Molecular mechanisms of the diabetogenic effects of arsenic inhibition of insulin signaling by arsenite and methylarsonous acid

Abstract

Background: Increased prevalences of diabetes mellitus have been reported among individuals chronically exposed to inorganic arsenic (iAs). However, the mechanisms underlying the diabetogenic effects of iAs have not been characterized. We have previously shown that trivalent metabolites of iAs, arsenite (iAsIII) and methylarsonous acid (MAsIII) inhibit insulin-stimulated glucose uptake (ISGU) in 3T3-L1 adipocytes by suppressing the insulin-dependent phosphorylation of protein kinase B (PKB/Akt). Objectives: Our goal was to identify the molecular mechanisms responsible for the suppression of PKB/Akt phosphorylation by iAsIII and MAsIII. Methods: The effects of iAsIII and MAsIII on components of the insulin-activated signal transduction pathway that regulate PKB/Akt phosphorylation were examined in 3T3-L1 adipocytes. Results: Subtoxic concentrations of iAsIII or MAsIII had little or no effect on the activity of phosphatidylinositol 3-kinase (PI-3K), which synthesizes phosphatidylinositol-3,4,5-triphosphate (PIP3), or on phosphorylation of PTEN (phosphatase and tensin homolog deleted on chromosome ten), a PIP3 phosphatase. Neither iAsIII nor MAsIII interfered with the phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK-1) located downstream from PI-3K. However, PDK-1 activity was inhibited by both iAsIII and MAsIII. Consistent with these findings, PDK-1-catalyzed phosphorylation of PKB/Akt(Thr308) and PKB/Akt activity were suppressed in exposed cells. In addition, PKB/Akt(Ser473) phosphorylation, which is catalyzed by a putative PDK-2, was also suppressed. Notably, expression of constitutively active PKB/Akt restored the normal ISGU pattern in adipocytes treated with either iAsIII or MAsIII. Conclusions: These results suggest that inhibition of the PDK-1/PKB/Akt-mediated transduction step is the key mechanism for the inhibition of ISCU in adipocytes exposed to iAsIII or MAsIII, and possibly for impaired glucose tolerance associated with human exposures to iAs.