The Interplay Between Cks, Pin1 and P27 in Tumourigenesis and Neuronal Development

This dissertation, "The Interplay Between CKS, PIN1 and P27 in Tumourigenesis and Neuronal Development" by Yiu-ming, Ng, 伍耀明, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled "The interplay between CKS, PIN1 and p27 in tumourigenesis and neuronal development" Submitted by NG Yiu Ming For the degree of Doctor of Philosophy at The University of Hong Kong in August 2015 Cyclin-Dependent Kinases Regulatory Subunit 1 and 2 (CKS1 and CKS2) and Peptidyl-Prolyl Cis-Trans Isomerase 1 (PIN1) are important cell-cycle regulatory proteins. CKS1 binds to cyclin-dependent kinases (CDKs) and other phospho-substrates via its anion-binding pocket. CKS1 also enhances the ability of CDKs binding to specific phosphorylated motifs. For example, CKS1 is an Kip1 essential adaptor for degrading p27 protein (p27) through ubiquitin proteasomal system by promoting its interaction with SCF-Skp2 E3 ligase. On the other hand, the functional role of CKS2 is less characterised. PIN1 binds to proteins with proline-directed phosphorylation and catalyses cis-trans isomerization of the peptide bond between the phosphorylated serine/threonine and proline. PIN1 induces conformational changes and regulates the function of its substrates, including many of the downstream targets of CDKs such as p27. In this dissertation, the effect of competitive binding and opposing actions of CKS proteins and PIN1 on p27 and the subsequent functional consequences in cell cycle regulation, neurodevelopment and tumorigenesis had been examined. -/- From the analysis of Cks2 knock-out (Cks2 ) mice, CKS2 was found to counteract the effect of CKS1, resulting in p27 accumulation. The low level of -/- p27 in Cks2 background shortened cell cycle and activation of CDK2-cyclin A. Interestingly, inhibition of PIN1 activity partly reversed the replicative DNA damage associated with the loss of CKS2. Therefore, PIN1 activity contributed to -/- an increased in CDK2-cyclin A activity and DNA damage was observed in Cks2 cells. One potential explanation was that PIN1 bound with p27 and decreased the inhibitory activity of p27 on CDK2-cyclin A. CKS1 was found to be expressed in post-mitotic neurons in adult Cks1 knock-out -/- -/- (Cks1 ) mice hippocampus. Interestingly, Cks1 mice exhibited poor memory consolidation, and diminished maintenance of long-term potentiation in the hippocampal circuits. Furthermore, neuronal accumulation of cofilin aggregates was found, which were associated with defective dendritic spine maturation and synaptic loss characteristic of some neurodegenerative diseases. The increased -/- p27 in Cks1 neuronal cells activated cofilin by suppressing the RhoA kinase-mediated inhibitory cofilin phosphorylation, resulting in cofilin aggregates formation. As PIN1 was known to compete with CKS1 for binding with p27, inhibition of PIN1 diminished the cofilin aggregates formation through decreasing p27 level, and thereby activated RhoA and increased cofilin phosphorylation. Therefore, CKS1 and PIN1 also competed for p27 binding in neuronal cells, which in turn regulated RhoA activity and cofilin comformation. Since there was an increased CDK2-cyclin A/E and RhoA activities in cancers, experiments were designed to investigate whether PIN1 modulates the p27-RhoA axis and CDK2-cyclin A/E activities in hepatocellular carcinoma (HCC) cells. Knocked down Pin1 in HCC cells decreased p27 level and activated RhoA. Moreover, with an overall decreased in p27 level, more p27 demonstrated to be bound to CDK2-cyclin A/E complex, causing an inhibition in CDK2-cyclin A/E a