Inhibition of nitric oxide synthase unmasks vigorous vasoconstriction in established pulmonary arterial hypertension

Increased Lung Uric Acid Deteriorates Pulmonary Arterial Hypertension

Background

Recent studies have demonstrated that uric acid (UA) enhances arginase activity, resulting in decreased NO in endothelial cells. However, the role of lung UA in pulmonary arterial hypertension (PAH) remains uncertain. We hypothesized that increased lung UA level contributes to the progression of PAH.

Methods and Results

In cultured human pulmonary arterial endothelial cells, voltage‐driven urate transporter 1 (URATv1) gene expression was detected, and treatment with UA increased arginase activity. In perfused lung preparations of VEGF receptor blocker (SU5416)/hypoxia/normoxia‐induced PAH model rats, addition of UA induced a greater pressure response than that seen in the control and decreased lung cGMP level. UA‐induced pressor responses were abolished by benzbromarone, a UA transporter inhibitor, or L‐norvaline, an arginase inhibitor. In PAH model rats, induction of hyperuricemia by administering 2% oxonic acid significantly increased lung UA level and induced greater elevation of right ventricular systolic pressure with exacerbation of occlusive neointimal lesions in small pulmonary arteries, compared with nonhyperuricemic PAH rats. Administration of benzbromarone to hyperuricemic PAH rats significantly reduced lung UA levels without changing XOR (xanthine oxidoreductase) activity, and attenuated right ventricular systolic pressure increase and occlusive lesion development. Topiroxostat, a XOR inhibitor, significantly reduced lung XOR activity in PAH rats, with no effects on increase in right ventricular systolic pressure, arterial elastance, and occlusive lesions. XOR‐knockout had no effects on right ventricular systolic pressure increase and arteriolar muscularization in hypoxia‐exposed mice.

Conclusions

Increased lung UA per se deteriorated PAH, whereas XOR had little impact. The mechanism of increased lung UA may be a novel therapeutic target for PAH complicated with hyperuricemia.

Chronic Inhibition of Toll-Like Receptor 9 Ameliorates Pulmonary Hypertension in Rats

Background

Recent accumulating evidence suggests that toll‐like receptor 9 (TLR9) is involved in the pathogenesis of cardiovascular diseases. However, its role in pulmonary hypertension remains uncertain. We hypothesized that TLR9 is involved in the development of pulmonary hypertension.

Methods and Results

A rat model of monocrotaline‐induced pulmonary hypertension was used to investigate the effects of TLR9 on hemodynamic parameters, vascular remodeling, and survival. Monocrotaline‐exposed rats significantly showed increases in plasma levels of mitochondrial DNA markers, which are recognized by TLR9, TLR9 activation in the lung, and interleukin‐6 mRNA level in the lung on day 14 after monocrotaline injection. Meanwhile, monocrotaline‐exposed rats showed elevated right ventricular systolic pressure, total pulmonary vascular resistance index and vascular remodeling, together with macrophage accumulation on day 21. In the preventive protocol, administration (days −3 to 21 after monocrotaline injection) of selective (E6446) or nonselective TLR9 inhibitor (chloroquine) significantly ameliorated the elevations of right ventricular systolic pressure and total pulmonary vascular resistance index as well as vascular remodeling and macrophage accumulation on day 21. These inhibitors also significantly reduced NF‐κB activation and interleukin‐6 mRNA levels to a similar extent. In the short‐term reversal protocol, E646 treatment (days 14–17 after monocrotaline injection) almost normalized NF‐κB activation and interleukin‐6 mRNA level, and reduced macrophage accumulation. In the prolonged reversal protocol, E6446 treatment (days 14–24 after monocrotaline injection) reversed total pulmonary vascular resistance index and vascular remodeling, and improved survival in monocrotaline‐exposed rats.

Conclusions

TLR9 is involved in the development of pulmonary hypertension concomitant via activation of the NF‐κB‒IL‐6 pathway. Inhibition of TLR9 may be a novel therapeutic strategy for pulmonary hypertension.

Evaluation of right coronary vascular dysfunction in severe pulmonary hypertensive rats using synchrotron radiation microangiography

Pulmonary hypertension (PH) causes cardiac hypertrophy in the right ventricle (RV) and eventually leads to RV failure due to persistently elevated ventricular afterload. We hypothesized that the mechanical stress on the RV associated with increased afterload impairs vasodilator function of the right coronary artery (RCA) in PH. Coronary vascular response was assessed using microangiography with synchrotron radiation (SR) in two well-established PH rat models, monocrotaline injection or the combined exposure to chronic hypoxia and vascular endothelial growth factor receptor blockade with Su5416 (SuHx model). In the SuHx model, the effect of the treatment with the nonselective endothelin-1 receptor antagonist (ERA), macitentan, was also examined. Myocardial viability was determined in SuHx model rats, using 18F-FDG Positron emission tomography (PET) and magnetic resonance imaging (MRI). Endothelium-dependent and endothelium-independent vasodilator responses were significantly attenuated in the medium and small arteries of severe PH rats. ERA treatment significantly improved RCA vascular function compared with the untreated group. ERA treatment improved both the decrease in ejection fraction and the increased glucose uptake, and reduced RV remodeling. In addition, the upregulation of inflammatory genes in the RV was almost suppressed by ERA treatment. We found impairment of vasodilator responses in the RCA of severe PH rat models. Endothelin-1 activation in the RCA plays a major role in impaired vascular function in PH rats and is partially restored by ERA treatment. Treatment of PH with ERA may improve RV function in part by indirectly attenuating right heart afterload and in part by associated improvements in right coronary endothelial function.NEW & NOTEWORTHY We demonstrated for the first time the impairment of vascular responses in the right coronary artery (RCA) of the dysfunctional right heart in pulmonary hypertensive rats in vivo. Treatment with an endothelin-1 receptor antagonist ameliorated vascular dysfunction in the RCA, enabled tissue remodeling of the right heart, and improved cardiac function. Our results suggest that impaired RCA function might also contribute to the early progression to heart failure in patients with severe pulmonary arterial hypertension (PAH). The endothelium of the coronary vasculature might be considered as a potential target in treatments to prevent heart failure in severe patients with PAH.

Downregulation of Soluble Guanylate Cyclase and Protein Kinase G With Upregulated ROCK2 in the Pulmonary Artery Leads to Thromboxane A2 Sensitization in Monocrotaline-Induced Pulmonary Hypertensive Rats

Thromboxane A2 (TXA2) promotes various physiological responses including pulmonary artery (PA) contraction, and pathophysiological implications have been suggested in cardiovascular diseases including pulmonary hypertension. Here, we investigated the role of TXA2 receptor (TP)-mediated signaling in the pathophysiology of pulmonary arterial hypertension (PAH). The sensitivity of PA to the contractile agonist could be set by relaxing signals such as the nitric oxide (NO), soluble guanylate cyclase (sGC), and cGMP-dependent kinase (PKG) pathways. Changes in the TP agonist (U46619)-induced PA contraction and its modulation by NO/cGMP signaling were analyzed in a monocrotaline-induced PAH rat model (PAH-MCT). In the myograph study, PA from PAH-MCT showed higher responsiveness to U46619, that is decreased EC50. Immunoblot analysis revealed a lower expression of eNOS, sGC, and PKG, while there was a higher expression of RhoA-dependent kinase 2 (ROCK2) in the PA from PAH-MCT than in the control. In PAH-MCT, the higher sensitivity to U46619 was reversed by 8-Br-cGMP, a membrane-permeable cGMP analog, but not by the NO donor, sodium nitroprusside (SNP 30 μM). In contrast, in the control PA, inhibition of sGC by its inhibitor (1H- [1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ), 10 μM) lowered the threshold of U46619-induced contraction. In the presence of ODQ, SNP treatment had no effect whereas the addition of 8-Br-cGMP lowered the sensitivity to U46619. The inhibition of ROCK by Y-27632 attenuated the sensitivity to U46619 in both control and PAH-MCT. The study suggests that the attenuation of NO/cGMP signaling and the upregulation of ROCK2 increase the sensitivity to TXA2 in the PAH animal, which might have pathophysiological implications in patients with PAH.

Metabolism of asymmetric dimethylarginine in hypoxia: from bench to bedside

Acute hypoxia and chronic hypoxia induce pulmonary vasoconstriction. While hypoxic pulmonary vasoconstriction is a physiological response if parts of the lung are affected, global exposure to hypoxic conditions may lead to clinical conditions like high-altitude pulmonary hypertension. Nitric oxide is the major vasodilator released from the vascular endothelium. Nitric oxide-dependent vasodilation is impaired in hypoxic conditions. Inhibition of nitric oxide synthesis is the most rapid and easily reversible molecular mechanism to regulate nitric oxide-dependent vascular function in response to physiological and pathophysiological stimuli. Asymmetric dimethylarginine is an endogenous, competitive inhibitor of nitric oxide synthase and a risk marker for major cardiovascular events and mortality. Elevated asymmetric dimethylarginine has been observed in animal models of hypoxia as well as in human cohorts under chronic and chronic intermittent hypoxia at high altitude. In lowlanders, asymmetric dimethylarginine is high in patients with pulmonary hypertension. We have recently shown that high asymmetric dimethylarginine at sea level is a predictor for high-altitude pulmonary hypertension. Asymmetric dimethylarginine is a highly regulated molecule, both by its biosynthesis and metabolism. Methylation of L-arginine by protein arginine methyltransferases was shown to be increased in hypoxia. Furthermore, the metabolism of asymmetric dimethylarginine by dimethylarginine dimethylaminohydrolases (DDAH1 and DDAH2) is decreased in animal models of hypoxia. Whether these changes are caused by transcriptional or posttranslational modifications remains to be elucidated. Current data suggest a major role of asymmetric dimethylarginine in regulating pulmonary arterial nitric oxide production in hypoxia. Further studies are needed to decipher the molecular mechanisms regulating asymmetric dimethylarginine in hypoxia and to understand their clinical significance.

Usefulness of bevacizumab-induced hypertension in patients with metastatic colorectal cancer: an updated meta-analysis

We tested the hypothesis that bevacizumab-induced hypertension may be a useful predictor for objective response rate, progression-free and overall survival in patients with metastatic colorectal cancer via a comprehensive meta-analysis. Search process, article selection and data extraction were independently performed by two investigators. Statistical analyses were conducted using the STATA/SE software. Fourteen independent studies and 2292 study subjects were synthesized. Overall relative risk of objective response rate for bevacizumab-induced hypertension was 2.03 (95% confidence interval [CI]: 1.18-3.48, p=0.01), with significant heterogeneity and publication bias, whereas unbiased estimate was nonsignificant after considering potentially missing studies. Overall hazard ratio for progression-free survival was 0.58 (95% CI: 0.43-0.77, p<0.001), with significant heterogeneity and publication bias, and unbiased estimate was significant (hazard ratio: 0.52, 95% CI: 0.41-0.66, p<0.001). Overall hazard ratio for overall survival was 0.51 (95% CI: 0.39-0.65, p<0.001), and this estimate was not likely confounded by heterogeneity or publication bias. Subgroup and meta-regression analyses suggested that hypertension grade of controls, sample size, age and gender were possible causes of heterogeneity. Taken together, our findings indicate that bevacizumab-induced hypertension can predict progress-free survival and overall survival in patients with metastatic colorectal cancer, whereas its prediction for objective response rate was nonsignificant.

Inhibition of Multidrug Resistance Proteins by MK 571 Enhances Bladder, Prostate, and Urethra Relaxation through cAMP or cGMP Accumulation

The biologic effect of cAMP and cGMP is terminated by phosphodiesterases and multidrug resistance proteins MRP4 and MRP5, which pump cyclic nucleotides out of the cell. Therefore, this study aimed to characterize the role of MRP inhibitor, MK 571 (3-[[[3-[(1E)-2-(7-chloro-2-quinolinyl)ethenyl]phenyl][[3-(dimethylamino)-3-oxopropyl]thio]methyl]thio]propanoic acid), in the bladder, prostate, and urethra of male mice by means of functional assays, protein expression, and cyclic nucleotide quantification. The cumulative addition of MK 571 (1-30 µM) produced only small relaxation responses (approximately 25%) in all studied tissues. In the bladder, isoprenaline/fenoterol and forskolin concentration-dependently relaxed and MK 571 (20 µM) increased the maximal response values by 37% and 24%, respectively. When MK 571 was coincubated with fenoterol or forskolin, intracellular levels of cAMP and protein expression of phospho-vasodilator-stimulated phosphoprotein (p-VASP) Ser157 were significantly greater compared with bladders stimulated with fenoterol or forskolin alone. In the prostate and urethra, sodium nitroprusside concentration-dependently relaxed and MK 571 (20 µM) significantly increased relaxation responses by 70% and 56%, respectively, accompanied by greater intracellular levels of cGMP and protein expression of p-VASP Ser239 in the prostate. Tadalafil and BAY 41-2272 (5-cyclopropyl-2-[1-[(2-fluorophenyl)methyl]-1H-pyrazolo[3,4-b]pyridin-3-yl]-4-pyrimidinamine) also relaxed the prostate and urethra, respectively, and MK 571 markedly enhanced this response. The stable analog of cGMP (8-Br-cGMP) induced concentration-dependent relaxation responses in the prostate and urethra, and MK 571 significantly increased the relaxation response. In conclusion, to our knowledge, this is the first study to show that efflux transporters are physiologically active in the bladder, prostate, and urethra to control intracellular levels of cAMP or cGMP.

Inhibition of nitric oxide synthase unmasks vigorous vasoconstriction in established pulmonary arterial hypertension

It is widely accepted that impaired bioavailability of endothelial nitric oxide (NO) plays a critical role in the pathophysiology of pulmonary arterial hypertension (PAH). However, there are published data that show that relatively many PAH patients respond favorably to acetylcholine‐induced pulmonary vasodilation during their follow‐up period, when diverse stages of the disorder are included. We hypothesized that NO bioavailability varies depending on the progression of PAH. Adult rats were exposed to the VEGF receptor blocker Sugen5416 and 3 weeks of hypoxia followed by return to normoxia for various additional weeks. All rats developed increased right ventricular systolic pressure (RVSP) and occlusive lesion formation at 1, 3, 5, and 8 weeks after the Sugen5416 injection. Acute NO synthase blockade did not change the elevated RVSP at the 1‐week time point, while it further increased RVSP markedly at the 3‐, 5‐, and 8‐week time points, leading to death in all rats tested at 8 weeks. Acetylcholine caused significant reduction in RVSP at the 8‐week but not the 1‐week time point, whereas sodium nitroprusside decreased the pressure similarly at both time points. Increased NO‐mediated cGMP production was found in lungs from the 8‐week but not the 1‐week time point. In conclusion, despite its initial impairment, NO bioavailability is restored and endogenous NO plays a critical protective role by counteracting severe pulmonary vasoconstriction in established stages of PAH in the Sugen5416/hypoxia/normoxia‐exposed rats. Our results provide solid pharmacological evidence for a major contribution of a NO‐suppressed vasoconstrictor component in the pathophysiology of established PAH.