PurposeLung cancer is the most common cancer worldwide, and approximately 30% of lung cancer patients will develop brain metastases. Serine/threonine kinase 39 (STK39) plays a significant role in various malignancies. However, the role and mechanism of STK39 in lung cancer brain metastasis have not been reported.MethodsThe expression levels of STK39 in lung cancer cells were detected using quantitative reverse transcription PCR (RT-qPCR) and Western blotting. STK39 expression was knocked down in lung cancer cell lines PC9 and H1299 using RNA interference. Cell proliferation, apoptosis, cell cycle, migration, and invasion abilities were assessed using the CCK-8 assay, colony formation assay, flow cytometry, and Transwell chamber assay, respectively. Phosphoproteomics analysis was performed to identify phosphorylated target proteins of STK39 and associated signaling pathways. PC9 and H1299 cells with knocked-down STK39 were injected into nude mice via the common carotid artery to observe the formation of brain metastases. Finally, RT-qPCR and Western blotting were used to detect the expression of STK39, CPSF4/NF kappa B/COX2, and epithelial-mesenchymal transition (EMT) markers in lung cancer and brain metastasis tissues, and to analyze the correlation between STK39 expression and the size of metastatic tumors.ResultsSTK39 was highly expressed in lung cancer cell lines PC9 and H1299. Knockdown of STK39 inhibited proliferation, migration, and invasion of lung cancer cells, induced apoptosis, and caused cell cycle arrest. Phosphoproteomics and Phos-tag analyses showed that knockdown of STK39 significantly downregulated the expression of phosphorylated CPSF4 protein in PC9 and H1299 cells, along with significant downregulation of NF kappa B, COX2, and EMT markers. Knockdown of STK39 inhibited the formation of brain metastases by PC9 and H1299 cells in nude mice. Lung cancer brain metastasis tissues exhibited high expression of STK39, CPSF4, NF kappa B, and COX2, with their expression levels showing a significant positive correlation with the size of metastatic tumors.ConclusionSTK39 is highly expressed in lung cancer brain metastasis tissues, and knockdown of STK39 significantly inhibits brain metastasis in experimental models, accompanied by the suppression of the CPSF4/NF kappa B/COX2 signaling pathway and EMT process. Therefore, STK39 may be a key factor promoting lung cancer brain metastasis and a potential therapeutic target.