Background and Aims IL‐6–induced tumor progression has been well established through the induction of antiapoptotic and proliferative genes. However, whether other mechanisms such as IL‐6 regulation of circular RNAs (circRNAs) may also contribute to tumor development remains unknown. Approach and Results High‐throughput RNA sequencing was used to identify the differentially expressed circRNAs on IL‐6 stimulation in intrahepatic cholangiocarcinoma (ICC) cells. CircRNA GGNBP2 (derived from ggnbp2 gene, termed as cGGNBP2) was up‐regulated by IL‐6 treatment in a time and concentration‐dependent manner. The biogenesis of cGGNBP2 was regulated by RNA‐binding protein DEx‐H Box Helicase 9, which was also mediated by IL‐6 exposure. Mass spectrometry and western blotting identified a protein cGGNBP2‐184aa encoded by cGGNBP2. cGGNBP2‐184aa promoted ICC cell proliferation and metastasis in vitro and in vivo. Mechanistically, cGGNBP2‐184aa directly interacted with signal transducers and activators of transduction‐3 (STAT3), phosphorylated STAT3Tyr705, and played a positive regulatory role in modulating IL‐6/STAT3 signaling. IL‐6/cGGNBP2‐184aa/STAT3 formed a positive feedback loop to sustain constitutive activation of IL‐6/STAT3 signaling. Elevated cGGNBP2 expression was correlated with poor prognosis of patients with ICC and was identified as an independent risk factor for patient prognosis. Conclusions Our study demonstrates that cGGNBP2‐184aa, a protein encoded by IL‐6–induced cGGNBP2, formed a positive feedback loop to facilitate ICC progression and may serve as an auxiliary target for clinical IL‐6/STAT3‐targeting treatments in ICC.

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Background Considerable evidence shows that circular RNAs (circRNAs) play an important role in tumor development. However, their function in intrahepatic cholangiocarcinoma (ICC) metastasis and the underlying mechanisms are incompletely understood. Methods circNFIB (hsa_circ_0086376, termed as cNFIB hereafter) was identified in human ICC tissues through circRNAs sequencing. The biological role of cNFIB was determined in vitro and in vivo by gain or loss of functional experiments. Fluorescence in situ hybridization (FISH), RNA immunoprecipitation (RIP) and RNA pull-down assays were conducted to analyze the interaction of cNFIB with dual specificity mitogen-activated protein kinase kinase1 (MEK1). Duolink in situ proximity ligation assay (PLA) and coimmunoprecipitation (co-IP) assay were used to investigate the effects of cNFIB on the interaction between MEK1 and mitogen-activated protein kinase 2 (ERK2). Finally, a series of in vitro and in vivo experiments were performed to explore the influences of cNFIB on the anti-tumor activity of trametinib (a MEK inhibitor). Results cNFIB was significantly down-regulated in human ICC tissues with postoperative metastases. The loss of cNFIB was highly associated with aggressive characteristics and predicted unfavorable prognosis in ICC patients. Functional studies revealed that cNFIB inhibited the proliferation and metastasis of ICC cells in vitro and in vivo. Mechanistically, cNFIB competitively interacted with MEK1, which induced the dissociation between MEK1 and ERK2, thereby resulting in the suppression of ERK signaling and tumor metastasis. Moreover, we found that ICC cells with high levels of cNFIB held the potential to delay the trametinib resistance. Consistently, in vivo and in vitro studies demonstrated that cotreatment with trametinib and lentivirus vector encoding cNFIB showed greater inhibitory effect than isolated trametinib treatment. Conclusions Our findings identified that cNFIB played a key role in ICC growth and metastasis by regulating MEK1/ERK signaling. Given the efficacy of cNFIB modulation on ICC suppression and trametinib sensitivity, cNFIB appears to be a potential therapeutic molecule for ICC treatment.

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