Lysophosphatidylcholine and arachidonic acid are required in the cytotoxic response of human natural killer cells to tumor target cells.

Treatment of human natural killer (NK) cells with phospholipase A(2) (PLA(2)) inhibitors, mepacrine and 4-bromophenacyl bromide (BPB), diminished their ability to lyse K562 target cells by as much as 100%. The ability of NK cells to bind to K562 cells was significantly affected by BPB above 2 microM, but not by mepacrine at any concentration tested. This indicates that BPB is having effects on NK cells unrelated to its inhibition of PLA(2) activity at concentrations above 2 microM. The activation of phospholipase C in response to K562 cell binding (as measured by inositol phosphate turnover) was unaffected by inhibition of the PLA(2) activity. The products of PLA(2) catabolism are a fatty acid (often arachidonic acid) and a lysophospholipid. Inhibition of NK cytotoxicity by mepacrine or BPB is reversed significantly when lysophosphatidylcholine, but no other lysolipid, is added back to the NK cells before assaying for cytotoxicity. Arachidonic acid, but not linoleic acid, also significantly reverses inhibition of NK cytotoxicity. Finally, the 15-lipoxygenase product, 15S-hydroperoxyeicosatetraenoic acid (15S-HPETE), is also able to reverse mepacrine-induced inhibition of NK cytotoxicity. The 5-lipoxygenase product 5S-HPETE was not effective. These data indicate that PLA(2) activation is a necessary signal in human NK cytotoxicity and that it is not involved in protein tyrosine kinase and subsequent phospholipase C activation; these latter two enzymes are also required in the cytotoxic response. Thus PLA(2) activation is either a more distal signal, dependent on activation of some earlier signal, or an independent cosignal stimulated by tumor-target binding which generates lysophosphatidylcholine, arachidonic acid, and/or a lipoxygenase product(s). Copyright 1999 S. Karger AG, Basel