Influence of a tumor promoting phorbol ester on the metabolism of 25-hydroxyvitamin D3.

When added to primary cultures of chick kidney cells, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) decreased the basal rate of production of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and increased that of 24,25-dihydroxyvitamin D3 (24,25(OH)2D3). The normal stimulatory effect of parathyroid hormone and forskolin on 1,25(OH)2D3 production was abolished or blunted by the presence of TPA and TPA overcame the inhibitory effect of PTH and forskolin on 24,25(OH)2D3 production. The evidence suggests that protein kinase C may be involved in the regulation of 25(OH)D3 metabolism by chick kidney cells.