AbstractGenetic mutations that initiate and drive the progression of colorectal cancer cause defects in key signalling pathways that affect cell function. The Wnt signalling pathway is a key pathway that is disrupted in colorectal cancer and β-catenin is the key mediator of Wnt signalling. β-catenin is a multifunctional protein with roles in Wnt signalling and cell adhesion. I aimed to understand the relationship between the main interactions and functions of β-catenin. Specifically, I investigated how a stabilising mutation in β-catenin, decreases in APC, E-cadherin or PTEN altered the interactions of β-catenin and cell behaviour.
I established that mutant, non-degradable β-catenin can be sequestered by APC and is less available for binding to and stabilising E-cadherin than wild-type β-catenin, which only transiently interacts with APC. I found that, similar to the β-catenin/E-cadherin protein complex, the β-catenin/APC complex also localises to the plasma membrane and cultured cells and tissue organoids.
I also established a direct role for APC in directed cell migration that was independent of other genetic changes commonly found in colorectal cancer, including loss of PTEN, which on its own increased migration. To compare results from a cell culture to the situation in human tumours, localisation of β-catenin was measured in adenomatous and serrated human colorectal polyps. In all polyps, the predominant location of β-catenin was the plasma membrane, even in polyps with a serrated histology and reduced PTEN signals. However, the changes in E-cadherin observed in cultured cells lacking PTEN were not reproduced in these tumours. I also discovered that changes in transcription did not always mirror changes in protein expression, suggesting a much more complex relationship between the two main functions of β-catenin than expected.
Together, my data reveal that the distribution of β-catenin between its main functions as an adhesion or transcriptional regulator, cannot be predicted by the mutation status of APC or β-catenin alone, but is affected by many other signalling pathways that are frequently dis-regulated in cancer.
|Date of Award
|Cancer Research UK
|Inke Nathke (Supervisor)