Insights into the Function of a Conserved Cys120 in Human Neuroglobin in Oxidative Stress Regulation of Breast Cancer Cells

Human neuroglobin (Ngb) is a globin featuring a disulfide bond (Cys46–Cys55) and a redox-active cysteine residue (Cys120) and plays a dual role in cellular stress responses. In this study, we investigated how wild-type (WT) Ngb and its two mutants, C120S Ngb, in which Cys120 is replaced by serine, and A15C Ngb, which contains an engineered Cys15–Cys120 disulfide bridge, modulate oxidative stress in triple-negative breast cancer (MDAMB231) and hormone receptor-positive breast cancer (MCF-7) cells. In both cell lines, WT Ngb enhanced cell survival under H2O2-induced oxidative stress by scavenging reactive oxygen species (ROS) through oxidation of Cys120. In contrast, the C120S and A15C mutants lost this protective capacity and instead promoted apoptosis. Mass spectrometry analysis confirmed the oxidation of Cys120 to sulfenic acid in WT Ngb, whereas both mutants exhibited impaired redox activity, leading to elevated ROS levels, lipid peroxidation, and activation of caspase-9/3. AO/EB staining further revealed that WT Ngb attenuated DNA damage, while the mutants exacerbated apoptosis in both MDAMB231 and MCF-7 cells. These results demonstrate that Cys120 acts as a critical redox switch, dictating whether Ngb exerts cytoprotective or pro-apoptotic effects across different breast cancer cell types. Our findings suggest that WT Ngb may help protect normal tissues during cancer therapy, whereas engineered Ngb mutants could be used to selectively sensitize both triple-negative and hormone receptor-positive breast cancer cells to oxidative damage, offering a novel redox-targeted therapeutic strategy.