Glucose stimulates translocation of the homeodomain transcription factor PDX1 from the cytoplasm to the nucleus in pancreatic beta-cells

One of the mechanisms whereby glucose stimulates insulin gene transcription in pancreatic beta-cells involves activation of the homeodomain transcript ion factor PDX1 (pancreatic/duodenal homeobox-1) via a stress-activated pat hway involving stress-activated protein kinase 2 (SAPK2, also termed RK/p38 , CSBP, and Mxi2). In the present study we show, by Western blotting and el ectrophoretic mobility shift assay, that in human islets of Langerhans incu bated in low glucose (3 mM) PDX1 exists as an inactive 31-kDa protein local ized exclusively in the cytoplasm. Transfer of the islets to high (16 mM) g lucose results in rapid (within 10 min) conversion of PDX1 to an active 46- kDa form that was present predominantly in the nucleus. Activation of PDX1 appears to involve phosphorylation, as shown by incorporation of P-32(i) in to the 46-kDa form of the protein. These effects of glucose could be mimick ed by chemical stress (sodium arsenite), or by overexpression of SAPK2 in t he beta-cell line MIN6. Overexpression of SAPK2 also stimulated PDX1-depend ent transcription of a -50 to -250 region of the human insulin gene promote r linked to a firefly luciferase reporter gene. The effects of glucose were inhibited by the SAPK2 inhibitor SB 203580, and by wortmannin and LY 29400 2, which inhibit phosphatidylinositol-3-kinase, although the effects of str ess (arsenite) were inhibited only by SE 203580, These results demonstrate that glucose regulates the insulin gene promoter through activation and nuc lear translocation of PDX1 via the SAPK2 pathway.