089 Mice Are Resistant to the Development of Pulmonary Arterial Hypertension After SU5416-Induced Endothelial Apoptosis and Chronic Hypoxia Despite Altering Background and Dose

Abstract

Pulmonary arterial hypertension (PAH) is believed to result from pulmonary endothelial cell (EC) injury and apoptosis followed by reactive vascular cell proliferation leading to obliteration of distal lung arterioles. SU5416 (SU) inhibits VEGFR2 causing EC apoptosis and, with chronic hypoxia (CH), severe PAH in rats. Moreover, athymic (nude) rats, lacking mature T-cells, exhibit severe PAH in response to SU5416, even in the absence of CH. Therefore, the aim of this study was to assess the susceptibility of wild type (WT) and athymic mice to SU and CH-induced severe PAH. Wild type C57Bl6/J mice were exposed to normoxia (N) or chronic hypoxia (CH) (9-10% O2), combined with a single injection of 20mg/kg SU5416 (SU), or multiple injections (3 or 6). Athymic nude mice (B6.Cg-Foxn1nu/J on C57Bl6/J background) were exposed to three weeks of CH combined with 3 SU injections (20mg/kg). To confirm the activity of the SU compound, Sprague-Dawley rats were treated with a single dose of SU and 3 weeks of CH. Hemodynamic assessments and lung tissue collection were performed at three weeks. In WT mice, CH induced a significant increase in right ventricle systolic pressure (RVSP) compared with N (38.2±2.3 vs. 25.4±1.2 mmHg, respectively; p<0.0001 n=8-9/group) and greater RV remodeling (RV/LV+S weight ratio 0.36±0.01 vs. 0.26±0.01, respectively; p<0.001 n=9/group). However, the administration of SU+CH failed to increase the PAH phenotype regardless of dose (1, 3 or 6 SU injections), assessed either by RVSP (43.1±1.2, 40.1±2.5, and 32.7±1.2 mmHg, respectively; n=5-9/group) or RV hypertrophy (0.4±0.01, 0.39±0.02, 0.45±0.03, respectively; n=5-10/group). Moreover, compared to CH alone, treatment of athymic mice with CH+SU did not increase RVSP (31.1±2 CH+SU vs. 29.9±1.0 CH, n=3) or RV remodeling (0.39±0.01 CH+SU vs. 0.42±0.01 CH, n=3). In contrast, in rats, SU+CH resulted in marked exaggeration of PAH compared with CH alone: RVSP (80.0±8.2 vs. 40±2mmHg, respectively); RV hypertrophy (0.56±0.34 and 0.33±0.1, respectively, all p< 0.001). Finally, despite the demonstration of EC apoptosis, there was no evidence of proliferative, intimal lesions in lungs of SU-CH treated WT or nude mice; whereas, typical obliterative, plexiform lesions were seen in the rat model. We now show for the first time that unlike rats, mice are highly resistant to PAH induced by EC apoptosis using SU5416, even in a T-cell deficient background and with up to 6 doses of SU, suggesting a fundamental difference in the response to vascular injury between these two species.