Molecular mechanism of bis(maltolato)oxovanadium(IV)-induced insulin signaling in 3T3-L1 and IM9 cells: impact of dexamethasone

The organovanadium compound bis(maltolato)oxovanadium(IV) (BMOV) enhanced the tyr-phosphorylation of major upstream insulin signaling proteins including the vital site-specific phosphorylation of insulin receptor beta (IR beta) in IM9 and 3T3-L1 cells in dose- and time-dependent manners more efficiently than insulin. Nevertheless, insulin in general had a synergistic impact on those phosphorylations in both cell lines, while its presence was obligatory to induce Tyr(972)- phosphorylation of 1190 in IM9 cells at 18-h treatment with BMOV. However, prolonged exposure of cells to BMOV caused depletion in IR level and using IM9 cells we found that this event was counteracted by insulin, where monensin, a monocarboxylic acid ionophore made an additive impact, suggesting that a novel mechanism is being involved in the recycling of internalized IR in BMOV-treated cells. On the other hand, dexamethasone elevated the IR level in both cell lines. However, no correlation was found between the cellular content and the degree of phosphorylation of IR beta in cells receiving combined treatment of BMOV, and dexamethasone with short insulin post-exposure. BMOV also induced the phosphorylation of Thr 308 and Ser(473) of Akt in both cell lines receiving insulin post-treatment, while dexamethasone decreased those phosphorylations. However, this activation/deactivation of Akt did not correlate with the phosphorylation status of Ser(9) and Ser(259) of glycogen synthase kinase (GSK)-3 beta and Raf respectively. Taken together, it is conceivable that BMOV and/or dexamethasone modulate insulin signaling by acting differentially on the components of the insulin signaling network. We also consider that the observed dexamethasone-mediated modulation of insulin receptor kinase in BMOV-treated 3T3-L1 cells probably occurs through the activation/deactivation of some mechanism which needs further studies for proper characterization.