Protein Kinase Cζ Activation Mediates Glucagon-Like Peptide-1–Induced Pancreatic β-Cell Proliferation
- Jean Buteau1,
- Sylvain Foisy1,
- Christopher J. Rhodes2,
- Lee Carpenter3,
- Trevor J. Biden3 and
- Marc Prentki1
- 1Molecular Nutrition Unit, Department of Nutrition, University of Montreal, the Centre de Recherche du CHUM and Institut du Cancer, Montreal, Quebec, Canada
- 2Pacific Northwest Research Institute & Department of Pharmacology, University of Washington, Seattle, Washington
- 3Garvan Institute of Medical Research, Darlinghurst, New South Wales, Australia
Abstract
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 β-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 β-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 β(INS 832/13) cells. GLP-1 also caused nuclear translocation of the atypical protein kinase C (aPKC) ζ isoform in INS as well as in dissociated normal rat β-cells as shown by immunolocalization and Western immunoblotting analysis. Tritiated thymidine incorporation measurements showed that the p38 MAPK inhibitor SB203580 suppressed GLP-1–induced β-cell proliferation. Further investigation was performed using isoform-specific pseudosubstrates of classical (α, β, and γ) or ζ aPKC isoforms. The PKCζ pseudosubstrate suppressed the proliferative action of GLP-1, whereas the inhibitor of classical PKC isoforms had no effect. Overexpression of a kinase-dead PKCζ acting as a dominant negative protein suppressed GLP-1–induced proliferation. In addition, ectopic expression of a constitutively active PKCζ 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 PKCζ is implicated in the β-cell proliferative signal of the insulinotropic hormone. The results are consistent with a model in which GLP-1–induced PI-3K activation results in PKCζ translocation to the nucleus, which may play a role in the pleiotropic effects (DNA synthesis, metabolic enzymes, and insulin gene expression) of the glucoincretin.
Footnotes
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Address correspondence and reprint requests to Dr. Marc Prentki, CR-CHUM, Pavillon de Sève, 4e, 1560 Sherbrooke Est, Montreal, PQ H2L 4M1, Canada. E-mail: marc.prentki{at}umontreal.ca.
Received for publication 16 November 2000 and accepted in revised form 29 June 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; NFκB, nuclear-factor κB; 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.
