Pulmonary arterial hypertension (PAH) is caused by functional and structural changes in the pulmonary vasculature, leading to increased pulmonary vascular resistance. The process of pulmonary vascular remodeling is accompanied by endothelial dysfunction, activation of fibroblasts and smooth muscle cells, crosstalk between cells within the vascular wall, and recruitment of circulating progenitor cells. Recent findings have reestablished the role of chronic vasoconstriction in the remodeling process. Although the pathology of PAH in the lung is well known, this article is concerned with the cellular and molecular processes involved. In particular we focus on the role of the Rho family guanosine triphosphatases in endothelial function and vasoconstriction. The crosstalk between endothelium and vascular smooth muscle is explored in the context of mutations in the bone morphogenetic protein type II receptor, alterations in angiopoietin-1/TIE2 signaling and the serotonin pathway. We also review the role of voltage-gated K+ (Kv) channels and transient receptor potential channels in the regulation of cytosolic [Ca2+] and [K+], vasoconstriction, proliferation and cell survival. We highlight the importance of the extracellular matrix as an active regulator of cell behavior and phenotype and evaluate the contribution of the glycoprotein tenascin-c as a key mediator of smooth muscle cell growth and survival. Finally, we discuss the origins of a cell type critical to the process of pulmonary vascular remodeling, the myofibroblast, and review the evidence supporting a contribution for the involvement of endothelial-mesenchymal transition and recruitment of circulating mesenchymal progenitor cells.

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) due to Ca(2+) release from intracellular Ca(2+) stores and Ca(2+) influx through plasmalemmal Ca(2+) channels plays a critical role in mitogen-mediated cell growth. Depletion of intracellular Ca(2+) stores triggers capacitative Ca(2+) entry (CCE), a mechanism involved in maintaining Ca(2+) influx and refilling intracellular Ca(2+) stores. Transient receptor potential (TRP) genes have been demonstrated to encode the store-operated Ca(2+) channels that are activated by Ca(2+) store depletion. In this study, we examined whether CCE, activity of store-operated Ca(2+) channels, and human TRP1 (hTRP1) expression are essential in human pulmonary arterial smooth muscle cell (PASMC) proliferation. Chelation of extracellular Ca(2+) and depletion of intracellularly stored Ca(2+) inhibited PASMC growth in media containing serum and growth factors. Resting [Ca(2+)](cyt) as well as the increases in [Ca(2+)](cyt) due to Ca(2+) release and CCE were all significantly greater in proliferating PASMC than in growth-arrested cells. Consistently, whole cell inward currents activated by depletion of intracellular Ca(2+) stores and the mRNA level of hTRP1 were much greater in proliferating PASMC than in growth-arrested cells. These results suggest that elevated [Ca(2+)](cyt) and intracellularly stored [Ca(2+)] play an important role in pulmonary vascular smooth muscle cell growth. CCE, potentially via hTRP1-encoded Ca(2+)-permeable channels, may be an important mechanism required to maintain the elevated [Ca(2+)](cyt) and stored [Ca(2+)] in human PASMC during proliferation.

Pulmonary vascular medial hypertrophy caused by excessive pulmonary artery smooth muscle cell (PASMC) proliferation is a major cause for the elevated pulmonary vascular resistance in patients with idiopathic pulmonary arterial hypertension (IPAH). Increased Ca 2؉ influx is an important stimulus for PASMC proliferation. Transient receptor potential (TRP) channel genes encode Ca 2؉ channels that are responsible for Ca 2؉ entry during cell proliferation. Normal human PASMC expressed multiple canonical TRP (TRPC) isoforms; TRPC6 was highly expressed and TRPC3 was minimally expressed. The protein expression of TRPC6 in normal PASMC closely correlated with the expression of Ki67, suggesting that TRPC6 expression is involved in the transition of PASMC from quiescent phase to mitosis. In lung tissues and PASMC from IPAH patients, the mRNA and protein expression of TRPC3 and -6 were much higher than in those from normotensive or secondary pulmonary hypertension patients. Inhibition of TRPC6 expression with TRPC6 small interfering RNA markedly attenuated IPAH-PASMC proliferation. These results demonstrate that expression of TRPC channels correlates with the progression of the cell cycle in PASMC. TRPC channel overexpression may be partially responsible for the increased PASMC proliferation and pulmonary vascular medial hypertrophy in IPAH patients.I diopathic pulmonary arterial hypertension (IPAH) is a fatal disease that causes right heart failure and death. The elevated pulmonary vascular resistance (PVR) and arterial pressure in IPAH patients result mainly from pulmonary vasoconstriction, vascular remodeling, and in situ thrombosis (1). A central aspect of pulmonary vascular remodeling is medial hypertrophy caused by sustained pulmonary vasoconstriction (2-4), excessive pulmonary artery smooth muscle cell (PASMC) proliferation (5), and inhibited PASMC apoptosis (6, 7), resulting in a narrowed vascular lumen and increased PVR. Although its etiology remains unclear, elevated levels of circulating mitogens, dysfunction or down-regulation of receptors and ion channels, upregulation of transporters, and heightened activity of elastases and glycoproteins have been implicated in IPAH (5,6,(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) Transient receptor potential (TRP) channel genes may encode subunits that form receptor-(ROC) and store-(SOC) operated Ca 2ϩ channels in many cell types, including PASMC and pulmonary artery endothelial cells (PAEC) (28,(30)(31)(32)(33)(34). Ca 2ϩ entry through ROC and SOC increases [Ca 2ϩ ] cyt , allowing for phosphorylation of signal transduction proteins and transcription factors (23,24,(35)(36)(37)(38), that are essential for the progression of the cell cycle (21). High levels of [Ca 2ϩ ] cyt and sufficient levels of Ca 2ϩ in the SR are required for vascular smooth muscle cell proliferation (22,25,39). Because they regulate SR and cytoplasmic Ca 2ϩ , CCE and SOC may play significant roles in regulating cell proliferation (28,29). This study tested the hypothesis that canonical TRP (TRPC...

Pulmonary hypertension (PH) is an unremitting disease defined by a progressive increase in pulmonary vascular resistance leading to right-sided heart failure. Using mice with genetic deletions of caveolin 1 (Cav1) and eNOS (Nos3), we demonstrate here that chronic eNOS activation secondary to loss of caveolin-1 can lead to PH. Consistent with a role for eNOS in the pathogenesis of PH, the pulmonary vascular remodeling and PH phenotype of Cav1 -/-mice were absent in Cav1 -/-Nos3 -/-mice. Further, treatment of Cav1 -/-mice with either MnTMPyP (a superoxide scavenger) or l-NAME (a NOS inhibitor) reversed their pulmonary vascular pathology and PH phenotype. Activation of eNOS in Cav1 -/-lungs led to the impairment of PKG activity through tyrosine nitration. Moreover, the PH phenotype in Cav1 -/-lungs could be rescued by overexpression of PKG-1. The clinical relevance of the data was indicated by the observation that lung tissue from patients with idiopathic pulmonary arterial hypertension demonstrated increased eNOS activation and PKG nitration and reduced caveolin-1 expression. Together, these data show that loss of caveolin-1 leads to hyperactive eNOS and subsequent tyrosine nitration-dependent impairment of PKG activity, which results in PH. Thus, targeting of PKG nitration represents a potential novel therapeutic strategy for the treatment of PH.

These results indicate that KV channel function in PPH-PASMCs is inhibited compared with SPH-PASMCs. The resulting membrane depolarization and increase in [Ca2+]cyt lead to pulmonary vasoconstriction and PASMC proliferation. Our data suggest that defects in PASMC KV channels in PPH patients may be a unique mechanism involved in initiating and maintaining pulmonary vasoconstriction and appear to play a role in the pathogenesis of PPH.