Cancer stem cells: controversies in multiple myeloma

CAR T cell therapies for patients with multiple myeloma

Despite several therapeutic advances over the past decade, multiple myeloma (MM) remains largely incurable, indicating a need for new treatment approaches. Chimeric antigen receptor (CAR) T cell therapy works by mechanisms distinct from those of other MM therapies and involves the modification of patient or donor T cells to target specific cell-surface antigens. B cell maturation antigen (BCMA) is expressed only on plasma cells, a small subset of B cells and MM cells, which makes it a suitable target antigen for such therapies. At the time of writing, data from >20 clinical trials involving anti-BCMA CAR T cells have demonstrated that patients with relapsed and/or refractory MM can achieve objective responses. These early investigations have been instrumental in demonstrating short-term safety and efficacy; however, most patients do not have disease remission lasting >18 months. Attempts to reduce or delay the onset of relapsed disease are underway and include identifying additional CAR T cell target antigens and methods of enhancing BCMA expression on MM cells. Engineering CAR T cells to enhance both the activity and safety of treatment continues to be a promising avenue for improvement. In this Review we summarize data from clinical trials that have been carried out to date, describe novel antigens that could be targeted in the future, and highlight potential future innovations that could enhance the efficacy and/or reduce the toxicities associated with CAR T cell therapies.

GRP78 modulates cell adhesion markers in prostate Cancer and multiple myeloma cell lines

Background

Glucose regulated protein 78 (GRP78) is a resident chaperone of the endoplasmic reticulum and a master regulator of the unfolded protein response under physiological and pathological cell stress conditions. GRP78 is overexpressed in many cancers, regulating a variety of signaling pathways associated with tumor initiation, proliferation, adhesion and invasion which contributes to metastatic spread. GRP78 can also regulate cell survival and apoptotic pathways to alter responsiveness to anticancer drugs. Tumors that reside in or metastasize to the bone and bone marrow (BM) space can develop pro-survival signals through their direct adhesive interactions with stromal elements of this niche thereby resisting the cytotoxic effects of drug treatment. In this study, we report a direct correlation between GRP78 and the adhesion molecule N-cadherin (N-cad), known to play a critical role in the adhesive interactions of multiple myeloma and metastatic prostate cancer with the bone microenvironment.

Methods

N-cad expression levels (transcription and protein) were evaluated upon siRNA mediated silencing of GRP78 in the MM.1S multiple myeloma and the PC3 metastatic prostate cancer cell lines. Furthermore, we evaluated the effects of GRP78 knockdown (KD) on epithelial-mesenchymal (EMT) transition markers, morphological changes and adhesion of PC3 cells.

Results

GRP78 KD led to concomitant downregulation of N-cad in both tumors types. In PC3 cells, GRP78 KD significantly decreased E-cadherin (E-cad) expression likely associated with the induction in TGF-β1 expression. Furthermore, GRP78 KD also triggered drastic changes in PC3 cells morphology and decreased their adhesion to osteoblasts (OSB) dependent, in part, to the reduced N-cad expression.

Conclusion

This work implicates GRP78 as a modulator of cell adhesion markers in MM and PCa. Our results may have clinical implications underscoring GRP78 as a potential therapeutic target to reduce the adhesive nature of metastatic tumors to the bone niche.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-5178-8) contains supplementary material, which is available to authorized users.

The role of Cancer/Testis Antigens in Multiple Myeloma pathogenesis and their application in disease monitoring and therapy

A unique group of genes, encoding tumour associated antigens, known as the Cancer/Testis Antigens (CTAs), have been explored as novel markers of disease progression and as targets of immunotherapy in several cancers, including the haematological malignancy Multiple Myeloma (MM). This review aims to update the knowledge of CTA involvement in MM pathogenesis and how their potential as biomarkers for disease monitoring and targets of immunotherapy has been explored in the MM disease arena. Despite the initial promise of these antigens, their use as immunotherapy targets has not been successful, yet with a greater understanding of their role in disease pathogenesis they may still have a significant role to play as biomarkers of disease and therapeutic targets.

Telomerase reverse transcriptase coordinates with the epithelial-to-mesenchymal transition through a feedback loop to define properties of breast cancer stem cells

Telomerase and its core component, telomerase reverse transcriptase (hTERT), are critical for stem cell compartment integrity. Normal adult stem cells have the longest telomeres in a given tissue, a property mediated by high hTERT expression and high telomerase enzymatic activity. In contrast, cancer stem cells (CSCs) have short telomeres despite high expression of hTERT, indicating that the role of hTERT in CSCs is not limited to telomere elongation and/or maintenance. The function of hTERT in CSCs remains poorly understood. Here, we knocked down hTERT expression in CSCs and observed a morphological shift to a more epithelial phenotype, suggesting a role for hTERT in the epithelial-to-mesenchymal transition (EMT) of CSCs. Therefore, in this study, we systematically explored the relationship between hTERT and EMT and identified a reciprocal, bi-directional feedback loop between hTERT and EMT in CSCs. We found that hTERT expression is mutually exclusive to the mesenchymal phenotype and that, reciprocally, loss of the mesenchymal phenotype represses hTERT expression. We also showed that hTERT plays a critical role in the expression of key CSC markers and nuclear β-catenin localization, increases the percentage of cells with side-population properties, and upregulates the CD133 expression. hTERT also promotes chemoresistance properties, tumorsphere formation and other important functional CSC properties. Subsequently, hTERT knockdown leads to the loss of the above advantages, indicating a loss of CSC properties. Our findings suggest that targeting hTERT might improve CSCs elimination by transitioning them from the aggressive mesenchymal state to a more steady epithelial state, thereby preventing cancer progression.

Summary: This study describe a reciprocal, bi-directional feedback loop between hTERT and EMT to regulate properties of CSCs, suggesting that targeting hTERT may eliminate CSCs, thereby preventing cancer progression.

Nonadherent Spheres With Multiple Myeloma Surface Markers Contain Cells that Contribute to Sphere Formation and Are Capable of Internalizing Extracellular Double-Stranded DNA

BACKGROUND

The most prominent features of cancer stem cells are asymmetric cell division, tumorigenicity, and clonogenicity. Recently one more feature of poorly differentiated cell types of various origin, including cancer stem cells, has been described. Namely, these cells can internalize extracellular DNA natively, without additional transfection procedures.

PATIENTS AND METHODS

Using our approach to trace internalization of a TAMRA (carboxy tetramethyl-rhodamine [fluorescent dye])-DNA labeled probe by poorly differentiated cell types, we isolated and characterized the cells from free-floating spheres derived from the bone marrow clonogenic aspirate of a multiple myeloma patient.

RESULTS

Nonadherent spheres display a B-cell phenotype (CD73/CD20⁺/CD45⁺/CD19^dim). Further, free-floating spheres contain 1% to 3% cells with a clonogenic potential, and these cells display a marker of poorly differentiated cell types (TAMRA⁺). Upon association with a group of ∼ 10 free-floating TAMRA^- cells, this peculiar cell type forms a sphere-forming cluster that initiates secondary aggregation of cells into a spheric structure. TAMRA⁺ and TAMRA^- cells secrete distinct sets of cytokines indicative of the paracrine regulation. Grafting experiments of intact whole spheres versus cell suspensions prepared from dispersed spheres indicate that successful engraftment only occurs in the former case.

CONCLUSION

Nonadherent 3-D cell colonies (spheres) encompass B cells with CD73/CD20⁺/CD45⁺/CD19^dim phenotype, as well as double-stranded DNA-internalizing cells. The latter cell type appears to function as a sphere-forming center. Different cells in the spheres communicate with each other by secreting specific sets of cytokines. For successful engraftment and tumor growth in mice, intact spheres containing ∼ 10⁶ cells must be used.

A CD138-independent strategy to detect minimal residual disease and circulating tumour cells in multiple myeloma

CD138 (also termed SDC1) has been the gold-standard surface marker to detect multiple myeloma (MM) cells for decades; however, drug-resistant residual and circulating MM cells were shown to have lower expression of this marker. In this study, we have shown that residual MM cells following bortezomib treatment are hypoxic. This combination of drug exposure and hypoxia down-regulates their CD138 expression, thereby making this marker unsuitable for detecting residual or other hypoxic MM cells, such as circulating tumour cells, in MM. Hence, we developed an alternative biomarker set which detects myeloma cells independent of their hypoxic and CD138 expression status in vitro, in vivo and in primary MM patients. The new markers were able to identify a clonal CD138-negative population as minimal residual disease in the bone marrow and peripheral blood of MM patients. Further investigation to characterize the role of this population as a prognostic marker in MM is warranted.

Immunotherapy in Multiple Myeloma Using Cancer-Testis Antigens

Context:

Multiple myeloma (MM) is a B-cell malignancy characterized by monoclonal expansion of abnormal plasma cells in the bone marrow. It accounts for 10% of hematological malignancies. Although patients respond to a wide range of anticancer modalities, relapse occurs in a significant number of the cases. Immunotherapeutic approaches have been evolved to tackle this problem. Cancer-testis antigens CTAs as a group of tumor-associated antigens are appropriate targets for cancer immunotherapy as they have restricted expression pattern in normal tissues except for testis which is an immune-privileged site. Expression of these antigens has been assessed in different malignancies including MM.

Evidence Acquisition:

We performed a computerized search of the MEDLINE/PubMed databases with key words: multiple myeloma, cancer-testis antigen, and cancer stem cell and immunotherapy.

Results:

Several CTAs including NY-ESO-1, MAGE and GAGE family have been shown to be expressed in MM patients. Cellular and humoral immune responses against these antigens have been detected in MM patients.

Conclusions:

The frequent and high expression level of CTAs in MM patients shows that these antigens can be applied as cancer biomarkers as well as targets for immunotherapy in these patients.

Myelomatous plasma cells display an aberrant gene expression pattern similar to that observed in normal memory B cells

Memory B cells (MBCs) remain in a quiescent state for years, expressing pro-survival and anti-apoptotic factors while repressing cell proliferation and activation genes. During their differentiation into plasma cells (PCs), their expression pattern is reversed, with a higher expression of genes related to cell proliferation and activation, and a lower expression of pro-survival genes. To determine whether myelomatous PCs (mPCs) share characteristics with normal PCs and MBCs and to identify genes involved in the pathophysiology of multiple myeloma (MM), we compared gene expression patterns in these three cell sub-types. We observed that mPCs had features intermediate between those of MBCs and normal PCs, and identified 3455 genes differentially expressed in mPCs relative to normal PCs but with a similar expression pattern to that in MBCs. Most of these genes are involved in cell death and survival, cell growth and proliferation and protein synthesis. According to our findings, mPCs have a gene expression pattern closer to a MBC than a PC with a high expression of genes involved in cell survival. These genes should be physiologically inactivated in the transit from MBC to PC, but remain overexpressed in mPCs and thus may play a role in the pathophysiology of the disease.

Drug resistance in multiple myeloma: latest findings and new concepts on molecular mechanisms

In the era of new and mostly effective therapeutic protocols, multiple myeloma still tends to be a hard-to-treat hematologic cancer. This hallmark of the disease is in fact a sequel to drug resistant phenotypes persisting initially or emerging in the course of treatment. Furthermore, the heterogeneous nature of multiple myeloma makes treating patients with the same drug challenging because finding a drugable oncogenic process common to all patients is not yet feasible, while our current knowledge of genetic/epigenetic basis of multiple myeloma pathogenesis is outstanding. Nonetheless, bone marrow microenvironment components are well known as playing critical roles in myeloma tumor cell survival and environment-mediated drug resistance happening most possibly in all myeloma patients. Generally speaking, however; real mechanisms underlying drug resistance in multiple myeloma are not completely understood. The present review will discuss the latest findings and concepts in this regard. It reviews the association of important chromosomal translocations, oncogenes (e.g. TP53) mutations and deranged signaling pathways (e.g. NFκB) with drug response in clinical and experimental investigations. It will also highlight how bone marrow microenvironment signals (Wnt, Notch) and myeloma cancer stem cells could contribute to drug resistance in multiple myeloma.

The role of the Hippo pathway in human disease and tumorigenesis

Understanding the molecular nature of human cancer is essential to the development of effective and personalized therapies. Several different molecular signal transduction pathways drive tumorigenesis when deregulated and respond to different types of therapeutic interventions. The Hippo signaling pathway has been demonstrated to play a central role in the regulation of tissue and organ size during development. The deregulation of Hippo signaling leads to a concurrent combination of uncontrolled cellular proliferation and inhibition of apoptosis, two key hallmarks in cancer development. The molecular nature of this pathway was first uncovered in Drosophila melanogaster through genetic screens to identify regulators of cell growth and cell division. The pathway is strongly conserved in humans, rendering Drosophila a suitable and efficient model system to better understand the molecular nature of this pathway. In the present study, we review the current understanding of the molecular mechanism and clinical impact of the Hippo pathway. Current studies have demonstrated that a variety of deregulated molecules can alter Hippo signaling, leading to the constitutive activation of the transcriptional activator YAP or its paralog TAZ. Additionally, the Hippo pathway integrates inputs from a number of growth signaling pathways, positioning the Hippo pathway in a central role in the regulation of tissue size. Importantly, deregulated Hippo signaling is frequently observed in human cancers. YAP is commonly activated in a number of in vitro and in vivo models of tumorigenesis, as well as a number of human cancers. The common activation of YAP in many different tumor types provides an attractive target for potential therapeutic intervention.

High throughput quantitative reverse transcription PCR assays revealing over-expression of cancer testis antigen genes in multiple myeloma stem cell-like side population cells

Multiple myeloma (MM) stem cells, proposed to be responsible for the tumourigenesis, drug resistance and recurrence of this disease, are enriched in the cancer stem cell-like side population (SP). Cancer testis antigens (CTA) are attractive targets for immunotherapy because they are widely expressed in cancers but only in limited types of normal tissues. We designed a high throughput assay, which allowed simultaneous relative quantifying expression of 90 CTA genes associated with MM. In the three MM cell lines tested, six CTA genes were over-expressed in two and LUZP4 and ODF1 were universally up-regulated in all three cell lines. Subsequent study of primary bone marrow (BM) from eight MM patients and four healthy donors revealed that 19 CTA genes were up-regulated in SP of MM compared with mature plasma cells. In contrast, only two CTA genes showed a moderate increase in SP cells of healthy BM. Furthermore, knockdown using small interfering RNA (siRNA) revealed that LUZP4 expression is required for colony-forming ability and drug resistance in MM cells. Our findings indicate that multiple CTA have unique expression profiles in MM SP, suggesting that CTA may serve as targets for immunotherapy that it specific for MM stem cells and which may lead to the long-term cure of MM.

Preclinical screening of histone deacetylase inhibitors combined with ABT-737, rhTRAIL/MD5-1 or 5-azacytidine using syngeneic Vk*MYC multiple myeloma

Multiple myeloma (MM) is an incurable malignancy with an unmet need for innovative treatment options. Histone deacetylase inhibitors (HDACi) are a new class of anticancer agent that have demonstrated activity in hematological malignancies. Here, we investigated the efficacy and safety of HDACi (vorinostat, panobinostat, romidepsin) and novel combination therapies using in vitro human MM cell lines and in vivo preclinical screening utilizing syngeneic transplanted Vk*MYC MM. HDACi were combined with ABT-737, which targets the intrinsic apoptosis pathway, recombinant human tumour necrosis factor-related apoptosis-inducing ligand (rhTRAIL/MD5-1), that activates the extrinsic apoptosis pathway or the DNA methyl transferase inhibitor 5-azacytidine. We demonstrate that in vitro cell line-based studies provide some insight into drug activity and combination therapies that synergistically kill MM cells; however, they do not always predict in vivo preclinical efficacy or toxicity. Importantly, utilizing transplanted Vk*MYC MM, we report that panobinostat and 5-azacytidine synergize to prolong the survival of tumor-bearing mice. In contrast, combined HDACi/rhTRAIL-based strategies, while efficacious, demonstrated on-target dose-limiting toxicities that precluded prolonged treatment. Taken together, our studies provide evidence that the transplanted Vk*MYC model of MM is a useful screening tool for anti-MM drugs and should aid in the prioritization of novel drug testing in the clinic.

Differential protein expression profile between CD20 positive and negative cells of the NCI-H929 cell line

At present, multiple myeloma (MM) remains an incurable disease and cologenic cells may be responsible for disease relapse. It has been proposed that CD20+/CD138- NCI-H929 cells could be hallmarks of MM clonogenic cells. Here, the immunology phenotype of NCI-H929 cells is described. Only a small population of CD20+/CD138- cells (<1%) was found in the NCI-H929 cell line, but CD20+/CD138- cells were not detected. We found that CD20+/CD138+ cells were able to exhibit cologenic capacity by colony formation assay and continuous passage culture. Proteins were analyzed by 1D-SDS-PAGE and TMT based quantitative differential liquid chromatography tandem mass spectrometry (LC-MS/MS). 1,082 non-redundant proteins were identified, 658 of which were differentially expressed with at least a 1.5-fold difference. 205 proteins in CD20+ cells were expressed at higher levels and 453 proteins were at lower levels compared with CD20- cells. Most proteins had catalytic and binding activity and mainly participated in metabolic processes, cell communication and molecular transport. These results proved that there are different biological features and protein expression profile between CD20+ and CD20- cells in the NCI-H929 cell line.

Heterogeneity and targeting of pancreatic cancer stem cells

Cancer stem cells (CSC) have been identified in an ever-increasing number of human malignancies on the basis of their ability to recapitulate tumors in the ectopic setting and maintain long-term tumorigenic potential. In addition, in pancreatic adenocarcinoma, CSCs may display additional properties, such as relative drug resistance and enhanced invasive and migratory potential that implicate a role in disease pathogenesis spanning initial tumor formation to metastatic disease progression. Importantly, these findings also indicate that the development of novel therapeutic strategies capable of inhibiting or eliminating CSCs will improve clinical outcomes. Preclinical studies have already described a wide array of potential approaches that target CSC-specific surface antigens and cellular pathways involved in cell survival, adhesion, self-renewal, and differentiation. Further, progress in this area should continue to move forward as the unique biology of CSCs is better understood. All preclinical studies to date have focused on targeting specific and phenotypically defined CSCs, but multiple cell populations with the ability to form tumors and self-renew have been identified in pancreatic carcinoma. As the clinical efficacy of CSC-directed therapies will depend on the inhibition of all sources of tumor self-renewal, better understanding of how specific CSC populations are related to one another and whether each possesses specific functional properties will be critical. In this CCR Focus article, we discuss the potential relationships between different pancreatic CSC populations and strategies to identify novel targeting approaches.

Relapsed/Refractory multiple myeloma: defining refractory disease and identifying strategies to overcome resistance

Despite the development of more effective therapies for multiple myeloma (MM) over the past decade, nearly all patients will eventually experience disease relapse and require further therapy. Designing the next generation of therapies for relapsed and refractory disease will depend on understanding the complex molecular pathogenesis of MM and mechanisms of resistance. Oncogenomic studies have identified many potential therapeutic targets and have led to emerging models of the multistep molecular pathogenesis of MM. The key to overcoming resistance may depend on interrupting the complex interactions between MM cells and the bone microenvironment. Direct interaction between MM cells and bone marrow cells activates pleiotropic signaling pathways that mediate growth, survival, and migration of MM cells as well as resistance to chemotherapy (known as cell adhesion-mediated drug resistance). The bone marrow also secretes growth factors and cytokines that maintain MM cells and inhibit apoptosis. Therefore, successful therapeutic strategies must target not only the MM plasma cell but also the bone microenvironment. The benefit of immunomodulatory drugs such as thalidomide and lenalidomide and the proteasome inhibitor bortezomib in relapsed/refractory MM is related to their ability to target both. Novel agents and combination strategies are building on the success of these agents and targeting synergistic pathways.

Canonical and noncanonical Hedgehog pathway in the pathogenesis of multiple myeloma

The Hedgehog (Hh) pathway is required for cell-fate determination during the embryonic life, as well as cell growth and differentiation in the adult organism, where the inappropriate activation has been implicated in several cancers. Here we demonstrate that Hh signaling plays a significant role in growth and survival of multiple myeloma (MM) cells. We observed that CD138(+) MM cells express Hh genes and confirmed Smoothened (Smo)-dependent Hh signaling in MM using a novel synthetic Smo inhibitor, NVP-LDE225 (Novartis), which decreased MM cell viability by inducing specific down-regulation of Gli1 and Ptch1, hallmarks of Hh activity. In addition, we detected a nuclear localization of Gli1 in MM cells, which is completely abrogated by Forskolin, a Gli1-modulating compound, confirming Smo-independent mechanisms leading to Hh activation in MM. Finally, we identified that bone marrow stromal cells are a source of the Shh ligand, although they are resistant to the Hh inhibitor because of defective Smo expression and Ptch1 up-regulation. Further in vitro as well as in vivo studies showed antitumor efficacy of NVP-LDE225 in combination with bortezomib. Altogether, our data demonstrate activation of both canonical and noncanonical Hh pathway in MM, thus providing the rationale for testing Hh inhibitors in clinical trials to improve MM patient outcome.

Autologous stem cell transplantation and multiple myeloma cancer stem cells

It is well established that high-dose therapy (HDT) combined with autologous stem cell transplantation (ASCT) produces superior response rates and progression-free survival compared with conventional chemotherapy in patients with multiple myeloma (MM). Accordingly, MM currently represents the most common indication for ASCT. Despite these clinical improvements, the impact of ASCT on overall survival is unclear because the vast majority of patients eventually experience disease relapse and progression. The continual risk of relapse suggests that malignant cells resistant to HDT possess the clonogenic growth potential to mediate tumor regrowth, and in several diseases cancer stem cells (CSCs) have been identified that are both highly tumorigenic and resistant to standard anticancer approaches. Putative CSCs have been identified in MM, and their characterization may lead to the development of novel maintenance strategies that inhibit the production of new tumor cells, prevent disease relapse, and improve overall survival.

Phenotypic detection of clonotypic B cells in multiple myeloma by specific immunoglobulin ligands reveals their rarity in multiple myeloma

In multiple myeloma, circulating “clonotypic” B cells, that express the immunoglobulin rearrangement of the malignant plasma cell clone, can be indirectly detected by PCR. Their role as potential “feeder” cells for the malignant plasma cell pool remains controversial. Here we established for the first time an approach that allows direct tracking of such clonotypic cells by labeling with patient-specific immunoglobulin ligands in 15 patients with myeloma. Fifty percent of patients showed evidence of clonotypic B cells in blood or bone marrow by PCR. Epitope-mimicking peptides from random libraries were selected on each patient's individual immunoglobulin and used as ligands to trace cells expressing the idiotypic immunoglobulin on their surface. We established a flow cytometry and immunofluorescence protocol to track clonotypic B cells and validated it in two independent monoclonal B cell systems. Using this method, we found clonotypic B cells in only one out of 15 myeloma patients. In view of the assay's validated sensitivity level of 10⁻³, this surprising data suggests that the abundance of such cells has been vastly overestimated in the past and that they apparently represent a very rare population in myeloma. Our novel tracing approach may open perspectives to isolate and analyze clonotypic B cells and determine their role in myeloma pathobiology.

Progress in myeloma stem cells

Multiple myeloma (MM) is the second most common hematologic malignancy in the United States and affects about 4 in 100,000 Americans. Even though much progress has been made in MM therapy, MM remains an incurable disease for the vast majority of patients. The existence of MM stem cell is considered one of the major causes of MM drug-resistance, leading to relapse. This highlights the importance and urgency of developing approaches to target MM stem cells. However, very little is known about the molecular characteristics of the MM stem cells, which makes it difficult to target MM stem cells therapeutically. Evidence of the existence of a myeloma stem cell has been provided by Matsui et al. showing that the CD138- and CD20+ fraction, which is a minor population of the MM cells, has a greater clonogenic potential and has the phenotype of a memory B-cell (CD19+, CD27+). In this review, we report recent progress of cell surface markers in cancer stem cells, especially in myeloma and the molecular mechanisms related to drug resistance and myeloma disease progression.

Experimental approaches in the treatment of multiple myeloma

Myeloma therapy has undergone significant advances in recent years resulting in a marked improvement in survival. Knowledge of the active pathways involved in myeloma pathogenesis has led to the discovery of novel agents and greatly expanded the potential armamentarium available for treatment. This better understanding of the disease and resistance mechanisms has resulted in new agent classes that are being evaluated in preclinical and early clinical studies. In addition, dosing for existing agents is being optimized, and they are being given in new combinations. In this article, we review experimental agents that are showing promise in multiple myeloma treatment. New biological agents in clinical trials hold the promise of efficacy through novel mechanisms of action, with a significant reduction of dose-limiting toxicities compared with classic cytotoxic chemotherapeutics. Second-generation proteasome inhibitors and immunomodulatory agents are furthest along in clinical development, and histone deacetylase inhibitors, heat shock protein 90 inhibitors, Akt inhibitors and monoclonal antibodies are some of the other agents entering later-phase clinical trials. We also review developments in targeting the myeloma stem cell as an exciting new treatment direction.

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.

Stem cell marker nestin is expressed in plasma cells of multiple myeloma patients

Nestin is considered to be a characteristic marker of multipotent proliferative precursors found in some embryonic and fetal tissues. Its expression might be a suitable diagnostic and prognostic indicator of malignancy and a potential marker of cancer stem cells in solid tumors. Unexpectedly, nestin protein was detected in mature CD138(+)CD38(+) plasma cells of multiple myeloma patients and statistical analysis confirmed significant differences between myeloma patients and control group without hematological malignancy. Our results represent the first evidence of nestin expression in multiple myeloma. Further studies are required to elucidate the role of this protein in multiple myeloma.

The hemopoietic stem cell niche versus the microenvironment of the multiple myeloma-tumor initiating cell

Multiple myeloma cells are reminiscent of hemopoietic stem cells in their strict dependence upon the bone marrow microenvironment. However, from all other points of view, multiple myeloma cells differ markedly from stem cells. The cells possess a mature phenotype and secrete antibodies, and have thus made the whole journey to maturity, while maintaining a tumor phenotype. Not much credence was given to the possibility that the bulk of plasma-like multiple myeloma tumor cells is generated from tumor-initiating cells. Although interleukin-6 is a major contributor to the formation of the tumor’s microenvironment in multiple myeloma, it is not a major factor within hemopoietic stem cell niches. The bone marrow niche for myeloma cells includes the activity of inflammatory cytokines released through osteoclastogenesis. These permit maintenance of myeloma cells within the bone marrow. In contrast, osteoclastogenesis constitutes a signal that drives hemopoietic stem cells away from their bone marrow niches. The properties of the bone marrow microenvironment, which supports myeloma cell maintenance and proliferation, is therefore markedly different from the characteristics of the hemopoietic stem cell niche. Thus, multiple myeloma presents an example of a hemopoietic tumor microenvironment that does not resemble the corresponding stem cell renewal niche.