INDUCTION OF 24-HYDROXYLASE CYTOCHROME-P450 MESSENGER-RNA BY 1,25-DIHYDROXYVITAMIN-D AND PHORBOL ESTERS IN NORMAL RAT-KIDNEY (NRK-52E) CELLS

The biologically active form of vitamin D, 1,25-dihydroxyvitamin D (1, 25(OH)(2)D), acts on intestinal, renal, and bone cells to regulate ske letal and mineral metabolism. 1,25(OH)(2)D also induces 24-hydroxylase activity in these target cells. The 24-hydroxylase hydroxylates 1,25( OH)(2)D to 1,24,25-trihydroxyvitamin D and 25(OH)D to 24,25-dihydroxyv itamin D. The production of 1,24,25-trihydroxyvitamin D is thought to be the first step in the inactivation of 1,25(OH)(2)D by its target ti ssues. Previous studies have characterized the induction of the 24-hyd roxylase by 1,25(OH)(2)D in clonal cell Lines from intestine and bone. The purpose of these studies was to characterize the induction of the 24-hydroxylase by 1,25(OH)(2)D in the kidney, using the clonal rat re nal cell line NRK-52E. 1,25(OH)(2)D (10(-7) M) increased the mRNA leve ls for the cytochrome P450 component of the 24-hydroxylase (P450cc24) by sevenfold after 36h in NRK-52E cells. 1,25(OH)(2)D increased P450cc 24 mRNA levels in a dose-dependent manner with an EC50 of 10(-8) M. I, parallel experiments, 1,25(OH)(2)D significantly increased 24-hydroxy lase enzyme activity after 48-72 h. The increase in P450cc24 mRNA indu ced by 1,25(OH)(2)D required ongoing transcription and translation and was inhibited by H-7, a protein kinase C inhibitor. Tetradecanoyl pho rbol acetate markedly increased the magnitude of the tissue responsive ness to 1,25(OH)(2)D by a protein kinase C-dependent pathway. These st udies demonstrate that 1,25(OH)(2)D increases P450cc24 mRNA levels in NRK-52E cells by a mechanism requiring new protein synthesis and invol ving protein kinase C. This is in contrast to the action of 1,25(OH)(2 )D in intestinal cells, which does not require new protein synthesis, and in osteoblastic cells, which does not involve protein kinase C.