Breast cancer frequently metastasizes to the skeleton. It is estimated that 85% of individuals with advanced disease harbor bone metastases [1]. While ductal carcinoma in situ detected early is 98% curable, bone metastases are basically incurable [2]. Metastatic cancer cells tend to colonize the heavily vascularized areas of the skeleton, such as the red marrow of the long bones, sternum, pelvis, ribs and vertebrae, where they disrupt not only bone physiology but also hematopoiesis and the immune system [3].

Metastases leading to overall bone loss are classified as osteolytic. Those leading to excess bone deposition are considered osteoblastic. However, both bone degradation and deposition likely occur early in the metastatic process. The majority of breast cancer metastases ultimately cause bone loss. The clinical outcomes of bone pain, pathologic fractures, nerve compression syndrome, and metabolic disturbances leading to hypercalcemia and acid/base imbalance severely reduce the quality of life [3].

In the 1960s and 70s it was proposed that bone degradation might result from the physical pressure of the tumor on the bone and/or direct resorption of the bone by tumor cells. It was also noted that tumor cells caused other cells in the bone (for example, lymphocytes) to produce molecules such as prostaglandins (PGs) that can affect bone [4]. While there is evidence that the breast cancer cell matrix metalloproteinases (MMPs) can resorb bone in vitro and contribute to bone degradation in vivo [5], it is now well accepted that osteoclasts are largely responsible for osteolytic metastatic lesions [6].

Bone provides support and protects vital organs but also is a metabolically active tissue. It is a reservoir of numerous growth factors as well as calcium and phosphorous, which are released from the matrix during bone remodeling. Cortical bone provides strength and protection while trabecular bone is the most metabolically active. Trabecular bone is the major site of bone turnover under normal conditions and in diseases of bone loss or formation.

The skeleton is constantly undergoing remodeling. Even in adults it is estimated that about 10% of the bone is renewed each year [7]. The normal processes of bone resorption and formation are remarkably well balanced. In the young adult, bone mass reaches its peak, but with increasing age there is a slow loss of mass. This loss is more precipitous in women, due to the decrease in estrogen at menopause [3]. However, the presence of metastatic breast cancer cells or other bone metastatic cancers, such as prostate, lung, renal, and myeloma, accelerates the remodeling process and disturbs the balance between bone depositing cells, osteoblasts, and bone degrading cells, osteoclasts. It is impossible to understand the growth and progression of cancer cells in the bone marrow without consideration of the interaction between osteoblasts and osteoclasts. A thorough review of bone remodeling is beyond the scope of this article, and there are several excellent, recent reviews [8, 9]. However, the process is described in brief in order to further consider the mechanisms of osteolytic metastasis.

Bone remodeling is often described as a cycle beginning with bone degradation and ending with bone deposition (Figure 1A). This process is effected by osteoblasts and osteoclasts within a functional and anatomic unit known as the basic multicellular unit (BMU). Cells of the osteoblast lineage are derived from mesenchymal stem cells, and are represented in this unit by osteoblasts, bone lining cells and osteocytes. Bone lining cells appear microscopically as relatively undifferentiated cells that line the bone. Their function is not clear except that their retraction is necessary for bone resorption to begin [10]. Osteocytes are terminally differentiated osteoblasts that become embedded in the bone matrix at the end of the deposition phase of remodeling. Once osteoblasts finish bone deposition, they undergo apoptosis, remain in the matrix as osteocytes or revert to thin bone-lining cells.

Osteoclasts derive from hematopoietic stem cells. Cells of the monocyte-macrophage lineage are stimulated to form osteoclast progenitor cells. These cells fuse to form multinucleated, but non-functional pre-osteoclasts. Further stimulation results in large multinuclear cells capable of bone resorption.

The entry of breast cancer cells into the bone micro-environment synergistically increases the complexity of cell-cell interactions. A working model to describe the bone remodeling compartment in the presence of metastatic cancer cells has been referred to as the 'vicious cycle of bone metastasis' [13] (Figure 1B). There are many excellent reviews describing this paradigm [14–17] from its inception in the 1990 s. The minimal essential components are osteoblasts, osteoclasts, tumor cells and the mineralized bone matrix. According to this paradigm, the tumor cells produce a variety of growth factors, most notably parathyroid hormone-related protein (PTHrP) [18]. The role of PTHrP in bone metabolism is not fully understood, but it is known to cause upregulation of RANKL and downregulation of OPG [19], thus enhancing osteoclast function leading to bone degradation. In the process, growth factors stored in the matrix, such as transforming growth factor (TGF)-β, vascular endothelial growth factor (VEGF), insulin-like growth factors (IGFs), bone morphogenic proteins and fibroblast-derived factors, as well as calcium, are released into the bone microenvironment. These factors can stimulate the tumor cells to proliferate and produce more growth factors and more PTHrP, further perpetuating the vicious cycle of bone metastasis.

In reality the system is much more complex (Table 1). Cytokines such as IL-6, IL-8 and IL-11 secreted by breast cancer cells also promote osteoclast differentiation and bone resorption. IL-11, normally produced by bone marrow stromal cells and osteoblasts, is an important regulator of hematopoiesis and a potent promoter of osteoclast formation. In addition, its expression is enhanced in the presence of TGF-β [20]. IL-8, a proinflammatory CXC chemokine, is secreted by monocytes, endothelial cells and osteoblasts. It can activate osteoclasts independent of RANKL [21]. Cancer cells also can elicit an increase in osteoblast production of several other osteoclastogenic cytokines, such as monocyte chemotactic protein-1 (MCP-1) and IL-6, IL-8 and TNF [22].

TGF-β is well-known for its role in osteolytic bone metastasis. It can activate both Smad-dependent and Smad-independent signal pathways to induce preosteolytic factors such as PTHrP [23]. Because of its significant role, TGF-β has been a tempting therapeutic target. Ganapathy and colleagues [24] found that TGF-β antagonists are able to reduce bone metastasis and the number and activity of differentiated osteoclasts [24]. However, because TGF-β plays a more global role in cell proliferation and differentiation, its utility as a therapeutic may be limited.

Just as osteoblasts are a critical partner in normal bone remodeling, they are vital to the metastatic osteolytic process. Because osteoblasts secrete both RANKL and OPG, they are major mediators of osteoclastogenesis [25]. Current therapies consist of blocking osteoclast activity as a means of disrupting the vicious cycle. Bisphosphonates such as zoledronic acid (Zoledronate™) bind to hydroxyapatite of the bone matrix and are ingested by osteoclasts, which then undergo apoptosis. However, this approach has not entirely solved the problem. Administration of bisphosphonates may slow osteolytic lesion progression and stabilize or increase overall bone density, but does not bring about healing [1, 16, 26]. There is evidence in both humans and animals that bone loss in osteolytic metastasis is partly due to the failure of the osteoblasts to produce new osteoid for the bone matrix.

The hypoactivity of osteoblasts has been known for some time in multiple myeloma. This is a disease of clonal malignancy of terminally differentiated plasma cells that accumulate in the bone marrow. It is estimated that osteolytic lesions occur in 60 to 95% of myeloma patients [1, 27]. In advanced disease, bone formation is essentially absent, and the processes of bone resorption and formation become uncoupled. Myeloma cells produce factors that upregulate osteoblast production of M-CSF and RANKL and downregulate production of OPG. Myeloma cells may also produce RANKL and directly affect osteoclasts [28]. The mechanisms for suppressed osteoblast activity are not clear but Dickkopf-1 (DKK1), an inhibitor of Wnt signaling, is believed to inhibit osteoblast differentiation [29]. Other molecules made by multiple myeloma cells, such as IL-3, IL-7 and soluble frizzle-related protein-2, also inhibit osteoblast differentiation [27]. Furthermore, Pozzi and colleagues [30] have recently reported that high doses of zoledronic acid, the current standard therapeutic for most osteolytic diseases, may also negatively affect osteoblast differentiation.

Recently, we have found that metastatic breast cancer cells have profound effects on osteoblasts in culture [22] and in animals [31, 32]. Metastatic breast cancer cells or their conditioned media increase osteoblast apoptosis, and suppress osteoblast differentiation and expression of proteins required for new bone matrix formation. Neutralization of TGF-β in conditioned medium from human metastatic MDA-MB-231 breast cancer cells permitted the differentiation of osteoblasts in culture, suggesting that TGF-β negatively affects osteoblasts while promoting growth of the metastatic cells [33]. In the presence of cancer cells, osteoblasts increase expression of pro-inflammatory cytokines such as IL-6, monocyte chemotactic protein-1 (MCP-1), macrophage inflammatory protein-2 (MIP-2; GRO alpha human), keratinocyte chemoattractant (KC; IL-8 human) and VEGF. These molecules not only help support tumor cells, but also are osteoclastogenic. When the bone loss is extensive, the osteoblasts are absent from the lesion [32]. Thus, in the course of the osteolytic process, the osteoblasts are unable to fulfill their role as bone building cells.

Breast cancer is often compared with prostate cancer, which metastasizes to the skeleton with a similar frequency. In contrast to breast cancer, prostate bone metastasis often results in osteoblastic lesions. While the outcome is predominantly osteoblastic, it is known that prostate cancer lesions display both blastic and lytic characteristics early in the process. There is evidence that osteoblastic metastases form at sites of osteolytic lesions, suggesting an overall increase of bone remodeling Accelerated osteoblastogenesis can be stimulated by factors secreted by prostate cancer cells, such as endothelin-1, TGF-β, and fibroblast growth factor (FGF) [1]. These molecules cause osteoblasts not only to form new bone but also to release RANKL and other osteoclastic mediators. Although the mechanisms of osteoteoblastic and osteolytic responses are not fully understood, it is clear that many factors involved in osteolytic breast cancer bone metastasis also regulate the osteolytic aspects of prostate cancer. Akech and colleagues [34] recently reported that Runx2 (Runt-related transcription factor 2) is produced by the highly metastatic prostate cancer cell PC-3, and positively correlates to the severity of osteolytic disease. There is also evidence that molecules in conditioned medium from PC-3 cells alone [34], or from both PC-3 cells and MC3T3-E1 osteoblasts [35], promote osteoclastogenesis. While some of the growth factors produced by breast and prostate cancers may be different, ultimately they engage the bone re-modeling process.

The bone remodeling microenvironment is a complex system in which the cell functions are controlled by multifunctional transcription factors, cytokines and growth factors. The dynamics of this system are interrupted when metastatic breast cancer cells are introduced, adding another layer of active molecules to the bone environment. In the section that follows, we will discuss in greater detail the key factors involved in metastatic breast cancer osteolysis. While they are categorized into functional groups, it should be noted that many of these factors are multifunctional and must be considered within the context of the bone remodeling system as a whole.

RANKL clearly holds the key to the osteolytic process. In fact, a new drug, denosumab (Prolia™), a fully human monoclonal antibody to RANKL, has been approved by the US Food and Drug Administration (FDA) for the treatment of postmenopausal women with high risk of osteoporotic fractures, and is under priority review for patients with bone metastases. Osteoblasts and bone stromal cells can respond to a variety of substances that upregulate RANKL. PTH/PTHrP, TNF-α, prostaglandins (PGE2), IL-1, IL-11, FGF-2, and IGF-1 have been reported to increase RANKL production. Cells of the immune system, T cells and dendritic cells can also express RANKL. In this context, RANKL increases in the presence of inflammatory agents from infectious organisms, such as lipopolysaccharide, CpGpDNA and viral double-stranded DNA [41]. Several of these RANKL inducers merit further discussion with respect to metastatic breast cancer-induced osteolysis.

While not directly responsible for osteolysis in metastatic breast cancer disease, there are physiological parameters that can amplify the degree of bone loss. Clinical studies of newly diagnosed breast cancer patients have revealed that high bone turnover correlates with a higher risk of skeletal complications [62]. For post-menopausal women, high bone turnover may be caused by estrogen deficiency. Estrogen profoundly affects bone remodeling by suppressing production of RANKL while increasing production of OPG. Estrogen also increases osteoblast pro-collagen synthesis and decreases osteoblast apoptosis [63]. In addition, production of inflammatory cytokines (that is, IL-6, TNF-α, M-CSF, IL-1) is suppressed by estrogen [64]. Estrogen has also been shown to promote osteoclast apoptosis and inhibit activation of mature osteoclasts. Unfortunately, some of the therapies used for breast cancer patients may exacerbate the problem. For example, the use of aromatase inhibitors increases the risk for osteoporosis. Chemotherapy may bring about ovarian failure and premature menopause [1].

As primary constituents in bone metabolism, calcium and vitamin D can not be overlooked as critical regulators of osteolysis in bone metastatic breast cancer. In middle aged and elderly women, calcium and/or vitamin D deficiencies are quite common, as is the incidence of breast cancer [65]. Epidemiological studies have also correlated the increase in breast cancer rates with decreasing sunlight exposure. It was recently reported that mice deficient in vitamin D or calcium showed increased metastatic tumor growth and accelerated rates of bone resorption [66, 67]. In light of these findings, correction of calcium and vitamin D deficiencies should be considered as adjuvant therapies in slowing or preventing osteolysis in breast cancer patients.

Chronic inflammation has long been considered a risk factor in cancer initiation [68]. Inflammation associated with bone fractures and arthritic joints has been anecdotally associated with the appearance of bone metastasis, often many years after the primary tumor has been treated. Recently, Roy and colleagues [69] investigated this association in a mouse model of autoimmune arthritis and found that arthritic mice had an increase in both lung and bone metastasis compared to the non-arthritic mice. Thus, inflammation is likely to be important in cancer initiation, metastasis and the resulting osteolysis.

Understanding the mechanisms of osteolysis should be the key to designing the cure. Of course, the best cure for bone metastasis is prevention. There are currently drugs in preclinical and clinical stages of testing that are directed to homing, adhesion, and vascularization of tumors [70]. However, once bone metastasis has occurred, the aim has been to break the osteolytic cycle by targeting osteoclasts. Drugs of the bisphosphonate family have been used for many years as the standard of care. Until recently they were the only FDA approved drugs for metastatic bone disease [71]. These molecules bind to hydroxyapatite of the bone matrix and are ingested by osteoclasts, which then undergo apoptosis. There is evidence that bisphosphonates also contribute to tumor cell death, especially in combination with chemotherapy [72]. There are conflicting reports regarding their effect on osteoblasts. At higher doses they may in fact prevent osteoblast differentiation [30]. Of the bisphosphonates, zoledronic acid is the most potent. Clinical evidence indicates that this drug can reduce the rate of bone loss, but is not curative. It improves the quality of life by preventing fractures but does not prolong life [73]. Denosumab (Prolia™), the latest drug to enter the field, is a monoclonal antibody to RANKL. It inhibits the differentiation of osteoclasts by competitive binding with RANKL. Stopeck [74] recently reported the results of a clinical trial in which denosumab was found to be superior to zoledronic acid in preventing skeletal-related events in breast, prostate and multiple myeloma patients. Denosumab has recently been approved by the FDA for treatment of osteoporosis in women with high risk of fractures and is being considered for treatment of bone metastasis. However, both drugs are associated with low incidence of osteonecrosis of the jaw [75]. Another drug, teriparatide (Forteo™), the amino-terminal 34 amino acids of parathyroid hormone, has been used for many years to treat osteoporosis. Teriparatide, in contrast to bisphosphonates and denosumab, acts on osteoblasts to stimulate bone formation. At first glance it would seem ideal to pair bisphosphonates or denosumab with teriparatide since the former two block bone resorption and the latter stimulates bone deposition. However, teriparatide is associated with an increased risk of osteosarcoma and exacerbation of skeletal metastases because of its effect on bone turnover [75]. Other drugs on the horizon target TGF-β, and cathepsin K. Various approaches, including kinase inhibitors, ligand-neutralizing antibodies and anti-sense molecules, are being investigated [33].

Most breast cancer metastasis to bone results in osteolytic lesions. Despite the role of the osteoclasts in this process, the outcome is due in large part to the impact of cancer cells directly and indirectly on osteoblasts. Induction of aberrant osteoclastogenesis is only part of the equation. Breast cancer cells also cause inhibition of osteoblast differentiation and adhesion, downregulation of collagen synthesis and increased osteoblast apoptosis. Thus, bone loss is the result of excessive bone degradation and insufficient bone replacement. In the final stages of metastatic osteolytic breast cancer disease, the cancer cells, fueled by growth factors released from the degraded matrix, expand unchecked. Eventually, bone remodeling ceases as both osteoblasts and osteoclasts are lost.

What can be done to stop osteolytic metastasis? To date, osteoclasts have been the primary target of drug therapies. Current treatments can improve bone density, decrease skeletal related events and ease bone pain, yet existing bone lesions do not heal. While drugs that inhibit osteoclast differentiation or activity are vital to treating osteolysis, therapies designed to restore osteoblast number and function will be required to fully resolve osteolytic lesions. Part of this uncertainty is because we do not fully understand all of the cell, cytokine and growth factor interactions that occur in the bone microenvironment.

Identification of a stimulator or protector of osteoblasts would be a major improvement in treatment for osteolytic breast cancer as well as other diseases of bone loss. However, there is no guarantee that inhibition of osteolytic lesions would prevent the growth of cancer cells in the bone or their spread to other organs. It is interesting that cancer cells often remain dormant in bone for many years before they begin to grow. Continuing research into the mechanisms of cancer cell dormancy could result in a treatment that would prevent cancer cell proliferation in the bone and the chain of events that leads to osteolysis.

Since the discovery of RANKL and its role in bone remodeling, the field of bone metastasis has moved rapidly. It is now generally accepted that the bone microenvironment is critical to the colonization and growth or dormancy of metastases. Nevertheless, the inaccessibility, opacity and size of the skeleton make it difficult to study even in laboratory animals. Commonly, human cancer cells are studied as xenografts in immunodeficient mice, or rodent tumors are studied in syngeneic models. However, more accessible and defined [76] models are needed. Several groups have developed in vivo models in which bone or bone substitutes are implanted in animals. Retrieval of the bone at specific times gives a snapshot of the status of metastases. For example, a hydroxyapatite scaold pre-loaded with bone morphogenetic protein-2 enhanced the growth rate of mammary tumor cells in the scaold [77]. Fragments of human fetal bone implanted in SCID mice allow one to examine human cancer with human bone [76]. These approaches still rely on animals. Recently we have begun developing an in vitro bioreactor [78]. Using this device, we have been able to grow osteoblasts into a mineralized tissue. Metastastic human breast cancer cells (MDA-MB-231) added to this culture attach, penetrate the tissue and form single cell files characteristic of metastases seen in pathologic tissues. The cancer cells affect osteoblast morphology and extracellular matrix. We are in the process of adding osteoclasts to the system to create a rudimentary in vitro bone remodeling unit. This approach will allow testing of components and drugs in a model less complex than an animal but more relevant than standard tissue culture.

AMM, the senior investigator and corresponding author, has worked in the area of breast cancer metastasis to bone for over 12 years. Along with colleagues and students she has focused particularly on the fate of osteoblasts in the metastatic bone environment. DMS is a senior research technician with many years experience in the bone field. Y-CC is a senior graduate student completing work on the studies of selenium in breast cancer metastasis.

This article is part of a review series on New pathways of metastasis, edited by Lewis Chodosh. Other articles in the series can be found online at http://breast-cancer-research.com/series/metastasis_pathway