Regulation of Deoxycytidine Kinase by Deoxycytidine and Deoxycytidine 5′ Triphosphate in Whole Leukemia and Tumor Cells

Abstract

The clinical importance of the dCTP salvage pathway is given by the fact that deoxycytidine (dCyd) analogs like cytosine arabinoside (ara-C) are activated via the salvage pathway. The non-toxic prodrug ara-C gains its cytotoxic potential by phosphorylation to the 5′-triphosphate ara-CTP. Not only cellular accumulation, but also retention of ara-CTP are known determinants of response to antileukemic treatment. Deoxycytdine kinase is the rate-limiting enzyme for intracellular ara-CTP formation. Experiments performed with purified dCK (dCK) demonstrated that dCTP, the final product of phosphorylation, acts as a potent feed-back inhibitor of dCyd kinase.^1,2 In fact, depletion of intracellular dCTP as induced by inhibition of the de novo pathway increases of ara-C phosphorylation.³ Therefore it was concluded that the release of dCTP-mediated feed-back inhibition was responsible for enhanced dCK activity. The present study investigates the role of dCTP for regulation of dCK activity in the cell lines CCRF-CEM (T-cell line), Raji (B-cell line), HL-60 (promyelocytic), and CHO (Chinese hamster ovary). Compared to T-cells, B-cells are characterised by a fast turnover of dNTP and a short retention of ara-CTP. CHO cells were included because of their deficiency in deoxycytidylate (dCMP) deaminase activity. Cellular accumulation of ara-CTP was evaluated as a measure of salvage pathway activity. Inhibition of ribonucleotide reductase as induced by hydroxyurea may cause an activation of the salvage pathway to maintain adequate dCTP levels for DNA synthesis. Cells with a fast dCTP turnover and a greater dependence on the de novo pathway should respond with a greater enhancement of the salvage pathway than cells with long retention of deoxynucleotides and a correspondingly lower dependence on the de novo pathway.⁴