Abstract
Acute lung injury (ALI) is a severe form of hypoxic lung disease responsible for a large number of deaths worldwide. Despite recent advances in supportive care, no reduction in mortality has been evident since the introduction of a standard consensus definition almost two decades ago. New strategies are urgently required to help design effective therapies for this condition. A key pathological feature of ALI involves regional alveolar hypoxia. Because alveolar hypoxia in isolation, such as that encountered at high altitude, causes an inflammatory pulmonary phenotype in the absence of any other pathogenic stimuli, these regions may not be passive bystanders but may actually contribute to the pathogenesis and progression of lung injury. Unique transcriptional responses to hypoxia in the lung apparently allow it to express an inflammatory phenotype at levels of hypoxia that would not produce such a response in other organs. We will review recent advances in our understanding of these unique transcriptional responses to moderate levels of alveolar hypoxia, which may provide new insights into the pathogenesis of ALI.
Our understanding of the underlying pathogenic mechanisms in hypoxic lung diseases such as acute lung injury (ALI) is incomplete. The important proinflammatory actions of pulmonary hypoxia have recently been recognized. This translational review explores potential mechanisms mediating these responses in the lung. The future elucidation of the molecular mechanisms behind pulmonary-specific hypoxia responses may provide a new paradigm in our search for therapeutic strategies in ALI.
Considering that alveolar hypoxia is an inevitable consequence of many respiratory diseases, it is notable that hypoxia is a potent proinflammatory stimulus in systemic organs (1, 2). In recent years, considerable advances have been made in our understanding of the molecular pathways underlying hypoxia-induced inflammation (3–6).
The concentrations of alveolar oxygen found in lung disease, although below normal, are higher than the profoundly reduced concentrations of oxygen responsible for hypoxia-induced inflammation in systemic organs. Nevertheless, alveolar hypoxia results in an inflammatory phenotype in the lung, both in humans exposed to hypobaric hypoxia and in rodents exposed to alveolar hypoxia mimicking that found in lung disease (2, 7–9). These findings suggest that hypoxia may exert damaging inflammatory effects in the lung, similar to its effects in systemic organs. Thus alveolar hypoxia in lung disease may not simply involve a consequence of the disease process, but may actively contribute to progressive lung damage after the disease process is initially established.
However, given that hypoxic responses in the lung occur at a much higher Po2 than is required to produce a response in systemic organs, unique pulmonary mechanisms must be contributing to this phenotype. In this review, we consider recent evidence supporting a role for hypoxia as an inflammatory stimulus in the lung, the specific distinct mechanisms by which it exerts these effects, and the potential role of these mechanisms in the pathogenesis of acute lung injury (ALI), a prototypical hypoxic lung disease.
Hypoxia and inflammation are intertwined at the molecular, cellular, and clinical levels (10). Hypoxia (when caused, for example, by vascular occlusion) results in organ ischemia and the expression of a proinflammatory phenotype (11–20). Although organ ischemia normally occurs in a sterile environment, the consequences of the resultant hypoxic responses share many phenotypic parallels with the activation of a host immune response directed toward invading microorganisms (17). This hypoxia-induced inflammatory response results in the recruitment of immune cells, the activation of downstream signaling pathways, and the induction of proinflammatory cytokines and chemokines (17, 18). In addition, these extremely low concentrations of oxygen prolong neutrophil survival and increase endothelial permeability and vascular leakage (11–16).
In addition to being caused by hypoxia, inflammation is an important cause of hypoxia. Inflamed tissue becomes severely hypoxic because of cell infiltration, the formation of edema, and microthrombi, which together lengthen the diffusion distance between the capillary vessels and the metabolically active cells. The consequent reduction in the rate of oxygen delivery reduces cellular Po2 and thus induces inflammation, contributing further to the propagation of damaging immune responses. In diseases such as rheumatoid arthritis and inflammatory bowel disease, hypoxia is not a bystander but influences the environment of the tissue by regulating oxygen-dependent gene expression, which includes proinflammatory networks. The mechanisms driving this phenomenon are currently the focus of intensive study in many diseases of systemic organs, and such studies have already identified pathways that can be therapeutically manipulated in in vivo models (10).
The mechanisms of hypoxic responses have been extensively investigated in animal models in vivo, where hypoxic–ischemic conditions are typically induced by vascular occlusion, and in vitro models in which oxygen has been reduced to 0.5–1.0%, mimicking the oxygen concentrations encountered during ischemia. In these models, several important transcriptional factors are known to be involved in the resultant changes in gene expression. The first to be identified was the hypoxia-inducible factor (HIF) family (11), which responds in a rapid manner to profound reductions in cellular oxygen concentration, thereby constituting key modulators of hypoxia-responsive genes (21). The prototypical transcription factor HIF-1 is highly conserved across species, is present in almost all cell types, is tightly regulated by oxygen availability, and alters the expression of hundreds of genes. HIF-1 exists as a heterodimer, consisting of HIF-1α and HIF-1β subunits. HIF-1β is constitutively present, whereas HIF-1α protein is found at very low concentrations under normoxic conditions. Under normoxic conditions, the hydroxylation of HIF-1α is catalyzed by prolyl hydroxylase domain proteins (PHDs), using molecular oxygen as a substrate, leading to interactions with the von Hipple–Lindau (VHL) protein and the addition of ubiquitin, thus targeting HIF-1α for proteasomal degradation (22). In hypoxia, PHD activity decreases, resulting in HIF-1α stabilization, interactions with HIF-1β, nuclear translocation, DNA-binding, and the recruitment of coactivator proteins to the HIF binding site within the hypoxia response element, activating the transcription of target genes. HIF-2α is a closely related protein that, like HIF-1α, is subject to PHD-dependent regulation and dimerizes with HIF-1β after stabilization. However, HIF-2α exhibits a much more restricted pattern of cellular expression (23). Different target genes are controlled by the two isoforms of HIF, and they show varying sensitivities to the different isoforms of PHDs (24–27).
Increased HIF-1α protein generally correlates with increased transcriptional activity, although the HIF-1 transactivation function is further regulated by the hydroxylation of HIF-1α at an asparagine residue through the action of a second oxygen-dependent hydroxylase enzyme, factor-inhibiting HIF-1 (FIH-1). Under normoxic conditions, this hydroxylation blocks the binding of the transcriptional coactivators CREB-binding protein/p300 (28). Because both PHD and FIH-1 activity require molecular oxygen, hypoxia reduces the activity of both enzymes, leading to the stabilization of HIF-1α and the transactivation of HIF-1 target genes.
Although HIF stabilization and activation by the inhibition of PHDs and FIH are clearly very important rapid-response mechanisms during the development of profound hypoxia within a cell, HIF can also be activated by longer-latency mechanisms involving the synthesis of new HIF-α protein (Figure 1). Significant crosstalk occurs between HIF and NF-κB, and the activation of the innate immune system increases HIF-1α in a hypoxia-independent manner through an NF-κB–dependent increase of HIF-1α gene transcription, acting over hours or days (29–36). A second hypoxia-independent mechanism of increased HIF protein expression involves regulation by the PI3K/Akt pathway in response to growth factors and cytokines, including insulin and insulin-like growth factors (Figure 2). This pathway is also linked to longer-term HIF expression in the colon and in lung cancer (37–40). Another mechanism contributing to the stabilization of HIF has recently been described in the ischemic heart. Hypoxia-induced increases in extracellular adenosine can cause an increase in HIF-1α protein via a protein period–2–dependent mechanism (41).
Figure 1. Three mechanisms of activating the hypoxia-inducible factor (HIF) are known. (A) Mechanism 1. Under normoxic conditions, HIF-1α (HIFA) is hydroxylated by prolyl hydroxylases (PHDs) and targeted for proteasomal degradation by Von Hippel–Lindau (VHL) proteins. Under profound hypoxic conditions, PHD is inhibited, HIF-1α increases, and both HIF-1α and HIF-1β (HIFB) translocate to the nucleus, where they bind as heterodimers to a hypoxia response promoter element (HRE) in genes, thus inducing the transcription of numerous genes. This rapid response mechanism takes place over minutes (22). (B) Mechanisms 2 and 3. In mechanism 2, NF-κB (NFKB) can be activated by many insults, including hypoxia. NF-κB is inactive in the cytosol because it is associated with nuclear factor of κ light polypeptide gene enhancer in B cells α (IκBα). At the time of cellular activation, the β subunit of the IκB kinase complex (IKKβ) phosphorylates the inhibitor IκBα, which thereby becomes degraded and liberates NF-κB for translocation in the nucleus, where it can activate the transcription of inflammatory genes and of HIF. This transcriptional process takes hours to days (42, 43). In mechanism 3, new HIF protein can also be transcribed in response to the cellular binding of growth factors, including epidermal growth factor (EGF), insulin-like growth factor (IGF)1/2, and fibroblast-like growth factor (FGF)2. This is a longer process requiring the transcription of new proteins (37–40).
[More] [Minimize]
Figure 2. cAMP response element binding protein (CREB) is activated by phosphorylation in response to a number of cellular signals. The classic mechanism involves the G-protein–mediated induction of adenyl cyclase via G-protein receptors. Increased cAMP promotes the dissociation of protein kinase A (i.e., the PKA heterotetramer, which consists of regulatory [R] and catalytic [C] subunits). Liberated C subunits migrate in to the nuclear compartment, and phosphorylate CREB at the single phospho-acceptor site (P), serine 133. Other mechanisms of activation occur via cellular kinases, such as protein kinase C (PKC) and calcium–calmodulin complexes. The mechanism of activation in response to hypoxia remains unclear, but may occur via the kinase Akt. The transcription of cellular genes after CREB phosphorylation is rapid, peaking within 30 minutes of stimulation with cAMP (92).
[More] [Minimize]However, HIF is not the sole transcription factor mediating transcriptional responses to hypoxia. The inhibition of PHDs by hypoxia also stabilizes proteins other than HIF, thus producing HIF-independent hypoxic gene responses (42–46). The activation of the proinflammatory transcription factor NF-κB in response to hypoxia appears, in part at least, to occur as a result of the inhibition of PHDs. I-κ B kinase–β (IKK-β) contains an evolutionarily conserved consensus motif for hydroxylation by PHDs. The hypoxic inhibition of PHD-1 or PHD-2 stabilizes IKK-β and increases its activity, thus leading to an increased phosphorylation-dependent degradation of I-κ B kinase–α (IKK-α) and the liberation and nuclear translocation of NF-κB (42–44). Thus hypoxia releases the repression of NF-κB activity through an HIF-independent molecular mechanism, contributing to an inflammatory phenotype. Interestingly, PHD-3 can act by a different HIF-independent mechanism to mediate a proinflammatory phenotype, prolonging neutrophil survival through a SIVA1/BCL-X(L)–dependent pathway (46). However, a deficiency of PHD3 was recently demonstrated to shorten the survival of mice subjected to various models of abdominal sepsis because of an overwhelming innate immune response, suggesting an anti-inflammatory action of PHD-3 in this context. By contrast, this phenotype was absent in mice deficient in PHD-1 (PHD-1−/−) or PHD-2 (PHD-2+/−). Thus, the role of PHDs in regulating hypoxia-induced inflammation may be both isoform-specific and tissue-specific or organ-specific (47).
Hypoxia also controls mRNA concentrations by inducing changes in microRNA (miRNA) transcription and expression. miRNAs conjugate with the RNA-induced silencing complex, and by annealing with complementary sequences in the 3′ untranslated region of target mRNAs, cause the suppression of translation and mRNA degradation. A set of hypoxia-responsive miRNAs (termed “hypoxamirs”) controls mRNA concentrations in hypoxic cells, and thus alters the phenotypic response. The transcription of hypoxamirs themselves is regulated by hypoxia-responsive transcription factors, because the miRNAs are embedded within regions of the genome whose transcription is regulated by such transcription factors, including HIF and NF-κB (48, 49). Interestingly, the action of hypoxamirs was also reported to be altered in a transcriptonally independent manner by posttranslational modifications of the miRNA-binding protein argonaute2 (Ago2) (50). Type 1 collagen proyly-4–hydroxylase (C-P4H9(I)) hydroxylates Ago2, thus activating and stabilizing the protein within 4 hours of the onset of hypoxia. This hydroxylation increases the expression of a large number of miRNAs, and increases Ago2 endonuclease activity. C-P4H9(I) is a prolyl hydroxylase that is distinct from the PHDs 1–3 already discussed, and has a much lower Km for oxygen, thus remaining active in profoundly hypoxic environments (50). In addition, the expression of C-P4H9(I) is increased during hypoxia by an HIF-independent protein-stabilization mechanism (51).
Taken together, these data demonstrate that profoundly low levels of hypoxia, such as those encountered in ischemic organs, inhibit prolyl hydroxylases, leading to altered gene transcription and mRNA stability though both HIF-dependent and HIF-independent mechanisms.
The classic definition of hypoxia as “a failure to supply adequate oxygen to maintain the tissue ATP supply” is frequently not useful in the lung, where hypoxic responses occur at concentrations of oxygen that are perfectly adequate to maintain normal oxidative metabolism. The normal alveolar Po2 (∼ 110 mm Hg) is uniquely high when compared with that of the other internal organs, in which oxygen tension ranges from 3–20 mm Hg in health (52–55). Furthermore, hypoxic responses are observed in the lung at Po2 values (50–60 mm Hg) that are higher than those in other organs under conditions of normal oxygenation, and markedly greater than those (Po2 < 10 mm Hg) producing hypoxic responses in any other organ under ischemic and inflammatory conditions (52, 56, 57).
In addition to uniquely high Po2 levels, the lung displays unique responses to hypoxia when compared with those observed in other organs. Two specific responses illustrate this clearly. First, the acute onset of hypoxia leads to vasoconstriction (hypoxic pulmonary vasoconstriction), that is, a marked contrast to the vasodilatation caused by hypoxia in all other organs (58, 59). This is a very rapid (i.e., in minutes) nongenomic response, and is a beneficial adaptation when disease is localized to a few regions in the lung because it improves ventilation–perfusion matching and enhances oxygen uptake (58). However, in some normal sea-level dwellers (5–10% of the population), an excessive vasoconstrictor response is associated with the development of high-altitude pulmonary edema (60). More persistent hypoxia in the lung (lasting weeks to months or longer) causes pulmonary hypertension attributable to remodeling of the pulmonary arterioles and a sustained ρ-kinase–dependent vasoconstrictor response, distinct from acute hypoxic vasoconstriction (61, 62). At high altitude, in isolation from any underlying lung disease, this hypertension can cause right heart failure and death in susceptible individuals, as seen in subacute mountain sickness and chronic mountain sickness (Monge disease) (60, 63).
The second unique aspect of the pulmonary response to hypoxia involves specific genes whose transcription is altered in the lung, but is not activated in other organs. Teng and colleagues reported that resistin-like molecule–α (RELM-α) was up-regulated in the lung at levels of hypoxia mimicking those found in lung disease, but not in any other organs (64). Furthermore, RELM-α exerted proinflammatory effects via vascular endothelial growth factor receptor–2 (VEGFR2)–dependent and IL-4–dependent pathways. Moreover, RELM-α was proangiogenic, and its overexpression in the rodent lung in vivo led to the development of pulmonary hypertension, increased pulmonary vascular resistance, and remodeling of the pulmonary arterioles (7, 65–67). Conversely, the siRNA-mediated knockdown of RELM-α attenuated the development of hypoxic pulmonary hypertension (66).
Using a unique subtractive gene microarray approach, we identified a set of genes up-regulated in the hypoxic pulmonary microvascular endothelium whose expression was unaltered or reduced in the hypoxic endothelium of systemic microvessels. To date, two of these genes have been investigated more extensively, gremlin1 and CXC chemokine receptor type 7 (CXCR7) (68). Both were up-regulated in the hypoxic lung, but were unaltered in other (systemic) organs (68, 69). A haplodeficiency of gremlin1 augments bone morphogenetic protein signaling in the hypoxic murine lung and reduces pulmonary vascular resistance by attenuating pulmonary vascular remodeling, thereby demonstrating an important mechanistic role for gremlin in the development of hypoxic pulmonary hypertension (69). Importantly, gremlin1 expression is increased in the vascular endothelium of explanted human lungs with pulmonary hypertension (69).
The second gene that we identified as selectively up-regulated in the hypoxic murine lung is CXCR7, a chemokine receptor that binds two ligands, chemokine (C-X-C motif) ligand (CXCL)11 and CXCL12 (70). CXCL12 is chemoattractant to neutrophils, T-lymphocytes, and monocytes, promotes transendothelial cell migration, and exerts a potent proangiogenic action (71–75). We recently reported that CXCR7 expression is increased in human pulmonary hypertensive lungs, and its ligand CXCL12 is significantly elevated in humans with pulmonary hypertension (76). The blockade of CXCR7 inhibits the development of hypoxic pulmonary hypertension in the murine lung, suggesting an important mechanistic role for this receptor (77, 78). Taken together, these reports demonstrate that lung-selective hypoxia response genes exert proinflammatory and pathogenic vascular actions that may play important roles in the development of hypoxic lung disease.
In addition to these lung-specific gene responses to hypoxia, other hypoxic responses occur that are not unique to the lung, but that are triggered at concentrations of oxygen much higher than those required in systemic organs, such as inflammation, angiogenesis, and the expression of an endothelial cell procoagulant phenotype (61, 79, 80). Moderate levels of alveolar hypoxia, such as those experienced during exposure to high altitude, result in an inflammatory phenotype. Increased macrophages, neutrophils, and inflammatory cytokines, including IL-1β, IL-6, IL-8, and TNF-α, are found in the bronchoalveolar lavage fluid of humans exposed to hypobaric hypoxia (60, 81). Similarly, rodents exposed to alveolar hypoxia mimicking that found in lung disease (FiO2 = 0.10) demonstrate a pulmonary infiltration of neutrophils, macrophage recruitment and activation, increased pulmonary vascular permeability, and elevations of inflammatory cytokines in the lung (2, 7–9). These findings demonstrate that hypoxia (albeit at a much higher Po2) in the lung may exert damaging inflammatory effects similar to those in the systemic organs. Moreover, in addition to local alveolar inflammation, pulmonary hypoxia induces the release of monocyte chemoattractant protein–1 from activated alveolar macrophages, which is then released into the circulation, stimulating inflammatory responses in systemic organs within minutes of the onset of alveolar hypoxia (82–84). Thus the lung-specific responses to hypoxia can induce important systemic effects.
In addition to the expression of a proinflammatory phenotype, moderate levels of alveolar hypoxia stimulate angiogenesis in the pulmonary circulation. However, during the process of new vessel formation, the capillary endothelium becomes leaky, potentially contributing to the formation of alveolar edema in the acutely hypoxic lung (61, 79). New vessel formation is an important pathogenic mechanism in the inflammatory processes of rheumatoid arthritis and inflammatory bowel disease, and in a similar way, new vessel formation may worsen inflammatory lung disease (85).
Given the unique network of genes activated in the hypoxic lung and the uniquely high Po2 at which these responses occur, lung-specific mechanisms must control the inflammatory responses of the lung to hypoxia. Transcription factors that are activated quickly after the onset of hypoxia are likely to play key roles in initiating changes in gene and protein expression. Three transcription factors, HIF, cyclic AMP (cAMP) response element binding protein (CREB), and NF-κB (9, 86–88), demonstrate such early responses to hypoxia in other organs, and each will be considered here.
Heterozygous mice partly deficient for HIF-1α expression (HIF-1α+/−) achieve partial short-term protection against the development of right ventricular hypertrophy, pulmonary hypertension, and pulmonary vascular remodeling when exposed to hypoxia (FiO2 = 0.1), but after more extended hypoxic exposure (> 3 weeks), these responses are the same as in their wild-type littermates (86). However, HIF-2α–deficient (HIF-2α+/−) animals gain long-term protection against hypoxic pulmonary hypertension (87). The importance of HIF in hypoxic pulmonary responses is further demonstrated by observations of patients with Chuvash disease. In this condition, the rate of degradation for HIF-1α and HIF-2α is reduced as a result of a mutation in the VHL gene, leading to constitutively increased concentrations of HIF. Patients with this inherited condition exhibit exaggerated pulmonary hypoxic responses and invariably develop pulmonary hypertension because of their exposure to elevated concentrations of HIF throughout their lives (89). Mice in which the genetic mutation of Chuvash disease has been reproduced also show the spontaneous development of pulmonary hypertension. When crossed with mice deficient in HIF-2α (HIF-2α+/−), these mice are protected against pulmonary hypertension, although they are not protected when crossed with HIF-1α+/− mice, providing further evidence for a key role of HIF-2α in the responses of the lung to hypoxia (90).
The evidence obtained in HIF-1α–deficient and HIF-2α–deficient mice and in patients with Chuvash syndrome clearly shows that pulmonary changes in response to sustained hypoxia over weeks and years involve the HIF pathway. However, whether those changes play an early primary role in these events remains unclear. The stabilization of HIF by PHDs has typically been demonstrated in cultured cell models, using concentrations of oxygen (0.5–1.0%) not encountered in vivo in the lung. Yu and colleagues demonstrated that hypoxia induced rapid, early increases of HIF-1α in the isolated ventilated ferret lung, but only when alveolar oxygen was reduced to 1.3% (∼ 9 mm Hg). In the intact organism, such concentrations of alveolar oxygen would not be compatible with survival for more than a few minutes (91). Therefore, although HIF is undoubtedly important for longer-term pulmonary hypoxic responses, rapid, PHD-dependent HIF stabilization is unlikely to be responsible for the early alterations of gene expression in the hypoxic lung in vivo.
The ubiquitous transcription factor CREB is activated via the phosphorylation of serine 133 by a number of mechanisms, including the prototypical cAMP-dependent activation of protein kinase A (Figure 1). This phosphorylation results in the modulation of expression for multiple genes (92).
We recently reported on a series of investigations undertaken to identify transcription factor pathways and genes selectively activated in the lung in response to the levels of alveolar hypoxia encountered in human lung disease, and we found that members of the CREB family of transcription factors (CREB-1 and activating transcription factor-1) were selectively activated by phosphorylation in the hypoxic lung within 3 hours of exposure, but not in other organs (88). Four CREB-dependent genes have been tested to date (brain-derived natriuretic factor, endothelin-1, follistatin, and VEGFR-1), and were shown to be up-regulated in response to hypoxia in the lung, but were unaltered in systemic organs, thus providing further support for an important role of a lung-selective CREB activation pathway in response to hypoxia (88).
Chava and colleagues (93) recently demonstrated the critical role played by CREB in maintaining vascular function in the lung under both basal conditions and in response to inflammatory mediators. The impairment of CREB function in a murine LPS-injured lung in vivo by the expression of a dominant-negative CREB mutant increased basal lung microvessel permeability, and prevented the resolution of lung edema (93).
Thus CREB is activated rapidly in the lung in response to levels of alveolar hypoxia mimicking those found in lung disease, and may play an important role in the lung-specific response to hypoxia.
NF-κB comprises a family of transcription factors that are typically activated after the stimulation of cells with proinflammatory ligands, but as already discussed, can also be activated in response to hypoxia (94, 95). Similar to CREB, NF-κB is a “rapidly acting” primary transcription factor constitutively present in the cytosol in its inactive form. Activating signals cause its dissociation from its inhibitory protein I-κB and its rapid translocation to the nucleus, where it regulates gene function (Figure 2).
NF-κB is activated rapidly (within 2 hours) in the lung in response to hypoxia (9). In vivo, at concentrations of alveolar oxygen commonly seen in ALI (10%), transgenic mice expressing luciferase under the control of NF-κB demonstrate that hypoxia selectively activates NF-κB in the lung, resulting in the transcriptional activation of cyclooxygenase-2 and other key proinflammatory genes (94). Sarada and colleagues found a robust increase in nuclear NF-κB in the lungs of rodents exposed to hypobaric hypoxia and significantly increased NF-κB DNA-binding activity (96). Furthermore, the blockade of NF-κB, using the polyphenol compound curcumin before hypoxic exposure, resulted in reduced nuclear NF-κB concentrations, with a concomitant reduction in lung water content and transvascular leakage (96).
However, the mechanism of NF-κB activation in the lung remains to be fully elucidated. Although it is activated in response to hypoxia in systemic organs via PHD-dependent mechanisms, PHDs are unlikely to be responsible for the activity of NF-κB or its translocation in the lung, given the uniquely high Po2. To understand its role more fully in the lung, the mechanisms of activation at alveolar levels of Po2 need to be identified and characterized. Nonetheless, considering that it is (1) activated early in response to hypoxia at concentrations of oxygen consistent with those seen in ALI, (2) controls a network of inflammatory cytokines, and (3) maintains significant crosstalk with the HIF pathway, NF-κB may play an important role in the initiation and propagation of early pulmonary hypoxic responses.
Recently, a number of publications documented a hypoxia-induced increase in specific microRNAs in the hypoxic lung, and showed that blocking their actions prevents the development of pulmonary hypertension (50, 97–101). However, to date, none of these miRNAs have been found to demonstrate early lung-selective activation, nor have any lung-specific mechanisms controlling miRNA transcription and expression been identified.
ALI is a prototypical hypoxic lung disease that presents with progressive arterial hypoxemia, dyspnea, and a marked increase in the work of breathing. Although several different initiating events are associated with its development, all result in a similar clinical and pathological profile that includes the development of areas of regional alveolar hypoxia and profound arterial hypoxemia (102).
Data obtained using the multiple inert-gas excretion technique demonstrated that lung regions in ALI range from those with high or normal ventilation-to-perfusion ratios to regions with low ventilation-to-perfusion ratios and unventilated areas (/ < 0.005, i.e., intrapulmonary shunt). Blood flow to regions of low ventilation and to areas of intrapulmonary shunt account for 5–38% and 15–63% of cardiac output, respectively, depending on disease severity (103–107). In poorly ventilated regions of the lung, the Po2 can vary from values close to the mean alveolar Po2 down to values similar to those of mixed venous blood (∼ 32 mm Hg in ALI) (108). Therefore, within these low / regions, large proportions of alveoli are exposed to levels of oxygen tension compatible with the occurrence of pulmonary-specific hypoxic responses.
Interestingly, modern lung-protective strategies and the application of higher levels of positive end-expiratory pressure (PEEP) do not reduce the areas of unventilated lung tissue in patients with ALI. Feihl and colleagues found that in a group ventilated in accordance with ARDSnet guidelines, encompassing low tidal volume permissive hypercapnia and moderate levels of PEEP, the shunt fraction (48% ± 5%) was increased compared with traditional higher tidal volume ventilation strategies (105). Therefore, despite modern techniques of lung-protective ventilation, up to half of the lung in ALI is exposed to hypoxia that cannot be overcome solely by mechanical ventilation or the delivery of supplemental oxygen. The exposure of a large proportion of alveoli and their adjacent pulmonary vascular endothelia to concentrations of oxygen known to produce hypoxic inflammatory responses raises the possibility that these regions play an important role in the progression of ALI. Hypoxic inflammatory responses in these regions, once initiated, may promote a self-perpetuating amplification cascade whereby hypoxia worsens inflammatory lung damage, in turn leading to further areas of alveolar hypoxia (Figure 3).
Figure 3. A variety of insults can lead to the development of lung injury. Regardless of the initiating insult, heterogeneous areas of tissue damage ensue, resulting in areas of regional alveolar hypoxia. The activation of damaging hypoxia-induced inflammatory responses in these regions leads to a self-perpetuating amplification cascade whereby hypoxia worsens tissue damage, leading to further areas of alveolar hypoxia.
[More] [Minimize]Pulmonary hypertension is a significant complication of ALI, and its importance was emphasized by the recent demonstration that the underlying increased pulmonary vascular resistance is an independent predictor of poor outcomes (109). Because alveolar hypoxia, in the absence of any other stimulus, can cause pulmonary hypertension, right heart failure, and death in susceptible individuals (e.g., subacute mountain sickness), it is likely to be a major contributor to this vascular pathology in severe cases of ALI (60, 63). Taken together, these data suggest that an improved understanding of lung-specific hypoxia response mechanisms may enhance our understanding of the pathogenic mechanisms in ALI, and may reveal new therapeutic targets in this disease. Moreover, the lung-selective nature of these responses may provide insights into mechanisms that could be targeted with minimal unwanted systemic effects.
Our understanding of the underlying pathogenic mechanisms of hypoxic lung diseases such as ALI is incomplete. The important proinflammatory actions of pulmonary hypoxia have recently been recognized, and research has begun regarding the organ-specific mechanisms mediating these responses. Future elucidation of the diverse functions of HIF-1 in the lung, the exploration of molecular mechanisms underlying hypoxic NF-κB activation, and an understanding of the pulmonary-specific activation and anti-inflammatory effects of CREB may provide a new paradigm in our search for therapeutic strategies in hypoxic pulmonary diseases, including ALI.
| 1. | Gonzalez NC, Wood JG. Alveolar hypoxia–induced systemic inflammation: what low PO(2) does and does not do. Adv Exp Med Biol 2010;662:27–32. |
| 2. | Minamino T, Christou H, Hsieh CM, Liu Y, Dhawan V, Abraham NG, Perrella MA, Mitsialis SA, Kourembanas S. Targeted expression of heme oxygenase–1 prevents the pulmonary inflammatory and vascular responses to hypoxia. Proc Natl Acad Sci USA 2001;98:8798–8803. |
| 3. | Cummins EP, Seeballuck F, Keely SJ, Mangan NE, Callanan JJ, Fallon PG, Taylor CT. The hydroxylase inhibitor dimethyloxalylglycine is protective in a murine model of colitis. Gastroenterology 2008;134:156–165. |
| 4. | Robinson A, Keely S, Karhausen J, Gerich ME, Furuta GT, Colgan SP. Mucosal protection by hypoxia-inducible factor prolyl hydroxylase inhibition. Gastroenterology 2008;134:145–155. |
| 5. | Hill P, Shukla D, Tran MG, Aragones J, Cook HT, Carmeliet P, Maxwell PH. Inhibition of hypoxia inducible factor hydroxylases protects against renal ischemia–reperfusion injury. J Am Soc Nephrol 2008;19:39–46. |
| 6. | Aragones J, Schneider M, Van Geyte K, Fraisl P, Dresselaers T, Mazzone M, Dirkx R, Zacchigna S, Lemieux H, Jeoung NH, et al.. Deficiency or inhibition of oxygen sensor PHD1 induces hypoxia tolerance by reprogramming basal metabolism. Nat Genet 2008;40:170–180. |
| 7. | Vergadi E, Chang MS, Lee C, Liang OD, Liu X, Fernandez-Gonzalez A, Mitsialis SA, Kourembanas S. Early macrophage recruitment and alternative activation are critical for the later development of hypoxia-induced pulmonary hypertension. Circulation 2011;123:1986–1995. |
| 8. | Carpenter TC, Stenmark KR. Hypoxia decreases lung neprilysin expression and increases pulmonary vascular leak. Am J Physiol Lung Cell Mol Physiol 2001;281:L941–L948. |
| 9. | Madjdpour C, Jewell UR, Kneller S, Ziegler U, Schwendener R, Booy C, Klausli L, Pasch T, Schimmer RC, Beck-Schimmer B. Decreased alveolar oxygen induces lung inflammation. Am J Physiol Lung Cell Mol Physiol 2003;284:L360–L367. |
| 10. | Eltzschig HK, Carmeliet P. Hypoxia and inflammation. N Engl J Med 2011;364:656–665. |
| 11. | Zampetaki A, Mitsialis SA, Pfeilschifter J, Kourembanas S. Hypoxia induces macrophage inflammatory protein–2 (MIP-2) gene expression in murine macrophages via NF-kappaB: the prominent role of p42/p44 and PI3 kinase pathways. FASEB J 2004;18:1090–1092. |
| 12. | Walmsley SR, Print C, Farahi N, Peyssonnaux C, Johnson RS, Cramer T, Sobolewski A, Condliffe AM, Cowburn AS, Johnson N, et al.. Hypoxia-induced neutrophil survival is mediated by HIF-1alpha–dependent NF-kappaB activity. J Exp Med 2005;201:105–115. |
| 13. | Hannah S, Mecklenburgh K, Rahman I, Bellingan GJ, Greening A, Haslett C, Chilvers ER. Hypoxia prolongs neutrophil survival in vitro. FEBS Lett 1995;372:233–237. |
| 14. | Mecklenburgh KI, Walmsley SR, Cowburn AS, Wiesener M, Reed BJ, Upton PD, Deighton J, Greening AP, Chilvers ER. Involvement of a ferroprotein sensor in hypoxia-mediated inhibition of neutrophil apoptosis. Blood 2002;100:3008–3016. |
| 15. | Eckle T, Faigle M, Grenz A, Laucher S, Thompson LF, Eltzschig HK. A2B adenosine receptor dampens hypoxia-induced vascular leak. Blood 2008;111:2024–2035. |
| 16. | Eltzschig HK, Ibla JC, Furuta GT, Leonard MO, Jacobson KA, Enjyoji K, Robson SC, Colgan SP. Coordinated adenine nucleotide phosphohydrolysis and nucleoside signaling in posthypoxic endothelium: role of ectonucleotidases and adenosine A2B receptors. J Exp Med 2003;198:783–796. |
| 17. | Chen GY, Nunez G. Sterile inflammation: sensing and reacting to damage. Nat Rev Immunol 2010;10:826–837. |
| 18. | Eltzschig HK, Eckle T. Ischemia and reperfusion: from mechanism to translation. Nat Med 2011;17:1391–1401. |
| 19. | Imtiyaz HZ, Williams EP, Hickey MM, Patel SA, Durham AC, Yuan LJ, Hammond R, Gimotty PA, Keith B, Simon MC. Hypoxia-inducible factor 2alpha regulates macrophage function in mouse models of acute and tumor inflammation. J Clin Invest 2011;120:2699–2714. |
| 20. | Shah YM, Ito S, Morimura K, Chen C, Yim SH, Haase VH, Gonzalez FJ: Hypoxia-inducible factor augments experimental colitis through an MIF-dependent inflammatory signaling cascade. Gastroenterology 2008;134:2036–2048. |
| 21. | Semenza GL. Life with oxygen. Science 2007;318:62–64. |
| 22. | Ivan M, Kondo K, Yang H, Kim W, Valiando J, Ohh M, Salic A, Asara JM, Lane WS, Kaelin WG. HIFalpha targeted for VHL-mediated destruction by proline hydroxylation: implications for O2 sensing. Science 2001;292:464–468. |
| 23. | Shimoda LA, Semenza GL. HIF and the lung: role of hypoxia-inducible factors in pulmonary development and disease. Am J Respir Crit Care Med 2011;183:152–156. |
| 24. | Rankin EB, Biju MP, Liu Q, Unger TL, Rha J, Johnson RS, Simon MC, Keith B, Haase VH. Hypoxia-inducible factor–2 (HIF-2) regulates hepatic erythropoietin in vivo. J Clin Invest 2007;117:1068–1077. |
| 25. | Ahmad A, Ahmad S, Glover L, Miller SM, Shannon JM, Guo X, Franklin WA, Bridges JP, Schaack JB, Colgan SP, et al.. Adenosine A2A receptor is a unique angiogenic target of HIF-2alpha in pulmonary endothelial cells. Proc Natl Acad Sci USA 2009;106:10684–10689. |
| 26. | Menrad H, Werno C, Schmid T, Copanaki E, Deller T, Dehne N, Brune B. Roles of hypoxia-inducible factor–1alpha (HIF-1alpha) versus HIF-2alpha in the survival of hepatocellular tumor spheroids. Hepatology 2010;51:2183–2192. |
| 27. | Minamishima YA, Moslehi J, Bardeesy N, Cullen D, Bronson RT, Kaelin WG. Somatic inactivation of the PHD2 prolyl hydroxylase causes polycythemia and congestive heart failure. Blood 2008;111:3236–3244. |
| 28. | Mahon PC, Hirota K, Semenza GL. FIH-1: a novel protein that interacts with HIF-1alpha and VHL to mediate repression of HIF-1 transcriptional activity. Genes Dev 2001;15:2675–2686. |
| 29. | Figueroa YG, Chan AK, Ibrahim R, Tang Y, Burow ME, Alam J, Scandurro AB, Beckman BS. NF-kappaB plays a key role in hypoxia-inducible factor–1–regulated erythropoietin gene expression. Exp Hematol 2002;30:1419–1427. |
| 30. | Jung YJ, Isaacs JS, Lee S, Trepel J, Neckers L. Microtubule disruption utilizes an NFkappa B-dependent pathway to stabilize HIF-1alpha protein. J Biol Chem 2003;278:7445–7452. |
| 31. | Zhou J, Schmid T, Brune B. Tumor necrosis factor–alpha causes accumulation of a ubiquitinated form of hypoxia inducible factor–1alpha through a nuclear factor–kappaB–dependent pathway. Mol Biol Cell 2003;14:2216–2225. |
| 32. | Tacchini L, De Ponti C, Matteucci E, Follis R, Desiderio MA. Hepatocyte growth factor–activated NF-kappaB regulates HIF-1 activity and ODC expression, implicated in survival, differently in different carcinoma cell lines. Carcinogenesis 2004;25:2089–2100. |
| 33. | Frede S, Stockmann C, Freitag P, Fandrey J. Bacterial lipopolysaccharide induces HIF-1 activation in human monocytes via p44/42 MAPK and NF-kappaB. Biochem J 2006;396:517–527. |
| 34. | Bonello S, Zahringer C, Belaiba RS, Djordjevic T, Hess J, Michiels C, Kietzmann T, Gorlach A. Reactive oxygen species activate the HIF-1alpha promoter via a functional NFkappaB site. Arterioscler Thromb Vasc Biol 2007;27:755–761. |
| 35. | Belaiba RS, Bonello S, Zahringer C, Schmidt S, Hess J, Kietzmann T, Gorlach A. Hypoxia up-regulates hypoxia-inducible factor–1alpha transcription by involving phosphatidylinositol 3-kinase and nuclear factor kappaB in pulmonary artery smooth muscle cells. Mol Biol Cell 2007;18:4691–4697. |
| 36. | Kim J, Shao Y, Kim SY, Kim S, Song HK, Jeon JH, Suh HW, Chung JW, Yoon SR, Kim YS, et al.. Hypoxia-induced IL-18 increases hypoxia-inducible factor–1alpha expression through a Rac1-dependent NF-kappaB pathway. Mol Biol Cell 2008;19:433–444. |
| 37. | Pez F, Dayan F, Durivault J, Kaniewski B, Aimond G, Le Provost GS, Deux B, Clezardin P, Sommer P, Pouyssegur J, et al.. The HIF-1–inducible lysyl oxidase activates HIF-1 via the Akt pathway in a positive regulation loop and synergizes with HIF-1 in promoting tumor cell growth. Cancer Res 2011;71:1647–1657. |
| 38. | Koshikawa N, Hayashi J, Nakagawara A, Takenaga K. Reactive oxygen species–generating mitochondrial DNA mutation up-regulates hypoxia-inducible factor–1alpha gene transcription via phosphatidylinositol 3-kinase–Akt/protein kinase C/histone deacetylase pathway. J Biol Chem 2009;284:33185–33194. |
| 39. | Zhong H, Chiles K, Feldser D, Laughner E, Hanrahan C, Georgescu MM, Simons JW, Semenza GL. Modulation of hypoxia-inducible factor 1alpha expression by the epidermal growth factor/phosphatidylinositol 3-kinase/PTEN/Akt/FRAP pathway in human prostate cancer cells: implications for tumor angiogenesis and therapeutics. Cancer Res 2000;60:1541–1545. |
| 40. | Feldser D, Agani F, Iyer NV, Pak B, Ferreira G, Semenza GL. Reciprocal positive regulation of hypoxia-inducible factor 1alpha and insulin-like growth factor 2. Cancer Res 1999;59:3915–3918. |
| 41. | Eckle T, Hartmann K, Bonney S, Reithel S, Mittelbronn M, Walker LA, Lowes BD, Han J, Borchers CH, Buttrick PM, et al.. Adora2b-elicited Per2 stabilization promotes a HIF-dependent metabolic switch crucial for myocardial adaptation to ischemia. Nat Med 2012;18:774–782. |
| 42. | Cummins EP, Berra E, Comerford KM, Ginouves A, Fitzgerald KT, Seeballuck F, Godson C, Nielsen JE, Moynagh P, Pouyssegur J, et al.. Prolyl hydroxylase–1 negatively regulates IkappaB kinase–beta, giving insight into hypoxia-induced NFkappaB activity. Proc Natl Acad Sci USA 2006;103:18154–18159. |
| 43. | Cockman ME, Lancaster DE, Stolze IP, Hewitson KS, McDonough MA, Coleman ML, Coles CH, Yu X, Hay RT, Ley SC, et al.. 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 2006;103:14767–14772. |
| 44. | Winning S, Splettstoesser F, Fandrey J, Frede S. Acute hypoxia induces HIF-independent monocyte adhesion to endothelial cells through increased intercellular adhesion molecule–1 expression: the role of hypoxic inhibition of prolyl hydroxylase activity for the induction of NF-kappa B. J Immunol 2010;185:1786–1793. |
| 45. | Siddiq A, Ayoub IA, Chavez JC, Aminova L, Shah S, LaManna JC, Patton SM, Connor JR, Cherny RA, Volitakis I, et al.. Hypoxia-inducible factor prolyl 4–hydroxylase inhibition: a target for neuroprotection in the central nervous system. J Biol Chem 2005;280:41732–41743. |
| 46. | Walmsley SR, Chilvers ER, Thompson AA, Vaughan K, Marriott HM, Parker LC, Shaw G, Parmar S, Schneider M, Sabroe I, et al.. Prolyl hydroxylase 3 (PHD3) is essential for hypoxic regulation of neutrophilic inflammation in humans and mice. J Clin Invest 2011;121:1053–1063. |
| 47. | Kiss J, Mollenhauer M, Walmsley SR, Kirchberg J, Radhakrishnan P, Niemietz T, Dudda J, Steinert G, Whyte MK, Carmeliet P, et al.. Loss of the oxygen sensor PHD3 enhances the innate immune response to abdominal sepsis. J Immunol 2012;189:1955–1965. |
| 48. | Zhang Y, Fei M, Xue G, Zhou Q, Jia Y, Li L, Xin H, Sun S. Elevated levels of hypoxia-inducible microRNA-210 in pre-eclampsia: new insights into molecular mechanisms for the disease. J Cell Mol Med 2012;16:249–259. |
| 49. | Chan SY, Loscalzo J. MicroRNA-210: a unique and pleiotropic hypoxamir. Cell Cycle 2010;9:1072–1083. |
| 50. | Wu C, So J, Davis-Dusenbery BN, Qi HH, Bloch DB, Shi Y, Lagna G, Hata A. Hypoxia potentiates microRNA-mediated gene silencing through posttranslational modification of Argonaute2. Mol Cell Biol 2011;31:4760–4774. |
| 51. | Fahling M, Mrowka R, Steege A, Nebrich G, Perlewitz A, Persson PB, Thiele BJ. Translational control of collagen prolyl 4-hydroxylase–alpha(I) gene expression under hypoxia. J Biol Chem 2006;281:26089–26101. |
| 52. | Tsai AG, Friesenecker B, Mazzoni MC, Kerger H, Buerk DG, Johnson PC, Intaglietta M. Microvascular and tissue oxygen gradients in the rat mesentery. Proc Natl Acad Sci USA 1998;95:6590–6595. |
| 53. | Rolett EL, Azzawi A, Liu KJ, Yongbi MN, Swartz HM, Dunn JF. Critical oxygen tension in rat brain: a combined (31)P-NMR and EPR oximetry study. Am J Physiol Regul Integr Comp Physiol 2000;279:R9–R16. |
| 54. | Richmond KN, Shonat RD, Lynch RM, Johnson PC. Critical PO(2) of skeletal muscle in vivo. Am J Physiol 1999;277:H1831–H1840. |
| 55. | Leong CL, Anderson WP, O’Connor PM, Evans RG. Evidence that renal arterial–venous oxygen shunting contributes to dynamic regulation of renal oxygenation. Am J Physiol Renal Physiol 2007;292:F1726–F1733. |
| 56. | Hyman AL, Higashida RT, Spannhake EW, Kadowitz PJ. Pulmonary vasoconstrictor responses to graded decreases in precapillary blood PO2 in intact-chest cat. J Appl Physiol 1981;51:1009–1016. |
| 57. | Loutzenhiser RD, Parker MJ. Hypoxia inhibits myogenic reactivity of renal afferent arterioles by activating ATP-sensitive K+ channels. Circ Res 1994;74:861–869. |
| 58. | Bergofsky EH, Bass BG, Ferretti R, Fishman AP. Pulmonary vasoconstriction in response to precapillary hypoxemia. J Clin Invest 1963;42:1201–1215. |
| 59. | Svanes K. Studies in hypothermia: VII. Anaphylactoid reaction to low-molecular-weight dextran in hypothermic mice. Acta Anaesthesiol Scand 1966;10:133–146. |
| 60. | West JB. The physiologic basis of high-altitude diseases. Ann Intern Med 2004;141:789–800. |
| 61. | Hyvelin JM, Howell K, Nichol A, Costello CM, Preston RJ, McLoughlin P. Inhibition of rho-kinase attenuates hypoxia-induced angiogenesis in the pulmonary circulation. Circ Res 2005;97:185–191. |
| 62. | Nagaoka T, Morio Y, Casanova N, Bauer N, Gebb S, McMurtry I, Oka M. Rho/rho kinase signaling mediates increased basal pulmonary vascular tone in chronically hypoxic rats. Am J Physiol Lung Cell Mol Physiol 2004;287:L665–L672. |
| 63. | Hopkins N, McLoughlin P. The structural basis of pulmonary hypertension in chronic lung disease: remodelling, rarefaction or angiogenesis? J Anat 2002;201:335–348. |
| 64. | Teng X, Li D, Champion HC, Johns RA. FIZZ1/RELMalpha, a novel hypoxia-induced mitogenic factor in lung with vasoconstrictive and angiogenic properties. Circ Res 2003;92:1065–1067. |
| 65. | Yamaji-Kegan K, Su Q, Angelini DJ, Champion HC, Johns RA. Hypoxia-induced mitogenic factor has proangiogenic and proinflammatory effects in the lung via VEGF and VEGF receptor–2. Am J Physiol Lung Cell Mol Physiol 2006;291:L1159–L1168. |
| 66. | Angelini DJ, Su Q, Yamaji-Kegan K, Fan C, Skinner JT, Champion HC, Crow MT, Johns RA. Hypoxia-induced mitogenic factor (HIMF/FIZZ1/RELMalpha) induces the vascular and hemodynamic changes of pulmonary hypertension. Am J Physiol Lung Cell Mol Physiol 2009;296:L582–L593. |
| 67. | Yamaji-Kegan K, Su Q, Angelini DJ, Myers AC, Cheadle C, Johns RA. Hypoxia-induced mitogenic factor (HIMF/FIZZ1/RELMalpha) increases lung inflammation and activates pulmonary microvascular endothelial cells via an IL-4–dependent mechanism. J Immunol 2010;185:5539–5548. |
| 68. | Costello CM, Howell K, Cahill E, McBryan J, Konigshoff M, Eickelberg O, Gaine S, Martin F, McLoughlin P. Lung-selective gene responses to alveolar hypoxia: potential role for the bone morphogenetic antagonist gremlin in pulmonary hypertension. Am J Physiol Lung Cell Mol Physiol 2008;295:L272–L284. |
| 69. | Cahill E, Costello CM, Rowan SC, Harkin S, Howell K, Leonard MO, Southwood M, Cummins EP, Fitzpatrick SF, Taylor CT, et al.. Gremlin plays a key role in the pathogenesis of pulmonary hypertension. Circulation 2012;125:920–930. |
| 70. | Naumann U, Cameroni E, Pruenster M, Mahabaleshwar H, Raz E, Zerwes HG, Rot A, Thelen M. CXCR7 functions as a scavenger for CXCL12 and CXCL11. PLoS ONE 2010;5:e9175. |
| 71. | Bleul CC, Farzan M, Choe H, Parolin C, Clark-Lewis I, Sodroski J, Springer TA. The lymphocyte chemoattractant SDF-1 is a ligand for LESTR/fusin and blocks HIV-1 entry. Nature 1996;382:829–833. |
| 72. | Burke DL, Frid MG, Kunrath CL, Karoor V, Anwar A, Wagner BD, Strassheim D, Stenmark KR. Sustained hypoxia promotes the development of a pulmonary artery–specific chronic inflammatory microenvironment. Am J Physiol Lung Cell Mol Physiol 2009;297:L238–L250. |
| 73. | Zabel BA, Lewen S, Berahovich RD, Jaen JC, Schall TJ. The novel chemokine receptor CXCR7 regulates trans-endothelial migration of cancer cells. Mol Cancer 2011;10:73. |
| 74. | Miao Z, Luker KE, Summers BC, Berahovich R, Bhojani MS, Rehemtulla A, Kleer CG, Essner JJ, Nasevicius A, Luker GD, et al.. CXCR7 (RDC1) promotes breast and lung tumor growth in vivo and is expressed on tumor-associated vasculature. Proc Natl Acad Sci USA 2007;104:15735–15740. |
| 75. | Struyf S, Noppen S, Loos T, Mortier A, Gouwy M, Verbeke H, Huskens D, Luangsay S, Parmentier M, Geboes K, et al.. Citrullination of CXCL12 differentially reduces CXCR4 and CXCR7 binding with loss of inflammatory and anti-HIV-1 activity via CXCR4. J Immunol 2009;182:666–674. |
| 76. | Costello CM, McCullagh B, Howell K, Sands M, Belperio JA, Keane MP, Gaine S, McLoughlin P: A role for the CXCL12 receptor, CXCR7, in the pathogenesis of human pulmonary vascular disease. Eur Respir J 2012;39:1415–1424. |
| 77. | Gambaryan N, Perros F, Montani D, Cohen-Kaminsky S, Mazmanian M, Renaud JF, Simonneau G, Lombet A, Humbert M: Targeting of C-kit+ haematopoietic progenitor cells prevents hypoxic pulmonary hypertension. Eur Respir J 2011;37:1392–1399. |
| 78. | Sartina E, Suguihara C, Ramchandran S, Nwajei P, Rodriguez M, Torres E, Hehre D, Devia C, Walters MJ, Penfold ME, et al.. Antagonism of CXCR7 attenuates chronic hypoxia-induced pulmonary hypertension. Pediatr Res 2012;71:682–688. |
| 79. | Howell K, Preston RJ, McLoughlin P. Chronic hypoxia causes angiogenesis in addition to remodelling in the adult rat pulmonary circulation. J Physiol 2003;547:133–145. |
| 80. | Yan SF, Mackman N, Kisiel W, Stern DM, Pinsky DJ. Hypoxia/hypoxemia-induced activation of the procoagulant pathways and the pathogenesis of ischemia-associated thrombosis. Arterioscler Thromb Vasc Biol 1999;19:2029–2035. |
| 81. | Kubo K, Hanaoka M, Hayano T, Miyahara T, Hachiya T, Hayasaka M, Koizumi T, Fujimoto K, Kobayashi T, Honda T. Inflammatory cytokines in BAL fluid and pulmonary hemodynamics in high-altitude pulmonary edema. Respir Physiol 1998;111:301–310. |
| 82. | Chao J, Wood JG, Blanco VG, Gonzalez NC. The systemic inflammation of alveolar hypoxia is initiated by alveolar macrophage–borne mediator(s). Am J Respir Cell Mol Biol 2009;41:573–582. |
| 83. | Chao J, Donham P, van Rooijen N, Wood JG, Gonzalez NC. Monocyte chemoattractant protein–1 released from alveolar macrophages mediates the systemic inflammation of acute alveolar hypoxia. Am J Respir Cell Mol Biol 2011;45:53–61. |
| 84. | Gonzalez NC, Allen J, Blanco VG, Schmidt EJ, van Rooijen N, Wood JG. Alveolar macrophages are necessary for the systemic inflammation of acute alveolar hypoxia. J Appl Physiol 2007;103:1386–1394. |
| 85. | Howell K, Costello CM, Sands M, Dooley I, McLoughlin P. L-arginine promotes angiogenesis in the chronically hypoxic lung: a novel mechanism ameliorating pulmonary hypertension. Am J Physiol Lung Cell Mol Physiol 2009;296:L1042–L1050. |
| 86. | Yu AY, Shimoda LA, Iyer NV, Huso DL, Sun X, McWilliams R, Beaty T, Sham JS, Wiener CM, Sylvester JT, et al.. Impaired physiological responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1alpha. J Clin Invest 1999;103:691–696. |
| 87. | Brusselmans K, Compernolle V, Tjwa M, Wiesener MS, Maxwell PH, Collen D, Carmeliet P. Heterozygous deficiency of hypoxia-inducible factor–2alpha protects mice against pulmonary hypertension and right ventricular dysfunction during prolonged hypoxia. J Clin Invest 2003;111:1519–1527. |
| 88. | Leonard MO, Howell K, Madden SF, Costello CM, Higgins DG, Taylor CT, McLoughlin P. Hypoxia selectively activates the CREB family of transcription factors in the in vivo lung. Am J Respir Crit Care Med 2008;178:977–983. |
| 89. | Smith TG, Brooks JT, Balanos GM, Lappin TR, Layton DM, Leedham DL, Liu C, Maxwell PH, McMullin MF, McNamara CJ, et al.. Mutation of von Hippel–Lindau tumour suppressor and human cardiopulmonary physiology. PLoS Med 2006;3:e290. |
| 90. | Hickey MM, Richardson T, Wang T, Mosqueira M, Arguiri E, Yu H, Yu QC, Solomides CC, Morrisey EE, Khurana TS, et al.. The von Hippel–Lindau Chuvash mutation promotes pulmonary hypertension and fibrosis in mice. J Clin Invest 2010;120:827–839. |
| 91. | Yu AY, Frid MG, Shimoda LA, Wiener CM, Stenmark K, Semenza GL. Temporal, spatial, and oxygen-regulated expression of hypoxia-inducible factor–1 in the lung. Am J Physiol 1998;275:L818–L826. |
| 92. | Mayr B, Montminy M. Transcriptional regulation by the phosphorylation-dependent factor CREB. Nat Rev Mol Cell Biol 2001;2:599–609. |
| 93. | Chava KR, Tauseef M, Sharma T, Mehta D. Cyclic AMP response element–binding protein prevents endothelial permeability increase through transcriptional controlling p190RhoGAP expression. Blood 2012;119:308–319. |
| 94. | Fitzpatrick SF, Tambuwala MM, Bruning U, Schaible B, Scholz CC, Byrne A, O’Connor A, Gallagher WM, Lenihan CR, Garvey JF, et al.. An intact canonical NF-kappaB pathway is required for inflammatory gene expression in response to hypoxia. J Immunol 2011;186:1091–1096. |
| 95. | Rius J, Guma M, Schachtrup C, Akassoglou K, Zinkernagel AS, Nizet V, Johnson RS, Haddad GG, Karin M. NF-kappaB links innate immunity to the hypoxic response through transcriptional regulation of HIF-1alpha. Nature 2008;453:807–811. |
| 96. | Sarada S, Himadri P, Mishra C, Geetali P, Ram MS, Ilavazhagan G. Role of oxidative stress and NFkB in hypoxia-induced pulmonary edema. Exp Biol Med (Maywood) 2008;233:1088–1098. |
| 97. | Caruso P, Dempsie Y, Stevens HC, McDonald RA, Long L, Lu R, White K, Mair KM, McClure JD, Southwood M, et al.. A role for miR-145 in pulmonary arterial hypertension: evidence from mouse models and patient samples. Circ Res 2012;111:290–300. |
| 98. | Caruso P, MacLean MR, Khanin R, McClure J, Soon E, Southgate M, MacDonald RA, Greig JA, Robertson KE, Masson R, et al.. Dynamic changes in lung microRNA profiles during the development of pulmonary hypertension due to chronic hypoxia and monocrotaline. Arterioscler Thromb Vasc Biol 2010;30:716–723. |
| 99. | Gou D, Ramchandran R, Peng X, Yao L, Kang K, Sarkar J, Wang Z, Zhou G, Raj JU. miR-210 has an anti-apoptotic effect in pulmonary artery smooth muscle cells during hypoxia. Am J Physiol Lung Cell Mol Physiol 2012;303:L682–L691. |
| 100. | Guo L, Qiu Z, Wei L, Yu X, Gao X, Jiang S, Tian H, Jiang C, Zhu D. The microRNA-328 regulates hypoxic pulmonary hypertension by targeting at insulin growth factor 1 receptor and L-type calcium channel–alpha1C. Hypertension 2012;59:1006–1013. |
| 101. | Yang S, Banerjee S, Freitas A, Cui H, Xie N, Abraham E, Liu G. miR-21 regulates chronic hypoxia-induced pulmonary vascular remodeling. Am J Physiol Lung Cell Mol Physiol 2012;302:L521–L529. |
| 102. | Ware LB, Matthay MA. The acute respiratory distress syndrome. N Engl J Med 2000;342:1334–1349. |
| 103. | Dantzker DR, Brook CJ, Dehart P, Lynch JP, Weg JG. Ventilation–perfusion distributions in the adult respiratory distress syndrome. Am Rev Respir Dis 1979;120:1039–1052. |
| 104. | Pappert D, Rossaint R, Slama K, Gruning T, Falke KJ. Influence of positioning on ventilation–perfusion relationships in severe adult respiratory distress syndrome. Chest 1994;106:1511–1516. |
| 105. | Feihl F, Eckert P, Brimioulle S, Jacobs O, Schaller MD, Melot C, Naeije R. Permissive hypercapnia impairs pulmonary gas exchange in the acute respiratory distress syndrome. Am J Respir Crit Care Med 2000;162:209–215. |
| 106. | Henzler D, Pelosi P, Bensberg R, Dembinski R, Quintel M, Pielen V, Rossaint R, Kuhlen R. Effects of partial ventilatory support modalities on respiratory function in severe hypoxemic lung injury. Crit Care Med 2006;34:1738–1745. |
| 107. | Cressoni M, Caironi P, Polli F, Carlesso E, Chiumello D, Cadringher P, Quintel M, Ranieri VM, Bugedo G, Gattinoni L. Anatomical and functional intrapulmonary shunt in acute respiratory distress syndrome. Crit Care Med 2008;36:669–675. |
| 108. | Mohsenifar Z, Goldbach P, Tashkin DP, Campisi DJ. Relationship between O2 delivery and O2 consumption in the adult respiratory distress syndrome. Chest 1983;84:267–271. |
| 109. | Bull TM, Clark B, McFann K, Moss M. Pulmonary vascular dysfunction is associated with poor outcomes in patients with acute lung injury. Am J Respir Crit Care Med 2010;182:1123–1128. |
S.F. is funded by grants from the St. Vincent’s Hospital Anesthesia Foundation, the Health Research Board of Ireland, the College of Anaesthetists Ireland, and the Intensive Care Society of Ireland. P.M. receives grants from the Health Research Board of Ireland and the Program for Research in Third Level Institutes Ireland.
Originally Published in Press as DOI: 10.1165/rcmb.2012-0137TR on October 18, 2012
Author disclosures
