AXL signaling pathway: biology and therapeutic perspectives for bladder cancer

Abstract

Bladder cancer is one of the most common cancers in the world, and the overall survival rate for patients with metastatic bladder cancer is low. One of the main risk factors of bladder cancer is age, which makes it a growing concern in developed countries with a high proportion of the elderly population. Also, Schistosoma haematobium-infected individuals are at high risk of bladder cancer development as a complication of schistosomiasis in countries across Africa and the Middle East. Therefore, the demand for more efficient treatment approaches for bladder cancer, especially its advanced forms, is high, and there is an unmet need to find new targets for therapies against bladder cancer. AXL is a TAM family receptor tyrosine kinase that is overexpressed in various cancers. AXL signalling is important for most of the cancer hallmarks, such as proliferation, survival, invasion, epithelial-mesenchymal transition, and angiogenesis. According to the analysis of publicly available databases, high expression of AXL is associated with poor survival of bladder cancer patients. The aim of the work was to characterize the molecular effects of AXL inhibition by its small molecule inhibitors. The experiments primarily focused on the type 1 small molecule RTK inhibitor TP-0903, which is currently undergoing clinical trials for the treatment of various solid cancers. The obtained results show that AXL and its ligand GAS6 are co-expressed in bladder cancer cell lines with a quasi-mesenchymal or mesenchymal phenotype. TP-0903 prevents ligand-dependent AXL phosphorylation, while its prolonged application results in GAS6-independent non-canonical phosphorylation of AXL. In contrast, type 2 small molecule RTK inhibitor LDC1267 has no such effect. I found that extended treatment of cells with TP-0903 increases the level of ROS in bladder cancer cells, and TP-0903-induced AXL phosphorylation within the kinase domain at Y702 and Y703 is ROS-dependent. In contrast, AXL phosphorylation at Y779, a tyrosine residue responsible for interaction with some adaptor proteins, is ROS-independent and can occur due to AXL heterodimerization with other RTK, such as ERBB2. Also, I show that TP-0903 has a dramatic effect on the kinome in bladder cancer cells. Overall, the results of this work demonstrate that the type 1 small molecule RTK inhibitor TP-0903 induces AXL phosphorylation that is independent of GAS6. This unexpected effect relies on ROS. Consequently, the application of TP-0903 rewires signalling in bladder cancer cells, and this finding should be taken into consideration when AXL-targeted therapeutics are applied in clinical settings.