Phosphorylation of muscle Akt, AS160, and S6K1 are reduced following 8 weeks of increased physical activity in fasting rats

Abstract

Overactivation of the mTOR pathway increases S6K1 activity resulting in an enhanced serine phosphorylation of IRS‐1 and an inhibition of insulin signaling in skeletal muscle. To characterize the effect of increased energy expenditure on regulatory proteins associated with both the insulin and mTOR signaling pathway we examined these proteins while in the fasting state. Soleus muscle was collected from 4 male Sprague‐Dawley sedentary rats and from 5 rats exposed to 8 weeks of free access to running wheels. We utilized immunoblotting methods to measure phosphorylation status and total protein abundance of mTOR, 4E‐BP1, Akt/PKB, S6K1, IRS‐1, TSC2 and AS160. Rats exposed to the chronic increase in energy expenditure tended to have a lower body weight as compared to sedentary rats (336±13g vs 356±30g, P=0.09), and soleus muscle weights were similar between groups (P>0.05). The phosphorylation status of S6K1 (Thr389), AKT/PKB (Ser473) and AS160 (Ser/Thr) were significantly reduced with an increase in energy expenditure (P<0.05). Phosphorylation of TSC2, 4E‐BP1, and IRS‐1 (Ser636/639) were unchanged. The total protein abundance of mTOR, S6K1, 4E‐BP1, Akt/PKB, TSC2, AS160 and IRS‐1 were unaffected with chronic energy expenditure (P>0.05). We conclude that a chronic increase in energy expenditure downregulates Akt signaling to AS160 and S6K1 phosphorylation in fasting skeletal muscle. Funded by NIH grants R01 HL074122 and R01 AR049877