The receptor tyrosine kinase Tyro3 as a novel drug target in cancer

Abstract

The TAMs are a subfamily of three homologous receptor tyrosine kinases (RTKs), Tyro3, Axl and MerTK, with their known ligands being two homologous, vitamin K-dependent proteins, Gas6 and protein S (ProS1). The TAMs are known to activate signalling pathways that mediate diverse cellular functions, including cell survival, proliferation, adhesion, phagocytosis and immune regulation. Dysregulation of TAM signalling has been associated with chronic inflammatory, autoimmune conditions and many cancers, across which all three TAMs have been shown to be upregulated. However, relative to Axl and MerTK, little has been studied on Tyro3 to date, including its role in oncogenic signalling. This thesis is based on experimental work using cultured human cancer cell lines to elucidate the role of Tyro3 RTK. These cells were used to determine the ligand efficacy and TAM receptor affinity of ProS1 as compared to Gas6 as well as the putative TAM ligand galectin-3. The intracellular signalling pathway activation and functional effects of Tyro3 activation were determined in both ligand-dependent and -independent manners. A key finding was that the ProS1-Tyro3 interaction is coupled to the pro-proliferative Erk intracellular signalling pathway in human cancer cells, but that Tyro3 also signals to Akt survival signalling in cells where it is the sole TAM receptor. In addition, functional ProS1 protein secreted from cancer cells was able to activate Tyro3 in other cancer cells, indicating an autocrine or paracrine role in tumour progression. Furthermore, experiments using domain swap and point mutant constructs of ProS1 revealed the essential role of the first LG domains of ProS1 and Gas6 for activating their respective TAMs. Finally, a combination of in silico molecular modelling and docking analysis, together with in vitro kinase and cancer cell growth assays, identified three novel compounds that selectively inhibited Tyro3 kinase activity, to be evaluated further for their potential in cancer therapy or as research tools.

Date of Award	1 Sept 2020
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Sassan Hafizi (Supervisor) & Paul Cox (Supervisor)