Osteoclast-like cell formation by circulating myeloma B lymphocytes: role of RANK-L

The immune cells in modulating osteoclast formation and bone metabolism

Osteoclasts are pivotal in regulating bone metabolism, with immune cells significantly influencing both physiological and pathological processes by modulating osteoclast functions. This is particularly evident in conditions of inflammatory bone resorption, such as rheumatoid arthritis and periodontitis. This review summarizes and comprehensively analyzes the research progress on the regulation of osteoclast formation by immune cells, aiming to unveil the underlying mechanisms and pathways through which diseases, such as rheumatoid arthritis and periodontitis, impact bone metabolism.

Molecular Features of the Mesenchymal and Osteoblastic Cells in Multiple Myeloma

Multiple myeloma (MM) is a monoclonal gammopathy characterized by biological heterogeneity and unregulated proliferation of plasma cells (PCs) in bone marrow (BM). MM is a multistep process based on genomic instability, epigenetic dysregulation and a tight cross-talk with the BM microenvironment that plays a pivotal role supporting the proliferation, survival, drug-resistance and homing of PCs. The BM microenvironment consists of a hematopoietic and a non-hematopoietic compartment, which cooperate to create a tumor environment. Among the non-hematopoietic component, mesenchymal stromal cells (MSCs) and osteoblasts (OBs) appear transcriptionally and functionally different in MM patients compared to healthy donors (HDs) and to patients with pre-malignant monoclonal gammopathies. Alterations of both MSCs and OBs underly the osteolytic lesions that characterize myeloma-associated bone disease. In this review, we will discuss the different characteristics of MSCs and OBs in MM patients, analyzing the transcriptome, the deregulated molecular pathways and the role performed by miRNAs and exosome in the pathophysiology of MM.

Epithelial Mesenchymal Transition (EMT) and Associated Invasive Adhesions in Solid and Haematological Tumours

In solid tumours, cancer cells that undergo epithelial mesenchymal transition (EMT) express characteristic gene expression signatures that promote invasive migration as well as the development of stemness, immunosuppression and drug/radiotherapy resistance, contributing to the formation of currently untreatable metastatic tumours. The cancer traits associated with EMT can be controlled by the signalling nodes at characteristic adhesion sites (focal contacts, invadopodia and microtentacles) where the regulation of cell migration, cell cycle progression and pro-survival signalling converge. In haematological tumours, ample evidence accumulated during the last decade indicates that the development of an EMT-like phenotype is indicative of poor disease prognosis. However, this EMT phenotype has not been directly linked to the assembly of specific forms of adhesions. In the current review we discuss the role of EMT in haematological malignancies and examine its possible link with the progression towards more invasive and aggressive forms of these tumours. We also review the known types of adhesions formed by haematological malignancies and speculate on their possible connection with the EMT phenotype. We postulate that understanding the architecture and regulation of EMT-related adhesions will lead to the discovery of new therapeutic interventions to overcome disease progression and resistance to therapies.

The Potential Role of RP105 in Regulation of Inflammation and Osteoclastogenesis During Inflammatory Diseases

Inflammatory diseases have a negative impact on bone homeostasis via exacerbated local and systemic inflammation. Bone resorbing osteoclasts are mainly derived from hematopoietic precursors and bone marrow monocytes. Induced osteoclastogenesis during inflammation, autoimmunity, metabolic diseases, and cancers is associated with bone loss and osteoporosis. Proinflammatory cytokines, pathogen-associated molecular patterns, or endogenous pathogenic factors induce osteoclastogenic differentiation by binding to the Toll-like receptor (TLR) family expressed on surface of osteoclast precursors. As a non-canonical member of the TLRs, radioprotective 105 kDa (RP105 or CD180) and its ligand, myeloid differentiation protein 1 (MD1), are involved in several bone metabolic disorders. Reports from literature had demonstrated RP105 as an important activator of B cells, bone marrow monocytes, and macrophages, which regulates inflammatory cytokines release from immune cells. Reports from literature had shown the association between RP105 and other TLRs, and the downstream signaling mechanisms of RP105 with different “signaling-competent” partners in immune cells during different disease conditions. This review is focused to summarize: (1) the role of RP105 on immune cells’ function and inflammation regulation (2) the potential regulatory roles of RP105 in different disease-mediated osteoclast activation and the underlying mechanisms, and (3) the different “signaling-competent” partners of RP105 that regulates osteoclastogenesis.

Mathematical modelling of the pathogenesis of multiple myeloma-induced bone disease

Multiple myeloma (MM) is the second most common haematological malignancy and results in destructive bone lesions. The interaction between MM cells and the bone microenvironment plays an important role in the development of the tumour cells and MM-induced bone disease and forms a 'vicious cycle' of tumour development and bone destruction, intensified by suppression of osteoblast activity and promotion of osteoclast activity. In this paper, a mathematical model is proposed to simulate how the interaction between MM cells and the bone microenvironment facilitates the development of the tumour cells and the resultant bone destruction. It includes both the roles of inhibited osteoblast activity and stimulated osteoclast activity. The model is able to mimic the temporal variation of bone cell concentrations and resultant bone volume after the invasion and then removal of the tumour cells and explains why MM-induced bone lesions rarely heal even after the complete removal of MM cells. The behaviour of the model compares well with published experimental data. The model serves as a first step to understand the development of MM-induced bone disease and could be applied further to evaluate the current therapies against MM-induced bone disease and even suggests new potential therapeutic targets.

Pasteurella multocida and immune cells

Pasteurella multocida was first discovered by Perroncito in 1878 and named after Louis Pasteur who first isolated and described this Gram-negative bacterium as the cause of fowl disease in 1880. Subsequently, P. multocida was also found to cause atrophic rhinitis in pigs, haemorrhagic septicaemia in cattle and respiratory diseases in many other animals. Among other factors such as lipopolysaccharide, outer membrane proteins and its capsule, the protein toxin (PMT) of P. multocida is an important virulence factor that determines the immunological response of the host's immune system. However, the exact molecular mechanisms taking place in cells of the innate and adaptive immune system are largely unknown for any of these virulence factors. Due to the obvious function of PMT on cells of the porcine skeletal system where it causes bone destruction, PMT was regarded as an osteolytic protein toxin. However, it remained unclear what the actual benefit for the bacteria would be. Recently, more attention was drawn to the osteoimmunological effects of PMT and the interplay between bone and immune cells. This review summarises the knowledge of effects of P. multocida virulence factors on the host's immune system.

Osteoblast function in myeloma

Multiple myeloma (MM) is the most frequent cancer to involve the skeleton and results in purely osteolytic lesions that rarely heal. MM bone disease is responsible for some of the most devastating complications of MM. The marrow microenvironment plays a key role in MM bone disease as well as in the initiation, expansion and chemoresistance of MM cells. How this microenvironment becomes so supportive of MM, and the contribution and interaction of the various components of the microenvironment to enhancing MM growth are only beginning to be understood. However, it is clear that suppression of osteoblast activity plays a key role in the bone destructive process as well as progression of the tumor burden in myeloma. The impairment of osteoblast activity in MM results primarily from blockade of osteogenic differentiation of mesenchymal progenitors to mature osteoblasts. MM patients have low to normal levels of bone formation markers, such as alkaline phosphatase and osteocalcin in the setting of increased bone resorption. In contrast, MM patients without bone lesions display balanced bone remodeling with increased osteoclastogenesis and normal or increased bone formation rates. Both soluble factors and cell-to-cell contact between MM cells and osteoblast progenitors are responsible for the suppression of osteoblast differentiation in MM. In this article, the mechanism responsible for osteoblast suppression will be reviewed, and the effects of novel bone anabolic agents on myeloma bone disease will be discussed.

Cell fusion and hyperactive osteoclastogenesis in multiple myeloma

Multiple myeloma (MM) is a hematologic malignancy whose progression may account for uncontrolled osteoclastogenesis promoted by the malignant plasma cells within the marrow microenvironment. Osteoclasts are multinucleated cells derived from the fusion of myeloid progenitors such as monocytes/macrophages, in response to specific differentiation factors released within the marrow niche, that are significantly deregulated in MM. In this malignancy DC-STAMP, a major fusogen protein enrolled by pre-osteoclasts, is highly expressed by peripheral macrophages, whereas dendritic cells and myeloma plasma cells show high fusogenic susceptibility and under specific conditions transdifferentiate to osteoclasts. In particular, the malignant plasma cells, besides altered ploidy, expression of cancer stem cell phenotype and high metastasizing capability, are able to express phenotypic markers of osteclasts, namely the proteolytic enzymes for the bone matrix, and to activate the β3 transcriptional pathway leading to ERK1/2 phosphorylation and initiation of the bone resorbing activity. Thus, based on the imbalanced osteoclast formation and activity that involve cells constitutively uncommitted to osteoclast differentiation, both homotypic and heterotypic cell fusions in myeloma marrow microenvironment represent a major pathogenetic event that drives the development and progression of the skeleton devastation typical of the myeloma bone disease.

Examining the metastatic niche: targeting the microenvironment

Some of the most common cancer types, including breast cancer, prostate cancer, and lung cancer, show a predilection to metastasize to bone. The molecular basis of this preferential growth of cancer cells in the bone microenvironment has been an area of active investigation. Although the precise molecular mechanisms underlying this process remain to be elucidated, it is now increasingly being recognized that the unique characteristics of the bone niche provide homing signals to cancer cells, and create a microenvironment conducive for the cancer cells to colonize. Concomitantly, cancer cells release several regulatory factors that result in abnormal bone destruction and/or formation. This complex bidirectional interplay between tumor cells and bone microenvironment establishes a "vicious cycle" that leads to a selective growth advantage for the cancer cells. The molecular insights gained on the underpinnings of bone metastasis in recent years have also provided us with avenues to devise innovative approaches for therapeutic intervention. The goal of this review is to describe our current understanding of molecular pathophysiology of cancer metastases to bone, as well as its therapeutic implications.

beta(3) Integrin subunit mediates the bone-resorbing function exerted by cultured myeloma plasma cells

alpha(v)beta(3) integrin was investigated in multiple myeloma in relation to the in vitro osteoclast-like activity of malignant plasma cells. Myeloma cells from patients with skeleton involvement overexpressed alpha(v)beta(3) and produced erosion pits on bone substrates, whereas this effect was not observed by cells from patients with no evidence of bone disease. We therefore explored the alpha(v)beta(3) transcriptional pathway in the bone-resorbing cells. Silencing of beta(3) chain abrogated the ability to produce erosion pits and extracellular signal-regulated kinase 1/2 phosphorylation resulting in the defective function of cFos and nuclear factor activator T cell 1, the terminal effectors of osteoclast activation. A similar defect occurred in constitutively beta(3)-deficient cells from patients with no skeleton disease. Microarray gene analysis of beta(3)(+) myeloma cells showed that several osteoclast-related genes were up-regulated. Their functions include the activation of receptor pathways beta(3) and c-fms that regulate several osteoclast functions. These data emphasize the postulated role of myeloma cells in multiple myeloma bone disease and suggest that their osteoclast-like activity is regulated, at least in vitro, by the beta(3) subunit of the integrin.

Bone lesions and macrophage inflammatory protein-1 alpha (MIP-1α) in human multiple myeloma

Bone lesions are a prominent feature accompanying multiple myeloma. Elucidation of the mechanisms regulating osteolysis is crucial in achieving a good quality of life, as such patients suffer from bone pain even after achieving improvement of the disease by high-dose chemotherapy. Recent research has revealed that bone lysis in myeloma patients is the result of both inhibited bone formation and enhanced bone destruction. It has been considered that bone absorption is regulated by activation of osteoclasts mediated by osteoclast activating factor (OAF) produced from myeloma cells. Macrophage inflammatory protein-1 alpha (MIP-1a) is a member of the chemokine family, and was originally determined as a soluble factor secreted from activated macrophages. Many candidates for OAF had been proposed and MIP-1a is now considered a major OAF. In this review, the significance of MIP-1a in myeloma bone disease is summarized.

Distribution of macrophages, osteoclasts and the B-lymphocyte lineage in osteolytic metastasis of mouse mammary carcinoma

The purpose of this study was to examine the localization of macrophages, B-lymphocytes and osteoclasts in tumoral lesions of mammary carcinoma metastasized to bone of non-immunocompromised mice. Mouse mammary carcinoma cells (BALB/c-MC) were injected through the left cardiac ventricle into 5-week-old female wild-type Balb/c mice. The femora and tibiae of mice with metastasized cancer were extracted, and thereafter processed for histochemical analyses. The foci of metastasized tumor cells occupied the metaphyseal area, and the cell death zones could be identified within the tumor mass. Abundant tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts were found among the alkaline phosphatase (ALP)-reactive osteoblastic cell layer that covered the bone surface neighboring the metastatic lesion. In contrast, F4/80-positive macrophages/monocytes were localized adjacent to, or invading the metastatic tissue. In addition, some F4/80-positive cells were found in the aforementioned cell death zones. Unlike F4/80-positive cells, CD45R-positive B-lymphocytes did not accumulate at the surfaces of the tumor lesions, nor infiltrate into them, but were found scattered over bone marrow. Interestingly, some CD45R-positive cells were observed close to TRAP-positive osteoclasts in the stromal tissue surrounding the tumor lesion. Our findings suggest that, in the bone metastatic lesions of non-immunocompromised mice, F4/80-positive macrophages/monocytes accumulated on and/or infiltrated into the tumor nests, while CD45R-positive B-lymphocytes were associated with osteoclasts, rather than attacking metastatic tumor cells.

Functional osteoclast-like transformation of cultured human myeloma cell lines

Hyperactive osteoclastogenesis is a hallmark of multiple myeloma, a B cell neoplasia homing to bone marrow and resulting in multiple osteolytic lesions and skeleton devastation. We provide evidence that myeloma cells can themselves act as osteoclasts in vitro. By extending standard cultures of U-266 and MCC-2 myeloma cell lines, we found that subsets of adherent cells also expressed the osteoclast phenotype, including multinuclear morphology, cytoplasmic tartrate-resistant acid phosphatase, the calcitonin receptor and a specific osteoclast antigen. These subsets resorbed bone substrates by producing osteoclast enzymes as well as the characteristic redistribution of F-actin in their cytoskeleton, thus forming the sealing zone that is adopted by adherent osteoclasts to generate the acidified environment essential for bone resorption. Neither the phenotype nor the functional properties of osteoclasts were detected in parental non-adherent cells. In adherent cultures osteoclastogenesis was associated with deregulated expression of both receptor activator of nuclear transcription factor (NF)-kappaB (RANK) and its ligand RANK-L, which triggers cell maturation in osteoclast precursors. Resorption of bone substrates was prevented by a neutralising anti-RANK-L antibody. Our data indicate that osteoclast-like transformation of both U-266 and MCC-2 cellular models of human myeloma is dependent on RANK-L stimulation.

Modulation of osteoclast formation

Osteoclasts are derived following the fusion of precursors of hematopoietic and myelomonocytic origin after appropriate stimulus, such as that afforded by RANKL and M-CSF. Thus the osteoclast can be considered as a specialized type of macrophage, and several of the factors that affect osteoclast formation also have affects upon macrophage differentiation. Inhibitors of osteoclast formation may perturb RANKL or M-CSF signalling or affect other signalling pathways. Several of these inhibitors are discussed with the view of their capacity to influence osteoclast differentiation, but not necessarily their activity.