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 skeletal 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-dihydroxyvitamin 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 tissues. Previous studies have characterized the induction of the 24-hydroxylase 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 renal cell line NRK-52E. 1,25(OH) ₂ D (10 ⁻⁷ m ) increased the mRNA levels for the cytochrome P450 component of the 24-hydroxylase (P450cc24) by sevenfold after 36 h in NRK-52E cells. 1,25(OH) ₂ D increased P450cc24 mRNA levels in a dose-dependent manner with an EC ₅₀ of 10 ⁻⁸ m . In parallel experiments, 1,25(OH) ₂ D significantly increased 24-hydroxylase enzyme activity after 48–72 h. The increase in P450cc24 mRNA induced by 1,25(OH) ₂ D required on-going transcription and translation and was inhibited by H-7, a protein kinase C inhibitor. Tetradecanoyl phorbol acetate markedly increased the magnitude of the tissue responsiveness to 1,25(OH) ₂ D by a protein kinase C-dependent pathway. These studies demonstrate that 1,25(OH) ₂ D increases P450cc24 mRNA levels in NRK-52E cells by a mechanism requiring new protein synthesis and involving 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.

Journal of Endocrinology (1997) 153, 199–205