The SphK1/S1P axis is a novel pathway in PAH that promotes PASMC proliferation, a major contributor to pulmonary vascular remodeling. Our results suggest that this pathway is a potential therapeutic target in PAH.

Rationale: Recent studies suggest that microRNAs (miRNAs) play important roles in regulation of pulmonary artery smooth muscle cell (PASMC) phenotype and are implicated in pulmonary arterial hypertension (PAH). However, the underlying molecular mechanisms remain elusive.Objectives: This study aims to understand the mechanisms regulating PASMC proliferation and differentiation by microRNA-17z92 (miR-17z92) and to elucidate its implication in PAH. Methods:We generated smooth muscle cell (SMC)-specific miR17z92 and PDZ and LIM domain 5 (PDLIM5) knockout mice and overexpressed miR-17z92 and PDLIM5 by injection of miR-17z92 mimics or PDLIM5-V5-His plasmids and measured their responses to hypoxia. We used miR-17z92 mimics, inhibitors, overexpression vectors, small interfering RNAs against PDLIM5, Smad, and transforming growth factor (TGF)-b to determine the role of miR-17z92 and its downstream targets in PASMC proliferation and differentiation.Measurements and Main Results: We found that human PASMC (HPASMC) from patients with PAH expressed decreased levels of the miR-17z92 cluster, TGF-b, and SMC markers. Overexpression of miR-17z92 increased and restored the expression of TGF-b 3 , Smad3, and SMC markers in HPASMC of normal subjects and patients with idiopathic PAH, respectively. Knockdown of Smad3 but not Smad2 prevented miR-17z92-induced expression of SMC markers. SMC-specific knockout of miR-17z92 attenuated hypoxiainduced pulmonary hypertension (PH) in mice, whereas reconstitution of miR-17z92 restored hypoxia-induced PH in these mice. We also found that PDLIM5 is a direct target of miR-17/20a, and hypertensive HPASMC and mouse PASMC expressed elevated PDLIM5 levels. Suppression of PDLIM5 increased expression of SMC markers and enhanced TGF-b/Smad2/3 activity in vitro and enhanced hypoxia-induced PH in vivo, whereas overexpression of PDLIM5 attenuated hypoxia-induced PH. Conclusions:We provided the first evidence that miR-17z92 inhibits PDLIM5 to induce the TGF-b 3 /SMAD3 pathway, contributing to the pathogenesis of PAH.

Pulmonary arterial hypertension (PAH) is a devastating disease without effective treatment. Despite decades of research and the development of novel treatments, PAH remains a fatal disease, suggesting an urgent need for better understanding of the pathogenesis of PAH. Recent studies suggest that microRNAs (miRNAs) are dysregulated in patients with PAH and in experimental pulmonary hypertension. Furthermore, normalization of a few miRNAs is reported to inhibit experimental pulmonary hypertension. We have reviewed the current knowledge about miRNA biogenesis, miRNA expression pattern, and their roles in regulation of pulmonary artery smooth muscle cells, endothelial cells, and fibroblasts. We have also identified emerging trends in our understanding of the role of miRNAs in the pathogenesis of PAH and propose future studies that might lead to novel therapeutic strategies for the treatment of PAH.

Human pulmonary artery smooth muscle cells (HPASMCs) express both adenosine monophosphate-activated protein kinase (AMPK) α1 and α2. We investigated the distinct roles of AMPK α1 and α2 in the survival of HPASMCs during hypoxia and hypoxia-induced pulmonary hypertension (PH). The exposure of HPASMCs to hypoxia (3% O2) increased AMPK activation and phosphorylation, and the inhibition of AMPK with Compound C during hypoxia decreased their viability and increased lactate dehydrogenase activity and apoptosis. Although the suppression of either AMPK α1 or α2 expression led to increased cell death, the suppression of AMPK α2 alone increased caspase-3 activity and apoptosis in HPASMCs exposed to hypoxia. It also resulted in the decreased expression of myeloid cell leukemia sequence 1 (MCL-1). The knockdown of MCL-1 or MCL-1 inhibitors increased caspase-3 activity and apoptosis in HPASMCs exposed to hypoxia. On the other hand, the suppression of AMPK α1 expression alone prevented hypoxia-mediated autophagy. The inhibition of autophagy induced cell death in HPASMCs. Our results suggest that AMPK α1 and AMPK α2 play differential roles in the survival of HPASMCs during hypoxia. The activation of AMPK α2 maintains the expression of MCL-1 and prevents apoptosis, whereas the activation of AMPK α1 stimulates autophagy, promoting HPASMC survival. Moreover, treatment with Compound C, which inhibits both isoforms of AMPK, prevented and partly reversed hypoxia-induced PH in mice. Taking these results together, our study suggests that AMPK plays a key role in the pathogenesis of pulmonary arterial hypertension, and AMPK may represent a novel therapeutic target for the treatment of pulmonary arterial hypertension.

In the pulmonary vasculature, the endothelial and smooth muscle cells are two key cell types that play a major role in the pathobiology of pulmonary vascular disease and pulmonary hypertension. The normal interactions between these two cell types are important for the homeostasis of the pulmonary circulation, and any aberrant interaction between them may lead to various disease states including pulmonary vascular remodeling and pulmonary hypertension. It is well recognized that the endothelial cell can regulate the function of the underlying smooth muscle cell by releasing various bioactive agents such as nitric oxide and endothelin-1. In addition to such paracrine regulation, other mechanisms exist by which there is cross-talk between these two cell types, including communication via the myoendothelial injunctions and information transfer via extracellular vesicles. Emerging evidence suggests that these nonparacrine mechanisms play an important role in the regulation of pulmonary vascular tone and the determination of cell phenotype and that they are critically involved in the pathobiology of pulmonary hypertension.Keywords: paracrine; myoendothelial injunction; microvesicles; vasoconstriction; vascular remodelingIn the pulmonary vasculature, endothelial cells (ECs) and smooth muscle cells (SMCs) are the key cell types involved in the regulation of vessel diameter in most of the pulmonary vascular tree and thereby they regulate pulmonary arterial pressure and total vascular resistance. ECs, which are strategically located as the innermost layer of the blood vessel and are in contact with the circulating blood, exert a delicate and constant influence on the underlying vascular smooth muscle cells (VSMCs). It is well recognized that the regulation of pulmonary vasoreactivity by the endothelium is predominantly accomplished by paracrine signaling through the release of various vasoactive agents such as nitric oxide (NO), endothelin-1 (ET-1) (1-3), and others (4-8). However, there is increasing evidence that the relationship between the EC and the SMC is not a simple one-way interaction from the endothelium to the SMC; rather, there are complicated interactions between them (9-11). Moreover, the communication patterns are not limited to paracrine signaling; cross-talk through myoendothelial gap junctions (MEJs), extracellular vesicles (EVs), and other mechanisms is critically involved (12)(13)(14)(15)(16)(17)(18)(19)(20). These mechanisms operate in a complicated but co-ordinated manner to maintain the homeostasis of the pulmonary circulation. Under pathological conditions, the interaction between ECs and VSMCs may be chronically altered so that a sustained increase in vasocontractility and abnormal vascular proliferation develops, which leads to high pulmonary artery pressure, vascular remodeling, right ventricular hypertrophy, and the clinical condition, pulmonary hypertension (PH) (1,(21)(22)(23)(24). This article discusses the recent progress in our knowledge of the interactions between ECs and SMCs thr...