Metastatic breast cancer cells suppress osteoblast adhesion and differentiation

Abstract

Bone is a primary target for colonization of metastatic breast cancer cells. Once present, the breast cancer cells activate osteoclasts, thereby stimulating bone loss. Bone degradation is accompanied by pain and increased susceptibility to fractures. However, targeted inhibition of osteoclasts does not completely prevent lesion progression, nor does it heal the lesions. This suggests that breast cancer cells may also affect osteoblasts, cells that build bone. The focus of this study was to determine the ability of breast cancer cells to alter osteoblast function. MC3T3-E1 osteoblasts were cultured with conditioned medium from MDA-MB-231 breast cancer cells and subsequently assayed for changes in differentiation. Osteoblast differentiation was monitored by expression of osteocalcin, bone sialoprotein and alkaline phosphatase, and by mineralization. Osteoblasts cultured with MDA-MB-231 conditioned medium did not express these mature bone proteins, nor did they mineralize a matrix. Inhibition of osteoblast differentiation was found to be due to transforming growth factor beta present in MDA-MB-231 conditioned medium. Interestingly, breast cancer conditioned medium also altered cell adhesion. When osteoblasts were assayed for adhesion properties using interference reflection microscopy and scanning acoustic microscopy, there was a reduction in focal adhesion plaques and sites of detachment were clearly visible. F-actin was disassembled and punctate in osteoblasts cultured with MDA-MB-231 conditioned medium rather than organized in long stress fibers. Taken together, these observations suggest that metastatic breast cancer cells alter osteoblast adhesion and prevent differentiation. These affects could account for the continued loss of bone after osteoclast inhibition in patients with bone-metastatic breast cancer.

In Vitro Mimics of Bone Remodeling and the Vicious Cycle of Cancer in Bone

“…Despite osteoclast inhibition, bone is not completely repaired or regenerated in breast cancer patients with bone metastases, suggesting an important role for osteoblasts in the progression of disease. There is evidence from clinical and animal studies that bone loss in osteolytic metastasis is partly due to the failure of the osteoblasts to produce the osteoid for bone matrix (Stewart et al, 1982;Sasaki et al, 1995;Mercer et al, 2004). Similarly, administration of bisphosphonates to humans with osteolytic metastasis slows lesion progression but does not bring about healing (Lipton, 2000).…”

“…Similarly, administration of bisphosphonates to humans with osteolytic metastasis slows lesion progression but does not bring about healing (Lipton, 2000). Also, in vitro studies have shown that metastatic breast cancer cells increase apoptosis of osteoblasts, suppress osteoblast differentiation and induce an osteoblast inflammatory stress response Mercer et al, 2004). We have previously shown that metastatic breast cancer cells suppress the adhesion and differentiation of osteoblasts .…”

Breast Cancer Metastases to Bone: Role of the Microenvironment

“…In vitro, breast cancer cells have been shown to produce soluble factors able to inhibit osteoblast differentiation (20,134), the effect that may be mediated at least in part by the dysregulation of Notch and Wnt developmental signalling pathways. Notch signalling is essential in embryogenesis but has distinct roles in bone homeostasis, regulating the proliferation of immature osteoblasts (135) and suppressing osteoblast differentiation (62,63).…”

Bisphosphonate treatment and radiotherapy in metastatic breast cancer

“…In addition to promoting invasion, and metastasis of breast cancer, TGF-b secreted by the extracellular bone matrix and osteoclasts suppresses late stages of osteoblast differentiation [3]. It has been shown that metastatic breast cancer cells alter osteoblast adhesion, prevent differentiation, and induce apoptosis, but fas/fas-ligand and TNF-a, two common initiators of cell death, are probably not involved in this aspect of the metastasis/bone cell axis [9,10]. Therefore, bone destruction in metastatic breast cancer results from asynchronous bone turnover wherein increased osteoclastic bone resorption is not accompanied by a comparable increase in bone formation.…”