title: Guanosine 5'-O-(thiotriphosphate)-dependent inositol trisphosphate formation in membranes is inhibited by phorbol ester and protein kinase C.
abstract: Phosphoinositide hydrolysis was studied in a washed membrane preparation of 1321N1 astrocytoma cells prelabeled with [3H]inositol. GTP gamma S stimulated the formation of [3H]inositol mono-, bis-, and trisphosphate ([3H]InsP, [3H]InsP2, and [3H]InsP3) with a half-maximal effect on [3H]InsP formation at 5 microM. Carbachol increased the accumulation of [3H]inositol phosphates only in the presence of added guanine nucleotide. Calcium increased [3H]InsP3 accumulation over a range of concentrations (10 nM-3 mM free calcium). When 1321N1 cells were treated with phorbol ester (100 nM 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA)) prior to preparation of the membranes, the maximal [3H]InsP formation induced by GTP gamma S or GTP gamma S plus carbachol was decreased by 50-75%. In contrast, the response to a maximal calcium concentration presumed to activate phospholipase C directly was minimally inhibited (approximately 15%). PMA treatment did not affect muscarinic receptor affinity for carbachol or the effect of GTP on agonist binding. PMA treatment was also without effect on the breakdown of exogenous [3H]InsP3 in homogenates, permeabilized cells, and membranes, indicating that the InsP3-phosphatase was not the site of phorbol ester action. PMA treatment inhibited [3H] InsP3 formation only in membranes and not in cytosol prepared from the same cells, suggesting a membrane site of PMA action. Membranes were also required to demonstrate GTP gamma S-stimulated [3H]InsP3 formation although calcium-stimulated [3H]InsP3 formation was demonstrable in both membranes and cytosol. The addition of purified protein kinase C to the membranes mimicked the effect of PMA treatment to decrease GTP gamma S-stimulated [3H]InsP3 production. These data indicate that the effect of PMA on phosphoinositide metabolism is demonstrable in a cell-free system and that it can be mimicked by protein kinase C. We suggest that the ability of PMA to block GTP gamma S-stimulated formation of [3H]InsP3 results from inhibition of the G protein interaction with phospholipase C.
mesh_terms: D001920:Bradykinin / Q000494:pharmacology; D002217:Carbachol / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006632:Histamine / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000096:biosynthesis*; D007700:Kinetics; D008566:Membranes / Q000378:metabolism; D010703:Phorbol Esters / Q000494:pharmacology*; D011493:Protein Kinase C / Q000378:metabolism*; D011813:Quinuclidinyl Benzilate / Q000378:metabolism; D011976:Receptors, Muscarinic / Q000378:metabolism; D013403:Sugar Phosphates / Q000096:biosynthesis*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D013873:Thionucleotides / Q000378:metabolism*
chemical_list: D007295:Inositol Phosphates; D010703:Phorbol Esters; D011976:Receptors, Muscarinic; D013403:Sugar Phosphates; D013873:Thionucleotides; D006165:Guanylyl Imidodiphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D011813:Quinuclidinyl Benzilate; D006632:Histamine; D015544:Inositol 1,4,5-Trisphosphate; D006160:Guanosine Triphosphate; D002217:Carbachol; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D001920:Bradykinin