Alternate signaling pathways selectively regulate binding of insulin-like growth factor I and II on fetal rat bone cells

Bone cells synthesize and respond to ICF-I and ICF-II which contribute to bone remodeling and linear growth. In osteoblasts, prostaglandin (PG)E-2 stimulates IGF-I but not IGF-II synthesis through a cAMP-dependent protein kinase A (PKA)-related event. However, protein kinase C (PKC) activation by PGE(2) enhances replication and protein synthesis by less differentiated periosteal cells more so than in osteoblast-enriched cultures from fetal rat bane. Using various PGs and other PKA and PKC pathway activators, the importance of these aspects of PGE(2) activity has now been examined on IGF receptors in these bone cell culture models. PGE(2) and other agents that activate PKA enhanced I-125-IGF-II binding to type 2 IGF receptors on both cell populations. In contrast, agents that activate PKC enhanced I-125-IGF-I binding to type 1 receptors on less differentiated bone cells, and of these, only phorbol myristate acetate (PMA), which activates PKC in a receptor-independent way, was effective in osteoblast-enriched cultures. No stimulator increased total type 1 receptor protein in either cell population. Consequently, ligand binding to type 1 and type 2 IGF receptors is differentially modulated by specific intracellular pathways in bone cells. Importantly, changes in apparent type 1 receptor number occur rapidly and may do so at least in part through post-translational effects. These results may help to predict new ways to manipulate autocrine or paracrine actions by IGFs in skeletal tissue. (C) 1998 Wiley-Liss, Inc.