Feeling the Squeeze: Live-Cell Extrusion Limits Cell Density in Epithelia

bind both of these cellular targets to enable ferroptosis? Previous studies showed that erastin inhibits VDACmediated transport of NADH into mitochondria, thus raising the possibility that erastin’s effects on VDAC may help to elevate cytosolic levels of NADPH to support the activity of cytosolic oxidases that generate lethal levels of ROS (Yagoda et al., 2007; Yang and Stockwell, 2008). Consequently, erastin might promote ROS production both by influencing the activity of NADPHrequiring oxidases and by reducing cystine-dependent glutathione production via decreased cystine transport, thereby lowering antioxidant defenses. However, given that ROS have been implicated in so many forms of cell death, one wonders whether ‘‘pure’’ ferroptosis is easily discerned, which is a question that would potentially yield to single-cell analysis rather than relying on bulk cell populations. The practical utility of chemical biology efforts that have led to the discovery of compounds such as erastin, which promotes iron-dependent oxidative killing of RAS-expressing cancer cells, is yet to be determined. Oncogenic RAS mutations occur in 20% of human cancers, but RAS itself has failed to yield to drug discovery efforts, prompting interest in downstream targets of the RAS pathway. An analog of erastin (PRLX-93936) was tested in phase I trials but was poorly tolerated, raising concerns about therapeutic index (Ramanathan et al., 2010). The FDA-approved drug Lanperisone has been reported to kill RAS-transformed tumor cells through a similar ironand ROS-dependent mechanism (Shaw et al., 2011), suggesting a potential drug repurposing opportunity. While sulfasalazine (another approved medicine) also inhibits cystine transport, its potency is probably too weak to trigger ferroptosis in vivo. More work is needed toward understanding the mechanism(s) of ferroptosis so that optimal targets can be identified for future drug discovery efforts.