The pathogenesis of breast cancer-induced osteolysis remains largely unknown. To evaluate the potential role of osteoblasts as target cells during this process, we incubated SaOS-2 human osteoblast-like cells (OBL) with culture media conditioned by proliferative (PM, "Proliferation Media") or confluent (CfM, "Confluence Media") MCF-7 human breast cancer cells. CfM decreased the growth of OBL by 26% while PM was without significant effect on this parameter. In contrast, both PM and CfM obtained from MCF-7 cultures increased the cyclic AMP (cAMP) response of OBL to the osteolytic agents PTH (10-8 M) and PTH-related peptide (PTHrP, 10-8 M) by a factor of about 3, and to prostaglandin E2 (PGE2, 10-6 M) by a factor of about 2. No significant modulation of OBL growth or sensitivity to PTH, PTHrP and PGE2 was induced by media obtained from HBL-100 non-malignant immortalized breast epithelial cells cultures. 17ß-estradiol (E2, 10-8 M) and the antiestrogen tamoxifen (Tam, 10-7 M) added for 48 h to MCF-7 cultures before collecting conditioned media attenuated and potentiated, respectively, the PM- but not the CfM-induced increase in the response of OBL to PTH or PTHrP. Along the same line, the addition to MCF-7 conditioned media of a polyclonal anti-transforming growth factor-ß (TGF-ß) antibody attenuated by about 25% the PM-induced increase in OBL response to PTH and PTHrP while abrogating the modulatory effects of E2 and Tam on that response. Together, our results indicate that MCF-7 breast cancer cells secrete factors which inhibit the growth of OBL and increase their sensitivity to various osteolytic agents. TGF-ß was only partly responsible for these effects and account for their modulation by E2 and Tam. The identification of other(s) osteoblast-modulatory factor(s) should contribute to a better understanding and treatment of breast cancer-induced osteolysis.
