Oxidant-Induced Cell Death Mediated By A Rho Gtpase In Saccharomyces cerevisiae

Rho GTPases comprise a subgroup of the Ras superfamily of GTPases that controls a wide range of cellular processes by participating in processes such as actin cytoskeleton organization, phagocytosis, cell adhesion, cell-cycle progression and cell survival (Reviewed by Ridley, 2001). In budding yeast, Rho5 is a less characterized member of the Rho family of GTPase, and it shares considerable homology (45%) with Rac GTPase (another member of the Rho family of GTPases). One of the better-established functions of Rac is activation of NADPH oxidase in neutrophils that is required for killing microbes (Abo et al., 1991). The Rho family of proteins have a unique “Rho insert region” and this domain has been implicated in Rac mediated NADPH oxidase activation (Freeman et al., 1996). Historically, these oxidase were thought to function specifically in immune cells. However, in recent years, studies have shown that superoxides and ROS (reactive oxygen species) have important roles as signaling molecules, and they regulate a number of biological processes (reviewed by Werner, 2004, Veal et al., 2007). Although there are a considerable number of reports on the events associated with the oxidative stress response, and the role of Rho GTPases in the production of ROS, the mechanism of sensing and oxidant-mediated signaling remains to be fully established.

In this study, the role of the Rho5 GTPase in hydrogen peroxide (H₂O₂)-induced cell death was explored. The presence of functional Rho5 in cells was found to confer sensitivity to various oxidants, including H₂O₂. Cells lacking Rho5 (i.e. Rho5-GTP bound form) exhibited much less cell death when exposed to H₂O₂, suggesting the presence of a Rho5-mediated cell death program. This Rho5-mediated cell death process was triggered by H₂O₂ and exhibited some hallmarks of mammalian apoptosis such as ROS accumulation and DNA fragmentation. Additionally, Rho5 was also found to interact with Trr1 (Thioredoxin reductase), an important component of the thioredoxin antioxidant system. Interestingly, Rho1 GTPase another member of the Rho family of GTPases was found to facilitate cell survival upon exposure to H₂O₂. These observations suggest that Rho GTPases control different cell fates depending on the oxidative stress level in promoting cell survival or cell death. This study uncovers a new role of a Rho GTPase in oxidant-induced cell death in yeast. This work also provides evidence for Rho5 interaction with Trr1, which is a novel link between a Rho GTPase and an antioxidant component.

This study raises important questions about the extent to which the apoptotic cell death machinery is conserved from yeast through higher multicellular systems, and also about the benefits that a unicellular organism like yeast gains by retaining this function. This study has opened avenues to explore the above possibilities and hopefully identify more intracellular players functioning in the oxidative stress response and cell death processes. Identification of players that have homologous partners in higher systems will help determine the extent to which these cellular processes are conserved from yeast to humans.