Interactions between intracellular signals involved in the regulation of 25-hydroxyvitamin D3 metabolism.

The tumor-promoting phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) increases 25-hydroxyvitamin D3 (25OHD3)-24-hydroxylase and decreases 25OHD3-1-hydroxylase activity in cultured kidney cells, effects similar to those exerted by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and opposite those of PTH, forskolin, and cAMP. In this paper it is shown that the effects of TPA and 1,25-(OH)2D3 are additive, suggesting that they operate through distinct mechanisms. TPA did not alter cAMP metabolism by cultured chick kidney cells, not did it alter their response, in terms of 25OHD3 metabolism, to cAMP, suggesting that these two regulators of 25OHD3 metabolism also operate through distinct pathways. Another presumed activator of protein kinase-C 1,oleoyl-2-acetyl-glycerol, was tested and found to have the same effect as TPA in decreasing 1-hydroxylase activity, but it does not increase 24-hydroxylase activity. In addition, 1-oleoyl-2-acetyl-glycerol increases intracellular cAMP levels to approximately 25% of those attained by stimulation with PTH. None of the treatments resulted in altered [3H]PDBu binding by the cells. The results, taken together, suggest that 25OHD3-1-hydroxylase and the 25OHD3-24-hydroxylase are subject to multifactorial regulation and can be regulated independently of one another.