Activation of protein kinase C modulates dihydroxycholecalciferol synthesis in rat renal tubules

1,25(OH)₂D₃, the biologically active metabolite of vitamin D, is produced from 25(OH)D₃, by the renal mitochondrial 25(OH)D₃ 1α-hydroxylase. Several studies have implicated reversible phosphorylation and a possible role for protein kinase C (PKC) in acute regulation of 1,25(OH)₂D₃ production. In the experiments described here, we studied 1,25(OH)₂D₃ production in freshly isolated rat renal tubules treated with activators and inhibitors of PKC. In this mammalian system, TPA, but not its inactive analogue 4αPDD, inhibited 1,25(OH)₂D₃ production in a dose-dependent fashion within 20 min. The acute inhibition of 1,25(OH)₂D₃ production by TPA exposure was preceded by an increase in membrane associated PKC activity, which was paralleled by a decrease in cytosolic PKC activity. Pre-incubation of tubules with staurosporine, a PKC inhibitor, abolished the inhibitory effect of TPA on 1,25(OH)₂D₃ production. Chronic (18 h) exposure of tubules to high dose TPA resulted in down regulation of both membrane and cytosolic PKC activity and re-exposure to TPA did not affect PKC translocation or 1,25(OH)₂D₃ production in down regulated tubules. Our data strongly suggest that modulation of renal PKC activity may be an important mechanism for acute regulation of 1,25(OH)₂D₃ production.