Identification of novel Focal Adhesion Kinase binding partners and their biological functions in cancer cells
Focal adhesion kinase (FAK) is a non-receptor protein tyrosine kinase that localises to focal adhesions. FAK is crucial for many cellular processes that are disturbed in malignancy, including proliferation, cell cycle, cell survival, adhesion, and migration. Mouse models have shown that FAK is involved in tumour formation and progression. Other studies demonstrated a functional correlation between FAK expression, tumour progression and malignancy in human cancer, making FAK a potentially important therapeutic target. Several FAK inhibitors have been developed most of which target the FAK kinase function. However, FAK may predominantly act as a scaffolding molecule rather than as a kinase, therefore, disruption of FAK’s interaction with protein binding partners could be a good strategy to inhibit some cancer processes. The identification and characterisation of novel FAK interactions may help to uncover important molecular mechanisms that, in turn, regulate key cellular processes involved in tumour formation and/or progression. Disruption of their function, or inhibition of their binding to FAK, will define their roles and identify whether they are good anti-cancer targets. In this thesis work, I set out to identify novel binding partners of FAK, and study the role of a sub-set of these in tumour biology by impairing them in squamous cell carcinoma cancer cells in vitro. To do this I employed protein microarray and phage display methodologies using FAKΔ375 and FAK-FERM recombinant proteins as bait, respectively. I identified a number of novel proteins that interact directly with FAK. Then I set out to characterise some of these proteins. The first of these, Axl, is a protein receptor tyrosine kinase that has previously been linked with tumour progression and metastasis in number of human cancers. I confirmed the interaction between FAK and Axl in SCC cells and showed that the FAK-Axl interaction is predominantly a scaffolding function of FAK, which seems to be unregulated, at least by any of the major phosphorylation events characterised for FAK. I also found that Axl controls cell spreading, cell polarisation and invasive migration in this cancer cell lines. The second protein I characterise is the autophagy protein Ambra1. I found that Ambra1 is required for selective targeting of active Src to the autophagy pathway – a process that SCC cancer cells use when they are under adhesion stress, such as when FAK is deleted. Thus, Axl and Ambra1 are potentially important proteins in SCC biology. They bind to FAK and function at cell adhesions to promote cancer-associated cellular processes. Analysis of FAK binding proteins may be a useful strategy to discover proteins that function in various aspects of cancer cell behaviour.