To reduce the burden of bone metastases, the pathophysiology of the metastatic niche should be elucidated and targeted. The aim of the present study was to assess the effect of tumor cells on osteoclast (OC) recruitment and activity in the presence of altered bone remodelling. Peripheral blood mononuclear cells (PBMC) were isolated from healthy and ovariectomized (OVX) rats and co-cultured with MRMT-1 rat breast carcinoma cells or with their conditioned medium for 1 and 2 weeks. Alamar Blue viability test, synthesis of cathepsin K, transforming growth factor-beta 1 (TGF-β1), tumor necrosis factor alpha (TNF-α), vascular endothelial growth factor (VEGF), metalloproteinase (MMP)-7, MMP-9, FITC-conjugate phalloidin staining and tartrate-resistant acid phosphatase (TRAP) staining were evaluated. The results indicate that breast carcinoma cells induced different responses in PBMC derived from rats affected by estrogen deficiency osteoporosis (OP) in comparison with healthy ones, with a significant increase in proliferation rate, OC differentiation, synthesis of TNF-α, MMP-7 and MMP-9. The data support the "proof of concept" that OP due to estrogen deficiency might offer a receptive site for cancer cells to form bone metastases.