Circ_0000079 Decoys the RNA-Binding Protein FXR1 to Interrupt Formation of the FXR1/PRCKI Complex and Decline Their Mediated Cell Invasion and Drug Resistance in NSCLC

Nonsmall cell lung cancer (NSCLC) has gradually become one of the deadliest threats to human health and life worldwide. Although reports have shown that circular RNAs (circRNAs) are associated with progression and metastasis of NSCLC, the biological functions of circRNAs during these processes remain largely unknown. Our study showed that circ_0000079 (CiR79) levels were significantly downregulated in NSCLC patients, especially in cisplatin (DDP)-resistant NSCLC patients, and low circ_0000079 levels were significantly associated with poor overall survival of NSCLC patients. Then, results from Cell Counting Kit-8 (CCK-8) cell viability assay and transwell cell invasion assay in A549/DDP and H460/DDP cells transfected with pCDH-CiR79 expression vector showed that circ_0000079 overexpression significantly inhibited cell proliferation and invasion of these DDP-resistant NSCLC cells. The online bioinformatic program StarBase and RNA-binding protein immunoprecipitation predicted and demonstrated that circ_0000079 could bind with the Fragile X-Related 1 (FXR1) protein rather than with protein kinase C, iota (PRKCI), which was shown to form a complex with FXR1 to promote invasion and growth of NSCLC cells. Co-immunoprecipitation combined with Western blot assays indicated that FXR1 levels were remarkably decreased, but PRKCI levels remained unchanged in pCDH-ciR79 transfected NSCLC cells. Moreover, circ_0000079 negatively regulated FXR1/PRKCI-mediated phosphorylation of glycogen synthesis kinase 3β and activator protein 1, thus suppressing the protein level of the Snail gene, an important promoter gene regulating cancer cell growth and epithelial-mesenchymal transition. Furthermore, DDP resistance of A549/DDP and H460/DDP cells was inhibited by circ_0000079 overexpression but was restored by FXR1. Hence, our findings demonstrated that circ_0000079 might inhibit cell invasion and drug resistance in NSCLC by interrupting the formation of the FXR1/PRCKI complex by interacting with FXR1, and circ_0000079 could act as a potential biomarker and therapeutic target for NSCLC.