circFAM120B functions as a tumor suppressor in esophageal squamous cell carcinoma via the miR-661/PPM1L axis and the PKR/p38 MAPK/EMT pathway

Extensive changes of circRNA expression underscore their essential contributions to multiple hallmarks of cancers; however, their functions and mechanisms of action in esophageal squamous cell carcinoma (ESCC) remain undetermined. Here, we adopted a three-stage approach by first screening for significantly differentially expressed circRNAs in ESCC and performing an external validation study, followed by the functional analyses. The properties of circRNAs were evaluated using Sanger sequencing, RNase R digestion, actinomycin D treatment, subcellular localization analysis, and fluorescence in situ hybridization. Target transcripts were predicted using online tools and verified by dual-luciferase, RNA immunoprecipitation, qRT-PCR, and western blot. Biotin-labeled RNA-protein pull-down, mass spectrometry, and RNA immunoprecipitation were employed to identify proteins interacting with circRNAs. Gain- and loss-of-function experiments were performed to uncover the roles of circRNAs, their target genes, and binding proteins in the proliferation, metastasis, and invasion. We observed that circFAM120B (hsa_circ_0001666) was frequently downregulated in cancer tissues and patient plasma, and its expression level was related to overall survival in ESCC patients. Overexpression of circFAM120B inhibited the proliferation, metastasis, and invasion of ESCC while silencing it enhanced malignant phenotypes. Mechanistically, circFAM120B was predominantly located in the cytoplasm, guarantying its sponging for miR-661 to restore the expression of PPM1L, a tumor suppressor. We observed that circFAM120B could reduce the stability of RNA-dependent protein kinase (PKR) by promoting its ubiquitination-dependent degradation and subsequently regulating the p38 MAPK signaling pathway, resulting in the repression of EMTs in ESCC cells. Our findings suggest that circFAM120B is a promising biomarker of ESCC, which acts as a tumor suppressor via the circFAM120B/miR-661/PPM1L axis and PKR/p38 MAPK/EMT pathway, supporting its significance as a candidate therapeutic target.