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Antidiabetogenic Effects of Chromium Mitigate Hyperinsulinemia-Induced Cellular Insulin Resistance via Correction of Plasma Membrane Cholesterol Imbalance

Departments of Cellular and Integrative Physiology (E.M.H., L.T., A.M.M., J.S.E.) and Biochemistry and Molecular Biology (J.T.B., J.S.E.), Indiana University School of Medicine, Center for Diabetes Research, Indianapolis, Indiana 46202; Department of Molecular Cardiology (P.R.), Joseph J. Jacobs Center for Thrombosis and Vascular Biology, Cleveland Clinic, Cleveland, Ohio 44195; and Eli Lilly and Company (J.T.B.), Indianapolis, Indiana 46285

Address all correspondence and requests for reprints to: Jeffrey S. Elmendorf, Ph.D., Department of Cellular and Integrative Physiology, Indiana University School of Medicine, VanNuys Medical Science Building Room 308A, Indianapolis, Indiana 46202. E-mail: jelmendo{at}iupui.edu.

Previously, we found that a loss of plasma membrane (PM) phosphatidylinositol 4,5-bisphosphate (PIP₂)-regulated filamentous actin (F-actin) structure contributes to insulin-induced insulin resistance. Interestingly, we also demonstrated that chromium picolinate (CrPic), a dietary supplement thought to improve glycemic status in insulin-resistant individuals, augments insulin-regulated glucose transport in insulin-sensitive 3T3-L1 adipocytes by lowering PM cholesterol. Here, to gain mechanistic understanding of these separate observations, we tested the prediction that CrPic would protect against insulin-induced insulin resistance by improving PM features important in cytoskeletal structure and insulin sensitivity. We found that insulin-induced insulin-resistant adipocytes display elevated PM cholesterol with a reciprocal decrease in PM PIP₂. This lipid imbalance and insulin resistance was corrected by the cholesterol-lowering action of CrPic. The PM lipid imbalance did not impair insulin signaling, nor did CrPic amplify insulin signal transduction. In contrast, PM analyses corroborated cholesterol and PIP₂ interactions influencing cytoskeletal structure. Because extensive in vitro study documents an essential role for cytoskeletal capacity in insulin-regulated glucose transport, we next evaluated intact skeletal muscle from obese, insulin-resistant Zucker (fa/fa) rats. Because insulin resistance in these animals likely involves multiple mechanisms, findings that cholesterol-lowering restored F-actin cytoskeletal structure and insulin sensitivity to that witnessed in lean control muscle were striking. Also, experiments using methyl-β-cyclodextrin to shuttle cholesterol into or out of membranes respectively recapitulated the insulin-induced insulin-resistance and protective effects of CrPic on membrane/cytoskeletal interactions and insulin sensitivity. These data predict a PM cholesterol basis for hyperinsulinemia-associated insulin resistance and importantly highlight the reversible nature of this abnormality.

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