Objective: Chemoresistance is the leading cause of recurrence in non-small cell lung cancer (NSCLC). The long non-coding RNA (lncRNA) cancer susceptibility candidate 2 (CASC2) inhibits the tumorigenesis of various cancers. However, the regulatory function of CASC2 on the chemoresistance of NSCLC remains unclear.
Patients and methods: The levels of CASC2 and miR-18a in cisplatin (DDP)-resistant NSCLC tissues and cell lines were evaluated by quantitative Polymerase Chain Reaction (qPCR). The role of low CASC2 levels on overall survival in patients with NSCLC was tested using the log-rank test. The Chi-squared test was used to assess the relation between CASC2 expression and clinicopathological features of NSCLC patients. Cell Counting Kit-8 (CCK-8) assays tested the cell proliferation of cisplatin-resistant NSCLC cells (H226/DDP and A549/DDP). The underlying regulatory mechanism between CASC2 and miR-18a or miR-18a and interferon regulatory factor 2 (IRF-2) was predicted by bioinformatics and verified by a Dual-Luciferase reporter assay, RNA transfection, qPCR, and Western blotting. Chromatin immunoprecipitation (ChIP) assay was done to exam the relation between E74 like factor 1 (ELF1) and CASC2 gene. Mice xenografts were applied to exam the function of CASC2 on chemosensitivity of cisplatin of NSCLC cells in vivo.
Results: Low CASC2 expression is more likely to present in patients with advanced TNM stage (IV), cisplatin-resistance, and poor overall survival. The expression of CASC2 sharply decreased in cisplatin-resistant NSCLC tissues and cell lines (H226/DDP and A549/DDP). CASC2 overexpression strongly inhibited proliferation, migration, and invasion of cisplatin-resistant NSCLC cells (H226/DDP and A549/DDP) in vitro and inhibited tumor growth in vivo. Besides, CASC2 repressed miR-18a function by binding to the complementary sites of miR-18a as competing endogenous RNAs (ceRNAs). MiR-18a released by the declining expression of CASC2 reduced the protein concentration of IRF-2 in NSCLC cells. Furthermore, the transcription factor ELF1 was found to be promotor of CASC2 and increased its levels in cisplatin-resistant NSCLC cells.
Conclusions: IRF-2 expression mediated by the ELF1/CASC2/miR-18a axis is markedly associated with the proliferation, migration, and invasion of cisplatin-resistant NSCLC, resulting in inferior survival. These findings suggest that this regulatory axis may serve as a novel therapeutic target in NSCLC.