Characterization of syntaxin 18 as a novel modulator of radioresistance in non-small-cell lung cancer

Lung cancer is the leading cause of cancer-related death in the world, and non-small cell lung cancers (NSCLC) account for 85% of all lung cancers. Despite recent advances in NSCLC treatment, the 5-year survival rate remains low. Deeper understanding of signaling pathways involved in the hallmarks of lung cancer, such as proliferation, resistance to cell death, immune escape, and metastasis, paved the way towards the development of molecularly targeted therapies. While modern drug therapies of NSCLC aim to target specific oncogenic aberrations or immune escape mechanisms, radiotherapy still is not taking into account the individual tumor biology. In order to identify new targets for “personalized” radiosensitization therapy of NSCLC, our group performed a functional in vitro short hairpin ribonucleic acid (shRNA) screen in NSCLC cells treated with irradiation. Syntaxin 18 (STX18) was one of the candidates emerging from this screen, which was selected for further validation. STX18 is a protein located on the endoplasmic reticulum (ER) membrane and involved in vesicular transport between the Golgi apparatus and ER. The shRNA-mediated suppression of STX18 in human NSCLC cell lines consistently increased apoptosis and reduced clonogenic survival following irradiation. RNA profiling of NSCLC cells with STX18 knockdown identified suppression of genes involved in deoxyribonucleic acid (DNA) replication and repair. Depletion of STX18 associated with decreased phosphorylation of the checkpoint kinase ataxia telangiectasia and Rad3-related (ATR) and its downstream target checkpoint kinase 1 (Chk1) in response to irradiation-induced DNA damage. Accordingly, stabilization of the tumor suppressor protein p53 was impaired. This identifies a role for STX18 in the regulation of the DNA damage checkpoint in lung cancer cells. In addition, a suppression of genes involved in epithelial-mesenchymal transition (EMT) and an increase in epithelial markers was observed upon STX18 knockdown, which was confirmed by protein expression ...