Advances in
Bone Cancer Treatment:
Preventing Metastasis and Bone Loss
Pathophysiology of Bone Metastasis
Bone is an extremely fertile soil for the growth of tumor cells. Bone is the largest storage site of growth factors in the body, which are activated when tumors metastasize to bone (Figure 1). Further, marrow stromal cells express chemotactic factors that attract tumor cells to bone and then provide a supportive microenvironment for tumor cells to grow. This "seed-and–soil" hypothesis for bone metastasis was originally proposed by Paget.1
In order to more effectively treat and prevent bone metastasis, bone loss, and related complications, it is important to identify and study the pathways and mediators cancer cells use to establish themselves, proliferate, and disseminate from the bone environment. While the biology of cancer metastasis to bone is complex, involving a multitude of factors and growth pathways, research in this area has been advancing rapidly and is already contributing to the development of new therapies and insights into cancer prevention. What is known is that tumors, like other self-renewing adult tissues, appear to be maintained by stem cells that facilitate cancer cell dormancy and, possibly, heterogeneity within a specific tumor. When cancer cells metastasize to bone, they home to endosteal niches in the bone marrow from where they are later disseminated. This process is facilitated by several factors, including SDF-1, AXII, and RANKL proteins that are expressed by both osteoblasts and stromal cells.2-4 Osteoclasts play a key role in bone metastasis as described below, and may function in tumor mobilization through release of MMP-9 and cathepsin-K, osteopontin, and stem cell factor into the surrounding bone marrow matrix.5
There are two distinct types of bone metastasis, osteolytic and osteoblastic, which represent the extremes of a continuum where either bone destruction (osteolytic) or bone formation (osteoblastic) predominate. Both processes occur in most types of bone metastasis, with the exception of multiple myeloma where lesions are exclusively osteolytic.
Increased bone resorption occurs in both osteolytic and osteoblastic metastasis, and is mediated by osteoclasts. The osteolytic bone destructive process releases activated growth factors from bone, including TGF-α, IGF-1 and -2, FGF, PDGF, bone morphogenetic proteins, and calcium.6 Osteoblastic factors, either produced by or induced by tumors, stimulate osteoblast proliferation, differentiation, and the secretion of more growth factors, which are deposited into the bone matrix and also enrich the local tumor cell microenvironment. Additionally, bone marrow stromal cells and osteoblasts produce growth factors and cytokines that promote tumor cell growth. Most of the osteolytic factors secreted by the tumor act via the osteoclast differentiation factor RANK L, which increases osteoclast formation, survival, and bone resorption. Interactions between tumor cells and bone marrow stromal cells further increase production of cytokines and soluble factors produced by the tumor and bone marrow stromal cells (Figure 2).7 The bone marrow microenvironment also plays a major role in maintaining tumor stem cells in a dormant state, where they are resistant to chemotherapy.
Factors Affecting Osteolytic Bone Metastasis
Osteoclast activity is increased in all osteolytic metastasis. However, the factors that increase osteoclast activity differ among different tumor types. RANKL is increased in the marrow of patients with multiple myeloma, breast cancer, and prostate cancer. Its expression is upregulated when tumor cells bind to marrow stromal cells, and by soluble factors produced by tumor cells.8 RANKL then induces osteoclastogenesis, with subsequent release of growth factors from matrix that enhance the growth and survival of tumor cells. A human monoclonal antibody to RANKL, denosumab, has been developed and is in clinical trials for the treatment and prevention of cancer-related bone disease. In multiple myeloma, MIP-1α, TNF-α, and IL-6 can induce RANKL expression, while in breast cancer RANKL is induced by PTHrP. Release of TGF-β in the bone microenvironment by osteoclasts induces PTHrP production by breast cancer cells to increase osteoclasts. Breast cancer cells also produce IL-6, IL-8, prostaglandin E2, M-CSF, IL-1, and TNF-α, which further increase osteoclast formation in bone metastasis.9-11
Because of the importance of TGF-β in osteolytic bone metastasis, inhibitors of TGF-β type 1 receptor kinase have been evaluated in preclinical models of skeletal metastasis. Systemic administration of a TGF-β receptor 1 kinase inhibitor reduces the number and size of lung and bone metastases and suppresses transcription of PTHrP and IL-11 mRNA in a model of metastatic breast cancer.12,13 Thus, blocking TGF-β signaling may be an important strategy for treating bone metastasis from breast cancer. TGF-α receptor kinase inhibitors are currently in clinical trials for patients with bone metastasis.
Kang and coworkers identified a set of genes necessary for breast cancer to metastasize to bone, and found that IL-11, OPN, CXCR4, and CTGF were required. In breast cancer cell lines, overexpression of IL-11 and OPN with either CTGF or CXCR4 was sufficient for bone metastasis.14 These results suggest that genes for homing (CXCR4), angiogenesis (CTGF), and osteolysis (IL-11 and OPN) are required for breast cancer bone metastasis.
In multiple myeloma, the suppression of osteoblast activity further exacerbates osteolysis. Primary CD138+ multiple myeloma cells, but not normal plasma cells, produce DKK1,15,16 an inhibitor of the Wnt signaling pathway. Wnt signaling promotes proliferation, expansion, and survival of osteoblastic precursors.17 DKK1 mRNA levels correlated with focal bone lesions in patients with multiple myeloma.15,16 Importantly, administration of anti-DKK1 antibody to SCID-hu mice injected with primary multiple myeloma cells inhibited tumor cell growth and increased bone formation in the implanted fetal bone.18 Edwards et al and Qiang et al have shown that stimulating Wnt signaling in a multiple myeloma model increases bone formation and suppresses tumor growth.19,20 Multiple myeloma cells also produce another Wnt antagonist, sFR2, that suppresses osteoblast differentiation.10
Factors Increasing Osteoblast Activity in Bone Metastasis
Prostate cancer predominantly forms osteoblastic bone metastasis, although there is still ongoing bone destruction.21 The factors responsible for increased osteoblast activity in bone metastasis are just beginning to be defined. In vitro studies of samples from patients with breast cancer showed that endothelin-1 stimulates bone formation and osteoblast proliferation.22,23 In addition, serum endothelin-1 levels are increased in patients with osteoblastic metastases from prostate cancer.24 The Wnt signaling pathway also plays an important role in the osteoblastic metastasis in prostate cancer.25 Osteoblasts produce several soluble inhibitors of the canonical Wnt pathway, including DKK1, secreted frizzled related proteins (sFRP), and Wnt inhibitor factor (Wif-1). The Wnt signaling pathway antagonist DKK1 decreases development of bone metastasis,26 and blocking DKK1 in an osteolytic prostate cancer cell line resulted in increased osteoblast activity in the metastases. In contrast, expressing DKK1 in a prostate cancer cell line that induced both osteoblastic and osteolytic metastasis resulted in conversion to a highly osteolytic tumor. Endothelin-1 may also increase osteoblast activity by inhibiting expression of DKK1 in marrow stromal cells.27 Furthermore, the Wnt stimulator, Wnt7b, is expressed by high-grade prostate cancer cells and not normal prostate cells.26 In patients with breast cancer, 16 of 38 bone metastases were shown to express Wnt-7b by gene array analysis.
Conclusion
The identification and characterization of the pathophysiologic mechanisms underlying bone metastasis have provided important new therapeutic targets for treating patients. Phase III clinical trials with denosumab are ongoing, as are trials with antagonists of endothelin-1 receptor and TGF-β receptor kinase, and antibodies to DKK1. The availability of new agents, used alone or in combination with bisphosphonates may prevent or eradicate bone metastasis in the future.
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