5-Fluorouracil (5-FU) continues to be widely used for treatment of
gastrointestinal cancers. Because many
tumors show primary or acquired resistance, it is important to understand the molecular basis underlying the mechanism of resistance to
5-FU. In addition to its effect on
thymidylate synthase inhibition and
DNA synthesis,
5-FU may also influence
RNA metabolism. Our previous studies revealed that
colorectal cancer cells resistant to bolus
5-FU (HCT-8/4hFU) showed significantly decreased incorporation of the drug into
RNA. Resistance to bolus
5-FU was associated with lower expression of
UMP kinase (UMPK), an
enzyme that plays an important role in the activation of
5-FU to 5-FUTP and its incorporation into
RNA. Activities of other 5-FU-metabolizing
enzymes (e.g.,
thymidine kinase,
uridine phosphorylase,
thymidine phosphorylase, and
orotate phosphoribosyltransferase) remained unchanged between sensitive and resistant cell lines. Herein, we show that UMPK down-regulation in 5-FU-sensitive cells (HCT-8/P) induces resistance to bolus
5-FU treatment. Moreover, HCT-8/4hFU cells are even more cross-resistant to treatment with
5-fluorouridine, consistent with the current understanding of
5-fluorouridine as a
RNA-directed drug. Importantly,
colorectal cancer hepatic
metastases isolated from patients clinically resistant to weekly bolus 5-FU/
leucovorin treatment exhibited decreased
mRNA expression of UMPK but not
thymidylate synthase or
dihydropyrimidine dehydrogenase compared with
tumor samples of patients not previously exposed to
5-FU. Our findings provide new insights into the mechanisms of acquired resistance to
5-FU in
colorectal cancer and implicate UMPK as an important mechanism of clinical resistance to pulse
5-FU treatment in some patients.