INDUCTION OF THE VITAMIN-D 24-HYDROXYLASE (CYP24) BY 1,25-DIHYDROXYVITAMIN D-3 IS REGULATED BY PARATHYROID-HORMONE IN UMR106 OSTEOBLASTIC CELLS

The expression of the vitamin D 24-hydroxylase is highly regulated in target tissues for 1,25-dihydroxyvitamin D-3 (1, 25(OH)(2)D), where it may modulate the action of 1,25(OH)(2)D. In UMR106 osteoblastic cells , 1,25(OH)(2)D and PTH synergistically induce 24-hydroxylase expressio n. The purpose of these studies was to characterize the interaction be tween 1,25(OH)(2)D and PTH with regard to the messenger RNA (mRNA) lev els of the cytochrome P450 component of the 24-hydroxylase (CYP24). PT H alone had no effect on CYP24 mRNA levels, and 1,25(OH)(2)D alone pro duced only a modest increase. However, 1,25(OH)(2)D and PTH together s ynergistically increased CYP24 mRNA levels 3-fold compared with 1,25(O H)(2)D alone. PTH also increased the sensitivity of UMR cells to 1,25( OH)(2)D from 10(-8) to 10(-10) M. PTH worked through the cAMP signalin g pathway as evidenced by the lack of effect of PTH (3-34) and by the full activity of 8-bromo-cAMP. PTH in the presence of 1,25(OH)(2)D inc reased CYP24 gene transcription as shown by nuclear run-on studies and by activation of a CYP24 promoter-reporter construct after transfecti on. PTH also increased vitamin D receptor number in UMR cells, but thi s occurred at times later than the increase in transcription. These st udies demonstrate that PTH in the presence of 1,25(OH)(2)D works throu gh the cAMP-dependent signaling pathway to increase transcription of t he CYP24 gene, to increase CYP24 protein levels, and to increase 24-hy droxylase activity.