Activation of aPCK is required for vanadate-induced phosphorylation of protein kinase B (Akt), but not p70^S6k in mouse epidermal JB6 cells

Vanadium is a metal widely distributed in the environment. Although vanadate-containing compounds exert potent toxic effects on a wide variety of biological systems, the mechanisms by which vanadate mediates adverse effects are not well understood. The present study investigated the vanadate-induced phosphorylation of Akt and p70^S6K, two kinases known to be vital for cell survival, growth, transformation, and transition of the cell cycle in mammals. Exposure of mouse epidermal JB6 cells to vanadium led to phosphorylation of Akt and p70^S6K in a time- and dose-dependent manner. Vanadium exposure also caused translocation of atypical isoforms of PKC ( from the cytosol to the membrane, but had no effect on PKCα translocation, suggesting that the atypical PKCs (aPKC) were specifically involved in vanadium-induced cellular response. Importantly, over expression of a dominant negative mutant PKCλ blocked Akt phosphorylation at Ser⁴⁷³ and Thr³⁰⁸, whereas it did not inhibit p70^S6k phosphorylation at Thr³⁸⁹ and Thr⁴²¹ /Ser⁴²⁴, suggesting that aPKC activation is specifically involved in vanadium-induced activation of Akt, but not in activation of p70^S6k. Furthermore, vanadium-induced p70^S6k phosphorylation at Thr³⁸⁹ and Thr⁴²¹/Ser⁴²⁴ and Akt phosphorylation at Thr³⁰⁸ occurred through a PI-3K-dependent pathway because a PI-3K dominant negative mutant inhibited induction as compared with vector control cells. These results indicate that there was a differential role of aPKC in vanadate-induced phosphorylation of Akt and p70^S6k, suggesting that signal transduction pathways leading to the activation of Akt and p706^S6k were different.