Molecular Nutrition Unit, Department of Nutrition, University of Montreal, the Centre de Recherche du CHUM and Institut du Cancer, Montreal, Quebec, Canada.
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-1-induced beta-cell proliferation. Further investigation was performed using isoform-specific pseudosubstrates of classical (alpha, beta, and gamma) or zeta aPKC isoforms. The PKCzeta pseudosubstrate suppressed the proliferative action of GLP-1, whereas the inhibitor of classical PKC isoforms had no effect. Overexpression of a kinase-dead PKCzeta acting as a dominant negative protein suppressed GLP-1-induced proliferation. In addition, ectopic expression of a constitutively active PKCzeta 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 GLP-1-induced activation of PKCzeta 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 PKCzeta translocation to the nucleus, which may play a role in the pleiotropic effects (DNA synthesis, metabolic enzymes, and insulin gene expression) of the glucoincretin.