EVIDENCE FOR MODULATION OF OSTEOCALCIN CONTAINING GAMMA-CARBOXYGLUTAMIC ACID RESIDUES SYNTHESIS BY INSULIN-LIKE GROWTH-FACTOR-I AND VITAMIN-K-2 IN HUMAN OSTEOSARCOMA CELL-LINE MG-63

The effect of insulin-like growth factor-I (IGF-I) and 2-methyl-3-all- trans-tetraphenyl-1,4-naphtoquinone (vitamin K-2) on the synthesis of osteocalcin containing gamma-carboxyglutamic acid (Gla) residues which is the physiologically relevant form in bone metabolism was studied i n cultured human osteoblast-like (MG-63) cells. Both IGF-I and vitamin K-2 stimulated 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3)-induced ost eocalcin containing Gla secretion in a concentration-dependent manner. This stimulatory effect of IGF-I and vitamin K-2 was additive. Vitami n K-2-enhanced osteocalcin containing Gla secretion was selectively su ppressed by 3-(alpha-acetonyl-benzyl)-4-hydroxy-coumarin (warfarin). T he stimulatory effect of IGF-I was completely abolished by the presenc e of cycloheximide; in contrast the effect of vitamin I;2 Was still ob served in the presence of cycloheximide. Treatment of MG-63 cells with IGF-I caused an approximately 2.2-fold increase in osteocalcin mRNA l evels (determined by reverse transcription-polymerase chain reaction). Vitamin 1;2 had no effect on either the stimulation of mRNA level by IGF-I or the basal level. IGF-I-stimulated osteocalcin containing Gla secretion was inhibited by one of its binding proteins (insulin-like g rowth factor binding protein-4) in a concentration-dependent manner. T hese findings suggest that the modes of action of IGF-I and vitamin K- 2 on 1,25(OH)(2)D-3-induced osteocalcin containing Gla secretion in MG -63 cells are different.