Introduction
Pulmonary hypertension (PH) is a progressive and presently incurable disorder, which complicates the majority of cardiovascular and respiratory diseases.1,2 The etiology of PH is classified by WHO into the following five distinct groups: (1) pulmonary artery hypertension (PAH), (2) PH due to left heart disease, (3) PH due to lung disease and/or hypoxia, (4) chronic thromboembolic pulmonary hypertension, and (5) unclear multifocal mechanisms.3All of these groups share a mean, resting, pulmonary arterial pressure (PAP) of greater than 20 mmHg.4 The pathogenesis of PH is complexed and multifactorial; proliferative vasculopathy, pulmonary vascular remodeling, vascular constriction, and endothelial cell dysfunction have been proven in several previous studies.5,6
As rodent models are beneficial for understanding the pathophysiology of PH and for testing experimental therapies, more and more rodent models have been developed and many of them have exhibited characteristics mimicking WHO’s classification of human PH.6 Traditionally, rat models of pulmonary artery hypertension (PAH) include monocrotaline (with or without pneumonectomy and/or hypoxia), chronic hypoxia together with SU5416 (3-(3,5-dimethyl-1H-pyrrol-2-ylmethylene)-1,3-dihydroindol-2-one; a VEGF (Vascular endothelial growth factor) receptor-2 inhibitor, and the fawn-hooded rat (FHR).7-9 A major advantage of the SU5416 hypoxia models over that of the monocrotaline hypoxia one is the remodeling of neointimal occlusive lesion, which accumulates unique tumorlets of endothelial cells and obliterates medium-sized precapillary arteries. It has also been shown that the SU5416 hypoxia model developed pulmonary arterial changes resembling plexiform-like lesions, which are characteristic features of human PAH, by increasing apoptosis of endothelial cells followed by converting them into apoptosis resistant and phenotypically altered endothelial cells.10Recently, a newly developed rodent PAH model that combined left pneumonectomy with SU5416 was reported.11 Unlike the traditional SU5416 hypoxia model in which a partial reversal of PAH is seen upon returning to normoxia, this new animal model does not reverse hypoxia-induced vasoconstriction and hemoconcentration under similar conditions. Rather, the right ventricular systolic pressure increased gradually over time, a feature that may favor the assessment of drug effects in preclinical trials.11
The Renin-angiotensin system (RAS) has been implicated in playing a causative role in PH.12,13 There are two opposing arms in RAS: the presser arm, composed of angiotensin converting enzyme (ACE), angiotensin II (Ang II) produced from Ang I by ACE, and the Ang II type 1 (AT1) receptor as the main protein mediating the biological actions of Ang II; and the vasodilator arm, consisting angiotensin converting enzyme 2 (ACE2), Ang-(1-7) generated through hydrolysis of Ang II by ACE2, and the Mas receptor as the protein conveying the vasodilatory, antiproliferative, antifibrotic, and antithrombotic effects of Ang-(1-7).12,13,14 Therefore, an increase in the levels of Ang-(1-7) should have beneficial effects for PAH. However, targeting the ACE2/Ang (1-7)/Mas receptor pathway in PAH is still under scrutiny.
ACE2 has exhibited its modulating ability in the balance between vasoconstriction and vasodilation in many experiments.15,16 Recently, diminazene aceturate (DIZE), a Food and Drug Administration (FDA) approved anti-trypanosomal drug, has been demonstrated to exert off-target effect on activating ACE2.17,18 This effect of DIZE has been confirmed through the analysis of cleavage of the vasoconstrictor peptide Ang II, the most physiologically relevant natural substrate for ACE2.17
On the basis of the benefits demonstrated by DIZE in other PH models,18-20 we aimed to evaluate its effects on a rat model of PAH induced by left pneumonectomy combined with SU5416. We reasoned that in this model there could be an up-regulated expression of ACE, leading to the imbalance between vasoconstriction and vasodilation. Treatment with DIZE would increase the levels of ACE2, and hence, the vasodilatory peptide Ang-(1-7) to prevent and ameliorate the development of PAH.