Glucose regulation of insulin gene expression

The function of the pancreatic beta cells is absolutely essential to the maintenance of glucose homeostasis in mammals. In order for these cells to function, they must be able to sense and response to blood glucose levels. This is imperative for the cells to produce and secrete mature insulin in response to high concentrations of glucose and to halt insulin secretion when glucose concentrations are low. Defects in this regulatory pathway can lead to a variety of metabolic diseases such as diabetes mellitus. Our studies have focused on the mechanisms by which glucose regulates gene expression in the pancreatic beta cell. We provide substantial evidence that the beta cell specific transcription factor Pdx-1 is vital to this process.; We first studied the effects of point mutations in Pdx-1 on its ability to function as a transcriptional activator in yeast, and found that these mutations severely disrupted the ability of Pdx-1 to activate transcription. Furthermore, we analyzed the regulation of Pdx-1 function by glucose in the pancreatic beta cell line MIN-6. We found that the recruitment of Pdx-1 to the insulin promoter was augmented by high concentrations of glucose. In addition, we provide evidence that Pdx-1 recruitment to the insulin promoter is regulated by insulin itself.; Since Pdx-1 has been previously shown to interact with the histone acetyltrasferase p300, we determined the role of Pdx-1 and p300 in histone acetylation at the insulin promoter. We demonstrated that the levels of histone H4 acetylation at the insulin promoter increase in response to high concentrations of glucose whereas the levels of histone H3 acetylation remain constant. In order to characterize the mechanism by which glucose regulate this hyper-acetylation event, we analyzed the interaction of Pdx-1 with the histone acetyltransferase p300 and found that the interaction in increased in response to high glucose concentrations. Using a dominant negative form of Pdx-1, we found that functional Pdx-1 is required for the recruitment of p300 to the insulin promoter, and that a disruption of p300 recruitment is associated with low levels of histone H4 acetylation in response to glucose. At low glucose concentrations histone H4 acetylation levels at the insulin promoter are decreased due to the action of histone deacetylases. Consistent with this data, the histone deacetylases Hdac-1 and Hdac-2 are recruited to the insulin promoter in response to low concentrations of glucose, and interact with Pdx-1 only under low glucose conditions. In summary, Pdx-1 controls the levels of histone H4 acetylation at the insulin promoter in response to changes in glucose concentration by recruiting p300 or Hdac-1 and Hdac-2.; To confirm the requirement of Pdx-1 for the observed histone H4 acetylation at the insulin promoter in response to high glucose, we have expressed Pdx-1 in the liver cell line Hepa1-6. Analysis of histone acetylation at the promoters of a number of beta cell specific genes revealed that Pdx-1 expression is sufficient to direct histone H4 acetylation to the insulin 1, insulin 11, GLUT-2, glucokinase, Pdx-1, Pax-4, and PC 1/3 promoter. These data show that Pdx-1 is a bona fide regulator of beta cell specific gene expression and that it functions to direct histone acetylation to a variety of promoters in response to high concentrations of glucose.