title: Arachidonic acid release from NIH 3T3 cells by group-I phospholipase A2: involvement of a receptor-mediated mechanism.
abstract: Group I pancreatic phospholipase A2 (PLA2 I) is primarily a digestive enzyme. Recently, however, in addition to its catalytic activity a receptor-mediated function has been described for this enzyme. PLA2 I binding to its receptor induces cellular chemokinesis, proliferation, and smooth muscle contraction. This enzyme also induces the production of prostaglandin E2 in certain cells and may have a proinflammatory role. However, despite its ability to hydrolyze phospholipids in in vitro assays, PLA2-I does not efficiently catalyze release of AA from intact cells. Here, we demonstrate that while short-term exposure of NIH 3T3 cells to PLA2-I is ineffective, exposure of 6 h or longer significantly increases the basal release of AA. Dose-response curve of PLA2-I-induced AA release was saturable with an EC50 of 14.01 +/- 1.36 nM (n = 3). [3H]-AA was preferentially released over [3H]-oleic acid by PLA2-I. PLA2-I, inactivated with 4-bromophenacyl bromide, was fully capable of mediating AA release. These data suggest that a non-catalytic, receptor-mediated mechanism is involved in PLA2-I-induced AA release in NIH-3T3 cells. This release of AA is not dependent on protein kinase C or Ca2+ concentration. Comparison of the effect of PLA2-I with those of ATP and platelet-derived growth factor indicates that each of these agonists regulates AA release via independent pathways. Neither the basal enzymatic activity of the 85-kDa cytosolic PLA2 nor the protein level of this enzyme was affected by treatment of cells with PLA2-I. However, the increase in basal enzymatic activity of 85 kDa PLA2 due to protein kinase C activation was further enhanced by pretreatment of cells with PLA2-I. We conclude that: (1) short-term exposure of cells to PLA2 I does not cause measurable AA release; (2) release of AA from intact cells by this enzyme requires long-term exposure; (3) AA release is not mediated by a direct catalytic effect of PLA2 I; and (4) AA release by PLA2 I is accomplished via a receptor-mediated process. Taken together, these results raise the possibility that PLA2 I, in addition to its digestive function, may also contribute to aggravate preexisting inflammatory processes and/or to initiate new ones when chronic exposure of cells to this enzyme occurs.
authors.affiliation: Section on Developmental Genetics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-1830, USA.
authors.forename: M
authors.initials: M
authors.lastname: Xing
authors.forename: L
authors.initials: L
authors.lastname: Miele
authors.forename: A B
authors.initials: AB
authors.lastname: Mukherjee
mesh_terms: D016475:3T3 Cells; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism*; D002462:Cell Membrane / Q000201:enzymology; D007249:Inflammation / Q000378:metabolism; D051379:Mice; D010741:Phospholipases A / Q000378:metabolism*; D054467:Phospholipases A2; D010982:Platelet-Derived Growth Factor / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D013997:Time Factors
chemical_list: D010982:Platelet-Derived Growth Factor; D016718:Arachidonic Acid; D000255:Adenosine Triphosphate; D010741:Phospholipases A; D054467:Phospholipases A2