Abstract 147: Monocyte-derived Extracellular Delta-like 4 Contributes To Pulmonary Hypertension Progression By Inhibiting Endothelial Notch1 Activation

Abstract

Background: We published that reduced Notch1 cleavage in lung endothelial cells (ECs) contributes to pulmonary hypertension (PH) development. However, the underlying mechanism of Notch1 inactivation remains elusive. We recently found enhanced circulating extracellular Delta-like ligand 4 (exDll4) from monocytes in HIV patients, the population with persistent systemic inflammation. Mobilized Notch ligands have demonstrated the ability to inhibit Notch signaling in both in vitro and in vivo studies. Hence, we hypothesize that inflammation increases monocyte-derived exDll4 and diminishes Notch1 cleavage in lung ECs, subsequently promoting the progression of PH. Methods and Results: Tumor necrosis factor-alpha (TNFα) is a major cytokine increase in the plasma of PAH patients and mouse models. Our in vitro studies revealed that TNFα induced a 17.1-fold and 23.6-fold increase of Dll4 mRNA and protein in human monocytes and a 7.2-fold elevation in exDll4 secretion. In PH mouse models induced by chronic hypoxia, Dll4 protein expression in circulating monocytes increased by 4.2 folds, and plasma exDll4 levels rose by 2.6 folds. In PAH patients, serum exDll4 was elevated by 3 folds compared to non-PAH patients. Mimicking enhanced exDll4 in PH patients; we found that exDll4 injection exacerbated PH progression in hypoxia-treated mice, accompanied by inhibited EC-Notch1 cleavage, leaky EC cell-cell junctions, and increased immune cell infiltration in the lungs. Notably, specific knock-out monocyte Dll4 in mice (mDll4 KO mice) significantly attenuated PH progression, indicated by reduced right ventricular systolic pressure and right ventricular remodeling. Mechanistically, exDll4 treatment significantly inhibited Notch1 activation and Notch target gene expression in human pulmonary microvascular ECs (HPMECs) and led to cell apoptosis, cell arrest, impaired barrier function, and enhanced inflammation. Conclusion: Our study demonstrates the role of monocyte-derived exDll4 in PH progression by inducing EC arrest and apoptosis and impairing barrier function in lung ECs through inhibition of Notch1 signaling. These findings imply that monocyte-derived Dll4 could be a potential therapeutic target and biomarker for PH.