NOX4 PROTECTIVE EFFECT IN ENDOTHELIAL FUNCTION INVOLVES ACTIVATION OF TRANSIENT RECEPTOR POTENTIAL MELASTATIN 2 (TRPM2) CATION CHANNEL

Abstract

Objective: Nox4 regulates endothelium-dependent relaxation through production of H2O2, which in turn acts as an EDHF. We demonstrated that TRPM2 is also a target of H2O2 in vascular cells. TRPM2 is a redox-sensitive Ca2+ channel and once activated leads to Ca2 influx. Hypertension-associated endothelial dysfunction involves reduction in eNOS activation and is potentiated in the absence of NOX4. As TRPM2 channel is a H2O2 sensor and regulates Ca2+ entry, we questioned whether impaired vascular relaxation in hypertensive NOX4 KO mice involves the H2O2-TRPM2-Ca2+ axis in endothelial cells. Design and method: WT and TTRhRen hypertensive mice were crossed with Nox4 KO mice. Vascular function was measured by wire myograph in resistance arteries. Ca2+ influx was assessed by fluorescence microscopy in aortic endothelial cells. eNOS activation was assessed by immunoblotting. Results: Blood pressure was significantly increased in TTRhRen (130.3 ± 7.0 mmHg) and TTRhRen/NOX4 KO mice (141.3 ± 18 mmHg) versus control mice (98.1 ± 8.0 mmHg). TTRhRen mice had reduced endothelium-dependent relaxation (Emax: WT 83.5 ± 4.03 vs TTRhRen 59.1 ± 3.5); an effect worsened by NOX4 KO (37.9 ± 5.4), p<0.05. The TRPM2 activator, ADPR, improved vascular relaxation in TTRhRen/NOX4 KO mice (75.9 ± 7.7); an effect recapitulated by H2O2 (74.2 ± 15.4), p < 0.05. Ang II-induced Ca2+ influx in endothelial cells, important in eNOS activation, was reduced by TRPM2 inhibitors (AUC - Ca2+: 8-br-cADPR 76161 ± 1052.0; Olaparib 74763 ± 843.2 and 2-APB 79625 ± 277.4, p < 0.05) and the NOX1/4 inhibitor GKT137831 (AUC - Ca2+: Ang II 107357 ± 1940.2 vs GKT 60270 ± 255.6, p < 0.05). Ang II-induced activation of eNOS (Ser1177) (%: 72.3 ± 24.7) was blocked by GKT137831, PEG-catalase and the TRPM2 inhibitor 8-br-cADPR (p < 0.05). Conclusions: Endothelial dysfunction in TTRhRen mice involves impaired TRPM2 activation. Inhibition/downregulation of Nox4 and reduced bioavailability of H2O2 exacerbates these processes. We identify a novel protective effect of NOX4 in endothelial cells involving H2O2-mediated activation of TRPM2-Ca2+-eNOS signalling.