5-Fluorouracil (5-FU) is a cornerstone drug used in the treatment of colorectal cancer (CRC). However, the development of resistance to 5-FU and its analogs remain an unsolved problem in CRC treatment. In this study, we investigated the molecular mechanisms and tumor biological aspects of 5-FU resistance in CRC HCT116 cells. We established an acquired 5-FU-resistant cell line, HCT116RF10. HCT116RF10 cells were cross-resistant to the 5-FU analog, fluorodeoxyuridine. In contrast, HCT116RF10 cells were collaterally sensitive to SN-38 and CDDP compared with the parental HCT16 cells. Whole-exome sequencing revealed that a cluster of genes associated with the 5-FU metabolic pathway were not significantly mutated in HCT116 or HCT116RF10 cells. Interestingly, HCT116RF10 cells were regulated by the function of thymidylate synthase (TS), a 5-FU active metabolite 5-fluorodeoxyuridine monophosphate (FdUMP) inhibiting enzyme. Half of the TS was in an active form, whereas the other half was in an inactive form. This finding indicates that 5-FU-resistant cells exhibited increased TS expression, and the TS enzyme is used to trap FdUMP, resulting in resistance to 5-FU and its analogs.

The major metabolite of the anticancer agent 5-fluorouracil (5-FU) is 5-fluorodeoxyuridine monophosphate (FdUMP), which is a potent inhibitor of thymidylate synthase (TS). Recently, we hypothesized that 5-FU-resistant colorectal cancer (CRC) cells have increased levels of TS protein relative to 5-FU-sensitive CRC cells and use a fraction of their TS to trap FdUMP, which results in resistance to 5-FU. In this study, we analyzed the difference between the regulation of the balance of the free, active form of TS and the inactive FdUMP-TS form in 5-FU-resistant HCT116 cells and parental HCT116 cells. Silencing of TYMS, the gene that encodes TS, resulted in greater enhancement of the anticancer effect of 5-FU in the 5-FU-resistant HCT116RF10 cells than in the parental HCT116 cells. In addition, the trapping of FdUMP by TS was more effective in the 5-FU-resistant HCT116RF10 cells than in the parental HCT116 cells. Our observations suggest that the regulation of the balance between the storage of the active TS form and the accumulation of FdUMP-TS is responsible for direct resistance to 5-FU. The findings provide a better understanding of 5-FU resistance mechanisms and may enable the development of anticancer strategies that reverse the sensitivity of 5-FU resistance in CRC cells.

5‐Fluorouracil (5‐FU) is widely used for colorectal cancer (CRC) treatment; however, continuous treatment of CRC cells with 5‐FU can result in acquired resistance, and the underlying mechanism of 5‐FU resistance remains unclear. We previously established an acquired 5‐FU‐resistant CRC cell line, HCT116RF10, and examined its biological features and 5‐FU resistance mechanisms. In this study, we evaluated the 5‐FU sensitivity and cellular respiration dependency of HCT116RF10 cells and parental HCT116 cells under conditions of high‐ and low‐glucose concentrations. Both HCT116RF10 and parental HCT116 cells were more sensitive to 5‐FU under low‐glucose conditions compared with high‐glucose conditions. Interestingly, HCT116RF10 and parental HCT116 cells exhibited altered cellular respiration dependence for glycolysis and mitochondrial respiration under high‐ and low‐glucose conditions. Additionally, HCT116RF10 cells showed a markedly decreased ATP production rate compared with HCT116 cells under both high‐ and low‐glucose conditions. Importantly, glucose restriction significantly reduced the ATP production rate for both glycolysis and mitochondrial respiration in HCT116RF10 cells compared with HCT116 cells. The ATP production rates in HCT116RF10 and HCT116 cells were reduced by approximately 64% and 23%, respectively, under glucose restriction, suggesting that glucose restriction may be effective at enhancing 5‐FU chemotherapy. Overall, these findings shed light on 5‐FU resistance mechanisms, which may lead to improvements in anticancer treatment strategies.

The properties of 5-fluorouracil (5-FU), including its chemical synthesis and widespread anticancer effects, in fundamental and clinical terms were first published in 1957. 1-3 5-FU is still widely used today, mainly for the treatment of gastrointestinal cancers, such as colorectal cancer (CRC). 4,5 5-FU is converted to the active metabolite 5-fluorodeoxyuridine monophosphate (FdUMP), which is a potent inhibitor of thymidylate synthase (TS). 3,[5][6][7][8] FdUMP forms a ternary complex with TS and 5, 10-methylenetetrahydrofolate (5, 10-CH 2 -THF). 4-9 TS catalyzes the conversion of dUMP to dTMP using the coenzyme 5, 10-CH 2 -THF as a methyl donor. 10 The ternary complex inhibits TS function, depletes intracellular dTTP, dNTP pools, and subsequently inhibits DNA synthesis. [3][4][5]8

Background: Poly(ADP-ribose) glycohydrolase (PARG) is a key enzyme in poly(ADPribose) (PAR) metabolism and a potential anticancer target. Many drug candidates have been developed to inhibit its enzymatic activity. Additionally, PDD00017273 is an effective and selective inhibitor of PARG at the first cellular level.Aims: Using human colorectal cancer (CRC) HCT116 cells, we investigated the molecular mechanisms and tumor biological aspects of the resistance to PDD00017273.Methods and results: HCT116R PDD , a variant of the human CRC cell line HCT116, exhibits resistance to the PARG inhibitor PDD00017273. HCT116R PDD cells contained specific mutations of PARG and PARP1, namely, PARG mutation Glu352Gln and PARP1 mutation Lys134Asn, as revealed by exome sequencing. Notably, the levels of PARG protein were comparable between HCT116R PDD and HCT116. In contrast, the PARP1 protein levels in HCT116R PDD were significantly lower than those in HCT116. Consequently, the levels of intracellular poly(ADP-ribosyl)ation were elevated in HCT116R PDD compared to HCT116. Interestingly, HCT116R PDD cells did not exhibit cross-resistance to COH34, an additional PARG inhibitor. Conclusion:Our findings suggest that the mutated PARG acquires PDD00017273 resistance due to structural modifications. In addition, our findings indicate that PDD00017273 resistance induces mutation and PARP downregulation. These discoveries collectively provide a better understanding of the anticancer candidate PARG inhibitors in terms of resistance mechanisms and anticancer strategies.