Circular RNA circIKBKB promotes breast cancer bone metastasis through sustaining NF-\u03baB/bone remodeling factors signaling

Yingru Xu,Suwen Chen,Meisongzhu Yang,Libing Song,Miaoling Tang,Man Li,Ziwen Li,Ruyuan Yu,Jun Li,Xingui Wu,Xincheng Li,Shuxia Zhang,Mudan Huang,Xinyi Liao,Yameng Hu

doi:10.1186/s12943-021-01394-8

Abstract

BackgroundBreast cancer (BC) has a marked tendency to spread to the bone, resulting in significant skeletal complications and mortality. Recently, circular RNAs (circRNAs) have been reported to contribute to cancer initiation and progression. However, the function and mechanism of circRNAs in BC bone metastasis (BC-BM) remain largely unknown.MethodsBone-metastatic circRNAs were screened using circRNAs deep sequencing and validated using in situ hybridization in BC tissues with or without bone metastasis. The role of circIKBKB in inducing bone pre-metastatic niche formation and bone metastasis was determined using osteoclastogenesis, immunofluorescence and bone resorption pit assays. The mechanism underlying circIKBKB-mediated activation of NF-κB/bone remodeling factors signaling and EIF4A3-induced circIKBKB were investigated using RNA pull-down, luciferase reporter, chromatin isolation by RNA purification and enzyme-linked immunosorbent assays.ResultsWe identified that a novel circRNA, circIKBKB, was upregulated significantly in bone-metastatic BC tissues. Overexpressing circIKBKB enhanced the capability of BC cells to induce formation of bone pre-metastatic niche dramatically by promoting osteoclastogenesis in vivo and in vitro. Mechanically, circIKBKB activated NF-κB pathway via promoting IKKβ-mediated IκBα phosphorylation, inhibiting IκBα feedback loop and facilitating NF-κB to the promoters of multiple bone remodeling factors. Moreover, EIF4A3, acted acting as a pre-mRNA splicing factor, promoted cyclization of circIKBKB by directly binding to the circIKBKB flanking region. Importantly, treatment with inhibitor eIF4A3-IN-2 reduced circIKBKB expression and inhibited breast cancer bone metastasis effectively.ConclusionWe revealed a plausible mechanism for circIKBKB-mediated NF-κB hyperactivation in bone-metastatic BC, which might represent a potential strategy to treat breast cancer bone metastasis.

Highlights

Breast cancer (BC) has a marked tendency to spread to the bone, resulting in significant skeletal complications and mortality
Overexpression of circIKBKB induces osteolytic bone metastasis of breast cancer To identify the critical circRNAs that contribute to BC bone metastasis (BC-BM), circRNA deep-sequencing was performed in six primary BC tissues without BM and six bone-metastatic BC tissues (BM/BC) (Fig. 1a)
Comparative analysis revealed that a total of 214 circRNAs were dysregulated, including 163 significantly upregulated and 51 downregulated circRNAs, in BM/BC tissues compared with non-BM/BC tissues (Fig. 1a and Table S1)

Summary

Introduction

Breast cancer (BC) has a marked tendency to spread to the bone, resulting in significant skeletal complications and mortality. The function and mechanism of circRNAs in BC bone metastasis (BC-BM) remain largely unknown. The clinical interventions for patients with BCbone metastasis (BC-BM) are bisphosphonates (BPs) or denosumab (an anti-receptor activator of nuclear factor kappa B ligand (RANKL) monoclonal antibody), which disrupt the vicious cycle by targeting osteoclasts [6]. These drugs have no effect on the patient survival rate and cause severe side effects, even leading to BC metastasis to visceral organs [7,8,9]. Understanding the mechanisms of BC-BM is essential to develop innovative therapeutic strategies and improve of patient outcomes

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