The TCF3-FBXL18 axis regulates KRAS stability to determine cetuximab response in colorectal cancer

Background Resistance to anti-EGFR therapy represents a major clinical challenge in metastatic colorectal cancer. Despite the use of RAS mutation status to guide cetuximab administration, a substantial proportion of RAS wild-type tumors develop primary or acquired resistance, underscoring the need to elucidate alternative mechanisms governing EGFR signaling. While protein ubiquitination critically regulates oncogenic protein stability, the specific E3 ubiquitin ligases mediating KRAS turnover in cetuximab resistance remain poorly defined. Objective This study aimed to identify novel molecular determinants of cetuximab resistance and elucidate the underlying transcriptional and post-translational regulatory mechanisms controlling KRAS protein stability in colorectal cancer. Methods We examined FBXL18 expression in cetuximab-sensitive and -resistant clinical specimens and established resistant cell models (HCT-8/CET-R and Caco-2/CET-R). Functional assays including CCK-8, colony formation, EDU incorporation, and flow cytometry were performed to evaluate the biological role of FBXL18. Ubiquitination assays, cycloheximide chase experiments, and co-immunoprecipitation were conducted to investigate KRAS degradation mechanisms. Bioinformatics analysis and dual-luciferase reporter assays were employed to identify upstream transcriptional regulators. In vivo xenograft experiments validated the therapeutic effects of FBXL18 restoration. Results FBXL18 was significantly downregulated in cetuximab-resistant CRC tissues and cell lines at both transcriptional and translational levels. Functionally, FBXL18 suppressed cell proliferation, promoted apoptosis, and enhanced cetuximab sensitivity. Mechanistically, FBXL18 directly interacted with KRAS and catalyzed its ubiquitin-mediated proteasomal degradation, thereby reducing KRAS protein stability and attenuating oncogenic signaling. Furthermore, integrated bioinformatics analysis identified transcription factor 3 (TCF3) as a critical upstream regulator that directly binds to the FBXL18 promoter and transcriptionally activates its expression. Disruption of the TCF3–FBXL18 axis led to KRAS accumulation and cetuximab resistance. Conclusion Our findings establish the TCF3–FBXL18–KRAS axis as a novel transcriptional–posttranslational regulatory network governing cetuximab response in colorectal cancer. These results provide a potential predictive biomarker and therapeutic target for overcoming anti-EGFR resistance.