Abstract B26: Long interspersed nuclear element-1 regulates malignant transformation of lung bronchial epithelial cells through epithelial-to-mesenchymal transition

Lung cancer has the highest cancer-related mortality in the United States. Non-small cell lung cancer (NSCLC) is the predominant form of lung cancer. More than 65% of NSCLC patients show cancer progression presenting with locally advanced or metastatic disease. There is an imminent need to identify novel molecular targets that can help improve the precision of current therapies. Long interspersed nuclear element-1 (L1) is an abundant genetic element that mobilizes via retrotransposition using L1-encoded ORF1p and ORF2p proteins. L1 is reactivated by the carcinogen benzo(a)pyrene (BaP) which is a polycyclic aromatic hydrocarbon (PAH) present in tobacco smoke and environment pollutant. L1 reactivation is highly mutagenic by insertion into different locations in the genome. Recent studies have implicated L1 in the onset and progression of lung cancer, but the molecular bases of this response remain largely unknown. Moreover, the genome of lung cancer is one of the most frequently affected by L1 insertions, with reports showing that 50% of NSCLC have L1 ORF1p expression across a panel of different human lung neoplasms. L1 proteins expression correlates with downregulation of differentiation genes and appearance of undifferentiated phenotypes. Epithelial-to-mesenchymal transition (EMT) is a process that promotes undifferentiated phenotypes, loss of cell–cell adhesion and acquisition of cell motility, promoting invasiveness, metastasis and apoptotic resistance. To assess whether L1 induces EMT phenotypes to promote malignant transformation and cancer progression, non-malignant human bronchial epithelial BEAS-2B cells were stably transfected with vectors that constitutively expressed wildtype L1, a mutant counterpart that lacked reverse trascriptase activity and thus was unable to retrotranpose (mutant L1), or empty vector as control. Immunoblotting showed that cells expressing L1 and mutant L1 proteins exhibited increased expression of the mesenchymal markers, N-Cadherin and Snail, coupled with decreased expression of the epithelial marker ZO-1 compared to control cells. The expression of E-Cadherin or Vimentin was unaffected by ectopic expression of L1. Interestingly, Claudin-1 was selectively induced in cells expressing wildtype L1, but not cells expressing the mutant L1 or empty vector. The changes in ZO-1 and N-Cadherin expression, but not Claudin-1, were reversible upon genetic knockdown of L1. Importantly, ectopic expression of wildtype and mutant L1 in BEAS-2B cells enhanced tyrosine kinase inhibitor chemoresistance and induces a tumorigenicity in vivo. We conclude that L1 may contribute to lung carcinogenesis and progression by promoting cell transformation to induce tumorigenecity and changing cell plasticity to induce EMT-like phenotype via retrotransposition-dependent and -independent mechanisms in non-malignant bronchial epithelial. Citation Format: Ivan O. Aispuro, Elsa M. Reyes-Reyes, Minerva SantaCruz, Kenneth S. Ramos. Long interspersed nuclear element-1 regulates malignant transformation of lung bronchial epithelial cells through epithelial-to-mesenchymal transition. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr B26.