Apoptosis: Demolition Cells Under Multi-level Control

Apoptosis is an important mode of cell death that is used by multicellular organisms to remove damaged or unwanted cells during development or in response to environmental stress. Both too much or too less of apoptosis might contribute to the pathogenesis of diverse diseases. Due to the importance of apoptosis is maintaing hemostasis, each step of the progression of apoptosis, from receiving the stimuli to the signaling cascade, is tightly orchachested in a cell and conserved during evolution.In a certain sense, apoptosis is an active process that depends on RNA and protein synthesis by the dying cell. Under normal growth conditions, the pro-apoptotic factors are inactivated or kept at low levels, and the suppressors of apoptosis usually play a dominant role. Upon stimulation by death signals, the balance between pro- and anti- apoptotic factors will be broke. This process is controlled at multiple pathways, which includes the transcriptional regulation, RNA processing, translational and post-translational regulation of proteins, as well as protein degradation. The goal of this dissertation is to reveal how the multiple regulatory pathways are coordinated during apoptosis, which finally determines the fate of cells under specific environmental conditions.In the first part of the dissertation, we reported the identification of a novel auto-cleavage site between two tandemly repeated DEDs of the caspase8/Mch5 protein. One of the cleavage products, the DEDa fragment could translocate into the nucleus by association with ERK1/2 in response to death stimuli. In the nucleus, DEDa interacts with TOPORS, and possibly displaces p53 form TOORS, allowing p53 to stimulate caspase-8 gene expression. In summary, this finding postulate a positive feed back loop involving DEDa, which enables the continual replenish of procaspase-8 during apoptosis.The above work also suggests an important role for the nucleolus in apoptosis. Our study on the functions of a key nucleolar protein, termed B23, led many interesting findings. We observed that B23 could translocate into the cytoplasm and interact with the mRNA-binding proteins hnRNP-U and hnRNPAl, and such protein-protein interaction is regulated by specific mRNA molecules. We further confirmed that the dynamic interplay between B23 and hnRNPs affects post-transcriptional regulation of gene expression through a microRNA involved pathway, which contribute to cellular response to nucleolus stress and cell cycle checkpoint control.The last part of the dissertation reports the work of a large-scale selection of anti-drug genes. By sequential selection of Taxol-resistant clones in HeLa cells, we successfully isolated a clone which was very insensitive to Taxol-induced cell cycle arrest and apoptosis. Using microarray analysis, we identified a novel factor termed TXR-3 which may contribute cell resistance to Taxol treatment. The detailed mechanisms are under investigation at the moment.