The silencing of the L1-MET chimeric transcript activates cancer cell death program and inhibits the expression of crucial oncoproteins in lung cancer cells

Abstract

Background Among the antisense chimeric sequences generated by intragenic long interspersed nuclear elements (LINE1s), L1-MET transcript, within MET oncogene, is of particular interest since its expression, activated by promoter hypomethylation, has been associated with the acquisition of cancer phenotype. L1-MET can originate several isoforms, but it is unable to form stable proteins. Presently, its biological functions remain unknown. Methods To investigate the role of L1-MET, we silenced its expression on selected lung and breast cancer cells, characterized by variable levels of L1-MET and MET mRNA, using specifically-modified targeting antisense oligonucleotides. In addition to viability and apoptotic rate, the transfected cells were compared for their gene expression profiles and the protein level of identified downregulated cancer genes. Results Besides a considerable decrease of cell viability and increase of apoptosis, transiently transfected cancer cells partly rewrote their gene expression profiles, with an effect related to the L1-MET/ MET mRNA level and the type of cells, being particularly strong in lung cancer cells. In particular, MET and EGFR genes, activated in EBC1 lung cancer cells, but at the steady-state level in the other tested cell lines, showed a significant downregulation of MET and EGFR oncoproteins, with a specific loss of the AKT phosphorylation and a decrease of phospho-ERK, in the case of EBC1 cells. No effects were evidenced in non-transformed fibroblasts and human lymphocytes, used as controls. Conclusions Our results clearly demonstrate the ability of L1-MET to interfere with the expression of MET and EGFR oncoproteins in selected cancer cells. The expression of L1-MET, strictly limited to transformed cells, makes its silencing an ideal approach to induce tumor cells to death and a potential inhibitor of crucial oncoproteins on which cancer cells rely for their survival and proliferation, such as lung cancer cells.