Breast cancer cells interact with osteoblasts to support osteoclast formation

Breast cancers commonly cause osteolytic metastases in bone, a process that is dependent upon osteoclast-mediated bone resorption. Recently the osteoc last differentiation factor (ODF), better termed RANKL (receptor activator of NF-kappa B ligand), expressed by osteoblasts has been cloned as well as its cognate signaling receptor, receptor activator of NF kappa B (RANK), an d a secreted decoy receptor osteoprotegerin (OPG) that limits RANKL's biolo gical action. We determined that the breast cancer cell lines MDA-MB-231, M CF-7, and T47D as well as primary breast cancers do not express RANKL but e xpress OPG and RANK. MCF-7, MDA-MB-231, and T47D cells did not act as surro gate osteoblasts to support osteoclast formation in coculture experiments, a result consistent with the fact that they do not express RANKL. When MCF- 7 cells overexpressing PTH-related protein (PTHrP) were added to cocultures of murine osteoblasts and hematopoietic cells, osteoclast formation result ed without the addition of any osteotropic agents; cocultures with MCF-7 or MCF-7 cells transfected with pcDNAIneo required exogenous agents for osteo clast formation. When MCF-7 cells overexpressing PTHrP were cultured with m urine osteoblasts, osteoblastic RANKL messenger RNA (mRNA) levels were enha nced and osteoblastic OPG mRNA levels diminished; MCF-7 parental cells had no effect on RANKL or OPG mRNA levels when cultured with osteoblastic cells . Using a murine model of breast cancer metastasis to bone, we established that MCF-7 cells that overexpress PTHrP caused significantly more bone meta stases, which were associated with increased osteoclast formation, elevated plasma PTHrP concentrations and hypercalcaemia compared with parental or e mpty vector controls.