Proteolytic processing of TAM receptors: implications for the biology of bladder cancer and use as urine biomarkers

Faculty-development competitive research grants program for 2023-2025

TAM (TYRO3, AXL, MER) family of receptor tyrosine kinases (RTKs) play an important role in inflammatory responses. They are activated by two homologous vitamin K-dependent ligands, growth arrest specific gene 6 (GAS6) and the anti-coagulation factor Protein S (PROS1). TAM receptors are expressed in professional phagocytes and drive efferocytosis, a process of clearance of apoptotic cells. Efferocytosis is initiated by the activation of TAM receptors by ligands bound to the surface of dead cells leading to the cytoskeletal reorganization, formation of cytoplasmic protrusions, engulfment and consumption of apoptotic cells. At the same time, activation of TAM signalling stimulates PI3K pathway, which is required for the survival of myeloid cells in toxic environment. TAM pathways are often hijacked by cancer cells and utilized to activate cancer cell motility, processes of epithelial-mesenchymal transition (EMT) and to cause drug resistance.
TAM receptors undergo posttranslational proteolytic cleavage leading to the formation of soluble extracellular forms and nuclear accumulation of C-terminal portions of the proteins. Soluble TAM proteins act as decoy receptors, they bind the ligands leading to the dampening of TAM signalling. Genetic ablation of TAM receptors in mice results in pathologies closely resembling autoimmune disorders in humans. Expectedly, elevated levels of soluble TAM receptors were detected in biological fluids of patients suffering from various autoimmune conditions. Although this seems counterintuitive, presence of soluble TAM receptors in the blood of cancer patients is associated with the most aggressive diseases. Here, we present a model aiming to explain this controversy. We propose that ectodomain shedding occurs after receptors are activated by the ligands leading to a switch from the canonical TAM signalling to nonconventional pathways, which are dependent on heterodimerization between TAM receptor membranous fragments and other RTKs. This oncogenic switch takes place in malignant cells only, where the concentration of RTKs is high. We also propose that while some TAM isoforms undergo proteolytic cleavage, some others do not. We hypothesize that EMT-driven reprogramming of RNA splicing controls TAM ectodomain shedding and oncogenic switch of TAM signalling.
We found that TAM receptors are highly expressed in bladder cancer, and soluble and nuclear TAM fragments are detectable in bladder cancer cells in vitro and in vivo. We propose to test our hypotheses explaining oncogenic function and regulation of TAM receptors in bladder cancer cell lines and patients-derived tumour organoids. We will study if soluble TAM receptors can be detected in urine and used as non-invasive biomarkers of bladder cancer. The proposed work is important for Kazakhstan, a country with progressively aging population. As bladder cancer is diagnosed in people with an average age of 73 years, it is anticipated that the incidence rates of this cancer type will be constantly increasing in this country.