Modulation of 5-FU metabolism in human MCF-7 breast carcinoma cells.

We have previously demonstrated a highly significant relationship (P less than 0.0001) between the incorporation of 5-fluorouracil (5-FU) into total cellular RNA and loss of clonogenic survival of the human MCF-7 breast carcinoma cell line. The present studies explore the applicability of this relationship to MCF-7 cells exposed to 5-FU and modulating agents such as PALA, MTX, and MMPR. PALA treatment produces a minimal increase in the absolute amount of 5-FU incorporated into total cellular RNA, but it results in a three-fold enhancement of the [3H]FU/32P ratio, which measures 5-FU misincorporation into newly synthesized RNA. MTX and MMPR increase intracellular PRPP levels up to four-fold; nevertheless these agents result in only minimal increases in absolute (5-FU)RNA formation. In contrast, the relative incorporation of 5-FU into newly synthesized RNA of MTX- or MMPR-treated cells is increased 2.5-fold. The combination of PALA/MMPR results in a two-fold absolute increase in (5-FU)RNA formation and a nine-fold enhancement of the [3H]FU/32P ratio. Combinations of modulating agents with 5-FU result in more than additive decreases in MCF-7 clonogenic survival. The relationship between 5-FU incorporation into RNA and loss of clonogenic survival was highly significant (P less than 0.0002) when corrected for newly synthesized RNA, while the correlation with absolute amounts of (5-FU)RNA formation was less significant (P less than 0.05). These studies demonstrate that the relationship previously established between (5-FU)RNA formation and loss of clonogenic survival should be corrected for the amount of newly synthesized RNA when 5-FU is combined with modulating agents that alter rates of RNA synthesis.