The role of the CXCL12-CXCR4/CXCR7 axis in the progression and metastasis of bone sarcomas

Bone sarcomas, which comprise less than 1% of all human malignancies, are a group of relatively rare mesenchymal-derived tumors. They are mainly composed of osteosarcoma, chondrosarcoma and Ewing's sarcoma. In spite of advances in adjuvant chemotherapy and wide surgical resection, prognosis remains poor due to the high propensity for lung metastasis, which is the leading cause of mortality in patients with bone sarcomas. Chemokines are a superfamily of small pro-inflammatory chemoattractant cytokines which can bind to specific G protein-coupled seven-span transmembrane receptors. Chemokine 12 (CXCL12), also designated as stromal cell-derived factor-1 (SDF-1), is able to bind to its cognate receptors, chemokine receptor 4 (CXCR4) and chemokine receptor 7 (CXCR7), with high affinity. The binding of CXCL12 to CXCR4/CXCR7 stimulates the activation of several downstream signaling pathways that regulate tumor progression and metastasis. In this review, the structure and function of CXCL12 and its receptors, CXCR4 and CXCR7, as well as many factors affecting their expression are discussed. Phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways are the two most important downstream pathways regulated by the CXCL12-CXCR4/CXCR7 interaction. CXCR4 expression in bone sarcomas, including tumor cells and samples and the correlation between CXCR4/CXCR7 expression and the survival of patients with bone sarcomas are also discussed. In addition, we review the involvement of the CXCL12-CXCR4/CXCR7 axis in the growth and metastasis of bone sarcomas and the targeting of this axis in preclinical studies.