Regulation of cancer cell proliferation and mitosis by NF-kB

PhD ThesisThe NF-κB family of transcription factors can induce or repress target gene expression by binding DNA as homo- or hetero-dimers. In some cancer cells the p52 (NF-κB2) subunit, which is derived by proteolytic processing of its precursor p100, regulates the expression of genes having a role in cell proliferation, such as Cyclin D1 and is also involved in the regulation of G2/M phase. Depletion of p100/p52 by siRNA leads to an increase of cells in G2/M phase and defects in mitosis. These defects in mitosis can be visualised as aberrant chromosome segregation, disruption of the microtubule network and poor alignment of chromosomes on the metaphasic plate. Moreover, p100/p52 depletion results in an increase in multinucleate cells, as well as aberrant centrosome structures. To elucidate this mechanism, I have investigated p100/p52 regulation of various genes involved in the cell cycle and centrosome duplication. I discovered that p100/p52 siRNA depletion reduces the expression of a number of cell cycle regulators, including Polo-like kinase 4 (PLK4) or Spindle assembly abnormal protein 6 (SAS6), a member of the SAS proteins family. PLK4, in conjunction with the cyclin-dependent kinase CDK2 and the PLK4 effector SAS6, is a key regulator of centriolar duplication. I demonstrated by reporter- gene and chromatin-immunoprecipitation analyses that the PLK4 promoter is a direct target for multiple NF-κB subunits and its activity depends upon NF-κB expression. Moreover PLK4 and p100/p52 mRNA and protein expression are cell cycle regulated and NF-κB subunits bind the PLK4 promoter in a cell cycle dependent manner. These results reveal PLK4 as a new NF-κB target, providing a direct link between NF-κB and centrosome duplication with implications for the role of p52 in tumorigenesis