NSUN2-mediated m5C modification of KDM6B mRNA enhances osteoclast differentiation and promotes breast cancer bone metastasis

Bone metastasis frequently occurs in advanced breast cancer (BCa) and is associated with poor prognosis. The pathogenesis of bone metastasis is driven by cross-communications within the tumor-bone microenvironment. In this study, we demonstrated that NSUN2, a 5-methylcytosine (m⁵C) methyltransferase, is significantly overexpressed in bone metastatic lesions of BCa compared to adjacent non-metastatic bone tissues. Overexpression of NSUN2 promoted osteoclast differentiation and osteolytic bone metastasis, whereas knockdown of NSUN2 had the opposite effects. Mechanistically, NSUN2 facilitates the degradation of lysine demethylase 6B (KDM6B) mRNA through m⁵C modification in a peptidylprolyl isomerase A (PPIA)-dependent manner. Reduced KDM6B leads to hypermethylation of NUMB, disrupting its interaction with the Notch1 intracellular domain (NICD1). Consequently, the Notch signaling pathway is activated, leading to upregulated RANKL expression and accelerated osteoclast differentiation, which contributes to osteolytic bone metastasis. Strikingly, activating KDM6B expression with paricalcitol or inhibiting the Notch pathway with DAPT effectively counteracted NSUN2-induced osteolytic bone metastasis in vivo. Collectively, our findings underscore the pivotal role of the NSUN2-KDM6B-Notch axis in promoting osteoclast differentiation and bone metastasis in BCa, providing potential therapeutic targets for BCa patients with bone metastasis.