Tumour‐necrosis‐factor‐mediated cytotoxicity is correlated with phospholipase‐A2 activity, but not with arachidonic acid release per se

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Abstract
L929, a murine fibrosarcoma cell line highly sensitive to the anti-proliferative and cytotoxic action of tumour necrosis factor (TNF), was used as a target cell in our studies. We [Suffys et al. (1987) Biochem. Biophys. Res. Commun. 149, 735–743], as well as others, have previously provided evidence that a phospholipase (PL), most probably a PL-A2-type enzyme, is likely to be involved in TNF-mediated cell killing. We now further document this conclusion and provide suggestive evidence that the enzyme activity specifically involved in TNF cytotoxicity differs from activities associated with the eventual cell death process itself or with non-toxic serum treatment. We also show that the 5,8,11,14-icosatetraenoic acid (arachidonic acid, 4Ach) released by PL, and possibly metabolized, is unlikely to be a key mediator of the TNF-mediated cytotoxicity. These conclusions are based on the following experimental findings. 1 TNF treatment of cells, prelabelled for 24 h with [3H]4Ach or [14C]3Ach (3Ach ≡ 5,8,11-icosatrienoic acid) resulted in an early, time-dependent and concentration-dependent release of radioactivity in the supernatant preceding actual cell death. The extent of this response was moderate, albeit reproducible and significant. Analysis of the total lipid fraction from cells plus supernatant revealed that only release of arachidonic acid from phospholipids, but not its metabolization was induced by TNF. However, the release of less unsaturated fatty acids, such as linoleic acid (Lin) or palmitic acid (Pam), was not affected during the first hours after TNF addition. 2 An L929 subclone, selected for resistance to TNF toxicity, was found to be defective in TNF-induced 4Ach liberation. 3 Interleukin-1 (IL1) was not cytotoxic for L929 and did not induce release of 4Ach. 4 Release of 4Ach was not restricted to TNF; the addition of serum to the cells also induced release of fatty acids into the medium. In this case, however, there was no specificity, as all fatty acids tested, including Lin and Pam, were released. 5 Inhibition of PL-A2 activity by appropriate drugs markedly diminished TNF-induced 4Ach release and resulted also in a strong decrease in TNF-induced cytotoxicity. 6 Other drugs, including serine protease inhibitors, which strongly inhibit TNF-induced cytotoxicity, also decreased the TNF-induced 4Ach release, whereas LiCl potentiated both TNF-mediated effects. 7 Protection of cells against TNF toxicity by means of various inhibitors was not counteracted by addition of exogenous fatty acids, including 4Ach. 8 We could not detect an increase in the amount of free inositol or any inositol phosphate after TNF treatment of cells, prelabelled with myo-[3H]inositol. This indicates that the phosphatidylinositol-specific PL-C is not involved in the TNF-induced fatty acid release. 9 Neither were we able to observe a decrease in fatty acid incorporation into phospholipids during TNF treatment. This excludes a decrease in acyltransferase activity as the reason for increased levels of free fatty acid. The above observations strongly suggest that an activated PL-A2 is involved in TNF-mediated cell killing. The limited extent of TNF-specific fatty acid release, the relative specificity for 4Ach of this early response which is not shared by serum treatment, and the characteristic drug-resistance profile of this activity, all argue for the involvement of a particular PL-A2 species. Moreover, our data also suggest that 4Ach itself and its metabolites do not play a key role in TNF cytotoxicity.

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