Screening of Differentially Expressed Genes in 5‐Fluorouracil‐resistant Human Gastrointestinal Tumor Cells

To identify genes differentially expressed in association with resistance to 5‐fluorouracil (5FU), an niRNA differential display (DD) analysis was used to compare transcripts from the NUGC‐3 human gastric tumor cell line and the NUGC‐3/5FU/L line, which had acquired 208‐fold resistance as a consequence of repeated exposure to escalating concentrations of 5FU. The 110 cDNA fragments differentially expressed in the DD analysis of either the NUGC‐3 or NUGC‐3/5FU/L cells were sequenced and subjected to a homology search, and 29 overexpressed and 22 underexpressed genes were identified in NUGC‐3/5FU/L as a result. To confirm whether the changes in the gene expression levels in the NUGC‐3/5FU/L cells were shared by other 5FU‐resistant cells, 35 genes were analyzed by northern hybridization in 3 pairs of parent/5FU‐resistant human gastrointestinal tumor cell lines. The analysis revealed 20 overexpressed and 10 underexpressed genes in at least one of the three 5FU‐resistant cells as compared with those in the parent cells. Among them, P‐glycoprotein, equilibrative nucleoside transporter 1, and methylenetetrahydrofolate dehydrogenase were highly expressed in two of the three 5FU‐resistant cells and cytidine deaminase and integrin o3 were underexpressed. The acquisition of resistance to 5FU by tumor cells may result from multiple changes in cellular functions.