Substrate-specific Deoxycytidine Kinase Deficiency in 1-β-d-Arabinofuranosylcytosine-resistant Leukemic Cells

In this study we describe the establishment of a leukemic cell line (BNML-CL/ara-C), originating from the 1-β-d-arabinofuranosylcytosine (ara-C)-resistant brown Norway rat myelocytic leukemia model (BNML/ara-C), that retains the in vivo generated ara-C resistance. Its biological and biochemical characteristics have been compared with a cell line, derived from the ara-C-sensitive BNML model (BNML-CL/O). Resistance to ara-C was attributed to a decrease in phosphorylation of ara-C. Deoxycytidine (dCyd) kinase activity in crude cell extracts with dCyd as substrate showed similar enzyme activities in both cell lines, whereas with ara-C as substrate no dCyd kinase activity was detectable in the ara-C-resistant cell line. Two isoenzymes of dCyd kinase with different substrate specificities have been described (Cheng, Y. C., Domin, B., and Lee, L. S. Biochim. Biophys. Acta, 481: 481–492, 1977), cytoplasmic (dCyd kinase I, substrates: dCyd and ara-C) and mitochondrial (dCyd kinase II, substrates: dCyd and thymidine). In the ara-C-sensitive BNML model, thymidine induced a reduction of dCyd kinase activity when dCyd was used as substrate. However, thymidine did not affect kinase activity with ara-C was used as substrate. In the BNML-CL/ara-C, thymidine even induces a dCyd kinase inhibition of 85% with dCyd as substrate. It is likely that the ara-C-specific dCyd kinase deficiency in BNML-CL/ara-C cells was due to a selective loss of dCyd kinase I, whereas dCyd kinase II activity remained intact.