Inhibition of human osteoblast proliferation and mineralization by breast cancer cell-derived paracrine factors

Breast cancer cells have a high propensity to metastasize to bone. Skeletal metastases cause complications such as hypercalcemia, severe bone pain, spinal cord compression and increased fracture frequency. While the mechanism of breast cancer metastasis to bone is not fully understood, evidence suggests that this process is influenced by paracrine factors secreted from both breast cancer cells and bone cells. Recent reports have demonstrated that medium conditioned by breast cancer cells decreases osteoblast proliferation, collagen production, and alters the response of osteoblasts to PTH, PTHrP, and PGE₂. Similarily, our studies have shown that the conditioned medium (CM) from MCF-7, T-47D and MDA-MB-231 breast cancer cells inhibits human fetal osteoblast (hFOB) cell proliferation and mineralization in culture. Initial FPLC purification experiments using a Superdex 30 (16/60) column indicate that this osteoblast inhibitory activity is due to factors of approximately 5-15 kD. Because PTHrP has been implicated in the process of breast cancer metastasis to bone, we examined the effect of polyclonal antibodies (pAb) to PTHrP on the inhibition of hFOB cell proliferation by breast cancer cells. Treatment of hFOB cells with PTHrP pAb (10-400 ng/mL) caused a dose-dependent increase in hFOB cell proliferation. When PTHrP pAb was added to CM from untreated MDA-MB-231 or T-47D cells prior to adding the CM to hFOB cells, no change in the inhibition of hFOB cell proliferation was observed. However, when MDA-MB-231 cells were treated with PTHrP pAb (10-400 ng/mL) during the production of the CM, the resulting CM further inhibited hFOB cell proliferation in a dose-dependent manner. CM from T-47D cells treated with 400 ng/mL PTHrP pAb also had a greater inhibitory effect on hFOB cell proliferation compared to CM from untreated T-47D cells. These results suggest that 1)PTHrP secreted from hFOB cells decreases hFOB cell proliferation in an autocrine manner, 2)the inhibition of hFOB cell proliferation by MDA-MB-231 and T-47D cell CM is not due to PTHrP in the breast cancer cell CM, 3)PTHrP secreted from MDA-MB-231 and T-47D cells regulates the secretion of osteoblast inhibitory factors secreted from these cells in an autocrine manner. In conclusion. Ihese results support the idea that factors secreted from breast cancer cells can alter osteoblast function. In addition, these results suggest that PTHrP can modulate the communication between breast cancer cells and osteoblasts.