CircRHBDD1(4,5)/CCL2 mRNA interaction promotes breast cancer bone metastasis via bone metastatic niche formation

Bone metastasis is a prevalent and severe complication of breast cancer, with limited effective preventive or therapeutic strategies. Recently, circular RNA-targeted therapeutics have shown considerable promise in cancer treatment; yet their application for breast cancer-bone metastases remain infrequent. Comprehending the mechanisms underlying breast cancer-bone metastasis could provide possible targets in the future. The biological characteristics of circRHBDD1(4,5) were validated via Sanger sequencing, RNase R, and actinomycin D treatment, and its expression was further validated using in situ hybridization in breast cancer tissues. The impact of circRHBDD1(4,5) in bone-metastatic niche formation and bone metastasis was investigated utilizing osteoclastogenesis bone resorption pit assays and xenografted tumor models. Additionally, the mechanism underlying circRHBDD1(4,5)-mediated CCL2 mRNA stability was investigated using RNA pull-down, RNA immunoprecipitation, and enzyme-linked immunosorbent assays. CircRHBDD1(4,5) was significantly elevated (P < 0.001) in bone-tropic metastatic breast cancer cells and bone-metastatic breast cancer tissues, which significantly enhanced osteoclastogenesis and induced the formation of bone metastatic niches by upregulating the chemokine CCL2. Mechanistically, circRHBDD1(4,5) directly interacted with CCL2 mRNA, whose expression was significantly positively correlated with immune cell infiltration and breast cancer-bone metastasis, and increased its stability and translation by recruiting the RNA-binding protein ELAVL1. Finally, treatment with locked nucleic acid (LNA) oligonucleotides, which targeted circRHBDD1(4,5) on CCL2 mRNA binding sites, namely LNA-based CMI-blockers, effectively inhibited circRHBDD1(4,5)-induced osteoclastogenesis and breast cancer-bone metastasis. We revealed a credible mechanism for circRHBDD1(4,5)-induced breast cancer-bone metastasis and hypothesized that disrupting circRHBDD1(4,5)-CCL2 mRNA interactions could be a promising strategy for treating breast cancer bone metastases.