Acidic Tumor Microenvironment Promotes 5-Fluorouracil Resistance Via Lactate-Driven HIF-1α/MCT4 Activation and Epigenetic Remodeling in Gastric Cancer.

INTRODUCTION

Tumor acidosis is associated with poor therapeutic response. Yet, it remains unclear whether 5-fluorouracil (5-FU) resistance in gastric cancer is driven by low extracellular pH per se or by lactate as a signaling metabolite.

METHODS

The human gastric cancer cell line AGS was exposed to extracellular acidosis (pH 6.6; short-term or chronic) or exogenous lactate at neutral pH. 5-FU sensitivity, apoptosis, and lactate production were assessed. Extracellular pH was buffered with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES); lactate metabolism/ export was disrupted by LDHA inhibition (FX11) or siRNA-mediated MCT4 knockdown. Global histone methylation was analyzed by Western blot, and qPCR quantified H3K4me3 and H3K27me3 occupancy at SLC16A3 (MCT4) and LDHA promoters.

RESULTS

Acidosis reduced 5-FU sensitivity, increased lactate output, suppressed apoptosis, and activated hypoxia- inducible factor 1 alpha (HIF-1α)/monocarboxylate transporter 4 (MCT4) signaling. Buffering pH or inhibiting lactate synthesis/export restored 5-FU sensitivity, indicating a reversible, metabolism-dependent phenotype. Under neutral pH, exogenous lactate alone induced reduced 5-FU sensitivity and activated HIF-1α signaling, supporting a pH-independent role for lactate. Acid exposure increased global H3K27me3 and H3K9me3, which normalized after buffering, whereas SLC16A3 and LDHA promoters showed H3K4me3 enrichment with reduced H3K27me3.

DISCUSSION: These findings suggest that acidic stress is transduced through lactate signaling to selectively activate chromatin at lactate-metabolism genes, stabilizing an adaptive survival program under chemotherapy.

CONCLUSION

Lactate signaling, rather than low pH alone, underlies a reversible 5-FU-resistant state in gastric cancer cells and may be mitigated by targeting lactate metabolism and/or extracellular pH control.