Response of mitochondrial reactive oxygen species generation to steady-state oxygen tension: implications for hypoxic cell signaling

Abstract

Hoffman DL, Salter JD, Brookes PS. Response of mitochondrial reactive oxygen species generation to steady-state oxygen tension: implications for hypoxic cell signaling. Am J Physiol Heart Circ Physiol 292: H101-H108, 2007. First published September 8, 2006; doi:10.1152/ajpheart.00699.2006.-Mitochondria are proposed to play an important role in hypoxic cell signaling. One currently accepted signaling paradigm is that the mitochondrial generation of reactive oxygen species (ROS) increases in hypoxia. This is paradoxical, because oxygen is a substrate for ROS generation. Although the response of isolated mitochondrial ROS generation to [O 2] has been examined previously, such investigations did not apply rigorous control over [O2] within the hypoxic signaling range. With the use of open-flow respirometry and fluorimetry, the current study determined the response of isolated rat liver mitochondrial ROS generation to defined steady-state [O2] as low as 0.1 M. In mitochondria respiring under state 4 (quiescent) or state 3 (ATP turnover) conditions, decreased ROS generation was always observed at low [O 2]. It is concluded that the biochemical mechanism to facilitate increased ROS generation in response to hypoxia in cells is not intrinsic to the mitochondrial respiratory chain alone but may involve other factors. The implications for hypoxic cell signaling are discussed.hypoxia-inducible factor; superoxide; free radicals; mitochondria; metabolism REACTIVE OXYGEN SPECIES (ROS) generated by mitochondria are key intermediates in a diverse array of cell signaling events, including regulation of the cell cycle (68), proliferation (51), metalloproteinases (65), apoptosis (54), protein kinases (8,16,54,64), phosphatases (62), growth factor signaling (78), and transcription factors (44, 52) (see Ref. 31 for review). One area of investigation that has been the focus of much recent interest is the potential role of mitochondrial ROS in the signaling events that occur during hypoxia, including the regulation of hypoxia-inducible factors such as HIF-1␣ (2, 4, 7, 10, 25, 32, 48, 56, 59, 67, 84 -86, 88, 89). Whereas the downstream effects of mitochondrially derived ROS are relatively well established (vide supra), the mechanisms and the directionality (i.e., increased or decreased) of mitochondrial ROS generation in hypoxia are currently under debate (59,86). This is an important area of study, since it appears that not only can mitochondria regulate HIF, but HIF and its downstream target genes (e.g., heme oxygenase) are important pathophysiological regulators of mitochondrial function (5, 42, 77). Thus, perturbing the mitochondria/HIF signaling loop may contribute to disease pathogenesis (10).A widely accepted model of hypoxic cell signaling is that mitochondrial ROS generation increases in hypoxia (7,25,32,67,85,86,88,89). This is proposed to occur via O 2 limitation at the terminal enzyme in the mitochondrial respiratory chain, cytochrome c oxidase (complex IV), causing a backup of electrons in the proximal chain and increase...

Hypoxia-cadmium interactions on rainbow trout (Oncorhynchus mykiss) mitochondrial bioenergetics: attenuation of hypoxia-induced proton leak by low doses of cadmium

“…1), the rainbow trout mitochondria became markedly uncoupled (reduced RCR) and inefficient (reduced P/O ratio) in line with previous findings in hypoxia-sensitive mammalian mitochondria (Gnaiger et al, 2000;Blomgren et al, 2003;Kim et al, 2003;Navet et al, 2006;Hoffman et al, 2007). In contrast, mitochondria from hypoxia-tolerant species maintain or increase the phosphorylation efficiency and coupling following hypoxia/reoxygenation (Storey and Storey, 1990;Kurochkin et al, 2009;Ivanina et al, 2012;Sussarellu et al, 2013).…”

“…Unsurprisingly, therefore, even the mechanisms via which hypoxia/reoxygenation activates proton leak pathways are not well known. Nonetheless, ROS, together with resultant products of oxidation, stimulate mitochondrial proton leak (Jastroch et al, 2010), and the proportion of electrons redirected to ROS production increases as the partial pressure of oxygen (P O2 ) decreases in isolated rat mitochondria (Hoffman et al, 2007). The role of ROS in stimulating proton leak was, at least in part, substantiated in the present study by the finding that NAC, an ROS scavenger, attenuated hypoxia/reoxygenation-stimulated state 4 and 4 ol respiration.…”

“…Although ROS scavengers are commonly used to implicate ROS in pathophysiological processes, unambiguous confirmation of ROS involvement in the stimulation of proton leak observed in the present study requires actual measurements of ROS generation. It is also worth noting that although there is wide acceptance of the notion that ROS production by the mitochondria increases in hypoxia (Bell et al, 2005;Waypa and Schumacker, 2002;Murphy, 2009), reduced ROS generation has also been demonstrated and convincingly justified (Weir et al, 2005;Hoffman et al, 2007).…”

Regulation of HIF-1α Activity by Overexpression of Thioredoxin is Independent of Thioredoxin Reductase Status

“…However, the mechanisms by which these pathways affect HIF-1α stability or function have not been clarified and their implication for HIF-1α signaling remains controversial (Alvarez-Tejado et al, 2002;Chua et al, 2010;Hoffman et al, 2007;Naranjo-Suarez et al, 2012;Srinivas et al, 2001;Vaux et al, 2001).…”

Promoting differentiation and survival of human c-kit+ cardiac progenitor cells ex vivo.

“…[34], ischemia generates detrimental high levels of reactive oxygen species (ROS) [59][60][61]. Indeed, Kolamunne et al have shown that hypoxia produces high amounts of mitochondrial superoxide, which mediate the toxicity observed in cardiac progenitors [62].…”