Bcl2-independent chromatin cleavage is a very early event during induction of apoptosis in mouse thymocytes after treatment with either dexamethasone or ionizing radiation.
We have quantified the emergence of early chromatin breaks during the signal transduction phase of apoptosis in mouse thymocytes after treatment with either ionizing radiation or dexamethasone. Dexamethasone at 1 microM can induce significant levels of DNA breaks (equivalent to the amount induced directly by 7.5 Gy ionizing radiation) within 0.5 h of treatment. The execution phase of apoptosis was not observed until 4-6 h after the same treatment. The presence of the Bcl2 transgene under the control of the p56lck promoter almost completely inhibited apoptosis up to 24 h after treatment, but it had virtually no effect on the early chromatin cleavage occurring in the first 6 h. Ionizing radiation induced chromatin cleavage both directly by damaging DNA and indirectly with kinetics similar to the induction of chromatin cleavage by dexamethasone. The presence of the Bcl2 transgene had no effect on the direct or indirect radiation-induced cleavage in the first 6 h, but after the first 6 h, the Bcl2 gene inhibited further radiation-induced chromatin cleavage. These results suggest that endonucleases are activated within minutes of treatment with either dexamethasone or ionizing radiation as part of the very early signal transduction phase of apoptosis, and prior to the irreversible commitment to cell death.
Hahn
Peter J
PJ
Departments of Radiation Oncology, SUNY Upstate Medical University, Syracuse, New York 13210, USA. hahnp@mail.upstate.edu
Lai
Zhi-Wei
ZW
Nevaldine
Barbara
B
Schiff
Ninel
N
Fiore
Nancy C
NC
Silverstone
Allen E
AE
D000818:Animals; D017209:Apoptosis / Q000187:drug effects* / Q000235:genetics / Q000528:radiation effects*; D002455:Cell Division; D002478:Cells, Cultured; D002843:Chromatin / Q000187:drug effects / Q000528:radiation effects; D004249:DNA Damage; D003907:Dexamethasone / Q000494:pharmacology*; D005260:Female; D019254:Genes, bcl-2 / Q000187:drug effects / Q000528:radiation effects; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2; D011839:Radiation, Ionizing; D015398:Signal Transduction / Q000187:drug effects / Q000528:radiation effects; D013950:Thymus Gland / Q000187:drug effects* / Q000473:pathology / Q000528:radiation effects*; D019076:Transgenes
D002843:Chromatin; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; C052617:Bcl2 protein, mouse; D003907:Dexamethasone