The Proteasome Inhibitor Bortezomib Maintains Osteocyte Viability in Multiple Myeloma Patients by Reducing Both Apoptosis and Autophagy: A New Function for Proteasome Inhibitors

Mechanistic insights into bone destruction in multiple myeloma: Cellular and molecular perspectives

Multiple myeloma (MM) is a hematological malignancy that leads to significant bone destruction, resulting in debilitating pain and skeletal-related events. The pathophysiology of osteolytic bone destruction in MM involves complex interactions between malignant plasma cells (PCs) and the bone marrow (BM) microenvironment. This review aims to provide a comprehensive synthesis of the cellular and molecular pathways underlying MM-associated bone disease. We discuss the role of osteoclast (OC), osteoblast (OB), osteocytes, along with the complex interactions between immune cells and the BM microenvironment in shaping disease progression. Additionally, we explore the molecular signaling pathways involved in bone disease as well as the influence of inflammatory cytokines, and the role of the metabolic alterations that characterize the MM BM. We also explore novel therapeutic strategies targeting these pathways to improve clinical outcomes. Understanding these mechanisms is crucial for the development of more effective treatments to prevent bone damage in MM patients.

A novel CCL3-HMGB1 signaling axis regulating osteocyte RANKL expression in multiple myeloma

Multiple myeloma (MM) is a clonal plasma cell proliferative malignancy characterized by a debilitating bone disease. Osteolytic destruction, a hallmark of MM, is driven by increased osteoclast number and exacerbated bone resorption, primarily fueled by the excessive production of RANKL, the master regulator of osteoclast formation, within the tumor niche. We previously reported that osteocytes, the most abundant cells in the bone niche, promote tumor progression and support MM bone disease by overproducing RANKL. However, the molecular mechanisms underlying RANKL dysregulation in osteocytes in the context of MM bone disease are not entirely understood. Here, we present evidence that MM-derived CCL3 induces upregulation of RANKL expression in both human and murine osteocytes. Through a combination of in vitro, ex vivo, and in vivo models and clinical data, we demonstrate that genetic or pharmacologic inhibition of CCL3 prevents RANKL upregulation in osteocytes and attenuates the bone loss induced by MM cells. Mechanistic studies revealed that MM-derived CCL3 triggers the secretion of HMGB1 by osteocytes, a process required for osteocytic RANKL upregulation by MM cells. These findings identify a previously unknown CCL3-HMGB1 signaling axis in the MM tumor niche that drives bone resorption by promoting RANKL overproduction in osteocytes.

Osteocytes contribute to sex-specific differences in osteoarthritic pain

Osteoarthritic (OA) pain affects 18% of females and 9.6% of males aged over 60 worldwide, with 62% of all OA patients being women. The molecular drivers of sex-based differences in OA are unknown. Bone is intricately coupled with the sensory nervous system and one of the only joint tissues known to show changes that correlate with patient pain in OA. There are fundamental sex differences in pain sensation and bone biology which may be intrinsic to OA disease progression, however these differences are vastly under researched. We have utilised three data sets to investigate the hypothesis that potential mediators responsible for sex dependent pain mechanisms displayed in OA are derived from mechanically stimulated osteocytes. Our published dataset of the in vitro human osteocyte mechanosome was independently compared with published data from, sex-based gene expression differences in human long bone, the sex-based gene expression differences during the skeletal maturation of the mouse osteocyte transcriptome and sex specific OA risk factors and effector genes in a large human GWAS. 80 of the 377 sex-specific genes identified in the mouse osteocyte transcriptome were mechanically regulated in osteocytes with enrichment associated with neural crest migration and axon extension, and DISEASES analysis enrichment for the rheumatoid arthritis pathway. 3861 mechanically regulated osteocytic genes displayed sex-specific differences in human long bone with enrichment for genes associated with the synapse, sensory perception of pain, axon guidance, immune responses, distal peripheral sensory neuropathy, sensory neuropathy, and poor wound healing. 32 of 77 effector genes and 1 of 3 female specific OA risk factor genes identified in the human GWAS were differentially expressed in the osteocyte mechanosome and male and female bone. This analysis lends support to the hypothesis that mechanically regulated genes in osteocytes could influence sex specific differences in osteoarthritic pain and highlights pain pathways with approved drugs that could potentially treat elevated pain susceptibility in females with OA.

Novel insights into osteocyte and inter-organ/tissue crosstalk

Osteocyte, a cell type living within the mineralized bone matrix and connected to each other by means of numerous dendrites, appears to play a major role in body homeostasis. Benefiting from the maturation of osteocyte extraction and culture technique, many cross-sectional studies have been conducted as a subject of intense research in recent years, illustrating the osteocyte-organ/tissue communication not only mechanically but also biochemically. The present review comprehensively evaluates the new research work on the possible crosstalk between osteocyte and closely situated or remote vital organs/tissues. We aim to bring together recent key advances and discuss the mutual effect of osteocyte and brain, kidney, vascular calcification, muscle, liver, adipose tissue, and tumor metastasis and elucidate the therapeutic potential of osteocyte.

Osteocytes: New Kids on the Block for Cancer in Bone Therapy

The tumor microenvironment plays a central role in the onset and progression of cancer in the bone. Cancer cells, either from tumors originating in the bone or from metastatic cancer cells from other body systems, are located in specialized niches where they interact with different cells of the bone marrow. These interactions transform the bone into an ideal niche for cancer cell migration, proliferation, and survival and cause an imbalance in bone homeostasis that severely affects the integrity of the skeleton. During the last decade, preclinical studies have identified new cellular mechanisms responsible for the dependency between cancer cells and bone cells. In this review, we focus on osteocytes, long-lived cells residing in the mineral matrix that have recently been identified as key players in the spread of cancer in bone. We highlight the most recent discoveries on how osteocytes support tumor growth and promote bone disease. Additionally, we discuss how the reciprocal crosstalk between osteocytes and cancer cells provides the opportunity to develop new therapeutic strategies to treat cancer in the bone.

Bone-targeted bortezomib increases bone formation within Calvarial trans-sutural distraction osteogenesis

The high rate of relapse in craniofacial disharmony treatment via trans-sutural distraction osteogenesis (TSDO) is due to the failure to form a stable bone bridge in the suture gap. Bisphosphonates (BP) have a high propensity to localize to hydroxyapatite in the bone matrix and are commonly used as targeting ligands for local delivery of therapeutics into bone microenvironment. Bone-targeted Bortezomib (BP-Btz) is chemosynthetic by linking Btz (Bortezomib) to a BP residue and could target bone tissue to promote osteoblast differentiation and inhibit osteoclastogenesis. Here, suture-derived mesenchymal stem cells (SuSCs) and osteoclasts were treated with Btz and BP-Btz. Aforesaid drugs were injected locally into the sagittal sutures to explore their effects in TSDO. Further, pharmacological properties of BP-Btz in the suture expansion model were assessed by fluorescent BP analogs and levels of total ubiquitinated (Ub)-proteins. The results showed that BP-Btz could stimulate osteogenic differentiation of SuSCs, bind to bone matrix and inhibit osteoclastogenesis. Biological effects of BP-Btz were similar with those of Btz in osteoblast differentiation and osteoclastogenesis inhibition in vitro. Activated bone metabolism were detected after 14 days in the sagittal suture expansion model. Increased osteoid area, remarkably decreased osteoclast surface and enhanced osteogenesis were detected in vivo after treatment with BP-Btz. Green fluorescence signal detection and pharmacodynamic studies revealed that BP-Btz bound to suture edge, released Btz in remodeling conditions, had a higher local concentration and sustained longer than free Btz. This study delineated the clinical potential of bone-targeted Btz conjugate as an efficacious strategy to promote trans-sutural distraction osteogenesis.

Myeloma bone disease: pathogenesis and management in the era of new anti-myeloma agents

INTRODUCTION

Multiple myeloma (MM) is a malignancy of plasma cells with characteristic bone disease. Despite recent great strides achieved in MM treatment owing to the implementation of new anti-MM agents, MM is still incurable and bone destruction remains a serious unmet issue in patients with MM.

APPROACH

In this review, we will summarize and discuss the mechanisms of the formation of bone disease in MM and the available preclinical and clinical evidence on the treatment for MM bone disease.

CONCLUSIONS

MM cells produce a variety of cytokines to stimulate receptor activator of nuclear factor-κB ligand-mediated osteoclastogenesis and suppress osteoblastic differentiation from bone marrow stromal cells, leading to extensive bone destruction with rapid loss of bone. MM cells alter the microenvironment through bone destruction where they colonize, which in turn favors tumor growth and survival, thereby forming a vicious cycle between tumor progression and bone destruction. Denosumab or zoledronic acid is currently recommended to be administered at the start of treatment in newly diagnosed patients with MM with bone disease. Proteasome inhibitors and the anti-CD38 monoclonal antibody daratumumab have been demonstrated to exert bone-modifying activity in responders. Besides their anti-tumor activity, the effects of new anti-MM agents on bone metabolism should be more precisely analyzed in patients with MM. Because prognosis in patients with MM has been significantly improved owing to the implementation of new agents, the therapeutic impact of bone-modifying agents should be re-estimated in the era of these new agents.

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.

Patient-Derived Multiple Myeloma 3D Models for Personalized Medicine-Are We There Yet?

Despite the wide variety of existing therapies, multiple myeloma (MM) remains a disease with dismal prognosis. Choosing the right treatment for each patient remains one of the major challenges. A new approach being explored is the use of ex vivo models for personalized medicine. Two-dimensional culture or animal models often fail to predict clinical outcomes. Three-dimensional ex vivo models using patients' bone marrow (BM) cells may better reproduce the complexity and heterogeneity of the BM microenvironment. Here, we review the strengths and limitations of currently existing patient-derived ex vivo three-dimensional MM models. We analyze their biochemical and biophysical properties, molecular and cellular characteristics, as well as their potential for drug testing and identification of disease biomarkers. Furthermore, we discuss the remaining challenges and give some insight on how to achieve a more biomimetic and accurate MM BM model. Overall, there is still a need for standardized culture methods and refined readout techniques. Including both myeloma and other cells of the BM microenvironment in a simple and reproducible three-dimensional scaffold is the key to faithfully mapping and examining the relationship between these players in MM. This will allow a patient-personalized profile, providing a powerful tool for clinical and research applications.

Notch3 signaling between myeloma cells and osteocytes in the tumor niche promotes tumor growth and bone destruction

In multiple myeloma (MM), communication via Notch signaling in the tumor niche stimulates tumor progression and bone destruction. We previously showed that osteocytes activate Notch, increase Notch3 expression, and stimulate proliferation in MM cells. We show here that Notch3 inhibition in MM cells reduced MM proliferation, decreased Rankl expression, and abrogated the ability of MM cells to promote osteoclastogenesis. Further, Notch3 inhibition in MM cells partially prevented the Notch activation and increased proliferation induced by osteocytes, demonstrating that Notch3 mediates MM-osteocyte communication. Consistently, pro-proliferative and pro-osteoclastogenic pathways were upregulated in CD138+ cells from newly diagnosed MM patients with high vs. low NOTCH3 expression. These results show that NOTCH3 signaling in MM cells stimulates proliferation and increases their osteoclastogenic potential. In contrast, Notch2 inhibition did not alter MM cell proliferation or communication with osteocytes. Lastly, mice injected with Notch3 knock-down MM cells had a 50% decrease in tumor burden and a 50% reduction in osteolytic lesions than mice bearing control MM cells. Together, these findings identify Notch3 as a mediator of cell communication among MM cells and between MM cells and osteocytes in the MM tumor niche and warrant future studies to exploit Notch3 as a therapeutic target to treat MM.

Advancing Treatment of Bone Metastases through Novel Translational Approaches Targeting the Bone Microenvironment

Bone metastases represent a lethal condition that frequently occurs in solid tumors such as prostate, breast, lung, and renal cell carcinomas, and increase the risk of skeletal-related events (SREs) including pain, pathologic fractures, and spinal cord compression. This unique metastatic niche consists of a multicellular complex that cancer cells co-opt to engender bone remodeling, immune suppression, and stromal-mediated therapeutic resistance. This review comprehensively discusses clinical challenges of bone metastases, novel preclinical models of the bone and bone marrow microenviroment, and crucial signaling pathways active in bone homeostasis and metastatic niche. These studies establish the context to summarize the current state of investigational agents targeting BM, and approaches to improve BM-targeting therapies. Finally, we discuss opportunities to advance research in bone and bone marrow microenvironments by increasing complexity of humanized preclinical models and fostering interdisciplinary collaborations to translational research in this challenging metastatic niche.

The osteocyte as a signaling cell

Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone and in distant tissues. Osteocytes are a major source of molecules that regulate bone homeostasis by integrating both mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in both rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of several disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte-secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of several bone therapeutics used in the clinic. Here we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematologic and metastatic cancers in the skeleton.

Myeloma-Bone Interaction: A Vicious Cycle via TAK1-PIM2 Signaling

Simple Summary

Myeloma cells interact with their ambient cells in the bone, such as bone marrow stromal cells, osteoclasts, and osteocytes, resulting in enhancement of osteoclastogenesis and inhibition of osteoblastogenesis while enhancing their growth and drug resistance. The activation of the TAK1–PIM2 signaling axis appears to be vital for this mutual interaction, posing it as an important therapeutic target to suppress tumor expansion and ameliorate bone destruction in multiple myeloma.

Abstract

Multiple myeloma (MM) has a propensity to develop preferentially in bone and form bone-destructive lesions. MM cells enhance osteoclastogenesis and bone resorption through activation of the RANKL–NF-κB signaling pathway while suppressing bone formation by inhibiting osteoblastogenesis from bone marrow stromal cells (BMSCs) by factors elaborated in the bone marrow and bone in MM, including the soluble Wnt inhibitors DKK-1 and sclerostin, activin A, and TGF-β, resulting in systemic bone destruction with loss of bone. Osteocytes have been drawn attention as multifunctional regulators in bone metabolism. MM cells induce apoptosis in osteocytes to trigger the production of factors, including RANKL, sclerostin, and DKK-1, to further exacerbate bone destruction. Bone lesions developed in MM, in turn, provide microenvironments suited for MM cell growth/survival, including niches to foster MM cells and their precursors. Thus, MM cells alter the microenvironments through bone destruction in the bone where they reside, which in turn potentiates tumor growth and survival, thereby generating a vicious loop between tumor progression and bone destruction. The serine/threonine kinases PIM2 and TAK1, an upstream mediator of PIM2, are overexpressed in bone marrow stromal cells and osteoclasts as well in MM cells in bone lesions. Upregulation of the TAK1–PIM2 pathway plays a critical role in tumor expansion and bone destruction, posing the TAK1–PIM2 pathway as a pivotal therapeutic target in MM.

Apheresis Platelet Rich-Plasma for Regenerative Medicine: An In Vitro Study on Osteogenic Potential

Background: Platelet-Rich Plasma (PRP) induces bone regeneration; however, there is low evidence supporting its efficacy in bone healing. The lack of a standardized protocol of administration represents the main obstacle to its use in the clinical routine for bone defects’ treatment. The purpose of this study was to characterize PRP and elucidate its osteogenic potential. Methods: Platelet count, fibrinogen levels, and growth factors concentration were measured in PRP obtained by four apheresis procedures. HOB-01-C1, a pre-osteocytic cell line, was used to examine the effects of different PRP dilutions (from 1% to 50%) on cell viability, growth, and differentiation. Gene expression of RUNX2, PHEX, COL1A1, and OCN was also assayed. Results: PRP showed a mean 4.6-fold increase of platelets amount compared to whole blood. Among the 36 proteins evaluated, we found the highest concentrations for PDGF isoforms, EGF, TGF-β and VEGF-D. PDGF-AA positively correlated with platelet counts. In three of the four tested units, 25% PRP induced a growth rate comparable to the positive control (10% FBS); whereas, for all the tested units, 10% PRP treatment sustained differentiation. Conclusions: This study showed that PRP from apheresis stimulates proliferation and differentiation of pre-osteocyte cells through the release of growth factors from platelets.

Role of Osteocytes in Cancer Progression in the Bone and the Associated Skeletal Disease

PURPOSE OF REVIEW

The goal of this manuscript is to review the current knowledge on the role of osteocytes in cancer in the bone, discuss the potential of osteocytes as a therapeutic target, and propose future research needed to understand the crosstalk between cancer cells and osteocytes in the tumor niche.

RECENT FINDINGS

Numerous studies have established that cancer cells manipulate osteocytes to facilitate invasion and tumor progression in bone. Moreover, cancer cells dysregulate osteocyte function to disrupt physiological bone remodeling, leading to the development of bone disease. Targeting osteocytes and their derived factors has proven to effectively interfere with the progression of cancer in the bone and the associated bone disease. Osteocytes communicate with cancer cells and are also part of the vicious cycle of cancer in the bone. Additional studies investigating the role of osteocytes on metastases to the bone and the development of drug resistance are needed.

New and Old Osteocytic Cell Lines and 3D Models

The aim of this review was to compile a list of tools currently available to study bone cells and in particular osteocytes. As the interest (and importance) in osteocyte biology has greatly expanded over the past decade, new tools and techniques have become available to study these elusive cells, RECENT FINDINGS: Osteocytes are the main orchestrators of bone remodeling. They control both osteoblasts and osteoclast activities via cell-to cell communication or through secreted factors. Osteocytes are also the mechanosensors of the bone and they orchestrate skeletal adaptation to loads. Recent discoveries have greatly expanded our knowledge and understanding of these cells and new models are now available to further uncover the functions of osteocytes. Novel osteocytic cell lines, primary cultures, and 3D scaffolds are now available to investigators to further unravel the functions and roles of these cells.

Ubiquitination in rheumatoid arthritis

Rheumatoid arthritis is a chronic, inflammatory joint disease leading to inflammation of synovial membrane that lines the joints. This inflammation further progresses and results in destruction of joints and surrounding cartilages. The underlying factors can be oxidative stress, pro-inflammatory mediators, imbalance and attenuation between various enzymes and proteins (like nuclear factor erythroid 2 related factor 2/Nrf2 and ubiquitin). Protein degradation pathways comprises of lysosomal, proteasomal pathway, and autophagosome (that are carried out in mammalian cells) are regulated through ubiquitin. Ubiquitin proteasomal system is dominating pathway for carrying out non-lysosomal proteolysis of intracellularly proteins. Fundamental processes including cell cycle progression, process of division, apoptosis, modulation of immune responses and cell trafficking are regulated by process of ubiquitination. Ubiquitin proteasomal pathway (UPP) includes ubiquitin moieties which are covalently attached to proteins and guides them proteasome for degradation. Misfolded, oxidized and damaged proteins which are responsible for critical processes, are major targets of degradation process. Any alteration in this system leads to dysregulated cellular homeostasis; progressively leading to numerous diseases including rheumatoid arthritis. Factors including TAK1, TRAF6 undergo are required for the progression of disease and thus contributes towards pathology of inflammatory disorders such as rheumatoid arthritis. This review will include all linked aspects which contribute its major role in rheumatoid arthritis.

Cell cycle exit during bortezomib-induced osteogenic differentiation of mesenchymal stem cells was mediated by Xbp1s-upregulated p21Cip1 and p27Kip1

Mesenchymal stem cells (MSCs) are multipotent cells capable of differentiating into a variety of cell types. Bortezomib, the first approved proteasome inhibitor used for the treatment of multiple myeloma (MM), has been shown to induce osteoblast differentiation, making it beneficial for myeloma bone disease. In the present study, we aimed to investigate the effects and underlying mechanisms of bortezomib on the cell cycle during osteogenic differentiation. We confirmed that low doses of bortezomib can induce MSCs towards osteogenic differentiation, but high doses are toxic. In the course of bortezomib-induced osteogenic differentiation, we observed cell cycle exit characterized by G₀ /G₁ phase cell cycle arrest with a significant reduction in cell proliferation. Additionally, we found that the cell cycle exit was tightly related to the induction of the cyclin-dependent kinase inhibitors p21^Cip1 and p27^Kip1 . Notably, we further demonstrated that the up-regulation of p21^Cip1 and p27^Kip1 is transcriptionally dependent on the bortezomib-activated ER stress signalling branch Ire1α/Xbp1s. Taken together, these findings reveal an intracellular pathway that integrates proteasome inhibition, osteogenic differentiation and the cell cycle through activation of the ER stress signalling branch Ire1α/Xbp1s.

Hematological Diseases and Osteoporosis

Secondary osteoporosis is a common clinical problem faced by bone specialists, with a higher frequency in men than in women. One of several causes of secondary osteoporosis is hematological disease. There are numerous hematological diseases that can have a deleterious impact on bone health. In the literature, there is an abundance of evidence of bone involvement in patients affected by multiple myeloma, systemic mastocytosis, thalassemia, and hemophilia; some skeletal disorders are also reported in sickle cell disease. Recently, monoclonal gammopathy of undetermined significance appears to increase fracture risk, predominantly in male subjects. The pathogenetic mechanisms responsible for these bone loss effects have not yet been completely clarified. Many soluble factors, in particular cytokines that regulate bone metabolism, appear to play an important role. An integrated approach to these hematological diseases, with the help of a bone specialist, could reduce the bone fracture rate and improve the quality of life of these patients.

Pathological Crosstalk between Metastatic Breast Cancer Cells and the Bone Microenvironment

Bone is the most common metastatic site in breast cancer. Upon arrival to the bone, disseminated tumor cells can undergo a period of dormancy but often eventually grow and hijack the bone microenvironment. The bone marrow microenvironment consists of multiple cell types including the bone cells, adipocytes, endothelial cells, and nerve cells that all have crucial functions in the maintenance of bone homeostasis. Tumor cells severely disturb the tightly controlled cellular and molecular interactions in the bone marrow fueling their own survival and growth. While the role of bone resorbing osteoclasts in breast cancer bone metastases is well established, the function of other bone cells, as well as adipocytes, endothelial cells, and nerve cells is less understood. In this review, we discuss the composition of the physiological bone microenvironment and how the presence of tumor cells influences the microenvironment, creating a pathological crosstalk between the cells. A better understanding of the cellular and molecular events that occur in the metastatic bone microenvironment could facilitate the identification of novel cellular targets to treat this devastating disease.

The Roles of Bone Marrow-Resident Cells as a Microenvironment for Bone Metastasis

It has been appreciated that the cross talk between bone metastatic cancer cells and bone marrow microenvironment influence one another to worsen bone metastatic disease progression. Bone marrow contains various cell types, including (1) cells of mesenchymal origin (e.g., osteoblasts, osteocytes, and adipocytes), (2) cells of hematopoietic origin (e.g., osteoclast and immune cells), and (3) others (e.g., endothelial cells and nerves). The recent studies have enabled us to discover many important cancer-derived factors responsible for the development of bone metastasis. However, many critical questions regarding the roles of bone microenvironment in bone metastatic progression remain elusive. To answer these questions, a deeper understanding of the cross talk between bone metastatic cancer and bone marrow microenvironment is clearly warranted.

Prevention Of Skeletal Related Events In Multiple Myeloma: Focus On The RANK-L Pathway In The Treatment Of Multiple Myeloma

More than 90% of patients with multiple myeloma (MM) have osteolytic bone lesions which increase the risk of skeletal-related events (SRE). The cytokine milieu in the bone marrow microenvironment (BMME) of MM plays a key role in myeloma bone disease by impairing the balance between osteoclastogenesis and osteoblastogenesis. This is orchestrated by the malignant plasma cell (MPC) with the ultimate outcome of MPC proliferation and survival at the expense of excess osteoclast activation resulting in osteolytic bone lesions. Prevention of SRE is currently accomplished by the inhibition of osteoclasts. Bisphosphonates (BPs) are pyrophosphate analogues that cause apoptosis of osteoclasts and have been proven to prevent and delay SRE. Denosumab, a fully humanized monoclonal antibody that binds and inhibits receptor activator of nuclear factor-ĸB ligand (RANKL), a key molecule in the BMME crucial for osteoclastogenesis, is also approved for the prevention of SRE in MM. The addition of BPs and denosumab to standard MM treatment affords a survival benefit for patients with MM. Specifically, the addition of denosumab to standard MM treatments results in superior PFS compared to BPs, highlighting the key role of the RANKL pathway in MM. This review focuses on the pathophysiology of myeloma bone disease as well as on the importance of targeting the RANK-L pathway for the treatment of MM and prevention of SRE.

Aplidin (plitidepsin) is a novel anti-myeloma agent with potent anti-resorptive activity mediated by direct effects on osteoclasts

Despite recent progress in its treatment, Multiple Myeloma (MM) remains incurable and its associated bone disease persists even after complete remission. Thus, identification of new therapeutic agents that simultaneously suppress MM growth and protect bone is an unmet need. Herein, we examined the effects of Aplidin, a novel anti-cancer marine-derived compound, on MM and bone cells. In vitro, Aplidin potently inhibited MM cell growth and induced apoptosis, effects that were enhanced by dexamethasone (Dex) and bortezomib (Btz). Aplidin modestly reduced osteocyte/osteoblast viability and decreased osteoblast mineralization, effects that were enhanced by Dex and partially prevented by Btz. Further, Aplidin markedly decreased osteoclast precursor numbers and differentiation, and reduced mature osteoclast number and resorption activity. Moreover, Aplidin reduced Dex-induced osteoclast differentiation and further decreased osteoclast number when combined with Btz. Lastly, Aplidin alone, or suboptimal doses of Aplidin combined with Dex or Btz, decreased tumor growth and bone resorption in ex vivo bone organ cultures that reproduce the 3D-organization and the cellular diversity of the MM/bone marrow niche. These results demonstrate that Aplidin has potent anti-myeloma and anti-resorptive properties, and enhances proteasome inhibitors blockade of MM growth and bone destruction.

The Emerging Role of Osteocytes in Cancer in Bone

Advances in the last decade have established the osteocyte, the most abundant cell in bone, as a dynamic and multifunctional cell capable of controlling bone homeostasis by regulating the function of both osteoblasts and osteoclasts. In addition, accumulating evidence demonstrates that osteocyte function is altered in several skeletal disorders, and targeting osteocytes and their derived factors improves skeletal health. Despite the remarkable progress in our understanding of osteocyte biology, there has been a paucity of information regarding the role of osteocytes in the progression of cancer in bone. Exciting, recent discoveries suggest that tumor cells communicate with osteocytes to generate a microenvironment that supports the growth and survival of cancer cells and stimulates bone destruction. This review features these novel findings and discussions regarding the impact of chemotherapy on osteocyte function and the potential of targeting osteocytes for the treatment of cancer in bone. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Myeloma bone disease: from biology findings to treatment approaches

Bone disease is a cardinal complication of multiple myeloma that affects quality of life and survival. Osteocytes have emerged as key players in the development of myeloma-related bone disease. Along with other factors, they participate in increased osteoclast activity, decreased osteoblast function, and immunosuppressed marrow microenvironment, which deregulate bone turnover and result in bone loss and skeletal-related events. Denosumab is a novel alternative to bisphosphonates against myeloma bone disease. Special considerations in this constantly evolving field are thoroughly discussed.

An engineered, quantifiable in vitro model for analysing the effect of proteostasis-targeting drugs on tissue physical properties

Cellular function depends on the maintenance of protein homeostasis (proteostasis) by regulated protein degradation. Chronic dysregulation of proteostasis is associated with neurodegenerative and age-related diseases, and drugs targeting components of the protein degradation apparatus are increasingly used in cancer therapies. However, as chronic imbalances rather than loss of function mediate their pathogenesis, research models that allow for the study of the complex effects of drugs on tissue properties in proteostasis-associated diseases are almost completely lacking. Here, to determine the functional effects of impaired proteostatic fine-tuning, we applied a combination of materials science characterisation techniques to a cell-derived, in vitro model of bone-like tissue formation in which we pharmacologically perturbed protein degradation. We show that low-level inhibition of VCP/p97 and the proteasome, two major components of the degradation machinery, have remarkably different effects on the bone-like material that human bone-marrow derived mesenchymal stromal cells (hMSC) form in vitro. Specifically, whilst proteasome inhibition mildly enhances tissue formation, Raman spectroscopic, atomic force microscopy-based indentation, and electron microscopy imaging reveal that VCP/p97 inhibition induces the formation of bone-like tissue that is softer, contains less protein, appears to have more crystalline mineral, and may involve aberrant micro- and ultra-structural tissue organisation. These observations contrast with findings from conventional osteogenic assays that failed to identify any effect on mineralisation. Taken together, these data suggest that mild proteostatic impairment in hMSC alters the bone-like material they form in ways that could explain some pathologies associated with VCP/p97-related diseases. They also demonstrate the utility of quantitative materials science approaches for tackling long-standing questions in biology and medicine, and could form the basis for preclinical drug testing platforms to develop therapies for diseases stemming from perturbed proteostasis or for cancer therapies targeting protein degradation. Our findings may also have important implications for the field of tissue engineering, as the manufacture of cell-derived biomaterial scaffolds may need to consider proteostasis to effectively replicate native tissues.

Myeloma Bone Disease: Update on Pathogenesis and Novel Treatment Strategies

Bone disease, including osteolytic lesions and/or osteoporosis, is a common feature of multiple myeloma (MM). The consequences of skeletal involvement are severe pain, spinal cord compressions, and bone fractures, which have a dramatic impact on patients’ quality of life and, ultimately, survival. During the past few years, several landmark studies significantly enhanced our insight into MM bone disease (MBD) by identifying molecular mechanisms leading to increased bone resorption due to osteoclast activation, and decreased bone formation by osteoblast inhibition. Bisphosphonates were the mainstay to prevent skeletal-related events in MM for almost two decades. Excitingly, the most recent approval of the receptor activator of NF-kappa B ligand (RANKL) inhibitor, denosumab, expanded treatment options for MBD, for patients with compromised renal function, in particular. In addition, several other bone-targeting agents, including bone anabolic drugs, are currently in preclinical and early clinical assessment. This review summarizes our up-to-date knowledge on the pathogenesis of MBD and discusses novel state-of-the-art treatment strategies that are likely to enter clinical practice in the near future.

Role of Osteocytes in Myeloma Bone Disease: Anti-sclerostin Antibody as New Therapeutic Strategy

Osteocytes are terminally differentiated cells of the osteoblast lineage. They are involved in the regulation of bone remodeling by increasing osteoclast formation or decreasing bone formation by the secretion of the osteoblast inhibitor sclerostin. Monoclonal antibody anti-sclerostin, Romosozumab, has been developed and tested in clinical trials in patients with osteoporosis. In the last years, the role of osteocytes in the development of osteolytic bone lesions that occurs in multiple myeloma, have been underlined. Myeloma cells increase osteocyte death through the up-regulation of both apoptosis and autophagy that, in turn, triggers osteoclast formation, and activity. When compared to healthy controls, myeloma patients with bone disease have higher osteocyte cell death, but the treatment with proteasome inhibitor bortezomib has been shown to maintain osteocyte viability. In preclinical mouse models of multiple myeloma, treatment with blocking anti-sclerostin antibody increased osteoblast numbers and bone formation rate reducing osteolytic bone lesions. Moreover, the combination of anti-sclerostin antibody and the osteoclast inhibitor zoledronic acid increased bone mass and fracture resistance synergistically. However, anti-sclerostin antibody did not affect tumor burden in vivo or the efficacy of anti-myeloma drugs in vitro. Nevertheless, the combination therapy of anti-sclerostin antibody and the proteasome inhibitor carfilzomib, displayed potent anti-myeloma activity as well as positive effects on bone disease in vivo. In conclusion, all these data suggest that osteocytes are involved in myeloma bone disease and may be considered a novel target for the use of antibody-mediated anti-sclerostin therapy also in multiple myeloma patients.

Chaperonin 60 sustains osteoblast autophagy and counteracts glucocorticoid aggravation of osteoporosis by chaperoning RPTOR

Glucocorticoid excess medication interrupts osteoblast homeostasis and exacerbates bone mass and microstructure loss ramping up the pathogenesis of osteoporotic disorders. Heat shock protein 60 (HSP60) is found to maintain protein function within cellular microenvironment upon encountering detrimental stress. In this study, we revealed that supraphysiological dexamethasone decreased HSP60 expression along with deregulated autophagy in osteoblasts cultures. This chaperonin is required to sustain autophagic markers Atg4, and Atg12 expression, LC3-II conversion, and autophagic puncta formation, and alleviated the glucocorticoid-induced loss of osteogenic gene expression and mineralized matrix accumulation. Regulator-associated protein of mTOR complex 1 (RPTOR) existed in HSP60 immunoprecipitate contributing to the HSP60-promoted autophagy and osteogenesis because knocking down RPTOR impaired autophagic influx and osteogenic activity. HSP60 shielded from RPTOR dysfunction by reducing the glucocorticoid-induced RPTOR de-phosphorylation, aggregation, and ubiquitination. In vivo, forced RPTOR expression attenuated the methylprednisolone-induced loss of osteoblast autophagy, bone mass, and trabecular microstructure in mice. HSP60 transgenic mice displayed increased cortical bone, mineral acquisition, and osteoblast proliferation along with higher osteogenesis of bone marrow mesenchymal cells than those of wild-type mice. HSP60 overexpression retained RPTOR signaling, sustained osteoblast autophagy, and compromised the severity of glucocorticoid-induced bone loss and sparse trabecular histopathology. Taken together, HSP60 is essential to maintain osteoblast autophagy, which facilitates mineralized matrix production. It fends off glucocorticoid-induced osteoblast apoptosis and bone loss by stabilizing RPTOR action to autophagy. This study offers a new insight into the mechanistic by which chaperonin protects against the glucocorticoid-induced osteoblast dysfunction and bone loss.

Bortezomib inhibited the progression of diffuse large B-cell lymphoma via targeting miR-198

Diffuse large B-cell lymphoma (DLBCL) is the most common subtype of non-Hodgkin lymphoma, which is an aggressive malignancy with high variance of clinical features and response to the treatment. The proteasome inhibitor bortezomib (BTZ) has been demonstrated to suppress the progression of DLBCL, however, the underlying molecular mechanisms by which BTZ regulates the growth of DLBCL cells remain largely unknown. Increasing evidence has suggested that microRNAs (miRNAs) are novel targets of anti-cancer drugs to modulate the progression of cancers. Here, we showed BTZ treatment significantly inhibited the proliferation of DLBCL CRL-2630 cells. Mechanistically, exposure of BTZ up-regulated the expression of miR-198 in DLBCL cells. Depletion of miR-198 significantly reversed the inhibitory effect of BTZ on the proliferation of CRL-2630 cells. To further characterize the involvement of miR-198 in BTZ-induced growth defects of CRL-2630 cells, the downstream targets of miR-198 were predicted with the bioinformatics tools. The results showed that miR-198 bound the 3'-untranslated region (UTR) of the high mobility group AT-hook 1 (HMGA1) and suppressed the expression of HMGA1 in DLBCL cells. Consistently, BTZ treatment decreased the level of HMAG1 and inhibited the migration of DLBCL cells. Our results provided the possible mechanism by which BTZ suppressed the growth of DLBCL cells.

Multiple Myeloma and Bone: The Fatal Interaction

Multiple myeloma (MM) is the second-most-common hematologic malignancy and the most frequent cancer to involve bone. MM bone disease (MMBD) has devastating consequences for patients, including dramatic bone loss, severe bone pain, and pathological fractures that markedly decrease the quality of life and impact survival of MM patients. MMBD results from excessive osteoclastic bone resorption and persistent suppressed osteoblastic bone formation, causing lytic lesions that do not heal, even when patients are in complete and prolonged remission. This review discusses the cellular and molecular mechanisms that regulate the uncoupling of bone remodeling in MM, the effects of MMBD on tumor growth, and potential therapeutic approaches that may prevent severe bone loss and repair damaged bone in MM patients.

Sphingosine kinase 2 inhibition synergises with bortezomib to target myeloma by enhancing endoplasmic reticulum stress

The proteasome inhibitor bortezomib has proven to be invaluable in the treatment of myeloma. By exploiting the inherent high immunoglobulin protein production of malignant plasma cells, bortezomib induces endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), resulting in myeloma cell death. In most cases, however, the disease remains incurable highlighting the need for new therapeutic targets. Sphingosine kinase 2 (SK2) has been proposed as one such therapeutic target for myeloma. Our observations that bortezomib and SK2 inhibitors independently elicited induction of ER stress and the UPR prompted us to examine potential synergy between these agents in myeloma. Targeting SK2 synergistically contributed to ER stress and UPR activation induced by bortezomib, as evidenced by activation of the IRE1 pathway and stress kinases JNK and p38MAPK, thereby resulting in potent synergistic myeloma apoptosis in vitro. The combination of bortezomib and SK2 inhibition also exhibited strong in vivo synergy and favourable effects on bone disease. Therefore, our studies suggest that perturbations of sphingolipid signalling can synergistically enhance the effects seen with proteasome inhibition, highlighting the potential for the combination of these two modes of increasing ER stress to be formally evaluated in clinical trials for the treatment of myeloma patients.

The Proteasome and Myeloma-Associated Bone Disease

Bone disease is the hallmark of multiple myeloma (MM), a hematological malignancy characterized by osteolytic lesions due to a severe uncoupled and unbalanced bone remodeling with pronounced osteoblast suppression. Bone metastasis is also a frequent complication of solid tumors including advanced breast or prostate cancer. In the past years, the ubiquitin-proteasome pathway has been proved critical in regulating the balance between bone formation and bone resorption. Proteasome inhibitors (PIs) are a new class of drugs, currently used in the treatment of MM, that affect both tumor cells and bone microenvironment. Particularly, PIs stimulate osteoblast differentiation by human mesenchymal stromal cells and increase bone regeneration in mice. Interestingly, in vitro data indicate that PIs block MM-induced osteoblast and osteocyte cell death by targeting both apoptosis and autophagy. The preclinical data are supported by the following effects observed in MM patients treated with PIs: increase of bone alkaline phosphatase levels, normalization of the markers of bone turnover, and reduction of the skeletal-related events. Moreover, the histomorphometric data indicate that the treatment with bortezomib stimulates osteoblast formation and maintains osteocyte viability in MM patients. This review updates the evidence on the effects of PIs on bone remodeling and on cancer-induced bone disease while focusing on MM bone disease.

The Expanding Life and Functions of Osteogenic Cells: From Simple Bone-Making Cells to Multifunctional Cells and Beyond

During the last three decades, important progress in bone cell biology and in human and mouse genetics led to major advances in our understanding of the life and functions of cells of the osteoblast lineage. Previously unrecognized sources of osteogenic cells have been identified. Novel cellular and molecular mechanisms controlling osteoblast differentiation and senescence have been determined. New mechanisms of communications between osteogenic cells, osteocytes, osteoclasts, and chondrocytes, as well as novel links between osteogenic cells and blood vessels have been identified. Additionally, cells of the osteoblast lineage were shown to be important components of the hematopoietic niche and to be implicated in hematologic dysfunctions and malignancy. Lastly, unexpected interactions were found between osteogenic cells and several soft tissues, including the central nervous system, gut, muscle, fat, and testis through the release of paracrine factors, making osteogenic cells multifunctional regulatory cells, in addition to their bone-making function. These discoveries considerably enlarged our vision of the life and functions of osteogenic cells, which may lead to the development of novel therapeutics with immediate applications in bone disorders. © 2017 American Society for Bone and Mineral Research.

Pathogenesis of bone disease in multiple myeloma: from bench to bedside

Osteolytic bone disease is the hallmark of multiple myeloma, which deteriorates the quality of life of myeloma patients, and it affects dramatically their morbidity and mortality. The basis of the pathogenesis of myeloma-related bone disease is the uncoupling of the bone-remodeling process. The interaction between myeloma cells and the bone microenvironment ultimately leads to the activation of osteoclasts and suppression of osteoblasts, resulting in bone loss. Several intracellular and intercellular signaling cascades, including RANK/RANKL/OPG, Notch, Wnt, and numerous chemokines and interleukins are implicated in this complex process. During the last years, osteocytes have emerged as key regulators of bone loss in myeloma through direct interactions with the myeloma cells. The myeloma-induced crosstalk among the molecular pathways establishes a positive feedback that sustains myeloma cell survival and continuous bone destruction, even when a plateau phase of the disease has been achieved. Targeted therapies, based on the better knowledge of the biology, constitute a promising approach in the management of myeloma-related bone disease and several novel agents are currently under investigation. Herein, we provide an insight into the underlying pathogenesis of bone disease and discuss possible directions for future studies.

Sclerostin: an Emerging Target for the Treatment of Cancer-Induced Bone Disease

PURPOSE OF REVIEW

This review provides a summary of the current knowledge on Sost/sclerostin in cancers targeting the bone, discusses novel observations regarding its potential as a therapeutic approach to treat cancer-induced bone loss, and proposes future research needed to fully understand the potential of therapeutic approaches that modulate sclerostin function.

RECENT FINDINGS

Accumulating evidence shows that sclerostin expression is dysregulated in a number of cancers that target the bone. Further, new findings demonstrate that pharmacological inhibition of sclerostin in preclinical models of multiple myeloma results in a robust prevention of bone loss and preservation of bone strength, without apparent effects on tumor growth. These data raise the possibility of targeting sclerostin for the treatment of cancer patients with bone metastasis. Sclerostin is emerging as a valuable target to prevent the bone destruction that accompanies the growth of cancer cells in the bone. Further studies will focus on combining anti-sclerostin therapy with tumor-targeted agents to achieve both beneficial skeletal outcomes and inhibition of tumor progression.

Mechanisms of bone destruction in multiple myeloma

Osteolytic bone disease is a frequent complication of multiple myeloma, resulting in skeletal complications that are a significant cause of morbidity and mortality. It is the result of an increased activity of osteoclasts, which is not followed by reactive bone formation by osteoblasts. Recent studies have revealed novel molecules and pathways that are implicated in osteoclast activation and osteoblast inhibition. Among them, the most important include the receptor activator of nuclear factor-kappa B ligand/osteoprotegerin pathway, the macrophage inflammatory proteins and the activin-A that play a crucial role in osteoclast stimulation in myeloma, while the wingless-type (Wnt) signalling inhibitors (sclerostin and dickkopf-1) along with the growth factor independence-1 are considered the most important factors for the osteoblast dysfunction of myeloma patients. Finally, the role of osteocytes, which is the key cell for normal bone remodelling, has also revealed during the last years through their interaction with myeloma cells that leads to their apoptosis and the release of RANKL and sclerostin maintaining bone loss in these patients. This review focuses on the latest available data for the mechanisms of bone destruction in multiple myeloma.

Targeting autophagy in multiple myeloma

Autophagy plays an important role in plasma cell ontogeny and in the pathophysiology of multiple myeloma. Autophagy is usually considered a pro-survival mechanism, and cooperates with the ubiquitin proteasome system in maintaining the homeostasis of myeloma cells by degrading excessive and misfolded proteins for energy recycling. Therefore, the inhibition of autophagy could effectively induce death in myeloma cells, and could synergize with proteasome inhibitors. However, the excessive activation of autophagy could also lead to the extreme degradation of the organelles that induce autophagic cell death. Hence, the activation of autophagic cell death might also represent a promising approach for treating myeloma. Recent studies have demonstrated that autophagy also mediates drug resistance in myeloma cells and the complications of myeloma, while the inhibition of autophagy may reverse the response to drugs. In this study, we have mainly reviewed recent research on autophagy in relationship to the therapeutic effect, the reversal of drug resistance, and the mediation of complications.

Genetic deletion of Sost or pharmacological inhibition of sclerostin prevent multiple myeloma-induced bone disease without affecting tumor growth

Multiple myeloma (MM) causes lytic bone lesions due to increased bone resorption and concomitant marked suppression of bone formation. Sclerostin (Scl) levels, an osteocyte-derived inhibitor of Wnt/β-catenin signaling, are elevated in MM patient sera and are increased in osteocytes in MM-bearing mice. We show here that genetic deletion of Sost, the gene encoding Scl, prevented MM-induced bone disease in an immune-deficient mouse model of early MM, and that administration of anti-Scl antibody (Scl-Ab) increased bone mass and decreases osteolysis in immune-competent mice with established MM. Sost/Scl inhibition increased osteoblast numbers, stimulated new bone formation and decreased osteoclast number in MM-colonized bone. Further, Sost/Scl inhibition did not affect tumor growth in vivo or anti-myeloma drug efficacy in vitro. These results identify the osteocyte as a major contributor to the deleterious effects of MM in bone and osteocyte-derived Scl as a promising target for the treatment of established MM-induced bone disease. Further, Scl did not interfere with efficacy of chemotherapy for MM suggesting that combined treatment with anti-myeloma drugs and Scl-Ab should effectively control MM growth and bone disease, providing new avenues to effectively control MM and bone disease in patients with active MM.