Disruptions in autophagy have been linked to metabolic complications, including insulin resistance (IR). Heat therapy has been shown to protect against IR; however, the effect of heat on autophagy and the role of autophagy on the insulin sensitizing effects of heat are unclear.
C2C12 myotubes were treated with high glucose and palmitate (HG+Palm) for 24 hours and either maintained at 37°C (thermoneutral) or heated at 40°C (HT) for 1 hour. Myotubes were harvested immediately (0hr) or 24 hours (24hr) after HT, in either the basal state or following stimulation with insulin for 15 min. In a subset of the cells, an autophagy inhibitor, Bafilomycin A1 (100 nM), was added 3 hours prior to cell harvest.
There was a main effect of HG+Palm to reduce autophagy initiation signals p-AMPK Thr172 and p-ULK1 Ser555, and increase markers of autophagasome accumulation, LC3 II and p62 (all p<0.05). HT attenuated the increase in p62 (p<0.05), suggesting a reduction in autophagasome accumulation. HG+Palm reduced insulin signaling, represented by decreased phosphorylation of AS160 at Thr642, increased inhibitory phosphorylation of IRS1 at Ser636/639, and decreased phosphorylation of Akt at Ser473 (all p<0.05). The suppression in insulin signaling induced by HG+Palm was partially attenuated by HT, including an attenuation in inhibitory phosphorylation of IRS1 (p<0.05) and trending increase in the phosphorylation of Akt (p<0.1). HT-induced reductions in p-IRS1 Ser636/639 were lost when autophagy was inhibited by Bafilomycin A1.
Conclusions: These data suggest that short bouts of heat may prevent autophagasome accumulation induced by nutrient excess, which plays a role in the insulin sensitizing effects of heat.