The Role of AMPK in The Regulation of Fatty Acid Transport in Adipocytes

Abstract

AMP-activated protein kinase (AMPK) is a Ser/Thr protein kinase that acts as a key regulator of mammalian metabolism. AMPK activation is associated with decreased energy charge and acts to promote ATP conservation and production. In myocytes, AMPK activation increases Glut4-mediated glucose uptake and fatty acid (FA) uptake via the FA transporter CD36. In adipocytes, we have previously demonstrated that AMPK activation by the indirect activator AICAR is associated with decreasd adipocyte glucose uptake although the AMPK-dependence of this effect was uncertain. Whether AMPK regulates FA uptake in adipocytes remains uncharacterised. Previous studies have reported that insulin stimulates FA uptake in adipocytes by a mechanism involving mTORC1 activation. This study aimed to characterise the effect of AMPK activation on adipocyte FA uptake and cell surface levels of CD36 in adipocytes. Intriguingly, insulin only modestly increased FA uptake in 3T3-L1 adipocytes. Direct and indirect AMPK activators inhibited both basal and insulin stimulated FA uptake in 3T3-L1 adipocytes. Similarly, the mTORC1 inhibitor rapamycin inhibited basal and insulin stimulated FA uptake in 3T3-L1 adipocytes. Both compound 991 and rapamycin tended to inhibit insulin stimulated CD36 cell surface expression. These data indicate that AMPK activators and mTORC1 inhibition decrease FA uptake in 3T3-L1 adipocytes. This mechanism may involve AMPK-mediated inhibition of mTORC1 and suppression of cell surface translocation of CD36 which was investigated using a range of approaches. These data highlight the contrasting actions of AMPK on nutrient influx in adipocytes and myocytes. Furthermore, the direct AMPK activator compound 991 had no rapid effect on basal or insulin-stimulated glucose uptake, suggesting that the previously reported action of AICAR on insulin-stimulated glucose uptake was AMPK- independent. We also report that global AMPKα1 downregulation has no effect on nutrient transporters level in adipose tissue, skeletal muscle, and heart tissue. Finally, this study demonstrated that there was an upregulation of AMPKβ2:AMPKβ1 levels during 3T3-L1 adipogenesis. Experiments where particular pro-adipogenic stimuli were omitted during 3T3-L1 differentiation demonstrated that dexamethasone was critical for the increase in AMPKβ2:AMPKβ1 ratio during adipogenesis. Taken together, this thesis has detailed several novel findings regarding AMPK regulation of FA uptake in adipocytes and the role of AMPKβ2 on adipogenesis.