Insulin stimulates pancreatic-duodenal homoeobox factor-1 (PDX1) DNA-binding activity and insulin promoter activity in pancreatic beta cells

Pancreatic-duodenal homoeobox factor-1 (PDX1) is a homoeo-domain transcript ion factor that plays an important role in linking glucose metabolism in pa ncreatic beta cells to the regulation of insulin gene transcription. Our pr evious results indicated that glucose activates PDX1 DNA-binding activity a nd insulin promoter activity via a stress-activated signalling pathway invo lving phosphatidylinositol 3-kinase (PtdIns 3-kinase) and stress-activated protein kinase 2 (SAPK2/p38). The present study was undertaken to determine the effects of other metabolizable and non-metabolizable nutrients. The re sults indicate that nonmetabolizable nutrients,with the exception of 2-deox yglucose, had no effect. Metabolizable nutrients that could stimulate calci um uptake and insulin release were shown to activate both PDX1 and the insu lin promoter. The possible role of insulin acting via an autoregulatory loo p was therefore examined. Insulin was shown to potently activate PDX1 DNA-b inding activity and insulin promoter activity. The effects of insulin were inhibited by the PtdIns 3-kinase inhibitors wortmannin and LY294002 and by the SAPK2 inhibitor SB203580, suggesting that its effects were mediated via activation of PtdIns 3-kinase and SAPK2. Further support for the insulin-m ediated activation of SAPK2 came from the observation that both glucose and insulin stimulated the phosphorylation of SAPK2. These results suggest tha t both glucose and insulin stimulate PDX1 DNA-binding activity and insulin promoter activity via a pathway involving PtdIns 3-kinase and SAPK2.