CircFN1 promotes invasion of papillary thyroid carcinoma cells by activating the PI3K/AKT pathway via the miR-29a/TRIB2 axis.

BACKGROUND: Circular RNAs regulate cancer progression, but their mechanisms in papillary thyroid carcinoma (PTC) invasion remain incompletely defined. This study investigated circFN1 expression and function and examined whether circFN1 promotes PTC invasion through microRNA-29a (miR-29a)/tribbles pseudokinase 2 (TRIB2)-mediated PI3K/AKT activation. METHODS: The abundance of TRIB2 and p-AKT in paired PTC and adjacent non-cancerous tissues was first assessed by immunohistochemistry (IHC), and circFN1, miR-29a, TRIB2, p-PI3K and p-AKT were further evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting. In TPC-1 cells, circFN1 overexpression/knockdown, miR-29a overexpression/inhibition and TRIB2 overexpression models were established. Cell proliferation, colony formation, apoptosis, migration and invasion were assessed. Dual-luciferase reporter and RNA immunoprecipitation (RIP) assays were used to validate miR-29a/TRIB2 targeting and circFN1-miR-29a association. The phosphorylation status of crucial proteins within the PI3K/AKT signaling cascade (p-PI3K, p-AKT) was detected by qRT-PCR and Western Blot to confirm the activation status of this signaling axis. RESULTS: Examination of clinical specimens indicated that the abundance of TRIB2 and p-AKT showed significant upregulation when compared with adjacent normal tissues. Additional paired-tissue validation further showed increased circFN1 and TRIB2 expression, decreased miR-29a expression, and enhanced PI3K/AKT phosphorylation in PTC tissues. In vitro functional experiments demonstrated that elevated expression of circFN1 markedly enhanced the growth, migration, and invasion of PTC cells while concurrently suppressing their apoptosis, whereas knockdown of circFN1 produced contrary results. Mechanistic studies revealed that circFN1 could function as a competitive endogenous RNA (ceRNA) for miR-29a, negatively regulating its expression through a sponging effect. The newly added RIP-based assay evidence supported the direct association between circFN1 and miR-29a. The dual-luciferase reporter assay further confirmed that TRIB2 is a direct target of miR-29a. TRIB2 overexpression was sufficient to increase PTC cell migration and invasion and to enhance p-PI3K/PI3K and p-AKT/AKT ratios. Consequently, circFN1 competitively binds to miR-29a, thereby relieving the inhibition of its downstream target gene TRIB2, leading to an upregulation of TRIB2 protein expression. Ultimately, the upregulation of TRIB2 activated the PI3K/AKT signaling pathway, as demonstrated by a substantial rise in the phosphorylation status of p-PI3K and p-AKT. CONCLUSIONS: circFN1 may promote PTC invasion through the miR-29a/TRIB2/PI3K/AKT axis. This axis represents a potential biomarker and therapeutic target for assessing invasion risk in PTC.