Greater Phosphorylation of AMPK and Multiple AMPK Substrates in the Skeletal Muscle of 24 Month-old Calorie Restricted Compared to Ad Libitum Fed Male Rats

Abstract AMP-activated protein kinase (AMPK), a highly conserved, heterotrimeric serine/threonine kinase with critical sensory and regulatory functions, is proposed to induce anti-aging actions of caloric restriction (CR). Although earlier studies assessed CR’s effects on AMPK in rodent skeletal muscle, the scope of these studies was narrow with limited focus on older animals. This study’s purpose was to fill important knowledge gaps related to CR’s influence on AMPK in skeletal muscle of older animals. Therefore, using epitrochlearis muscles from 24 month-old ad libitum fed (AL) and CR (consuming 65% of AL intake for 8 weeks), male Fischer-344 x Brown Norway F1 rats, we determined: 1) AMPK Thr172 phosphorylation (a key regulatory site) by immunoblot; 2) AMPKα1 and AMPKα2 activity (representing the two catalytic α-subunits of AMPK), and AMPKγ3 activity (representing AMPK complexes that include the skeletal muscle-selective regulatory γ3 subunit) using enzymatic assays; 3) phosphorylation of multiple protein substrates that are linked to CR-related effects (acetyl CoA carboxylase, ACC, that regulates lipid oxidation; Beclin-1 and ULK1 that are autophagy regulatory proteins; Raptor, mTORC1 complex protein that regulates autophagy; TBC1D1 and TBC1D4 that regulate glucose uptake) by immunoblot; and 4) ATP and AMP concentrations (key AMPK regulators) by mass spectrometry. The results revealed significant CR-associated increases in the phosphorylation of AMPK Thr172 and four AMPK substrates (ACC, Beclin-1, TBC1D1, TBC1D4), without significant diet-related differences in ATP or AMP concentration or AMPKα1-, AMPKα2-, or AMPKγ3-associated activity. The enhanced phosphorylation of multiple AMPK substrates provides novel mechanistic insights linking AMPK to functionally important consequences of CR.