Interestingly, apelin (APLN) and apelin cleaving enzyme lysosomal pro-X carboxypeptidase (PRCP) were elevated in PAH samples [74], contradicting the previously reported reduction in APLN and PRCP expression in pulmonary artery ECs isolated from PAH patients in vitro and the regression of PAH in mice after APLN administration [75,76]. Here, PRCP is linked to pulmonary arterial hypertension.