GPX4 Inhibition Enhances the Pro-Oxidant and ER Stress Effects of Tempol in Colon and Gastric Cancer Cell Lines

Tempol, a synthetic nitroxide, exhibits dual antioxidant and pro-oxidant activity, requiring millimolar concentrations to induce oxidative stress, which limits its therapeutic use. Glutathione Peroxidase 4 (GPX4) is a critical lipid peroxidase that prevents ferroptosis, and its inhibition has emerged as a strategy to sensitize cancer cells to oxidative stress. To enhance Tempol’s efficacy, we investigated its interaction with ML210, a GPX4 inhibitor, in human colon (HT29) and gastric (CRL-1739) cancer cell lines. We quantified cell viability, oxidative stress markers (H₂O₂, Total Oxidant Status (TOS), and Total Antioxidant Status (TAS)) and endoplasmic reticulum (ER) stress proteins (ATF6, GRP78, and IRE1α) in in vitro assays. Synergy was assessed using Bliss independence analysis. The combination of Tempol (2 mM) and ML210 (0.05 μM) markedly reduced viability in both cell lines. Bliss analysis revealed slight/moderate synergy for cytotoxicity (Δ = +0.15 in HT29; Δ = +0.26 in CRL-1739) and strong synergy for H₂O₂ accumulation (Δ = +1.92–2.23 across replicates). In contrast, TOS showed moderate-to-strong antagonism across both cell lines, and TAS demonstrated slight synergistic or antagonistic effects. ER stress markers exhibited marker and cell line specific synergy: ATF6 showed strong synergy, IRE1α slight synergy in both lines, and GRP78 activation was highly variable, showing strong synergy in CRL-1739 cells but moderate antagonism in HT29 cells. These findings indicate that the cooperative action of Tempol and ML210 is ROS-pool–specific and pathway-selective in the ER. These findings demonstrate that ML210 potentiates Tempol’s pro-oxidant pressure by targeting GPX4, selectively amplifying H₂O₂ accumulation and ER stress engagement without collapsing global redox balance. This study provides mechanistic rationale for redox–proteostasis co-targeting in gastric and colon cancers and establishes a foundation for in vivo validation.