Insulin stimulates primary beta-cell proliferation via Raf-1 kinase.
A relative decrease in beta-cell mass is key in the pathogenesis of type 1 diabetes, type 2 diabetes, and in the failure of transplanted islet grafts. It is now clear that beta-cell duplication plays a dominant role in the regulation of adult beta-cell mass. Therefore, knowledge of the endogenous regulators of beta-cell replication is critical for understanding the physiological control of beta-cell mass and for harnessing this process therapeutically. We have shown that concentrations of insulin known to exist in vivo act directly on beta-cells to promote survival. Whether insulin stimulates adult beta-cell proliferation remains unclear. We tested this hypothesis using dispersed primary mouse islet cells double labeled with 5-bromo-2-deoxyuridine and insulin antisera. Treating cells with 200-pm insulin significantly increased proliferation from a baseline rate of 0.15% per day. Elevating glucose from 5-15 mm did not significantly increase beta-cell replication. beta-Cell proliferation was inhibited by somatostatin as well as inhibitors of insulin signaling. Interestingly, inhibiting Raf-1 kinase blocked proliferation stimulated by low, but not high (superphysiological), insulin doses. Insulin-stimulated mouse insulinoma cell proliferation was dependent on both phosphatidylinositol 3-kinase/Akt and Raf-1/MAPK kinase pathways. Overexpression of Raf-1 was sufficient to increase proliferation in the absence of insulin, whereas a dominant-negative Raf-1 reduced proliferation in the presence of 200-pm insulin. Together, these results demonstrate for the first time that insulin, at levels that have been measured in vivo, can directly stimulate beta-cell proliferation and that Raf-1 kinase is involved in this process. These findings have significant implications for the understanding of the regulation of beta-cell mass in both the hyperinsulinemic and insulin-deficient states that occur in the various forms of diabetes.
Beith
Jennifer L
JL
Laboratory of Molecular Signalling in Diabetes, Diabetes Research Group, Department of Cellular and Physiological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3.
Alejandro
Emilyn U
EU
Johnson
James D
JD
D000818:Animals; D019898:Autocrine Communication / Q000187:drug effects / Q000502:physiology; D018929:Cell Culture Techniques; D002461:Cell Line, Transformed; D049109:Cell Proliferation / Q000187:drug effects*; D002478:Cells, Cultured; D002867:Chromones / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology*; D050417:Insulin-Secreting Cells / Q000187:drug effects* / Q000378:metabolism / Q000502:physiology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009025:Morpholines / Q000494:pharmacology; D051058:Oncogene Protein v-akt / Q000037:antagonists & inhibitors; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism / Q000502:physiology*
D002867:Chromones; D004791:Enzyme Inhibitors; D007328:Insulin; D009025:Morpholines; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; D051058:Oncogene Protein v-akt; D019908:Proto-Oncogene Proteins c-raf