Background: Chronic hypoxia (CH) induces pulmonary hypertension (PH), a progressive disease of the pulmonary arterioles characterized by increased pulmonary arterial pressure and right ventricular failure (RVF). In the clinical setting, the disease is often detected at later stages characterized by full-blown RVF. Although advancements in the pathogenesis and treatment of RVF have been done, it remains a severe and devastating disease with a poor prognosis and a high mortality rate, justifying the need to identify novel therapeutic targets. Autophagy, a homeostatic process of intracellular catabolism involved in response to a wide variety of stresses, is a double-edged sword in cardiovascular disease, being either beneficial or detrimental depending on the context. However, the role of autophagy in RVF occurring in CH has not been investigated so far. Here, we propose to evaluate the role of autophagy in RVF in response to CH.
Methods: Adult male Sprague-Dawley rats were exposed 2 weeks to CH (10% O2, n=10) or normoxia (N, 21%O2, n=10). For estimation of PH, Doppler imaging of pulmonary artery (PA) was obtained. The PA acceleration time (PAAT) was measured as the time from the onset of systolic flow to peak pulmonary outflow velocity, while the RV ejection time (ET) was measured as the time from the onset to completion of systolic pulmonary flow. PVR was estimated using the formula: [1/(PAAT/ET)]. The RV pressure was obtained by catheterization. Cell proliferation in RV and LV frozen sections was detected by BrdU immunofluorescence. The levels of the autophagy markers, LC3-II (a marker for autophagosome) and SQSTM1/p62 (selectively degraded by autophagy), were measured by Western blot analyses.
Results: Doppler echocardiography analysis revealed a significant reduction of PAAT/ET ratio (0.15±0.01 vs 0.35±0.01) in CH compared to N rats, indicating the development of PH. As expected, CH enhanced RV systolic pressure and hypertrophy (expressed as RV/LV+Septum ratio) and determined a 1.5-fold increase in medial wall thickness of pulmonary arterioles. A marked enhancement of cell proliferation in RV tissue exposed to CH was found in CH in respect to N rats (25.9±4.2% vs 7.8±4.2%). Notably, the BrdU incorporation did not change in LV tissue. More interestingly, in our rat model of PH, CH caused a significant increase in LC3-II and SQSTM1/p62 expression selectively in RV tissue. These results were also confirmed by electron microscopy, showing increased accumulation of autophagosome-like structures in RV and not in LV in CH conditions.
Conclusion: Our data demonstrate, for the first time that PH, when caused by prolonged and continuous exposure to CH, activates distinct response mechanisms in RV and LV. Specifically, the impairment of autophagy is a RV-specific event that occurs in CH condition. This observation may be useful to develop RV-specific therapeutic strategies to treat RVF in pulmonary hypertension patients.