Coagulation-independent effects of thrombin and Factor Xa: role of protease-activated receptors in pulmonary hypertension

Aims: Pulmonary arterial hypertension (PAH) is a devastating disease with limited therapeutic options. Vascular remodelling of pulmonary arteries, characterized by increased proliferation and migration of pulmonary arterial smooth muscle cells (PASMCs), is a hallmark of PAH. Here, we aimed to systematically characterize coagulation-independent effects of key coagulation proteases thrombin and Factor Xa (FXa) and their designated receptors, protease-activated receptor (PAR)-1 and -2, on PASMCs in vitro and experimental PAH in vivo.

Methods and results: In human and murine PASMCs, both thrombin and FXa were identified as potent mitogens, and chemoattractants. FXa mediated its responses via PAR-1 and PAR-2, whereas thrombin signalled through PAR-1. Extracellular-signal regulated kinases 1/2, protein kinase B (AKT), and sphingosine kinase 1 were identified as downstream mediators of PAR-1 and PAR-2. Inhibition of FXa or thrombin blunted cellular responses in vitro, but unexpectedly failed to protect against hypoxia-induced PAH in vivo. However, pharmacological inhibition as well as genetic deficiency of both PAR-1 and PAR-2 significantly reduced vascular muscularization of small pulmonary arteries, diminished right ventricular systolic pressure, and right ventricular hypertrophy upon chronic hypoxia compared to wild-type controls.

Conclusion: Our findings indicate a coagulation-independent pathogenic potential of thrombin and FXa for pulmonary vascular remodelling via acting through PAR-1 and PAR-2, respectively. While inhibition of single coagulation proteases was ineffective in preventing experimental PAH, our results propose a crucial role for PAR-1 and PAR-2 in its pathobiology, thus identifying PARs but not their dedicated activators FXa and thrombin as suitable targets for the treatment of PAH.