Androgen Receptor Regulates the Proliferation of Stretched Myoblasts and Its Mechanisms: IGF-1R-mediated p38 and ERK1/2 Pathways

Abstract BackgroundOur previous study has indicated that in C2C12 myoblasts androgen receptor (AR) plays a crucial role in 15% stretch-induced proliferation through insulin-like growth factor (IGF-1)-modulated PI3K/Akt, p38 and ERK1/2 pathways. In this study we further explored AR's effects on 15% and 20% stretches-modulated myoblasts proliferation and involved pathways in L6 (no detectable AR) and C2C12 (abundant AR) myoblasts.MethodsThe proliferation and the above indicators were compared between stretched L6 and C2C12 myoblasts by CCK8, ELISA and Western blot, then were detected again in stretched L6 myoblasts after transfection with AR overexpression plasmid utilizing LipoPlusTM 2000 reagent or treatment with different concentrations (200, 500, 1000 ng/ml) IGF-1 recombinant polypeptide.ResultsWe found that ①Under un-stretched states, the proliferation rate of L6 cells was slower than that of C2C12 cells, and the proliferation of C2C12 cells was repressed by AR knockdown using siRNA interference, while L6 myoblasts proliferation was promoted by overexpression AR. ②The proliferation rate as well as the activities of p38 and ERK1/2 increased by 15% stretch were much lower in L6 than that in C2C12 cells; AR overexpression enhanced the proliferation of 15% stretched L6 cells accompanied with increased activities of p38 and ERK1/2; ③20% stretch decreased the proliferation and activity of p38 in L6 more than that in C2C12 myoblasts; overexpression of AR totally reversed the anti-proliferation of 20% stretch and significantly enhanced p38 activity in L6 myoblasts;④In L6 myoblasts, overexpression AR increased IGF-1R expression despite no detectable IGF-1 was secreted; and recombinant IGF-1 increased not only the proliferation, but also the protein level of IGF-1R and activities of p38 and ERK1/2 in a dose-dependent manner in 15% stretched L6 myoblasts. ConclusionsAR fulfilled 15% stretch-induced pro-proliferation of myoblasts by activating IGF-1R-p38 and ERK1/2 pathways, while 20% stretch-induced anti-proliferation was mediated by inhibiting AR-IGF-1R-p38 pathway. This study is beneficial to understand in depth the role and mechanisms of AR on appropriate exercise increases while excessive exercise decreases muscle mass.