor-kappaB) and JNK (c-Jun N-terminal kinase) signaling pathways. The second project focused on the mechanisms by which JNK and NF-kappaB regulate the expression of CHOP, a mediator of ER stress-induced apoptosis, upon IL-1beta stimulation. Exposure of beta cells to IL-1beta markedly increased CHOP messenger RNA and protein. Electrophoretic mobility shift assays showed that IL-1beta-activated NF-kappaB bound to the CHOP promoter. Furthermore, immunoblot data indicated that expression of c-Jun was strongly increased, and that multiple residues on c-Jun were phosphorylated after IL-1beta treatment. IL-1beta also increased c-Fos expression in beta cells. These data suggest that IL-1beta-induced activation of NF-kappaB and JNK controls CHOP gene expression in pancreatic beta cells, and that IL-1beta influences beta-cell function through a variety of signaling pathways.