| Abstract
| - Vanadium(IV) oxo-bis(maltolato) (BMOV), an organovanadium compound, is a potentinsulinomimetic agent and improves glucose homeostasis in various models of diabetes. We have shownpreviously that BMOV stimulates the phosphorylation of PKB which may contribute as one of themechanisms for the insulinomimetic effect of this compound. However, the upstream mechanism of BMOV-induced PKB phosphorylation remains elusive. Therefore, in this study, we examine the upstream eventsleading to BMOV-induced PKB phosphorylation in HepG2 cells. Since BMOV is an inhibitor of proteintyrosine phosphatases and through enhanced tyrosine phosphorylation may activate various protein tyrosinekinases (PTK), we have investigated the potential role of different receptor or nonreceptor PTK in mediatingBMOV-induced PKB phosphorylation. Among several pharmacological inhibitors that were tested, onlyAG1024, a selective inhibitor of IGF-1R-PTK, almost completely blocked BMOV-stimulated phosphorylation of PKB. In contrast, AG1295 and AG1478, specific inhibitors of PDGFR and EGFR, respectively,were unable to block the BMOV response. Moreover, efficient reduction of the level of IGF-1R proteinexpression by antisense oligonucleotides (ASO) attenuated BMOV-induced PKB phosphorylation. BMOV-induced PKB phosphorylation was associated with an increased level of tyrosine phosphorylation of theIRβ subunit, IGF-1Rβ subunit, IRS-1, and p85α subunit of PI3-kinase. However, this response wasindependent of IR-PTK activity because in cells overexpressing a PTK-inactive form of IR, insulin responsewas attenuated while the effect of BMOV remained intact. A role of PKC in BMOV-induced responsewas also tested. Pharmacological inhibition with chelerythrine, a nonselective PKC inhibitor, or rottlerin,a PKCδ inhibitor, as well as chronic treatment with PMA attenuated BMOV-induced PKB phosphorylation.In contrast, GÖ6976 and RO31-8220 PKCα/β selective inhibitors failed to alter the BMOV effect. Takentogether, these data suggest that IGF-1R and PKCδ are required to stimulate PKB phosphorylation inresponse to BMOV in HepG2 cells and provide new insights into the molecular mechanism by which thiscompound exerts its insulinomimetic effects.
|
