A SPECIFIC INCREASED EXPRESSION OF INSULIN-RECEPTOR SUBSTRATE-2 IN PANCREATIC BETA-CELL LINES IS INVOLVED IN MEDIATING SERUM-STIMULATED BETA-CELL GROWTH

Certain nutrients and growth factors can stimulate pancreatic beta-cel l growth. However, the appropriate mitogenic signaling pathways in bet a-cells have been relatively undefined. In this study, differential ge ne expression in NEDH rat insulinoma was compared with NEDH rat primar y islet beta-cells. Differential mRNA display analysis revealed an ele vated expression in insulinoma of VL30 transposons, S24 ribosomal prot ein, and cytochrome-C oxidase VIIc that is typical for cells undergoin g mitosis. A gene candidate approach revealed that mRNA levels of the oncogenes c-fos and c-jun were equivalently expressed in insulinoma an d islet cells, as was the mRNA for the mitogenic signal transduction m olecule insulin receptor substrate (IRS)-1. However, in contrast to th at of IRS-1, IRS-2 gene expression was 60- to 70-fold higher in the in sulinoma tissue compared with islets, which was reflected at the prote in as well as the mRNA level. The specific elevated IRS-2 expression w as a consistent observation across all rodent pancreatic beta-cell lin es. To investigate whether IRS-2 was functional, serum-stimulated beta -cell proliferation was examined in isolated insulinoma. cells. After a 48-h period of serum withdrawal, 24 h of serum refeeding rendered an 8- to 10-fold increase in [H-3]thymidine incorporation into insulinom a cells. This serum-stimulated DNA synthesis was prevented by inhibito rs of tyrosine protein kinase and phosphatidylinositol (PI) 3-kinase a ctivities, as well as the activation of mitogen-activated protein (MAP ) kinase and p70(S6K). Examination of IRS-mediated signal transduction pathways indicated that after 10-15 min of serum refeeding, there was increased tyrosine phosphorylation of IRS-2 and pp60, and PI 3-kinase recruitment to IRS-2. Serum also increased the association of growth factor-bound protein 2/ murine sons of sevenless 1 protein to a PI 3-k inase/IRS-2 protein complex. Moreover, serum also activated MAP-kinase (erk-1 and erk-2 isoforms) and 70 kD S6 kinase. Thus IRS-mediated sig nal transduction pathways are functional in pancreatic beta-cells. It is conceivable that IRS-2 expression in beta-cells contributes to main taining the islet beta-cell population, complementary to observations in the IRS-2 knockout mouse in which beta-cell mass is markedly reduce d.