Inhibition of PLK1 by p53 and by PLK1-targeted drugs
Polo like kinase 1 (PLK1) has many functions in the orchestration of cell division. It is an oncogene and its overexpression is associated with poor prognosis. PLK1 is a cell cycle-dependent protein; its levels are low in the early phases of the cell cycle and increase as cells go through the cycle, peaking in G2/M. In this thesis, our focus has been on two critical areas in PLK1 research. 1. Tumour suppressor p53 mediates transcriptional repression of PLK1. There have been several mechanisms suggested for this repression both by direct and indirect p53 involvement. In light of the recently proposed mechanism suggesting that p53-p21-DREAM-CDE/CHR is responsible for p53 mediated repression of G2/M proteins (including PLK1), we reassessed the event(s) by which p53 downregulates PLK1. While investigating the effect of different p53 stabilising agents on PLK1 levels, different agents were found to give rise to different cell cycle profiles. This could account for differences in the extent of PLK1 downregulation in response to different agents: i.e. that the levels of PLK1 after treatment reflect the phase in which the cycle is arrested. Further investigation showed that p53 mediated repression of PLK1 is partly p21 dependent, consistent with repression occurring (partly) through DREAM. Also, we found that different cells use different or overlapping mechanisms for this repression. Thus While in HCT116 cells, mutation of CDE/CHR elements (through which DREAM acts) abolished the PLK1 repression, U2OS cells were only partly dependent on CDE/CHR elements for this repression. Additionally, Serine 15 phosphorylation was found to be partly required for the repression and only wild type p53 (but not mutated forms of p53 that lack transactivation capacity) could repress PLK1 expression. These data are consistent with the idea that p53 repression is indirect, but do not rule out other mechanisms. 2. PLK1 has been extensively studied as a target for cancer therapeutics. PLK1 inhibition causes arrest in prometaphase and activation of the DNA damage response. Part of this thesis aimed at investigating the DNA damage response (DDR) induced in mitosis by PLK1 inhibitor and the consequences of such inhibition. Also, the results were compared with the results obtained/reported in response to microtubule poison, nocodazole. Our investigations showed some differences in the mitotic arrest-induced DDR observed by PLK1 inhibition and microtubule poisons. PLK1 inhibition resulted in both telomeric and non-telomeric γ-H2AX foci which were not caspase/CAD dependent, whereas nocodazole treatment resulted in caspase dependent DNA damage which was mostly on telomeres. DNA damage sensing protein kinases involved in DDR by PLK1 inhibition were found to be ATR and DNA-PK, whereas for nocodazole DNA-PK was the main kinase involved. Also, in clonogenic survival assays, more surviving colonies were observed in response to PLK1 inhibitor as compared with nocodazole treatment. Interestingly, PLK1 inhibition resulted in recruitment of the 53BP1 (one of the components of the DNA repair pathway) in mitosis which may not be beneficial and suggests further investigations on chromosomal abnormalities that PLK1 inhibition may bring about.
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