Phosphorylation-dependent nucleocytoplasmic shuttling of pancreatic duodenal homeobox-1

Pancreatic duodenal homeobox-1 (PDX-1) is a homeodomain protein that plays an important role in the development of the pancreas and in maintaining the identity and function of the islets of Langerhans. It also regulates the e xpression of the insulin gene in response to changes in glucose and insulin concentrations. Glucose and insulin regulate PDX-1 by way of a signaling p athway involving phosphatidylinositol 3-kinase (PI 3-kinase) and SAPK2/p38. Activation of this pathway leads to phosphorylation of PDX-1 and its movem ent into the nucleus. To investigate the intracellular trafficking of PDX-1 , immunocytochemistry was used to localize PDX-1 in the human beta -cell li ne NesPDX-1, in which PDX-1 is overexpressed, and in MIN6 beta -cells. In l ow-glucose conditions, PDX-1 localized predominantly to the nuclear periphe ry, with some staining in the cytoplasm. After stimulation with glucose, PD X-1 was present in the nucleoplasm. The translocation of PDX-1 to the nucle oplasm was complete within 15 min and occurred in 5-10 mmol/l glucose. Insu lin and sodium arsenite, an activator of the stress-activated pathway, also stimulated PDX-1 movement from the nuclear periphery to the nucleoplasm. W hen cells were transferred between high glucose - and low glucose-containin g medium, PDX-1 rapidly shuttled between the nuclear periphery and the nucl eoplasm. Glucose- and insulin-stimulated translocation of PDX-1 to the nucl eoplasm was inhibited by wortmannin and SB 203580, indicating that a pathwa y involving PI 3-kinase and SAPK2/p38 was involved; translocation was unaff ected by PD 098959 and rapamycin, suggesting that neither mitogen-activated protein kinase nor p70(s6k) were involved. Arsenite-stimulated import of P DX-1 into the nucleus was inhibited by SB 203580 but not by wortmannin. Exp ort from the nucleoplasm to the nuclear periphery was inhibited by calyculi n A and okadaic acid, suggesting that dephosphorylation of PDX-1 was involv ed. These results demonstrated that PDX-1 shuttles between the nuclear peri phery and nucleoplasm in response to changes in glucose and insulin concent rations and that these events are dependent on PI 3-kinase, SAPK2/p38, and a nuclear phosphatase(s).