Free docosahexaenoic acid promotes ferroptotic cell death via lipoxygenase dependent and independent pathways in cancer cells.

PURPOSE: Ferroptosis is a form of regulated cell death that has the potential to be targeted as a cancer therapeutic strategy. But cancer cells have a wide range of sensitivities to ferroptosis, which limits its therapeutic potential. Accumulation of lipid peroxides determines the occurrence of ferroptosis. However, the type of lipid involved in peroxidation and the mechanism of lipid peroxide accumulation are less studied. METHODS: The effects of fatty acids (10 µM) with different carbon chain length and unsaturation on ferroptosis were evaluated by MTT and LDH release assay in cell lines derived from prostate cancer (PC3, 22RV1, DU145 and LNCaP), colorectal cancer (HT-29), cervical cancer (HeLa) and liver cancer (HepG2). Inhibitors of apoptosis, necroptosis, autophagy and ferroptosis were used to determine the type of cell death. Then the regulation of reactive oxygen species (ROS) and lipid peroxidation by docosahexaenoic acid (DHA) was measured by HPLC-MS and flow cytometry. The avtive form of DHA was determined by siRNA mediated gene silencing. The role of lipoxygenases was checked by inhibitors and gene silencing. Finally, the effect of DHA on ferroptosis-mediated tumor killing was verified in xenografts. RESULTS: The sensitivity of ferroptosis was positively correlated with the unsaturation of exogenously added fatty acid. DHA (22:6 n-3) sensitized cancer cells to ferroptosis-inducing reagents (FINs) at the highest level in vitro and in vivo. In this process, DHA increased ROS accumulation, lipid peroxidation and protein oxidation independent of its membrane receptor, GPR120. Inhibition of long chain fatty acid-CoA ligases and lysophosphatidylcholine acyltransferases didn't affect the role of DHA. DHA-involved ferroptosis can be induced in both arachidonate lipoxygenase 5 (ALOX5) negative and positive cells. Down regulation of ALOX5 inhibited ferroptosis, while overexpression of ALOX5 promoted ferroptosis. CONCLUSION: DHA can effectively promote ferroptosis-mediated tumor killing by increasing intracellular lipid peroxidation. Both ALOX5 dependent and independent pathways are involved in DHA-FIN induced ferroptosis. And during this process, free DHA plays an important role.

Do lipoxygenases modulate normal or aberrant lympho-hematopoiesis?

Whether 5- (and the 12- or 15-) lipoxygenases participate in normal or malignantly transformed hematopoietic cell proliferation and differentiation, or contribute to programmed or necrotic cell death has been difficult to decide. Recent evidence concerning these questions is reviewed and some reasons for these difficulties are considered.

Mitochondrial free radical production induces lipid peroxidation during myohemoglobinuria.

Iron catalyzed free radical formation and lipid peroxidation are accepted mechanisms of heme protein-induced acute renal failure. However, the source(s) of those free radicals which trigger lipid peroxidation in proximal tubular cells remains unknown. This study tested the potential involvement of mitochondrial electron transport, xanthine oxidase activity, and arachidonic acid metabolism in the heme-induced peroxidative state. The impact of cytosolic Ca2+ loading also was assessed. Rhabdomyolysis was induced in mice by glycerol injection, and two hours later heme-laden proximal tubular segments (PTS) were isolated for study. PTS from normal mice served as controls. During 30 to 60 minute incubations, heme loaded PTS developed progressive cytotoxicity (LDH release) and iron-dependent lipid peroxidation (malondialdehyde, MDA, generation; inhibited by deferoxamine). Site 2 (antimycin A) or site 3 (cyanide, hypoxia) mitochondrial respiratory chain inhibition completely blocked lipid peroxidation, whereas site 1 inhibition (rotenone) doubled its extent (presumably by shunting NADH through NADH dehydrogenase, a free radical generating system). Conversely, these agents did not substantially alter MDA in normal PTS. Normal and heme loaded PTS developed comparable degrees of LDH release during respiratory blockade irrespective of increased or decreased MDA production (indicating that lipid peroxidation was not a critical determinant of cell death). Neither increasing free arachidonic acid (PLA2 treatment) nor adding cyclooxygenase/lipoxygenase/cytochrome p450 inhibitors conferred a consistent protective effect. Altering free Ca2+ status (chelators; ionophore addition) and xanthine oxidase inhibition had no discernible impacts. Despite mitochondrial free radical production, mitochondrial function, as assessed by the ATP/ADP ratio, seemingly remained intact. In conclusion, (1) the terminal mitochondrial respiratory chain is the dominant source of free radicals which trigger PTS lipid peroxidation; (2) iron is a required secondary factor; (3) although mitochondria fuel lipid peroxidation, they do not appear to be critical targets of the heme-induced oxidant attack.