Molecular mechanisms of the diabetogenic effects of arsenic: inhibition of insulin signaling by arsenite and methylarsonous acid.
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 (iAs(III)) and methylarsonous acid (MAs(III)) inhibit insulin-stimulated glucose uptake (ISGU) in 3T3-L1 adipocytes by suppressing the insulin-dependent phosphorylation of protein kinase B (PKB/Akt).


OBJECTIVE
Our goal was to identify the molecular mechanisms responsible for the suppression of PKB/Akt phosphorylation by iAs(III) and MAs(III).


METHODS
The effects of iAs(III) and MAs(III) on components of the insulin-activated signal transduction pathway that regulate PKB/Akt phosphorylation were examined in 3T3-L1 adipocytes.


RESULTS
Subtoxic concentrations of iAs(III) or MAs(III) had little or no effect on the activity of phosphatidylinositol 3-kinase (PI-3K), which synthesizes phosphatidylinositol-3,4,5-triphosphate (PIP(3)), or on phosphorylation of PTEN (phosphatase and tensin homolog deleted on chromosome ten), a PIP(3) phosphatase. Neither iAs(III) nor MAs(III) 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 iAs(III) and MAs(III). 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 iAs(III) or MAs(III).


CONCLUSIONS
These results suggest that inhibition of the PDK-1/PKB/Akt-mediated transduction step is the key mechanism for the inhibition of ISGU in adipocytes exposed to iAs(III) or MAs(III), and possibly for impaired glucose tolerance associated with human exposures to iAs.
Paul
David S
DS
Department of Nutrition, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7461, USA. styblo@med.unc.edu
Harmon
Anne W
AW
Devesa
Vicenta
V
Thomas
David J
DJ
Stýblo
Miroslav
M
D064413:3-Phosphoinositide-Dependent Protein Kinases; D016475:3T3 Cells; D017667:Adipocytes / Q000378:metabolism; D000818:Animals; D001152:Arsenicals / Q000494:pharmacology*; D018053:Arsenites / Q000633:toxicity*; D003920:Diabetes Mellitus / Q000139:chemically induced*; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4 / Q000378:metabolism; D015151:Immunoblotting; D007328:Insulin / Q000378:metabolism*; D051379:Mice; D008856:Microscopy, Fluorescence; D051059:PTEN Phosphohydrolase / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects*
D064413:3-Phosphoinositide-Dependent Protein Kinases; D001152:Arsenicals; D018053:Arsenites; D005947:Glucose; D051275:Glucose Transporter Type 4; D007328:Insulin; D051059:PTEN Phosphohydrolase; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; C020300:monomethylarsonic acid; C015001:arsenite