EFFECTS OF TRANSFORMING GROWTH-FACTOR BETA(1) ON THE REGULATION OF OSTEOCALCIN SYNTHESIS IN HUMAN MG-63 OSTEOSARCOMA CELLS

Treatment of human MG-63 osteosarcoma cells with human recombinant tra nsforming growth factor beta(1) (TGF-beta(1)) was found to inhibit cel l proliferation. In addition, 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D- 3]-induced osteocalcin synthesis was greatly influenced by TGF-P,. Dos e- and time-dependent inhibition was seen both in medium osteocalcin a nd the corresponding mRNA concentrations. Furthermore, TGF-P, decrease d osteocalcin synthesis modulated negatively by dexamethasone or posit ively by retinoic acid. The stability of osteocalcin mRNA was not decr eased by the TGF-beta(1) treatment, but in vitro transcription assays demonstrated diminished osteocalcin gene transcription caused by the T GF-beta(1) treatment. Binding of vitamin D receptor (VDR) to an oligon ucleotide probe containing the osteocalcin vitamin D response element (VDRE) was not influenced by TGF-beta(1) however. Incubation of the ce lls with the serine/threonine kinase inhibitor H-7 did not block the a bility of TGF-beta(1) to decrease osteocalcin synthesis but caused a f urther inhibition. Also, the 1,25(OH)(2)D-3-induced osteocalcin synthe sis was decreased by H-7 treatment, suggesting that phosphorylation as such is involved in the transcriptional activation mechanism of VDR. These results demonstrate that TGF-beta(1) is a strong inhibitor of th e synthesis of osteocalcin, a calcium binding protein participating in bone mineralization, by counteracting the stimulatory effects of othe r hormones on its synthesis. We further suggest that TGF-beta(1) affec ts the synthesis of osteocalcin at the level of transcription through mechanism(s) different from the serine/threonine kinase pathway.