Abstract 4: Apolipoprotein (apo) A-I Increases Insulin-Dependent Glucose Disposal in Skeletal Muscle by Stimulating the PI3K/Akt/AS160 Signaling Pathway

Abstract

Introduction: Inhibition of CETP increases plasma high density lipoprotein (HDL) levels and improves glycaemic control in people with type 2 diabetes. Recent in vitro studies have also established that the main HDL apolipoprotein, apoA-I, increases insulin secretion from pancreatic β-cells. However, the effect of apoA-I on glucose uptake by skeletal muscle, the main site of glucose disposal, is not understood. Hypothesis: Lipid-free apoA-I increases insulin-dependent glucose disposal in skeletal muscle. Methods: Primary human skeletal muscle cells were differentiated into myotubes, then incubated for 16 h with lipid-free apoA-I (final concentration 1 mg/ml). Glucose uptake was initiated by adding a tracer amount of [3H]2-deoxy-glucose to the myotubes and continuing the incubation for up to 5 h with or without insulin (0.1 μM final conc.). IRS-1, PI3K, Akt and AS160 protein levels were quantified by subjecting total cell lysates to SDS-PAGE and immunoblotting. Results: Incubation of skeletal muscle myotubes in the absence or presence of insulin resulted in [3H]2-deoxy-glucose uptake of 1000 and 1103±68.9 pmol/mg protein/h, respectively (p<0.1). Pre-incubation of the myotubes with lipid-free apoA-I, followed by incubation in the absence or presence of insulin resulted in [3H]2-deoxy-glucose uptake of 1089±51.35 and 1358±57.9 pmol/mg protein/h (p<0.01), respectively. The apoA-I-mediated increase in insulin-dependent glucose uptake into the myotubes was time- and concentration-dependent. Incubation of myotubes in the presence of apoA-I and insulin increased IRS-1 phosphorylation and total IRS-1 protein by 27.8±5% and 30.8±1.3% (p<0.01 for both), respectively. PI3K, Akt and AS160 phosphorylation increased by 55.1±22%, 52.5±20.9%, and 32±8%, respectively (p<0.01 for all) without any change in total protein. Conclusion: Lipid-free apoA-I increases insulin-dependent glucose uptake into human skeletal muscle cells by stimulating the PI3K/Akt/AS160 insulin signaling pathway. These results suggest that HDL-raising therapies may improve glucose disposal in skeletal muscle, and thus be beneficial in type 2 diabetes.