Browsing by Subject "hyperinsulinemia"

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    Evidence that Hyperinsulinemia, known to Accelerate Diabetes Progression, may also Contribute to Dyslipidemia via Impairing ApoA1/ABCA1-Mediated Cholesterol Efflux
    (Office of the Vice Chancellor for Research, 2010-04-09) Penque, Brent; Sealls, Whitney; Elmendorf, Jeffrey S.
    Low levels of plasma high-density lipoprotein cholesterol (HDL-C) are associated with insulin resistance and type 2 diabetes (T2D). As it is well appreciated that hyperinsulinemia contributes to the progression/worsening of insulin resistance, we tested here if this key metabolic derangement impaired cellular mechanisms of HDL-C generation. An initial event in this process is the binding of apolipoprotein A1 (ApoA1) to the plasma membrane (PM)-localized ATPbinding cassette cholesterol transporter protein ABCA1. Subcellular fractionation analyses revealed that 3T3-L1 adipocytes exposed to chronic insulin (12h, 5nM) displayed a 25% decrease (P<0.05) in PM ABCA1 content and a reciprocal increase in endosomal ABCA1 content. These insulin-induced changes in cellular ABCA1 distribution occurred concomitantly with a decrease in ApoA1-mediated cellular cholesterol efflux. Consistent with endosomal/cytosolic cycling of the small molecular GTPase Rab8 playing a functional role in ABCA1 vesicle trafficking, we found a 50% increase (P<0.05) in endosomal Rab8 content and a 30% decrease (P<0.05) in cytosolic Rab8 content. New data shows that increased HBP activity increases cholesterol biosynthesis and increased endosomal cholesterol content inhibits the functional cycling of Rab proteins. In line with these observations, we found that cells treated with the cholesterol-lowering agent methyl-β-cyclodextrin were protected against insulininduced defects in ABCA1/Rab8 vesicle trafficking and PM cholesterol accrual. These data are consistent with the concept that the coexistence of low plasma HDL-C with insulin resistance and T2D may reflect a negative influence of hyperinsulinemia on Rab8-mediated trafficking of ABCA1 to the PM for ApoA1-mediated cholesterol efflux.
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    Identification of an Actin-Based Antidiabetic Action of Chromium in Skeletal Muscle
    (Office of the Vice Chancellor for Research, 2010-04-09) Hoffman, Nolan J.; Habegger, Kirk M.; Elmendorf, Jeffrey S.
    We recently demonstrated that cortical filamentous actin (F-actin) loss contributes to cellular insulin resistance induced by hyperinsulinemia. New animal and human analyses suggest a similar loss of F-actin is present in insulin-resistant skeletal muscle and results from cellular cholesterol accrual. Interestingly, we found that chromium picolinate (CrPic), a dietary supplement recognized to improve insulin action, lowers plasma membrane cholesterol in cultured adipocytes. Understanding whether CrPic can improve F-actin structure in insulinresistant skeletal muscle via lowering membrane cholesterol is not known, yet significant, as skeletal muscle is responsible for a large majority of insulin-stimulated glucose transport. In L6 myotubes stably expressing the insulin-responsive glucose transporter GLUT4 carrying an exofacial myc-epitope tag, acute insulin stimulation (20 min, 100 nM) increased myc-epitope labeling at the surface of intact cells by ~2-fold (P<0.05). In contrast, the ability of insulin to stimulate this process was inhibited 25% (P<0.05) by sustained exposure of L6 myotubes to insulin (12 h, 5 nM). Defects in insulin signaling did not readily account for the observed disruption. However, we found that insulin-induced insulin-resistant myotubes displayed a 28% elevation (P<0.05) in membrane cholesterol with a reciprocal 14% loss (P<0.05) in F-actin. This cholesterol/actin imbalance and insulin/GLUT4 dysfunction was corrected by the cholesterollowering action of CrPic. Mechanistically, CrPic increased the activity of the AMP-activated protein kinase (AMPK). Tests also revealed that other well-recognized activators of AMPK (e.g., AICAR, DNP) lowered membrane cholesterol and that, in a fashion similar to that witnessed for CrPic, improved regulation of GLUT4 in insulin-induced insulin-resistant myotubes. These data, as well as findings from ongoing siRNA-mediated AMPK knockdown experiments, are consistent with AMPK mediating its antidiabetic action by lowering cellular cholesterol. We predict that chromium, via AMPK activation, protects against cholesterol accrual that induces skeletal muscle F-actin loss and insulin resistance.