Endothelial and Smooth Muscle Cell Interactions in the Pathobiology of Pulmonary Hypertension

Abstract

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: microvesicles; myoendothelial injunction; paracrine; vascular remodeling; vasoconstriction.

Figure 1.

Altered signaling for endothelin-1 (ET-1) and nitric oxide (NO) in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) in pulmonary hypertension. ET-1 is synthesized as preproendothelin (pp-ET-1), which is cleaved to big–ET-1 by the enzyme furin convertase and then to ET-1 (134). ET-1 is released to extracellular space in response to various stimuli (63). ET-1 exerts its actions on VSMCs via endothelin A receptor (ET_A) or endothelin B receptor (ET_B), predominately the former. It may cause sustained vasoconstriction by increasing intracellular Ca²⁺ concentration resulting from inositol 1,4,5-trisphosphate–induced Ca²⁺ release from the sarcoplasmic reticulum and by sensitizing the myofilaments to Ca²⁺ through the Rho kinase (ROCK) pathway. It also promotes vascular remodeling through the activation of the mitogen-activated protein kinase (MAPK) pathway. ET-1 also causes the release of NO from ECs after binding to the ET_B receptor (134). The formation and release of ET-1 is inhibited by NO (–72). Such action is diminished in pulmonary hypertension, in part because of reduced production of NO by uncoupled endothelial nitric oxide synthase (eNOS) (34). The uncoupled eNOS generates superoxide anions, which reduce the bioavailability of NO (53). NO can counteract the vasoconstriction and vascular remodeling caused by ET-1 action through the stimulation of soluble guanylyl cyclase (sGC) and the elevation of cyclic guanosine monophosphate (cGMP). Such effects are impaired in pulmonary hypertension (56, 134). The thick and thin arrows denote enhanced and suppressed activity, respectively. The red and green arrows denote stimulatory and inhibitory effect, respectively. l-Arg, l-arginine; O^•−, superoxide anion.

Figure 2.

Possible mechanism for endothelial–smooth muscle cross-talk via the myoendothelial junctions (MEJs) in pulmonary hypertension exemplified by serotonin actions. Serotonin is synthesized primarily from l-tryptophan (l-Trp) by tryptophan hydroxylase 1 (Tph1) in the ECs (99). It may diffuse to the VSMCs through MEJs and dissociate transforming growth factor β (TGF-β) from latency-associated peptide (LAP), resulting in the binding of TGF-β to its receptor (R) activin receptor-like kinase 5 (ALK5) (92, 99), which leads to increased vascular smooth muscle remodeling through the activation of similar mothers against decapentaplegic protein (SMAD) signaling (92, 99, 135) and phosphorylated TGF–associated kinase 1 (TAK1-p)–MAPK signaling pathways (136). Serotonin may also stimulate the production of superoxide anions by nicotinamide adenine dinucleotide phosphate reduced oxidase (NOX) in VSMCs (101, 102). The increased number of superoxide anions may diffuse into ECs through MEJs and may lead to eNOS uncoupling and reduced NO bioavailability and the resultant decreased activation of sGC–cGMP signaling (91). Accordingly, the antiproliferation effect of the NO–cGMP–cGMP-dependent protein kinase pathway is diminished. The thick and thin arrows denote enhanced and suppressed activity, respectively. The red and green arrows denote stimulatory and inhibitory effects, respectively.

Figure 3.

ECs and smooth muscle cells (SMCs) release extracellular vesicles (EVs) and interact through the transfer of EVs. Increased circulating levels of endothelium-derived microparticles (MPs) have been documented in patients with pulmonary hypertension (PH) (a). Visovatti and colleagues (128) demonstrated increased CD39 expression and function in the circulating MPs of patients with idiopathic pulmonary arterial hypertension, which may be associated with the increased ATPase/ADPase activity in MPs (b). Tual-Chalot and colleagues (126) showed that circulating MPs from hypoxic rats can suppress endothelial-dependent vascular relaxation in rat aorta and pulmonary arteries by decreasing NO production (c). More recently, Aliotta and colleagues (127) reported that healthy mice injected with circulating or lung EVs isolated from monocrotaline-treated mice show elevated right ventricular–to–body weight ratio and pulmonary arterial wall thickness-to-diameter ratio, compared with that of mice injected with control EVs (d). Deng and colleagues (130) showed a high abundance of microRNA (miR)-143-3p in pulmonary arterial smooth muscle cell (PASMC)-derived exosomes and a paracrine promigratory and proangiogenic effect of these miR-143-3p–enriched PASMC-derived exosomes on pulmonary arterial endothelial cell (e). However, the cross-talk between ECs and SMCs through EV transfer, especially from ECs to SMCs, and the underlying molecular mechanisms remain unclear (f).

Figure 4.

Cross-talk between pulmonary arterial endothelial cells and pulmonary arterial smooth muscle cells through paracrine effect (A), MEJ (B), and EV transfer (C).