A role for reactive oxygen species in apoptotic cell death.

Apoptosis is a well studied mechanism of controllable cell death, implicated in various disease states. Among other features, it can be characterised morphologically by cell shrinkage, nucleolar disintegration, chromatin condensation and membrane blebbing. Biochemically, apoptotic cell death is most commonly characterised by DNA fragmentation. Originally, it was thought that the cleavage of chromatin between nucleosomes was the biochemical hallmark of apoptosis. More recently it has been shown that the formation of large DNA fragments (30 kilobase-pairs) occurs in the absence of and as a precursor to internucleosomal cleavage. It has also been suggested that reactive oxygen species may play a role in the control of apoptosis. However, conflicting data has been accumulated regarding this role. In this thesis two aspects of apoptosis were studied. Firstly, a technique of labelling DNA termini, caused by strand breakage, was used to detect apoptotic cells. Cells possessing the larger DNA fragments, without concomitant internucleosomal cleavage, were labelled, demonstrating the ability of the technique to detect apoptotic cell death prior to the formation of oligonucleosomal fragments. Secondly, various metal ion chelators and other agents which decrease the formation of intracellular reactive oxygen species were used to modulate apoptosis in a human leukaemic cell line and in primary rat thymocytes. It was found that although some features of apoptosis could be inhibited, others were not. In addition, it was also demonstrated that a redox-active transcription factor, NFkB, thought to be a general modulator of apoptotic cell death, is neither required nor sufficient for apoptosis. This lead to the conclusion that reactive oxygen species are not involved in a common pathway of apoptosis.