Endocrinology, Vol 132, 1782-1788, Copyright © 1993 by Endocrine Society

ARTICLES

Effects of 1,25-dihydroxyvitamin D3 and phorbol ester on 25- hydroxyvitamin D3 24-hydroxylase cytochrome P450 messenger ribonucleic acid levels in primary cultures of rat renal cells

ML Chen, MA Boltz and HJ Armbrecht
Geriatric Research, Education, and Clinical Center, Veterans Affairs Medical Center, St. Louis, Missouri 63125.

The renal 25-hydroxyvitamin D3 24-hydroxylase enzyme, which may be the starting point in the catabolic pathway for vitamin D metabolism, is markedly induced by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], the hormonal form of vitamin D. The purpose of this study was to investigate the regulation of the cytochrome P450 component of this enzyme (P450cc24) by 1,25-(OH)2D3 and phorbol 12-myristate 13-acetate (TPA). P450cc24 messenger RNA (mRNA) levels were measured using the full-length rat complementary DNA probe (p108). In primary cultures of rat renal tubular cells, 1,25-(OH)2D3 produced a 26-fold increase in P450cc24 mRNA which was detectable at 4 h, maximal at 24 h, and returned almost to baseline by 48 h. The induction was inhibited by actinomycin D, 5,6-dichloro-1-b-D-ribofuranosyl benzimidazole (DRB), and cycloheximide, and it was specific for vitamin D compounds containing a 1-hydroxyl group. TPA alone had no effect, but TPA in the presence of 1,25-(OH)2D3 produced an increase in P450cc24 mRNA within 30 min, and this increase peaked at 2 h. TPA also shifted the dose- response curve of 1,25-(OH)2D3 to the left, so that 1,25-(OH)2D3 was effective at a concentration as low as 1 nM. In the same experiments, TPA increased c-fos mRNA levels, and this increase was accelerated by 1,25-(OH)2D3. These studies suggest that the induction of P450cc24 mRNA by 1,25-(OH)2D3 is a receptor-mediated genomic event and that this induction may account for the stimulation of 24-hydroxylase enzyme activity by 1,25-(OH)2D3. In addition, TPA accelerates the effect of 1,25-(OH)2D3 by a mechanism which may involve protein kinase C.

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