Insulin-like growth factor (IGF)-binding protein-related protein-1: An autocrine/paracrine factor that inhibits skeletal myoblast differentiation but permits proliferation in response to IGF

Skeletal myogenic cells respond to the insulin-like growth factors (IGF-I and IGF-II) by differentiating or proliferating, which are mutually exclusive pathways. What determines which of these responses to IGF skeletal myoblast undergo is unclear. IGF-binding protein-related protein 1 (IGFBP-rP1) is a secreted protein with close homology to the IGF-binding proteins (IGFBPs) in the N-terminal region. IGFBP-rP1, previously called mac25 and IGFBP-7, is highly expressed in C2 skeletal myoblasts during the proliferative phase, but is down-regulated during myoblast differentiation. To determine the role of IGFBP-rP1 in myogenesis, IGFBP-rP1 was overexpressed in C2 myoblasts using a retroviral vector. Western blots indicated that the resulting C2-rP1 myoblasts secreted approximately 27-fold higher levels of IGFBP-rP1 than control CB-LX myoblasts that were transduced with a control vector (LXSN). Compared with CB-LX myoblasts, the differentiation responses of C2-rP1 myoblasts to IGF-I, IGF-II, insulin, and des(1-3)IGF-I were significantly reduced (P < 0.05). However, proliferation responses of C2-rP1 and CB-LX myoblasts to these same factors were not significantly different. Exposure of control CB-LX myoblasts to factors secreted by C2-rP1 myoblasts using a transwell coculture system reduced CB-LX myoblast differentiation significantly (P < 0.05). Experiments with the mitogen-activated protein kinase (MAPK) kinase inhibitor PD098059 suggested that IGFBP-rP1 inhibits a MAPK-dependent differentiation pathway. In confirmation of this idea, levels of phosphorylated extracellular signal-regulated kinase-fl (a MAPK) were reduced in C2-rP1 myoblasts compared with those in CB-LX myoblasts. These findings indicate that IGFBP-rP1 may function as an autocrine/paracrine factor that specifies the proliferative response to the IGFs in myogenesis.