Carnosic acid alleviates 5-Fluorouracil chemoresistance and enhances CD8+ T cell cytotoxicity in gastric cancer by regulating the CXCL12/CXCR7 axis.

BACKGROUND: Key features of gastric cancer (GC) include its aggressive nature, high metastatic capability, and a tendency for relapse. While 5-fluorouracil (5-FU) remains a cornerstone of GC treatment, its efficacy is substantially undermined by the emergence of acquired drug resistance. Carnosic acid (CA) has antitumor and immunomodulatory potential, but its role in 5-FU-resistant GC remains unclear. This study investigated whether CA enhances 5-FU sensitivity and CD8+ T cell-mediated antitumor immunity by regulating the CXCL12/CXCR7 axis. METHODS: The effects of CA combined with 5-FU on cell viability, proliferation, apoptosis, migration, invasion, and EMT-related features were evaluated using 5-FU-sensitive and 5-FU-resistant GC cell models and MFC/MFC-R mouse GC models. CXCR7 overexpression, co-immunoprecipitation, immunofluorescence, and Western blotting were performed to examine CXCL12/CXCR7-axis regulation. GC cell/CD8+ T cell co-culture, subcutaneous tumor, lung metastasis, anti-PD-L1 combination therapy, and safety models were further used for validation. RESULTS: 5-FU-resistant GC cells showed reduced drug sensitivity and enhanced malignant phenotypes. CA plus 5-FU suppressed proliferation, migration, invasion, and EMT, while promoting apoptosis. Mechanistically, CA restored 5-FU sensitivity by inhibiting aberrant CXCL12/CXCR7-axis activation, whereas CXCR7 overexpression partially reversed these effects. The combined treatment also enhanced CD8+ T cell proliferation, cytotoxicity, and IFN-γ and Granzyme B expression, reduced immunosuppression-associated molecules, inhibited tumor growth and lung metastasis, potentiated anti-PD-L1 efficacy, and caused no obvious systemic toxicity. CONCLUSION: CA enhances 5-FU sensitivity, suppresses EMT and metastasis-associated phenotypes, and improves CD8+ T cell-mediated antitumor immunity in resistant GC by targeting the CXCL12/CXCR7 axis.