Myeloma cells (5TMM) and their interactions with the marrow microenvironment

Hypermethylation of the Promoter Region of miR-23 Enhances the Metastasis and Proliferation of Multiple Myeloma Cells via the Aberrant Expression of uPA

Multiple myeloma has a long course, with no obvious symptoms in the early stages. However, advanced stages are characterized by injury to the bone system and represent a severe threat to human health. The results of the present work indicate that the hypermethylation of miR-23 promoter mediates the aberrant expression of uPA/PLAU (urokinase plasminogen activator, uPA) in multiple myeloma cells. miR-23, a microRNA that potentially targets uPA’s 3’UTR, was predicted by the online tool miRDB. The endogenous expressions of uPA and miR-23 are related to disease severity in human patients, and the expression of miR-23 is negatively related to uPA expression. The hypermethylation of the promoter region of miR-23 is a promising mechanism to explain the low level of miR-23 or aberrant uPA expression associated with disease severity. Overexpression of miR-23 inhibited the expression of uPA by targeting the 3’UTR of uPA, not only in MM cell lines, but also in patient-derived cell lines. Overexpression of miR-23 also inhibited in vitro and in vivo invasion of MM cells in a nude mouse model. The results therefore extend our knowledge about uPA in MM and may assist in the development of more effective therapeutic strategies for MM treatment.

The Association between Metabolic Syndrome and Multiple Myeloma

Multiple myeloma (MM) is a haematological malignancy arising from monoclonal proliferation of plasma cells in the bone marrow, resulting in the presence of paraproteins or M-protein in serum. The involvement of paraproteins produced by malignant plasma cells in the development of hyperlipidaemia and low-HDL cholesterol has been described, as has an association with MM and obesity, hypertension, and type 2 diabetes mellitus, and insulin resistance, that is, features of the metabolic syndrome (MS). There is an association between MS components, inflammatory cytokines, and the development of MM, and some drugs used in the treatment of MS such as statins and metformin may improve outcomes in MM.

Bone Marrow CX3CL1/Fractalkine is a New Player of the Pro-Angiogenic Microenvironment in Multiple Myeloma Patients

C-X3-C motif chemokine ligand 1 (CX3CL1)/fractalkine is a chemokine released after cleavage by two metalloproteases, ADAM metallopeptidase domain 10 (ADAM10) and ADAM metallopeptidase domain 17 (ADAM17), involved in inflammation and angiogenesis in the cancer microenvironment. The role of the CX3CL1/ C-X3-C motif chemokine receptor 1(CX3CR1) axis in the multiple myeloma (MM) microenvironment is still unknown. Firstly, we analyzed bone marrow (BM) plasma levels of CX3CL1 in 111 patients with plasma cell disorders including 70 with active MM, 25 with smoldering myeloma (SMM), and 16 with monoclonal gammopathy of undetermined significance (MGUS). We found that BM CX3CL1 levels were significantly increased in MM patients compared to SMM and MGUS and correlated with BM microvessel density. Secondly, we explored the source of CX3CL1 in MM and BM microenvironment cells. Primary CD138⁺ cells did not express CXC3L1 but up-regulated its production by endothelial cells (ECs) through the involvement of tumor necrosis factor alpha (TNFα). Lastly, we demonstrated the presence of CX3CR1 on BM CD14⁺CD16⁺ monocytes of MM patients and on ECs, but not on MM cells. The role of CX3CL1 in MM-induced angiogenesis was finally demonstrated in both in vivo chick embryo chorioallantoic membrane and in vitro angiogenesis assays. Our data indicate that CX3CL1, present at a high level in the BM of MM patients, is a new player of the MM microenvironment involved in MM-induced angiogenesis.

Constitutive Activation of STAT3 in Myeloma Cells Cultured in a Three-Dimensional, Reconstructed Bone Marrow Model

Malignant cells cultured in three-dimensional (3D) models have been found to be phenotypically and biochemically different from their counterparts cultured conventionally. Since most of these studies employed solid tumor types, how 3D culture affects multiple myeloma (MM) cells is not well understood. Here, we compared MM cells (U266 and RPMI8226) in a 3D culture model with those in conventional culture. While the conventionally cultured cells were present in single cells or small clusters, MM-3D cells grew in large spheroids. We discovered that STAT3 was the pathway that was more activated in 3D in both cell lines. The active form of STAT3 (phospho-STAT3 or pSTAT3), which was absent in MM cells cultured conventionally, became detectable after 1–2 days in 3D culture. This elevated pSTAT3 level was dependent on the 3D environment, since it disappeared after transferring to conventional culture. STAT3 inhibition using a pharmacological agent, Stattic, significantly decreased the cell viability of MM cells and sensitized them to bortezomib in 3D culture. Using an oligonucleotide array, we found that 3D culture significantly increased the expression of several known STAT3 downstream genes implicated in oncogenesis. Since most primary MM tumors are naturally STAT3-active, studies of MM in 3D culture can generate results that are more representative of the disease.

Primary myeloma interaction and growth in coculture with healthy donor hematopoietic bone marrow

Background

Human primary myeloma (MM) cells do not survive in culture; current in vitro and in vivo systems for growing these cells are limited to coculture with a specific bone marrow (BM) cell type or growth in an immunodeficient animal model. The purpose of the study is to establish an interactive healthy donor whole BM based culture system capable of maintaining prolonged survival of primary MM cells. This normal BM (NBM) coculture system is different from using autologous BM that is already affected by the disease.

Methods

Whole BM from healthy donors was cultured in medium supplemented with BM serum from MM patients for 7 days, followed by 7 days of coculture with CD138-selected primary MM cells or MM cell lines. MM cells in the coculture were quantified using flow cytometry or bioluminescence of luciferase-expressing MM cells. T-cell cytokine array and proteomics were performed to identify secreted factors.

Results

NBM is composed of adherent and nonadherent compartments containing typical hematopoietic and mesenchymal cells. MM cells, or a subset of MM cells, from all examined cases survived and grew in this system, regardless of the MM cells’ molecular risk or subtype, and growth was comparable to coculture with individual stromal cell types. Adherent and nonadherent compartments supported MM growth, and this support required patient serum for optimal growth. Increased levels of MM growth factors IL-6 and IL-10 along with MM clinical markers B2M and LDHA were detected in supernatants from the NBM coculture than from the BM cultured alone. Levels of extracellular matrix factors (e.g., MMP1, HMCN1, COL3A1, ACAN) and immunomodulatory factors (e.g., IFI16, LILRB4, PTPN6, AZGP1) were changed in the coculture system. The NBM system protected MM cells from dexamethasone but not bortezomib, and effects of lenalidomide varied.

Conclusions

The NBM system demonstrates the ability of primary MM plasma cells to interact with and to survive in coculture with healthy adult BM. This model is suitable for studying MM-microenvironment interactions, particularly at the early stage of engagement in new BM niches, and for characterizing MM cell subpopulations capable of long-term survival through secretion of extracellular matrix and immune-related factors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1892-7) contains supplementary material, which is available to authorized users.

The use of animal models in multiple myeloma

In myeloma, the understanding of the tissular, cellular and molecular mechanisms of the interactions between tumor plasma cells and bone cells have progressed from in vitro and in vivo studies. However none of the known animal models of myeloma reproduce exactly the human form of the disease. There are currently three types of animal models: (1) injection of pristane oil in BALB/c mice leads to intraperitoneal plasmacytomas but without bone marrow colonization and osteolysis; (2) injection of malignant plasma cell lines in immunodeficient mice SCID or NOD/SCID; the use of the SCID-hu or SCID-rab model allows the use of fresh plasma cells obtained from MM patients; (3) injection of allogeneic malignant plasma cells (5T2MM, 5T33) in the C57BL/KalwRij mouse induces bone marrow proliferation and osteolytic lesions. These cells did not grow in vitro and can be propagated by injection of plasma cells isolated from bone marrow of a mouse at end stage of the disease into young recipient mice. The 5TGM1 is a subclone of 5T33MM cells and can grow in vitro. Among the different models, the 5TMM models and SCID-hu/SCID-rab models were extensively used to test pathophysiological hypotheses and to assess anti-osteoclastic, anti-osteoblastic or anti-tumor therapies in myeloma. In the present review, we report the different types of animal models of MM and describe their interests and limitations.

Myeloid-derived suppressor cells as therapeutic target in hematological malignancies

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of immature myeloid cells that accumulate during pathological conditions such as cancer and are associated with a poor clinical outcome. MDSC expansion hampers the host anti-tumor immune response by inhibition of T cell proliferation, cytokine secretion, and recruitment of regulatory T cells. In addition, MDSC exert non-immunological functions including the promotion of angiogenesis, tumor invasion, and metastasis. Recent years, MDSC are considered as a potential target in solid tumors and hematological malignancies to enhance the effects of currently used immune modulating agents. This review focuses on the characteristics, distribution, functions, cell–cell interactions, and targeting of MDSC in hematological malignancies including multiple myeloma, lymphoma, and leukemia.

Augmented expression of urokinase plasminogen activator and extracellular matrix proteins associates with multiple myeloma progression

Multiple myeloma (MM) represents a B cell malignancy, characterized by a monoclonal proliferation of malignant plasma cells. Interactions between tumor cells and extracellular matrix (ECM) are of importance for tumor invasion and metastasis. Protein levels of urokinase plasminogen activator (uPA) and fibulin 1, nidogen and laminin in plasma and serum respectively and mRNA levels of these molecules in peripheral blood mononuclear cells were determined in 80 subjects by using ELISA and quantitative PCR and data was analyzed with severity of disease. Pearson correlation was determined to observe interrelationship between different molecules. A statistical significant increase for ECM proteins (laminin, nidogen and fibulin 1) and uPA at circulatory level as well as at mRNA level was observed compared to healthy controls. The levels of these molecules in serum might be utilized as a marker of active disease. Significant positive correlation of all ECM proteins with uPA was found and data also correlates with severity of disease. Strong association found between ECM proteins and uPA in this study supports that there might be interplay between these molecules which can be targeted. This study on these molecules may help to gain insight into processes of growth, spread, and clinical behavior of MM.

A novel mouse model for multiple myeloma (MOPC315.BM) that allows noninvasive spatiotemporal detection of osteolytic disease

Multiple myeloma (MM) is a lethal human cancer characterized by a clonal expansion of malignant plasma cells in bone marrow. Mouse models of human MM are technically challenging and do not always recapitulate human disease. Therefore, new mouse models for MM are needed. Mineral-oil induced plasmacytomas (MOPC) develop in the peritoneal cavity of oil-injected BALB/c mice. However, MOPC typically grow extramedullary and are considered poor models of human MM. Here we describe an in vivo-selected MOPC315 variant, called MOPC315.BM, which can be maintained in vitro. When injected i.v. into BALB/c mice, MOPC315.BM cells exhibit tropism for bone marrow. As few as 10(4) MOPC315.BM cells injected i.v. induced paraplegia, a sign of spinal cord compression, in all mice within 3-4 weeks. MOPC315.BM cells were stably transfected with either firefly luciferase (MOPC315.BM.Luc) or DsRed (MOPC315.BM.DsRed) for studies using noninvasive imaging. MOPC315.BM.Luc cells were detected in the tibiofemoral region already 1 hour after i.v. injection. Bone foci developed progressively, and as of day 5, MM cells were detected in multiple sites in the axial skeleton. Additionally, the spleen (a hematopoietic organ in the mouse) was invariably affected. Luminescent signals correlated with serum myeloma protein concentration, allowing for easy tracking of tumor load with noninvasive imaging. Affected mice developed osteolytic lesions. The MOPC315.BM model employs a common strain of immunocompetent mice (BALB/c) and replicates many characteristics of human MM. The model should be suitable for studies of bone marrow tropism, development of osteolytic lesions, drug testing, and immunotherapy in MM.

Targeting the bone microenvironment in multiple myeloma

Multiple myeloma (MM) is a plasma cell malignancy characterized by the frequent development of osteolytic lesions, osteopenia, pathological fractures, and/or severe bone pain. In the past few years several potential factors involved in this process have been identified and, with the increased knowledge of the signaling pathways involved in the regulation of normal osteoblast and osteoclast function, have provided us with a better understanding of the contributions of the marrow microenvironment to MM bone disease. These studies have identified several potential novel targets for treating MM bone disease in addition to the current standard treatment of bisphosphonates. In this article, we discuss several potential targets for treating MM bone disease as well as novel therapies that are in clinical trials for these patients.

Brain-derived neurotrophic factor induces proliferation, migration, and VEGF secretion in human multiple myeloma cells via activation of MEK-ERK and PI3K/AKT signaling

This study investigated the signaling pathways involved in the different biological effects of brain-derived neurotrophic factor (BDNF) in multiple myeloma (MM). The effects of BDNF on proliferation of MM cell lines and primary myeloma cells were examined by [(3)H]thymidine incorporation assay. The effects of BDNF on MM cells migration were studied by transwell migration assay. Stimulation by BDNF of vascular endothelial growth factor (VEGF) production was analyzed by reverse transcriptase-polymerase chain reaction and enzyme-linked immunosorbent assay. The signal-transduction pathways that are activated in response to BDNF were determined by Western blots. VEGF is induced by BDNF in a dose-dependent manner in MM cells. Stimulation of MM cells with BDNF led to the activation of the phosphatidylinositol 3-kinase (PI3K)/Akt and the MEK-extracellular signal-regulated protein kinase pathways. Using specific signal-transduction inhibitors, we demonstrated that MEK is required for BDNF-induced proliferation, whereas activation of PI3K is required for BDNF-stimulated migration and VEGF production. BDNF affects different cell signaling pathways mediating growth, migration, and VEGF secretion in MM cells. Our observations provided the framework for novel therapeutic strategies targeting BDNF signaling cascades in MM.

Serum levels of total-RANKL in multiple myeloma

BACKGROUND

Receptor activator of nuclear factor-kappaB ligand (RANKL) plays a key role in osteoclast activation in myeloma bone disease. The increased expression of RANKL in the bone marrow microenvironment was demonstrated in several studies, but there are only rare data on circulating RANKL levels in patients with multiple myeloma (MM).

PATIENTS AND METHODS

In the current study, we investigated the clinical significance of serum RANKL levels, using an enzyme-linked immunosorbent assay test that detects both free and osteoprotegerin (OPG)-bound RANKL (total-RANKL, tRANKL) in patients with newly diagnosed MM (n = 93) and monoclonal gammopathy of undetermined significance (MGUS; n = 20) compared with healthy controls (n = 20).

RESULTS

Circulating serum tRANKL was significantly elevated in patients with MM compared with controls (P < .001) or MGUS (P < .001). Furthermore, tRANKL levels were higher in smoldering MM versus MGUS (P = .031) and in symptomatic versus smoldering MM (P < .001). Serum tRANKL increased parallel to International Staging System stages I to III (P = .004) and correlated with the presence of lytic bone lesions (P < .001). Total-RANKL was a prognostic factor for overall survival in symptomatic MM (P = .043). A significantly longer progression-free survival was observed in patients with a > 50% decrease in tRANKL levels after 3 months of combined chemotherapy and bisphosphonate treatment.

CONCLUSION

Our study demonstrates for the first time that serum tRANKL reflects advanced disease, lytic bone destruction, and poor prognosis in MM.

The role of the insulin-like growth factor 1 receptor axis in multiple myeloma

Multiple myeloma remains a fatal B cell malignancy with severe clinical features such as anaemia and bone fractures, caused by the predominant localization of the myeloma cells in the bone marrow (BM). The MM cells first migrate towards the BM, followed by their clonal expansion and induction of angiogenesis and osteolysis. Insulin-like growth factor 1 or IGF-1 is a cytokine which plays a role in myeloma development. Besides serving as a growth and survival factor, it attracts the cells towards the BM, and is involved in the angiogenesis process. This makes the IGF-1R an interesting target for therapeutical interventions. Apart from mediating aspects of the malignant phenotype, it also appears not to be an absolute requirement for normal cell homeostasis. Various strategies targeting the IGF-1R have emerged with the two main strategies being blocking antibodies and small molecule inhibitors. After encouraging preclinical results both strategies are now in clinical trials.

T-lymphocyte interaction with stromal, bone and hematopoietic cells in the bone marrow

Mature T cells in the bone marrow (BM) are in constant exchange with the blood pool. Within the BM, T-cell recognition of antigen presented by dendritic cell (DC) can occur, nevertheless it is thought that BM T cells mostly receive non-antigenic signals by either stimulatory, for example, interleukin (IL)-7, IL-15, tumor necrosis factor family members, or inhibitory molecules, for example, transforming growth factor-beta. The net balance is in favor of T-cell proliferation. Indeed, the percentage of proliferating T cells is higher in the BM than in spleen and lymph nodes, both within CD4 and CD8 T cells. High numbers of memory T cells proliferate in the BM, as they preferentially home to the BM and have an increased turnover as compared with naive T cells. I propose here that the BM plays an essential role in maintaining normal peripheral T-lymphocyte numbers and antigen-specific memory for both CD4 and CD8 T cells. I also discuss BM T-cell contribution to the homeostasis of bone metabolism as well as of hematopoiesis. It emerges that BM T cells play unexpected roles in several diseases, for example AIDS and osteoporosis. A better knowledge on BM T cells has implications for currently used clinical interventions, for example, vaccination, BM transplantation, mesenchymal stem cell-based therapies.

Heparanase stimulation of protease expression implicates it as a master regulator of the aggressive tumor phenotype in myeloma

High levels of heparanase are an indicator of poor prognosis in myeloma patients, and up-regulation of the enzyme enhances tumor growth, angiogenesis, and metastasis in animal models. At least part of the impact of heparanase in driving the aggressive tumor phenotype is due to its effect on increasing the expression and shedding of the heparan sulfate proteoglycan syndecan-1, a molecule known to promote myeloma progression. The present work demonstrated that elevation in heparanase expression in myeloma cells stimulates sustained ERK phosphorylation that in turn drives MMP-9 expression. In addition, urokinase-type plasminogen activator (uPA) and uPA receptor expression levels increased, and blocking the proteolytic activation of either MMP-9 or uPA inhibited the heparanase-induced increase in syndecan-1 shedding. Together these data provide a mechanism for heparanase-induced syndecan-1 shedding and, more importantly, demonstrate that heparanase activity in myeloma cells can lead to increased levels of proteases that are known to play important roles in the aggressive behavior of myeloma tumors. This in addition to its other known biological roles, indicates that heparanase acts as a master regulator of the aggressive tumor phenotype by up-regulating protease expression and activity within the tumor microenvironment.

Role of CCR1 and CCR5 in homing and growth of multiple myeloma and in the development of osteolytic lesions: a study in the 5TMM model

Multiple myeloma (MM) is a plasma cell malignancy, characterized by the localization of the MM cells in the bone marrow (BM), where they proliferate and induce osteolysis. The MM cells first need to home or migrate to the BM to receive necessary survival signals. In this work, we studied the role of CCR1 and CCR5, two known chemokine receptors, in both chemotaxis and osteolysis in the experimental 5TMM mouse model. A CCR1-specific (BX471) and a CCR5-specific (TAK779) antagonist were used to identify the function of both receptors. We could detect by RT-PCR and flow cytometric analyses the expression of both CCR1 and CCR5 on the cells and their major ligand, macrophage inflammatory protein 1alpha (MIP1alpha) could be detected by ELISA. In vitro migration assays showed that MIP1alpha induced a 2-fold increase in migration of 5TMM cells, which could only be blocked by TAK779. In vivo homing kinetics showed a 30% inhibition in BM homing when 5TMM cells were pre-treated with TAK779. We found, in vitro, that both inhibitors were able to reduce osteoclastogenesis and osteoclastic resorption. In vivo end-term treatment of 5T2MM mice with BX471 resulted in a reduction of the osteolytic lesions by 40%; while TAK779 treatment led to a 20% decrease in lesions. Furthermore, assessment of the microvessel density demonstrated a role for both receptors in MM induced angiogenesis. These data demonstrate the differential role of CCR1 and CCR5 in MM chemotaxis and MM associated osteolysis and angiogenesis.

Role of the microenvironment in multiple myeloma bone disease

Multiple myeloma bone disease occurs in over 70-80% of patients with myeloma and represents a significant source of morbidity and mortality. Early in multiple myeloma bone disease there is a balance between osteoclast activation and osteoblast suppression. However, this balance appears to be lost in advanced disease, resulting in the development of lytic lesions and bone destruction. Osteoclast activation occurs through a variety of factors, including receptor activator of nuclear factor-kappaB ligand, macrophage inflammatory protein-1alpha, interleukin-3 and interleukin-6, resulting in osteoclast stimulation and bone resorption. There is also significant osteoblast suppression through the inhibitory actions of interleukin-3, dickkopf 1, secreted frizzled-related protein-2 and interleukin-7. Understanding the mechanisms behind myeloma bone disease will help to identify potential future therapeutic interventions to help ameliorate or prevent osteoblast suppression and decrease osteoclast activation, with the goal of improving the overall quality of life for patients with multiple myeloma.

Endothelial cell-driven regulation of CD9 or motility-related protein-1 expression in multiple myeloma cells within the murine 5T33MM model and myeloma patients

The cell surface expression of CD9, a glycoprotein of the tetraspanin family influencing several processes including cell motility and metastasis, inversely correlates with progression in several solid tumors. In the present work, we studied the expression and role of CD9 in multiple myeloma (MM) biology using the 5T33MM mouse model. The 5T33MMvitro cells were found to be CD9 negative. Injection of these cells in mice caused upregulation of CD9 expression, while reculturing them resulted in downregulation of CD9. Coculturing of CD9-negative 5T33MMvitro cells with BM endothelial cells (BMECs) resulted in a partial retrieval of CD9. Laser microdissection followed by real-time polymerase chain reaction and immunohistochemistry performed on bone sections of 5T33MMvivo diseased mice demonstrated strong local expression of CD9 on MM cells in contact with BMEC compared to MM cells further away. These findings were also confirmed by immunohistochemistry in MM patients. Neutralizing anti-CD9 antibodies inhibited transendothelial invasion of CD9-expressing human MM5.1 and murine 5T33MMvivo cells. In conclusion, we provide evidence that CD9 expression by the MM cells is upregulated in vivo by close interaction of the cells with BMEC and that CD9 is involved in transendothelial invasion, thus possibly mediating homing and/or spreading of the MM cells.

Bone marrow angiogenesis in multiple myeloma

Angiogenesis is a constant hallmark of multiple myeloma (MM) progression and has prognostic potential. It is induced by plasma cells via angiogenic factors with the transition from monoclonal gammopathy of undetermined significance (MGUS) to MM, and probably with loss of angiostatic activity on the part of MGUS. The pathophysiology of MM-induced angiogenesis is complex and involves both direct production of angiogenic cytokines by plasma cells and their induction within the microenvironment. The latter are secreted by stromal cells, endothelial cells (EC) and osteoclasts, and promote plasma cell growth, survival and migration, as well as paracrine cytokine secretion and angiogenesis in the bone marrow milieu. Angiogenesis is also supported by inflammatory cells following their recruitment and activation by plasma cells. Finally, circulating EC and endothelial precursor cells (EPC) contribute to the neovascularization, and the presence of EPC suggests that vasculogenesis (new vessel formation from EPC) may also contribute to the full MM vascular tree.

The involvement of osteopontin and its receptors in multiple myeloma cell survival, migration and invasion in the murine 5T33MM model

Multiple myeloma (MM) is a malignancy characterised by the accumulation of monoclonal plasma cells in the bone marrow. Different reports indicate the expression of CD44 variant isoforms (CD44v) by MM cells. Osteopontin (OPN), which is expressed by MM cells, is known to be a ligand for CD44v6. In this study, we investigated the role of OPN with emphasis on a functional correlation between OPN and CD44v in the 5T33MM model. Our group reported the expression of CD44v by 5T33MM cells. Using this model, we have demonstrated the secretion of OPN by 5T33MM cells. OPN affected 5T33MM cell survival by increasing proliferation and inhibiting apoptosis. OPN also stimulated 5T33MM cell migration, transendothelial migration and matrix metalloproteinase-9 activity. We confirmed the proliferative and migratory effects of OPN on human MM cells. By applying inhibiting anti-CD44v6 antibodies, we found that OPN stimulated cell proliferation by engaging this isoform. Anti-CD44v antibodies and RGD peptides both inhibited cell migration, suggesting an involvement of both, CD44v isoforms and integrins. In conclusion, OPN may act as a mediator of MM cell survival by engaging CD44v. The protein is further involved in migration and invasion of MM cells through the activation of either alphavbeta3 integrin or CD44v isoforms.

Synergistic activity of the proteasome inhibitor PS-341 with non-myeloablative 153-Sm-EDTMP skeletally targeted radiotherapy in an orthotopic model of multiple myeloma

Multiple myeloma is a highly radiosensitive skeletal malignancy, but bone-seeking radionuclides have not yet found their place in disease management. We previously reported that the proteasome inhibitor PS-341 selectively sensitizes myeloma cells to the lethal effects of ionizing radiation. To extend these observations to an in vivo model, we combined PS-341 with the bone-seeking radionuclide 153-Sm-EDTMP. In vitro clonogenic assays demonstrated synergistic killing of myeloma cells exposed to both PS-341 and 153-Sm-EDTMP. Using the orthotopic, syngeneic 5TGM1 myeloma model, the median survivals of mice treated with saline, 2 doses of PS-341 (0.5 mg/kg), or a single nonmyeloablative dose of 153-Sm-EDTMP (22.5 MBq) were 21, 22, and 28 days, respectively. In contrast, mice treated with combination therapy comprising 2 doses of PS-341 (0.5 mg/kg), 1 day prior to and 1 day following 153-Sm-EDTMP (22.5 MBq) showed a significantly prolonged median survival of 49 days (P < .001). In addition to prolonged survival, this treatment combination yielded reduced clonogenicity of bone marrow-resident 5TGM1 cells, reduced serum myeloma-associated paraprotein levels, and better preservation of bone mineral density. Myelosuppression, determined by peripheral blood cell counts and clonogenicity assays of hematopoietic progenitors, did not differ between animals treated with 153-Sm-EDTMP alone versus those treated with the combination of PS-341 plus 153-Sm-EDTMP. PS-341 is a potent, selective in vivo radiosensitizer that may substantially affect the efficacy of skeletal-targeted radiotherapy in multiple myeloma.