Involvement of Phospholipase A 2 Activation and Arachidonic Acid Metabolism in the Cytotoxic Functions of Rat NK Cells

Abstract

Arachidonic acid (AA) release via phospholipase A 2 (PLA 2) activation and generation of eicosanoids have been implicated as playing signaling roles in a variety of cell types. Here we show evidence that interaction of fresh NK cells with membranes from sensitive or antibody (Ab)-coated targets generates eicosanoids through both cycloxygenase (CO) and lipoxygenase (LO) pathways. Eicosanoid generation is attributable to PLA 2 activation since pretreatment with PLA 2 irreversible inhibitors, such as mepacrine or parabromophenacylbromide (pBPB), completely blocks AA metabolite generation. The involvement of PLA 2 or AA metabolites in the cytotoxic functions of rat NK cells has also been investigated. Treatment of effector cells with mepacrine or pBPB resulted in complete, irreversible, dose-dependent inhibition of both NK and ADCC activities, which were completely reversed by the addition of exogenous PLA 2 or its hydrolysis products, AA and lysophosphatidylcholine (lysoPC). Among the metabolites of AA released by NK cells, the 5-LO product leukotriene B 4 (LTB 4) seems to play an important role in cytolytic activities of NK cells. Indeed, the LO inhibitor, nordihydroguaiaretic acid (NDGA), totally abrogated both NK and ADCC activities, which were restored by the addition of exogenous LTB 4. However, the failure of LTB 4 to reverse mepacrine or pBPB-induced inhibition of NK and ADCC suggests that its effects could be mediated by PLA 2. The results are consistent with a crucial role for the target-stimulated AA release as a fundamental step in the signal transduction pathway in NK cell. Moreover, LTB 4 generation seems to be responsible for further PLA 2 activation in a second step leading to the amplification of response.