Regulation of rat yolk sac 25-hydroxy- and 1,25-dihydroxyvitamin D3 24-hydroxylase activities.
The yolk sac of the pregnant rat which functions as a true placenta is a target organ for vitamin D. This tissue can hydroxylate in position 24 both 25-hydroxy- and 1,25-dihydroxyvitamin D3 (25-OHD3 and 1,25-(OH)2D3). The present report describes an in vitro model for the study of 1,25-(OH)2D3 action on the further metabolism of 25-OH[3H]D3 and 1,25-(OH)2[3H]D3 by yolk sac. The tissue explants were preincubated with 1,25-(OH)2D3 for 18 h in a serum-free culture medium. Physiological concentrations of 1,25-(OH)2D3 were the most effective in stimulating (7.5-fold) the 1,25-(OH)2D3 24-hydroxylase, while the 25-OHD3 24-hydroxylase stimulation (4-fold) required a 1,25-(OH)2D3 concentration of 10(-7) M. The stimulating effect of 1,25-(OH)2D3 on the 1,25-(OH)2D3 24-hydroxylase was temperature-dependent, and, since its was inhibited by actinomycin D and cycloheximide, required de novo protein synthesis. 1,24,25-(OH)3D3, 25-OHD3, and 24,25-(OH)2D3 were 10- to 1000-fold less potent than 1,25-(OH)2D3 in inducing the 1,25-(OH)2D3 hydroxylase. Our results strongly suggest that 1,25-(OH)2D3 regulated the 1,25-(OH)2D3 24-hydroxylase by a receptor-mediated process. Furthermore, 1,25-(OH)2D3 at 10(-9) M induced within 4 h an increase of its own degradation and the formation of an as yet unidentified major 1,25-(OH)2[3H]D3 metabolite. We conclude that the yolk sac can participate in the regulation of 1,25-(OH)2D3 concentration in the fetoplacental unit.
D015650:24,25-Dihydroxyvitamin D 3; D000818:Animals; D002117:Calcitriol / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D003513:Cycloheximide / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System; D003609:Dactinomycin / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006887:Hydroxycholecalciferols / Q000378:metabolism; D011247:Pregnancy; D051381:Rats; D011919:Rats, Inbred Strains; D013250:Steroid Hydroxylases / Q000302:isolation & purification* / Q000378:metabolism*; D065668:Vitamin D3 24-Hydroxylase; D015017:Yolk Sac / Q000201:enzymology*
D006887:Hydroxycholecalciferols; C030711:24-hydroxyvitamin D3; D003609:Dactinomycin; D015650:24,25-Dihydroxyvitamin D 3; D003577:Cytochrome P-450 Enzyme System; D003513:Cycloheximide; D013250:Steroid Hydroxylases; D065668:Vitamin D3 24-Hydroxylase; D002117:Calcitriol