Bone metastases are a common complication of many solid tumors and are clinically responsible ofbone fractures, hypercalcemia, and pain. Currently, there is no curative treatment; understanding themechanisms involved in the formation and development of bone metastases is therefore necessary toconsider new therapeutic approaches.A comparative transcriptomic analysis between the human breast cancer cell line MDA-MB-231, andits osteotropic subpopulation, B02, has shown an up-regulation of Slit/Robo signaling pathway.Robo receptors and their ligands Slits were initially identified as key factors in axon guidance duringdevelopment. However, these proteins are also involved in the migration of cancer cells.To investigate the role of these receptors Robo1 and 4 in breast cancer bone metastasis, theexpression of these genes was inhibited in B02 cells. In vitro and in vivo experiments point out antagonisticrole of Robo1/4. Indeed, inhibition of Robo1 expression increases primary tumors growth, while Robo4invalidation reduces it. These receptors also differently regulate the formation of osteolytic lesions and extentof skeletal tumor burden. In vitro, Robo1 depletion induces an increased invasion, whereas Robo4 depletiondecreases it. We also showed that inhibition of Robo4 reduces survival and growth of B02 cells in the bonemarrow at early times of invasion. Finally, a cohort study of breast cancer patients shows a strong correlationbetween Robo4 expression and metastatic relapse in bone.This study shows the involvement of the Slit/Robo pathway in bone metastasis, and therefore seemsto be a judicious therapeutic approach to treat these metastases.
