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EDITORIAL FOCUS
. Focus on "Induction of HIF-2 is dependent on mitochondrial O2 consumption in an O2-sensitive adrenomedullary chromaffin cell line"UCD Conway Institute, University College Dublin, Dublin, Ireland
OXYGEN GRADIENTS IN TISSUES and cells are critical signals in a range of physiological processes including development, wound healing, and adaptation to hypoxia (12). Eukaryotic cells have evolved mechanisms for sensing changes in local oxygen tension and initiation of a response designed to support the maintenance of metabolic homeostasis. Brown and Nurse (1) have investigated the role of mitochondria in such oxygen-sensing mechanisms in mammalian cells.
In the normal physiological state, the majority of cellular oxygen consumption occurs in mitochondria where oxygen functions as the terminal electron acceptor for the electron transport chain during aerobic respiration. This role of oxygen is critical in the generation of sufficient levels of ATP to satisfy cellular bioenergetic requirements. However, there is also a critical level of "spare" oxygen available to facilitate the activity of nonmitochondrial dioxygenases elsewhere in the cell. When oxygen demand exceeds supply (hypoxia), mitochondria cannot meet cellular ATP requirements, thereby leading to the potential for metabolic crisis. Under such conditions, the high affinity of cytochrome-c oxidase (complex IV of the electron transport chain) for molecular oxygen (Km < 1 µM) likely dictates that virtually all oxygen available to the cell is consumed by the process of mitochondrial respiration, leaving minimal oxygen available for other cellular dioxygenases. In such cases, mitochondria become akin to an oxygen sink without a plug (13). One such family of dioxygenases, the activity of which is inhibited in hypoxia, are the 2-oxoglutarate-dependent hydroxylases, which confer hypoxic sensitivity to the hypoxia-inducible factor (HIF) family of transcription factors (10). These HIF-hydroxylases consist of at least three proline hydroxylases and one asparagine hydroxylase (known as factor inhibiting HIF) and are active in the presence of available oxygen as well as the cofactors Fe2+ and 2-oxoglutarate. In normoxia, these hydroxylases repress the HIF pathway by targeting HIF-
-subunits for degradation and inhibiting transcriptional activity, respectively (10). However, in hypoxia, when oxygen availability is reduced, hydroxylases are inactivated and the HIF pathway is derepressed, leading to a rapid activation of the expression of genes, thereby facilitating oxygen delivery and anaerobic metabolism in the hypoxic region. The central importance of the HIF pathway in mediation of the adaptive cellular response to hypoxia has become clear, with over 300 HIF target genes, including angiogenic, metabolic, and vasoactive genes, being identified to date. The consequence of increasing expression of such genes depends on the nature of the cause of hypoxia. For example, in a healthy tissue, such a pathway will promote tissue survival, whereas in a growing tumor, activation of the pathway can promote tumor survival and growth. It should be noted that while the HIF pathway is clearly a master regulator of the hypoxic response (11), it is not the only oxygen-sensitive transcription factor (4).
HIF is dimeric, with an oxygen-labile HIF--subunit and a constitutively expressed HIF-β-subunit (ARNT). The oxygen-sensitive component, HIF-, is a direct target of HIF-hydroxylases, which target it for ubiquitination and proteosomal degradation in the presence of molecular oxygen. Hydroxylase inhibition by hypoxia leads to stabilization of HIF-, the critical step in formation of the HIF complex, with subsequent induction of hypoxia-dependent gene expression. Two HIF- isoforms (HIF-1 and HIF-2) share similar structures, bind the same DNA motif, and regulate overlapping yet distinct groups of genes. Significant differences exist between the tissue expression profiles of HIF-1 and HIF-2, and they are thought to play different roles in tumorigenesis (9).
A key question that has remained controversial in the field of oxygen sensing relates to the nature of the mechanism(s) linking decreased local oxygen tension to activation of the HIF pathway. It has become clear, however, that mitochondrial inhibition under conditions of hypoxia (but not anoxia) leads to a reversal of hypoxia-induced HIF-1 activation; such results have led to a proposed role for mitochondria in oxygen sensing in the HIF pathway. However, two schools of thought have evolved over the past ten years as to the mechanisms underlying this. First, Schumacker and colleagues have proposed that a paradoxical release of mitochondrial reactive oxygen species (ROS) at complex III in hypoxia leads to HIF activation and that mitochondrial inhibition reverses this (7). While studies in a number of models of hypoxia have found that mitochondrial ROS levels may be increased, the mechanistic link between elevated ROS and decreased activity of the HIF-hydroxylases remains unclear, although it may involve changes in the availability of Fe2+, leading to diminished cofactor availability (12). A second explanation as to why mitochondrial inhibition in hypoxia reverses HIF activation was proposed by Moncada and colleagues (8) and is based on studies of the role of the endogenous inhibitor of respiration, nitric oxide (NO), in the regulation of the cellular response to hypoxia. According to this model, inhibition of oxygen consumption in hypoxia by mitochondrial inhibitors (including NO) results in the redistribution of available oxygen to the cytoplasm, where it reactivates hydroxylases, leading to a reversal of HIF activation and causing the cell to fail to sense hypoxia (Fig. 1). While offering different explanations for the role of mitochondria in oxygen sensing in the HIF pathway, these models are not necessarily mutually exclusive (12).
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in an oxygen-sensitive adrenomedullary cell line. The authors convincingly demonstrate that hypoxia-induced HIF-2 accumulation is effectively blocked when mitochondria are depleted (rho zero cells) or when mitochondria are genetically inhibited at complex III or IV or pharmacologically inhibited at complex I, III, or IV. Thus, regardless of the point in the mitochondrial electron transport chain at which the inhibition is applied, hypoxia-induced HIF-2 is reversed. This is consistent with previous studies that favor the oxygen redistribution hypothesis (6, 8). Interestingly, the effectiveness of pharmacological inhibitors of mitochondria on HIF-2 accumulation depends on the degree of hypoxia experienced, possibly reflecting the level of oxygen that is available for redistribution. The authors also speculate that the effects of mitochondrial inhibition on HIF activation may be independent of ROS and conclude that the effects of mitochondrial inhibition in hypoxia on the HIF pathway are due to decreased oxygen consumption leading to increased cellular oxygen availability.
While results of this study further understanding of the role of mitochondria in oxygen sensing in the HIF pathway, some issues should be considered. First, insights regarding the role of ROS in the activation of HIF in hypoxia (as well as in signaling in general) are limited by the lack of availability of probes that accurately and reproducibly measure intracellular (in particular, intramitochondrial) ROS levels. Furthermore, the addition of pro- and antioxidants to cells in culture is unlikely to accurately reproduce alterations in intracellular redox potentials experienced in physiological hypoxia. Developments of tools to measure and manipulate intracellular ROS levels will be critical for full understanding of the role of ROS in HIF signaling. Second, while the current study provides evidence for oxygen redistribution to HIF-2-hydroxylases as a mechanism for the effects of mitochondrial inhibition, it does not demonstrate reactivation of these enzymes. These issues notwithstanding, this study makes an important contribution to the understanding of oxygen sensing at the cellular level.
Precise knowledge of cellular oxygen-sensing mechanisms in the HIF pathway is important for furthering insights of the physiology of adaptation to hypoxia. However, it also has implications in disease where HIF activation can impact on disease manifestations and progression. Future studies will have to address a number of key issues, including the role of mitochondrial inhibition on the availability of hydroxylase cofactors, such as Fe2+ and 2-oxoglutarate, and how this availability impacts on hydroxylase activity and ultimately HIF signaling. Furthermore, the regulation of gene expression in response to hypoxia is not confined to the HIF pathway; for example, a number of other transcription factors, including NF-
B and Notch, have been shown to be hypoxia sensitive in a hydroxylase-dependent manner (2,3, 5, 14).
Perhaps the most important challenge in this field is the need for increased understanding of the role of mitochondria in oxygen sensing in more complex systems including tissues and whole animals. In such conditions, the dynamics of physiological tissue oxygen supply and demand will have to be taken into consideration along with fluctuating gradients of oxygen and NO, the endogenous inhibitor of respiration. Understanding the complexity of these processes will likely require investigators to shed traditional reductionist approaches and, instead, to embrace more integrated, systems biology-led strategies.
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FOOTNOTES |
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REFERENCES |
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 TOP REFERENCES |
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is dependent on mitochondrial O2 consumption in an O2-sensitive adrenomedullary chromaffin cell line. Am J Physiol Cell Physiol (March 19, 2008). doi: 10.1152/ajpcell.00007.2008.2. Cockman ME, Lancaster DE, Stolze IP, Hewitson KS, McDonough MA, Coleman ML, Coles CH, Yu X, Hay RT, Ley SC, Pugh CW, Oldham NJ, Masson N, Schofield CJ, Ratcliffe PJ. Posttranslational hydroxylation of ankyrin repeats in IkappaB proteins by the hypoxia-inducible factor (HIF) asparaginyl hydroxylase, factor inhibiting HIF (FIH). Proc Natl Acad Sci USA 103: 14767–14772, 2006.
3. Coleman ML, McDonough MA, Hewitson KS, Coles C, Mecinovic J, Edelmann M, Cook KM, Cockman ME, Lancaster DE, Kessler BM, Oldham NJ, Ratcliffe PJ, Schofield CJ. Asparaginyl hydroxylation of the Notch ankyrin repeat domain by factor inhibiting hypoxia-inducible factor. J Biol Chem 282: 24027–24038, 2007.
4. Cummins EP, Taylor CT. Hypoxia-responsive transcription factors. Pflügers Arch 450: 363–371, 2005.[CrossRef][Web of Science][Medline]
5. Cummins EP, Berra E, Comerford KM, Ginouves A, Fitzgerald KT, Seeballuck F, Godson C, Nielsen JE, Moynagh P, Pouyssegur J, Taylor CT. Prolyl hydroxylase-1 negatively regulates IkappaB kinase-beta, giving insight into hypoxia-induced NFkappaB activity. Proc Natl Acad Sci USA 103: 18154–18159, 2006.
6. Doege K, Heine S, Jensen I, Jelkmann W, Metzen E. Inhibition of mitochondrial respiration elevates oxygen concentration but leaves regulation of hypoxia-inducible factor (HIF) intact. Blood 106: 2311–2317, 2005.
7. Guzy RD, Schumacker PT. Oxygen sensing by mitochondria at complex III: the paradox of increased reactive oxygen species during hypoxia. Exp Physiol 91: 807–819, 2006.
8. Hagen T, Taylor CT, Lam F, Moncada S. Redistribution of intracellular oxygen in hypoxia by nitric oxide: effect on HIF1alpha. Science 302: 1975–1978, 2003.
9. Löfstedt T, Fredlund E, Holmquist-Mengelbier L, Pietras A, Ovenberger M, Poellinger L, Påhlman S. Hypoxia inducible factor-2alpha in cancer. Cell Cycle 6: 919–926, 2007.[Web of Science][Medline]
10. Schofield CJ, Ratcliffe PJ. Oxygen sensing by HIF hydroxylases. Nat Rev Mol Cell Biol 5: 343–354, 2004.[CrossRef][Web of Science][Medline]
11. Semenza GL. Life with oxygen. Science 318: 62–64, 2007.
12. Taylor CT. Mitochondria and cellular oxygen sensing in the HIF pathway. Biochem J 409: 19–26, 2008.[CrossRef][Web of Science][Medline]
13. Wenger RH. Mitochondria: oxygen sinks rather than sensors? Med Hypotheses 66: 380–383, 2006.[CrossRef][Web of Science][Medline]
14. Zheng X, Linke S, Dias JM, Zheng X, Gradin K, Wallis TP, Hamilton BR, Gustafsson M, Ruas JL, Wilkins S, Bilton RL, Brismar K, Whitelaw ML, Pereira T, Gorman JJ, Ericson J, Peet DJ, Lendahl U, Poellinger L. Interaction with factor inhibiting HIF-1 defines an additional mode of cross-coupling between the Notch and hypoxia signaling pathways. Proc Natl Acad Sci USA 105: 3368–3373, 2008.
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