Rapid action of 1,25-dihydroxyvitamin D3 on hepatocyte phospholipids

Recent studies have reported cellular effects of 1,25-dihydroxyvitamin D3 within 15 minutes, a time period too rapid to be mediated by nuclear activation. The vitamin increases hepatocyte cytosolic calcium levels in the absence of extracellular calcium within 5 minutes. Since metabolites of phosphatidylinositol have been implicated as second messengers in the regulation of cytosolic calcium, we examined the effect of 1,25-dihy-droxyvitamin D3 on hepatocyte phosphatidylinositol turnover and compared these effects to those produced by vasopressin. In isolated hepatocytes labeled with [3H]inositol, 1,25-dihydroxyvitamin D3 (4 nM) increased [3H]glycerophosphoryIinositol by 16% (p < 0.01) within 2.5 minutes, by 18% (p < 0.01) after 5 minutes, and by 11% (p < 0.05) after 10 minutes. At a concentration of 20 nM, 1,25-dihydroxyvitamin D3 increased [3H]glycerophosphorylinositol by 27% (p < 0.01) after 5 minutes. Vitamin D did not affect [3H]inositol polyphosphates. Conversely, vasopressin had no effect on [3H]glycerophosphorylinositol but significantly increased [3H]inositol phosphate, [3H]inositol bisphosphate, and [3H]inositol trisphosphate. 1,25-Dihy-droxyvitamin D3 (4 nM) decreased [3H]phosphatidylinositol by 10% (p < 0.05) after 5 minutes and by 16% (p < 0.01) after 10 minutes. At a concentration of 20 nM, 1,25-dihydroxyvitamin D decreased [3H]phosphatidylinositol by 18% (p < 0.01) after 5 minutes. The vitamin did not affect [3H]phosphatidylinositol bis-phosphate or [3H]phosphatidylinositol trisphosphate. 24,25-Dihydroxyvitamin D had no effect on inositol phospholipids. The effects of 1,25-dihydroxyvitamin D3 on inositol phospholipids were blocked by quinacrine. Bromophenacylbromide inhibited the effects of 1,25-dihydroxyvitamin D3 on inositol phospholipids and also blocked the vitamin-induced increments in cytosolic calcium. In isolated hepatocytes labeled with [3H]arachidonic acid, 1,25-dihydroxyvitamin D3 (20 nM) decreased 3H-labeled phosphatidylcholine and phosphatidylethanolamine, as well as phosphatidylinositol. The data indicate that 1,25-dihydroxyvitamin D3 may enhance deacylation of hepatocyte phospholipids, perhaps by activation of phospholipase A activity. The ability of vitamin D to increase cytosolic calcium requires activation of phospholipase A. Although both 1,25-dihydroxyvitamin D and vasopressin rapidly affect phospholipid metabolism, their mechanisms of action differ.