24R,25-DIHYDROXYVITAMIN D-3 INCREASES CYCLIC-GMP CONTENTS, LEADING TOAN ENHANCEMENT OF OSTEOCALCIN SYNTHESIS BY 1,25-DIHYDROXYVITAMIN D-3 IN CULTURED HUMAN OSTEOBLASTIC CELLS

The effect of the physiological vitamin D metabolite 24R,25-dihydroxyv itamin D-3 [24R,25(OH)(2)D-3] on human osteoblastic cells was assessed . Physiological concentrations (10(-9)-10(-8) M) of 24R,25(OH)(2)D-3 s ignificantly increased the cyclic guanosine 5'-monophosphate (cGMP) co ntent in the human osteoblastic cells by approximately 200% in 5 to 15 min. In contrast, 24S,25-dihydroxyvitamin D-3 had only a weak effect on the cGMP content, and 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3] di d not affect the content. The production of osteocalcin was not induce d by 10(-9)-10(-8) M of 24R,25(OH)(2)D-3 in the absence of 1,25(OH)(2) D-3. However, the same concentration of 24R,25(OH)(2)D-3 showed stimul atory effects on osteocalcin synthesis in the presence of 10(-9) M 1,2 5(OH)(2)D-3. Rp-8Br-cyclic GMP, a specific inhibitor of cyclic GMP-dep endent protein kinase, significantly inhibited the cooperative effect of 24R,25(OH)(2)D-3 with 1,25(OH)(2)D-3 on the osteocalcin synthesis, although Rp-8Br-cyclic AMP, a specific inhibitor of cyclic ARLP-depend ent protein kinase, did not affect the cooperative effect. In addition , okadaic acid enhanced the osteocalcin synthesis induced by 1,25(OH)( 2)D-3. These observations suggest that 24R,25(OH)(2)D-3 has a unique a ctivity of increasing cGMP contents in osteoblastic cells, and that th e increase in cGMP contents may lead to the cooperative effect of 24R, 25(OH)(2)D-3 with 1,25(OH)(2)D-3 on osteocalcin synthesis, These data support the hypothesis that 24R,25(OH)(2)D-3 has a physiological role in human bone and mineral metabolism. (C) 1998 Academic Press.