The endosteal niche regulates breast cancer cell dormancy in bone:identification of new molecular determinantsOA

Cellular dormancy compromises the long-term survival of breast cancer patients.Bone represents a frequent site for metastasis,where Spindle-shaped N-cadherin+CD45−osteoblasts(SNOs)hold dormant metastatic cells in the endosteal niche with a Notch2-dependent mechanism.In this work,we excluded the involvement of Notch1 in SNO-induced breast cancer cellular dormancy by immunofluorescence/immunohistochemistry and molecular approaches,using breast cancer tissues and cell lines.RNAdSeq in human bone metastatic MDA-MB231 breast cancer cells sorted for Notch1^(HIGH)and Notch2^(HIGH)expression demonstrated that,compared to their low counterpart,only Notch2^(HIGH)cells expressed enriched pathways relevant for the metastatic process,including pluripotency and Hematopoietic Stem Cell(HSC)gene signatures.They expressed the HSC-associated genes CXCR4,CD34 and TIE2 and MDA-MB231 cells enriched in the encoded proteins showed lesser proliferation ability.Reduced incidence of osteolytic lesions was induced by CXCR4^(HIGH)cells intratibially injected in immunocompromised mice,while lower lesion extension was induced by CXCR4^(HIGH)and TIE2^(HIGH)injected cells compared to CXCR4^(LOW)and TIE2^(LOW)cells.Notch2^(HIGH)cells were enriched in endoplasmic reticulum stress and unfolded protein response genes and overexpressed the CD177 protein,while the CD177 ligands,Plaur,Itgam and Ceacam 1,were highly expressed in SNOs.Kaplan-Meier plots showed positive correlation between high expression of CD177,ITGAM and CEACAM 1-but not PLAUR-and overall survival of patients.CD177^(HIGH)cells were also CXCR4^(HIGH),CD34^(HIGH)and Notch2^(HIGH)and proliferated less than CD177^(LOW)cells.These results support the relevance of Notch2 in SNOmediated cellular dormancy and identified new pathways implicated in bone metastatic breast cancer cell quiescence.