Circ_RUSC2 Sequesters miR-661 and Elevates TUSC2 Expression to Suppress Colorectal Cancer Progression

Despite advancements in diagnostic efficiency, colorectal cancer (CRC) remains a leading cause of cancer-related mortality, with increasing incidence rates. Circular RNA (circRNA) is a closed-loop, generally stable noncoding RNA that functions as a sponge for microRNAs in CRC. The purpose of this study was to investigate the function and underlying mechanism of circ_RUSC2, a new circRNA, in CRC. The expression levels of circ_RUSC2, miR-661, and TUSC2 were assessed using qRT-PCR, Western blot, and immunohistochemistry. Functional assays, including CCK-8, Transwell, and scratch wound healing, were performed to evaluate cell proliferation, migration, and invasion. RNA pull-down and actinomycin D assays were used to study RNA interactions and stability. In both CRC cells and tissues, miR-661 was markedly elevated, while circ_RUSC2 expression was considerably reduced. Poor differentiation, distant metastases, lymph node metastases, and an advanced stage were all strongly correlated with either miR-661 overexpression or circ_RUSC2 downregulation. circ_RUSC2 was more stable compared to its linear RUSC2 mRNA. CRC cell invasion, migration, and proliferation were suppressed by circ_RUSC2 ectopic expression; this inhibitory effect was restored by a miR-661 mimic. Circ_RUSC2 served as miR-661's sponge. TUSC2 counteracted the effects of miR-661, which stimulated CRC cell proliferation, migration, and invasion. At the post-transcriptional level, miR-661 controlled the expression of TUSC2 in CRC cells. In comparison to the negative control, circ_RUSC2 expression was markedly reduced, and its half-life was shortened by methyltransferase-like 3 (METTL3) knockdown. Circ_RUSC2 is a stable cytoplasmic circRNA. Circ_RUSC2 inhibits CRC cell malignant phenotypes via the miR-661/TUSC2 axis. The onset and progression of CRC are linked to the downregulation of Circ_RUSC2. circ_RUSC2 might become more stable through N6-methyladenosine (m6A) methylation regulated by METTL3. According to our research, circ_RUSC2 might be a new biomarker and treatment target for CRC.