br Results br Discussion Our

Results
Discussion Our studies highlight MUFAs as key lipid modulators of both non-apoptotic and apoptotic cell death. Recent studies show that a more mesenchymal phenotype, de-differentiation, and acquired resistance to targeted inhibitors, can all promote a ferroptosis-sensitive cell state (Hangauer et al., 2017, Tsoi et al., 2018, Viswanathan et al., 2017). Here we show that exposure to exogenous MUFAs can drive teva 3026 mg into a state that makes them resistant to ferroptosis induced by system xc− inhibition, cystine deprivation, or GPX4 inactivation. These results are consistent with the observation that exogenous OA could inhibit cell death in response to the covalent GPX4 inhibitor RSL3 (Yang et al., 2016), and we now provide mechanistic insight into this protective mechanism. We propose that, over time, certain exogenous MUFAs, once activated by ACSL3, can displace PUFAs from PLs located at the plasma membrane and possibly other sites, reducing the sensitivity of these membranes to oxidation and lethal ROS accumulation following GPX4 inactivation (Figure 6F). By contrast, MUFAs inhibit SFA-induced apoptotic lipotoxicity through a distinct mechanism that appears independent of ACSL3, at least in the models examined here. It has been shown that lysosomes, endoplasmic reticulum, and mitochondria are sites of lipid ROS accumulation during ferroptosis (Gaschler et al., 2018, Kagan et al., 2017, Krainz et al., 2016, Torii et al., 2016). We confirm widespread lipid ROS accumulation on internal membranes, most likely including these organelles. However, the ability of exogenous MUFAs to inhibit ferroptosis and preferentially block lipid ROS accumulation at the plasma membrane suggests that the accumulation of lipid ROS at this site is necessary for ferroptosis. One possibility is that a triacsin C-insensitive ACSL3 pool is localized to the plasma membrane where it generates MUFA-CoAs that directly compete with PUFA-CoAs for insertion into lysophospholipids during plasma membrane lipid remodeling. This could explain why exogenous MUFAs inhibit lipid ROS accumulation at the plasma membrane more effectively than at intracellular sites. Lipid ROS accumulation at the plasma membrane may be linked to the blistering of this membrane just prior to frank membrane permeabilization, and may link lipid ROS accumulation to membrane damage and permeabilization during ferroptosis. Whether lipid ROS accumulation at the plasma membrane is sufficient for ferroptosis remains to be determined. For example, it is possible that following GPX4 inactivation lipid ROS first accumulate on one or more internal organelle membranes, and that lipid ROS then spread to the plasma membrane to induce lethal membrane permeabilization. The ability of exogenous MUFAs to inhibit ferroptosis could conceivably be relevant in several contexts in vivo, especially for cells that can extract exogenous MUFAs directly from the bloodstream. In healthy individuals, raising serum MUFA levels could provide a means to prevent ferroptosis in various pathological cell death contexts (e.g., neurodegeneration or kidney failure; Stockwell et al., 2017). However, increased levels of exogenous MUFAs could also enable existing cancer cells to evade ferroptosis. KRAS-driven lung tumors express high levels of ACSL3 (Padanad et al., 2016) and RAS pathway mutations also enable cancer cells to scavenge MUFA-containing lysophospholipids from the environment (Kamphorst et al., 2013). RAS-driven ACSL3 expression and increased lipid scavenging could together drive cancer cells into a more ferroptosis-resistant state. Treatments that inhibit MUFA uptake, activation or de novo synthesis could potentially help overcome this state and improve the lethality of existing pro-ferroptotic agents against cancer cells. The observation that low ACSL3 expression is strongly correlated with sensitivity to ferroptosis-inducing agents across hundreds of cancer cell lines supports the potential utility of such an approach.