A study of the apoptotic pathways affected by lipoxygenase enzyme inhibitors

Although oxidative processes and mitochondrial stress have been implicated, the exact mechanisms by which a broad spectrum lipoxygenase (LOX) inhibitor, nordihydroguaiaretic acid (NDGA) and a 5-LOX activating protein (FLAP) inhibitor, MK886 induce apoptosis are poorly understood. The effects of NDGA on the MAP kinase and Akt signaling pathways were investigated. NDGA increased phosphorylation of extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase (JNK), and p38 in murine pro-B lymphocytes (FL5.12 cells). Using pharmacologic inhibitors, it was found that only p38 was involved in the induction of apoptosis by NDGA. The antioxidant N acetylcysteine (NAC) attenuated NDGA-driven p38 phosphorylation, indicating involvement of a redox sensitive target(s) upstream of p38. NAC pretreatment also inhibited NDGA-induced release of cytochrome c and caspase-3 cleavage. Further experimentation revealed that NDGA inhibited phosphorylation of the survival kinase Akt at the Ser473 and Thr308 sites in both FL5.12 cells and human Jurkat T lymphocytes. NDGA treatment also activated the FOXO3a transcription factor, which is usually maintained in an inactive state via its phosphorylation by Akt. Overexpression of constitutively active Akt conferred some protection against NDGA-induced apoptosis in FL5.12 cells. The concentration of NDGA that induced apoptosis was found to be 5 times lower than the IC50 for inhibiting phosphoinositide-dependent kinase 1, the primary activator of Akt, suggesting involvement of other kinases. These data suggest that several independent mechanisms including oxidative reactions, p38 kinase activation, Akt activation, and cytochrome c release contribute to NDGA-induced apoptosis in FL5.12 cells. In contrast to its protective effects on NDGA-induced apoptosis in Jurkat cells, NAC enhanced apoptosis induced by MK886. This enhancement involved augmentation of mitochondrial membrane depolarization and significantly increased caspase-3 activity, implicating the mitochondrial pathway. The fact that neutralized NAC was also able to enhance MK886-induced apoptosis suggested that NAC might have direct effects on other apoptotic pathways initiated by MK886. These data support the conclusion that, in addition to LOX and FLAP inhibitory activities, NDGA and MK886 induce apoptosis via several independent mechanisms.