Na+/H+ Exchanger Isoform 1 Regulates Apoptosis Susceptibility in Pulmonary Arterial Smooth Muscle from the Sugen/Hypoxia model of Pulmonary Hypertension

Abstract

Pulmonary hypertension (PH) is characterized by vascular remodeling driven in part by apoptosis-resistant pulmonary arterial smooth muscle cells (PASMCs). Na/H exchanger isoform 1 (NHE1) regulates intracellular pH and plasma membrane cytoskeleton anchoring, influencing PASMC migration and proliferation, but the role of NHE1 in apoptosis remains unclear. NHE activity and NHE1 surface expression were increased in PASMCs from the Sugen/Hypoxia (SuHx) rat model compared to controls. Despite increased endoplasmic reticulum (ER) stress at baseline, SuHx PASMCs were resistant to apoptosis following H2O2 challenge. Pharmacological inhibition of NHE activity with ethyl-isopropyl amiloride (EIPA) and silencing with siRNA restored apoptosis in SuHx PASMCs. Conversely, NHE1 overexpression in control PASMCs conferred apoptosis resistance. Expression of mutant NHE1 constructs lacking ion translocation or binding to the adaptor protein, ezrin, also reduced H2O2-induced apoptosis. Mechanistically, apoptotic stimulation with H2O2 increased p38 phosphorylation in PASMCs from control, but not SuHx, rats, indicating impaired activation of this pro-apoptotic pathway. NHE1 suppression via EIPA or siRNA restored p38 phosphorylation in SuHx PASMCs, while overexpression of NHE1 (wild-type or mutants) suppressed p38 activation following apoptotic stimulation. Inhibition of p38 with SB203580 prevented the pro-apoptotic effect of EIPA, validating a role for p38 signaling in NHE1-mediated apoptosis resistance in SuHx PASMCs. These findings identify NHE1 as necessary and sufficient for PASMC apoptosis resistance in PH, by a mechanism independent of ion transport or ezrin-binding functions but involving suppression of p38 phosphorylation. Targeting NHE1-dependent pathways may restore PASMC apoptosis and offer a novel therapeutic strategy to reverse pulmonary vascular remodeling in PH.