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.