RTK AXL and its Isoforms: Regulation and Implications in Cancer

As a member of the TAM family of receptor kinases, the AXL protein plays an essential role in biological processes that maintain tissue homeostasis. Deregulated AXL signalling in tumour cells is linked to cancer progression, poor prognosis, metastasis, and reduced sensitivity to anti-cancer therapies. The underlying mechanisms are the activation of downstream signalling routes that promote cell survival, invasion and epithelial-mesenchymal transition. Two major AXL isoforms are expressed in human and rodent cells due to alternative splicing. Despite extensive research on AXL in cancer, little is known regarding the functional differences between these isoforms and whether they contribute to cancer differently. This review paper first outlines the structural and functional aspects of TAM biology with a particular focus on AXL. Next, we discuss the different levels of AXL regulation in tumour cells, including proteolytic cleavage, which leads to the formation of both extracellular and nuclear forms of AXL. Finally, we review articles investigating the variations in the function of AXL isoforms and report their associations with cancer. Notably, the formation of isoform 1 is likely to determine the presence of soluble AXL, elevated levels of which have been correlated with cancer progression in several tumour types. The review identifies areas deserving further investigation, such as how changes in isoform expression impact levels of soluble AXL in cancer. Additionally, isoform-specific downstream signalling effects and their impact on metastasis and drug resistance warrant more in-depth investigation.