EFFECT OF INTRACELLULAR FOLATE CONCENTRATION ON THE MODULATION OF 5-FLUOROURACIL CYTOTOXICITY BY THE ELEVATION OF PHOSPHORIBOSYLPYROPHOSPHATE IN CULTURED HUMAN KB CELLS

  • 15 December 1987
    • journal article
    • research article
    • Vol. 47  (24) , 6444-6450
Abstract
The effects of extracellular folate concentration on intracellular folate and phosphoribosylpyrophosphate (PRPP) levels and the cytotoxicity of methotrexate and 5-fluorouracil were studied in human KB cells grown in fetal bovine serum-supplemented Eagle''s minimum essential medium, which contained standard high folic acid levels (2.3 .mu.M) (standard or S medium), or folic acid-free serum-supplemented medium containing approximately 4 nM 5-methyltetrahydrofolate (physiological or P medium), a folate level and form more comparable to that in normal human serum. Macrocytosis and prolongation of the doubling time by 150% were observed after 5-10 serial passes in P medium, but after 10-15 serial passes, KB cells became "adapted" to P medium with return of size and doubling time to values indistinguishable from cells maintained in S medium. Cellular folate levels fell, and marked elevations in PRPP levels from 68 .+-. 43 to 642 .+-. 287 pmol/mg cell protein (mean .+-. SD) were observed as KB cells were serially passed through P medium. Human leukemia HL-60 and K562 cells and MJY-.alpha. mouse mammary tumor cells serially passed in P medium also exhibited 10- to 20-fold elevations in PRPP levels. Glucose consumption, glucose decarboxylation, thymidine and adenosine specific uptake, thymidine incorporation into DNA, and 5-fluorouracil uptake were studied in KB cells with elevated and control PRPP levels. As determined by clonal assay, despite elevated PRPP levels, KB cells cultured in P medium were less sensitive to 5-fluorouracil than cells cultured in S medium unless exogenous folate was added. These data support the concept that endogenous folate levels may be inadequate for optimal 5-FU pharmacological action in KB cells with a modulated increase in PRPP levels.