Hypoxia converts the myogenic action of IGF-II into mitogenic action. (A and B) C2C12 myoblasts were induced to differentiate by switching to differentiation medium supplemented with or without IGF-II (300 ng/mL). After culturing under normoxic (N) or hypoxic (H) conditions for 36, 72, and 96 h, cells were fixed and the differentiation index, defined as the percentage of MHC-positive cells, was determined (A). Total cell number was also quantified and shown in B. Data are mean ± SE, n = 3–9. (C and D) IGF1R inhibitor NVP-AEW541 abolishes myogenic action (C) and mitogenic action (D) of IGF-II. Cells were cultured in differentiation medium supplemented with or without IGF-II (300 ng/mL) and/or NVP-AEW541 (1 μM) under normoxic or hypoxic conditions for 36 h. Data are means ± SE, n = 4–9. *P < 0.05, ** P < 0.01; *** P <0.001. ns, Not significant.