Mitogenic G(i) protein-MAP kinase signaling cascade in MC3T3-E1 osteogeniccells: Activation by C-terminal pentapeptide of osteogenic growth peptide [OGP(10-14)] and attenuation of activation by cAMP

In osteogenic and other cells the mitogen-activated protein (MAP) kinases h ave a key role in regulating proliferation and differentiated functions. Th e osteogenic growth peptide (OGP) is a 14 mer mitogen of osteogenic and fib roblastic cells that regulates bone turnover, fracture healing, and hematop oiesis, including the engraftment of bone marrow transplants. It is present in the serum and extracellular fluid either free or complexed to OGP-bindi ng proteins (OGPBPs). The free immunoreactive OGP consists of the full leng th peptide and its C-terminal pentapeptide OGP(10-14). In the present study , designed to probe the signaling pathways triggered by OGP, we demonstrate in osteogenic MC3T3 El cells that mitogenic doses of OGP(10-14). but not O GP, enhance MAP kinase activity in a time-dependent manner. The OGP(10-14)- induced stimulation of both MAP kinase activity and DNA synthesis were abro gated by pertusis toxin, a G(i) protein inhibitor. These data offer direct evidence for the occurrence in osteogenic cells of a peptide-activated, mit ogenic Gi protein-MAP kinase-signaling cascade. Forskolin and dBu(2)-cAMP a brogated the OGP(10-14)stimulated proliferation, but induced only 50% inhib ition of the OGP(10-14)-mediated MAP kinase activation, suggesting addition al MAP kinase-dependent, OGP(10-14)-regulated, cellular functions. Finally, it is demonstrated that OGP(10-14) is the active form of OGP, apparently g enerated proteolytically in the extracellular milieu upon dissociation of O GP-OGPBP complexes. I. Cell. Biochem. 81:594-603, 2001. (C) 2001 Wiley-Liss , Inc.