Multiple myeloma cancer stem cells

MicroRNA-138 promotes the progression of multiple myeloma through targeting paired PAX5

BACKGROUND

Multiple myeloma cancer stem cells (MMSC) have been considered as the leading cause of multiple myeloma (MM) drug resistance and eventual relapse, microRNAs (miRNAs) collectively participate in the progression of MM. However, the pathogenesis of miR-138 in MMSC is still not fully understood.

OBJECTIVE

The intention of this study was to investigate the mechanism and role of miR-138 in multiple myeloma.

METHOD

Bone marrow samples and peripheral blood from patients and normal controls were collected. Use Magnet-based Cancer Stem Cell Isolation Kit to separate and extract MMSC. Real-time quantitative PCR (RT-qPCR) was carried out to determine mRNA level. Western blot was applied to detect protein levels. MTT and flow cytometry were conducted to examine the proliferation and apoptosis of MMSC. Finally, dual-luciferase reporter gene assays were performed to confirm that paired box 5 (PAX5) is a direct target for miR-138.

RESULTS

Compared with normal group, the expression of miR-138 in patients was significantly up-regulated, and the expression of miR-138 was in a negative correlation with PAX5. Additionally, downregulated miR-138 facilitated the apoptosis and inhibited the proliferation of MMSC in vitro and in vivo. Downregulated miR-138 moderated the expression of PAX5, Bcl-2, Bax, and Caspase-3. PAX5 was a direct target of miR-138.

CONCLUSION

Taken together, miR-138 plays a carcinogenic role in MM, and miR-138 adjusted the proliferation and apoptosis of MMSC by targeting PAX5. miR-138 has the probability of becoming a new medicinal target for the treatment of MM.

Leukemic Stem Cells and Hematological Malignancies

The association between leukemic stem cells (LSCs) and leukemia development has been widely established in the context of genetic alterations, epigenetic pathways, and signaling pathway regulation. Hematopoietic stem cells are at the top of the bone marrow hierarchy and can self-renew and progressively generate blood and immune cells. The microenvironment, niche cells, and complex signaling pathways that regulate them acquire genetic mutations and epigenetic alterations due to aging, a chronic inflammatory environment, stress, and cancer, resulting in hematopoietic stem cell dysregulation and the production of abnormal blood and immune cells, leading to hematological malignancies and blood cancer. Cells that acquire these mutations grow at a faster rate than other cells and induce clone expansion. Excessive growth leads to the development of blood cancers. Standard therapy targets blast cells, which proliferate rapidly; however, LSCs that can induce disease recurrence remain after treatment, leading to recurrence and poor prognosis. To overcome these limitations, researchers have focused on the characteristics and signaling systems of LSCs and therapies that target them to block LSCs. This review aims to provide a comprehensive understanding of the types of hematopoietic malignancies, the characteristics of leukemic stem cells that cause them, the mechanisms by which these cells acquire chemotherapy resistance, and the therapies targeting these mechanisms.

Vienna Cancer Stem Cell Club (VCSCC): 20 year jubilee and future perspectives

INTRODUCTION

The Vienna Cancer Stem Cell Club (VCSCC) was launched by a group of scientists in Vienna in 2002.

AREAS COVERED

Major aims of the VCSCC are to support research on cancer stem cells (CSC) in hematopoietic malignancies and to translate CSC-related markers and targets into clinical application. A primary focus of research in the VCSCC is the leukemic stem cell (LSC). Between 2013 and 2021, members of the VCSCC established a special research program on myeloproliferative neoplasms and since 2008, members of the VCSCC run the Ludwig Boltzmann Institute for Hematology and Oncology. In all these years, the VCSCC provided a robust intellectual platform for translational hematology and LSC research in Vienna. Furthermore, the VCSCC interacts with several national and international study groups and societies in the field. Representatives of the VCSCC also organized a number of international meetings and conferences on neoplastic stem cells, including LSC, in the past 15 years, and contributed to the definition and classification of CSC/LSC and related pre-malignant and malignant conditions.

EXPERT OPINION

The VCSCC will continue to advance the field and to develop LSC-detecting and LSC-eradicating concepts through which diagnosis, prognostication, and therapy of blood cancer patients should improve.

Single-cell technologies in multiple myeloma: new insights into disease pathogenesis and translational implications

Multiple myeloma (MM) is a hematological malignancy characterized by clonal proliferation of plasma cells. Although therapeutic advances have been made to improve clinical outcomes and to prolong patients' survival in the past two decades, MM remains largely incurable. Single-cell sequencing (SCS) is a powerful method to dissect the cellular and molecular landscape at single-cell resolution, instead of providing averaged results. The application of single-cell technologies promises to address outstanding questions in myeloma biology and has revolutionized our understanding of the inter- and intra-tumor heterogeneity, tumor microenvironment, and mechanisms of therapeutic resistance in MM. In this review, we summarize the recently developed SCS methodologies and latest MM research progress achieved by single-cell profiling, including information regarding the cancer and immune cell landscapes, tumor heterogeneities, underlying mechanisms and biomarkers associated with therapeutic response and resistance. We also discuss future directions of applying transformative SCS approaches with contribution to clinical translation.

Antiproliferative activity of compounds isolated from the root bark of Lannea acida in multiple myeloma cell lines

Lannea acida A. Rich. is a native plant of West Africa used in traditional medicine against diarrhea, dysentery, rheumatism, and women infertility. Eleven compounds were isolated from the dichloromethane root bark extract using various chromatographic techniques. Among those, nine compounds have not been previously reported, i.e. one cardanol derivative, two alkenyl 5-hydroxycyclohex-2-en-1-ones, three alkenyl cyclohex-4-ene-1,3-diols, two alkenyl 7-oxabicyclo[4.1.0]hept-4-en-3-ols, and one alkenyl 4,5-dihydroxycyclohex-2-en-1-one, together with two known cardanols. The structure of the compounds was elucidated using NMR, HRESIMS, ECD, IR, and UV. Their antiproliferative activity was evaluated in three multiple myeloma cell lines: RPMI 8226, MM.1S, and MM.1R. Two compounds showed activity in all cell lines with IC₅₀ values < 5 μM. Further investigations are needed to understand the mechanism of action.

Isolation and Structure Elucidation of Compounds from Sesamum alatum and Their Antiproliferative Activity against Multiple Myeloma Cells

The phytochemical investigation of the dichloromethane root extract of Sesamum alatum led to the isolation of 18 compounds. Among these, compounds 3-8, defined as 9-hydroxy-2,2-dimethyl-2H-benzo[g]chromene-5,10-dione 6-O-β-d-glucopyranoside (3), (2S,3R)-3,4,7-trihydroxy-2-(3'-methylbut-2'-en-1'-yl)-2,3-dihydro-1H-inden-1-one (4), (Z)-2-(1',4'-dihydroxy-4'-methylpent-2'-en-1'-ylidene)-4,7-dihydroxy-1H-indene-1,3(2H)-dione (5), (S)-2,5,8-trihydroxy-3-(2'-hydroxy-3'-methylbut-3'-en-1'-yl)naphthalene-1,4-dione (6), 6-hydroxy-3-(3'-methylbut-2'-en-1'-yl)-4-oxo-4H-chromene-5-carboxylic acid (7), and (S)-2-(1'-hydroxy-4'-methylpent-3'-en-1'-yl)anthracene-9,10-dione (8), respectively, have not yet been described. Their structures were elucidated based on spectroscopic data analysis, including IR, NMR, HRESIMS and ECD measurements. Additional known compounds, namely, hydroxysesamone (1), anthrasesamone A (2), 2,6-dimethoxy-1,4-benzoquinone (9), syringic acid (10), syringaresinol (11), 2,3-epoxysesamone 8-O-β-d-glucopyranoside (12), 2,3-diacetylmartinoside (13), 2,3-epoxy-4,5,8-trihydroxy-2-prenyl-1-tetralone (14), ursolic acid (15), chlorosesamone (16), 2,3-epoxysesamone (17), and 2-(4-methyl-3-pentenyl)anthraquinone (18) were isolated. The antiproliferative activity of the compounds was tested against the RPMI 8226 multiple myeloma cell line. When compounds presented an IC₅₀ value <10 μM, they were tested against two other multiple myeloma cell lines, MM.1S and MM.1R. Compound 17 was found to be the most potent, with IC₅₀ values of 0.6, 0.7, and 0.9 μM, respectively, for the three cell lines.

Development of a novel Bruton's tyrosine kinase inhibitor that exerts anti-cancer activities potentiates response of chemotherapeutic agents in multiple myeloma stem cell-like cells

Despite recent improvements in multiple myeloma (MM) treatment, MM remains an incurable disease and most patients experience a relapse. The major reason for myeloma recurrence is the persistent stem cell-like population. It has been demonstrated that overexpression of Bruton's tyrosine kinase (BTK) in MM stem cell-like cells is correlated with drug resistance and poor prognosis. We have developed a novel small BTK inhibitor, KS151, which is unique compared to other BTK inhibitors. Unlike ibrutinib, and the other BTK inhibitors such as acalabrutinib, orelabrutinib, and zanubrutinib that covalently bind to the C481 residue in the BTK kinase domain, KS151 can inhibit BTK activities without binding to C481. This feature of KS151 is important because C481 becomes mutated in many patients and causes drug resistance. We demonstrated that KS151 inhibits in vitro BTK kinase activities and is more potent than ibrutinib. Furthermore, by performing a semi-quantitative, sandwich-based array for 71-tyrosine kinase phosphorylation, we found that KS151 specifically inhibits BTK. Our western blotting data showed that KS151 inhibits BTK signaling pathways and is effective against bortezomib-resistant cells as well as MM stem cell-like cells. Moreover, KS151 potentiates the apoptotic response of bortezomib, lenalidomide, and panobinostat in both MM and stem cell-like cells. Interestingly, KS151 inhibits stemness markers and is efficient in inhibiting Nanog and Gli1 stemness markers even when MM cells were co-cultured with bone marrow stromal cells (BMSCs). Overall, our results show that we have developed a novel BTK inhibitor effective against the stem cell-like population, and potentiates the response of chemotherapeutic agents.

Single-Cell Proteomics and Tumor RNAseq Identify Novel Pathways Associated With Clofazimine Sensitivity in PI- and IMiD- Resistant Myeloma, and Putative Stem-Like Cells

Multiple myeloma (MM) is an incurable plasma cell malignancy with dose-limiting toxicities and inter-individual variation in response/resistance to the standard-of-care/primary drugs, proteasome inhibitors (PIs), and immunomodulatory derivatives (IMiDs). Although newer therapeutic options are potentially highly efficacious, their costs outweigh the effectiveness. Previously, we have established that clofazimine (CLF) activates peroxisome proliferator-activated receptor-γ, synergizes with primary therapies, and targets cancer stem-like cells (CSCs) in drug-resistant chronic myeloid leukemia (CML) patients. In this study, we used a panel of human myeloma cell lines as in vitro model systems representing drug-sensitive, innate/refractory, and clonally-derived acquired/relapsed PI- and cereblon (CRBN)-negative IMiD-resistant myeloma and bone marrow-derived CD138+ primary myeloma cells obtained from patients as ex vivo models to demonstrate that CLF shows significant cytotoxicity against drug-resistant myeloma as single-agent and in combination with PIs and IMiDs. Next, using genome-wide transcriptome analysis (RNA-sequencing), single-cell proteomics (CyTOF; Cytometry by time-of-flight), and ingenuity pathway analysis (IPA), we identified novel pathways associated with CLF efficacy, including induction of ER stress, autophagy, mitochondrial dysfunction, oxidative phosphorylation, enhancement of downstream cascade of p65-NFkB-IRF4-Myc downregulation, and ROS-dependent apoptotic cell death in myeloma. Further, we also showed that CLF is effective in killing rare refractory subclones like side populations that have been referred to as myeloma stem-like cells. Since CLF is an FDA-approved drug and also on WHO's list of safe and effective essential medicines, it has strong potential to be rapidly re-purposed as a safe and cost-effective anti-myeloma drug.

Phytochemical Investigation of the Roots of Ipomoea asarifolia and Antiproliferative Activity of the Isolated Compounds against Multiple Myeloma Cells

Ipomoea asarifolia is a herbaceous plant belonging to the family Convolvulaceae and is native to tropical regions of Africa, America, and Asia. A dichloromethane root extract showed antiproliferative activity against multiple myeloma cells (RPMI 8226). The phytochemical investigation led to the isolation of 15 compounds. Compounds 1-4, named (4S,8S)-1-(furan-3-yl)-9-hydroxy-4,8-dimethylnonane-1,6-dione, isoferulic acid hexadecyl ester, caffeic acid hexadecyl ester, and asarifolin I, respectively, are described for the first time. The structures of these molecules were established from their NMR, UV, IR spectroscopic, and MS data. 4-Hydroxycinnamic acid hexadecyl ester (5), 4-hydroxycinnamic acid octadecyl ester (6), 4-hydroxycinnamic acid eicosyl ester (7), caffeic acid octadecyl ester (8), pescapreins III, IV, XXI, XXIII, XXV, and XXVI (9-14), and stoloniferin III (15) were also isolated. All compounds were tested against a multiple myeloma cell line (RPMI 8226). When their IC₅₀ value was lower than 10 μM, the compounds were also tested against two other multiple myeloma cell lines, MM.1S and MM.1R. Compound 3 was the most potent, with an IC₅₀ value of 3.0 μM against RPMI 8226 cells.

Ofatumumab and Granzyme B as immunotoxin against CD20 antigen

Anti-CD20 antibodies such as ofatumumab has demonstrated efficacy in relapsed/refractory chronic lymphocytic leukemia, are among the most successful therapies to date. In this study, we have designed an immunotoxin composed of Granzyme B and the high affinity variant of Ofatumumab. Different simulation software applied to explore the structure of Granzyme B, a serine protease in cytotoxic lymphocytes granules as an apoptosis mediator was attached to its specific antibody structure (Ofatumumab) via an adaptor sequence. The accuracy, energy minimization and characterization of biological properties of the final structure were evaluated. Our computational outcomes indicated that the employed method for structure prediction has been successfully managed to design the immunotoxin structure. The precise and accurate design of the immune-therapeutic agents against cancer cells can be confirmed by employment of in-silico approaches. Consequently, based on this approach we could introduce a capable immunotoxin which specifically targeting CD20 in an accurate orientation and initiates cancer cell destruction by its toxin domain.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-022-00120-6.

Identification and Characterization of Multiple Myeloma Stem Cell-Like Cells

Multiple myeloma (MM) is a B-cell tumor of the blood system with high incidence and poor prognosis. With a further understanding of the pathogenesis of MM and the bone marrow microenvironment, a variety of adjuvant cell therapies and new drugs have been developed. However, the drug resistance and high relapse rate of MM have not been fundamentally resolved. Studies have shown that, in patients with MM, there is a type of poorly differentiated progenitor cell (MM stem cell-like cells, MMSCs). Although there is no recognized standard for identification and classification, it is confirmed that they are closely related to the drug resistance and relapse of MM. This article therefore systematically summarizes the latest developments in MMSCs with possible markers of MMSCs, introduces the mechanism of how MMSCs work in MM resistance and recurrence, and discusses the active pathways that related to stemness of MM.

Tumor cells in light-chain amyloidosis and myeloma show different transcriptional rewiring of normal plasma cell development

Although light-chain amyloidosis (AL) and multiple myeloma (MM) are characterized by tumor plasma cell (PC) expansion in bone marrow (BM), their clinical presentation differs. Previous attempts to identify unique pathogenic mechanisms behind such differences were unsuccessful, but there are no studies investigating the differentiation stage of tumor PCs in patients with AL and MM. We sought to define a transcriptional atlas of normal PC development (n=11) in secondary lymphoid organs (SLO), peripheral blood (PB) and BM for comparison with the transcriptional programs (TPs) of tumor PCs in AL (n=37), MM (n=46) and MGUS (n=6). Based on bulk and single-cell RNAseq, we observed thirteen TPs during transition of normal PCs throughout SLO, PB and BM; that CD39 outperforms CD19 to discriminate new-born from long-lived BM-PCs; that tumor PCs expressed the most advantageous TPs of normal PC differentiation; that AL shares greater similarity to SLO-PCs whereas MM is transcriptionally closer to PB-PCs and new-born BM-PCs; that AL and MM patients enriched in immature TPs had inferior survival; and that TPs related with protein N-linked glycosylation are upregulated in AL. Collectively, we provide a novel resource to understand normal PC development and the transcriptional reorganization of AL and other monoclonal gammopathies.

A Journey Through Myeloma Evolution: From the Normal Plasma Cell to Disease Complexity

The knowledge of cancer origin and the subsequent tracking of disease evolution represent unmet needs that will soon be within clinical reach. This will provide the opportunity to improve patient's stratification and to personalize treatments based on cancer biology along its life history. In this review, we focus on the molecular pathogenesis of multiple myeloma (MM), a hematologic malignancy with a well-known multi-stage disease course, where such approach can sooner translate into a clinical benefit. We describe novel insights into modes and timing of disease initiation. We dissect the biology of the preclinical and pre-malignant phases, elucidating how knowledge of the genomics of the disease and the composition of the microenvironment allow stratification of patients based on risk of disease progression. Then, we explore cell-intrinsic and cell-extrinsic drivers of MM evolution to symptomatic disease. Finally, we discuss how this may relate to the development of refractory disease after treatment. By integrating an evolutionary view of myeloma biology with the recent acquisitions on its clonal heterogeneity, we envision a way to drive the clinical management of the disease based on its detailed biological features more than surrogates of disease burden.

miR-296-5p suppresses stem cell potency of hepatocellular carcinoma cells via regulating Brg1/Sall4 axis

Epithelial-mesenchymal transition (EMT), a pivotal event during cancer progression such as relapse and metastasis, is positively correlated with the stemness potency of tumor cells. Our previous study showed that miR-296-5p attenuated EMT program of hepatocellular carcinoma cells (HCC) through NRG1/ERBB2/ERBB3 signaling. In the present study, we uncovered that miR-296-5p was able to inhibit the stemness potency of HCC by decreasing the number and size of tumorspheres, downregulating the expression of CSC biomarkers and hampering the ability of tumorigenesis in NOD/SCID mice. Brahma-related gene-1 (Brg1), as the target protein of miR-296-5p detected by bioinformatics methods, activates a series of downstream cascades through directly binding to Sall4 promoter and enhancing Sall4 transcription. Importantly, the higher expressions of Brg1 and Sall4 in tumor tissues of HCC patients suggest poorer prognoses after surgical extraction. In conclusion, miR-296-5p exerts an inhibitory effect on stemness potency of HCC cells via Brg1/Sall4 axis.

Identification of Key Genes and Pathways in Myeloma side population cells by Bioinformatics Analysis

Background: Multiple myeloma (MM) is the second most common hematological malignancy, which is still incurable and relapses inevitably, highlighting further understanding of the possible mechanisms. Side population (SP) cells are a group of enriched progenitor cells showing stem-like phenotypes with a distinct low-staining pattern with Hoechst 33342. Compared to main population (MP) cells, the underlying molecular characteristics of SP cells remain largely unclear. This bioinformatics analysis aimed to identify key genes and pathways in myeloma SP cells to provide novel biomarkers, predict MM prognosis and advance potential therapeutic targets. Methods: The gene expression profile GSE109651 was obtained from Gene Expression Omnibus database, and then differentially expressed genes (DEGs) with P-value <0.05 and |log2 fold-change (FC)| > 2 were selected by the comparison of myeloma light-chain (LC) restricted SP (LC/SP) cells and MP CD138⁺ cells. Subsequently, gene ontology (GO) and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analysis, protein-protein interaction (PPI) network analysis were performed to identify the functional enrichment analysis of the DEGs and screen hub genes. Cox proportional hazards regression was used to select the potential prognostic DEGs in training dataset (GSE2658). The prognostic value of the potential prognostic genes was evaluated by Kaplan-Meier curve and validated in another external dataset (MMRF-CoMMpass cohort from TCGA). Results: Altogether, 403 up-regulated and 393 down-regulated DEGs were identified. GO analysis showed that the up-regulated DEGs were significantly enriched in innate immune response, inflammatory response, plasma membrane and integral component of membrane, while the down-regulated DEGs were mainly involved in protoporphyrinogen IX and heme biosynthetic process, hemoglobin complex and erythrocyte differentiation. KEGG pathway analysis suggested that the DEGs were significantly enriched in osteoclast differentiation, porphyrin and chlorophyll metabolism and cytokine-cytokine receptor interaction. The top 10 hub genes, identified by the plug-in cytoHubba of the Cytoscape software using maximal clique centrality (MCC) algorithm, were ITGAM, MMP9, ITGB2, FPR2, C3AR1, CXCL1, CYBB, LILRB2, HP and FCER1G. Modules and corresponding GO enrichment analysis indicated that myeloma LC/SP cells were significantly associated with immune system, immune response and cell cycle. The predictive value of the prognostic model including TFF3, EPDR1, MACROD1, ARHGEF12, AMMECR1, NFATC2, HES6, PLEK2 and SNCA was identified, and validated in another external dataset (MMRF-CoMMpass cohort from TCGA). Conclusions: In conclusion, this study provides reliable molecular biomarkers for screening, prognosis, as well as novel therapeutic targets for myeloma LC/SP cells.

Heparanase promotes myeloma stemness and in vivo tumorigenesis

Heparanase is known to enhance the progression of many cancer types and is associated with poor patient prognosis. We recently reported that after patients with multiple myeloma were treated with high dose chemotherapy, the tumor cells that emerged upon relapse expressed a much higher level of heparanase than was present prior to therapy. Because tumor cells having stemness properties are thought to seed tumor relapse, we investigated whether heparanase had a role in promoting myeloma stemness. When plated at low density and grown in serum-free conditions that support survival and expansion of stem-like cells, myeloma cells expressing a low level of heparanase formed tumor spheroids poorly. In contrast, cells expressing a high level of heparanase formed significantly more and larger spheroids than did the heparanase low cells. Importantly, heparanase-low expressing cells exhibited plasticity and were induced to exhibit stemness properties when exposed to recombinant heparanase or to exosomes that contained a high level of heparanase cargo. The spheroid-forming heparanase-high cells had elevated expression of GLI1, SOX2 and ALDH1A1, three genes known to be associated with myeloma stemness. Inhibitors that block the heparan sulfate degrading activity of heparanase significantly diminished spheroid formation and expression of stemness genes implying a direct role of the enzyme in regulating stemness. Blocking the NF-κB pathway inhibited spheroid formation and expression of stemness genes demonstrating a role for NF-κB in heparanase-mediated stemness. Myeloma cells made deficient in heparanase exhibited decreased stemness properties in vitro and when injected into mice they formed tumors poorly compared to the robust tumorigenic capacity of cells expressing higher levels of heparanase. These studies reveal for the first time a role for heparanase in promoting cancer stemness and provide new insight into its function in driving tumor progression and its association with poor prognosis in cancer patients.

Macrophage Inhibitory Factor-1 (MIF-1) controls the plasticity of multiple myeloma tumor cells

Multiple Myeloma (MM) is the second most common hematological malignancy with a median survival of 5–10 years. While current treatments initially cause remission, relapse almost always occurs, leading to the hypothesis that a chemotherapy-resistant cancer stem cell (CSC) remains dormant, and undergoes self-renewal and differentiation to reestablish disease. Our finding is that the mature cancer cell (CD138+, rapidly proliferating and chemosensitive) has developmental plasticity; namely, the ability to dedifferentiate back into its own chemoresistant CSC progenitor, the CD138–, quiescent pre-plasma cell. We observe multiple cycles of differentiation and dedifferentiation in the absence of niche or supportive accessory cells, suggesting that soluble cytokines secreted by the MM cells themselves are responsible for this bidirectional interconversion and that stemness and chemoresistance are dynamic characteristics that can be acquired or lost and thus may be targetable. By examining cytokine secretion of CD138- and CD138+ RPMI-8226 cells, we identified that concomitant with interconversion, Macrophage Migration Inhibitory Factor (MIF-1) is secreted. The addition of a small molecule MIF-1 inhibitor (4-IPP) or MIF-1 neutralizing antibodies to CD138+ cells accelerated dedifferentiation back into the CD138- progenitor, while addition of recombinant MIF-1 drove cells towards CD138+ differentiation. A similar increase in the CD138- population is seen when MM tumor cells isolated from primary bone marrow aspirates are cultured in the presence of 4-IPP. As the CD138+ MM cell is chemosensitive, targeting MIF-1 and/or the pathways that it regulates could be a viable way to modulate stemness and chemosensitivity, which could in turn transform the treatment of MM.

Ludwig Boltzmann Cluster Oncology (LBC ONC): first 10 years and future perspectives

In 2008 the Ludwig Boltzmann Cluster Oncology (LBC ONC) was established on the basis of two previous Ludwig Boltzmann Institutes working in the field of hematology and cancer research. The general aim of the LBC ONC is to improve treatment of hematopoietic neoplasms by eradicating cancer-initiating and disease-propagating cells, also known as leukemic stem cells (LSC) in the context of leukemia. In a first phase, the LBC ONC characterized the phenotype and molecular aberration profiles of LSC in various malignancies. The LSC phenotypes were established in acute and chronic myeloid leukemia, in acute lymphoblastic leukemia and in chronic lymphocytic leukemia. In addition, the concept of preleukemic (premalignant) neoplastic stem cells (pre-L-NSC) was coined by the LBC ONC and was tested in myelodysplastic syndromes and myeloproliferative neoplasms. Phenotypic characterization of LSC provided a solid basis for their purification and for the characterization of specific target expression profiles. In a second phase, molecular markers and targets were validated. This second phase is ongoing and should result in the development of new diagnostics parameters and novel, more effective, LSC-eradicating, treatment strategies; however, many issues still remain to be solved, such as sub-clonal evolution, LSC niche interactions, immunologic control of LSC, and LSC resistance. In the forthcoming years, the LBC ONC will concentrate on developing LSC-eradicating strategies, with special focus on LSC resistance, precision medicine and translation of LSC-eradicating concepts into clinical application.

Differential effects of lenalidomide during plasma cell differentiation

Thalidomide, lenalidomide and pomalidomide have greatly improved the outcome of patients with multiple myeloma. However, their effects on plasma cells, the healthy counterpart of myeloma cells, are unknown. Here, we investigated lenalidomide effects on normal human plasma cell generation using an in vitro model. Lenalidomide inhibited the generation of pre-plasmablasts and early plasma cells, while it moderately affected plasmablast production. It also reduced the expression level of Ikaros, Aiolos, and IRF4 transcription factors, in plasmablasts and early plasma cells. This suggests that their differential sensitivity to lenalidomide is not due to a difference in Ikaros or Aiolos degradation. Lenalidomide also inhibited long-lived plasma cell generation, but did not impair their long-term survival once generated. This last observation is in agreement with the finding that lenalidomide treatment for 3-18 months did not affect the bone marrow healthy plasma cell count in allografted patients with multiple myeloma. Our findings should prompt to investigate whether lenalidomide resistance in patients with multiple myeloma could be associated with the emergence of malignant plasmablasts or long-lived plasma cells that are less sensitive to lenalidomide.

VS-5584 mediates potent anti-myeloma activity via the upregulation of a class II tumor suppressor gene, RARRES3 and the activation of Bim

The PI3K/mTOR/AKT pathway is an integral regulator of survival and drug resistance in multiple myeloma (MM). VS-5584 was synthesized with dual-specific and equipotent activity against mTORC1/2 and all four Class I PI3K isoforms so as to durably inhibit this pathway. We show that VS-5584 is highly efficacious against MM cell lines even in the presence of IL-6 and IGF-1 and that this growth inhibition is partially dependent on Bim. Importantly, VS-5584 triggers apoptosis in patient cells with a favorable therapeutic index. Gene expression profiling revealed a VS-5584-induced upregulation of RARRES3, a class II tumor suppressor gene. MM patient databases, UAMS and APEX, show that RARRES3 is under-expressed in 11q13 subsets which correlates with the reduced effectiveness of VS-5584 in 11q13 cell lines. Silencing RARRES3 expression significantly rescues VS-5584-induced cell death and increases cyclin D2 expression but not cyclin D1 or other cyclins implying a role for RARRES3 in cell cycle arrest. In vivo, VS-5584 significantly reduces the tumor burden of MM mouse xenografts. We further identified that VS-5584 synergised with Dexamethasone, Velcade, and exceptionally so with HDAC inhibitor, Panobinostat. Interestingly, this was consistently observed in several patient samples, proposing a promising novel clinical strategy for combination treatment especially in relapsed/refractory patients.

Withanolide D Exhibits Similar Cytostatic Effect in Drug-Resistant and Drug-Sensitive Multiple Myeloma Cells

In spite of recent therapeutic advances, multiple myeloma (MM) remains a malignancy with very low curability. This has been partly attributed to the existence of a drug-resistant subpopulation known as cancer stem cells (CSCs). MM-CSCs are equipped with the necessary tools that render them highly resistant to virtually all conventional therapies. In this study, the growth inhibitory effects of withanolide D (WND), a steroidal lactone isolated from Withania somnifera, on drug-sensitive tumoral plasma cells and drug-resistant MM cells have been investigated. In MTT/XTT assays, WND exhibited similar cytostatic effects between drug-resistant and drug-sensitive cell lines in the nM range. WND also induced cell death and apoptosis in MM-CSCs and RPMI 8226 cells, as examined by the calcein/ethidium homodimer and annexin V/propidium iodide stainings, respectively. To determine whether P-glycoprotein (P-gp) efflux affected the cytostatic activity of WND, P-gp was inhibited with verapamil and results indicated that the WND cytostatic effect in MM-CSCs was independent of P-gp efflux. Furthermore, WND did not increase the accumulation of the fluorescent P-gp substrate rhodamine 123 in MM-CSCs, suggesting that WND may not inhibit P-gp at the tested relevant doses. Therefore, the WND-induced cytostatic effect may be independent of P-gp efflux. These findings warrant further investigation of WND in MM-CSC animal models.

Blockade of Deubiquitylating Enzyme USP1 Inhibits DNA Repair and Triggers Apoptosis in Multiple Myeloma Cells

Purpose: The ubiquitin proteasome pathway is a validated therapeutic target in multiple myeloma. Deubiquitylating enzyme USP1 participates in DNA damage response and cellular differentiation pathways. To date, the role of USP1 in multiple myeloma biology is not defined. In the present study, we investigated the functional significance of USP1 in multiple myeloma using genetic and biochemical approaches.Experimental Design: To investigate the role of USP1 in myeloma, we utilized USP1 inhibitor SJB3-019A (SJB) for studies in myeloma cell lines and patient multiple myeloma cells.Results: USP1-siRNA knockdown decreases multiple myeloma cell viability. USP1 inhibitor SJB selectively blocks USP1 enzymatic activity without blocking other DUBs. SJB also decreases the viability of multiple myeloma cell lines and patient tumor cells, inhibits bone marrow plasmacytoid dendritic cell-induced multiple myeloma cell growth, and overcomes bortezomib resistance. SJB triggers apoptosis in multiple myeloma cells via activation of caspase-3, caspase-8, and caspase-9. Moreover, SJB degrades USP1 and downstream inhibitor of DNA-binding proteins as well as inhibits DNA repair via blockade of Fanconi anemia pathway and homologous recombination. SJB also downregulates multiple myeloma stem cell renewal/survival-associated proteins Notch-1, Notch-2, SOX-4, and SOX-2. Moreover, SJB induced generation of more mature and differentiated plasma cells. Combination of SJB and HDACi ACY-1215, bortezomib, lenalidomide, or pomalidomide triggers synergistic cytotoxicity.Conclusions: Our preclinical studies provide the framework for clinical evaluation of USP1 inhibitors, alone or in combination, as a potential novel multiple myeloma therapy. Clin Cancer Res; 23(15); 4280-9. ©2017 AACR.

Epigenetic strategies to reverse drug resistance in heterogeneous multiple myeloma

Multiple myeloma (MM) is a hematological malignancy, which remains incurable because most patients eventually relapse or become refractory to current treatments. Due to heterogeneity within the cancer cell microenvironment, cancer cell populations employ a dynamic survival strategy to chemotherapeutic treatments, which frequently results in a rapid acquisition of therapy resistance. Besides resistance-conferring genetic alterations within a tumor cell population selected during drug treatment, recent findings also reveal non-mutational mechanisms of drug resistance, involving a small population of "cancer stem cells" (CSCs) which are intrinsically more refractory to the effects of a variety of anticancer drugs. Other studies have implicated epigenetic mechanisms in reversible drug tolerance to protect the population from eradication by potentially lethal exposures, suggesting that acquired drug resistance does not necessarily require a stable heritable genetic alteration. Clonal evolution of MM cells and the bone marrow microenvironment changes contribute to drug resistance. MM-CSCs may not be a static population and survive as phenotypically and functionally different cell types via the transition between stem-like and non-stem-like states in local microenvironments, as observed in other types of cancers. Targeting MM-CSCs is clinically relevant, and different approaches have been suggested to target molecular, metabolic and epigenetic signatures, and the self-renewal signaling characteristic of MM CSC-like cells. Here, we summarize epigenetic strategies to reverse drug resistance in heterogeneous multiple myeloma.

The myeloma stem cell concept, revisited: from phenomenology to operational terms

The concept of the myeloma stem cell may have important therapeutic implications, yet its demonstration has been hampered by a lack of consistency in terms and definitions. Here, we summarize the current documentation and propose single-cell in vitro studies for future translational studies. By the classical approach, a CD19^-/CD45^low/-/CD38^high/CD138⁺ malignant plasma cell, but not the CD19⁺/CD38^low/- memory B cell compartment, is enriched for tumorigenic cells that initiate myeloma in xenografted immunodeficient mice, supporting that myeloma stem cells are present in the malignant PC compartment. Using a new approach, analysis of c-DNA libraries from CD19⁺/CD27⁺/CD38^- single cells has identified clonotypic memory B cell, suggested to be the cell of origin. This is consistent with multiple myeloma being a multistep hierarchical process before or during clinical presentation. We anticipate that further characterization will require single cell geno- and phenotyping combined with clonogenic assays. To implement such technologies, we propose a revision of the concept of a myeloma stem cell by including operational in vitro assays to describe the cellular components of origin, initiation, maintenance, and evolution of multiple myeloma. These terms are in accordance with recent (2012) consensus statements on the definitions, assays, and nomenclature of cancer stem cells, which is technically precise without completely abolishing established terminology. We expect that this operational model will be useful for future reporting of parameters used to identify and characterize the multiple myeloma stem cells. We strongly recommend that these parameters include validated standard technologies, reproducible assays, and, most importantly, supervised prospective sampling of selected biomaterial which reflects clinical stages, disease spectrum, and therapeutic outcome. This framework is key to the characterization of the cellular architecture of multiple myeloma and its use in precision medicine.

MicroRNA-451 regulates stemness of side population cells via PI3K/Akt/mTOR signaling pathway in multiple myeloma

Side population (SP) cells are an enriched source of cancer-initiating cells with stemness characteristics, generated by increased ABC transporter activity, which has served as a unique hallmark for multiple myeloma (MM) stem cell studies. Here we isolated and identified MM SP cells via Hoechst 33342 staining. Furthermore, we demonstrate that SP cells possess abnormal cell cycle, clonogenicity, and high drug efflux characteristics-all of which are features commonly seen in stem cells. Interestingly, we found that bortezomib, As₂O₃, and melphalan all affected apoptosis and clonogenicity in SP cells. We followed by characterizing the miRNA signature of MM SP cells and validated the specific miR-451 target tuberous sclerosis 1 (TSC1) gene to reveal that it activates the PI3K/Akt/mTOR signaling in MM SP cells. Inhibition of miR-451 enhanced anti-myeloma novel agents' effectiveness, through increasing cells apoptosis, decreasing clonogenicity, and reducing MDR1 mRNA expression. Moreover, the novel specific PI3K/Akt/mTOR signaling inhibitor S14161 displayed its prowess as a potential therapeutic agent by targeting MM SP cells. Our findings offer insights into the mechanisms regulating MM SP cells and provide a novel strategy to overcome resistance to existing therapies against myeloma.

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.

Mechanisms of Drug Resistance in Relapse and Refractory Multiple Myeloma

Multiple myeloma (MM) is a hematological malignancy that remains incurable because most patients eventually relapse or become refractory to current treatments. Although the treatments have improved, the major problem in MM is resistance to therapy. Clonal evolution of MM cells and bone marrow microenvironment changes contribute to drug resistance. Some mechanisms affect both MM cells and microenvironment, including the up- and downregulation of microRNAs and programmed death factor 1 (PD-1)/PD-L1 interaction. Here, we review the pathogenesis of MM cells and bone marrow microenvironment and highlight possible drug resistance mechanisms. We also review a potential molecular targeting treatment and immunotherapy for patients with refractory or relapse MM.

Immunotherapy based on bispecific T-cell engager with hIgG1 Fc sequence as a new therapeutic strategy in multiple myeloma

Bispecific antibodies play an important role in immunotherapy. They have received intense interest from pharmaceutical enterprises. The first antibody drug, OKT3 (muromonab-CD3), showed great performance in clinical treatment. We have successfully developed a single-chain variable fragment (ScFv) combination of anti-CD3 ScFv and anti-CD138 ScFv with the hIgG1 Fc (hIgFc) sequence. The novel bispecific T-cell engager (BiTE) with an additional hIgFc (BiTE-hIgFc, STL001) can target T cells, natural killer cells, and multiple myeloma cells (RPMI-8226 or U266). In addition, BiTE-hIgFc (STL001) has nanomolar-level affinity to recombinant human CD138 protein and shows more potent antitumor activity against RPMI-8226 cells than that of separate aCD3-ScFv-hIgFc and aCD138-ScFv-hIgFc, or the isotype mAb in vitro or in vivo.

Minimal residual disease in multiple myeloma: bringing the bench to the bedside

Outcomes for patients with multiple myeloma (MM) have improved substantially in the past decade, with improvements in both progression-free survival and overall survival. Many patients are now achieving a complete response to treatment, and consequently highly sensitive assays are needed for detection of minimal residual disease (MRD) in patients with MM. Results of multicolour flow cytometry and deep-sequencing studies suggest that among patients achieving a complete response, MRD-negative status is associated with significant improvements in progression-free survival and overall survival. Despite the increasing need for MRD testing in patients with MM, considerable heterogeneity in techniques for MRD detection hinders the clinical interpretation of their results. The criteria used to define MRD, strengths and weaknesses of the major types of tests (flow cytometry versus molecular testing), and the optimal sample type (bone marrow aspirate versus peripheral blood) are all unresolved dilemmas in MRD testing. This Review presents an overview of the various techniques for MRD detection in patients with MM. In addition, this article discusses challenges and opportunities for the routine use of MRD testing, possible future directions for clinical trials and implications for drug approval processes.

Leukemia and ABC transporters

Acute myeloid leukemia (AML) is a heterogeneous disease caused by aberrant proliferation and/or differentiation of myeloid progenitors. However, only ~65% of AML patients respond to induction chemotherapy and the overall survival rate for AML remains low (~24% for 5-year survival). The conventional view suggests that ATP-binding cassette (ABC) transporters contribute to treatment failure due to their drug-effluxing capabilities. This might be overly simplistic. Some ABC transporters export endogenous substrates that have defined roles in normal hematopoietic progenitors. It is conceivable that these substances also provide an advantage to leukemic progenitors. This review will highlight how certain endogenous substrates impact normal hematopoietic cells and suggest that ABC transporters facilitate export of these substances to affect both normal hematopoietic and leukemic progenitors. For example, the ability to export certain endogenous ligands may facilitate leukemogenesis by modifying leukemic progenitor cell proliferation or survival. If so, the addition of ABC transporter inhibitors to traditional chemotherapy might improve therapeutic efficacy by not just increasing intracellular drug accumulation but also blocking the beneficial effects ABC transporter ligands have on cell survival.

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.

The development of potential antibody-based therapies for myeloma

With optimal target antigen selection antibody-based therapeutics can be very effective agents for hematologic malignancies, but none have yet been approved for myeloma. Rituximab and brentuximab vedotin are examples of success for the naked antibody and antibody-drug conjugate classes, respectively. Plasma cell myeloma is an attractive disease for antibody-based targeting due to target cell accessibility and the complementary mechanism of action with approved therapies. Initial antibodies tested in myeloma were disappointing. However, recent results from targeting well-characterized antigens have been more encouraging. In particular, the CD38 and CD138 targeted therapies are showing single-agent activity in early phase clinical trials. Here we will review the development pipeline for naked antibodies and antibody-drug conjugates for myeloma. There is clear clinical need for new treatments, as myeloma inevitably becomes refractory to standard agents. The full impact is yet to be established, but we are optimistic that the first FDA-approved antibody therapeutic(s) for this disease will emerge in the near future.

The cancer stem cell hypothesis: a guide to potential molecular targets

Common cancer theories hold that tumor is an uncontrolled somatic cell proliferation caused by the progressive addition of random mutations in critical genes that control cell growth. Nevertheless, various contradictions related to the mutation theory have been reported previously. These events may be elucidated by the persistence of residual tumor cells, called Cancer Stem Cells (CSCs) responsible for tumorigenesis, tumor maintenance, tumor spread, and tumor relapse. Herein, we summarize the current understanding of CSCs, with a focus on the possibility to identify specific markers of CSCs, and discuss the clinical application of targeting CSCs for cancer treatment.

New insight into hedgehog signaling in hematological malignancies

The hedgehog (Hh) signaling pathway is well established as being evolutionarily conserved across vertebrates, and is involved in organogenesis, hematopoiesis, embryogenesis and homeostasis of adult tissues. At a microscopic level, the Hh signaling pathway controls the proliferation, apoptosis, cell-cycle and differentiation programs of stem and progenitor cells. Increasing evidence suggests that aberrant activation of the Hh signaling pathway is related to neoplasm, including solid tumors and hematologic malignancies. Currently the Hh signaling pathway has become one of the most studied potential therapeutic targets in hematological malignancies. In this review, we focus on findings related to Hh signaling in the initiation, maintenance, progression and chemoresistance of hematological malignancies, looking forward to better targeted treatment strategies.

Clonogenic multiple myeloma cells have shared stemness signature associated with patient survival

Multiple myeloma is the abnormal clonal expansion of post germinal B cells in the bone marrow. It was previously reported that clonogenic myeloma cells are CD138⁻. Human MM cell lines RPMI8226 and NCI H929 contained 2-5% of CD138⁻ population. In this study, we showed that CD138⁻ cells have increased ALDH1 activity, a hallmark of normal and neoplastic stem cells. CD138⁻ALDH⁺ cells were more clonogenic than CD138⁺ALDH⁻ cells and only CD138⁻ cells differentiated into CD138⁺ population. In vivo tumor initiation and clonogenic potentials of the CD138⁻ population was confirmed using NOG mice. We derived a gene expression signature from functionally validated and enriched CD138⁻ clonogenic population from MM cell lines and validated these in patient samples. This data showed that CD138⁻ cells had an enriched expression of genes that are expressed in normal and malignant stem cells. Differentially expressed genes included components of the polycomb repressor complex (PRC) and their targets. Inhibition of PRC by DZNep showed differential effect on CD138⁻ and CD138⁺ populations. The ‘stemness’ signature derived from clonogenic CD138⁻ cells overlap significantly with signatures of common progenitor cells, hematopoietic stem cells, and Leukemic stem cells and is associated with poorer survival in different clinical datasets.

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.

Next-generation sequencing of peripheral B-lineage cells pinpoints the circulating clonotypic cell pool in multiple myeloma

The identity of the proliferative compartment of myeloma progenitor cells remains a matter of debate. Polymerase chain reaction-based studies suggested pre-switch "clonotypic" B cells sharing the immunoglobulin (Ig) rearrangement of the malignant plasma cell (M-PC), to circulate in the blood and possess stem cell-like properties. Here, we disprove this hypothesis. We screened peripheral blood IgM, IgG, and IgA repertoires of myeloma patients for the clonotypic rearrangement by next-generation sequencing. None of 12 cases showed pre-switch clonotypic transcripts. In the post-switch IgG/IgA repertoires, however, the clonotypic rearrangement was detected at high frequency in 6 of 8 patients with active disease, whereas it was undetectable after treatment, correlating with flow cytometric presence or absence of circulating M-PCs. Minor subclones with alternative post-switch isotypes suggested ongoing switch events and clonal evolution at the M-PC level. Our findings consistently show an absence of pre-switch clonotypic B cells, while M-PCs circulate in the peripheral blood and may contribute to spreading of the disease.

Emerging role for leucine-rich repeat-containing G-protein-coupled receptors LGR5 and LGR4 in cancer stem cells

The concept of cancer stem cells has gained considerable interest in the last few decades, partly because of their potential implication in therapy resistance. However, the lack of specific cellular surface markers for these cells has impeded their isolation, making the characterization of this cellular subpopulation technically challenging. Recent studies have indicated that leucine-rich repeat-containing G-protein-coupled receptor 4 and 5 (LGR4 and LGR5) expression in multiple organs may represent a global marker of adult stem cells. This review aims to give an overview of LGR4 and LGR5 as cancer stem cell markers and their function in development.

Targeting the biophysical properties of the myeloma initiating cell niches: a pharmaceutical synergism analysis using multi-scale agent-based modeling

Multiple myeloma, the second most common hematological cancer, is currently incurable due to refractory disease relapse and development of multiple drug resistance. We and others recently established the biophysical model that myeloma initiating (stem) cells (MICs) trigger the stiffening of their niches via SDF-1/CXCR4 paracrine; The stiffened niches then promote the colonogenesis of MICs and protect them from drug treatment. In this work we examined in silico the pharmaceutical potential of targeting MIC niche stiffness to facilitate cytotoxic chemotherapies. We first established a multi-scale agent-based model using the Markov Chain Monte Carlo approach to recapitulate the niche stiffness centric, pro-oncogenetic positive feedback loop between MICs and myeloma-associated bone marrow stromal cells (MBMSCs), and investigated the effects of such intercellular chemo-physical communications on myeloma development. Then we used AMD3100 (to interrupt the interactions between MICs and their stroma) and Bortezomib (a recently developed novel therapeutic agent) as representative drugs to examine if the biophysical properties of myeloma niches are drugable. Results showed that our model recaptured the key experimental observation that the MBMSCs were more sensitive to SDF-1 secreted by MICs, and provided stiffer niches for these initiating cells and promoted their proliferation and drug resistance. Drug synergism analysis suggested that AMD3100 treatment undermined the capability of MICs to modulate the bone marrow microenvironment, and thus re-sensitized myeloma to Bortezomib treatments. This work is also the first attempt to virtually visualize in 3D the dynamics of the bone marrow stiffness during myeloma development. In summary, we established a multi-scale model to facilitate the translation of the niche-stiffness centric myeloma model as well as experimental observations to possible clinical applications. We concluded that targeting the biophysical properties of stem cell niches is of high clinical potential since it may re-sensitize tumor initiating cells to chemotherapies and reduce risks of cancer relapse.

A replicative self-renewal model for long-lived plasma cells: questioning irreversible cell cycle exit

Plasma cells are heterogenous in terms of their origins, secretory products, and lifespan. A current paradigm is that cell cycle exit in plasma cell differentiation is irreversible, following a pattern familiar in short-lived effector populations in other hemopoietic lineages. This paradigm no doubt holds true for many plasma cells whose lifespan can be measured in days following the completion of differentiation. Whether this holds true for long-lived bone marrow plasma cells that are potentially maintained for the lifespan of the organism is less apparent. Added to this the mechanisms that establish and maintain cell cycle quiescence in plasma cells are incompletely defined. Gene expression profiling indicates that in the transition of human plasmablasts to long-lived plasma cells a range of cell cycle regulators are induced in a pattern that suggests a quiescence program with potential for cell cycle re-entry. Here a model of relative quiescence with the potential for replicative self-renewal amongst long-lived plasma cells is explored. The implications of such a mechanism would be diverse, and the argument is made here that current evidence is not sufficiently strong that the possibility should be disregarded.

Decreased affinity of recombinant human tumor necrosis factor-related apoptosis-inducing ligand (rhTRAIL) D269H/E195R to osteoprotegerin (OPG) overcomes TRAIL resistance mediated by the bone microenvironment

The bone marrow microenvironment provides important signals for the survival and proliferation of hematopoietic and malignant cells. In multiple myeloma, plasma cells are surrounded by stromal cells including osteoblasts. These stromal cells protect multiple myeloma cells from apoptosis induced by chemotherapeutic agents. Osteoprotegerin (OPG), a soluble receptor of the cytokine TNF-related apoptosis-inducing ligand (TRAIL), is secreted by osteoblasts and has been implicated in the prevention of cell death induced by TRAIL in malignant cells. Previously, we have designed death receptor-specific TRAIL variants that induce apoptosis exclusively via one of its death receptors. Here, we have studied in detail the interaction between recombinant human (rhTRAIL) variants and OPG. We show that a DR5-specific variant (rhTRAIL D269H/E195R) displays a significantly decreased affinity to OPG. Furthermore, this rhTRAIL variant shows a much higher activity when compared with rhTRAIL WT and retains its effectiveness in inducing cell death in multiple myeloma cell lines, in the presence of OPG secreted by stromal cells. We also demonstrate that stromal cells are largely insensitive to high concentrations of this rhTRAIL variant. In conclusion, rhTRAIL D269H/E195R is a potential therapy for multiple myeloma due to its high effectiveness and diminished binding to OPG.

Myeloma stem cell concepts, heterogeneity and plasticity of multiple myeloma

Multiple myeloma (MM) is a haematological malignancy characterized by the accumulation of clonal plasma cells (PCs) in the bone marrow (BM). Although novel therapeutic strategies have prolonged survival of patients, the disease remains difficult to treat with a high risk of relapse. The failure of therapy is thought to be associated with a persistent population of the so-called MM stem cells or myeloma initiating cells (MIC) that exhibit tumour-initiating potential, self-renewal and resistance to chemotherapy. However, the population responsible for the origin and sustainability of tumour mass has not been clearly characterized so far. This review summarizes current myeloma stem cell concepts and suggests that high phenotypic and intra-clonal heterogeneity, together with plasticity potential of MM might be other contributing factors explaining discrepancies among particular concepts and contributing to the treatment failure.

Immunotherapy strategies for multiple myeloma: the present and the future

Growing knowledge of the complexities of the immune system have led to a better understanding of how it can be harnessed for the purpose of anticancer therapy. Moreover, recent success with immunotherapies for solid tumors, combined with novel therapeutic strategies against myeloma, heighten excitement at the prospect of improving clinical outcomes for myeloma by improving antitumor immunity. Increased understanding of myeloma tumor-associated antigens, availability of more potent vaccines, expanded immune-modulating therapies, development of agents that block immune-suppressive pathways, increased sophistication of adoptive cell therapy techniques and capitalization upon standard autologous transplant are all important standalone or combination strategies that might ultimately improve prognosis of patients with multiple myeloma.

Expression analysis of multiple myeloma CD138 negative progenitor cells using single molecule microarray readout

We present a highly sensitive bioanalytical microarray assay that enables the analysis of small genomic sample material. By combining an optimized cDNA purification step with single molecule cDNA detection on the microarray, the platform has improved sensitivity compared to conventional systems, allowing amplification-free determination of expression profiles with as little as 600ng total RNA. Total RNA from cells was reverse transcribed into fluorescently labeled cDNA and purified employing a precipitation method that minimizes loss of cDNA material. The microarray was scanned on a fluorescence chip-reader with single molecule sensitivity. Using the newly developed platform we were able to analyze the RNA expression profile of a subpopulation of rare multiple myeloma CD138 negative progenitor (MM CD138(neg)) cells. The high-sensitivity microarray approach led to the identification of a set of 20 genes differentially expressed in MM CD138(neg) cells. Our work demonstrates the applicability of a straight-forward single-molecule DNA array technology to current topics of molecular and cellular cancer research, which are otherwise difficult to address due to the limited amount of sample material.