Insulin inhibits autophagy signaling independent of counterregulatory hormone levels but does not affect the effects of exercise

Acute exercise increases autophagic signaling through Unc-51 like kinase-1 (ULK1) in human skeletal muscle during both anabolic and catabolic conditions. The aim of the present study was to investigate if changes in ULK1 Ser(555) phosphorylation during exercise are reflected by changes in phosphorylation of a newly identified ULK1 substrate (ATG14 Ser(29)) and to elucidate the involvement of circulatory hormones in the regulation of autophagy in human skeletal muscle. We show that 1 h of cycling exercise increases ATG14 Ser(29) phosphorylation during both hyperinsulinemic euglycemic and euinsulinemic euglycemic conditions. This could suggest that counterregulatory hormones stimulate autophagy in skeletal muscle, as circulating concentrations of these hormones are highly elevated during exercise. Furthermore, ATG14 Ser(29) correlated positively with ULK1 phosphorylation, suggesting that ULK1 Ser(555) (activating site) phosphorylation reflects ULK1 kinase activity. In a separate series of experiments, we show that insulin stimulates ULK1 phosphorylation at Ser(757) (inhibitory site) in both hypoglycemic and euglycemic conditions, suggesting that counterregulatory hormones (such as epinephrine, norepinephrine, growth hormone, and glucagon) have limited effects on autophagy signaling in human skeletal muscle. In conclusi on, 1 h of cycling exercise increases phosphorylation of ATG14 at Ser(29) in a pattern that mirrors ULK1 phosphorylation at Ser(555). Moreover, insulin effects on autophagy signaling in human skeletal muscle are independent of hypoglycemic and euglycemic conditions. NEW & NOTEWORTHY Autophagy signaling is regulated in a hierarchical order by exercise, insulin, and counterregulatory hormones. Exercise-induced autophagy signaling is stimulated by local factors in skeletal muscle rather than circulatory hormones. Unc-51 like kinase-1 (ULK1) phosphorylation at Ser(555) reflects ULK1 kinase activity.