The HIF1 target gene NOX2 promotes angiogenesis through urotensin-II

Urotensin-II (U-II) has been considered as one of the most potent vasoactive peptides, although its physiological and pathophysiological role is still not finally resolved. Recent evidence suggests that it promotes angiogenic responses in endothelial cells, although the underlying signalling mechanisms are unclear. Reactive oxygen species derived from NADPH oxidases are major signalling molecules in the vasculature. Because NOX2 is functional in endothelial cells, we investigated the role of the NOX2-containing NADPH oxidase in U-II-induced angiogenesis and elucidated a possible contribution of hypoxia-inducible factor-1 (HIF-1), the master regulator of hypoxic angiogenesis, in the response to U-II. We found that U-II increases angiogenesis in vitro and in vivo, and these responses were prevented by antioxidants, NOX2 knockdown and in Nox2(-/-) mice. In addition, U-II-induced angiogenesis was dependent on HIF-1. Interestingly, U-II increased NOX2 transcription involving HIF-1, and chromatin immunoprecipitation confirmed NOX2 as a target gene of HIF-1. In support, NOX2 levels were greatly diminished in U-II-stimulated isolated vessels derived from mice deficient in endothelial HIF-1. Conversely, reactive oxygen species derived from NOX2 were required for U-II activation of HIF and upregulation of HIF-1. In line with this, U-II-induced upregulation of HIF-1 was absent in Nox2(-/-) vessels. Collectively, these findings identified HIF-1 and NOX2 as partners acting in concert to promote angiogenesis in response to U-II. Because U-II has been found to be elevated in cardiovascular disorders and in tumour tissues, this feed-forward mechanism could be an interesting anti-angiogenic therapeutic option in these disorders.

Complement factor C5 deficiency significantly delays the progression of biliary fibrosis in bile duct-ligated mice

Fibrogenesis represents the universal response of the liver to chronic liver injury. Complement factor C5 has been linked to fibrosis in murine toxic liver injury and human chronic hepatitis C. C5 may also play a central role in chronic cholestatic disorders, since the BA receptor FXR has been characterized as an activator of the C3 gene. We aimed to investigate, whether C5 deficiency is able to prevent biliary fibrosis in the mouse bile-duct-ligation model. BDL for 1-4 weeks was performed in either Hc(0)/Hc(0) mice (deficient for C5) or WT controls. BA levels were measured by RIA. Histological examination included H&E, sirius-red and immunohistochemistry. mRNA expression was quantified by RT-PCR. Protein expression levels were determined by Western blotting or ELISA. Enzymatic MMP-activity was analysed by zymography. One week BDL leads to fibrosis in WT (F2.0 ± 0), while it is almost absent in Hc(0)/Hc(0) mice (F0.5 ± 0.5). No differences in fibrosis can be detected at week-4. Together with delayed fibrogenesis at week-1, fibrotic markers are decreased in Hc(0)/Hc(0) mice. Expression of the inflammatory cytokine TNF-α is decreased in Hc(0)/Hc(0) mice. In parallel C5 deficiency leads to an attenuated peribiliary infiltration of CD45(+) cells in fibrotic areas together with decreased MMP-9 expression and gelatinase activity. The present study proves a functional role of C5 during biliary fibrogenesis. C5 deficiency leads to attenuated inflammation and normalized MMP-9 activity concomitantly with a significant reduction of fibrosis. C5 appears to be an attractive target for future therapeutic intervention in chronic cholestatic liver disease.

Chronic endothelium-dependent regulation of arterial blood pressure by atrial natriuretic peptide: role of nitric oxide and endothelin-1

Atrial natriuretic peptide (ANP), via its guanylyl cyclase (GC)-A receptor, plays a key role in the regulation of arterial blood pressure (ABP) and volume. Endothelial-restricted deletion of GC-A in mice [endothelial cell (EC) GC-A knockout (KO)] resulted in hypervolemic hypertension, demonstrating that the endothelium participates in the hypotensive and hypovolemic actions of ANP. Published studies showed that ANP modulates the release of the vasoactive factors nitric oxide (NO) and endothelin-1 (ET-1) from cultured endothelia. Based on these observations, we examined the role of these endothelial factors in ANP-dependent vasodilatation (studied in isolated arteries) and chronic regulation of ABP (measured in awake mice by tail-cuff plethysmography). ANP induced concentration-dependent vasorelaxations of aortic, carotid, and pulmonary arteries. These responses were not different between control and EC GC-A KO mice, and were significantly enhanced after inhibition of NO synthase [by N(G)-nitro-L-arginine-methyl ester]. Intravenous administration of N(G)-nitro-L-arginine-methyl ester to conscious mice significantly increased ABP. The extent of these hypertensive reactions was similar in EC GC-A KO mice and control littermates (increases in systolic blood pressure by approximately 25 mm Hg). Conversely, antagonism of ET-1/endothelin-A receptors with BQ-123 reduced ABP significantly and comparably in both genotypes (by approximately 11 mm Hg). Finally, the vascular and tissue expression levels of components of the NO system and of immunoreactive ET-1 were not different in control and EC GC-A KO mice. We conclude that the endothelium, but not modulation of endothelial NO or ET-1, participates in the chronic regulation of ABP by ANP.

Vascular endothelium is critically involved in the hypotensive and hypovolemic actions of atrial natriuretic peptide

Atrial natriuretic peptide (ANP), via its vasodilating and diuretic effects, has an important physiological role in the maintenance of arterial blood pressure and volume. Its guanylyl cyclase-A (GC-A) receptor is highly expressed in vascular endothelium, but the functional relevance of this is controversial. To dissect the endothelium-mediated actions of ANP in vivo, we inactivated the GC-A gene selectively in endothelial cells by homologous loxP/Tie2-Cre-mediated recombination. Notably, despite full preservation of the direct vasodilating effects of ANP, mice with endothelium-restricted deletion of the GC-A gene (EC GC-A KO) exhibited significant arterial hypertension and cardiac hypertrophy. Echocardiographic and Doppler flow evaluations together with the Evan's blue dilution technique showed that the total plasma volume of EC GC-A KO mice was increased by 11-13%, even under conditions of normal dietary salt intake. Infusion of ANP caused immediate increases in hematocrit in control but not in EC GC-A KO mice, which indicated that ablation of endothelial GC-A completely prevented the acute contraction of intravascular volume produced by ANP. Furthermore, intravenous ANP acutely enhanced the rate of clearance of radio-iodinated albumin from the circulatory system in control but not in EC GC-A KO mice. We conclude that GC-A-mediated increases in endothelial permeability are critically involved in the hypovolemic, hypotensive actions of ANP.

Hypervolemic hypertension in mice with systemic inactivation of the (floxed) guanylyl cyclase-A gene by alphaMHC-Cre-mediated recombination

To dissect the tissue-specific functions of atrial natriuretic peptide (ANP), we recently introduced loxP sites into the murine gene for its receptor, guanylyl cyclase-A (GC-A), by homologous recombination (tri-lox GC-A). For either smooth-muscle or cardiomyocyte-restricted deletion of GC-A, floxed GC-A mice were mated to transgenic mice expressing Cre-recombinase under the control of the smooth-muscle SM22 or the cardiac alphaMHC promoter. As shown in these studies, Cre-mediated recombination of the floxed GC-A gene fully inactivated GC-A function in a cell-restricted manner. In the present study we show that alphaMHC-Cre, but not SM22-Cre, with high frequency generates genomic recombinations of the floxed GC-A gene segments which were transmitted to the germline. Alleles with partial or complete deletions were readily recovered from the next generation, after segregation of the Cre-transgene. We took advantage of this strategy to generate a new mouse line with global, systemic deletion of GC-A. Doppler-echocardiographic and physiological studies in these mice demonstrate for the first time the tremendous impact of ANP/GC-A dysfunction on chronic blood volume homeostasis.

Diverging vasorelaxing effects of C-type natriuretic peptide in renal resistance arteries and aortas of GC-A-deficient mice

This study aimed to characterize the vasorelaxing effects of ANP, BNP and CNP in isolated renal resistance arteries (RRA) from wild-type mice and mice with either systemic (GC-A -/-) or smooth muscle-restricted deletion of GC-A (SMC GC-A KO). In RRA from wild-type (GC-A +/+) mice natriuretic peptides (NP) induced concentration-dependent vasorelaxations with the rank order of potency ANP>BNP>CNP. In RAA obtained from mice with systemic or smooth muscle-restricted deletion of GC-A, the effects of ANP and BNP were abolished. In contrast, CNP induced concentration-dependent vasorelaxations of GC-A -/- and SMC GC-A KO RRA. However, the efficacy of CNP for vasorelaxation was markedly diminished compared with wild-type RRA. Such changes in CNP responsiveness did not affect large arteries as the aorta and they were not due to vascular changes secondary to chronic arterial hypertension in GC-A -/- mice. Unaltered vasorelaxing effects of acetylcholine and sodium nitroprusside demonstrated unaltered function of downstream targets regulated by cGMP in vascular smooth muscle. An increased expression of the clearance receptor (NPR-C) or diminished expression of GC-B were not found to account for the differences in CNP responsiveness. In conclusion, observations in isolated aortic rings do not necessarily allow conclusions concerning the physiology of natriuretic peptides in the smaller resistance size arteries. Changes at the GC-B receptor level are likely to explain the diminished responsiveness of GC-A-deficient RRA to CNP.

Increased sensitivity to endothelial nitric oxide (NO) contributes to arterial normotension in mice with vascular smooth muscle-selective deletion of the atrial natriuretic peptide (ANP) receptor

Atrial natriuretic peptide (ANP) plays a key regulatory role in arterial blood pressure homeostasis. We recently generated mice with selective deletion of the ANP receptor, guanylyl cyclase-A (GC-A), in vascular smooth muscle (SMC GC-A knockout (KO) mice) and reported that resting arterial blood pressure was completely normal in spite of clear abolition of the direct vasodilating effects of ANP (Holtwick, R., Gotthardt, M., Skryabin, B., Steinmetz, M., Potthast, R., Zetsche, B., Hammer, R. E., Herz, J., and Kuhn M. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 7142-7147). The purpose of this study was to clarify mechanisms compensating for the missing vasodilator responses to ANP. In particular, we analyzed the effect of the endothelial, cGMP-mediated vasodilators C-type natriuretic peptide and nitric oxide (NO). In isolated arteries from SMC GC-A KO mice, the vasorelaxing sensitivity to sodium nitroprusside and the endothelium-dependent vasodilator, acetylcholine, was significantly greater than in control mice. There was no difference in responses to C-type natriuretic peptide or to the activator of cGMP-dependent protein kinase I, 8-para-chlorophenylthio-cGMP. The aortic expression of soluble GC (sGC), but not of endothelial NO synthase or cGMP-dependent protein kinase I, was significantly increased in SMC GC-A KO mice. Chronic oral treatment with the NO synthase inhibitor N(w)-nitro-l-arginine methyl ester increased arterial blood pressure, the effect being significantly enhanced in SMC GC-A KO mice. We conclude that SMC GC-A KO mice exhibit a higher vasodilating sensitivity to NO. This can be attributed to an enhanced expression of sGC, whereas the expression and/or activity levels of downstream cGMP-effector pathways are not involved. Increased vasodilating responsiveness to endothelial NO contributes to compensate for the missing vasodilating effect of ANP in SMC GC-A KO mice.