Dihydropyrimidine dehydrogenase overexpression correlates with potential resistance to 5-fluorouracil-based treatment in head and neck squamous cell carcinoma

Background: Although dihydropyrimidine dehydrogenase (DPD) expression has been reported to correlate with 5-fluorouracil (5-FU) resistance in several cancers, the relationship between DPD expression and chemotherapeutic resistance to 5-FU remains unclear in head and neck squamous cell carcinoma (HNSCC). The purpose of this study was to determine the impact of DPD expression on 5-FU sensitivity and survival in HNSCC patients receiving 5-FU-based treatment. Methods: To study the correlation between DPD expression and clinical outcome of HNSCC patients who had undergone concurrent chemoradiotherapy with 5-FU-based regimens, we first examined DPD mRNA expression of these patient samples. We next developed a 5-FU-resistant HNSCC cell line (HSC-3R) to clarify the association between DPD expression and 5-FU resistance. Clonogenic survival assay was performed to determine the sensitivity of HSC-3 cells and HSC-3R cells to 5-FU. DPD expression levels in parental HSC-3 and HSC-3R cell lines were then examined by Western blotting and real-time quantitative polymerase chain reaction analysis. Lastly, we performed WST-8 assay to examine the effects of 5-Chloro2,4-dihydroxypyridine (CDHP), a 5-FU modulator to competitively inhibit DPD, on 5-FU cytotoxicity in the HSC-3 and HSC-3R cells, to evaluate if inhibition of DPD can restore the sensitivity of HNSCC cells to 5-FU. Results: Nineteen HNSCC patients who had undergone concurrent chemoradiotherapy with 5-FU-based regimens were enrolled in this study. The cut-off value for DPD mRNA expression calculated from the ROC curve was 8.53 against cancer-specific survival of these patients. The high-level DPD expression group showed significantly shorter overall and cancer-specific survival compared to the low-level DPD expression group (P=0.0018 and 0.0004, respectively). Both of protein and mRNA expression levels were greater in the HSC-3R cells than that in the HSC-3 cells. While HSC-3R cells showed 12.64-fold greater resistance to 5-FU compared with HSC-3 cells, the combination of 5-FU with CDHP had a significant inhibitory effect on 5-FU cytotoxicity in HSC-3R cells in CDHP exposure. Conclusions: In this study, we clarified that the high-level DPD expression group of HNSCC patients undergoing concurrent chemoradiotherapy with 5-FU-based regimens showed significantly shorter overall survival. The 5-FU-resistant cells established from HNSCC cells showed increased DPD expression and attenuated 5-FU resistance induced by CDHP. Our results suggested that a high level of DPD expression was correlated with 5-FU resistance and that DPD expression level might be a predictive biomarker in 5-FU-based chemoradiotherapy for HNSCC patients.