Evaluating the Role of the Receptor Tyrosine Kinase AXL in Bladder Cancer

Introduction & Objectives: There is mounting evidence that expression of the receptor tyrosine kinase, AXL, correlates with poor prognosis in many human cancers. We aimed to evaluate this association in bladder cancer and to examine the in vitro effect of foretinib, an oral inhibitor of VEGF, c-MET and AXL, in bladder cancer cell lines. Material & Methods: Immunohistochemistry was performed on 65 bladder cancer specimens collected from TURBT prior to radical cystectomy (series 1), in addition to a TMA (n=284) comprising bladder cancers of differing stages and grades encompassing both NMIBC and MIBC (series 2). AXL expression was correlated with pathological stage, grade, subsequent need for radical treatment in NMIBC and survival. We used Western blotting and densitometry to evaluate the effects of foretinib on the phosphorylation of signalling molecules downstream of AXL, namely MEK, AKT and MAPK in J82 bladder cancer cells. Results: AXL immunopositivity was associated with invasion in both patient series. In patients undergoing cystectomy, AXL positivity was associated with pre-operatively unsuspected lymph node metastasis. Furthermore, in patients with NMIBC in series 2, AXL expression was associated with an eventual need for cystectomy and with reduced disease-specific survival. Incubation with foretinib (10nM) produced a significant reduction in phosphorylation of MEK and AKT but not MAPK in serum starved J82 cells following stimulation with GAS6, the ligand for AXL. Conclusions: We provide evidence that AXL expression is aligned with the phenotypes of bladder cancers associated with poor outcomes and that AXL signalling can be abrogated with foretinib. Taken together this supports the continued study of AXL inhibitors in bladder cancer.