title: Effect of intracellular folate concentration on the modulation of 5-fluorouracil cytotoxicity by the elevation of phosphoribosylpyrophosphate in cultured human KB cells.
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 microM) (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.
mesh_terms: D000241:Adenosine / Q000378:metabolism; D002294:Carcinoma, Squamous Cell / Q000378:metabolism; D002470:Cell Survival / Q000187:drug effects; D005472:Fluorouracil / Q000494:pharmacology*; D005492:Folic Acid / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D006801:Humans; D007624:KB Cells / Q000187:drug effects* / Q000378:metabolism; D002955:Leucovorin / Q000494:pharmacology; D015470:Leukemia, Myeloid, Acute / Q000378:metabolism; D008727:Methotrexate / Q000494:pharmacology; D009303:Nasopharyngeal Neoplasms / Q000378:metabolism; D010428:Pentosephosphates / Q000378:metabolism*; D010754:Phosphoribosyl Pyrophosphate; D013936:Thymidine / Q000378:metabolism
chemical_list: D010428:Pentosephosphates; D010754:Phosphoribosyl Pyrophosphate; D005492:Folic Acid; D005947:Glucose; D000241:Adenosine; D002955:Leucovorin; D005472:Fluorouracil; D013936:Thymidine; D008727:Methotrexate