MITOGENIC ACTION OF OSTEOGENIC GROWTH PEPTIDE (OGP) - ROLE OF AMINO AND CARBOXY-TERMINAL REGIONS AND CHARGE

We have recently reported the discovery of a 14-amino-acid osteogenic growth peptide (OGP). In vivo OGP increases bone formation and trabecu lar bone density. Physiologically it is found in serum complexed to an OGP binding protein (OGPBP). In vitro OGP has a biphasic effect on os teoblastic MC 3T3 El and fibroblastic NIH 3T3 cell proliferation; at l ow concentrations (0.01-1.0 and 1.0-100.0 pM, respectively) it is high ly stimulatory with an inhibition at higher doses. To assess possibili ties of labeling synthetic OGP to obtain radio- or fluorescent ligands , OGP analogues were extended at the N- or C-termini with Cys or Cys(S -NEtSucc) or the OGP Tyr-10 replaced by 3-I(Tyr). All analogues with N -terminal modifications, as well as the [Cys15]OGP-NH2 retained the OG P-like dose-dependent effect on proliferation of the MC 3T3 E1 and NIH 3T3 cells, although the magnitude of stimulation was lower, approx. 2 /3 that of the native-like synthetic OGP. The [Cys15(S-NEtSucc)]OGP-NH 2 and [3-I(Tyr10)]OGP shared only the inhibitory activity of OGP. This suppression is further shared by a number of other positively and neg atively net charged, but not net neutral, peptides. Both N-terminal-mo dified analogues displayed a decreased binding activity to the OGPBP. All analogues except reverse OGP, [3-1(Tyr10)]OGP and [Cys15(S-NEtSucc )]OGP-NH2 reacted with anti-OGP antibodies. These data are not only im portant for labeling purposes but suggest a respective role for the OG P N- and C-terminal regions in binding to the OGPBP and putative OGP r eceptor. It appears that the OGP proliferative activity represents the net effect of stimulation specific to the OGP structure and nonspecif ic inhibition associated with the peptide's high positive net charge.