Evidence for functional activity and changes in degradation of p53 following 5-fluorouracil or doxorubicin treatment of human breast cancer cells

The accumulation of the tumor suppressor p53 is important for cell-cycle regulation and initiation of DNA repair following DNA damage. This thesis sought to determine the reason p53 was not undergoing degradation and if the accumulating p53 was functional in MCF-7 breast cancer cells following doxorubicin (dox) or 5-fluorouracil (5FU). Treatment with the proteasome inhibitor MG132 showed an appreciable additional accumulation of p53 following treatment. We next investigated the p53 regulatory proteins, Mdm2 and MdmX. Following treatment with 5FU, we observed that Mdm2 was able to bind to p53 and that Mdm2 appears to form heterocomplexes with MdmX, that have been shown by others to robustly poly-ubiquitinate p53. These results were not observed in dox-treated cells. We also investigated the functionality of the accumulated p53. We observed a p53-dependent up-regulation of p21 and Mdm2 following both treatments. These data suggest major differences in the p53 response following 5FU or dox.