Jcb_201612078 303..304

303 The Rockefeller University Press $30.00 J. Cell Biol. Vol. 216 No. 2 303–304 https://doi.org/10.1083/jcb.201612078 A decade ago, apoptosis was assumed to be the exclusive mode of cellular demise that followed genetically determined signaling pathways. Today, however, we understand that in most pathophysiological circumstances, necrosis also represents a regulated process of cell death that can follow distinct signaling pathways that define necroptosis, pyroptosis, or ferroptosis (Vanden Berghe et al., 2014). In contrast to apoptosis, in which the contents of dying cells remain sequestered within apoptotic bodies, necrosis is characterized by the rupture of the plasma membrane and the release of damage-associated molecular patterns that may elicit an immune response. Different effector molecules execute the endpoint of these pathways of regulated necrosis; for example, membrane rupture during necroptosis and pyroptosis may be mediated by pore formation driven by the mixed lineage kinase domain-like protein (Sun et al., 2012) and gasdermin D (Ding et al., 2016), respectively. In contrast, ferroptosis results from the iron-dependent generation of toxic lipid reactive oxygen species (ROS) in the plasma membrane by lipoxygenases when glutathione peroxidase activity is diminished (Dixon et al., 2012; Yang et al., 2014). Whether or not ferroptotic cell death involves formation of a specific protein membrane pore is unknown. Therefore, these three pathways of regulated necrosis are clearly distinct but how their upstream signaling pathways may be intercalated and how they differ in their effects on the immune system are currently matters of debate (Vanden Berghe et al., 2014). The reason for the conservation of several regulated pathways of necrosis is unclear and, potentially, other unknown pathways may exist. As a means to protect the organism in general, necrosis may appear counterintuitive at first glance given that the dying cells violently burst. However, as with many cellular processes that arise during evolution, host microbe interactions may be key to our understanding. Indeed, the most likely reason for the evolutionary conservation of pathways of regulated necrosis is for defense against microbes as it is very clear that necroptosis functions to fight viruses (Kaiser et al., 2013) and pyroptosis defends against bacteria (Lamkanfi and Dixit, 2010). However, why and how we have conserved the ferroptosis pathway currently remains elusive as a function to defend against microbes has not yet been convincingly demonstrated. Instead, ferroptosis can be induced artificially as a potent driver of tumor cell death (Dixon et al., 2012) and by ischemic injury during kidney damage or transplantation (Linkermann et al., 2014). But why would eukaryotes preserve such a dangerous program for cell death in their genome? In this issue, Distéfano et al. add important insights to our understanding of this question by identifying ferroptosis-like death by plant cells in response to a more physiological stimulus: heat stress. Distéfano et al. (2017) used models of plant stress and cell death in Arabidopsis thaliana to ask whether a process similar to ferroptosis might occur because plant cell death processes can be necrotic and driven by ROS (Huysmans et al., 2017). Whereas plant cell death during root development or reproduction was independent of any obvious ferroptosis-like features, heat shock–induced regulated cell death (HS-RCD) exhibited striking similarities to ferroptosis. When 6-d-old seedlings were pretreated with ferrostatins (Fer-1), small molecules that inhibit ferroptosis by blocking lipid ROS (Dixon et al., 2012), the plant cell death in root hairs that normally occurs when they are subjected to a temperature of 55°C was completely prevented. In contrast, ferrostatins had no effect on the induction of cell death at 77°C, or by H2O2 or high salt treatment, which are thought to initiate nonregulated forms of oxidative cellular necrosis. HS-RCD at 55°C could also be inhibited by treatment of plant cells with the iron chelator ciclopiroxolamine or polyunsaturated fatty acids resistant to oxidation, just like ferroptosis in mammalian cells. Closer examination of the morphology of root cortical cells by transmission electron microscopy revealed that the cytoplasm of plant cells dying by HS-RCD at 55°C had retracted from the cell wall and accumulated lytic vacuoles, and the mitochondria displayed a more condensed matrix, which resembled changes in the mitochondria of mammalian tumor cells undergoing ferroptosis (Dixon et al., 2012). Distéfano et al. (2017) observed that HS-RCD occurred in the parts of the root that are known to contain more iron and correlated with the disappearance of the antioxidant reduced glutathione (GSH) and the appearance of lipid and cytoplasmic ROS. In line with a critical role for GSH in preventing ferroptosis, inhibition of GSH biosynthesis accelerated HS-RCD, whereas the exogenous application of GSH to plants blocked HS-RCD as effectively as the ferrostatins. Given the striking overlap in these requirements for iron and ROS to trigger HS-RCD in plant cells with mammalian ferroptosis, the authors termed this form of In recent years, our knowledge of how cells die by regulated pathways of necrosis has increased tremendously. In this issue, Distéfano et al. (2017. J. Cell Biol. https ://doi .org /10 .1083 /jcb .201605110) provide yet another milestone in our understanding of regulated necrosis as they identify a ferroptosis-like cell death in Arabidopsis thaliana. Back to the roots of regulated necrosis