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

Abstract

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.