Biochemical basis for the differential sensitivity of human T- and B-lymphocyte lines to 5-fluorouracil.

The metabolism of 5-fluorouracil (FUra) was examined in two human lymphocyte cell lines, CCRF-CEM (T-leukemic) and LAZ-007 (Epstein-Barr virus-transformed B), which have widely differing FUra sensitivities. CEM cells have orotate phosphoribosyltransferase but no uridine or thymidine phosphorylase activity. Consistent with this, the FUra growth inhibition and the synthesis of fluorouridine triphosphate (FUTP) and fluorodeoxyuridine monophosphate (FdUMP) in CEM cells was substan tially reduced by hypoxanthine and allopurinol. FUra growth inhibition of CEM cells was due to inhibition of both DMA and RNA synthesis, since thymidine only partially restored growth. Growth inhibition of LAZ cells, which possess all of the above FUra-activating enzymes, occurred at a lower FUra concentra tion than for CEM cells and was due (at FUra concentrations giving 95% growth inhibition) to inhibition of thymidylate syn thesis, since thymidine completely restored growth. Consistent with this, the LAZ cells synthesize FdUMP at much higher rates than do CEM cells. FdUMP synthesis was not significantly inhibited by hypoxanthine but was activated by deoxyinosine and strongly inhibited by thymidine, leading us to conclude that LAZ cells synthesize FdUMP predominantly via thymidine phos phorylase. At higher (>10~6 M) FUra concentrations, thymidine only partially restored LAZ cell growth, indicating impairment of RNA metabolism apparently due to FUTP synthesized via orotate phosphoribosyltransferase since (a) substantial protec tion was obtained by addition of hypoxanthine to FUra plus thymidine and (b) FUTP synthesis in LAZ cells was strongly inhibited by hypoxanthine. Thus, the increased FUra sensitivity of LAZ cells is due primarily to their greater rate of FdUMP synthesis via thymidine phosphorylase. Impairment of RNA metabolism occurred in both cell lines at similar FUra concen trations and was due to FUTP synthesis via orotate phospho ribosyltransferase, the activity of which is similar in LAZ and CEM cells.