Role of Dephosphorylation in Accumulation of 1-β-d-Arabinofuranosylcytosine 5′-Triphosphate in Human Lymphoblastic Cell Lines with Reference to Their Drug Sensitivity

After a 3-day exposure to 0.1 µm 1-β-d-arabinofuranosylcytosine (ara-C) in culture, growth was inhibited to 5.6% in MOLT-4F, 25% in Raji, and 91% in Daudi cells compared with control. Growth inhibition was more profound when exposure time was extended up to 7 days. Inhibition of DNA synthesis varied with sensitivity to ara-C. Plateau levels of intracellular 1-β-d-arabinofuranosylcytosine 5′-triphosphate (ara-CTP) were 35.5, 13.4, and 3.6 nmol/10⁹ cells exposed to 0.1 µm ara-C in MOLT-4F, Raji, and Daudi cells, respectively, corresponding to the sensitivity to ara-C. The nucleotide levels at the plateau, however, did not correspond to the initial levels of the ara-CTP or to the calculated rate of ara-CTP synthesis, which decreased from Raji to MOLT-4F to Daudi cells. ara-C deamination had negligible effect on the differential accumulation of ara-CTP. ara-CTP degradation due to dephosphorylation was marked in Raji and Daudi cells but slight in MOLT-4F cells. The half-life of intracellular ara-CTP was 204, 26.4, and 31.1 min in MOLT-4F, Raji, and Daudi cells, respectively. The ara-CTP level was considered to be maintained bimodally by synthesis and degradation of the nucleotide. This conclusion was supported by the fact that, in Raji and Daudi cells exposed to 0.1 µm ara-C in the presence of 1 mm hydroxyurea, the plateau levels of ara-CTP increased 3-fold through inhibition of the nucleotide degradation. Thus, not only ara-C phosphorylation but also subsequent ara-CTP dephosphorylation was important in the accumulation and maintenance of ara-CTP and in the sensitivity to ara-C.