IGF-I secretion by prostate carcinoma cells does not alter tumor-bone cell interactions in vitro or in vivo

Dynamic interplay between bone and multiple myeloma: emerging roles of the osteoblast

Multiple myeloma is a B-cell malignancy characterized by the unrelenting proliferation of plasma cells. Multiple myeloma causes osteolytic lesions and fractures that do not heal due to decreased osteoblastic and increased osteoclastic activity. However, the exact relationship between osteoblasts and myeloma cells remains elusive. Understanding the interactions between these dynamic bone-forming cells and myeloma cells is crucial to understanding how osteolytic lesions form and persist and how tumors grow within the bone marrow. This review provides a comprehensive overview of basic and translational research focused on the role of osteoblasts in multiple myeloma progression and their relationship to osteolytic lesions. Importantly, current challenges for in vitro studies exploring direct osteoblastic effects on myeloma cells, and gaps in understanding the role of the osteoblast in myeloma progression are delineated. Finally, successes and challenges in myeloma treatment with osteoanabolic therapy (i.e., any treatment that induces increased osteoblastic number or activity) are enumerated. Our goal is to illuminate novel mechanisms by which osteoblasts may contribute to multiple myeloma disease progression and osteolysis to better direct research efforts. Ultimately, we hope this may provide a roadmap for new approaches to the pathogenesis and treatment of multiple myeloma with a particular focus on the osteoblast.

Expression of osteoprotegerin from a replicating adenovirus inhibits the progression of prostate cancer bone metastases in a murine model

Metastatic involvement of the skeleton is a frequent consequence of advanced prostate cancer. These skeletal metastases cause a number of debilitating complications and are refractory to current treatments. New therapeutic options are being explored, including conditionally replicating adenoviruses (CRAds). CRAds are engineered to selectively replicate in and destroy tumor cells and can be 'armed' with exogenous transgenes for enhanced potency. We hypothesized that a CRAd armed with osteoprotegerin (OPG), an inhibitor of osteoclastogenesis, would inhibit the progression of prostate cancer bone metastases by directly lysing tumor cells and by reducing osteoclast activity. Although prostate cancer bone metastases are predominantly osteoblastic in nature, increased osteoclast activity is critical for the growth of these lesions. Ad5-Δ24-sOPG-Fc-RGD is a CRAd that carries a fusion of the ligand-binding domains of OPG and the Fc region of human IgG1 in place of the viral E3B genes. To circumvent low tumor cell expression of the native adenoviral receptor, an arginine-glycine-aspartic acid (RGD) peptide insertion within the viral fiber knob allows infection of cells expressing α(v) integrins. A 24-base pair deletion (Δ24) within viral E1A limits replication to cells with aberrant retinoblastoma cell cycle regulator/tumor suppressor expression. We have confirmed that Ad5-Δ24-sOPG-Fc-RGD replicates within and destroys prostate cancer cells and, in both murine and human coculture models, that infection of prostate cancer cells inhibits osteoclastogenesis in vitro. In a murine model, progression of advanced prostate cancer bone metastases was inhibited by treatment with Ad5-Δ24-sOPG-Fc-RGD but not by an unarmed control CRAd.

Steps in prostate cancer progression that lead to bone metastasis

Prostate cancer is a complex disease in which metastasis to the bone is the main cause of death. Initial stages of metastasis are generally similar to those for most solid tumors; however, the mechanisms that underlie the homing of prostate tumor cells to the bone are not completely understood. Prostate cancer bone metastasis is also a microenvironment-driven disease, involving bidirectional interactions between the tumor and the bone microenvironment. In this review, we discuss the current understanding of the biologic processes and regulatory factors involved in the metastasis of prostate cancer cells, and their specific properties that promote growth in bone. Although many of these processes still need to be fully elucidated, a better understanding of the complex tumor/microenvironment interplay is slowly leading to more effective therapies for patients with prostate cancer bone metastases.

The critical role of the bone microenvironment in cancer metastases

Bone metastatic disease is a late-stage event of many common cancers, such as those of prostate and breast. It is incurable and causes severe morbidity. Tumor and bone interact in a vicious cycle, where tumor-secreted factors stimulate bone cells, which in turn release growth factors and cytokines that act back on the tumor cells. Within the vicious cycle are many potential therapeutic targets for novel treatment of bone metastatic disease. Therapeutic strategies can be oriented to inhibit bone cells (osteoclasts and osteoblasts) or tumor responses to factors enriched in the bone microenvironment. Many publications, especially from pre-clinical animal models, show that this approach, especially combination treatments, can reduce tumor burden and tumor-derived bone lesions. This supports a novel paradigm: tumor growth can be effectively inhibited by targeting the bone and its microenvironment rather than the tumor itself alone.

Androgen receptor-negative human prostate cancer cells induce osteogenesis in mice through FGF9-mediated mechanisms

In prostate cancer, androgen blockade strategies are commonly used to treat osteoblastic bone metastases. However, responses to these therapies are typically brief, and the mechanism underlying androgen-independent progression is not clear. Here, we established what we believe to be the first human androgen receptor-negative prostate cancer xenografts whose cells induced an osteoblastic reaction in bone and in the subcutis of immunodeficient mice. Accordingly, these cells grew in castrated as well as intact male mice. We identified FGF9 as being overexpressed in the xenografts relative to other bone-derived prostate cancer cells and discovered that FGF9 induced osteoblast proliferation and new bone formation in a bone organ assay. Mice treated with FGF9-neutralizing antibody developed smaller bone tumors and reduced bone formation. Finally, we found positive FGF9 immunostaining in prostate cancer cells in 24 of 56 primary tumors derived from human organ-confined prostate cancer and in 25 of 25 bone metastasis cases studied. Collectively, these results suggest that FGF9 contributes to prostate cancer-induced new bone formation and may participate in the osteoblastic progression of prostate cancer in bone. Androgen receptor-null cells may contribute to the castration-resistant osteoblastic progression of prostate cancer cells in bone and provide a preclinical model for studying therapies that target these cells.

Skeletal metastases: decreasing tumor burden by targeting the bone microenvironment

Several common cancers often metastasize to the skeleton in advanced disease. Bone metastases are incurable and cause protracted, severe symptoms. Growth of tumor in bone is driven by a vicious cycle: tumor-secreted factors stimulate bone cells, which in turn release growth factors and cytokines. The bone-derived factors fuel the vicious cycle by acting back on the tumor cells. The vicious cycle offers novel targets for the treatment of advanced cancers. Treatments can inhibit bone cells (osteoclasts and osteoblasts) that are stimulated by tumor-secreted factors. Drugs can also inhibit tumor responses to factors enriched in the bone microenvironment, such as transforming growth factor-beta. Animal models show that these approaches, especially combination treatments, can reduce tumor burden. The results suggest a novel paradigm in which tumor growth can be effectively inhibited by drugs that target cells in the bone microenvironment and not the tumor cells themselves.

Molecular biology of bone metastasis

Metastasis is a final stage of tumor progression. Breast and prostate cancer cells preferentially metastasize to bone, wherein they cause incurable osteolytic and osteoblastic lesions. The bone matrix is rich in factors, such as transforming growth factor-beta and insulin-like growth factors, which are released into the tumor microenvironment by osteolysis. These factors stimulate the growth of tumor cells and alter their phenotype, thus promoting a vicious cycle of metastasis and bone pathology. Physical factors within the bone microenvironment, including low oxygen levels, acidic pH, and high extracellular calcium concentrations, may also enhance tumor growth. These elements of the microenvironment are potential targets for chemotherapeutic intervention to halt tumor growth and suppress bone metastasis.

Hormonal regulation of IGFBP-2 proteolysis is attenuated with progression to androgen insensitivity in the LNCaP progression model

The identification of molecular determinants involved in the promotion of metastasis and development of androgen insensitive prostate cancer (AI-PCa) is necessary to discriminate aggressive from indolent disease and to identify therapeutic targets for advanced disease. Overexpression of one particular member of the insulin like growth factor (IGF) axis, IGFBP-2, is implicated in the development of AI-PCa and other cancers. Using the LNCaP human PCa progression model, we show that the AI and metastatic prostate cancer cell line C4-2B4 expresses greater amounts of secreted IGFBP-2 than the androgen sensitive (AS), non-metastatic LNCaP progenitor cell line. Further, the ability of androgens to decrease extracellular IGFBP-2 levels is attenuated in the AI and metastatic C4-2 cell line. The ability of androgen to negatively regulate extracellular IGFBP-2 levels was blocked by Casodex in a dose-dependent manner. The mechanism underlying the androgen-induced downregulation of secreted IGFBP-2 appears to involve extracellular proteolysis, resulting in the production of IGFBP-2 fragments lacking the ability to bind IGF-I and IGF-II. As C4-2 cells have an attenuated ability to proteolyze IGFBP-2 in response to androgen and C4-2B4 cells express greater amounts of IGFBP-2, our data implies that the diminished regulation of IGFBP-2 and loss of associated proteolytic fragments play a role in the increased metastatic behavior of these cells in vivo. Furthermore, our results suggest that either increased levels of intact IGFBP-2 or decreased levels of IGFBP-2 proteolytic fragments could serve as a biomarker to monitor for progression to AI-PCa.

The role of tumor microenvironment in prostate cancer bone metastasis

Prostate cancer (PCa) epithelial cells require a number of factors to facilitate their establishment and growth at a distant site of metastasis. Their ability to adapt to their microenvironment, proliferate and recruit an underlying stroma is integral to the survival and growth of the metastasis. PCa predominantly metastasizes to the bone, and bone metastases are the main cause of morbidity. The bone marrow provides a permissive environment for the formation of a metastasis. In some cases, the cells may remain dormant for some time, eventually proliferating in response to an unknown "trigger." The marrow is rich in progenitor cells that differentiate into numerous cell types, producing new blood vessels, supporting fibroblasts, and an underlying extracellular matrix (ECM) that form the reactive stroma. By secreting a number of cytokines, growth factors and proteases they recruit auxiliary cells required to produce a functional stroma. These components are involved in a reciprocal interaction between the stroma and the PCa cells, allowing for the growth and survival of the tumor. Left unchecked, once a PCa tumor has established itself in the bone marrow it will eventually replace the marrow, interrupting bone homeostasis and typically promoting an osteoblastic response in the bone including osteoclastic events. The abundant deposition of new woven bone results in nerve compression, bone pain and an increase in fractures in patients with PCa bone metastases. This review will examine the tumor microenvironment, its role in facilitating tumor dissemination, growth and the resultant pathologies associated with PCa bone metastasis.

Molecular interactions between breast cancer cells and the bone microenvironment drive skeletal metastases

Breast cancer cells preferentially spread to bone. Bone metastases are currently incurable and therefore better treatments need to be developed. Metastasis is an inefficient, multi-step process. Specific aspects of both breast cancer cells and the bone microenvironment contribute to the development of bone metastases. Breast cancers express chemokine receptors, integrins, cadherins, and bone-resorbing and bone-forming factors that contribute to the successful and preferential spread of tumor to bone. Bone is rich in growth factors and cell types that make it a hospitable environment for breast cancer growth. Once breast cancer cells enter the bone, a highly complex vicious cycle develops, in which breast cancer cells secrete factors that act on bone cells and other cells within the bone (stem cells, T cells, platelets, adipocytes, fibroblasts, and endothelial cells), causing them to secrete factors that act on adjacent cancer cells. The steps in the metastatic cascade and the vicious cycle within bone offer unique targets for adjuvant treatments to treat and cure bone metastases.