CKAP4 is a potential exosomal biomarker and therapeutic target for lung cancer

BACKGROUND: Globally, lung cancer causes the most cancer death. While molecular therapy progress, including epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), has provided remarkable therapeutic effects, some patients remain resistant to these therapies and therefore new target development is required. Cytoskeleton-associated membrane protein 4 (CKAP4) is a receptor of the secretory protein Dickkopf-1 (DKK1) and the binding of DKK1 to CKAP4 promotes tumor growth via Ak strain transforming (AKT) activation. We investigated if CKAP4 functions as a diagnostic biomarker and molecular therapeutic target for lung cancer. METHODS: CKAP4 secretion with exosomes from lung cancer cells and the effect of CKAP4 palmitoylation on its trafficking to the exosomes were examined. Serum CKAP4 levels were measured in mouse xenograft models, and 92 lung cancer patients and age- and sex-matched healthy controls (HCs). The lung cancer tissues were immunohistochemically stained for DKK1 and CKAP4, and their correlation with prognosis and serum CKAP4 levels were investigated. Roles of CKAP4 in the lung cancer cell proliferation were examined, and the effects of the combination of an anti-CKAP4 antibody and osimertinib, a third generation TKI, on anti-tumor activity were tested using in vitro and in vivo experiments. RESULTS: CKAP4 was released from lung cancer cells with exosomes, and its trafficking to exosomes was regulated by palmitoylation. CKAP4 was detected in sera from mice inoculated with lung cancer cells overexpressing CKAP4. In 92 lung cancer patients, positive DKK1 and CKAP4 expression patients showed worse prognoses. Serum CKAP4 positivity was higher in lung cancer patients than in HCs. After surgical operation, serum CKAP4 levels were decreased. CKAP4 overexpression in lung cancer cells promoted in vitro cell proliferation and in vivo subcutaneous tumor growth, which were inhibited by an anti-CKAP4 antibody. Moreover, treatment with this antibody or osimertinib, a third generation TKI, inhibited AKT activity, sphere formation, and xenograft tumor growth in lung cancer cells harboring EGFR mutations and expressing both DKK1 and CKAP4, while their combination showed stronger inhibition. CONCLUSIONS: CKAP4 may represent a novel biomarker and molecular target for lung cancer, and combination therapy with an anti-CKAP4 antibody and osimertinib could provide a new lung cancer therapeutic strategy.