A Chronic Increase in Energy Expenditure Inhibits S6K1 and Reduces IRS-1 Serine Phosphorylation in Muscle

Abstract

It is well established that chronic endurance exercise training can improve insulin sensitivity and glucose tolerance in insulin resistant and Type 2 diabetic patients. On the other hand, over-activation of the mTOR signaling pathway is associated with increased insulin resistance primarily via mTOR and S6K1 induced phosphorylation of serine residues on IRS-1. The molecular mechanisms for the improved action of insulin during chronic increases in energy expenditure, such as endurance training, remain unclear. PURPOSE: To determine if a chronic increase in energy expenditure alters expression and activation of regulatory proteins associated with both the insulin and mTOR signaling pathways. METHODS: Soleus muscle was collected from male Sprague-Dawley sedentary rats and from rats exposed to 8 weeks of free access to running wheels (peak running distance of 5.4 ± 1.4 km/day). We utilized immunoblotting methods to measure changes in total protein abundance of mTOR, 4E-BP1, eEF2, Akt/PKB, SKAR, and AMPKα. Phosphorylation status of S6K1 and IRS-1 (Ser636/639) was also examined with immunoblotting. Serum fed glucose and lactate levels were also measured. RESULTS: Rats exposed to a chronic increase in energy expenditure had a significantly lower body weights as compared to sedentary rats (411 ± 11gvs 338 ± 5g, P<0.05), however, soleus weights were similar between groups (P>0.05). The phosphorylation status of S6K1 (Thr389) and IRS-1 (Ser636/639) were significantly reduced with an increase in energy expenditure (P<0.05). The total protein abundance of mTOR, S6K1, 4E-BP1, eEF2, Akt/PKB or AMPKα were unaffected with chronic energy expenditure (P>0.05), however, total SKAR protein was increased (P<0.05). There was no difference in fed glucose or lactate levels between groups (P>0.05). CONCLUSION: A reduction in S6K1 activation during chronic increases in energy expenditure may be playing an important regulatory role in the serine phosphorylation status of IRS-1. Future studies are required to determine if reduced mTOR signaling may be partially responsible for the enhanced insulin sensitivity in endurance trained individuals.