Adenosine Modulates NR4A Orphan Nuclear Receptors To Attenuate Hyperinflammatory Responses in Monocytic Cells

Adenosine receptor-mediated regulation of monocyte/macrophage inflammatory responses is critical in the maintenance of tissue homeostasis. In this study, we reveal that adenosine potently modulates the expression of NR4A1, 2, and 3 orphan nuclear receptors in myeloid cells, and this modulation is primarily through the adenosine A2a receptor subtype. We demonstrate that A2a receptor activation of NR4A1-3 receptor synthesis is further enhanced in TLR4-stimulated monocytes. After TLR4 stimulation, NR4A receptor-depleted monocyte/macrophage cells display significantly altered expression of cell-surface markers and produce increased inflammatory cytokine and chemokine secretion rendering the cells an enhanced proinflammatory phenotype. Exposure of TLR4 or TNF-α-stimulated monocytes to adenosine analogs directs changes in the expression of MIP-3α and IL-23p19, with NR4A2 depletion leading to significantly enhanced expression of these factors. Furthermore, we establish that nuclear levels of NF-κB/p65 are increased in TLR/adenosine-stimulated NR4A2-depleted cells. We show that, after TLR/adenosine receptor stimulation, NR4A2 depletion promotes significant binding of NF-κB/p65 to a κB consensus binding motif within the MIP-3α proximal promoter leading to increased protein secretion, confirming a pivotal role for NF-κB activity in controlling cellular responses and gene expression outcomes in response to these mediators. Thus, these data demonstrate that during an inflammatory response, adenosine modulation of NR4A receptor activity acts to limit NF-κB-mediated effects and that loss of NR4A2 expression leads to enhanced NF-κB activity and hyperinflammatory responses in myeloid cells.

Carbon dioxide-sensing in organisms and its implications for human disease

The capacity of organisms to sense changes in the levels of internal and external gases and to respond accordingly is central to a range of physiologic and pathophysiologic processes. Carbon dioxide, a primary product of oxidative metabolism is one such gas that can be sensed by both prokaryotic and eukaryotic cells and in response to altered levels, elicit the activation of multiple adaptive pathways. The outcomes of activating CO2-sensitive pathways in various species include increased virulence of fungal and bacterial pathogens, prey-seeking behavior in insects as well as taste perception, lung function, and the control of immunity in mammals. In this review, we discuss what is known about the mechanisms underpinning CO2 sensing across a range of species and consider the implications of this for physiology, disease progression, and the possibility of developing new therapeutics for inflammatory and infectious disease.

A dynamic model of the hypoxia-inducible factor 1α (HIF-1α) network

Activation of the hypoxia-inducible factor (HIF) pathway is a critical step in the transcriptional response to hypoxia. Although many of the key proteins involved have been characterised, the dynamics of their interactions in generating this response remain unclear. In the present study, we have generated a comprehensive mathematical model of the HIF-1α pathway based on core validated components and dynamic experimental data, and confirm the previously described connections within the predicted network topology. Our model confirms previous work demonstrating that the steps leading to optimal HIF-1α transcriptional activity require sequential inhibition of both prolyl- and asparaginyl-hydroxylases. We predict from our model (and confirm experimentally) that there is residual activity of the asparaginyl-hydroxylase FIH (factor inhibiting HIF) at low oxygen tension. Furthermore, silencing FIH under conditions where prolyl-hydroxylases are inhibited results in increased HIF-1α transcriptional activity, but paradoxically decreases HIF-1α stability. Using a core module of the HIF network and mathematical proof supported by experimental data, we propose that asparaginyl hydroxylation confers a degree of resistance upon HIF-1α to proteosomal degradation. Thus, through in vitro experimental data and in silico predictions, we provide a comprehensive model of the dynamic regulation of HIF-1α transcriptional activity by hydroxylases and use its predictive and adaptive properties to explain counter-intuitive biological observations.

HIF-2α - a mediator of stem cell altruism?

Human embryonic stem cells (hESCs) have been reported to confer cytoprotection in the context of tissue injury. This is somewhat counterintuitive given that microenvironmental factors such as hypoxia and oxidative stress may activate p53 and result in death and differentiation of these hESCs. In this article, we discuss a novel mechanism through which hESCs can be re-programmed (through exposure to hypoxia/oxidative stress) to transiently suppress p53, enhance 'stemness', and exist in a highly cytoprotective and undifferentiated state.

Hypercapnia induces cleavage and nuclear localization of RelB protein, giving insight into CO2 sensing and signaling

Carbon dioxide (CO₂) is increasingly being appreciated as an intracellular signaling molecule that affects inflammatory and immune responses. Elevated arterial CO₂ (hypercapnia) is encountered in a range of clinical conditions, including chronic obstructive pulmonary disease, and as a consequence of therapeutic ventilation in acute respiratory distress syndrome. In patients suffering from this syndrome, therapeutic hypoventilation strategy designed to reduce mechanical damage to the lungs is accompanied by systemic hypercapnia and associated acidosis, which are associated with improved patient outcome. However, the molecular mechanisms underlying the beneficial effects of hypercapnia and the relative contribution of elevated CO₂ or associated acidosis to this response remain poorly understood. Recently, a role for the non-canonical NF-κB pathway has been postulated to be important in signaling the cellular transcriptional response to CO₂. In this study, we demonstrate that in cells exposed to elevated CO₂, the NF-κB family member RelB was cleaved to a lower molecular weight form and translocated to the nucleus in both mouse embryonic fibroblasts and human pulmonary epithelial cells (A549). Furthermore, elevated nuclear RelB was observed in vivo and correlated with hypercapnia-induced protection against LPS-induced lung injury. Hypercapnia-induced RelB processing was sensitive to proteasomal inhibition by MG-132 but was independent of the activity of glycogen synthase kinase 3β or MALT-1, both of which have been previously shown to mediate RelB processing. Taken together, these data demonstrate that RelB is a CO₂-sensitive NF-κB family member that may contribute to the beneficial effects of hypercapnia in inflammatory diseases of the lung.

Gremlin plays a key role in the pathogenesis of pulmonary hypertension

BACKGROUND

Pulmonary hypertension occurs in chronic hypoxic lung diseases, significantly worsening morbidity and mortality. The important role of altered bone morphogenetic protein (BMP) signaling in pulmonary hypertension was first suspected after the identification of heterozygous BMP receptor mutations as the underlying defect in the rare heritable form of pulmonary arterial hypertension. Subsequently, it was demonstrated that BMP signaling was also reduced in common forms of pulmonary hypertension, including hypoxic pulmonary hypertension; however, the mechanism of this reduction has not previously been elucidated.

METHODS AND RESULTS

Expression of 2 BMP antagonists, gremlin 1 and gremlin 2, was higher in the lung than in other organs, and gremlin 1 was further increased in the walls of small intrapulmonary vessels of mice during the development of hypoxic pulmonary hypertension. Hypoxia stimulated gremlin secretion from human pulmonary microvascular endothelial cells in vitro, which inhibited endothelial BMP signaling and BMP-stimulated endothelial repair. Haplodeficiency of gremlin 1 augmented BMP signaling in the hypoxic mouse lung and reduced pulmonary vascular resistance by attenuating vascular remodeling. Furthermore, gremlin was increased in the walls of small intrapulmonary vessels in idiopathic pulmonary arterial hypertension and the rare heritable form of pulmonary arterial hypertension in a distribution suggesting endothelial localization.

CONCLUSIONS

These findings demonstrate a central role for increased gremlin in hypoxia-induced pulmonary vascular remodeling and the increased pulmonary vascular resistance in hypoxic pulmonary hypertension. High levels of basal gremlin expression in the lung may account for the unique vulnerability of the pulmonary circulation to heterozygous mutations of BMP type 2 receptor in pulmonary arterial hypertension.

Regulation of gene expression by carbon dioxide

Carbon dioxide (CO(2)) is a physiological gas found at low levels in the atmosphere and produced in cells during the process of aerobic respiration. Consequently, the levels of CO(2) within tissues are usually significantly higher than those found externally. Shifts in tissue levels of CO(2) (leading to either hypercapnia or hypocapnia) are associated with a number of pathophysiological conditions in humans and can occur naturally in niche habitats such as those of burrowing animals. Clinical studies have indicated that such altered CO(2) levels can impact upon disease progression. Recent advances in our understanding of the biology of CO(2) has shown that like other physiological gases such as molecular oxygen (O(2)) and nitric oxide (NO), CO(2) levels can be sensed by cells resulting in the initiation of physiological and pathophysiological responses. Acute CO(2) sensing in neurons and peripheral and central chemoreceptors is important in rapidly activated responses including olfactory signalling, taste sensation and cardiorespiratory control. Furthermore, a role for CO(2) in the regulation of gene transcription has recently been identified with exposure of cells and model organisms to high CO(2) leading to suppression of genes involved in the regulation of innate immunity and inflammation. This latter, transcriptional regulatory role for CO(2), has been largely attributed to altered activity of the NF-B family of transcription factors. Here, we review our evolving understanding of how CO(2) impacts upon gene transcription.

An intact canonical NF-κB pathway is required for inflammatory gene expression in response to hypoxia

Hypoxia is a feature of the microenvironment in a number of chronic inflammatory conditions due to increased metabolic activity and disrupted perfusion at the inflamed site. Hypoxia contributes to inflammation through the regulation of gene expression via key oxygen-sensitive transcriptional regulators including the hypoxia-inducible factor (HIF) and NF-κB. Recent studies have revealed a high degree of interdependence between HIF and NF-κB signaling; however, the relative contribution of each to hypoxia-induced inflammatory gene expression remains unclear. In this study, we use transgenic mice expressing luciferase under the control of NF-κB to demonstrate that hypoxia activates NF-κB in the heart and lungs of mice in vivo. Using small interfering RNA targeted to the p65 subunit of NF-κB, we confirm a unidirectional dependence of hypoxic HIF-1α accumulation upon an intact canonical NF-κB pathway in cultured cells. Cyclooxygenase-2 and other key proinflammatory genes are transcriptionally induced by hypoxia in a manner that is both HIF-1 and NF-κB dependent, and in mouse embryonic fibroblasts lacking an intact canonical NF-κB pathway, there is a loss of hypoxia-induced inflammatory gene expression. Finally, under conditions of hypoxia, HIF-1α and the p65 subunit of NF-κB directly bind to the cyclooxygenase-2 promoter. These results implicate an essential role for NF-κB signaling in inflammatory gene expression in response to hypoxia both through the regulation of HIF-1 and through direct effects upon target gene expression.

Small ubiquitin-related modifier (SUMO)-1 promotes glycolysis in hypoxia

Under conditions of hypoxia, most eukaryotic cells undergo a shift in metabolic strategy, which involves increased flux through the glycolytic pathway. Although this is critical for bioenergetic homeostasis, the underlying mechanisms have remained incompletely understood. Here, we report that the induction of hypoxia-induced glycolysis is retained in cells when gene transcription or protein synthesis are inhibited suggesting the involvement of additional post-translational mechanisms. Post-translational protein modification by the small ubiquitin related modifier-1 (SUMO-1) is induced in hypoxia and mass spectrometric analysis using yeast cells expressing tap-tagged Smt3 (the yeast homolog of mammalian SUMO) revealed hypoxia-dependent modification of a number of key glycolytic enzymes. Overexpression of SUMO-1 in mammalian cancer cells resulted in increased hypoxia-induced glycolysis and resistance to hypoxia-dependent ATP depletion. Supporting this, non-transformed cells also demonstrated increased glucose uptake upon SUMO-1 overexpression. Conversely, cells overexpressing the de-SUMOylating enzyme SENP-2 failed to demonstrate hypoxia-induced glycolysis. SUMO-1 overexpressing cells demonstrated focal clustering of glycolytic enzymes in response to hypoxia leading us to hypothesize a role for SUMOylation in promoting spatial re-organization of the glycolytic pathway. In summary, we hypothesize that SUMO modification of key metabolic enzymes plays an important role in shifting cellular metabolic strategies toward increased flux through the glycolytic pathway during periods of hypoxic stress.

Loss of prolyl hydroxylase-1 protects against colitis through reduced epithelial cell apoptosis and increased barrier function

BACKGROUND & AIMS

Hypoxia inducible factor (HIF) prolyl hydroxylase inhibitors are protective in mouse models of inflammatory bowel disease (IBD). Here, we investigated the therapeutic target(s) and mechanism(s) involved.

METHODS

The effect of genetic deletion of individual HIF-prolyl hydroxylase (PHD) enzymes on the development of dextran sulphate sodium (DSS)-induced colitis was examined in mice.

RESULTS

PHD1(-/-), but not PHD2(+/-) or PHD3(-/-), mice were less susceptible to the development of colitis than wild-type controls as determined by weight loss, disease activity, colon histology, neutrophil infiltration, and cytokine expression. Reduced susceptibility of PHD1(-/-) mice to colitis was associated with increased density of colonic epithelial cells relative to wild-type controls, which was because of decreased levels of apoptosis that resulted in enhanced epithelial barrier function. Furthermore, with the use of cultured epithelial cells it was confirmed that hydroxylase inhibition reversed DSS-induced apoptosis and barrier dysfunction. Finally, PHD1 levels were increased with disease severity in intestinal tissue from patients with IBD and in colonic tissues from DSS-treated mice.

CONCLUSIONS

These results imply a role for PHD1 as a positive regulator of intestinal epithelial cell apoptosis in the inflamed colon. Genetic loss of PHD1 is protective against colitis through decreased epithelial cell apoptosis and consequent enhancement of intestinal epithelial barrier function. Thus, targeted PHD1 inhibition may represent a new therapeutic approach in IBD.

The role of NF-kappaB in hypoxia-induced gene expression

Hypoxia is a common physiologic and pathophysiologic stimulus that activates the expression of genes through oxygen-sensitive transcription factors including the hypoxia-inducible factor (HIF) and nuclear factor-kappaB (NF-kappaB). Hypoxia-dependent gene expression can have important physiologic or pathophysiologic consequences for an organism, depending upon the cause of the hypoxic insult. Consequently, this pathway represents an attractive therapeutic target in a number of disease states. While the mechanism linking hypoxia to the activation of HIF has been extensively studied, our understanding of how hypoxia activates NF-kappaB is limited. Recent studies have demonstrated that similar oxygen-sensing mechanisms are employed in conferring oxygen sensitivity to both HIF and NF-kappaB-dependent gene expression. Furthermore, there is an extensive degree of cross-talk occurring between NF-kappaB and HIF. Investigations into mechanisms of hypoxic activation of HIF and NF-kappaB and how these signaling pathways interact will uncover new therapeutic modalities in a diverse range of disease states where hypoxia is a feature of the microenvironment including cancer, vascular disease, and chronic inflammation.

Hypoxia activates NF-kappaB-dependent gene expression through the canonical signaling pathway

Hypoxia and inflammation are coincidental events in a diverse range of disease states including tumor growth, ischemia, and chronic inflammation. Hypoxia contributes to the development of inflammation, at least in part through the activation and/or potentiation of NF-kappaB, a master regulator of genes involved in innate immunity, inflammation, and apoptosis. NF-kappaB can be activated through two distinct signaling pathways termed the canonical and noncanonical pathways, respectively. The canonical pathway is activated through the IKKalpha/beta/gamma complex, while the noncanonical pathway involves NIK-mediated activation of IKKalpha homodimers. In the current study, we have investigated the relative roles of these two pathways in hypoxia-dependent NF-kappaB activation. Lymphotoxin alpha1beta2 (LTalpha1beta2) activated both the canonical and noncanonical NF-kappaB signaling pathways in HeLa cells. Sustained hypoxia enhanced basal and LTalpha1beta2-induced NF-kappaB activity in a manner that was dependent upon the canonical but not the noncanonical signaling pathway. Intermittent hypoxia activated NF-kappaB in a manner that was also primarily dependent upon the canonical pathway. Knockdown of the p65 subunit of the canonical NF-kappaB pathway was sufficient to abolish the effects of hypoxia on LTalpha1beta2-induced NF-kappaB activity. Furthermore, in synovial biopsies obtained at arthroscopy from patients with active inflammatory arthritis, the canonical pathway was preferentially activated in those patents with lower joint pO2 values. In summary, we hypothesize that hypoxia enhances NF-kappaB activity primarily through affecting the canonical pathway.

Hypoxia. Regulation of NFkappaB signalling during inflammation: the role of hydroxylases

NFκB is a master regulator of innate immunity and inflammatory signalling. Microenvironmental hypoxia has long been identified as being coincident with chronic inflammation. The contribution of microenvironmental hypoxia to NFκB-induced inflammation has more recently been appreciated. Identification of the co-regulation of NFκB and hypoxia inducible factor (HIF) pathways by 2-oxo-glutarate-dependent hydroxylase family members has highlighted an intimate relationship between NFκB inflammatory signalling and HIF-mediated hypoxic signalling pathways. Adding another layer of complexity to our understanding of the role of NFκB inflammatory signalling by hypoxia is the recent recognition of the contribution of basal NFκB activity to HIF-1α transcription. This observation implicates an important and previously unappreciated role for NFκB in inflammatory disease where HIF-1α is activated. The present review will discuss recent literature pertaining to the regulation of NFκB inflammatory signalling by hypoxia and some of the inflammatory diseases where this may play an important role. Furthermore, we will discuss the potential for prolylhydroxylase inhibitors in inflammatory disease.

The hydroxylase inhibitor dimethyloxalylglycine is protective in a murine model of colitis

BACKGROUND & AIMS

Prolyl and asparaginyl hydroxylases are key oxygen-sensing enzymes that confer hypoxic sensitivity to transcriptional regulatory pathways including the hypoxia inducible factor 1 (HIF-1) and nuclear factor-kappaB (NF-kappaB). Knockout of either HIF-1 or (IKKbeta-dependent) NF-kappaB pathways in intestinal epithelial cells promotes inflammatory disease in murine models of colitis. Both HIF-1 and NF-kappaB pathways are repressed by the action of hydroxylases through the hydroxylation of key regulatory molecules.

METHODS

In this study we have investigated the effects of the hydroxylase inhibitor dimethyloxalylglycine (DMOG) on Caco-2 intestinal epithelial cells in vitro and in a dextran sodium sulfate-induced model of murine colitis.

RESULTS

DMOG induces both HIF-1 and NF-kappaB activity in cultured intestinal epithelial cells, and is profoundly protective in dextran-sodium sulfate colitis in a manner that is at least in part reflected by the development of an anti-apoptotic phenotype in intestinal epithelial cells, which we propose reduces epithelial barrier dysfunction.

CONCLUSIONS

These data show that hydroxylase inhibitors such as DMOG represent a new strategy for the treatment of inflammatory bowel disease.

Enhanced sensitivity of protein kinase B/Akt to insulin in hypoxia is independent of HIF1α and promotes cell viability

Maintenance of oxygen homeostasis is a key requirement to ensure normal mammalian cell growth and differentiation. Hypoxia arises when oxygen demand exceeds supply, and is a feature of multiple human diseases including stroke, cancer and renal fibrosis. We have investigated the effect of hypoxia on kidney cells, and observed that insulin-induced cell viability is increased in hypoxia. We have characterized the role of protein kinase B (PKB/Akt) in these cells as a potential mediator of this effect. PKB/Akt activity was increased by low oxygen concentrations in kidney cells, and insulin-stimulated activation of PKB/Akt was stronger, more rapid and more sustained in hypoxia. Reduction of HIF1alpha levels using antimycin-A or siRNA targeting HIF1alpha did not affect PKB/Akt activation in hypoxia. Pharmacologic stabilization of HIF1alpha independent of hypoxia did not increase insulin-stimulated PKB/Akt activation. Although increased insulin-stimulated cell viability was observed in hypoxia, no differences in the degree of insulin-stimulated glucose uptake were observed in L6 muscle cells in hypoxia compared to normoxia. Thus, PKB/Akt may regulate specific cellular responses to growth factors such as insulin under adverse conditions such as hypoxia.

Hypoxic regulation of NF-kappaB signaling

Hypoxia and inflammation are coincidental events in an array of diseased tissues, including chronically inflamed sites (e.g., inflammatory bowel disease, rheumatoid arthritis), growing tumors, myocardial infarcts, atherosclerotic plaques, healing wounds, and sites of bacterial infection (Murdoch et al., 2005). An understanding of how hypoxia modulates the inflammatory response is critical in developing our fundamental understanding of inflammatory disease and identifying new windows of therapeutic opportunity. Nuclear factor-kappaB (NF-kappaB) is a master transcriptional regulator of inflammatory and antiapoptotic gene expression, the activation of which has significant implications in disease development. Recent work has uncovered mechanisms by which hypoxia modulates the activation of NF-kappaB in cells through decreased oxygen-dependent suppression of the key regulators of this pathway. This work has implicated a novel role for proline and asparagine hydroxylases in the modulation of NF-kappaB activity. Here, we describe methodologies used to demonstrate and interrogate hypoxic induction of the NF-kappaB pathway.

Prolyl hydroxylase-1 negatively regulates IkappaB kinase-beta, giving insight into hypoxia-induced NFkappaB activity

Hypoxia is a feature of the microenvironment of a growing tumor. The transcription factor NFkappaB is activated in hypoxia, an event that has significant implications for tumor progression. Here, we demonstrate that hypoxia activates NFkappaB through a pathway involving activation of IkappaB kinase-beta (IKKbeta) leading to phosphorylation-dependent degradation of IkappaBalpha and liberation of NFkappaB. Furthermore, through increasing the pool and/or activation potential of IKKbeta, hypoxia amplifies cellular sensitivity to stimulation with TNFalpha. Within its activation loop, IKKbeta contains an evolutionarily conserved LxxLAP consensus motif for hydroxylation by prolyl hydroxylases (PHDs). Mimicking hypoxia by treatment of cells with siRNA against PHD-1 or PHD-2 or the pan-prolyl hydroxylase inhibitor DMOG results in NFkappaB activation. Conversely, overexpression of PHD-1 decreases cytokine-stimulated NFkappaB reporter activity, further suggesting a repressive role for PHD-1 in controlling the activity of NFkappaB. Hypoxia increases both the expression and activity of IKKbeta, and site-directed mutagenesis of the proline residue (P191A) of the putative IKKbeta hydroxylation site results in a loss of hypoxic inducibility. Thus, we hypothesize that hypoxia releases repression of NFkappaB activity through decreased PHD-dependent hydroxylation of IKKbeta, an event that may contribute to tumor development and progression through amplification of tumorigenic signaling pathways.

Regulation of protein phosphatase 1gamma activity in hypoxia through increased interaction with NIPP1: implications for cellular metabolism

Eukaryotic cells sense decreased oxygen levels and respond by altering their metabolic strategy to sustain non-respiratory ATP production through glycolysis, and thus promote cell survival in a hypoxic environment. Protein phosphatase 1 (PP1) has been recently implicated in the governance of the rational use of energy when metabolic substrates are abundant and contributes to cellular recovery following metabolic stress. Under conditions of hypoxia, the expression of the gamma isoform of PP1 (PP1gamma), is diminished, an event we have hypothesized to be involved in the adaptive cellular response to hypoxia. Decreased PP1gamma activity in hypoxia has a profound impact on the activity of the cAMP response element binding protein (CREB), a major transcriptional regulator of metabolic genes and processes. Here, we demonstrate a further mechanism leading to inhibition of PP1 activity in hypoxia which occurs at least in part through increased association with the nuclear inhibitor of PP1 (NIPP1), an event dependent upon decreased basal cAMP/PKA-dependent signaling. Using a dominant negative NIPP1 construct, we provide evidence that NIPP1 plays a major role in the regulation of both CREB protein expression and CREB-dependent transcription in hypoxia. Furthermore, we demonstrate functional sequellae of such events including altered gene expression and recovery of cellular ATP levels. In summary, we demonstrate that interaction with NIPP1 mediates decreased PP1gamma activity in hypoxia, an event which may constitute an inherent part of the cellular oxygen-sensing machinery and may play a role in physiologic adaptation to hypoxia.

Hypoxia-responsive transcription factors

Hypoxia is a common pathophysiological occurrence with a profound impact on the cellular transcriptome. The consequences of hypoxia-induced or hypoxia-repressed gene expression have important implications in disease processes as diverse as tumour development and chronic inflammation. While the hypoxia-inducible factor (HIF-1) plays a major role in controlling the ubiquitous transcriptional response to hypoxia, it is clear that a number of other transcription factors are also activated either directly or indirectly. In this review, we comprehensively discuss the transcription factors that have been reported to be hypoxia-responsive and the signalling mechanisms leading to their activation. Understanding such events will enhance our understanding of cellular oxygen sensing.

c-Jun NH2-Terminal Kinase Activation Contributes to Hypoxia-Inducible Factor 1α–Dependent P-Glycoprotein Expression in Hypoxia

We previously have shown that hypoxia increases the expression of P-glycoprotein, which in turn increases tumor cell capacity to actively extrude chemotherapeutic agents and may contribute to tumor drug resistance. This event is mediated through the hypoxia-inducible factor (HIF-1). Here, we investigated the role of the stress-activated protein kinase c-Jun NH(2)-terminal kinase (JNK) in the signaling mechanisms underlying these events. Hypoxia activates JNK activity in vitro and in vivo. Overexpression of mitogen-activated protein kinase (MAPK) kinase kinase (MEKK-1), which preferentially activates JNK, mimics, in a nonadditive way, hypoxia-induced activity of the MDR1 promoter and expression of MDR1 mRNA and P-glycoprotein. Furthermore, the JNK inhibitor SP600125 selectively and specifically inhibits hypoxia- and MEKK-1-induced MDR1 promoter activity in a dose-dependent manner. JNK inhibition also reversed hypoxia- and MEKK-1-induced activity of an HIF-1-dependent reporter gene. MEKK-1-induced MDR1 expression depends on a functional HIF-1 binding site (hypoxia-responsive element). Hypoxia- but not cobalt chloride-dependent HIF-1-DNA binding and transcriptional activation was inhibited by SP600125, indicating that hypoxia-induced signaling to HIF-1 depends on JNK activation. Because it has been reported that reactive oxygen species are increased in hypoxia and related to JNK activation, we investigated their role in signaling this response. Whereas exogenous addition of H(2)O(2) was sufficient to activate JNK, reactive oxygen species scavengers were without effect on hypoxia-induced JNK or HIF-1 activation. Thus, hypoxia-elicited MDR1 expression, which depends on HIF-1 activation, depends at least in part on signaling via activation of JNK. Furthermore, these events are independent of the generation of reactive oxygen intermediates. Thus, JNK may represent a therapeutic target in the prevention of tumor resistance to chemotherapeutic treatment.