USP33 Promotes Lung Adenocarcinoma Brain Metastasis by Inhibiting the K48-Linked Ubiquitination and Degradation of S100A9 and Facilitating Vimentin Secretion.

Brain metastasis has a deterministic influence on the evolutionary trajectory of lung adenocarcinoma (LUAD). Given the profound clinical importance of LUAD brain metastases and the formidable obstacles that hinder their precise prediction, novel therapeutic targets are urgently needed. Notably, a substantial fraction of lung carcinoma cells in brain metastatic foci demonstrates marked S100A9 overexpression. However, whether these cells represent the primary drivers of brain metastasis requires further elucidation. Coculture of S100A9-overexpressing LUAD cells with human brain microvascular endothelial cells (HBMECs) markedly suppressed the expression of the tight junction components ZO-1, Occludin, and Claudin-5 within the endothelial barrier. These cocultured HBMECs also exhibited compromised angiogenic potential, elevated levels of reactive oxygen species, and increased endothelial-to-mesenchymal transition. Additionally, these cells displayed attenuated proliferative and migratory capacities and mitochondrial membrane potential depolarization along with increased autophagy characterized by increased lysosomal acidification. Conversely, LUAD cells with elevated S100A9 expression exhibited increased proliferative, migratory, and invasive capacities and increased tumorigenicity and potential for brain metastasis. In LUAD cell lines, USP33 stabilizes S100A9 via K48-linked deubiquitination, which promotes the expression and extracellular secretion of vimentin. LUAD cells reduce blood‒brain barrier integrity, increase permeability, and disrupt the intra- and extracellular functions of HBMECs, thus promoting parenchymal infiltration and the establishment of metastatic lesions.