Arachidonic acid release from NIH 3T3 cells by group‐I phospholipase A2: Involvement of a receptor‐mediated mechanism

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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, 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 relase 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. © 1995 Wiley‐Liss Inc. 1 This article is a US Government work and, as such, is in the public domain in the United States of America.

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