EFFECTS OF ASCORBIC-ACID, CALCITRIOL, AND RETINOIC ACID ON THE DIFFERENTIATION OF PREOSTEOBLASTS

The responses of the immortalized rat preosteoblast UMR-201-10B to ascorbic acid (AA), 1,25(OH)(2)D-3 (calcitriol), and retinoic acid (RA) were examined. UMR-201-10B cells have an undetectable basal alkaline phosphatase (ALP) activity that is induced after 24 h of treatment with 10(-6) M RA (4.64 +/- 0.06 mu mol/h/mg of protein). The addition of 10(-8) M calcitriol resulted in a slight induction of ALP activity after 72 h (0.43 +/- 0.07 mu mol/h/mg of protein). When calcitriol was added to RA, however, over the same period ALP activity was enhanced significantly compared with treatment with RA alone (RA and calcitriol, 12.29 +/- 0.86 mu mol/h/mg of protein). Treatment with AA (50 mu g/ml) alone had no effect on ALP activity but increased RA-induced ALP activity to 6.78 +/- 0.28 mu mol/h/mg of protein at 24 h. In contrast, AA inhibited calcitriol-induced ALP activity after 7 days of combined treatment with calcitriol (calcitriol, 7.73 +/- 0.16 mu mol/h/mg of protein; AA and calcitriol, 1.44 +/- 0.06 mu mol/h/mg of protein). Individually, RA and calcitriol induced mRNA expression for ALP, matrix-gla protein (MGP), and osteopontin (OP). The steady state level of pro-alpha 1(I) collagen mRNA also was increased significantly by treatment with RA and AA individually. The combination of RA and calcitriol had a synergistic effect on ALP, OP, and especially MGP mRNA expression but significantly reduced the expression of pro-alpha 1(I) collagen mRNA. AA enhanced the effect of RA on the expression of pro-alpha 1(I) collagen, MGP, and ALP mRNAs as well as the effect of calcitriol on OP and MGP. The addition of AA to RA resulted in a decrease in the steady state level of OP, whereas its cotreatment with calcitriol caused a decrease in pro-alpha 1(I) collagen and ALP mRNA. In conclusion, these studies identify RA, calcitriol, and AA as regulators of differentiated osteoblast function.