INFLUENCE OF VITAMIN-D AND RETINOIDS ON THE GAMMACARBOXYLATION OF OSTEOCALCIN IN HUMAN OSTEOSARCOMA MG63 CELLS

Osteocalcin (OC) is a bone matrix protein, synthesized by osteoblasts, which contains three residues of gammacarboxy-glutamic acid (GLA), A fraction of circulating OC, which is not fully carboxylated and does n ot bind to hydroxyapatite, is called undercarboxylated OC (ucOC), In e lderly institutionalized women, we have shown an increase of circulati ng ucOC level which may result not only from vitamin K deficiency but also from vitamin D deficiency (Szulc et al., J Clin Invest 91:1769; 1 993), This intriguing finding prompted us to study the effect of vitam in D on the secretion of ucOC by osteoblastic cells in vitro in the pr esence of warfarin, an inhibitor of gammacarboxylation of GLA-containi ng proteins, The potential influence of retinoic acid (RA) was also st udied, because its mechanism of action involves pathways that are simi lar to vitamin D, In the presence of warfarin (0.05 mu g/ mL), 1 alpha ,25(OH)(2)D (10(-8)-10(-6) mol/L) decreased dose dependently ucOC secr etion by human osteosarcoma MG63 cells (from 3.87 +/- 0.96 to 2.12 +/- 0.13 ng/10(6) cells), When expressed as a fraction of total OC, secre tion ucOC decreased from 47.4 +/- 1.4% to 24.8 +/- 3.2% in the MG63 ce lls, The secretion of total OC was stimulated by RA and by Ro 13-7410, which is a specific ligand of retinoic acid receptor (RAR), but not b y 9-cis retinoic acid (9-cisRA), which is a physiologic ligand of reti noid X receptor (RXR), RA and Ro 13-7410 decreased ucOC secretion and ucOC% in warfarin-treated MG63 cells (RA: from 50.4 +/- 13.3% to 13.5 +/- 2.8%; Ro 13-7410: from 28.4 +/- 8.2% to 11.3 +/- 8.4%); 9-cisRA ha d no effect on OC gammacarboxylation, These results show that vitamin D, RA, and Ro 13-7410, but not 9-cisRA, may modify the gammacarboxylat ion of OC in human MG63 cells. (C) 1996 by Elsevier Science Inc.