Involvement of insulin-like growth factor type 1 receptor and protein kinase Cdelta in bis(maltolato)oxovanadium(IV)-induced phosphorylation of protein kinase B in HepG2 cells

Abstract

Vanadium(IV) oxo-bis(maltolato) (BMOV), an organovanadium compound, is a potent insulinomimetic agent and improves glucose homeostasis in various models of diabetes. We have shown previously that BMOV stimulates the phosphorylation of PKB which may contribute as one of the mechanisms 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 events leading to BMOV-induced PKB phosphorylation in HepG2 cells. Since BMOV is an inhibitor of protein tyrosine phosphatases and through enhanced tyrosine phosphorylation may activate various protein tyrosine kinases (PTK), we have investigated the potential role of different receptor or nonreceptor PTK in mediating BMOV-induced PKB phosphorylation. Among several pharmacological inhibitors that were tested, only AG1024, 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 protein expression by antisense oligonucleotides (ASO) attenuated BMOV-induced PKB phosphorylation. BMOV-induced PKB phosphorylation was associated with an increased level of tyrosine phosphorylation of the IRbeta subunit, IGF-1Rbeta subunit, IRS-1, and p85alpha subunit of PI3-kinase. However, this response was independent of IR-PTK activity because in cells overexpressing a PTK-inactive form of IR, insulin response was attenuated while the effect of BMOV remained intact. A role of PKC in BMOV-induced response was also tested. Pharmacological inhibition with chelerythrine, a nonselective PKC inhibitor, or rottlerin, a PKCdelta inhibitor, as well as chronic treatment with PMA attenuated BMOV-induced PKB phosphorylation. In contrast, GO6976 and RO31-8220 PKCalpha/beta selective inhibitors failed to alter the BMOV effect. Taken together, these data suggest that IGF-1R and PKCdelta are required to stimulate PKB phosphorylation in response to BMOV in HepG2 cells and provide new insights into the molecular mechanism by which this compound exerts its insulinomimetic effects.