title: 1,2-Diacylglycerols but not phorbol esters stimulate sphingomyelin hydrolysis in GH3 pituitary cells.
abstract: It has recently been proposed that degradation products of sphingolipids may serve as physiologic inhibitors of protein kinase C. The present study was performed to determine the effect of 1,2-diacylglycerols and phorbol esters, known activators of protein kinase C, on sphingomyelin metabolism. 1,2-Dioctanoylglycerol (diC8) caused time- and concentration-dependent reduction in the level of sphingomyelin labeled to equilibrium with [3H]choline. diC8 (200 micrograms/ml) reduced [3H]sphingomyelin to 81 +/- 3% of control (p less than 0.005) by 15 min, and the level was 58 +/- 5% of control after 1 h; an EC50 for this event was 56 micrograms/ml. To evaluate the mechanisms of stimulated hydrolysis, the sphingoid base backbone of sphingomyelin was labeled with [14C] serine, and the effects of diC8 were quantitated. diC8 (100 micrograms/ml) reduced the level of sphingomyelin to 66 +/- 7% of control by 1 h from 375 +/- 12 pmol/10(6) cells to 245 +/- 26 pmol/10(6) cells. There was a concomitant increase in ceramide from 89 +/- 4 pmol/10(6) cells to 252 +/- 27 pmol/10(6) cells consistent with activation of the enzyme, sphingomyelinase (EC 3.1.4.12). In support of this contention, 1,2-diacylglycerols appeared to enhance the activity of an acid, but not a neutral, sphingomyelinase in homogenates of GH3 cells. The 1,2-diacylglycerol, 1-oleyl-2-acetylglycerol, produced similar effects. In contrast, the phorbol esters, 12-O-tetradecanoylphorbol 13-acetate and phorbol 12,13-dibutyrate, failed to stimulate sphingomyelin hydrolysis. Further, these effects of the 1,2-diacylglycerols occurred in cells down-modulated for protein kinase C. These studies demonstrate that 1,2-diacylglycerols stimulate sphingomyelin hydrolysis by a mechanism independent of the protein kinase C which mediates phorbol ester action. This is the first report of stimulated sphingomyelin hydrolysis by a physiologic effector molecule.
mesh_terms: D000818:Animals; D002518:Ceramides / Q000378:metabolism; D002794:Choline / Q000378:metabolism; D004075:Diglycerides / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D005989:Glycerides / Q000494:pharmacology*; D015240:Phorbol 12,13-Dibutyrate; D010703:Phorbol Esters / Q000494:pharmacology*; D010911:Pituitary Neoplasms / Q000378:metabolism*; D051381:Rats; D013108:Sphingomyelin Phosphodiesterase / Q000378:metabolism; D013109:Sphingomyelins / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013997:Time Factors; D014407:Tumor Cells, Cultured / Q000187:drug effects*
chemical_list: C011439:1,2-diacylglycerol; D002518:Ceramides; D004075:Diglycerides; D005989:Glycerides; D010703:Phorbol Esters; D013109:Sphingomyelins; D015240:Phorbol 12,13-Dibutyrate; D013108:Sphingomyelin Phosphodiesterase; D002794:Choline; D013755:Tetradecanoylphorbol Acetate