Phospholipase activation and secretion: evidence that PLA2, PLC, and PLD are not essential to exocytosis.
Numerous studies have identified phospholipase metabolites as membrane fusogens, and phospholipase D (PLD) (J.R. Coorssen and R.J. Haslam. FEBS Lett. 316: 170-174, 1993), C (PLC), and A2 (PLA2) activities correlate with secretion. Do these enzymes have essential or modulatory roles? This study confirms that secretion does not require Ca2+ or PLC (Coorssen et al. Cell Regul. 1: 1027-1041, 1990). Arachidonic acid (AA), phosphatidic acid (PA) and analogues, exogenous metabolites of PLA2 and PLD, were tested in electropermeabilized human platelets. AA potentiated guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S)-induced secretion, and eicosanoids were not essential. Endogenous [3H]AA formation correlated with GTP gamma S-induced secretion, and phorbol 12-myristate 13-acetate (PMA) promoted these effects. Inhibitors were used to probe phospholipase influences on secretion. Only PLD inhibitors blocked secretion. However, PMA blocked inhibition of protein kinase C (PKC) and secretion by quercetin, suggesting that PA formed by PLD supports PKC activation and GTP gamma S-induced secretion. Thus PA analogues had no effect alone but enhanced GTP gamma S-induced PKC activity and secretion. Slower PLD activation compared with secretion also indicates a nonessential role. This is the first report of a Ca(2+)-independent PLA2 activity in human platelets, use of quercetin as a PLD inhibitor, and dissociation of PLA2, PLC, and PLD activities from secretion. No major phospholipase activities are essential to the final steps in exocytosis, but modulatory roles are indicated.
Coorssen
J R
JR
Department of Pathology, McMaster University, Hamilton, Ontario, Canada.
D016718:Arachidonic Acid / Q000494:pharmacology; D001792:Blood Platelets / Q000201:enzymology; D002118:Calcium / Q000502:physiology; D004075:Diglycerides / Q000378:metabolism; D004789:Enzyme Activation; D005089:Exocytosis / Q000502:physiology*; D006801:Humans; D010739:Phospholipase D / Q000502:physiology*; D010741:Phospholipases A / Q000502:physiology*; D054467:Phospholipases A2; D010743:Phospholipids / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism; D010738:Type C Phospholipases / Q000502:physiology*
D004075:Diglycerides; D010743:Phospholipids; D016718:Arachidonic Acid; D011493:Protein Kinase C; D010741:Phospholipases A; D054467:Phospholipases A2; D010738:Type C Phospholipases; D010739:Phospholipase D; D002118:Calcium