Overexpression of Circ_0004496 is Associated with Pathological Bone Formation in Ankylosing Spondylitis via the miR-145/ACTG1 Axis

Background: The functional role of circular ribonucleic acids in ankylosing spondylitis (AS) remains poorly understood. In our previous study, circ_0004496 and actin gamma 1 (ACTG1) were found to be upregulated in ossified tissues from patients with AS. Aims: To investiagate the regulatory role of circ_0004496 in pathological bone formation through the miR-145/ACTG1 axis. Study Design: Combined in vitro and in vivo experimental study. Methods: Tissue samples were obtained from 15 patients with AS and hip ankylosis and 15 control patients with femoral neck fractures. Quantitative real-time polymerase chain reaction was performed to measure circ_0004496 and miR-145 expression. Western blot analysis was used to determine the protein levels of alkaline phosphatase (ALP), osteocalcin (OCN), runt-related transcription factor 2 (Runx2), and ACTG1. Cell proliferation was evaluated using Cell Counting Kit-8 and 5-ethynyl-2′-deoxyuridine assays. Osteogenic differentiation was assessed by measuring ALP activity and performing Alizarin Red S staining. Interactions among circ_0004496, miR-145, and ACTG1 were examined using RNA immunoprecipitation (RIP), RNA pull-down, dual-luciferase reporter, and fluorescence in situ hybridization (FISH) assays. Results: Circ_0004496 and ACTG1 expression levels were significantly elevated in AS hip capsule tissues, whereas miR-145 expression was reduced. Overexpression of circ_0004496 in AS-derived fibroblasts enhanced cell proliferation, accelerated cell cycle progression, increased calcium deposition, and upregulated ALP, OCN, and Runx2 protein levels. Mechanistically, circ_0004496 functioned as a molecular sponge for miR-145, which directly targets ACTG1. These regulatory interactions were confirmed by dual-luciferase reporter, RIP, RNA pulldown, and FISH assays. In vivo, circ_0004496 overexpression was associated with sacroiliac joint fusion in proteoglycan-induced arthritis mice. Conclusion: Circ_0004496 overexpression promotes pathological bone formation in AS by regulating the miR-145/ACTG1 axis both in vitro and in vivo.