Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling.

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

Pathogenic Mechanisms of Pulmonary Arterial Hypertension: Homeostasis Imbalance of Endothelium-Derived Relaxing and Contracting Factors.

Pulmonary arterial hypertension (PAH) is a progressive and fatal disease. Sustained pulmonary vasoconstriction and concentric pulmonary vascular remodeling contribute to the elevated pulmonary vascular resistance and pulmonary artery pressure in PAH. Endothelial cells regulate vascular tension by producing endothelium-derived relaxing factors (EDRFs) and endothelium-derived contracting factors (EDCFs). Homeostasis of EDRF and EDCF production has been identified as a marker of the endothelium integrity. Impaired synthesis or release of EDRFs induces persistent vascular contraction and pulmonary artery remodeling, which subsequently leads to the development and progression of PAH. In this review, the authors summarize how EDRFs and EDCFs affect pulmonary vascular homeostasis, with special attention to the recently published novel mechanisms related to endothelial dysfunction in PAH and drugs associated with EDRFs and EDCFs.

TRPC6, a therapeutic target for pulmonary hypertension.

Idiopathic pulmonary arterial hypertension (PAH) is a fatal and progressive disease. Sustained vasoconstriction due to pulmonary arterial smooth muscle cell (PASMC) contraction and concentric arterial remodeling due partially to PASMC proliferation are the major causes for increased pulmonary vascular resistance and increased pulmonary arterial pressure in patients with precapillary pulmonary hypertension (PH) including PAH and PH due to respiratory diseases or hypoxemia. We and others observed upregulation of TRPC6 channels in PASMCs from patients with PAH. A rise in cytosolic Ca2+ concentration ([Ca2+]cyt) in PASMC triggers PASMC contraction and vasoconstriction, while Ca2+-dependent activation of PI3K/AKT/mTOR pathway is a pivotal signaling cascade for cell proliferation and gene expression. Despite evidence supporting a pathological role of TRPC6, no selective and orally bioavailable TRPC6 antagonist has yet been developed and tested for treatment of PAH or PH. In this study, we sought to investigate whether block of receptor-operated Ca2+ channels using a nonselective blocker of cation channels, 2-aminoethyl diphenylborinate (2-APB, administered intraperitoneally) and a selective blocker of TRPC6, BI-749327 (administered orally) can reverse established PH in mice. The results from the study show that intrapulmonary application of 2-APB (40 µM) or BI-749327 (3-10 µM) significantly and reversibly inhibited acute alveolar hypoxia-induced pulmonary vasoconstriction. Intraperitoneal injection of 2-APB (1 mg/kg per day) significantly attenuated the development of PH and partially reversed established PH in mice. Oral gavage of BI-749327 (30 mg/kg, every day, for 2 wk) reversed established PH by ∼50% via regression of pulmonary vascular remodeling. Furthermore, 2-APB and BI-749327 both significantly inhibited PDGF- and serum-mediated phosphorylation of AKT and mTOR in PASMC. In summary, the receptor-operated and mechanosensitive TRPC6 channel is a good target for developing novel treatment for PAH/PH. BI-749327, a selective TRPC6 blocker, is potentially a novel and effective drug for treating PAH and PH due to respiratory diseases or hypoxemia.

Mouse model of experimental pulmonary hypertension: Lung angiogram and right heart catheterization.

Pulmonary arterial hypertension is a progressive and fatal disease and rodents with experimental pulmonary hypertension (PH) are often used to study pathogenic mechanisms, identify therapeutic targets, and develop novel drugs for treatment. Here we describe a hands-on set of experimental approaches including ex vivo lung angiography and histology and in vivo right heart catheterization (RHC) to phenotypically characterize pulmonary hemodynamics and lung vascular structure in normal mice and mice with experimental PH. We utilized Microfil polymer as contrast in our ex vivo lung angiogram to quantitatively examine pulmonary vascular remodeling in mice with experimental PH, and lung histology to estimate pulmonary artery wall thickness. The peripheral lung vascular images were selected to determine the total length of lung vascular branches, the number of branches and the number of junctions in a given area (mm-2). We found that the three parameters determined by angiogram were not significantly different among the apical, middle, and basal regions of the mouse lung from normal mice, and were not influenced by gender (no significant difference between female and male mice). We conducted RHC in mice to measure right ventricular systolic pressure, a surrogate measure for pulmonary artery systolic pressure and right ventricle (RV) contractility (RV ± dP/dtmax) to estimate RV function. RHC, a short time (4-6 min) procedure, did not alter the lung angiography measurements. In summary, utilizing ex vivo angiogram to determine peripheral vascular structure and density in the mouse lung and utilizing in vivo RHC to measure pulmonary hemodynamics are reliable readouts to phenotype normal mice and mice with experimental PH. Lung angiogram and RHC are also reliable approaches to examine pharmacological effects of new drugs on pulmonary vascular remodeling and hemodynamics.

Upregulation of Calcium Homeostasis Modulators in Contractile-To-Proliferative Phenotypical Transition of Pulmonary Arterial Smooth Muscle Cells.

Excessive pulmonary artery (PA) smooth muscle cell (PASMC) proliferation and migration are implicated in the development of pathogenic pulmonary vascular remodeling characterized by concentric arterial wall thickening and arteriole muscularization in patients with pulmonary arterial hypertension (PAH). Pulmonary artery smooth muscle cell contractile-to-proliferative phenotypical transition is a process that promotes pulmonary vascular remodeling. A rise in cytosolic Ca2+ concentration [(Ca2+) cyt ] in PASMCs is a trigger for pulmonary vasoconstriction and a stimulus for pulmonary vascular remodeling. Here, we report that the calcium homeostasis modulator (CALHM), a Ca2+ (and ATP) channel that is allosterically regulated by voltage and extracellular Ca2+, is upregulated during the PASMC contractile-to-proliferative phenotypical transition. Protein expression of CALHM1/2 in primary cultured PASMCs in media containing serum and growth factors (proliferative PASMC) was significantly greater than in freshly isolated PA (contractile PASMC) from the same rat. Upregulated CALHM1/2 in proliferative PASMCs were associated with an increased ratio of pAKT/AKT and pmTOR/mTOR and an increased expression of the cell proliferation marker PCNA, whereas serum starvation and rapamycin significantly downregulated CALHM1/2. Furthermore, CALHM1/2 were upregulated in freshly isolated PA from rats with monocrotaline (MCT)-induced PH and in primary cultured PASMC from patients with PAH in comparison to normal controls. Intraperitoneal injection of CGP 37157 (0.6 mg/kg, q8H), a non-selective blocker of CALHM channels, partially reversed established experimental PH. These data suggest that CALHM upregulation is involved in PASMC contractile-to-proliferative phenotypical transition. Ca2+ influx through upregulated CALHM1/2 may play an important role in the transition of sustained vasoconstriction to excessive vascular remodeling in PAH or precapillary PH. Calcium homeostasis modulator could potentially be a target to develop novel therapies for PAH.

Divergent changes of p53 in pulmonary arterial endothelial and smooth muscle cells involved in the development of pulmonary hypertension.

The tumor-suppressive role of p53, a transcription factor that regulates the expression of many genes, has been linked to cell cycle arrest, apoptosis, and senescence. The noncanonical function or the pathogenic role of p53 has more recently been implicated in pulmonary vascular disease. We previously reported that rapid nuclear accumulation of hypoxia-inducible factor (HIF)-1α in pulmonary arterial smooth muscle cells (PASMCs) upregulates transient receptor potential channels and enhances Ca2+ entry to increase cytosolic Ca2+ concentration ([Ca2+]cyt). Also, we observed differences in HIF-1α/2α expression in PASMCs and pulmonary arterial endothelial cells (PAECs). Here we report that p53 is increased in PAECs, but decreased in PASMCs, isolated from mice with hypoxia-induced pulmonary hypertension (PH) and rats with monocrotaline (MCT)-induced PH (MCT-PH). The increased p53 in PAECs from rats with MCT-PH is associated with an increased ratio of Bax/Bcl-2, while the decreased p53 in PASMCs is associated with an increased HIF-1α. Furthermore, p53 is downregulated in PASMCs isolated from patients with idiopathic pulmonary arterial hypertension compared with PASMCs from normal subjects. Overexpression of p53 in normal PASMCs inhibits store-operated Ca2+ entry (SOCE) induced by passive depletion of intracellularly stored Ca2+ in the sarcoplasmic reticulum, while downregulation of p53 enhances SOCE. These data indicate that differentially regulated expression of p53 and HIF-1α/2α in PASMCs and PAECs and the cross talk between p53 and HIF-1α/2α in PASMCs and PAECs may play an important role in the development of PH via, at least in part, induction of PAEC apoptosis and PASMC proliferation.

Alterations in human neutrophil function caused by bisphenol A.

Bisphenol A (BPA) is a synthetic, organic compound frequently present in consumer plastics, including plastic-lined cans, water bottles, toys, and teeth sutures. Previous studies have shown that BPA can produce adverse health effects that include defects in reproductive function and altered prenatal/childhood development. However, little is known regarding the effects of BPA on immune function. In this study, we assessed the effect of BPA on human neutrophils, a critical component of the innate immune system's defense against pathogens. We found that BPA induces a concentration-dependent increase in reactive oxygen species (ROS) generation by neutrophils, which is inhibited by the estrogen receptor-ß antagonist PHTPP. Furthermore, incubation with the membrane-permeable calcium chelator BAPTA-AM and/or removal of extracellular calcium inhibited BPA-induced ROS production, indicating that the process is calcium dependent. Transwell chemotaxis assays revealed that BPA exposure reduces the chemotactic capacity of neutrophils in a gradient of the bacterial cell wall component f-Met-Leu-Phe, a potent chemoattractant. Exposure to BPA also inhibits the ability of neutrophils to kill methicillin-resistant Staphylococcus aureus, a leading human pathogen. Our findings reveal that BPA alters the in vitro function of neutrophils, including ROS production, chemotaxis, and bacterial killing, and raises the possibility of altered innate immunity in vivo, especially in those with compromised immune function and who can be exposed to BPA in a wide variety of products.

Overexpression of hexokinase 2 reduces mitochondrial calcium overload in coronary endothelial cells of type 2 diabetic mice.

Coronary microvascular rarefaction, due to endothelial cell (EC) dysfunction, is one of the causes of increased morbidity and mortality in diabetes. Coronary ECs in diabetes are more apoptotic due partly to mitochondrial calcium overload. This study was designed to investigate the role of hexokinase 2 (HK2, an endogenous inhibitor of voltage-dependent anion channel) in coronary endothelial dysfunction in type 2 diabetes. We used mouse coronary ECs (MCECs) isolated from type 2 diabetic mice and human coronary ECs (HCECs) from type 2 diabetic patients to examine protein levels and mitochondrial function. ECs were more apoptotic and capillary density was lower in the left ventricle of diabetic mice than the control. MCECs from diabetic mice exhibited significant increase in mitochondrial Ca2+ concentration ([Ca2+]mito) compared with the control. Among several regulatory proteins for [Ca2+]mito, hexokinase 1 (HK1) and HK2 were significantly lower in MCECs from diabetic mice than control MCECs. We also found that the level of HK2 ubiquitination was higher in MCECs from diabetic mice than in control MCECs. In line with the data from MCECs, HCECs from diabetic patients showed lower HK2 protein levels than HCECs from nondiabetic patients. High-glucose treatment, but not high-fat treatment, significantly decreased HK2 protein levels in MCECs. HK2 overexpression in MCECs of diabetic mice not only lowered the level of [Ca2+]mito, but also reduced mitochondrial reactive oxygen species production toward the level seen in control MCECs. These data suggest that HK2 is a potential therapeutic target for coronary microvascular disease in diabetes by restoring mitochondrial function in coronary ECs.

STIM2 (Stromal Interaction Molecule 2)-Mediated Increase in Resting Cytosolic Free Ca2+ Concentration Stimulates PASMC Proliferation in Pulmonary Arterial Hypertension.

An increase in cytosolic free Ca2+ concentration ([Ca2+]cyt) in pulmonary artery smooth muscle cells (PASMCs) triggers pulmonary vasoconstriction and stimulates PASMC proliferation leading to vascular wall thickening. Here, we report that STIM2 (stromal interaction molecule 2), a Ca2+ sensor in the sarcoplasmic reticulum membrane, is required for raising the resting [Ca2+]cyt in PASMCs from patients with pulmonary arterial hypertension (PAH) and activating signaling cascades that stimulate PASMC proliferation and inhibit PASMC apoptosis. Downregulation of STIM2 in PAH-PASMCs reduces the resting [Ca2+]cyt, whereas overexpression of STIM2 in normal PASMCs increases the resting [Ca2+]cyt The increased resting [Ca2+]cyt in PAH-PASMCs is associated with enhanced phosphorylation (p) of CREB (cAMP response element-binding protein), STAT3 (signal transducer and activator of transcription 3), and AKT, increased NFAT (nuclear factor of activated T-cell) nuclear translocation, and elevated level of Ki67 (a marker of cell proliferation). Furthermore, the STIM2-associated increase in the resting [Ca2+]cyt also upregulates the antiapoptotic protein Bcl-2 in PAH-PASMCs. Downregulation of STIM2 in PAH-PASMCs with siRNA (1) decreases the level of pCREB, pSTAT3, and pAKT and inhibits NFAT nuclear translocation, thereby attenuating proliferation, and (2) decreases Bcl-2, which leads to an increase of apoptosis. In summary, these data indicate that upregulated STIM2 in PAH-PASMCs, by raising the resting [Ca2+]cyt, contributes to enhancing PASMC proliferation by activating the CREB, STAT3, AKT, and NFAT signaling pathways and stimulating PASMC proliferation. The STIM2-associated increase in the resting [Ca2+]cyt is also involved in upregulating Bcl-2 that makes PAH-PASMCs resistant to apoptosis, and thus plays an important role in sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in patients with PAH.

Pathogenic Role of mTORC1 and mTORC2 in Pulmonary Hypertension.

Concentric lung vascular wall thickening due to enhanced proliferation of pulmonary arterial smooth muscle cells is an important pathological cause for the elevated pulmonary vascular resistance reported in patients with pulmonary arterial hypertension. We identified a differential role of mammalian target of rapamycin (mTOR) complex 1 and complex 2, two functionally distinct mTOR complexes, in the development of pulmonary hypertension (PH). Inhibition of mTOR complex 1 attenuated the development of PH; however, inhibition of mTOR complex 2 caused spontaneous PH, potentially due to up-regulation of platelet-derived growth factor receptors in pulmonary arterial smooth muscle cells, and compromised the therapeutic effect of the mTOR inhibitors on PH. In addition, we describe a promising therapeutic strategy using combination treatment with the mTOR inhibitors and the platelet-derived growth factor receptor inhibitors on PH and right ventricular hypertrophy. The data from this study provide an important mechanism-based perspective for developing novel therapies for patients with pulmonary arterial hypertension and right heart failure.

Endothelial HIF-2α contributes to severe pulmonary hypertension due to endothelial-to-mesenchymal transition.

Pulmonary vascular remodeling characterized by concentric wall thickening and intraluminal obliteration is a major contributor to the elevated pulmonary vascular resistance in patients with idiopathic pulmonary arterial hypertension (IPAH). Here we report that increased hypoxia-inducible factor 2α (HIF-2α) in lung vascular endothelial cells (LVECs) under normoxic conditions is involved in the development of pulmonary hypertension (PH) by inducing endothelial-to-mesenchymal transition (EndMT), which subsequently results in vascular remodeling and occlusive lesions. We observed significant EndMT and markedly increased expression of SNAI, an inducer of EndMT, in LVECs from patients with IPAH and animals with experimental PH compared with normal controls. LVECs isolated from IPAH patients had a higher level of HIF-2α than that from normal subjects, whereas HIF-1α was upregulated in pulmonary arterial smooth muscle cells (PASMCs) from IPAH patients. The increased HIF-2α level, due to downregulated prolyl hydroxylase domain protein 2 (PHD2), a prolyl hydroxylase that promotes HIF-2α degradation, was involved in enhanced EndMT and upregulated SNAI1/2 in LVECs from patients with IPAH. Moreover, knockdown of HIF-2α (but not HIF-1α) with siRNA decreases both SNAI1 and SNAI2 expression in IPAH-LVECs. Mice with endothelial cell (EC)-specific knockout (KO) of the PHD2 gene, egln1 (egln1EC-/-), developed severe PH under normoxic conditions, whereas Snai1/2 and EndMT were increased in LVECs of egln1EC-/- mice. EC-specific KO of the HIF-2α gene, hif2a, prevented mice from developing hypoxia-induced PH, whereas EC-specific deletion of the HIF-1α gene, hif1a, or smooth muscle cell (SMC)-specific deletion of hif2a, negligibly affected the development of PH. Also, exposure to hypoxia for 48-72 h increased protein level of HIF-1α in normal human PASMCs and HIF-2α in normal human LVECs. These data indicate that increased HIF-2α in LVECs plays a pathogenic role in the development of severe PH by upregulating SNAI1/2, inducing EndMT, and causing obliterative pulmonary vascular lesions and vascular remodeling.

Chloroquine is a potent pulmonary vasodilator that attenuates hypoxia-induced pulmonary hypertension.

BACKGROUND AND PURPOSE: Sustained pulmonary vasoconstriction and excessive pulmonary vascular remodelling are two major causes of elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension. The purpose of this study was to investigate whether chloroquine induced relaxation in the pulmonary artery (PA) and attenuates hypoxia-induced pulmonary hypertension (HPH). EXPERIMENTAL APPROACH: Isometric tension was measured in rat PA rings pre-constricted with phenylephrine or high K+ solution. PA pressure was measured in mouse isolated, perfused and ventilated lungs. Fura-2 fluorescence microscopy was used to measure cytosolic free Ca2+ concentration levels in PA smooth muscle cells (PASMCs). Patch-clamp experiments were performed to assess the activity of voltage-dependent Ca2+ channels (VDCCs) in PASMC. Rats exposed to hypoxia (10% O2 ) for 3 weeks were used as the model of HPH or Sugen5416/hypoxia (SuHx) for in vivo experiments. KEY RESULTS: Chloroquine attenuated agonist-induced and high K+ -induced contraction in isolated rat PA. Pretreatment with l-NAME or indomethacin and functional removal of endothelium failed to inhibit chloroquine-induced PA relaxation. In PASMC, extracellular application of chloroquine attenuated store-operated Ca2+ entry and ATP-induced Ca2+ entry. Furthermore, chloroquine also inhibited whole-cell Ba2+ currents through VDCC in PASMC. In vivo experiments demonstrated that chloroquine treatment ameliorated the HPH and SuHx models. CONCLUSIONS AND IMPLICATIONS: Chloroquine is a potent pulmonary vasodilator that may directly or indirectly block VDCC, store-operated Ca2+ channels and receptor-operated Ca2+ channels in PASMC. The therapeutic potential of chloroquine in pulmonary hypertension is probably due to the combination of its vasodilator, anti-proliferative and anti-autophagic effects.