Molecular biology of primary pulmonary hypertension

Acid-base transporters modulate cell migration, growth and proliferation: Implications for structure development and remodeling of resistance arteries?

Disturbed acid-base transport across the plasma membrane affects intracellular pH control and has been shown--primarily based on studies with non-vascular cells--to interfere with a number of fundamental cell functions including cell migration, growth and proliferation. Here, we evaluate the effects of acid-base transport and intracellular pH on the morphology of the resistance artery wall, which is altered in a number of physiological and pathological conditions and is an independent predictor of cardiovascular risk. The current evidence supports that disturbed function and/or expression of acid-base transporters can alter resistance artery morphology--and potentially atherosclerosis-prone conduit arteries--and hence should be considered as possible mechanistic components and targets for treatment in cardiovascular disease. More experimental evidence is required, however, to evaluate the cell biological effects of acid-base transport in vascular cells, the roles of specific acid-base transporters in artery remodeling, the relative mechanistic importance of acid-base transporters in the vascular wall compared to other organs, and the therapeutic potential of modifying acid-base transport activity pharmacologically or genetically.

Dependence of Golgi apparatus integrity on nitric oxide in vascular cells: implications in pulmonary arterial hypertension

Although reduced bioavailability of nitric oxide (NO) has been implicated in the pathogenesis of pulmonary arterial hypertension (PAH), its consequences on organellar structure and function within vascular cells is largely unexplored. We investigated the effect of reduced NO on the structure of the Golgi apparatus as assayed by giantin or GM130 immunofluorescence in human pulmonary arterial endothelial (HPAECs) and smooth muscle (HPASMCs) cells, bovine PAECs, and human EA.hy926 endothelial cells. Golgi structure was also investigated in cells in tissue sections of pulmonary vascular lesions in idiopathic PAH (IPAH) and in macaques infected with a chimeric simian immunodeficiency virus containing the human immunodeficiency virus (HIV)-nef gene (SHIV-nef) with subcellular three-dimensional (3D) immunoimaging. Compounds with NO scavenging activity including 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), methylene blue, N-acetylcysteine, and hemoglobin markedly fragmented the Golgi in all cell types evaluated as did monocrotaline pyrrole, while LY-83583, sildenafil, fasudil, Y-27632, Tiron, Tempol, or H(2)O(2) did not. Golgi fragmentation by NO scavengers was inhibited by diethylamine NONOate, was evident in HPAECs after selective knockdown of endothelial nitric oxide synthase using small interfering RNA (siRNA), was independent of microtubule organization, required the GTPase dynamin 2, and was accompanied by depletion of α-soluble N-ethylmaleimide-sensitive factor (NSF) acceptor protein (α-SNAP) from Golgi membranes and codispersal of the SNAP receptor (SNARE) Vti1a with giantin. Golgi fragmentation was confirmed in endothelial and smooth muscle cells in pulmonary arterial lesions in IPAH and the SHIV-nef-infected macaque with subcellular 3D immunoimaging. In SHIV-nef-infected macaques Golgi fragmentation was observed in cells containing HIV-nef-bearing endosomes. The observed Golgi fragmentation suggests that NO plays a significant role in modulating global protein trafficking patterns that contribute to changes in the cell surface landscape and functional signaling in vascular cells.

Nitrite potently inhibits hypoxic and inflammatory pulmonary arterial hypertension and smooth muscle proliferation via xanthine oxidoreductase-dependent nitric oxide generation

BACKGROUND

Pulmonary arterial hypertension is a progressive proliferative vasculopathy of the small pulmonary arteries that is characterized by a primary failure of the endothelial nitric oxide and prostacyclin vasodilator pathways, coupled with dysregulated cellular proliferation. We have recently discovered that the endogenous anion salt nitrite is converted to nitric oxide in the setting of physiological and pathological hypoxia. Considering the fact that nitric oxide exhibits vasoprotective properties, we examined the effects of nitrite on experimental pulmonary arterial hypertension.

METHODS AND RESULTS

We exposed mice and rats with hypoxia or monocrotaline-induced pulmonary arterial hypertension to low doses of nebulized nitrite (1.5 mg/min) 1 or 3 times a week. This dose minimally increased plasma and lung nitrite levels yet completely prevented or reversed pulmonary arterial hypertension and pathological right ventricular hypertrophy and failure. In vitro and in vivo studies revealed that nitrite in the lung was metabolized directly to nitric oxide in a process significantly enhanced under hypoxia and found to be dependent on the enzymatic action of xanthine oxidoreductase. Additionally, physiological levels of nitrite inhibited hypoxia-induced proliferation of cultured pulmonary artery smooth muscle cells via the nitric oxide-dependent induction of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). The therapeutic effect of nitrite on hypoxia-induced pulmonary hypertension was significantly reduced in the p21-knockout mouse; however, nitrite still reduced pressures and right ventricular pathological remodeling, indicating the existence of p21-independent effects as well.

CONCLUSIONS

These studies reveal a potent effect of inhaled nitrite that limits pathological pulmonary arterial hypertrophy and cellular proliferation in the setting of experimental pulmonary arterial hypertension.

CPU0213, a non-selective ETA/ETB receptor antagonist, improves pulmonary arteriolar remodeling of monocrotaline-induced pulmonary hypertension in rats

1. The aim of the present study was to explore the effects of CPU0213, a dual endothelin ET(A)/ET(B) receptor antagonist, and nifedipine, a calcium antagonist, in relieving pulmonary hypertension (PH). Both endothelin receptor and calcium antagonists have been reported to be effective in alleviating the remodelling of pulmonary arteries induced by monocrotaline (MCT) in rats. 2. After an initial single dose of 60 mg/kg, s.c., MCT, CPU0213 was administered to rats at doses of 25, 50 or 100 mg/kg, p.o., for 28 days. In addition, nifedipine was administered to another group of rats at a dose of 10 mg/kg, p.o., for 28 days. The haemodynamics of the right ventricle, pulmonary vascular activity, remodelling of the pulmonary arterioles (< 150 microm) and biochemical changes were evaluated. 3. Right ventricular systolic pressure (RVSP), central venous pressure (CVP), the maximum rate of uprising pressure (dP/dT(max)) and the weight index of the right ventricle were significantly elevated in MCT-treated rats. In addition, increases in pulmonary endothelin-1, malonyldialdehyde (MDA) and hydroxyproline content and a reduction in superoxide dismutase activity was found after MCT treatment. The thickness and area of the pulmonary arterial wall were significantly increased in MCT-treated rats compared with control rats. At all three doses tested, CPU0213 ameliorated these changes in a dose-dependent manner and the effects were associated with a greater reduction in the remodelling of pulmonary arterioles. However, nifedipine was only partially effective in amelerioating biochemical and haemodynamic changes induced by MCT, significantly reducing RVSP, CVP, +dp/dt(max), tissue MDA, inducible nitric oxide synthase and hydroxyproline content, increasing -dp/dt(min) and having no effect on the other parameters investigated. In addition, nifedipine had no effect on remodelling of the arterial wall. 4. In conclusion, CPU0213 is more effective than nifedipine in suppressing the remodelling of pulmonary arterioles in PH induced by MCT treatment of rats. Furthermore, CPU0213 may have promise in treating PH secondary to connective tissue disease.

Rho kinase-mediated vasoconstriction in rat models of pulmonary hypertension

There is current controversy regarding whether vasoconstriction plays a significant role in the elevated pressure of severe, advanced stages of pulmonary hypertension. Results of acute vasodilator testing using conventional vasodilators in such patients suggest there is only a minor contribution of vasoconstriction. However, there is a possibility that these results may underestimate the contribution of vasoconstriction because the most effective vasodilators have not yet been tested. This issue has not been addressed even experimentally, due mainly to a lack of appropriate animal models. A few animal models that mimic the pathology of human severe pulmonary hypertension more closely (i.e., development of occlusive neointimal lesions in small pulmonary arteries/arterioles) have been introduced, including rat models of left lung pneumonectomy plus monocrotaline injection and vascular endothelial growth factor inhibition plus exposure to chronic hypoxia. We have observed that Rho kinase inhibitors, a novel class of potent vasodilators, reduce the high pulmonary artery pressure of these models acutely and markedly, suggesting that vasoconstriction can significantly be involved in pulmonary hypertension with severely remodeled (occluded) pulmonary vessels. This chapter describes methods used for evaluation of the involvement of Rho kinase-mediated vasoconstriction in rat models of pulmonary hypertension.

Serotonin transporter protein in pulmonary hypertensive rats treated with atorvastatin

HMG-CoA-reductase inhibitors (statins) influence lipid metabolism and have pleiotropic effects. Several statins reduce various forms of pulmonary hypertension (PH) in animal models. The relationship between atorvastatin and expression of serotonin transporter protein (5-HTT) remains unknown. This study focused on the effects of atorvastatin on the course of monocrotaline (MCT)-induced PH and its relation to 5-HTT expression. Male Sprague-Dawley rats were challenged with MCT with or without subsequent daily oral treatment with 0.1, 1, and 10 mg/kg of atorvastatin for 28 days. Over the 4-wk course, the progression of PH was followed by transthoracic echocardiography [pulmonary artery pressure was assessed by pulmonary artery flow acceleration time (PAAT), an estimate reciprocal to pulmonary artery pressure], and, at the end of the 4-wk course, invasive right ventricular pressure, right ventricular weight, quantitative morphology, and 5-HTT expression were measured. MCT caused significant PH as early as 7 days after injection. Atorvastatin treatment increased PAAT and reduced right ventricular pressure, right ventricular hypertrophy, and vascular remodeling over the 4-wk course. MCT challenge was associated with increased pulmonary vascular 5-HTT expression, and atorvastatin treatment reduced the 5-HTT expression. MCT-induced PH over the course of 4 wk can be easily followed by transthoracic echocardiography, and atorvastatin is effective in reducing the PH. Atorvastatin's effects are associated with a decrease of 5-HTT expression.

BMP-dependent activation of caspase-9 and caspase-8 mediates apoptosis in pulmonary artery smooth muscle cells

Germ line mutations in the bone morphogenetic protein (BMP) receptor type II (BMPRII) gene have been found in >50% of familial idiopathic pulmonary arterial hypertension (IPAH) patients and in 30% of sporadic cases of IPAH. Mutations of BMPRII occur in the extracellular ligand-binding domain, in the cytoplasmic serine/threonine kinase domain, or in the long carboxy terminus domain of unknown function. In this study, we demonstrate that BMPs promote apoptotic cell death in normal human pulmonary artery smooth muscle cells (PASMCs) by activation of caspases-3, -8, and -9, cytochrome c release, and downregulation of Bcl-2. Normal PASMCs expressing a kinase domain mutant or a carboxy-terminal domain deletion mutant of BMPRII identified in IPAH patients are resistant to BMP-mediated apoptosis. This dominant-negative effect may act in heterozygous patients and lead to the development of the pulmonary vascular medial hypertrophy found in IPAH patients. Our study also demonstrates an essential role of the carboxy terminus domain of BMPRII in the activation of the apoptotic signaling cascade.

Carbon monoxide reverses established pulmonary hypertension

Pulmonary arterial hypertension (PAH) is an incurable disease characterized by a progressive increase in pulmonary vascular resistance leading to right heart failure. Carbon monoxide (CO) has emerged as a potently protective, homeostatic molecule that prevents the development of vascular disorders when administered prophylactically. The data presented in this paper demonstrate that CO can also act as a therapeutic (i.e., where exposure to CO is initiated after pathology is established). In three rodent models of PAH, a 1 hour/day exposure to CO reverses established PAH and right ventricular hypertrophy, restoring right ventricular and pulmonary arterial pressures, as well as the pulmonary vascular architecture, to near normal. The ability of CO to reverse PAH requires functional endothelial nitric oxide synthase (eNOS/NOS3) and NO generation, as indicated by the inability of CO to reverse chronic hypoxia-induced PAH in eNOS-deficient (nos3^−/−) mice versus wild-type mice. The restorative function of CO was associated with a simultaneous increase in apoptosis and decrease in cellular proliferation of vascular smooth muscle cells, which was regulated in part by the endothelial cells in the hypertrophied vessels. In conclusion, these data demonstrate that CO reverses established PAH dependent on NO generation supporting the use of CO clinically to treat pulmonary hypertension.

OAZ regulates bone morphogenetic protein signaling through Smad6 activation

The intensity and duration of activation of a signal transduction system are important determinants of the specificity of the cellular response to the stimulus. It is unclear how different cells can generate a signal of varying intensity and duration in response to the same cytokine. We investigated the role of the transcriptional activator and Smad1/4 cofactor OAZ in regulating bone morphogenetic protein (BMP) signaling. We demonstrate that upon BMP4 stimulation, an OAZ-Smad1/4 complex binds to and activates the gene encoding Smad6, a specific inhibitor of the BMP pathway. Removal of endogenous OAZ from pluripotent embryonal carcinoma cells prevents the induction of Smad6 by BMP4 and extends the period of detection of phosphorylated Smad1 after BMP stimulation. Conversely, in cells that do not normally express OAZ, such as myoblasts and smooth muscle cells, forced OAZ expression leads to faster and higher Smad6 induction in response to BMP4, decrease of Smad1 phosphorylation, and attenuation of BMP-mediated responses. Our results demonstrate that OAZ can alter the intensity and duration of the BMP stimulus through Smad6 and indicate that the tissue-specific expression of OAZ is a critical determinant of the cellular response to the BMP signal.