Background: Gastric cancer (GC) has a poor response to current chemo-immunotherapy. Ferroptosis is a newly recognised form of iron-dependent cell death that can enhance drug efficacy; however, the key regulator factors and mechanisms governing ferroptosis in gastric cancer remain unclear. This study aimed to investigate the synergistic antitumor effect of oxaliplatin combined with a programmed death-1 (PD-1) inhibitor in gastric cancer and elucidate the regulatory role of six-transmembrane epithelial antigen of prostate 4 (STEAP4) in ferroptosis.

Methods: Subcutaneous mouse forestomach carcinoma (MFC) gastric cancer xenograft models were successfully developed using immunodeficient BALB/c nude mice and randomized into control, oxaliplatin, PD-1 inhibitor, and combination groups for in vivo efficacy assessment. In vitro, STEAP4 was silenced in MFC cells via siRNA to evaluate its impact on drug sensitivity. Western blotting, qRT-PCR (Quantitative Reverse Transcription Polymerase Chain Reaction), transmission electron microscopy, and biochemical assays were used to detect STEAP4 expression and ferroptosis-related markers.

Results: The combination treatment significantly suppressed tumor growth compared with monotherapies (p < 0.01). Mechanistically, it induced ferroptosis, characterized by mitochondrial shrinkage, increased Fe²⁺ and ROS (Reactive Oxygen Species) levels, upregulation of ACSL4, and downregulation of glutathione peroxidase 4 (GPX4) and SLC7A11. STEAP4 expression was markedly elevated in tumor tissues after combination therapy (p < 0.01). STEAP4 knockdown reduced the sensitivity of MFC cells to the combination and reversed ferroptosis induction by suppressing ACSL4 and restoring GPX4 and SLC7A11 expression.

Conclusion: The combination of oxaliplatin and a PD-1 inhibitor exerts potent synergistic effects in gastric cancer through STEAP4 upregulation, which promotes ferroptosis and enhances treatment sensitivity. STEAP4 may serve as a potential biomarker and therapeutic target for optimizing combination therapy in gastric cancer.