Association of cell lethality with incorporation of 5-fluorouracil and 5-fluorouridine into nuclear RNA in human colon carcinoma cells in culture.

The cytokinetic and biochemical effects of 5-fluorouracil and 5-fluorouridine were examined in a human colon carcinoma cell line (HT-29) in culture. Logarithmically growing cells were approximately 100 times more sensitive to the lethal effects of 5-fluorouridine than 5-fluorouracil as measured by colony formation in soft agar medium. A 2-hr exposure of cells to 10(-3) M 5-fluorouracil or 10(-5) M 5-fluorouridine produced a 2-log reduction in colony formation, a 31--33% inhibition of [14C]deoxyguanosine incorporation into DNA, and 30--40% inhibition of [3H]adenosine incorporation into total RNA. Increasing the duration of drug exposure to 24 hr produced a proportional reduction in the drug concentration required to produce similar biochemical and cytocidal effects. However, cell lethality produced by either drug did not correlate quantitatively with inhibition of DNA or RNA synthesis. Examination of nuclear rRNA and 4 S RNA synthesis by agarose gel electrophoresis following 2-hr and 24-hr exposure to 5-fluorouracil or 5-fluorouridine indicated that processing of rRNA was not impaired, rRNA synthesis was inhibited by 10--40%, and 4 S RNA synthesis was unaffected. In contrast to these results, measurements of the incorporation fo [3H]5-fluorouracil or [3H]5-fluorouridine into nuclear RNA showed that a significant correspondence existed between the amount of drug incorporated into nuclear RNA and cell lethality. These results indicate that the primary determinant of cell lethality in HT-29 cells is the degree of fluoropyrimidine substitution in nuclear RNA and not inhibition of either DNA or RNA synthesis.