Overexpression of CD9 in Human Breast Cancer Cells Promotes the Development of Bone Metastases

Background: Bone is a preferred target for circulating metastatic breast cancer cells. We found that the CD9 protein was up-regulated in the B02 osteotropic cell line, derived from the aggressive parental MDA-MB-231 breast cancer cell line. Here, we investigated the putative relationship between CD9 expression and the osteotropic phenotype. Materials and Methods: Overexpression of CD9 was analyzed by immunoblotting in different cell lines. Immunohistochemistry was used to assess CD9 expression in primary tumors and metastatic lesions. In vivo experiments were conducted in mice using a monoclonal antibody against CD9. Results: CD9 overexpression was confirmed in osteotropic cells. CD9 was significantly overexpressed in bone metastases versus primary tumors and visceral metastatic lesions. Finally, in vivo experiments showed that an antibody against CD9 delays homing of B02 cells in bone marrow, slowing down bone destruction. Conclusion: Our study reveals a potential implication of CD9 in the formation of bony metastases from breast cancer cells. Disseminated metastases represent the main cause of morbidity and mortality for patients with cancer. The skeleton is a preferred site for the seeding and growth of metastases originating from tumors identified as osteotropic, such as breast, prostate and lung cancer (1). The majority of patients with breast cancer at an advanced, stage experience bone metastasis (2). These secondary lesions are associated with substantial morbidity, including bone pain, pathological fracture, hypercalcemia, myelopathy and spinal cord compression (3). In this context, identifying the proteins involved in the acquisition of an osteotropic phenotype by cancer cells would represent a major step towards the development of both new prognostic markers and therapeutic improvements. Cell membrane proteins represent the most likely key actors in the specific tropism of circulating cancer cells. Indeed, membrane proteins control cell-matrix and cell-cell interactions, and therefore participate in cell signaling and adaptation to environment. In this regard, it is not a surprise that many clinical biomarkers and therapeutic targets are cell-surface proteins. In an attempt to reveal differentially expressed proteins that