Crucial role of androgen receptor in resistance and endurance trainings-induced muscle hypertrophy through IGF-1/IGF-1R- PI3K/Akt- mTOR pathway

BACKGROUND: Androgen receptor (AR) has been reported to play vital roles in exercise-induced increase of muscle mass in rats, but needs to be further verified and the mechanism behind remains unclear. As AR target genes, insulin growth factor-1 (IGF-1) and IGF-1 receptor (IGF-1R) promote muscle hypertrophy through activating PI3K/Akt- mammalian target of rapamycin (mTOR) pathway, a classic pathway of muscle hypertrophy. So the main purpose of this study was using AR antagonist flutamide to demonstrate AR’s effect on training-induced muscle hypertrophy and its possible mechanism: IGF-1/IGF-1R- PI3K/Akt- mTOR pathway? METHODS: Forty-eight Sprague Dawley male rats aged 7 weeks were randomly divided into six groups: control (C), flutamide (F), resistance training (R), resistance training plus flutamide (R + F), endurance training (E), and endurance training plus flutamide (E + F) groups. Flutamide was used to block AR in rats. Rats in R and R + F groups fulfilled 3 weeks of ladder climbing with progressively increased load, while E and E + F rats completed 3-week moderate intensity aerobic exercise on a treadmill. The relative muscle mass (muscle mass/body weight) of rats was detected. Serum levels of testosterone and IGF-1 of rats were determined by ELISA, and mRNA levels of IGF-1R and mTOR in muscles by real-time PCR. Protein levels of AR, IGF-1, IGF-1R, mTOR, PI3K, Akt, p-PI3K and p-Akt in muscles were detected by Western blot. RESULTS: (1) The training-induced rise in the relative muscle mass and the expression levels of AR were only found in the gastrocnemius of R rats and in the soleus of E rats (selective muscle hypertrophy), which were blocked by flutamide. (2) Serum testosterone in the R and E rat were increased, and flutamide exerted no effect. (3) The levels of IGF-1, IGF-1R and mTOR as well as the activities of PI3K and Akt were enhanced selectively (in the gastrocnemius of R rats and in the soleus of E rats), which were reduced by flutamide. Conclusions: AR exerted an essential role in both resistance training and endurance training-induced muscle hypertrophy, which was mediated at least partly through IGF-1/IGF-1R- PI3K/Akt- mTOR pathway.