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

RUNX2 promotes the suppression of osteoblast function and enhancement of osteoclast activity by multiple myeloma cells

RUNX2 is a transcription factor that participates in osteoblast differentiation and chondrocyte maturation and plays an important role in the invasion and metastasis of cancers. With the deepening of research, evidence has indicated the correlation between RUNX2 and bone destruction in cancers. However, the mechanisms underlying its role in multiple myeloma remain unclear. By observing the induction effects of conditioned medium from myeloma cells on preosteoblasts (MC3T3-E1) and preosteoclasts (RAW264.7) and constructing myeloma-bearing mice, we found that RUNX2 promotes bone destruction in multiple myeloma. In vitro, conditioned medium from RUNX2-overexpressing myeloma cells reduced osteoblast activity and increased osteoclast activity. In vivo, RUNX2 expression was positively correlated with bone loss in myeloma-bearing mice. These results suggest that therapeutic inhibition of RUNX2 may protect against bone destruction by maintaining the balance between osteoblast and osteoclast activity in multiple myeloma.

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

Mechanical regulation of bone remodeling

Bone remodeling is a lifelong process that gives rise to a mature, dynamic bone structure via a balance between bone formation by osteoblasts and resorption by osteoclasts. These opposite processes allow the accommodation of bones to dynamic mechanical forces, altering bone mass in response to changing conditions. Mechanical forces are indispensable for bone homeostasis; skeletal formation, resorption, and adaptation are dependent on mechanical signals, and loss of mechanical stimulation can therefore significantly weaken the bone structure, causing disuse osteoporosis and increasing the risk of fracture. The exact mechanisms by which the body senses and transduces mechanical forces to regulate bone remodeling have long been an active area of study among researchers and clinicians. Such research will lead to a deeper understanding of bone disorders and identify new strategies for skeletal rejuvenation. Here, we will discuss the mechanical properties, mechanosensitive cell populations, and mechanotransducive signaling pathways of the skeletal system.

The EGFL7-ITGB3-KLF2 axis enhances survival of multiple myeloma in preclinical models

Angiogenic factors play a key role in multiple myeloma (MM) growth, relapse, and drug resistance. Here we show that malignant plasma cells (cell lines and patient-derived MM cells) express angiocrine factor EGF like-7 (EGFL7) mRNA and protein. MM cells both produced EGFL7 and expressed the functional EGFL7 receptor integrin β 3 (ITGB3), resulting in ITGB3 phosphorylation and focal adhesion kinase activation. Overexpression of ITGB3 or EGFL7 enhanced MM cell adhesion and proliferation. Intriguingly, ITGB3 overexpression upregulated the transcription factor Krüppel-like factor 2 (KLF2), which further enhanced EGFL7 transcription in MM cells, thereby establishing an EGFL7-ITGB3-KLF2-EGFL7 amplification loop that supports MM cell survival and proliferation. EGFL7 expression was found in certain plasma cells of patients with refractory MM and of patients at primary diagnosis. NOD.CB17-Prkdc/J mice transplanted with MM cells showed elevated human plasma EGFL7 levels. EGFL7 knockdown in patient-derived MM cells and treatment with neutralizing antibodies against EGFL7 inhibited MM cell growth in vitro and in vivo. We demonstrate that the standard-of-care MM drug bortezomib upregulates EGFL7, ITGB3, and KLF2 expression in MM cells. Inhibition of EGFL7 signaling in synergy with BTZ may provide a novel strategy for inhibiting MM cell proliferation.

Mechanical sensing protein PIEZO1 regulates bone homeostasis via osteoblast-osteoclast crosstalk

Wolff’s law and the Utah Paradigm of skeletal physiology state that bone architecture adapts to mechanical loads. These models predict the existence of a mechanostat that links strain induced by mechanical forces to skeletal remodeling. However, how the mechanostat influences bone remodeling remains elusive. Here, we find that Piezo1 deficiency in osteoblastic cells leads to loss of bone mass and spontaneous fractures with increased bone resorption. Furthermore, Piezo1-deficient mice are resistant to further bone loss and bone resorption induced by hind limb unloading, demonstrating that PIEZO1 can affect osteoblast-osteoclast crosstalk in response to mechanical forces. At the mechanistic level, in response to mechanical loads, PIEZO1 in osteoblastic cells controls the YAP-dependent expression of type II and IX collagens. In turn, these collagen isoforms regulate osteoclast differentiation. Taken together, our data identify PIEZO1 as the major skeletal mechanosensor that tunes bone homeostasis.

Mechanical forces induce bone remodeling, but how bone cells sense mechanical signaling is unclear. Here, the authors show that loss of the mechanotransduction channel Piezo1 in osteoblastic cells impairs osteoclast activity via YAP signaling and collagen expression, leading to reduced bone mass and spontaneous fractures.

Novel Pure αVβ3 Integrin Antagonists That Do Not Induce Receptor Extension, Prime the Receptor, or Enhance Angiogenesis at Low Concentrations

The integrin αVβ3 receptor has been implicated in several important diseases, but no antagonists are approved for human therapy. One possible limitation of current small-molecule antagonists is their ability to induce a major conformational change in the receptor that induces it to adopt a high-affinity ligand-binding state. In response, we used structural inferences from a pure peptide antagonist to design the small-molecule pure antagonists TDI-4161 and TDI-3761. Both compounds inhibit αVβ3-mediated cell adhesion to αVβ3 ligands, but do not induce the conformational change as judged by antibody binding, electron microscopy, X-ray crystallography, and receptor priming studies. Both compounds demonstrated the favorable property of inhibiting bone resorption in vitro, supporting potential value in treating osteoporosis. Neither, however, had the unfavorable property of the αVβ3 antagonist cilengitide of paradoxically enhancing aortic sprout angiogenesis at concentrations below its IC50, which correlates with cilengitide's enhancement of tumor growth in vivo.

The induction of myeloma cell death and DNA damage by tetrac, a thyroid hormone derivative

Multiple myeloma (MM) is a plasma cell malignancy in which involvement of the thyroid hormone-integrin αvβ3 pathway was shown, and pharmacologic inhibition of this pathway is a rational approach to disease management. A thyroid hormone derivative, tetraiodothyroacetic acid (tetrac), which inhibits l-thyroxine (T₄) and 3,5,3'-triiodo-l-thyronine (T₃) binding to αvβ3 integrin, was studied in five MM cell lines and primary bone marrow (BM) MM cells. Tetrac inhibited MM cell proliferation (absolute cell number/viability) and induced caspase-dependent apoptosis (annexin-V/PI and cell cycle). Activation of caspase-9 and caspase-3 was further demonstrated. Moreover, DNA damage markers, ataxia-telangiectasia-mutated (ATM) kinase, poly ADP-ribose polymerase (PARP-1) and histone γH2AX were induced by tetrac. The various tetrac-initiated effects were attenuated by Arg-Gly-Asp (RGD) peptide, suggesting integrin involvement. Primary BM mononuclear cells were harvested from MM patients (n = 39) at various disease stages. Tetrac-induced apoptosis (12/17 samples) and sensitized the cytotoxic action of bortezomib (6/9 samples). Lastly, expression of plasma membrane integrin αvβ3 was shown not only in the malignant plasma clone, but also in other cell populations within the BM samples (n = 25). Tetrac is anti-proliferative and pro-apoptotic in MM and cells may offer a therapeutic approach for this disease.

Osteotropism of neuroendocrine tumors: role of the CXCL12/ CXCR4 pathway in promoting EMT in vitro

Neuroendocrine tumors (NETs) metastasize to the skeleton in approximately 20% of patients. We have previously shown that the epithelial-mesenchymal transition (EMT) regulates the NET osteotropism and that CXCR4 overexpression predicts bone spreading. Here, we unravel the molecular mechanisms linking the activation of the CXCL12/CXCR4 axis to the bone colonization of NETs using cell lines representative of pancreatic (BON1, CM, QGP1), intestinal (CNDT 2.5), and bronchial origin (H727). By combining flow cytometry and ELISA, BON1, CM and QGP1 cells were defined as CXCR4^high/CXCL12^low, while H727 and CNDT 2.5 were CXCR4^low/CXCL12^high. CXCL12 was inert on cell proliferation, but significantly increased the in vitro osteotropism of CXCR4^high/CXCL12^low cells, as assessed by transwell assays with or without Matrigel membranes. In these cells, CXCL12 induced in vitro a marked EMT-like transcriptional shift with acquirement of a mesenchymal shape. The nuclei of CXCR4^high/CXCL12^low NET cells were typically enriched in non-phosphorylated CXCR4, particularly upon agonist stimulation. Silencing of CXCR4 via siRNA prevented the CXCL12-induced EMT in CXCR4^high/CXCL12^low NET cell lines resulting in the abrogation of both migration and transcriptional mesenchymal patterns. Our data suggest that CXCL12 conveys EMT-promoting signals in NET cells through CXCR4, which in turn regulates transcriptional, morphologic and functional modifications resulting in enhanced in vitro osteotropism of NET cells. Unique functions of CXCR4 may be segregated in relation to its subcellular localization and may acquire potential relevance in future in vivo studies.

Integrin αvβ3 Signaling in Tumor-Induced Bone Disease

Tumor-induced bone disease is common among patients with advanced solid cancers, especially those with breast, prostate, and lung malignancies. The tendency of these cancers to metastasize to bone and induce bone destruction is, in part, due to alterations in integrin expression and signaling. Substantial evidence from preclinical studies shows that increased expression of integrin αvβ3 in tumor cells promotes the metastatic and bone-invasive phenotype. Integrin αvβ3 mediates cell adhesion to several extracellular matrix proteins in the bone microenvironment which is necessary for tumor cell colonization as well as the transmission of mechanical signals for tumor progression. This review will discuss the αvβ3 integrin receptor in the context of tumor-induced bone disease. Specifically, the focus will be the role of αvβ3 in modulating cancer metastasis to bone and tumor cell response to the bone microenvironment, including downstream signaling pathways that contribute to tumor-induced osteolysis. A better understanding of integrin dysregulation in cancer is critical to developing new therapeutics for the prevention and treatment of bone metastases.

1,25(OH)2 vitamin D(3) contributes to osteoclast-like trans-differentiation of malignant plasma cells

1,25-dihydroxyvitamin D (1,25(OH)₂D) exerts pleiotropic effects including bone turnover and immune system regulation. It inhibits both T and B cell proliferation while decreasing the secretion of inflammatory cytokines and immunoglobulins. 1,25(OH)₂D also modulates monocyte-macrophage and osteoclast (OC) maturation. Since we have previously described that malignant plasma cells may trans-differentiate towards the myeloid lineage participating to skeletal devastation in multiple myeloma (MM), we here evaluated in vitro the role of 1,25(OH)₂D in this lineage switch. We investigated the gene and protein expression of vitamin D receptor (VDR) in MM cell lines. Thus, after cell treatment with 1,25(OH)₂D, we analyzed their morphology and the expression of myeloid and OC markers. Finally, we assessed their bone resorption property on calcium phosphate slices. All MM cells expressed VDR in nuclear and perinuclear sites. Treatment with 1,25(OH)₂D altered their morphology from round to fusiform, while inducing paxillin focalization. 1,25(OH)₂D administration also up-regulated myeloid and OC genes, including C/EBPα, RANK, M-CSFR and V-ATPase, whose promoters contain potential 1,25(OH)₂D responsive elements. Finally, 1,25(OH)₂D increased MM cell capability to generate pits of erosion on calcium phosphate discs. This data suggest that myeloma cells may undergo a functional trans-differentiation into OCs and, under appropriate experimental conditions, 1,25(OH)₂D triggers this lineage switch.

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

Avβ3 integrin: Pathogenetic role in osteotropic tumors

The interplay of cancer cells and accessory cells within the microenvironment drives signals regulating the proliferation, migration and skeleton colonization. Osteotropism of tumor cells depends on chemokine activation, production of soluble factors and defective gene expression that cooperate within the metastatic niche to the bone resorbing functions of osteoclasts. Adhesion of cancer cells to the extracellular matrix is regulated by integrins as αvβ3 that enhances their invasiveness, pro-tumor angiogenesis and skeleton invasion. Therefore, αvβ3 signaling is implicated in enhancing osteotropism of breast and prostate cancers as well as of multiple myeloma. Targeting of αvβ3 has been adopted to restrain the tumor progression in several cancer models leading to improvement of overall survival as effect of the reduction of both tumor burden and osteotropism by malignant cells. Here, we review both the role of αvβ3 in malignant osteoclastogenesis and its potential targeting to restrain the bone colonization by skeleton invading cancers.

Relevance of the thyroid hormones-αvβ3 pathway in primary myeloma bone marrow cells and to bortezomib action

Thyroid hormones (T3 and T4) induce proliferation in multiple myeloma (MM) cell lines via the αvβ3 integrin-mitogen-activated protein kinase (MAPK) pathway. We further show in primary MM bone marrow (BM) samples (n = 9) induction of cell viability by 1 nM T3 (13%, p < 0.002) and more potently by 100 nM T4 (21-45%, p < 0.0002) and a quick (1 h) and long-lasting (24 h) pERK activation, which was inhibited in the presence of β3 but not β1 blocking antibodies. Involvement of the integrin was further shown by two disintegrins, Arg-Gly-Asp (RGD) and echistatin peptides, which occluded the effects of T3/T4 on viability, proliferating cell nuclear antigen (PCNA) (proliferation marker) and apoptotic gene expression. Lastly, T3/T4 significantly opposed bortezomib (25 nM) cytotoxicy, as confirmed by several methods. In summary, our results imply that endogenous thyroid hormones in myeloma are factors that may support cell growth, with relevance to bortezomib action.

Multiple myeloma mesenchymal stromal cells: Contribution to myeloma bone disease and therapeutics

Multiple myeloma is a hematological malignancy in which clonal plasma cells proliferate and accumulate within the bone marrow. The presence of osteolytic lesions due to increased osteoclast (OC) activity and suppressed osteoblast (OB) function is characteristic of the disease. The bone marrow mesenchymal stromal cells (MSCs) play a critical role in multiple myeloma pathophysiology, greatly promoting the growth, survival, drug resistance and migration of myeloma cells. Here, we specifically discuss on the relative contribution of MSCs to the pathophysiology of osteolytic lesions in light of the current knowledge of the biology of myeloma bone disease (MBD), together with the reported genomic, functional and gene expression differences between MSCs derived from myeloma patients (pMSCs) and their healthy counterparts (dMSCs). Being MSCs the progenitors of OBs, pMSCs primarily contribute to the pathogenesis of MBD because of their reduced osteogenic potential consequence of multiple OB inhibitory factors and direct interactions with myeloma cells in the bone marrow. Importantly, pMSCs also readily contribute to MBD by promoting OC formation and activity at various levels (i.e., increasing RANKL to OPG expression, augmenting secretion of activin A, uncoupling ephrinB2-EphB4 signaling, and through augmented production of Wnt5a), thus further contributing to OB/OC uncoupling in osteolytic lesions. In this review, we also look over main signaling pathways involved in the osteogenic differentiation of MSCs and/or OB activity, highlighting amenable therapeutic targets; in parallel, the reported activity of bone-anabolic agents (at preclinical or clinical stage) targeting those signaling pathways is commented.

RNA therapeutics targeting osteoclast-mediated excessive bone resorption

RNA interference (RNAi) is a sequence-specific post-transcriptional gene silencing technique developed with dramatically increasing utility for both scientific and therapeutic purposes. Short interfering RNA (siRNA) is currently exploited to regulate protein expression relevant to many therapeutic applications, and commonly used as a tool for elucidating disease-associated genes. Osteoporosis and their associated osteoporotic fragility fractures in both men and women are rapidly becoming a global healthcare crisis as average life expectancy increases worldwide. New therapeutics are needed for this increasing patient population. This review describes the diversity of molecular targets suitable for RNAi-based gene knock down in osteoclasts to control osteoclast-mediated excessive bone resorption. We identify strategies for developing targeted siRNA delivery and efficient gene silencing, and describe opportunities and challenges of introducing siRNA as a therapeutic approach to hard and connective tissue disorders.

Therapeutic approaches to myeloma bone disease: an evolving story

Bone disease is a major morbidity factor in patients with multiple myeloma and significantly affects their overall survival. A complex interplay between malignant plasma cells and other marrow cells results in the generation of a microenvironment capable of enhancing both tumor growth and bone destruction. Bisphosphonates have consistently reduced the incidence of skeletal-related events in patients with multiple myeloma and other osteotropic tumors as well. However, their use is burdened with side-effects, including the risks of osteonecrosis of the jaw and kidney failure, suggesting that they should be discontinued after prolonged administration. New molecular targets of cell cross-talk in myeloma bone marrow are therefore under intensive investigation and new drugs are being explored in preclinical and clinical studies of myeloma bone disease. Compounds targeting osteoclast activation pathways, such as receptor activator of nuclear factor-κB/receptor activator of nuclear factor-κB ligand/osteoprotegerin, B-cell activating factor, mitogen-activated protein kinase and macrophage inflammatory protein-1α/chemokine receptor for macrophage inflammatory protein-1α axes, or soluble agents that improve osteoblast differentiation by modulating specific inhibitors such as Dickkopf-1 and transforming growth factor-β, as well as novel approaches of cytotherapy represent a new generation of promising drugs for the treatment of myeloma bone disease.

Thyroid hormone is a MAPK-dependent growth factor for human myeloma cells acting via αvβ3 integrin

Experimental and clinical observations suggest that thyroid hormone [l-thyroxine (T(4)) and 3,5,3'-triiodo-l-thyronine (T(3))] can support cancer cell proliferation. T(3) and T(4) promote both tumor cell division and angiogenesis by activating mitogen-activated protein kinase (MAPK) via binding to a hormone receptor on the αvβ3 integrin, overexpressed on many cancer cells. We have studied the responsiveness of several MM cell lines to T(3) and T(4) and characterized hormonal effects on cell survival, proliferation, and MAPK activation. Overnight T(3) (1-100 nmol/L) and T(4) (100 nmol/L) incubation enhanced, up to 50% (P < 0.002), MM cell viability (WST-1 assay) and increased cell proliferation by 30% to 60% (P < 0.01). Short exposure (10 minutes) to T(3) and T(4) increased MAPK activity by 2.5- to 3.5-fold (P < 0.03). Pharmacologic MAPK inhibition blocked the proliferative action of T(3) and T(4). Antibodies to the integrin αvβ3 dimer and αv and β3 monomers (but not β1) inhibited MAPK activation and subsequent cell proliferation in response to thyroid hormone, indicating dependence upon this integrin. Moreover, tetraiodothyroacetic acid (tetrac), a non-agonist T(4) analogue previously shown to selectively block T(3)/T(4) binding to αvβ3 receptor site, blocked induction of MAPK by the hormones in a dose-dependent manner. This demonstration of the role of thyroid hormones as growth factors for MM cells may offer novel therapeutic approaches.

Osteoclasts and odontoclasts: signaling pathways to development and disease

Osteoclasts are cells essential for physiologic remodeling of bone and also play important physiologic and pathologic roles in the dentofacial complex. Osteoclasts and odontoclasts are necessary for tooth eruption yet result in dental compromise when associated with permanent tooth internal or external resorption. The determinants that separate their physiologic and pathologic roles are not well delineated. Clinical cases of primary eruption failure and root resorption are challenging to treat. Mineralized tissue resorbing cells undergo a fairly well characterized series of differentiation stages driven by transcriptional mediators. Signal transduction via cytokines and integrin-mediated events comprise the detailed pathways operative in osteo/odontoclastic cells and may provide insights to their targeted regulation. A better understanding of the unique aspects of osteoclastogenesis and osteo/odontoclast function will facilitate effective development of new therapeutic approaches. This review presents the clinical challenges and delves into the cellular and biochemical aspects of the unique cells responsible for resorption of mineralized tissues of the craniofacial complex.

Are bone defects in rare patients with Glanzmann's thrombasthenia associated with ITGB3 or ITGA2B mutations?

The question as to whether Glanzmann thrombasthenia patients with ITGB3 defects and deficiencies of both αIIbβ3 and αvβ3 show phenotypic differences to those with abnormalities exclusive to αIIbβ3 is unresolved. Studies on β3-deficient mice have shown an increased bone mass. Here we review the literature on bone defects in thrombasthenia patients and report the molecular analysis of a patient associating a lifelong thrombasthenia-like syndrome with skeletal defects. We show that the patient is compound heterozygote for Arg327His and Gly391Arg mutations in αIIb, with one mutation inherited from each parent. Modelling strongly suggested that both mutations act by destabilizing the αIIb beta propeller. So it appears likely that this patient has a combination of co-expressed genetic defects.

Pathogenesis of myeloma

Multiple myeloma (MM) is a neoplasm of post-germinal center, terminally differentiated B cells. It is characterized by a multifocal proliferation of clonal, long-lived plasma cells within the bone marrow (BM) and associated skeletal destruction, serum monoclonal gammopathy, immune suppression, and end-organ sequelae. MM is preceded by an age-progressive premalignant condition termed monoclonal gammopathy of undetermined significance. Unlike the genomes of most hematological malignancies, and similar to those of solid-tissue neoplasms, MM genomes are typified by numerous structural and numerical chromosomal aberrations as well as mutations in a number of oncogenes and tumor-suppressor genes, some of which have been linked to disease pathogenesis and clinical behavior. Recent studies have also defined the importance of interactions between the MM cells and their BM microenvironment, dysregulation in signaling pathways and in a specialized subpopulation of cells within the tumor (termed myeloma cancer stem cells) for tumor cell growth and survival, and the development of resistance to therapy.

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.

Adhesion molecules--The lifelines of multiple myeloma cells

Multiple myeloma is an incurable hematological malignancy of terminally differentiated immunoglobulin-producing plasma cells. As a common presentation of the disease, the malignant plasma cells accumulate and proliferate in the bone marrow, where they disrupt normal hematopoiesis and bone physiology. Multiple myeloma cells and the bone marrow microenvironment are linked by a composite network of interactions mediated by soluble factors and adhesion molecules. Integrins and syndecan-1/CD138 are the principal multiple myeloma receptor systems of extracellular matrix components, as well as of surface molecules of stromal cells. CD44 and RHAMM are the major hyaluronan receptors of multiple myeloma cells. The SDF-1/CXCR4 axis is a key factor in the homing of multiple myeloma cells to the bone marrow. The levels of expression and activity of these adhesion molecules are controlled by cytoplasmic operating mechanisms, as well as by extracellular factors including enzymes, growth factors and microenvironmental conditions. Several signaling responses are activated by adhesive interactions of multiple myeloma cells, and their outcomes affect the survival, proliferation and migration of these cells, and in many cases generate a drug-resistant phenotype. Hence, the adhesion systems of multiple myeloma cells are attractive potential therapeutic targets. Several approaches are being developed to disrupt the activities of adhesion molecules in multiple myeloma cells, including small antagonist molecules, direct targeting by immunoconjugates, stimulation of immune responses against these molecules, and signal transduction inhibitors. These potential novel therapeutics may be incorporated into current treatment schemes, or directed against minimal residual malignant cells during remission.