ToxWatch Cite This: Chem. Res. Toxicol. XXXX, XXX, XXX−XXX
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Working Together: Redox Signaling between the Endoplasmic Reticulum and Mitochondria Kevin D. Siegenthaler and Carolyn S. Sevier* Department of Molecular Medicine, Cornell University, Ithaca, New York 14853, United States
Chem. Res. Toxicol. Downloaded from pubs.acs.org by 95.181.217.90 on 02/05/19. For personal use only.
ABSTRACT: The concept that reactive oxygen species (ROS) are primarily toxic, mitochondria-generated molecules has persisted for decades. Here we highlight the emerging complexity for ROS-based events, emphasizing the evolving importance of the endoplasmic reticulum as a source and platform for redox signaling.
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of hydrogen peroxide per minute, as a byproduct of insulin production alone.1 Like the mitochondria, the ER contains various systems to limit ROS accumulation. In mammals, an ER-localized peroxiredoxin (PRX4) and a glutathione peroxidase (GPX8) accept electrons from PDI and in turn donate these electrons to reduce hydrogen peroxide to water. Resourcefully, these proteins use the peroxide generated by ERO1 to facilitate a second round of nascent disulfide bond formation (via PDI).2 Additional ER sources of ROS include members of the NADPH oxidase (NOX) family and microsomal monooxygenases (MMOs). Although most NOX enzymes have been localized to the plasma membrane, NOX enzymes have been identified also within the mitochondria, nucleus, and ER. Specifically, human NOX4 and yeast NADPH oxidase 1 (Yno1) are found associated with the ER membrane. NOX enzymes are polytopic membrane proteins that use electrons from NADPH to catalyze the reduction of oxygen to superoxide. The superoxide (or hydrogen peroxide) formed by ER NOX enzymes can influence a myriad of cell signaling and host defense pathways.3 MMOs are multienzyme systems that connect ROS generation to a variety of actions, including the oxidative metabolism of xenobiotics and the oxygenation of endogenous cellular substrates (e.g., heme and fatty acids). As its core, an MMO contains a NADPH-cytochrome P450 reductase and a cytochrome P450, which serves as the terminal oxidase. The abundant levels of MMO protein components in cells have led to the suggestion that these systems may account for a significant fraction of endogenously produced ROS. Redox Interplay: Redox-Crosstalk between the ER and Mitochondria. While the ER as a source of ROS may be unfamiliar to many, it is arguably even less appreciated that there is a significant redox interplay between the various cellular sources of ROS. Here we highlight examples of redox crosstalk; these examples are a just few of many redox feedback situations involving the ER that have recently appeared in the literature. The complex inter-relationship between the various sources of cellular ROS is aptly illustrated through the emerging role for redox signaling in calcium homeostasis. The ER is a well-
eactive oxygen species (ROS) are well recognized for their reactivity and potential cellular toxicity. The mitochondrial electron-transport chain (ETC) was identified early on as a major site of cellular ROS production, and over the last 50 years, ROS generated by the mitochondrial ETC have been highlighted for their negative impact on aging and their potential to accelerate various neurodegenerative pathologies. More recently, a second role for low levels of cellular ROS in redox signaling and normal cell function was revealed. An importance for ROS in redox signaling has been widely embraced, and numerous functions for ROS in promoting cell growth and cell death pathways have been uncovered. The roles for ROS in both cell damage and cell signaling have sparked a renewed interest in determining how to harness the range of ROS activities to develop successful therapeutic strategies. While the significance and complexity of the roles for ROS in cell physiology appear generally accepted, we argue that it remains less appreciated that mitochondria are not the sole (and not necessarily the major) source of cellular ROS. Specifically, here we highlight the endoplasmic reticulum (ER) as a significant source of ROS and the importance for the ER as a platform capable of initiating, receiving, and transmitting redox signals, which can also further influence cellular ROS dynamics via mitochondrial pathways. We propose that it is essential to recognize and consider all sources of cellular ROS to facilitate the development of successful future therapeutics. ER-Derived Sources of ROS: Beyond the Mitochondrial ETC. The ER is an organelle known for its importance in protein folding. As part of the ER folding process, disulfide bonds are formed between adjacent cysteine residues in many secretory proteins; this basic process generates ROS (Figure 1).1 Electrons lost during the formation of a disulfide bond in a nascent secretory protein are transferred between two ER enzymes (PDI and ERO1), which can be thought of as an electron transport chain similar to the mitochondrial ETC. Electrons are ultimately transferred from ERO1 to molecular oxygen, forming hydrogen peroxide. Although absolute concentrations of ROS generated via oxidative folding remain to be determined, various back-of-the-envelope calculations suggest the potential for copious amounts of cellular ROS. For example, extrapolating from the established secretion rates for insulin (a protein with three disulfide bonds), one anticipates that a stimulated β-cell would generate three million molecules © XXXX American Chemical Society
Special Issue: Redox Pathways in Chemical Toxicology
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DOI: 10.1021/acs.chemrestox.8b00379 Chem. Res. Toxicol. XXXX, XXX, XXX−XXX
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Chemical Research in Toxicology
Figure 1.
dysfunction and peroxide generation by an ER-localized NOX.5 These data again emphasize the potential complexity as to the origins of ROS and the importance of the ER. The authors observed that reduced cytochrome c oxidase (COX) function leads to an increase in cellular ROS. It was anticipated that increases in superoxide originated from the mitochondrial ETC; yet, the source was instead traced to the ER-localized NOX Yno1p. A suppression of Yno1p protein turnover was observed in cells lacking COX activity; the authors postulate that the elevated ROS levels in a COX-deficient strain are a consequence of the unchecked production of superoxide by the stabilized ER-localized Yno1p. Furthermore, the authors suggest the generated superoxide persists as hydrogen peroxide due to higher levels of superoxide dismutase (Sod1), and an absence of catalase (which breaks peroxide down to water), in the COX-deficient cells. Last but not least, we bring attention to the vicious redox cycles that can connect oxidative folding and ROS accumulation within the ER. The protein folding capacity of the ER is buffered through the activity of the unfolded protein response (UPR): a cellular stress response pathway that augments chaperone levels when the ER folding capacity is compromised.1 Paradoxically, ROS seem to amplify within the ER upon UPR induction, although the UPR is expected to lessen ER stress.1,2 At face value, ROS production seems to be an inadvertent consequence of reestablishing ER homeostasis; the increase in ER ROS is a likely byproduct of increased levels of ERO1-PDI, and also NOX enzymes, that are UPR targets. Yet, data suggest that elevated ROS may not only be a consequence of UPR induction but also may act to signal and tune the cellular stress response. For example, ROS-mediated oxidation of cysteines within UPR sensors can augment or diminish UPR
established calcium store. Calcium is actively pumped into the ER from the cytoplasm by the sarco/endoplasmic reticulum calcium ATPase (SERCA). When stimulated, calcium release from the ER occurs via the ryanodine receptor (RyR) and the inositol 1,4,5-triphosphate receptor (IP3R). Notably, the activities of all three proteins can be regulated through the oxidation of lumenal cysteines by ROS; oxidation of these transporters (as a consequence of elevated ROS) results in a net efflux of calcium from the ER into the cytoplasm.2 The complex interplay between ROS, calcium, the ER, and the mitochondria is nicely illustrated by recent work of Booth and co-workers.4 These authors followed the impact of calcium released from the ER, stimulated by an IP3 agonist or a SERCA inhibitor; a similar calcium efflux is observed upon oxidative modification of the IP3R or SERCA (Figure 1). The authors established that calcium released from the ER was taken up by the mitochondria, leading to a stimulation of the mitochondrial ETC and an accumulation of mitochondrial peroxide. Mitochondrial calcium uptake led also to a compression of the mitochondrial cristae, which the authors connected to a forced release of peroxide from the mitochondria into the ER− mitochondria interface.4 Notably, the release of mitochondrial peroxide brought about sustained calcium oscillations, due to a successive release of ER calcium (the efflux of ER calcium here was anticipated to be a byproduct of ROS-mediated thioloxidation of the ER calcium receptors, although this remains to be confirmed).4 It has been suggested that redox events connecting the ER and mitochondria can culminate in cell death through apoptosis, allowing for the clearance of cells with excessive ROS levels. An intriguing study from Leadsham and co-workers highlights a distinct but clear interplay between mitochondrial B
DOI: 10.1021/acs.chemrestox.8b00379 Chem. Res. Toxicol. XXXX, XXX, XXX−XXX
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Chemical Research in Toxicology signaling.2 Cells that show a prolonged UPR activation (failing to reestablish ER homeostasis) may co-opt mitochondrial activities to initiate cell death, by triggering ER calcium release to facilitate apoptosis. Summary. The complexity and intricacy of the highlighted cellular redox-signaling networks underscores the need to consider ROS sources and signaling events outside the mitochondria in therapeutic strategies (including the ER). Moreover, the extensive signaling pathways that work to correct redox imbalance within cells suggest that approaches that aim to blindly deplete all cellular ROS may be ineffective due to the potential disruption of important signaling systems. We argue that future attempts to down-regulate select sources of cellular ROS may be the key to limit excessive ROS accumulation, yet these selective approaches must be considered carefully for their potential to influence important redox signaling events through interconnected redox pathways.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Carolyn S. Sevier: 0000-0003-3245-6988 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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REFERENCES
(1) Shimizu, Y., and Hendershot, L. M. (2009) Oxidative folding: cellular strategies for dealing with the resultant equimolar production of reactive oxygen species. Antioxid. Redox Signaling 11, 2317−2331. (2) Eletto, D., Chevet, E., Argon, Y., and Appenzeller-Herzog, C. (2014) Redox controls UPR to control redox. J. Cell Sci. 127, 3649− 3658. (3) Laurindo, F. R., Araujo, T. L., and Abrahão, T. B. (2014) Nox NADPH oxidases and the endoplasmic reticulum. Antioxid. Redox Signaling 20, 2755−2775. (4) Booth, D. M., Enyedi, B., Geiszt, M., Várnai, P., and Hajnóczky, G. (2016) Redox nanodomains are induced by and control calcium signaling at the ER-mitochondrial interface. Mol. Cell 63, 240−248. (5) Leadsham, J. E., Sanders, G., Giannaki, S., Bastow, E. L., Hutton, R., Naeimi, W. R., Breitenbach, M., and Gourlay, C. W. (2013) Loss of cytochrome c oxidase promotes RAS-dependent ROS production from the ER resident NADPH oxidase, Yno1p, in yeast. Cell Metab. 18, 279−286.
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DOI: 10.1021/acs.chemrestox.8b00379 Chem. Res. Toxicol. XXXX, XXX, XXX−XXX
