Background

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.

Methods

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.

Results

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

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.