Glucose-stimulated preproinsulin gene expression and nuclear trans-location of pancreatic duodenum homeobox-1 require activation of phosphatidylinositol 3-kinase but not p38 MAP/SAPK2

Exposure of islet beta-cells to elevated glucose concentrations (30 versus 3 mM) prompts enhanced preproinsulin (PPI) gene transcription and the trans-location to the nucleoplasm of pancreatic duodenum homeobox-1 (PDX-1; Rafiq, I., Kennedy, H., and Rutter, G. A. (1998) J. Biol. Chem 273, 23241-23247). Here, we show that in MIN6 beta-cells, over-expression of p110.CAAX, a constitutively active form of phosphatidylinositol 3-kinase (PI3K) mimicked the activatory effects of glucose on PPI promoter activity, whereas Delta p85, a dominant negative form of the p85 subunit lacking the p110-binding domain, and the P13K inhibitor LY 294002, blocked these effects. Similarly, glucose-stimulated nuclear trans-location of endogenous PDX-1 was blocked by Delta p85 expression, and wortmannin or LY 294002 blocked the trans-location from the nuclear membrane to the nucleoplasm of epitope-tagged PDX-1.c-myc. By contrast, SE 203580, an inhibitor of stress-activated protein kinase-a (SAPK2)/p38 MAP kinase, had no effect on any of the above parameters, and PPI promoter activity and PDX-1,c-myc localization were unaffected by over-expression of the upstream kinase MKK6 (MAP kinase kinase-6) or wild-type p38/SAPK2, respectively. Furthermore, no change in the activity of extracted pS8/SAPK2 could be detected after incubation of cells at either 3 or 30 mM glucose. These data suggest that stimulation of PI3K is necessary and sufficient for the effects of glucose on PPI gene transcription, acting via a downstream signaling pathway that does not involve p58/SAPK2.