1-BETA-D-ARABINOSYLCYTOSINE AND 5-AZACYTIDINE INDUCE INTERNUCLEOSOMALDNA FRAGMENTATION AND CELL-DEATH IN THYMOCYTES

Incubation of mouse thymocytes with arabinosylcytosine or 5-azacytidin e induced dose-dependent internucleosomal DNA cleavage followed by cel l death. This process was RNA- and protein synthesis-dependent, since DNA fragmentation and cell death was inhibited by actinomycin D and cy cloheximide. The results suggest that the cytidine analogs induce apop tosis, a programmed cell death, in thymocytes. The DNA cleavage induce d by arabinosylcytosine and 5-azacytidine was inhibited by deoxycytidi ne and cytidine, respectively, suggesting that phosphorylation of thes e antimetabolites is required to induce DNA cleavage. DNA fragmentatio n was unaffected by the addition of aphidicolin or 3-aminobenzamide, i ndicating that DNA cleavage is not due to the inhibition of DNA synthe sis or repair. Other antimetabolites, including methotrexate, fluoropy rimidines and thiopurines, failed to induce DNA fragmentation. Arabino sylguanine induced DNA fragmentation similar to that produced by the c ytidine analogs, suggesting similarity to the selective sensitivity of T lymphocytes to deoxyguanosine toxicity. The precise mechanism by wh ich DNA cleavage is induced remains unclear, but the present study sho ws that certain antimetabolites act on cells not only by inhibiting pr oliferation, but by inducing apoptosis with internucleosomal DNA fragm entation.