Tumor Exosomal L1 Cell Adhesion Molecule Promotes Brain Metastasis of Lung Cancer

Brain metastasis (BrM) is a common occurrence in lung cancer and substantially worsens the prognosis due to the blood–brain barrier (BBB), which restricts drug entry into the brain. Here, we found that exosomes secreted by lung cancer cells that had acquired epidermal growth factor receptor tyrosine kinase inhibitor resistance and undergone epithelial–mesenchymal transition (osimertinib- and WZ4002-resistant H1975) exhibited enhanced brain-specific distribution and a concomitant increase in BrM compared with exosomes from parental H1975 cells. To identify exosomal mediators of this phenotype, liquid chromatography-tandem mass spectrometry-based proteomic analysis was performed. Exosomes derived from resistant cell lines exhibited distinct protein profiles relative to parental cells, with 744 exosomal proteins significantly altered (fold change ≥ 2; P ≤ 0.05). Prioritization of membrane proteins and ligand–receptor interaction analysis identified ITGAV (integrin αV), ITGB3 (intergrin β3), and L1CAM (L1 cell adhesion molecule) as candidates interacting with brain-specific ligands, including neural cell adhesion molecule 1 (NCAM1) and contactin 2. Validation of exosomal association by Western blotting identified ITGB3 and L1CAM as final candidates. Subsequent functional modulation studies demonstrated that exosomal L1CAM plays a dominant role in brain distribution and metastatic progression. Exosomal L1CAM increased BBB permeability by disrupting endothelial tight-junction integrity both in vitro and in vivo. This effect was associated with the involvement of NCAM1 on BBB endothelial cells, as suggested by an exosomal L1CAM masking experiment. Clinically, exosomal L1CAM demonstrated diagnostic potential for BrM (area under the curve [AUC] = 0.80), and a combined exosomal L1CAM/ITGB3 panel significantly improved diagnostic accuracy (AUC = 0.98). Collectively, these findings identify exosomal L1CAM as a key regulator of brain-specific metastasis and support the clinical utility of the L1CAM/ITGB3 panel as a noninvasive biomarker for BrM in lung cancer.