RSPO2/RANKL-LGR4 signaling regulates osteoclastic pre-metastatic niche formation and bone metastasis.

Therapeutics targeting osteoclasts are commonly used treatments for bone metastasis; however, whether and how osteoclasts regulate pre-metastatic niche and bone tropism is largely unknown. In this study, we report that osteoclast precursors (OPs) can function as a pre-metastatic niche component that facilitates breast cancer (BCa) bone metastasis at early stages. At the molecular level, unbiased GPCR ligand/agonist screening in BCa cells suggested that R-spondin 2 (RSPO2) and RANKL, through interacting with their receptor LGR4, promoted osteoclastic pre-metastatic niche formation and enhanced BCa bone metastasis. This was achieved by RSPO2/RANKL-LGR4 signal modulating WNT inhibitor DKK1 through Gαq and β-catenin signaling. DKK1 directly facilitated OP recruitment through suppressing its receptor low-density lipoprotein-related receptors 5 (LRP5) but not LRP6, upregulating Rnasek expression via inhibiting canonical WNT signaling. In clinical samples, RSPO2, LGR4 and DKK1 expression showed positive correlation with BCa bone metastasis. Furthermore, soluble LGR4 extracellular domain (ECD) protein, acting as a decoy receptor for RSPO2 and RANKL, significantly alleviated bone metastasis and osteolytic lesions in mouse bone metastasis model. These findings provide unique insights into the functional role of OPs as key components of pre-metastatic niche for BCa bone metastasis, indicate RSPO2/RANKL-LGR4 signaling as a promising target for inhibiting BCa bone metastasis.