Evidence against Calcium as a Mediator of Mitochondrial Dysfunction during Apoptosis Induced by Arachidonic Acid and Other Free Fatty Acids

Abstract

Apoptosis is often accompanied by activation of phospholipase A2, causing release of free fatty acids (FFAs), which in turn are thought to contribute to the loss of mitochondrial transmembrane potential (Δψm). In these experiments, we asked whether calcium plays a role as an intermediate in this process. A total of 14 FFAs were compared for their ability to cause loss of Δψm and for their ability to affect levels of intracellular calcium. Among the FFAs, unsaturated FFAs tended to induce apoptosis while saturated FFAs did not. Arachidonic acid (AA) was most damaging, causing loss of Δψm and cell death in 8–10 h while linoleic acid, γ-linolenic acid, and docosapentaenoic also strongly induced apoptosis. Effects of the FFAs on levels of intracellular calcium were very different. Many caused strong calcium responses; however, the ability to induce a strong calcium response was not predictive of ability to induce apoptosis, and overall, we did not find a correlation between apoptosis and calcium induction. Also, verapamil and TMB-8 were able to block the calcium response, but these inhibitors did not prevent loss of Δψm, indicating that the calcium response is not necessary for FFA-induced loss of Δψm. In contrast, we found that cyclosporine A could inhibit the AA-induced loss of Δψm with both whole cells and isolated mitochondria, confirming that the antimitochondrial effects of FFA can stem from direct effects on the mitochondrial permeability transition pore. Finally, we show that the strong apoptosis-inducing activity of AA may stem from its ability to selectively induce its own release.