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Background  Polycystic ovary syndrome (PCOS) is characterized by a hyperandrogenic state and frequently develops skeletal muscle insulin resistance. We determined whether testosterone adversely affects insulin action by increasing serine phosphorylation of IRS-1 636/639  in differentiated rat skeletal muscle myotubes. The phosphorylation of Akt, mTOR, and S6K, downstream targets of the PI3-kinase-IRS-1 complex were also studied.    Methods  Primary differentiated rat skeletal muscle myotubes were subjected to insulin for 30 min after 16-hour pre-exposure to either low (20 ng/ml) or high (200 ng/ml) doses of testosterone. Protein phosphorylation of IRS-1 Ser 636/639 , Akt Ser 473 , mTOR-Ser 2448 , and S6K-Thr 389  were measured by Western blot with signal intensity measured by immunofluorescence.    Results  Cells exposed to 100 nM of insulin had increased IRS-1 Ser 636/639  and Akt Ser 473  phosphorylation. Cells pre-exposed to low-dose testosterone had significantly increased insulin-induced mTOR-Ser 2448  and S6K-Thr 389  phosphorylation ( p <0.05), and further increased insulin-induced IRS-1 Ser 636/639  phosphorylation ( p  = 0.042) compared to control cells. High-dose testosterone pre-exposure attenuated the insulin-induced mTOR-Ser 2448  and S6K-Thr 389  phosphorylation.    Conclusions  The data demonstrated an interaction between testosterone and insulin on phosphorylation of intracellular signaling proteins, and suggests a link between a hyperandrogenic, hyperinsulinemic environment and the development of insulin resistance involving serine phosphorylation of IRS-1 Ser 636/639 . These results may guide further investigations of potential mechanisms of PCOS-related insulin resistance.

Effect of Testosterone on Insulin Stimulated IRS1 Ser Phosphorylation in Primary Rat Myotubes—A Potential Model for PCOS-Related Insulin Resistance