Deoxynucleoside overproduction in deoxyadenosine-resistant, adenosine deaminase-deficient human histiocytic lymphoma cells

Oral antilymphocyte activity and induction of apoptosis by 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine

2-Chlorodeoxyadenosine (CdA) is active in chronic lymphocytic leukemia, hairy-cell leukemia, and low-grade lymphomas. In part, this spectrum of activity may be attributable to the selective toxicity of CdA to nondividing lymphocytes and monocytes. However, CdA is unstable at acidic pH and is degraded by bacterial nucleoside phosphorylases. The present experiments demonstrate that the 2'-arabino-fluoro derivative of CdA, designated CAFdA, is also directly toxic to quiescent lymphocytes and macrophages. Unlike CdA, CAFdA was stable at pH 2 and resisted degradation by Escherichia coli nucleoside phosphorylase. Cell killing was preceded by the formation of DNA strand breaks and could be prevented by supplementation of the medium with deoxycytidine. The initial DNA damage initiated the pattern of oligonucleosomal DNA fragmentation characteristic of apoptosis. Mutant lymphoblasts, deficient in deoxycytidine kinase, with elevated cytoplasmic 5'-nucleotidase, or with expanded deoxynucleotide pools secondary to increased ribonucleotide reductase activity, were cross-resistant to both CAFdA and CdA toxicity. One-week oral treatment with CAFdA (1 mg/ml in drinking water) achieved an average plasma concentration of 0.56 microM and eliminated 90% of chronic lymphocytic leukemia cells transplanted into severe combined immunodeficiency (scid) mice. Under the same conditions, CdA was much less active. Collectively, these results suggest that CAFdA could be effective as an oral agent in indolent lymphoproliferative diseases and in autoimmune diseases where lymphocyte and monocyte depletion is desirable.

Deoxyadenosine-resistant human T lymphoblasts with elevated 5'-nucleotidase activity

Although several different enzymes with 5'-nucleotidase activity have been described in mammalian cells, their functions in nucleotide metabolism have not been clearly distinguished. In the present experiments, a mutant human T lymphoblastoid cell line (CEM-dAdoR) was selected specifically for resistance to deoxyadenosine toxicity. Compared to parental CEM cells, the variant had 4-fold elevated ATP-activated cytosolic 5'-nucleotidase activity. Other enzymes of potential importance for deoxyadenosine metabolism were indistinguishable in the two cell types. In medium supplemented with the adenosine deaminase inhibitor deoxycoformycin, the T cells with increased 5'-nucleotidase accumulated less nucleotides from exogenously added deoxyadenosine, or 9-beta-D-arabinofuranosyladenine, than did parental T lymphocytes. These metabolic changes were associated with resistance to the growth inhibitory effects of these nucleosides, and also to deoxyguanosine and to 9-beta-D-arabinofuranosylguanine. The T cells with elevated 5'-nucleotidase activity formed more 2',3'-dideoxyadenosine than did parental cells, in deoxycoformycin-supplemented medium. The accumulation of 2',3'-dideoxyadenosine 5'-triphosphate from 2',3'-dideoxyinosine was similarly augmented in the mutant. These data establish the importance of the cytosolic 5'-nucleotidase for the metabolism of purine 2'-deoxyribonucleosides, arabinonucleosides and 2',3'-dideoxyribonucleosides in T lymphoblasts.

Differential modulation of 1-beta-D-arabinofuranosylcytosine metabolism by hydroxyurea in human leukemic cell lines

The ability of hydroxyurea (HU) to modulate 1-beta-D-arabinofuranosylcytosine (Ara-C) metabolism was investigated in human leukemic cell lines. Exposure of HL-60 cells to 1 mM HU enhanced the accumulation of Ara-CTP up to 2.5-fold, whereas HU did not have significant effects on Ara-C metabolism in CEM cells. In addition, two adenine nucleosides, deoxyadenosine (dAdo) and 9-beta-D-arabinofuranosyladenine (Ara-A), which are known to be activated by deoxycytidine (dCyd) kinase as Ara-C, were more effectively phosphorylated after the addition of HU only in HL-60 cells. However, the changes of intracellular dCTP and TTP pools induced by HU, i.e. decrease in dCTP and increase in TTP, were the same in both cell lines. Finally, dCyd production under normal culture conditions was at least 3- to 4-fold higher in HL-60 cells and was inhibited significantly by HU administration. These results suggest that the modulation of Ara-C metabolism by HU occurs at the level of dCyd kinase through the regulation of de novo dCyd generation.