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Glucagon-like peptide-1 (GLP-1), an insulinotropic and glucoincretin hormone, is a potentially important therapeutic agent in the treatment of diabetes. We previously provided evidence that GLP-1 induces pancreatic beta-cell growth nonadditively with glucose in a phosphatidylinositol-3 kinase (PI-3K)-dependent manner. In the present study, we investigated the downstream effectors of PI-3K to determine the precise signal transduction pathways that mediate the action of GLP-1 on beta-cell proliferation. GLP-1 increased extracellular signal-related kinase 1/2, p38 mitogen-activated protein kinase (MAPK), and protein kinase B activities nonadditively with glucose in pancreatic beta(INS 832/13) cells. GLP-1 also caused nuclear translocation of the atypical protein kinase C (aPKC) zeta isoform in INS as well as in dissociated normal rat beta-cells as shown by immunolocalization and Western immunoblotting analysis. Tritiated thymidine incorporation measurements showed that the p38 MAPK inhibitor SB203580 suppressed GLP-1induced beta-cell proliferation. Further investigation was performed using isoform-specific pseudosubstrates of classical (alpha, beta, and gamma) or zeta aPKC isoforms. The PKC(zeta) pseudosubstrate suppressed the proliferative action of GLP-1, whereas the inhibitor of classical PKC isoforms had no effect. Overexpression of a kinase-dead PKC(zeta) acting as a dominant negative protein suppressed GLP-- 1-induced proliferation. In addition, ectopic expression of a constitutively active PKC(zeta) mutant stimulated tritiated thymidine incorporation to the same extent as GLP-1, and the glucoincretin had no growth-promoting action under this condition. The data indicate that GLP1-induced activation of PKC(zetz) is implicated in the beta-cell proliferative signal of the insulinotropic hormone. The results are consistent with a model in which GLP-1-- induced PI-3K activation results in PKC(zeta) translocation to the nucleus, which may play a role in the pleiotropic
effects (DNA synthesis, metabolic enzymes, and insulin gene expression) of the glucoincretin. Diabetes 50: 2237-2243, 2001
aPKC, atypical protein kinase C; BSA, bovine serum albumin; CA, constitutively active; cPKC, classical protein kinase C; DN, dominant-negative; DTT, dithiothreitol; ERK, extracellular signal-related kinases; GLP-1, glucagon-like peptide-1; MAPK, mitogen-activated protein kinase; MEK, mitogenic-extracellular signal-regulated kinase; MOI, multiplicity of infection; NF(kappa)B, nuclear-- factor (kappa)B; PBS, phosphate-buffered saline; PDK, phosphoinositide-dependent kinases; PDX-1, pancreatic and duodenal homeobox gene-1; PI-3K, phosphatidylinositol-3 kinase; PKB, protein kinase B; PKC, protein kinase C; PMSF, phenylmethylsulfonyl fluoride; WT, wild-type.
Clucagon-like peptide-1 (GLP-1)-(7-36) amide, a potent glucoincretin hormone (1,2), is secreted by the intestinal L-cells in response to fat meals...