MODULATION OF ARABINOSYLNUCLEOSIDE METABOLISM BY ARABINOSYLNUCLEOTIDES IN HUMAN-LEUKEMIA CELLS

  • 15 January 1988
    • journal article
    • research article
    • Vol. 48  (2) , 329-334
Abstract
Previous studies have indicated that deoxycytidine kinase (dCK) is requisite and rate limiting in the phosphorylation of 1-.beta.-D-arabinofuranosylcytosine (ara-C) and 9-.beta.-D-arabinofuranosyl-2-fluoroadenine (F-ara-A) on the pathway to their respective cytotoxic 5''-triphosphates. In K562 cells, the rate of triphosphate accumulation was maximal during incubation with 10 .mu.M ara-C (35 .mu.M/h) and 300 .mu.M F-ara-A (102 .mu.M/h). Under these conditions, accumulation of cellular ara-CTP plateaued at about 110 .mu.M after 3 h, whereas in separate cultures, F-ara-ATP continued to accumulate at a linear rate to cellular concentrations greater than 500 .mu.M after 5 h. Other laboratories have demonstrated that dCK activity in cell-free extracts was inhibited by ara-CTP. To determine whether ara-CTP exhibited ths same activity in whole cells, K562 cells were preincubated with ara-C to accumulate 110 .mu.M ara-CTP. After washing into medium containing F-ara-A, the rate of F-ara-ATP accumulation was significantly decreased (37 .mu.M/h). However, cells loaded with F-ara-ATP exhibited an increased rate of ara-CTP accumulation (110 .mu.M/h) that resulted in cellular ara-CTP concentrations in excess of 400 .mu.M after 5 h. This stimulation was proportional to the cellular concentrations of F-ara-ATP, achieving a maximum effect between 75 and 100 .mu.M. Phosphorylation of ara-C by cell-free extracts supplemented with physiological levels of ribo- and deoxyribonucleoside 5''-triphosphates was stimulated by addition of F-ara-ATP. The decreased rate of accumulation of products to dCK in intact cells containing 110 .mu.M ara-CTP suggests that this active triphosphate may limit its own synthesis and phosphorylation of other substrates. In contrast, stimulation of the accumulation of ara-CTP in cells containing F-ara-ATP suggests new possibilities for the design of combination chemotherapy regimens.