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The hypoxia-inducible factor-2alpha (HIF-2alpha) contributes to the vascular response to hypoxia. Hypoxia inhibits prolyl hydroxylation of the N-terminal transactivation domain (N-TAD), thus preventing binding of the von Hippel-Lindau protein (pVHL) and proteasomal degradation; additionally, hypoxia inhibits asparagyl hydroxylation of the C-TAD, thus diminishing cofactor recruitment. Reactive oxygen species (ROS) derived from NADPH oxidases (NOXs) have been shown to control vascular functions and to promote vascular remodeling. However, whether HIF-2alpha, ROS, and NOXs are linked under such nonhypoxic conditions is unclear. We found that activation of NOX4 by thrombin or H(2)O(2) increased HIF-2alpha protein because of decreased pVHL binding in pulmonary artery smooth muscle cells (PASMCs). Thrombin, H(2)O(2), and NOX4 overexpression increased HIF-2alpha N-TAD and C-TAD activity, which was prevented by ascorbate treatment or mutation of the hydroxylation sites in the TADs. HIF-2alpha also mediated induction of plasminogen activator inhibitor-1 and the proliferative response to thrombin, H(2)O(2), or NOX4 overexpression. Thus, ROS derived from NOX4 in response to thrombin stabilize HIF-2alpha by preventing hydroxylation of the N- and C-TAD, thus allowing formation of transcriptionally active HIF-2alpha, which promotes PASMC proliferation. Together, these findings present the first evidence that HIF-2alpha is critically involved in the ROS-regulated vascular remodeling processes.

Vascular remodelling is a complex phenomenon associated with restructuring of the vessel wall as a consequence of disruption of vascular homeostasis. Alterations of the vascular wall have been linked to a variety of cardiovascular disorders including atherosclerosis, vascular injury and pulmonary hypertension. Plasminogen activator inhibitor-1 (PAI-1) is a member of the serpin (serine proteinase inhibitor) family and acts as an important inhibitor of fibrinolysis by interfering with the plasminogen system. In addition to its anti-fibrinolytic effects, PAI-1 appears to modulate cellular responses linked to vascular remodelling. Since PAI-1 levels have been shown to be altered in various disorders associated with vascular remodelling of the systemic and pulmonary vascular bed, this serpin may play a pivotal role in the pathogenesis of these diseases.

HIF-1 (hypoxia-inducible factor-1) has been shown to essentially control the cellular response to hypoxia. Hypoxia stabilizes the inducible alpha-subunit, preventing post-translational hydroxylation and subsequent degradation via the proteasome. In recent years, clear evidence has emerged that HIF-1alpha is also responsive to many stimuli under normoxic conditions, including thrombin, growth factors, vasoactive peptides, insulin, lipopolysaccharide and cytokines such as TNF-alpha (tumour necrosis factor-alpha), and in many cases reactive oxygen species are involved. One important mechanism underlying these responses is the transcriptional regulation of HIF-1alpha by the redox-sensitive transcription factor NF-kappaB (nuclear factor kappaB), which binds at a distinct element in the proximal promoter of the HIF-1alpha gene. More recently, NF-kappaB binding to this site in the HIF-1alpha promoter has been shown also under hypoxic conditions. Thus these two major pathways regulating the responses to inflammation and oxidative stress on the one hand, and hypoxia on the other hand, appear to be intimately linked. In this issue of the Biochemical Journal, a study by van Uden et al. has supported these findings further, in which they have confirmed the binding of several proteins of the NF-kappaB family at the previously identified consensus site in the HIF-1alpha promoter and shown that TNF-alpha can also transcriptionally induce HIF-1alpha by this previously described pathway. The identification of HIF-1alpha as a target gene of NF-kappaB will have important implications for a variety of disorders related to hypoxia-ischaemia and/or inflammation and oxidative stress.

The oxygen sensitive alpha-subunit of the hypoxia-inducible factor-1 (HIF-1) is a major trigger of the cellular response to hypoxia. Although the posttranslational regulation of HIF-1alpha by hypoxia is well known, its transcriptional regulation by hypoxia is still under debate. We, therefore, investigated the regulation of HIF-1alpha mRNA in response to hypoxia in pulmonary artery smooth muscle cells. Hypoxia rapidly enhanced HIF-1alpha mRNA levels and HIF-1alpha promoter activity. Furthermore, inhibition of the phosphatidylinositol 3-kinase (PI3K)/AKT but not extracellular signal-regulated kinase 1/2 pathway blocked the hypoxia-dependent induction of HIF-1alpha mRNA and HIF-1alpha promoter activity, suggesting involvement of a PI3K/AKT-regulated transcription factor. Interestingly, hypoxia also induced nuclear factor-kappaB (NFkappaB) nuclear translocation and activity. In line, expression of the NFkappaB subunits p50 and p65 enhanced HIF-1alpha mRNA levels, whereas blocking of NFkappaB by an inhibitor of nuclear factor-kappaB attenuated HIF-1alpha mRNA induction by hypoxia. Reporter gene assays revealed the presence of an NFkappaB site within the HIF-1alpha promoter, and mutation of this site abolished induction by hypoxia. In line, gel shift analysis and chromatin immunoprecipitation confirmed binding of p50 and p65 NFkappaB subunits to the HIF-1alpha promoter under hypoxia. Together, these findings provide a novel mechanism in which hypoxia induces HIF-1alpha mRNA expression via the PI3K/AKT pathway and activation of NFkappaB.

Objective: Reactive oxygen species have been implicated as signaling molecules modulating the activity of redox-sensitive transcription factors such as nuclear factor kappa B (NF-kappaB). Recently, the transcription factor hypoxia-inducible factor-1 (HIF-1), known to mediate gene expression by hypoxia, has been found to be also activated by nonhypoxic factors in a redox-sensitive manner. We therefore aimed to elucidate the link between these 2 important redox-sensitive transcription factors., Methods and Results: In pulmonary artery smooth muscle cells, reactive oxygen species generated either by exogenous H2O2 or by a NOX4-containing NADPH oxidase stimulated by thrombin activated or induced NF-kappaB and HIF-1alpha. The reactive oxygen species-mediated HIF-1alpha induction occurred on the transcriptional level and was dependent on NF-kappaB. Transfection experiments with wild-type or mutant HIF-1alpha promoter constructs revealed the presence of a yet unidentified NF-kappaB binding element. Gel shift analyses and chromatin immunoprecipitation verified binding of NF-kappaB to this site. Furthermore, reactive oxygen species enhanced expression of plasminogen activator inhibitor-1, which was prevented by dominant-negative IkappaB or mutation of the HIF-1 binding site within the plasminogen activator inhibitor-1 promoter., Conclusion: These findings show for the first time to our knowledge that reactive oxygen species directly link HIF-1alpha and NF-kappaB, implicating an important pathophysiological role of this novel pathway in disorders associated with elevated levels of reactive oxygen species.

NADPH oxidases have been identified as sources of reactive oxygen species (ROS) in vascular cells. In addition to the initially described enzyme containing gp91phox (NOX2), several homologues to NOX2 have been identified. Whereas NOX1, NOX2, and NOX4 are expressed in endothelial cells, a functional role of NOX5 containing additional N-terminal calcium-binding domains of varying sequences has not been reported in these cells. NOX5 protein was found in the endoplasmic reticulum of human microvascular endothelial cells (HMEC-1) and in the vascular wall. HMEC-1 cells expressed NOX5beta and NOX5delta as well as a variant lacking calcium-binding domains (NOX5S). NOX5beta and NOX5S increased basal ROS levels. Ionomycin exclusively enhanced NOX5beta-mediated ROS production. Although p22phox, when overexpressed, interacted with both NOX5 proteins, it was not essential for NOX5-mediated ROS production. NOX5 proteins stimulated endothelial cell proliferation and the formation of capillary-like structures whereas depletion of NOX5 by siRNA prevented these responses to thrombin. These data show that endothelial cells express different NOX5 variants including NOX5S lacking calcium-binding domains. NOX5 proteins are functional, promoting endothelial ROS production, proliferation, and the formation of capillary-like structures and contribute to the endothelial response to thrombin. These findings suggest that NOX5 variants play a novel role in controlling ROS-dependent processes in the vasculature.

The stress-responsive serum- and glucocorticoid-inducible kinase Sgk-1 is involved in osmoregulation and cell survival and may contribute to fibrosis and hypertension. However, the function of Sgk-1 in vascular remodeling and thrombosis, 2 major determinants of pulmonary hypertension (PH), has not been elucidated. We investigated the role of Sgk-1 in thrombin signaling and tissue factor (TF) expression and activity in pulmonary artery smooth muscle cells (PASMC). Thrombin increased Sgk-1 activity and mRNA and protein expression. H2O2 similarly induced Sgk-1 expression. Antioxidants, dominant-negative Rac, and depletion of the NADPH oxidase subunit p22phox diminished thrombin-induced Sgk-1 expression. Inhibition of p38 mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and phosphoinositide-dependent kinase-1 prevented thrombin-induced Sgk-1 expression. Thrombin or Sgk-1 overexpression enhanced TF expression and procoagulant activity, whereas TF upregulation by thrombin was diminished by kinase-deficient Sgk-1 and was not detectable in fibroblasts from mice deficient in sgk-1 (sgk1(-/-)). Similarly, dexamethasone treatment failed to induce TF expression and activity in lung tissue from sgk1(-/-) mice. Transcriptional induction of TF by Sgk-1 was mediated through nuclear factor kappaB. Finally, Sgk-1 and TF proteins were detected in the media of remodeled pulmonary vessels associated with PH. These data show that thrombin potently induces Sgk-1 involving NADPH oxidases, phosphatidylinositol 3-kinase, p38 mitogen-activated protein kinase, and phosphoinositide-dependent kinase-1, and that activation of nuclear factor kappaB by Sgk-1 mediates TF expression and activity by thrombin. Because enhanced procoagulant activity can promote pulmonary vascular remodeling, and Sgk-1 and TF were present in the media of remodeled pulmonary vessels, this pathway may play a critical role in vascular remodeling in PH.

Background: Human urotensin II (hU-II) is a potent vasoactive peptide possibly involved in pulmonary hypertension. Because the signaling mechanisms activated by this peptide in the pulmonary vasculature are largely unknown, we investigated the role of hU-II in the activation of NADPH oxidase and the control of redox-sensitive kinase pathways, expression of plasminogen activator inhibitor-1 (PAI-1), and proliferation in pulmonary artery smooth muscle cells (PASMCs)., Methods and Results: hU-II upregulated expression of the NADPH oxidase subunits p22phox and NOX4 and increased the levels of reactive oxygen species (ROS), which were abrogated by transfecting p22phox or NOX4 antisense vectors. p22phox and NOX4 also contributed to hU-II-induced activation of extracellular signal-regulated kinase 1/2, p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, and protein kinase B (Akt). Furthermore, hU-II increased the expression of PAI-1 and enhanced PASMC proliferation in an NADPH oxidase- and kinase-dependent manner., Conclusions: hU-II is a potent activator of ROS generation by NADPH oxidase in PASMCs, leading to redox-sensitive activation of mitogen-activated protein kinases and Akt and subsequently to enhanced PAI-1 expression and increased proliferation. These findings suggest that hU-II may play a novel role in pulmonary hypertension by promoting remodeling processes via activation of NADPH oxidases.

Endothelial dysfunction is characterized by increased levels of reactive oxygen species (ROS) and a prothrombotic state. The mechanisms linking thrombosis to ROS production in the endothelium are not well understood. We investigated the role of thrombin in regulating NADPH oxidase-dependent ROS production and expression of its subunit p22phox in the endothelial cell line EaHy926. Thrombin elicited a biphasic increase in ROS generation peaking within 15 min, but also at 3 h. The delayed response was accompanied by increased p22phox mRNA and protein expression. Two-photon confocal laser microscopy showed colocalization between p22phox and ROS production. Antioxidant treatment with vitamin C or diphenyleneiodonium abrogated thrombin-induced ROS production and p22phox expression, whereas H2O2 elevated ROS production and p22phox levels. Both responses were dependent on p38 MAP kinase and phosphatidylinositol-3-kinase (PI3 kinase)/Akt. Finally, p22phox was required for thrombin- or H2O2-stimulated proliferation. These data show that thrombin rapidly increases ROS production in endothelial cells, resulting, via activation of p38 MAP kinase and PI3 kinase/Akt, in upregulation of p22phox accompanied by a delayed increase in ROS generation and enhanced proliferation. These findings suggest a positive feedback mechanism whereby ROS, possibly generated by the NADPH oxidase, lead to elevated levels of p22phox and, thus, sustained ROS generation as is observed in endothelial dysfunction.

Pulmonary hypertension and vascular remodeling processes are associated with oxidative stress, hypoxia and enhanced levels of thrombin and vascular endothelial growth factor (VEGF). The hypoxia-inducible transcription factor HIF regulates the expression of VEGF under hypoxia. The HIF pathway is also activated by thrombin or CoCl2, likely via reactive oxygen species (ROS). In this study we investigated whether the redox-modifying enzymes superoxide dismutase (SOD), glutathione peroxidase (GPX) and catalase affect HIF levels and the expression of VEGF mRNA in pulmonary artery smooth muscle cells (PASMC). Stimulation of PASMC with thrombin or CoCl2 increased ROS production and enhanced HIF-alpha protein and VEGF mRNA levels as well as HIF-dependent reporter gene activity. These responses were inhibited by vitamin C and by overexpression of GPX and catalase, whereas the opposite effects were observed in SOD-expressing cells. These findings suggest that an 'antioxidant' state with reduced levels of H2O2 limits the activation of the HIF pathway, whereas a 'prooxidant' state allowing elevated H2O2 levels promotes it. Thus, shifting the redox balance to a more reduced environment, thereby limiting VEGF expression, may be beneficial for treating remodeling processes during pulmonary hypertension.