Abstract
FDCP-Mix, a pluripotent routine hemopoietic stem cell line undergoes internucleosomal cleavage of DNA when induced to apoptosis either by drugs or by withdrawal of growth factor (IL-3), and also displays a pattern of nuclear morphology that is typical for apoptosis. However, increased autolytic activity in the cytoplasm precedes the nuclear changes. For etoposide-treated FDCP-Mix cells, mitochondria were identified as a target for autolytic digestion in large autolytic vacuoles, but during this period an increase in the number of mitochondria was observed. The autolytic vacuoles displayed variations in their content. Large, electron-dense inclusions resembling "condensed chromatin" could regularly be found in FDCP-Mix cells treated with low concentrations of etoposide (<4 microM). Confocal fluorescence microscopy and DNAse-gold labeling were employed to demonstrate the presence of DNA in the formation of the electron-dense inclusions within autolytic vacuoles. The identification of mitochondrial macroautophagy, the evidence for an etoposide-induced proliferation of mitochondria, and the fact that electron-dense inclusions are formed at a stage when the morphology of the nucleus is still not effected, suggests that the DNA within the autolytic vacuoles may be of mitochondrial origin.