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-cell growth. However, the appropriate mitogenic signaling pathways in beta-cells have been relatively undefined. In this study, differential gene expression in NEDH rat insulinoma was compared with NEDH rat primary islet beta-cells. Differential mRNA display analysis revealed an elevated expression in insulinoma of VL30 transposons, S24 ribosomal protein, and cytochrome-C oxidaseVIIc that is typical for cells undergoing mitosis. A gene candidate approach revealed that mRNA levels of the oncogenes c-fos and c-jun were equivalently expressed in insulinoma and islet cells, as was the mRNA for the mitogenic signal transduction molecule insulin receptor substrate (IRS)-1. However, in contrast to that of IRS-1, IRS-2 gene expression was 60- to 70-fold higher in the insulinoma tissue compared with islets, which was reflected at the protein as well as the mRNA level. The specific elevated IRS-2 expression was a consistent observation across all rodent pancreatic beta-cell lines. 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 [3H]thymidine incorporation into insulinoma cells. This serum-stimulated DNA synthesis was prevented by inhibitors of tyrosine protein kinase and phosphatidylinositol (PI) 3-kinase activities, as well as the activation of mitogen-activated protein (MAP) kinase and p70S6K. 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-kinase/IRS-2 protein complex. Moreover, serum also activated MAP-kinase (erk-1 and erk-2 isoforms) and 70 kD S6 kinase. Thus IRS-mediated signal transduction pathways are functional in pancreatic beta-cells. It is conceivable that IRS-2 expression in beta-cells contributes to maintaining the islet beta-cell population, complementary to observations in the IRS-2 knockout mouse in which beta-cell mass is markedly reduced.
Schuppin
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GT
Department of Internal Medicine, Gifford Laboratories for Diabetes Research, University of Texas Southwestern Medical Center, Dallas 75235-8854, USA.
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Hügl
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Aiello
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White
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D000818:Animals; D001483:Base Sequence; D001769:Blood; D002454:Cell Differentiation; D002455:Cell Division / Q000502:physiology*; D004247:DNA / Q000096:biosynthesis; D015870:Gene Expression; D016762:Genes, fos / Q000235:genetics; D016758:Genes, jun / Q000235:genetics; D055504:Insulin Receptor Substrate Proteins; D007340:Insulinoma; D047908:Intracellular Signaling Peptides and Proteins; D007515:Islets of Langerhans / Q000378:metabolism* / Q000473:pathology*; D008934:Mitogens; D008969:Molecular Sequence Data; D010190:Pancreatic Neoplasms; D010750:Phosphoproteins / Q000235:genetics*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D018626:Retroelements; D015398:Signal Transduction; D014407:Tumor Cells, Cultured
D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; C526298:Irs2 protein, mouse; C526299:Irs2 protein, rat; D008934:Mitogens; D010750:Phosphoproteins; D012333:RNA, Messenger; D018626:Retroelements; D004247:DNA