Targeting HECT Ubiquitin Ligases to Treat Pulmonary Arterial Hypertension

Background: Pulmonary arterial hypertension (PAH) is a progressive disease characterised by obliterative vascular remodelling, leading to increased pulmonary vascular resistance and heart failure. Despite current vasodilator therapies, mortality remains high because treatments fail to address the fundamental molecular driver of the disease. Genetic and molecular evidence show that a deficiency in Bone Morphogenetic Protein (BMP) pathway signalling, mediated by the loss of the type II receptor (BMPR2), causes endothelial dysfunction and smooth muscle cell hyperproliferation—the central molecular mechanism in PAH. This thesis investigated SMAD ubiquitin regulatory factor 1 (SMURF1) as a critical negative regulator of this pathway and its potential therapeutic target.
Methods: This study combined human tissue, primary cell models, and patient-specific blood outgrowth endothelial cells (BOECs). SMURF1 expression was characterised in explanted lung tissue and primary pulmonary artery smooth muscle cells (PASMCs) using immunohistochemistry and Western blotting. The mechanism of action of a novel allosteric SMURF1 inhibitor was validated in HEK-293 cells. Apoptosis and proliferation phenotypes were assessed in patient-derived PASMCs and pulmonary artery endothelial cells (PAECs) using Western blotting and real-time functional assays. Finally, BOECs served as a non-invasive model to validate the inhibitor's effect on BMP transcriptional activity via qPCR.
Result: SMURF1 was significantly upregulated in patients with remodelled vascular PAH. In the mammalian cell model, the allosteric inhibitor prevented BMPR2 and SMAD1 ubiquitin-mediated degradation via a hinge-locking mechanism. In patient-derived PASMCs, the inhibitor restored phospho-SMAD1/5/8 and ID1 expression, reversing PAH-PASMC hypertrophy and protecting PAECs from apoptosis. In patient BOECs, the SMURF1 inhibitor successfully restored BMP transcriptional activity (BMPR2, SMAD1, ID1).
Conclusion: These findings confirm that SMURF1 is a pathogenic driver in PAH and demonstrate that its inhibition restores vascular haemostasis, offering a distinct strategy to reverse vascular remodelling and improve outcomes in PAH.