Transforming growth factor β induction of insulin gene expression is mediated by pancreatic and duodenal homeobox gene-1 in rat insulinoma cells

Abstract

Although transforming growth factor-beta (TGF-β) stimulates pancreatic islet cells to synthesize and secret insulin, the mechanism underlying this effect is not known. To investigate this question, we examined the insulin promoter activity focusing on a transcription factor, pancreatic and duodenal homeobox gene-1 (PDX-1) that binds to the A3 element of the rat insulin promoter. Studies performed using the rat insulinoma cell line, INS-1 showed that TGF-β stimulation of endogenous insulin mRNA expression correlated with increased activity of a reporter construct containing the insulin promoter. A potential mechanism for this increase arose from, electrophoretic mobility shift assay showing that the nuclear extract from TGF-β treated cells contained higher levels of A3 binding activity. Western blot analysis confirmed that PDX-1 was increased in the nuclear extract from INS-1 cells treated with TGF-β. As expected, a mutant insulin promoter that lacked the PDX-1 binding site was not stimulated by TGF-β. In summary, the results of these studies show that TGF-β stimulates the transcription of insulin gene and this action is mediated by the transcription factor, PDX-1.