Phospholipid hydroperoxide glutathione peroxidase plays a role in protecting cancer cells from docosahexaenoic acid-induced cytotoxicity

Docosahexaenoic acid (DHA; 22:6, n-3) is known to exert cytotoxic effects against various types of tumors via lipid peroxidation. Whereas several enzymes influence the response of cells to oxidative stress, only one enzyme, phospholipid hydroperoxide glutathione peroxidase (GPx4), directly reduces lipid hydroperoxides in mammalian cells. The present study was designed to examine the involvement of GPx-4 in determining the effects of DHA addition to various human cancer cell lines. Although baseline levels of GPx-4 did not correlate with the relative sensitivity of human cancer cell lines to DHA, DHA reduced the level of protein expression of GPx-4 by at least 50% in all six lines. Knockdown of GPx-4 by small interfering RNA technique in a human ovarian cancer cell line significantly enhanced the cytotoxic effect of DHA in a time- and concentration-dependent manner. This cytotoxic effect of DHA was reversed by pretreatment with vitamin E, suggesting that the enhanced toxicitiy of GPx-4 knockdown is due to changes in the abilitiy of the cells to handle oxidative stress. Neither baseline superoxide dismutase-1 nor catalase expression correlated with the relative sensitivity of the cells to DHA treatment. These results illustrate that susceptibility to the oxidative stress imposed by DHA, and possibly other therapeutic agents, is due to complex interactions among multiple andoxidant systems. The modulation of GPx-4 levels by DHA administration is of potential importance and may influence the cellular response to other oxidant stresses.