Rationale
Pulmonary arterial hypertension (PAH) is a disease of the pulmonary vascular system, characterized by elevated mean pulmonary arterial pressure (mPAP), increased pulmonary vascular resistance (PVR), and right ventricular (RV) hypertrophy and dysfunction. The pathogenesis involves enhanced proliferation and reduced apoptosis of vascular cells, as well as inflammatory processes. Recent findings indicate a central role of the class IB phosphatidylinositol 3-kinase (PI3K) isoform PI3Kg, which is activated via G protein–coupled receptors. PI3Kg influences leukocyte recruitment, angiogenesis, nitric oxide (NO) production via endothelial NO synthase (eNOS) and regulates cardiac contractility. The aim of this study was to investigate the role of PI3Kg in pulmonary vessels and in the heart in the context of PAH.
Methods
The relevance of PI3Kg for the pathogenesis of PAH was investigated in vivo using a hypoxia-induced mouse model (21 days at 10% O₂; hypoxia, HOX vs. normoxia, NOX) in PI3Kg knockout mice (PI3Kg-/-). Following echocardiographic characterization, right ventricular systolic pressure (RVSP) was measured using a Millar catheter inserted into the jugular vein. RV hypertrophy was determined as the ratio of RV to left ventricle plus septum weight. The RV proteome was analyzed by mass spectrometry. Effects on the vascular tonus of pre-capillary resistance vessels was analyzed using functional lung slices from PI3Kg-/- mice.
Results
After three weeks of hypoxia, PI3Kg inactivation in mice led to a significantly increased RVSP compared to wild-type (WT) controls (PI3Kg+/+ HOX: 34.2 ± 3.5 mmHg, PI3Kg-/- HOX: 37.0 ± 2.4 mmHg, p = 0.005). Even under normoxic conditions an elevated RVSP was observed (PI3Kg+/+ NOX: 26.1 ± 1.3 mmHg, PI3Kg-/- NOX: 28.1 ± 1.0 mmHg, p = 0.016). RV hypertrophy was evident only when comparing NOX to HOX but it was not further increased in PI3Kg-/- mice (PI3Kg+/+ NOX: 0.26 ± 0.03; PI3Kg+/+ HOX: 0.33 ± 0.04, p = 0.0005; PI3Kg-/- NOX: 0.26 ± 0.01; PI3Kg-/- HOX: 0.33 ± 0.06, p = 0.0001). Additionally, the ratio of tricuspid annular plane systolic excursion (TAPSE) to pulmonary pressure (TAPSE/RVSP) was analyzed as a parameter of RV–PA coupling. A trend toward impaired coupling was observed in PI3Kg-/- mice after hypoxia (PI3Kg+/+ HOX: 0.019 ± 0.004; PI3Kg-/- HOX: 0.016 ± 0.002, p = 0.065). Proteomic analysis showed a 2.3-fold reduction in atrial natriuretic peptide (ANP) expression and an 8-fold reduction in CDK5 regulatory subunit–associated protein (CDK5RAP3) expression in hypoxic PI3Kg-/- mice compared to hypoxic PI3Kg+/+ animals. Functional lung slices from PI3Kg-/- mice showed impaired vasorelaxation in response to isoproterenol after maximal contraction (PI3Kg+/+: 38.7 ± 14.3%; PI3Kg-/-: 21.4 ± 6.2%; p = 0.005).
Conclusion
The findings demonstrate that inactivation of PI3Kg in vivo does not attenuate but rather aggravates the pathogenesis of PAH. The elevated RVSP may result from impaired relaxation of pulmonary vessels, while the right ventricle appears to be protected from excessive hypertrophy and possibly from right heart failure. Ongoing proteomic analyses aim to elucidate the underlying molecular mechanisms.
