Skeleton is the most common organ targeted by breast cancer cells, especially from estrogen receptor alpha (ER)-positive neoplasms. Metastatic cells can stimulate directly or indirectly osteoclast-mediated bone resorption. Tumor-induced osteolysis is often extensive and leads to the release of large quantities of calcium. Metastatic cancer cells can be thus exposed to high calcium concentrations (40 mM has been reported at the resorption site). However, the effects of Ca2+ on breast cancer cells have been minimally examined. We showed that 20-mM extracellular Ca2+ induced a downregulation of ER protein in MCF-7 cells and caused ER-mediated transactivation of a reporter gene by 55 +/- 10% (mean +/- SD) in MVLN cells (MCF-7 cells stably transfected with ERE and luciferase reporter gene). Moreover, 3 mM Ca2+ increased progesterone receptor (PgR) expression by 45 +/- 8%. Mg2+ tested at up to 20 mM did not exert any effects, while 17beta-estradiol downregulated ER, transactivated the reporter gene, and enhanced PgR expression. The pure antiestrogen ICI 182,780 was able to abrogate the transactivation of the reporter gene and the increase in PgR levels induced by Ca2+, indicating that Ca2+ may exert a weak and specific estrogenic effect in MCF-7 cells. Ca2+ effects on ER probably start at the cell membrane level since a large Ca2+ influx caused by the ionophore A23187 failed to activate ER. We have thus studied the involvement of the membrane calcium-sensing receptor (CaR) that is known to be expressed notably in MCF-7 cells. We first tested the effects of a specific activator of CaR. Exposure to 10(-4) M calcimimetic NPS R-467 mirrored the changes observed with extracellular Ca2+ by inducing a marked decrease in ER protein levels, increasing the transcriptional activity of ER (67 +/- 12%) and stimulating PgR expression (41 +/- 4%). As expected, the NPS S-467 isomer was less effective. Furthermore, a highly selective CaR antagonist partly suppressed the downregulation of ER as well as transactivation of the reporter gene induced by Ca(2+). Our results suggest that the effects of extracellular Ca2+ on ER expression and activity are mediated, at least in part, by the CaR. In summary, calcium released during the process of metastatic bone destruction could modulate the functions of the estrogen receptor, a key receptor involved in breast cancer cells growth and function, and thus participate in the pathogenesis of tumor-induced osteolysis.