The 5-lipoxygenase-activating protein (FLAP) inhibitor, MK886, induces apoptosis independently of FLAP

The ability of various inhibitors of lipoxygenase (LOX) enzymes and 5-lipox ygenase-activating protein (FLAP) to induce apoptosis has implicated these pathways in the mechanism(s) of this form of cell death. Although FLAP play s an important role in 5-LOX activity, this protein is found at high levels in some cells lacking LOX, suggesting it might mediate other effects. Furt hermore, the concentration of MK886, a FLAP inhibitor, required to induce a poptosis is approximate to 100-fold more than that required to inhibit LOX, and this compound remains effective in cells lacking LOX. The present stud y examines the role of FLAP in MK886-induced apoptosis. MK886 induced apopt osis in WSU cells, a human chronic lymphocytic leukaemia cell line that lac ks FLAP protein and mRNA, suggesting that this agent is acting independentl y of FLAP. This conclusion was further supported by the fact that a more sp ecific FLAP inhibitor, MK591, induced only minimal apoptosis in FL5.12 cell s, a murine prolymphoid cell line containing FLAP. The role of FLAP was exa mined more directly by decreasing its expression by more than 50%;, in FL5. 12 cells treated with 10 mu M of an antisense oligonucleotide for 48 h. Thi s change in FLAP was not accompanied by any increase in apoptosis. Furtherm ore, FLAP-depleted cells exhibited the same level of apoptosis 8 h after tr eatment with 10 mu M MK886, as did control cells. The increased fluorescenc e seen in MK886-treated cells loaded with carboxydichlorofluorescein indica tes that oxidative reactions are stimulated by this compound, possibly via the release of fatty acids from fatty acid-binding proteins and their subse quent oxidation.