Exploring the Endothelin-1 pathway in chronic thromboembolic pulmonary hypertension microvasculopathy

Targeted vasopeptide therapies have significantly advanced the management of pulmonary arterial hypertension (PAH). However, due to insufficient preclinical evidence regarding the involvement of the endothelin-1 (ET-1) pathway in chronic thromboembolic pulmonary hypertension (CTEPH) pathophysiology, the potential of ET-1 receptor antagonism in treating CTEPH remains uncertain. In this study, we investigated the role of the ET-1 pathway in CTEPH microvasculopathy using a multifaceted approach. Plasma ET-1 levels were measured in a cohort of 59 CTEPH patients and 41 healthy controls. Additionally, we evaluated the expression of key ET-1 pathway members in pulmonary explants from CTEPH, idiopathic PAH, and control patients. We used an in vitro system to test the hypothesis that the turbulent flow, observed near the vascular obstructions pathognomonic of CTEPH, enhances ET-1 expression. Our findings were further validated in vivo using a CTEPH piglet model that contains distinct regions representing pre- and post-thrombus lung territories. We found a twofold increase in circulating ET-1 levels in CTEPH patients compared to healthy subjects. Pulmonary explants from CTEPH patients exhibited pronounced overexpression of ET-1, endothelin receptor A (ET_A), and phosphorylated myosin light chain (p-MLC) in muscularized pulmonary microvessels, suggesting heightened vascular contraction. In vitro experiments showed that turbulent flow facilitates ET-1 secretion compared to laminar flow regions. Additionally, in the CTEPH piglet model, elevated plasma ET-1 levels were observed compared to controls. Immunofluorescence and confocal microscopy analyses confirmed increased ETA and p-MLC in remodeled arteries from both pulmonary territories. However, ET-1 protein elevation was exclusively observed in the obstructed territory. These findings collectively indicate impaired vascular tone in microvessels of CTEPH patients and the CTEPH piglet model. Furthermore, our data implicates the ET-1 pathway in microvasculopathy, with turbulent flow playing a pathological role. These insights underscore the potential utility of ET-1 receptor antagonists as a promising therapeutic approach for CTEPH treatment.