Gamma-ray-induced transcription and apoptosis-associated loss of 28S rRNA in interphase human lymphocytes.

Apoptosis, related to a naturally-occurring or programmed cellular death process, can be physiologically or exogenously induced. In vertebrate cells undergoing apoptosis, initiated by any of these ways, one of the numerous biochemical changes is an endogenous endonuclease activation that cleaves the chromatin DNA into oligonucleosome-sized 'ladder' fragments. In the present study we show that in parallel to chromatin DNA cleavage, ribosomal RNA is lost in gamma-ray-mediated apoptotic human lymphocytes. We demonstrate that 28S rRNA gene transcription is induced early (15 min) after irradiation, followed by a selective disappearance in apoptotic cells only. The fact that newly synthesized rRNA turns over at the same rate in irradiated and untreated cell fractions, highly suggests that the observed loss of 28S rRNA in the apoptotic cell fraction at the ribosome level is due to degradation occurring at a late stage of the apoptotic death process. These results suggest that, in addition to first-stage apoptosis-associated rDNA gene activation, cellular self-destruction at late stages is associated with processes occurring simultaneously at the ribosome level involving an endogenous RNase-like activity, and at the chromatin level involving DNA-nuclease activity.