Jessica A. Fowler

Host-derived adiponectin is tumor-suppressive and a novel therapeutic target for multiple myeloma and the associated bone disease.

The contributions of the host microenvironment to the pathogenesis of multiple myeloma, including progression from the non-malignant disorder monoclonal gammopathy of undetermined significance, are poorly understood. In the present study, microarray analysis of a murine model requiring a unique host microenvironment for myeloma development identified decreased host-derived adiponectin compared with normal mice. In support, clinical analysis revealed decreased serum adiponectin concentrations in monoclonal gammopathy of undetermined significance patients who subsequently progressed to myeloma. We investigated the role of adiponectin in myeloma pathogenesis and as a treatment approach, using both mice deficient in adiponectin and pharmacologic enhancement of circulating adiponectin. Increased tumor burden and bone disease were observed in myeloma-bearing adiponectin-deficient mice, and adiponectin was found to induce myeloma cell apoptosis. The apolipoprotein peptide mimetic L-4F was used for pharmacologic enhancement of adiponectin. L-4F reduced tumor burden, increased survival of myeloma-bearing mice, and prevented myeloma bone disease. Collectively, our studies have identified a novel mechanism whereby decreased host-derived adiponectin promotes myeloma tumor growth and osteolysis. Furthermore, we have established the potential therapeutic benefit of increasing adiponectin for the treatment of myeloma and the associated bone disease.

Bone marrow stromal cells create a permissive microenvironment for myeloma development: a new stromal role for Wnt inhibitor Dkk1.

The rapid progression of multiple myeloma is dependent upon cellular interactions within the bone marrow microenvironment. In vitro studies suggest that bone marrow stromal cells (BMSC) can promote myeloma growth and survival and osteolytic bone disease. However, it is not possible to recreate all cellular aspects of the bone marrow microenvironment in an in vitro system, and the contributions of BMSCs to myeloma pathogenesis in an intact, immune competent, in vivo system are unknown. To investigate this, we used a murine myeloma model that replicates many features of the human disease. Coinoculation of myeloma cells and a BMSC line, isolated from myeloma-permissive mice, into otherwise nonpermissive mice resulted in myeloma development, associated with tumor growth within bone marrow and osteolytic bone disease. In contrast, inoculation of myeloma cells alone did not result in myeloma. BMSCs inoculated alone induced osteoblast suppression, associated with an increase in serum concentrations of the Wnt signaling inhibitor, Dkk1. Dkk1 was highly expressed in BMSCs and in myeloma-permissive bone marrow. Knockdown of Dkk1 expression in BMSCs decreased their ability to promote myeloma and the associated bone disease in mice. Collectively, our results show novel roles of BMSCs and BMSC-derived Dkk1 in the pathogenesis of multiple myeloma in vivo.

Tumor-host cell interactions in the bone disease of myeloma

Multiple myeloma is a hematological malignancy that is associated with the development of a destructive osteolytic bone disease, which is a major cause of morbidity for patients with myeloma. Interactions between myeloma cells and cells of the bone marrow microenvironment promote both tumor growth and survival and bone destruction, and the osteolytic bone disease is now recognized as a contributing component to tumor progression. Since myeloma bone disease is associated with both an increase in osteoclastic bone resorption and a suppression of osteoblastic bone formation, research to date has largely focused upon the role of the osteoclast and osteoblast. However, it is now clear that other cell types within the bone marrow, including cells of the immune system, mesenchymal stem cells and bone marrow stromal cells, can contribute to the development of myeloma bone disease. This review discusses the cellular mechanisms and potential therapeutic targets that have been implicated in myeloma bone disease.

Myeloma cells exhibit an increase in proteasome activity and an enhanced response to proteasome inhibition in the bone marrow microenvironment in vivo.

The proteasome inhibitor bortezomib has a striking clinical benefit in patients with multiple myeloma. It is unknown whether the bone marrow microenvironment directly contributes to the dramatic response of myeloma cells to proteasome inhibition in vivo. We have used the well‐characterized 5TGM1 murine model of myeloma to investigate myeloma growth within bone and response to the proteasome inhibitor bortezomib in vivo. Myeloma cells freshly isolated from the bone marrow of myeloma‐bearing mice were found to have an increase in proteasome activity and an enhanced response to in vitro proteasome inhibition, as compared with pre‐inoculation myeloma cells. Treatment of myeloma‐bearing mice with bortezomib resulted in a greater reduction in tumor burden when the myeloma cells were located within the bone marrow when compared with extra‐osseous sites. Our results demonstrate that myeloma cells exhibit an increase in proteasome activity and an enhanced response to bortezomib treatment when located within the bone marrow microenvironment in vivo. Am. J. Hematol., 2009.

A murine model of myeloma that allows genetic manipulation of the host microenvironment

SUMMARY Multiple myeloma, and the associated osteolytic bone disease, is highly dependent upon cellular interactions within the bone marrow microenvironment. A major limitation of existing myeloma models is the requirement for a specific host strain of mouse, preventing molecular examination of the bone marrow microenvironment. The aim of the current study was to develop a model of myeloma in which the host microenvironment could be modified genetically. The Radl 5T murine model of myeloma is well characterized and closely mimics human myeloma. In the current study, we demonstrate 5T myeloma establishment in recombination activating gene 2 (RAG-2)-deficient mice, which have improper B- and T-cell development. Importantly, these mice can be easily bred with genetically modified mice to generate double knockout mice, allowing manipulation of the host microenvironment at a molecular level. Inoculation of 5TGM1 myeloma cells into RAG-2−/− mice resulted in myeloma development, which was associated with tumor growth within bone and an osteolytic bone disease, as assessed by microcomputed tomography (microCT), histology and histomorphometry. Myeloma-bearing RAG-2−/− mice displayed many features that were similar to both human myeloma and the original Radl 5T model. To demonstrate the use of this model, we have examined the effect of host-derived matrix metalloproteinase 9 (MMP-9) in the development of myeloma in vivo. Inoculation of 5TGM1 myeloma cells into mice that are deficient in RAG-2 and MMP-9 resulted in a reduction in both tumor burden and osteolytic bone disease when compared with RAG-2-deficient wild-type myeloma-bearing mice. The establishment of myeloma in RAG-2−/− mice permits molecular examination of the host contribution to myeloma pathogenesis in vivo.

Diet-induced obesity promotes a myeloma-like condition in vivo.

Multiple myeloma remains a fatal haematological malignancy associated with clonal plasma cell expansion within the bone marrow, osteolytic bone disease, anaemia and renal failure. In almost all cases, myeloma is preceded by the nonmalignant plasma cell disorder monoclonal gammopathy of undetermined significance (MGUS), characterized by an increase in monoclonal immunoglobulin secretion, <10% plasma cells within the bone marrow and a lack of lytic bone lesions. The mechanisms underlying the pathogenesis of MGUS and progression to myeloma are complex with a major role attributed to the host microenvironment. A loss of host-derived adiponectin is known to promote myeloma development in vivo, both in murine models and in patients with MGUS.1 Adiponectin is inversely linked to obesity, with increasing epidemiological evidence supporting an association between obesity and MGUS or myeloma.2, 3, 4, 5, 6, 7 In the present study, we have combined a well-characterized murine model of myeloma with diet-induced obesity and a genetic model of obesity to determine the effect of increased obesity on myeloma development in vivo.

A loss of host-derived MMP-7 promotes myeloma growth and osteolytic bone disease in vivo

Matrix metalloproteinases (MMPs) play a critical role in cancer pathogenesis, including tumor growth and osteolysis within the bone marrow microenvironment. However, the anti-tumor effects of MMPs are poorly understood, yet have significant implications for the therapeutic potential of targeting MMPs. Host derived MMP-7 has previously been shown to support the growth of bone metastatic breast and prostate cancer. In contrast and underscoring the complexity of MMP biology, here we identified a tumor-suppressive role for host MMP-7 in the progression of multiple myeloma in vivo. An increase in tumor burden and osteolytic bone disease was observed in myeloma-bearing MMP-7 deficient mice, as compared to wild-type controls. We observed that systemic MMP-7 activity was reduced in tumor-bearing mice and, in patients with multiple myeloma this reduced activity was concomitant with increased levels of the endogenous MMP inhibitor, tissue inhibitor of metalloproteinases-1 (TIMP-1). Our studies have identified an unexpected tumour-suppressive role for host-derived MMP-7 in myeloma bone disease in vivo, and highlight the importance of elucidating the effect of individual MMPs in a disease-specific context.

Animal Models of Choroidal Neovascularization

Choroidal neovascularization (CNV) is a pathological condition in which proliferating choroidal blood vessels grow through Bruch’s membrane, penetrate the retinal pigment epithelium (RPE) and extend into the subretinal space. There, the blood vessels leak fluid, ultimately leading to serous retinal detachment. CNV associated with the wet form of age-related macular degeneration (AMD) is the major cause of vision loss in the elderly. However, in spite of its prevalence, relatively little is known concerning the pathogenesis of CNV. In order to better understand this disease process and explore therapies to treat it, several experimental animal models of CNV have been developed. The most widely used of these models is laser-induced CNV in primates and rodents, but several knockout and transgenic mouse models exist as well. The aim of this chapter is to explore the historical background and significance of these animal models of CNV.

The Role of the Immune System in Hematologic Malignancies that Affect Bone

Publisher Summary Osteoimmunology in the context of multiple myeloma is a newly emerging field, and it is clear that interactions between myeloma cells and cells of the immune system are important both in terms of tumor growth and the development of the osteolytic bone disease. Increasing our understanding of the role of the immune system in myeloma bone disease, for example, the effect of the immunosuppression that is found in patients with myeloma will ultimately identify new therapeutic targets for the treatment of myeloma. Current knowledge is limited due to the difficulty of verifying intriguing in vitro observations in appropriate in vivo systems. Immune cells can have both a deleterious and advantageous role in myeloma pathogenesis. Macrophages and T cells, for example, can become altered by the presence of myeloma cells within the bone marrow cavity and act to support further cancer progression; however, the immune system can also be utilized for anticancer therapies. Within a healthy individual, the immune system provides defense mechanisms critical for protecting the body against foreign pathogens as well as tumor formation. The manipulation or stimulation of the bodys natural immune system to fight cancer provides an attractive area for therapeutic potential. Emerging cancer research of recent years not only demonstrates genetic alterations to the cancer cells but also to the surrounding microenvironment. The future of effective cancer therapeutics will have a dual focus; treating both the tumor cells and the altered microenvironment.