title: Differential effects of phorbol 12-myristate 13-acetate on GTP gamma S-induced diacylglycerol formation and arachidonic acid liberation in saponin-permeabilized rabbit platelets.
abstract: In rabbit platelets, collagen (50 micrograms/ml)- or thrombin (0.5 U/ml)-induced diacylglycerol formation was dose-dependently prevented by phorbol 12-myristate 13-acetate (PMA, 2-50 nM). However, collagen-induced arachidonic acid liberation and lysophosphatidylcholine formation were rather enhanced by PMA, while the thrombin-induced liberation was not. We also demonstrated with saponin-permeabilized platelets that collagen (100 micrograms/ml)-induced arachidonic acid liberation was enhanced by GTP gamma S and inhibited by GDP beta S, both dose-dependently. Since these results lead us to consider that protein kinase C affects a guanine-nucleotide-binding protein (G-protein) to modulate phospholipase A2 and C, we investigated this dual effect of PMA on arachidonic acid liberation and diacylglycerol formation induced by G-protein activator. Addition of GTP gamma S (100 microM) to saponin-permeabilized platelets significantly induced these responses, and PMA (2-10 nM)-pretreatment before the cell permeabilization inhibited diacylglycerol formation and enhanced arachidonic acid liberation and lysophosphatidylcholine formation, dose-dependently. Likewise, PMA (20 nM) had differential effects on the similar NaF (20 mM)-induced responses in intact platelets. Contrarily, 10 nM PMA had no effect on diacylglycerol formation caused by an addition of high concentration of Ca2+ (1 mM) alone after the cell permeabilization, while it still had a potentiating effect on arachidonic acid liberation under the condition. These results suggest that protein kinase C may have a dual regulatory effect on the activation of phospholipase A2 (positive feedback) and phospholipase C (negative feedback), probably through influences on two distinct G-proteins associated separately with these two enzymes.
authors.affiliation: Department of Biochemistry, Kyoto Pharmaceutical University, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Akiba
authors.forename: T
authors.initials: T
authors.lastname: Sato
authors.forename: T
authors.initials: T
authors.lastname: Fujii
mesh_terms: D000818:Animals; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000378:metabolism; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D004075:Diglycerides / Q000097:blood; D019204:GTP-Binding Proteins / Q000097:blood; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D008055:Lipids / Q000097:blood; D010539:Permeability; D010741:Phospholipases A / Q000097:blood; D054467:Phospholipases A2; D011817:Rabbits; D012503:Saponins; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D013873:Thionucleotides / Q000494:pharmacology; D010738:Type C Phospholipases / Q000097:blood
chemical_list: D001095:Arachidonic Acids; D004075:Diglycerides; D008055:Lipids; D012503:Saponins; D013873:Thionucleotides; D016718:Arachidonic Acid; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D010741:Phospholipases A; D054467:Phospholipases A2; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D013755:Tetradecanoylphorbol Acetate