DOXORUBICIN-INDUCED DNA-DEGRADATION IN MURINE THYMOCYTES

Exposure of murine thymocytes to doxorubicin (Dox) (0.5-1.0 mu M, 24 h r) triggered rapid DNA degradation, as indicated by the appearance of a major subdiploid population demonstrated by DNA flow cytometry. Elec tron microscopic comparison of samples with large subdiploid populatio ns versus those with little or no such subset revealed significantly m ore cells with the characteristic features of apoptosis, the morpholog ically definable stage of programmed cell death. These features includ e unipolar condensed chromatin, zeiosis, and electron-dense cytoplasm. Dox-induced apoptosis occurred without prior S or G(2)/M phase arrest or cell size increase. The subset most susceptible to Dox-induced apo ptosis in vitro and in vivo was CD3(-)CD4(+)CD8(+). The same subset is affected by dexamethasone (Dex); as reported for Dex-induced apoptosi s, actinomycin D and cycloheximide also blocked Dox-induced apoptosis. Thymocytes exposed to higher Dox concentrations (2-10 mu M) did not h ave a subdiploid population. Although at 2-10 mu M Dox significantly r educed cell numbers (probably as a result of necrosis), at least 5-10% of the population was viable at 24 hr. Thymocytes exposed to low conc entrations of Dox (0.001-1.0 mu M) plus Dex (0.1 mu M) exhibited addit ive induction of apoptosis, whereas those exposed to high concentratio ns of Dox (2-10 mu M) plus Dex were completely devoid of any evidence of apoptosis. These results indicate that the Dox-induced killing in t hymocytes (mostly noncycling cells) occurs via different mechanisms de pending upon the Dox concentration.