Figure 10.

The IGF-II and IGFBP-5 genes exhibit similar induction patterns and IGFBP-5 knockdown inhibits myogenic differentiation in primary skeletal muscle cells. (A) Total RNA was extracted from primary mouse skeletal myoblasts at the time indicated after the induction of differentiation. IGF-II mRNA and IGFBP-5 mRNA levels were measured by qRT-PCR. Values are expressed as relative levels to that of 12 h. Data shown are means ± SE of three independent experiments each performed in duplicate. Groups labeled with different symbols are different from each other at P < 0.05. (B) Neonatal mouse myoblasts were transfected with the empty pSUPER vector (a) or pSUPER-BP5 (b). 30 h after transfection, cells were induced to differentiate. Representative images from two independent groups are shown. Bar, 200 μm. (C) Quantified results of B. (D) A proposed model on how IGFBP-5 and IGF-II act in concert to stimulate myogenic differentiation. IGFBP-5 is induced in early stages of myogenesis and is located on the cell surface. Cell surface–associated IGFBP-5 binds to IGF-II and targets IGF-II to the close proximity of the IGF-IR receptor, thereby enhancing IGF-IR–mediated signaling activity, leading to a further increase in IGF-II gene expression. This, in turn, results in an accelerated increase in the IGF-IR-PI3K-Akt signaling activity, leading to increases in Myogenin expression, and promoting myogenic differentiation.