Dual influence of nitric oxide on gene regulation during hypoxia

GPER mediates cardiotropic effects in spontaneously hypertensive rat hearts

Estrogens promote beneficial effects in the cardiovascular system mainly through the estrogen receptor (ER)α and ERβ, which act as ligand-gated transcription factors. Recently, the G protein-coupled estrogen receptor (GPER) has been implicated in the estrogenic signaling in diverse tissues, including the cardiovascular system. In this study, we demonstrate that left ventricles of male Spontaneously Hypertensive Rats (SHR) express higher levels of GPER compared to normotensive Wistar Kyoto (WKY) rats. In addition, we show that the selective GPER agonist G-1 induces negative inotropic and lusitropic effects to a higher extent in isolated and Langendorff perfused hearts of male SHR compared to WKY rats. These cardiotropic effects elicited by G-1 involved the GPER/eNOS transduction signaling, as determined by using the GPER antagonist G15 and the eNOS inhibitor L-NIO. Similarly, the G-1 induced activation of ERK1/2, AKT, GSK3β, c-Jun and eNOS was abrogated by G15, while L-NIO prevented only the eNOS phosphorylation. In hypoxic Langendorff perfused WKY rat heart preparations, we also found an increased expression of GPER along with that of the hypoxic mediator HIF-1α and the fibrotic marker CTGF. Interestingly, G15 and L-NIO prevented the ability of G-1 to down-regulate the expression of both HIF-1α and CTGF, which were found expressed to a higher extent in SHR compared to WKY rat hearts. Collectively, the present study provides novel data into the potential role played by GPER in hypertensive disease on the basis of its involvement in myocardial inotropism and lusitropism as well as the expression of the apoptotic HIF-1α and fibrotic CTGF factors. Hence, GPER may be considered as a useful target in the treatment of some cardiac dysfunctions associated with stressful conditions like the essential hypertension.

Nitrosative stress and transcription

Low NO concentrations synthesized by constitutively expressed NO synthases act on several signaling pathways activating transcription factors (TF), such as NF-kappaB or AP-1, and thereby influence gene expression. In contrast, during inflammatory reactions the inducible NO synthase produces NO for prolonged periods of time. The resulting nitrosative stress directly affects redox-sensitive TF like NF-kappaB, AP-1, Oct-1, c-Myb, or zinc finger-containing TF, but also additional mechanisms have been identified. Nitrosative stress in some cases induces expression of TF (AP-1, p53), indirectly modulates activity or stability of TF (HIF-1, p53) or their inhibitors (NF-kappaB), or modulates accessibility of promoters via increased DNA methylation or histone deacetylation. Depending on the promoter the result is induced, increased, decreased or even totally inhibited expression of various target genes. In unstimulated cells nitrosative stress increases NF-kappaB- or AP-1-dependent transcription, while in activated cells nitrosative stress rather abolishes NF-kappaB- or AP-1-dependent transcription. Sometimes the oxygen concentration also is of prime importance, since under normoxic conditions nitrosative stress activates HIF-1-dependent transcription, while under hypoxic conditions nitrosative stress leads to inhibition of HIF-1-dependent transcription. This review summarizes what is known about effects of physiological NO levels as well as of nitrosative stress on transcription.

Superoxide dismutase-3 promotes full expression of the EPO response to hypoxia

Extracellular superoxide dismutase (SOD3) is the primary extracellular enzymatic scavenger of superoxide (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(^{{\cdot}}\mathrm{O}_{2}^{-}\) \end{document}). SOD3's expression is highest in the kidney, but its distribution and biologic functions there are unknown. To investigate the function of renal SOD3, we colocalized it with erythropoietin (EPO) to proximal tubules using in situ hybridization and immunohistochemistry. We then exposed wild-type (Wt) and SOD3 knock-out (KO) mice to hypoxia and found a late hematocrit response in the KO strain. EPO mRNA expression was attenuated in KO mice during the first 6 hours of hypoxia preceded at 2 hours by less accumulation of nuclear hypoxia-inducible transcription factor 1 α (HIF-1α) protein. Meanwhile KO mice exposed to hypoxia showed increases in renal mRNA for superoxide-producing nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX4) and early significant increases in glutathione disulfide (GSSG)/glutathione (GSH), a marker of oxidative stress, compared with Wt mice. Plasma nitrite/nitrate and renal 3-nitrotyrosine (3-NTyr), indicating peroxynitrite formation, increased later in hypoxia, and renal endothelial nitric oxide synthase protein induction was similar in both strains. These data show that hypoxic activation of HIF-1α and its target gene EPO in mouse kidney is regulated closely by the oxidant/antioxidant equilibrium involving SOD3, thus identifying renal SOD3 as a regulatory element in the body's innate adaptation to hypoxia.

Radiation activates HIF-1 to regulate vascular radiosensitivity in tumors: role of reoxygenation, free radicals, and stress granules

Through a poorly understood mechanism, tumors respond to radiation by secreting cytokines capable of inhibiting apoptosis in endothelial cells, thereby diminishing treatment response by minimizing vascular damage. We reveal here that this pathway is governed by a major angiogenesis regulator, HIF-1. Following radiotherapy, tumor reoxygenation leads to: (1) nuclear accumulation of HIF-1 in response to reactive oxygen, and (2) enhanced translation of HIF-1-regulated transcripts secondary to stress granule depolymerization. The resulting increase in HIF-1-regulated cytokines enhances endothelial cell radioresistance. Inhibiting postradiation HIF-1 activation significantly increases tumor radiosensitivity as a result of enhanced vascular destruction. These data describe novel pathways contributing significantly to our understanding of HIF-1 regulation which may be major determinants of tumor radiosensitivity, potentially having high clinical relevance.

Regulation of vascular endothelial growth factor synthesis by nitric oxide: facts and controversies

Vascular endothelial growth factor (VEGF) is the major molecule governing angiogenesis, defined as the growth of blood vessels from vascular structure. There is abundant evidence that nitric oxide (NO) is an effector molecule mediating the activity of VEGF. By binding to its receptors, VEGF initiates the signaling cascades leading to NO production and angiogenic activation of endothelial cells. Recent data show that NO induces VEGF synthesis in numerous cell types, including vascular smooth muscle cells, macrophages, keratinocytes, and tumor cells. NO enhances VEGF production by augmenting its expression through activation of Akt kinase, followed by induction of several transcription factors, of which stabilization of hypoxia-inducible factor (HIF-1) is the critical step. With respect to its effect on VEGF expression, NO mimics hypoxia, the classical activator of HIF-1 and VEGF synthesis. The effect of NO on VEGF production is also mediated by heme oxygenase, an enzyme generating carbon monoxide, which appears to stimulate VEGF release. In this review, we attempt to elucidate the molecular mechanisms underlying the effects of NO on VEGF synthesis. We also discuss some discrepant data and suggest explanations for various aspects of the NO-VEGF relationship.

17Beta-estradiol decreases hypoxic induction of erythropoietin gene expression

Exposure to chronic hypoxia induces erythropoietin (EPO) production to facilitate oxygen delivery to hypoxic tissues. Previous studies from our laboratory found that ovariectomy (OVX) exacerbates the polycythemic response to hypoxia and treatment with 17beta-estradiol (E2-beta) inhibits this effect. We hypothesized that E2-beta decreases EPO gene expression during hypoxia. Because E2-beta can induce nitric oxide (NO) production and NO can attenuate EPO synthesis, we further hypothesized that E2-beta inhibition of EPO gene expression is mediated by NO. These hypotheses were tested in OVX catheterized rats treated with E2-beta (20 microg/day) or vehicle for 14 days and exposed to 8 or 12 h of hypoxia (12% O(2)) or normoxia. We found that E2-beta treatment significantly decreased EPO synthesis and gene expression during hypoxia. E2-beta treatment did not induce endothelial NO synthase (eNOS) expression in the kidney but potentiated hypoxia-induced increases in plasma nitrates. We conclude that E2-beta decreases hypoxic induction of EPO. However, this effect does not appear to be related to changes in renal eNOS expression.