Is “Preparation for Oxidative Stress” a Case of Physiological Conditioning Hormesis?

Many animal species endure hypoxic or even anoxic stresses, when faced with harsh environmental conditions including freezing, severe dehydration and air exposure of aquatic organisms. Hypoxia in those animals induces a set of physiological/biochemical adaptive responses, allowing organisms to cope with low oxygen levels. Such responses are mediated by (i) arrest of transcriptional and translational activity, (ii) depression of metabolic rate, (iii) re-wiring of energy metabolism pathways toward fermentative rather than oxidative routes, (iv) activation of mechanisms involved in both macromolecular repair and detoxification of cellular-derived oxidants (Storey and Storey, 2011; Storey, 2015). In this regard, a transient up-regulation of endogenous antioxidant enzymes aiming the improvement of reactive species (RS) detoxification has emerged as a hallmark for many organisms to tolerate hypoxic stresses. Such phenomenon was coined “preparation for oxidative stress” (POS) 20 years ago, and numerous examples have supported POS as a physiological mechanism to deal with environmental stresses (Hermes-Lima and Storey, 1995, 1996; Hermes-Lima et al., 1998, 2015; Hermes-Lima and Zenteno-Savin, 2002; Lushchak et al., 2005; Welker et al., 2013). So far, we have identified POS as an adaptive physiological mechanism in 83 animal species from 8 different phyla when exposed to low oxygen stress and during estivation (Moreira et al., 2016, 2017). The phenotypes generated by POS include the up-regulation of superoxide dismutase (SOD), catalase and glutathione transferase (GST) activities by ~80% in Otala lactea snails during estivation (Hermes-Lima and Storey, 1995). Interestingly, snails that return to active state decrease all antioxidant enzyme activities to pre-estivation levels. Similar observations were reported when Rana pipiens frogs were challenged with 30 h anoxia, causing transient catalase, and GST activation (Hermes-Lima and Storey, 1996). Also, transient increases of catalase and glutathione peroxidase (GPX) activities by 30–70% were observed in the brain of common carp during hypoxia (Lushchak et al., 2005). Increases by ~60% in muscular SOD activity were also observed in Lacerta vivipara lizards upon freezing, which returns to control levels after thawing (Voituron et al., 2006). Evidence suggests the existence of common mechanisms underlying dormancy states induced by hypoxia, hypoxic-like conditions and aerobic hypometabolism. For example, it is known that hypoxia maintains the redox state of mitochondrial electron transport system (ETS) toward a reduced state, favoring the production of superoxide radicals (Chandel et al., 1998; Vanden Hoek et al., 1998; Hernansanz-Agustin et al., 2014). Thus, against the common-sense, reduced oxygenation increases, rather than decreases, cellular oxidants production (Murphy, 2009; Smith et al., 2017; see legend of Figure ​Figure1A).1A). Accordingly, the proposed mechanism by which POS confers tolerance to oxidant insults, considers an increase in mitochondrial RS formation during low oxygen stress, followed by redox imbalance that activates redox-sensitive transcription factors, such as NF-κB, FoxOs, and Nrf2 (Schreck et al., 1991; Ishii et al., 2000; Essers et al., 2004). Additionally, redox imbalance also shifts protein phosphorylation levels toward a higher phosphorylated state, by either reducing protein phosphatase and/or increasing protein kinase activities (Staal et al., 1994; Meng et al., 2002; Howe et al., 2004; Corcoran and Cotter, 2013) (Figure ​(Figure1A).1A). In this regard, oxidants can inhibit multiple protein tyrosine phosphatases including PTP1B and PTEN (Leslie et al., 2003; Salmeen et al., 2003), with direct consequences to cell function. Conversely, oxidant conditions activate several protein kinases such as Src (Devary et al., 1992), MAPK (Goldstone and Hunt, 1997) and calcium/calmodulin-dependent protein kinases (Howe et al., 2004). However, it seems that maintenance of the higher phosphorylated state of protein targets by redox imbalance may occur through protein phosphatase inhibition rather than direct protein kinase activation by oxidants (Lee and Esselman, 2002). The consequences of higher protein phosphorylation to cellular redox homeostasis are: (i) the activation of redox-sensitive transcription factors (Shirakawa and Mizel, 1989), and/or (ii) regulation of antioxidant enzymes activities by direct phosphorylation. Examples include the demonstration that Nrf2 expression depends on low PTEN phosphatase activity, rendering tumor cells more proliferative (Rojo et al., 2014). Likewise, maintenance of oxidant conditions indirectly activates antioxidant enzymes through their phosphorylation, acting independently of redox-sensitive transcription factors (Rhee and Woo, 2011; Rafikov et al., 2014; Tsang et al., 2014). Ultimately, higher tolerance to multiple redox stresses is afforded by increasing endogenous antioxidant levels mediated by either activation of redox-sensitive transcription factors or by activation of antioxidant enzymes through phosphorylation or other covalent modifications (Figure ​(Figure1A1A). Open in a separate window Figure 1 Molecular oxygen is absolutely required for maintenance of cellular energy and redox homeostasis across different animal species. Although some organisms cannot tolerate slight hypoxia, others can adapt to and survive strong shortages in oxygen supply even for long periods of time. A common trend observed in some hypoxia-tolerant animals is their enhanced capacity to boost antioxidant defenses during a number of stresses, a phenomenon known as “preparation for oxidative stress” (POS). POS was identified in animals from 8 distinct phyla and despite the molecular mechanisms are not fully understood, we have recently proposed an explanation (Hermes-Lima et al., 2015), where the role of phosphatases and kinases in POS is highlighted herein, as well as the increased cellular oxidant production under hypoxia (A). During hypoxia, the redox state of ETS and mitochondrial dehydrogenases shifts toward a reduced state due to limited electron transfer from cytochrome c oxidase to oxygen. This leads to increased electron availability in many enzymes/complexes involved in redox reactions, consequently favoring superoxide production (Smith et al., 2017). Importantly, given that a very small percentage of molecular oxygen is converted to superoxide in isolated mammalian mitochondria (about 0.2%, Tahara et al., 2009) and that this figure is likely to be much lower in vivo (Murphy, 2009), it is suggestive that the electron availability, not oxygen concentration, would be the limiting factor in mitochondrial superoxide production (Campian et al., 2007). Therefore, even in hypoxia, increases in electron availability should boost mitochondrial superoxide production—at least until molecular oxygen concentration becomes so low that electron availability ceases to be the limiting factor. Thus, the overall pattern observed is an increase in oxidant formation during hypoxia. The pattern of transient activation of antioxidant defenses along hypoxic challenges, and the improved protection against stressful insults generated afterwards, follows the same trend observed in many cases of physiological conditioned hormesis. Limited time and magnitude exposure of animals to insults including hypoxia/anoxia, freezing and severe dehydration, as well as to conditions inducing estivation, activates a “physiological program” that reduces adaptive failure and/or mortality upon stronger challenges (the “hormetic zone”), as proposed in the hormesis concept. “Conditioned re-oxygenation” (or reoxygenation-like, during dehydration/rehydration and freezing/thawing), shown in (B), is a state where the protective POS-response range is maximum. However, longer and/or stronger exposure to these insults revert the protective hormetic effects (the “harmful zone”), increasing adaptive failure. Therefore, given their remarkable similarities in biological and biochemical outputs, we propose that POS should be included as a new example of physiological conditioning hormesis. Graphic elements adapted from Servier Medical Art.