Effects of a modulation of the urokinase-type plasminogen activator (u-PA) system in chronic hypoxia-induced pulmonary vascular remodeling and right ventricular hypertrophy (RVH) in mice

Abstract

Pulmonary hypertension, a devastating disease of complex and multifactorial pathogenesis, is characterized by sustained elevation in pulmonary artery pressure, pulmonary vascular remodeling and subsequent progressive right heart hypertrophy. The structure of the pulmonary vascular bed severely altered. A marked elevation in plasma levels of plasminogen activator inhibitor (PAI)-1 has consistently been reported in patients with severe primary pulmonary hypertension, and accordingly, an induction of PAI-1 and diminution of plasminogen activator (PA) have been described in hypoxia-exposed murine lungs. Hence, as also suggested by the clinical efficacy of warfarin treatment in PAH, alterations of the hemostatic balance towards predominance of procoagulant and antifibrinolytic activity in the pulmonary vascular compartment might potentially play an important role in the pathogenesis of pulmonary hypertension. In the present study, we aimed to investigate the effects of a modulation of u-PA system in pulmonary vascular remodeling. We firstly analyzed u-PA and PAI-1 expression, and u-PA activity in lung homogenates from patients with different forms of pulmonary hypertension (IPAH, CTEPH) as well as from mice exposed to chronic hypoxia. Secondly, we investigated the potential role of u-PA in pulmonary vascular remodeling in a mouse model of hypoxia-induced pulmonary hypertension by employing wild type, u-PA and PAI-1 knock out (KO), specific u-PA inhibitor (CJ463)-treated and continuously u-PA-infused mice. Overall, the expression of u-PA and PAI-1 at transcript and protein level in the lungs from chronic hypoxia-exposed mice showed some similarities to that in the lungs from patients with IPAH and CTEPH. u-PA was induced at protein level in the lung homogenates from the patients as compared to donor lungs. However, the u-PA activity was either increased (CTEPH) or unchanged (IPAH). In line with the difference in u-PA activity, a different spatial distribution of u-PA and PAI-1 was observed in the lungs from patients with IPAH and CTEPH. On the other hand, reduced u-PA activity was observed in the lungs from chronically hypoxic mice, suggesting a differential regulation of u-PA activity in the lungs from hypoxic mice and patients with pulmonary hypertension. By employing u-PA ko and specific u-PA inhibitor treated mice, we demonstrated that neither the inhibition of u-PA activity nor the absence of u-PA exerted a major effect on the course of chronic hypoxia-induced pulmonary vascular remodeling and RVH. Moreover, pulmonary vascular remodeling and subsequent RVH was not impaired also in PAI-1 ko mice exposed to chronic hypoxia. To further clarify the potential role of u-PA, we applied exogenous u-PA infusion into mice exposed to chronic hypoxia. Somewhat contradictory, we could observe a beneficial role of a permanent u-PA infusion on pulmonary vascular remodeling in chronically hypoxic mice. Our results suggest that endogenous regulation of u-PA and PAI-1 does not alter the course of pulmonary vascular remodeling induced by chronic hypoxia. This could probably be attributable to existence of redundant factors and regulation of u-PA functions at multiple levels in vivo. However, exogenous application of u-PA did attenuate pulmonary vascular remodeling, probably by yielding a high endoluminal to vascular wall u-PA gradient and thus excessive proteolytic activities. Further studies to precisely delineate the underlying mechanism are warranted. Our findings may have an important implication for future investigation of plasminogen activation system based therapeutic strategies with regard to pulmonary hypertension.