INDUCTION OF APTOPTOSIS BY 5-AZACYDINE - DRUG CONCENTRATION-DEPENDENTDIFFERENCES IN CELL-CYCLE SPECIFICITY

There are conflicting data in the literature as to whether cytotoxicit y of the cytidine antimetabolite 5'-azacytidine (AZC) is a consequence of its incorporation into RNA, DNA, or both. Because apoptosis appear s to be the predominant mode of tumor cell death after treatment with most antitumor drugs, and in the case of some drugs, the proclivity of the cell to undergo apoptosis varies depending on the cell cycle posi tion, this study was aimed toward elucidating whether induction of apo ptosis by AZC is cell cycle phase specific. Human promyelocytic leukem ic HL-60 cells were treated with varying concentrations of AZC, and fl ow cytometric methods that identify apoptotic cells and provide inform ation about the cell cycle distribution of the apoptotic and nonapopto tic cell populations were used, At 2-6 mu M concentrations of AZC, the cells in the G(1) phase preferentially underwent apoptosis, whereas t he cells in G(2)-M were particularly resistant. Although incorporation of bromouridine into RNA was suppressed at that tow AZC concentration , the rate of 5'-bromo-2-deoxyuridine incorporation into DNA was not s ignificantly affected. At an AZC concentration of 8-40 mu M, no cell c ycle phase specificity in induction of apoptosis was apparent, but bot h the rate of 5'-bromo-2-deoxyuridine incorporation into DNA and bromo uridine into RNA were reduced in proportion to drug concentration. The data suggest that the mechanism of cell killing by AZC may be differe nt, depending on its concentration. Namely, whereas incorporation of A ZC into RNA may play a predominant role in the induction of cytotoxici ty of G(1) cells at low drug concentrations, the perturbation of both RNA and DNA metabolism may be responsible for triggering cell death in the G(1) and S phases, as is seen at higher concentrations of this an timetabolite.