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

Skeletal myogenic cells respond to the insulin-like growth factors (IGF-I a nd IGF-II) by differentiating or proliferating, which are mutually exclusiv e pathways. What determines which of these responses to IGF skeletal myobla st 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 phas e, but is down-regulated during myoblast differentiation. To determine the role of IGFBP-rP1 in myogenesis, IGFBP-rP1 was overexpressed in C2 myoblast s using a retroviral vector. Western blots indicated that the resulting C2- rP1 myoblasts secreted approximately 27-fold higher levels of IGFBP-rP1 tha n control CB-LX myoblasts that were transduced with a control vector (LXSN) . Compared with CB-LX myoblasts, the differentiation responses of C2-rP1 my oblasts to IGF-I, IGF-II, insulin, and des(1-3)IGF-I were significantly red uced (P < 0.05). However, proliferation responses of C2-rP1 and CB-LX myobl asts to these same factors were not significantly different. Exposure of co ntrol CB-LX myoblasts to factors secreted by C2-rP1 myoblasts using a trans well coculture system reduced CB-LX myoblast differentiation significantly (P < 0.05). Experiments with the mitogen-activated protein kinase (MAPK) ki nase inhibitor PD098059 suggested that IGFBP-rP1 inhibits a MAPK-dependent differentiation pathway. In confirmation of this idea, levels of phosphoryl ated extracellular signal-regulated kinase-fl (a MAPK) were reduced in C2-r P1 myoblasts compared with those in CB-LX myoblasts. These findings indicat e that IGFBP-rP1 may function as an autocrine/paracrine factor that specifi es the proliferative response to the IGFs in myogenesis.