The Yin and Yang of the Bone Marrow Microenvironment: Pros and Cons of Mesenchymal Stromal Cells in Acute Myeloid Leukemia

Homoharringtonine sensitized resistant acute myeloid leukemia cells to venetoclax-induced apoptosis

Venetoclax (VEN), a B-cell lymphoma 2 (BCL-2) selective inhibitor, is widely used for treating acute myeloid leukemia (AML) with promising results. However, the anti-leukemic effect of VEN in relapsed/refractory (R/R)- AML requires improvement. In this study, we observed that combining homoharringtonine (HHT) with VEN plus azacitidine resulted in a significantly higher response and better survival than VA alone in patients with R/R-AML. Basic research indicates that HHT combined with VEN has a highly synergistic effect against both resistant AML cells and primary cells with/without mesenchymal stem cell (MSC) co-culture in vivo, inhibiting proliferation and colony-forming capacity of AML cells associated with concomitant cell cycle arrest. Mechanistically, HHT sensitizes AML cells to VEN by downregulating the anti-apoptotic proteins MCL-1/BCL-xL, activating reactive oxygen species (ROS), leading to mitochondrial membrane potential loss, and attenuating fatty acid (FA) uptake. These findings adding HHT to VEN-based regimens may enhance outcomes in R/R-AML patients.

Paediatric bone marrow mesenchymal stem cells support acute myeloid leukaemia cell survival and enhance chemoresistance via contact-independent mechanism

Children diagnosed with acute myeloid leukaemia (paediatric AML [pAML]) have limited treatment options and relapse rates due to chemoresistance and refractory disease are over 30%. Current treatment is cytotoxic and in itself has long-lasting harsh side effects. New, less toxic treatments are needed. The bone marrow microenvironment provides chemoprotection to leukaemic cells through cell communication and interaction with mesenchymal stem cells (MSCs), but this is not well defined in pAML. Using primary patient material, we identify a cell contact-independent mechanism of MSC-mediated chemoprotection involving extrinsic soluble factors that is abrogated through inhibition of the JAK/STAT and ERK pathways.

Effect of AML-exosomes on the cellular and molecular properties of bone marrow mesenchymal stromal cells: Expression of JAK/STAT signaling genes

PURPOSE OF STUDY

Despite the various therapeutic options introduced for AML treatment, therapy resistance and relapse are still the main obstacles. It is well known that alterations in the bone marrow microenvironment (BMM) play a crucial role in leukemia growth and the treatment failure of AML. Evidence shows that exosomes alter the components of BMM in a way that support leukemia survival, leading to chemoresistance. In this study, we evaluated the effect of AML exosomes on the biological functions of human bone marrow mesenchymal stromal cells (h BM-MSCs), especially alteration in the expression of the JAK/STAT signaling genes, as a leukemia-favoring pathway.

METHOD

Exosomes were isolated from the HL-60 cell line and characterized using flow cytometry, Transmission Electron Microscopy (TEM), and Dynamic Light Scattering (DLS) technique. The exosome protein content was assessed using a bicinchoninic acid (BCA) protein assay kit in order to determine the concentration of exosomes. Subsequently, MSCs were treated with varying concentrations of AML exosomes, and data was obtained using MTT, cell cycle, apoptosis, and ki67 assays. Additionally, gene expression analysis was conducted through qRT-PCR.

RESULT

AML exosomes regulated the viability and survival of MSCs in a concentration-dependent manner. The qRT-PCR data revealed that treatment with AML exosomes at a concentration of 50 μg/mL led to a significant upregulation of JAK2, STAT3, and STAT5 genes in MSCs.

CONCLUSION

Because the JAK/STAT signaling pathway has been shown to play a role in the proliferation and survival of leukemic cells, our results suggest that AML exosomes stimulate MSCs to activate this pathway. This activation may impede AML cell apoptosis, potentially leading to chemoresistance and relapse.

SLPI overexpression in hMSCs could be implicated in the HSC gene expression profile in AML

Acute myeloid leukaemia (AML) is a severe haematological neoplasm that originates from the transformation of haematopoietic stem cells (HSCs) into leukaemic stem cells (LSCs). The bone marrow (BM) microenvironment, particularly that of mesenchymal stromal cells (hMSCs), plays a crucial role in the maintenance of HSCs. In this context, we explored whether alterations in the secretome of hMSCs derived from AML patients (hMSC-AML) could impact HSC gene expression. Proteomic analysis revealed that the secretome of coculture assays with hMSC-AMLs and HSC from healthy donor is altered, with increased levels of secretory leukocyte protease inhibitor (SLPI), a protein associated with important processes for maintenance of the haematopoietic niche that has already been described to be altered in several tumours. Increased SLPI expression was also observed in the BM plasma of AML patients. Transcriptome analysis of HSCs cocultured with hMSC-AML in comparison with HSCs cocultured with hMSC-HD revealed altered expression of SLPI target genes associated with the cell cycle, proliferation, and apoptosis. Important changes were identified, such as increased expression levels of CCNA2, CCNE2, CCND2, CD133 and CDK1 and decreased levels of CDKN2A and IGFBP3, among others. Overall, these findings suggest that the altered secretome of coculture assays with hMSC-AMLs and HSC from healthy donor, particularly increased SLPI expression, can contribute to gene expression changes in HSCs, potentially influencing important molecular mechanisms related to AML development and progression.

Mesenchymal stromal cells in myeloid malignancies: Immunotherapeutic opportunities

Myeloid malignancies are clonal disorders of the progenitor cells or hematopoietic stem cells, including acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative malignancies, and chronic myelomonocytic leukemia. Myeloid neoplastic cells affect the proliferation and differentiation of other hematopoietic lineages in the bone marrow and peripheral blood, leading to severe and life-threatening complications. Mesenchymal stromal cells (MSCs) residing in the bone marrow exert immunosuppressive functions by suppressing innate and adaptive immune systems, thus creating a supportive and tolerant microenvironment for myeloid malignancy progression. This review summarizes the significant features of MSCs in myeloid malignancies, including their role in regulating cell growth, cell death, and antineoplastic resistance, in addition to their immunosuppressive contributions. Understanding the implications of MSCs in myeloid malignancies could pave the path for potential use in immunotherapy.

In Vitro Insights Into the Influence of Marrow Mesodermal/Mesenchymal Progenitor Cells on Acute Myelogenous Leukemia and Myelodysplastic Syndromes

The study of marrow-resident mesodermal progenitors can provide important insight into their role in influencing normal and aberrant hematopoiesis as occurs in acute myelogenous leukemia (AML) and myelodysplastic syndromes (MDS). In addition, the chemokine competency of these cells provides links to the inflammatory milieu of the marrow microenvironment with additional implications for normal and malignant hematopoiesis. While in vivo studies have elucidated the structure and function of the marrow niche in murine genetic models, corollary human studies have not been feasible, and thus the use of culture-adapted mesodermal cells has provided insights into the role these rare endogenous niche cells play in physiologic, malignant, and inflammatory states. This review focuses on culture-adapted human mesenchymal stem/stromal cells (MSCs) as they have been utilized in understanding their influence in AML and MDS as well as on their chemokine-mediated responses to myeloid malignancies, injury, and inflammation. Such studies have intrinsic limitations but have provided mechanistic insights and clues regarding novel druggable targets.

An Updated Overview of the Role of CYP450 during Xenobiotic Metabolization in Regulating the Acute Myeloid Leukemia Microenvironment

Oxidative stress is associated with several acute and chronic disorders, including hematological malignancies such as acute myeloid leukemia, the most prevalent acute leukemia in adults. Xenobiotics are usually harmless compounds that may be detrimental, such as pharmaceuticals, environmental pollutants, cosmetics, and even food additives. The storage of xenobiotics can serve as a defense mechanism or a means of bioaccumulation, leading to adverse effects. During the absorption, metabolism, and cellular excretion of xenobiotics, three steps may be distinguished: (i) inflow by transporter enzymes, (ii) phases I and II, and (iii) phase III. Phase I enzymes, such as those in the cytochrome P450 superfamily, catalyze the conversion of xenobiotics into more polar compounds, contributing to an elevated acute myeloid leukemia risk. Furthermore, genetic polymorphism influences the variability and susceptibility of related myeloid neoplasms, infant leukemias associated with mixed-lineage leukemia (MLL) gene rearrangements, and a subset of de novo acute myeloid leukemia. Recent research has shown a sustained interest in determining the regulators of cytochrome P450, family 2, subfamily E, member 1 (CYP2E1) expression and activity as an emerging field that requires further investigation in acute myeloid leukemia evolution. Therefore, this review suggests that CYP2E1 and its mutations can be a therapeutic or diagnostic target in acute myeloid leukemia.

Effect of Acute Myeloid Leukemia-derived Extracellular Vesicles on Bone Marrow Mesenchymal Stromal Cells: Expression of Poor Prognosis Genes

OBJECTIVE

Acute myeloid leukemia (AML) is a heterogeneous clonal disorder resulting from a complex interplay between leukemic cells and supporting factors from their microenvironment. In this context, extracellular vesicles (EVs) have been shown to play an essential role in forming a tumor-protective microenvironment. In this study, we examined the influence of AML-derived EVs on cellular and molecular characterization of bone marrow mesenchymal stromal cells (BM-MSCs), particularly alteration in the expression of genes (IL-6, Gas-6, and Galectin-3) relating to relapse and chemoresistance.

METHODS

MSCs were co-cultured with different concentrations of AML-EVs. Our data has been achieved by MTT assay, ROS assay, proliferation assay and apoptosis assay. RT-qPCR was also performed for gene expression analysis.

RESULTS

Our results demonstrated that AML-EVs impact the MSCs characterization in a concentration-dependent manner. We revealed higher viability, increased Ki-67 and BCL-2, and lower ROS levels in MSCs treated with a 40 µg/mL dose of EVs. On the other hand, the rate of MSCs apoptosis and BAX expression exposed to a 60 µg/mL dose of EVs were increased compared with the control group. In addition, RT-qPCR results showed that the expression of IL-6, Gas-6, and Galectin-3 was significantly up-regulated in treated MSCs with a 40 µg/mL dose of EVs.

CONCLUSION

Because the overexpression of IL-6, Gas-6, and Galectin-3 has contributed to chemoresistance and relapse, our findings suggest that AML-EVs propel MSCs to express these genes, which in turn could guard leukemic cells from chemotherapy-inflicted damages and eventually lead to relapse.

Allogeneic stem cell transplantation in the treatment of acute myeloid leukemia: An overview of obstacles and opportunities

As an important treatment for acute myeloid leukemia, allogeneic hematopoietic stem cell transplantation (allo-HSCT) plays an important role in reducing relapse and improving long-term survival. With rapid advancements in basic research in molecular biology and immunology and with deepening understanding of the biological characteristics of hematopoietic stem cells, allo-HSCT has been widely applied in clinical practice. During allo-HSCT, preconditioning, the donor, and the source of stem cells can be tailored to the patient’s conditions, greatly broadening the indications for HSCT, with clear survival benefits. However, the risks associated with allo-HSCT remain high, i.e. hematopoietic reconstitution failure, delayed immune reconstitution, graft-versus-host disease, and post-transplant relapse, which are bottlenecks for further improvements in allo-HSCT efficacy and have become hot topics in the field of HSCT. Other bottlenecks recognized in the current treatment of individuals diagnosed with acute myeloid leukemia and subjected to allo-HSCT include the selection of the most appropriate conditioning regimen and post-transplantation management. In this paper, we reviewed the progress of relevant research regarding these aspects.

Rationale for Combining the BCL2 Inhibitor Venetoclax with the PI3K Inhibitor Bimiralisib in the Treatment of IDH2- and FLT3-Mutated Acute Myeloid Leukemia

In October 2020, the FDA granted regular approval to venetoclax (ABT-199) in combination with hypomethylating agents for newly-diagnosed acute myeloid leukemia (AML) in adults 75 years or older, or in patients with comorbidities precluding intensive chemotherapy. The treatment response to venetoclax combination treatment, however, may be short-lived, and leukemia relapse is the major cause of treatment failure. Multiple studies have confirmed the upregulation of the anti-apoptotic proteins of the B-cell lymphoma 2 (BCL2) family and the activation of intracellular signaling pathways associated with resistance to venetoclax. To improve treatment outcome, compounds targeting anti-apoptotic proteins and signaling pathways have been evaluated in combination with venetoclax. In this study, the BCL-XL inhibitor A1331852, MCL1-inhibitor S63845, dual PI3K-mTOR inhibitor bimiralisib (PQR309), BMI-1 inhibitor unesbulin (PTC596), MEK-inhibitor trametinib (GSK1120212), and STAT3 inhibitor C-188-9 were assessed as single agents and in combination with venetoclax, for their ability to induce apoptosis and cell death in leukemic cells grown in the absence or presence of bone marrow stroma. Enhanced cytotoxic effects were present in all combination treatments with venetoclax in AML cell lines and AML patient samples. Elevated in vitro efficacies were observed for the combination treatment of venetoclax with A1331852, S63845 and bimiralisib, with differing response markers for each combination. For the venetoclax and bimiralisib combination treatment, responders were enriched for IDH2 and FLT3 mutations, whereas non-responders were associated with PTPN11 mutations. The combination of PI3K/mTOR dual pathway inhibition with bimiralisib and BCL2 inhibition with venetoclax has emerged as a candidate treatment in IDH2- and FLT3-mutated AML.

Targeting the Tumor Microenvironment in Acute Myeloid Leukemia: The Future of Immunotherapy and Natural Products

The tumor microenvironment (TME) plays an essential role in the development, proliferation, and survival of leukemic blasts in acute myeloid leukemia (AML). Within the bone marrow and peripheral blood, various phenotypically and functionally altered cells in the TME provide critical signals to suppress the anti-tumor immune response, allowing tumor cells to evade elimination. Thus, unraveling the complex interplay between AML and its microenvironment may have important clinical implications and are essential to directing the development of novel targeted therapies. This review summarizes recent advancements in our understanding of the AML TME and its ramifications on current immunotherapeutic strategies. We further review the role of natural products in modulating the TME to enhance response to immunotherapy.

The Bone Marrow Microenvironment in B-Cell Development and Malignancy

B lymphopoiesis is characterized by progressive loss of multipotent potential in hematopoietic stem cells, followed by commitment to differentiate into B cells, which mediate the humoral response of the adaptive immune system. This process is tightly regulated by spatially distinct bone marrow niches where cells, including mesenchymal stem and progenitor cells, endothelial cells, osteoblasts, osteoclasts, and adipocytes, interact with B-cell progenitors to direct their proliferation and differentiation. Recently, the B-cell niche has been implicated in initiating and facilitating B-cell precursor acute lymphoblastic leukemia. Leukemic cells are also capable of remodeling the B-cell niche to promote their growth and survival and evade treatment. Here, we discuss the major cellular components of bone marrow niches for B lymphopoiesis and the role of the malignant B-cell niche in disease development, treatment resistance and relapse. Further understanding of the crosstalk between leukemic cells and bone marrow niche cells will enable development of additional therapeutic strategies that target the niches in order to hinder leukemia progression.

Proliferative and anti‐apoptotic fractions in maturing hematopoietic cell lineages and their role in homeostasis of normal bone marrow

Recent developments in clinical flow cytometry allow the simultaneous assessment of proliferative and anti‐apoptotic activity in the different hematopoietic cell lineages and during their maturation process. This can further advance the flow cytometric diagnosis of myeloid malignancies. In this study we established indicative reference values for the Ki‐67 proliferation index and Bcl‐2 anti‐apoptotic index in blast cells, as well as maturing erythroid, myeloid, and monocytic cells from normal bone marrow (BM). Furthermore, the cell fractions co‐expressing both proliferation and anti‐apoptotic markers were quantified. Fifty BM aspirates from femoral heads of patients undergoing hip replacement were included in this study. Ten‐color/twelve‐parameter flow cytometry in combination with a software‐based maturation tool was used for immunophenotypic analysis of Ki‐67 and Bcl‐2 positive fractions during the erythro‐, myelo‐, and monopoiesis. Indicative reference values for the Ki‐67 and Bcl‐2 positive fractions were established for different relevant hematopoietic cell populations in healthy BM. Ki‐67 and Bcl‐2 were equally expressed in the total CD34 positive blast cell compartment and 30% of Ki‐67 positive blast cells also showed Bcl‐2 positivity. The Ki‐67 and Bcl‐2 positive fractions were highest in the more immature erythroid, myeloid and monocytic cells. Both fractions then gradually declined during the subsequent maturation phases of these cell lineages. We present a novel application of an earlier developed assay that allows the simultaneous determination of the Ki‐67 proliferative and Bcl‐2 anti‐apoptotic indices in maturing hematopoietic cell populations of the BM. Their differential expression levels during the maturation process were in accordance with the demand and lifespan of these cell populations. The indicative reference values established in this study can act as a baseline for further cell biological and biomedical studies involving hematological malignancies.

Release of IFN-γ by acute myeloid leukemia cells remodels bone marrow immune microenvironment by inducing regulatory T cells

PURPOSE

The stromal and immune bone marrow (BM) landscape is emerging as a crucial determinant for acute myeloid leukemia (AML). Regulatory T cells (Tregs) are enriched in the AML microenvironment, but the underlying mechanisms are poorly elucidated. Here, we addressed the effect of IFN-γ released by AML cells in BM Tregs induction and its impact on AML prognosis.

EXPERIMENTAL DESIGN

BM aspirates from AML patients were subdivided according to IFNG expression. Gene expression profiles in INFGhigh and IFNGlow samples were compared by microarray and NanoString analysis and used to compute a prognostic index. The IFN-g release effect on the BM microenvironment was investigated in mesenchymal stromal cell (MSC)/AML cell co-cultures. In mice, AML cells silenced for IFN-γ expression were injected intrabone.

RESULTS

IFNGhigh AMLsamples showed an upregulation of inflammatory genes, usually correlated with a good prognosis in cancer. By contrast, in AML patients, high IFNG expression associated with poor overall survival. Notably, IFN-g release by AML cells positively correlated with a higher BM suppressive Tregs' frequency. In co-culture experiments, IFNGhigh AML cells modified MSC transcriptome by up-regulating IFN-γ-dependent genes related to Treg induction, including indoleamine 2,3-dioxygenase 1 (IDO1). IDO1 inhibitor abrogated the effect of IFN-γ release by AML cells on MSC-derived Treg induction. Invivo, the genetic ablation of IFN-γ production by AML cells reduced MSC IDO1 expression and Treg infiltration, hindering AML engraftment.

CONCLUSIONS

IFN-g release by AML cells induces an immune-regulatory program in MSCs and remodels BM immunological landscape toward Treg induction, contributing to an immunotolerant microenvironment.

The mechanisms of mutual relationship between malignant hematologic cells and mesenchymal stem cells: Does it contradict the nursing role of mesenchymal stem cells?

Mesenchymal stem/stromal cells (MSCs) are known as the issue in biology because of some unpredictable characteristics in the different microenvironments especially in their bone marrow niche. MSCs are used in the regenerative medicine because of their unique potentials for trans-differentiation, immunomodulation, and paracrine capacity. But, their pathogenic and pro-survival effects in tumors/cancers including hematologic malignancies are indisputable. MSCs and/or their derivatives might be involved in tumor growth, metastasis and drug resistance in the leukemias. One of important relationship is MSCs and hematologic malignancy-derived cells which affects markedly the outcome of disease. The communication between these two cells may be contact-dependent and/or contact-independent. In this review, we studied the crosstalk between MSCs and malignant hematologic cells which results the final feedback either the progression or suppression of blood cell malignancy. Video abstract.

Targeting Thyrointegrin αvβ3 Using Fluorobenzyl Polyethylene Glycol Conjugated Tetraiodothyroacetic Acid (NP751) in Acute Myeloid Leukemia

Background

Acute myeloid leukemia (AML) is associated with poor long-term survival, even with newer therapeutic agents. Here, we show the results of our preclinical study, in which we evaluated the efficacy of a new thyrointegrin αvβ3 antagonist, named fluorobenzyl polyethylene glycol conjugated tetraiodothyroacetic acid (fb-PMT).

Methods and Results

fb-PMT (NP751) is a potent αvβ3 antagonist of molecular weight of 2,478.9 Da. it represents a conjugate of tetraiodothyroacetic acid (tetrac) and monodisperse polyethylene glycol (PEG36), with a 4-fluorobenzyl group capping the other end of the PEG. fb-PMT effectively suppresses the malignant growth of human acute myeloid leukemia (AML) after successful engraftment in transgenic NSG-S xenograft mouse models of either established human AML cell line or primary AML cells. Daily treatment with fb-PMT (1–10 mg/kg body weight) subcutaneously (s.c.) for 3–4 weeks was associated with marked regression of leukemogenesis and extended survival in both models. The efficiency of the fb-PMT therapy was verified using in vivo imaging system (IVIS) imaging, flow cytometry, and histopathological examination to monitor the engraftment of leukemic cells in the bone marrow and other organs. fb-PMT therapy for 3–4 weeks at 3 and 10 mg/kg daily doses exhibited significant reduction (p < 0.0001) of leukemic cell burden of 74% and >95%, respectively. All fb-PMT-treated mice in the 10 mg/kg treatment arm successfully maintained remission after discontinuing the daily treatment. Comprehensive fb-PMT safety assessments demonstrated excellent safety and tolerability at multiple folds above the anticipated human therapeutic doses. Lastly, our genome-wide microarray screens demonstrated that fb-PMT works through the molecular interference mechanism with multiple signaling pathways contributing to growth and survival of leukemic cells.

Conclusion

Our preclinical findings of the potent anticancer activities of fb-PMT and its favorable safety profiles warrant its clinical investigation for the effective and safe management of AML.

Put in a “Ca2+ll” to Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a clonal disorder characterized by genetic aberrations in myeloid primitive cells (blasts) which lead to their defective maturation/function and their proliferation in the bone marrow (BM) and blood of affected individuals. Current intensive chemotherapy protocols result in complete remission in 50% to 80% of AML patients depending on their age and the AML type involved. While alterations in calcium signaling have been extensively studied in solid tumors, little is known about the role of calcium in most hematologic malignancies, including AML. Our purpose with this review is to raise awareness about this issue and to present (i) the role of calcium signaling in AML cell proliferation and differentiation and in the quiescence of hematopoietic stem cells; (ii) the interplay between mitochondria, metabolism, and oxidative stress; (iii) the effect of the BM microenvironment on AML cell fate; and finally (iv) the mechanism by which chemotherapeutic treatments modify calcium homeostasis in AML cells.

The Yin-Yang Dynamics in Cancer Pharmacogenomics and Personalized Medicine

The enormous heterogeneity of cancer systems has made it very challenging to overcome drug resistance and adverse reactions to achieve personalized therapies. Recent developments in systems biology, especially the perception of cancer as the complex adaptive system (CAS), may help meet the challenges by deciphering the interactions at various levels from the molecular, cellular, tissue-organ, to the whole organism. The ubiquitous Yin-Yang interactions among the coevolving components, including the genes and proteins, decide their spatiotemporal features at various stages from cancer initiation to metastasis. The Yin-Yang imbalances across different systems levels, from genetic mutations to tumor cells adaptation, have been related to the intra- and inter-tumoral heterogeneity in the micro- and macro-environments. At the molecular and cellular levels, dysfunctional Yin-Yang dynamics in the cytokine networks, mitochondrial activities, redox systems, apoptosis, and metabolism can contribute to tumor cell growth and escape of immune surveillance. Up to the organism and system levels, the Yin-Yang imbalances in the cancer microenvironments can lead to different phenotypes from breast cancer to leukemia. These factors may be considered the systems-based biomarkers and treatment targets. The features of adaptation and nonlinearity in Yin-Yang dynamical interactions should be addressed by individualized drug combinations, dosages, intensities, timing, and frequencies at different cancer stages. The comprehensive "Yin-Yang dynamics" framework would enable powerful approaches for personalized and systems medicine strategies.

Emerging Bone Marrow Microenvironment-Driven Mechanisms of Drug Resistance in Acute Myeloid Leukemia: Tangle or Chance?

Simple Summary

Despite high rates of remission obtained with conventional chemotherapy, the persistence of leukemic cells after treatments, eventually exiting in disease relapse, remains the main challenge in acute myeloid leukemia (AML). Increasing evidence indicates that, besides AML cell mutations, stromal and immune cells, as leukemic microenvironment components, may protect AML cells from therapies. Here, we will recapitulate emerging bone marrow (BM) microenvironment-dependent mechanisms of therapy resistance. The understanding of these processes will help find new drug combinations and conceive novel and more effective treatments.

Abstract

Acute myeloid leukemia (AML) has been considered for a long time exclusively driven by critical mutations in hematopoietic stem cells. Recently, the contribution of further players, such as stromal and immune bone marrow (BM) microenvironment components, to AML onset and progression has been pointed out. In particular, mesenchymal stromal cells (MSCs) steadily remodel the leukemic niche, not only favoring leukemic cell growth and development but also tuning their responsiveness to treatments. The list of mechanisms driven by MSCs to promote a leukemia drug-resistant phenotype has progressively expanded. Moreover, the relative proportion and the activation status of immune cells in the BM leukemic microenvironment may vary by influencing their reactivity against leukemic cells. In that, the capacity of the stroma to re-program immune cells, thus promoting and/or hampering therapeutic efficacy, is emerging as a crucial aspect in AML biology, adding an extra layer of complexity. Current treatments for AML have mainly focused on eradicating leukemia cells, with little consideration for the leukemia-damaged BM niche. Increasing evidence on the contribution of stromal and immune cells in response to therapy underscores the need to hold the mutual interplay, which takes place in the BM. A careful dissection of these interactions will help provide novel applications for drugs already under experimentation and open a wide array of opportunities for new drug discovery.

Bone Marrow Mesenchymal Stem Cells-Derived Exosomal miR-425-5p Inhibits Acute Myeloid Leukemia Cell Proliferation, Apoptosis, Invasion and Migration by Targeting WTAP

Introduction

Acute myeloid leukemia (AML) is a predominant blood malignancy with high mortality and severe morbidity. AML is affected by microRNAs (miRNAs) loaded in exosomes derived from bone marrow mesenchymal stem cells (BM-MSCs). MiR-425-5p has been reported to participate in different cancer models. However, the function of BM-MSCs-derived exosomal miR-425-5p in AML is unclear.

Methods

The expression of miR-425-5p was measured by qRT-PCR in clinical AML samples. The immunophenotype of BM-MSCs was analyzed using antibodies against CD44, CD90, and CD105. The exosome was isolated from BM-MSCs. The effect of BM-MSCs-derived exosomal miR-425-5p on AML was analyzed by CCK-8 assay, Edu assay, transwell assay, flow cytometry in AML cells. qRT-PCR, luciferase reporter gene assay and Western blot analysis were also conducted in AML cells.

Results

The expression levels of miR-425-5p were decreased in CD34 + CD38-AML cells from primary AML patients compared to that from the bone marrow of healthy cases, and were reduced in exosomes from AML patients compared that from healthy cases. Similarly, miR-425-5p was also down-regulated in AML cell lines compared with BM-MSCs. MiR-425-5p was able to express in exosomes from BM-MSCs. CCK-8, Edu, transwell assay and flow cytometry analysis revealed that BM-MSCs-derived exosomal miR-425-5p significantly inhibited cell viability, Edu positive cells, invasion and migration, and induced apoptosis of AML cells. Meanwhile, the expression levels of cleaved PARP and cleaved caspase3 were increased by BM-MSCs-derived exosomal miR-425-5p in cells. MiR-425-5p inhibited the expression of Wilms tumor 1-associated protein (WTAP). Moreover, overexpression of WTAP could reverse the miR-425-5p-induced inhibition effect on AML cell proliferation, apoptosis, migration and invasion.

Conclusion

BM-MSCs-derived exosomal miR-425-5p inhibits proliferation, invasion and migration of AML cells and induced apoptosis of AML cells by targeting WTAP. Therapeutically, BM-MSCs-derived exosomal miR-425-5p may serve as a potential target for AML therapy.

Primary mesenchymal stromal cells in co-culture with leukaemic HL-60 cells are sensitised to cytarabine-induced genotoxicity, whilst leukaemic cells are protected

Tumour microenvironments are hallmarked in many cancer types. In haematological malignancies, bone marrow (BM) mesenchymal stromal cells (MSC) protect malignant cells from drug-induced cytotoxicity. However, less is known about malignant impact on supportive stroma. Notably, it is unknown whether these interactions alter long-term genotoxic damage in either direction. The nucleoside analogue cytarabine (ara-C), common in haematological therapies, remains the most effective agent for acute myeloid leukaemia, yet one third of patients develop resistance. This study aimed to evaluate the bidirectional effect of MSC and malignant cell co-culture on ara-C genotoxicity modulation. Primary MSC, isolated from patient BM aspirates for haematological investigations, and malignant haematopoietic cells (leukaemic HL-60) were co-cultured using trans-well inserts, prior to treatment with physiological dose ara-C. Co-culture genotoxic effects were assessed by micronucleus and alkaline comet assays. Patient BM cells from chemotherapy-treated patients had reduced ex vivo survival (P = 0.0049) and increased genotoxicity (P = 0.3172) than untreated patients. It was shown for the first time that HL-60 were protected by MSC from ara-C-induced genotoxicity, with reduced MN incidence in co-culture as compared to mono-culture (P = 0.0068). Comet tail intensity also significantly increased in ara-C-treated MSC with HL-60 influence (P = 0.0308). MSC sensitisation to ara-C genotoxicity was also demonstrated following co-culture with HL60 (P = 0.0116), which showed significantly greater sensitisation when MSC-HL-60 co-cultures were exposed to ara-C (P = 0.0409). This study shows for the first time that malignant HSC and MSC bidirectionally modulate genotoxicity, providing grounding for future research identifying mechanisms of altered genotoxicity in leukaemic microenvironments. MSC retain long-term genotoxic and functional damage following chemotherapy exposure. Understanding the interactions perpetuating such damage may inform modifications to reduce therapy-related complications, such as secondary malignancies and BM failure.

Myeloid-Derived Suppressor Cells and Mesenchymal Stem/Stromal Cells in Myeloid Malignancies

Myeloid malignancies arise from an altered hematopoietic stem cell and mainly comprise acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative malignancies, and chronic myelomonocytic leukemia. Myeloid neoplastic leukemic cells may influence the growth and differentiation of other hematopoietic cell lineages in peripheral blood and bone marrow. Myeloid-derived suppressor cells (MDSCs) and mesenchymal stromal cells (MSCs) display immunoregulatory properties by controlling the innate and adaptive immune systems that may induce a tolerant and supportive microenvironment for neoplasm development. This review analyzes the main features of MDSCs and MSCs in myeloid malignancies. The number of MDSCs is elevated in myeloid malignancies exhibiting high immunosuppressive capacities, whereas MSCs, in addition to their immunosuppression contribution, regulate myeloid leukemia cell proliferation, apoptosis, and chemotherapy resistance. Moreover, MSCs may promote MDSC expansion, which may mutually contribute to the creation of an immuno-tolerant neoplasm microenvironment. Understanding the implication of MDSCs and MSCs in myeloid malignancies may favor their potential use in immunotherapeutic strategies.

Myeloid-Derived Suppressor Cells and Mesenchymal Stem/Stromal Cells in Myeloid Malignancies

Myeloid malignancies arise from an altered hematopoietic stem cell and mainly comprise acute myeloid leukemia, myelodysplastic syndromes, myeloproliferative malignancies, and chronic myelomonocytic leukemia. Myeloid neoplastic leukemic cells may influence the growth and differentiation of other hematopoietic cell lineages in peripheral blood and bone marrow. Myeloid-derived suppressor cells (MDSCs) and mesenchymal stromal cells (MSCs) display immunoregulatory properties by controlling the innate and adaptive immune systems that may induce a tolerant and supportive microenvironment for neoplasm development. This review analyzes the main features of MDSCs and MSCs in myeloid malignancies. The number of MDSCs is elevated in myeloid malignancies exhibiting high immunosuppressive capacities, whereas MSCs, in addition to their immunosuppression contribution, regulate myeloid leukemia cell proliferation, apoptosis, and chemotherapy resistance. Moreover, MSCs may promote MDSC expansion, which may mutually contribute to the creation of an immuno-tolerant neoplasm microenvironment. Understanding the implication of MDSCs and MSCs in myeloid malignancies may favor their potential use in immunotherapeutic strategies.

Directly targeting c-Myc contributes to the anti-multiple myeloma effect of anlotinib

Despite the significant advances in the treatment of multiple myeloma (MM), this disease is still considered incurable because of relapse and chemotherapy resistance, underscoring the need to seek novel therapies with different mechanisms. Anlotinib, a novel multi-targeted tyrosine kinase inhibitor (TKI), has exhibited encouraging antitumor activity in several preclinical and clinical trials, but its effect on MM has not been studied yet. In this study, we found that anlotinib exhibits encouraging cytotoxicity in MM cells, overcomes the protective effect of the bone marrow microenvironment and suppresses tumor growth in the MM mouse xenograft model. We further examined the underlying molecular mechanism and found that anlotinib provokes cell cycle arrest, induces apoptosis and inhibits multiple signaling pathways. Importantly, we identify c-Myc as a novel direct target of anlotinib. The enhanced ubiquitin proteasomal degradation of c-Myc contributes to the cell apoptosis induced by anlotinib. In addition, anlotinib also displays strong cytotoxicity against bortezomib-resistant MM cells. Our study demonstrates the extraordinary anti-MM effect of anlotinib both in vitro and in vivo, which provides solid evidence and a promising rationale for future clinical application of anlotinib in the treatment of human MM.

Therapeutic Potential of Niche-Specific Mesenchymal Stromal Cells for Spinal Cord Injury Repair

The use of mesenchymal stem/stromal cells (MSCs) for transplant-mediated repair represents an important and promising therapeutic strategy after spinal cord injury (SCI). The appeal of MSCs has been fuelled by their ease of isolation, immunosuppressive properties, and low immunogenicity, alongside the large variety of available tissue sources. However, despite reported similarities in vitro, MSCs sourced from distinct tissues may not have comparable biological properties in vivo. There is accumulating evidence that stemness, plasticity, immunogenicity, and adaptability of stem cells is largely controlled by tissue niche. The extrinsic impact of cellular niche for MSC repair potential is therefore important, not least because of its impact on ex vivo expansion for therapeutic purposes. It is likely certain niche-targeted MSCs are more suited for SCI transplant-mediated repair due to their intrinsic capabilities, such as inherent neurogenic properties. In addition, the various MSC anatomical locations means that differences in harvest and culture procedures can make cross-comparison of pre-clinical data difficult. Since a clinical grade MSC product is inextricably linked with its manufacture, it is imperative that cells can be made relatively easily using appropriate materials. We discuss these issues and highlight the importance of identifying the appropriate niche-specific MSC type for SCI repair.

Mesenchymal Stromal Cell Therapy in Solid Organ Transplantation

Transplantation is the gold-standard treatment for the failure of several solid organs, including the kidneys, liver, heart, lung and small bowel. The use of tailored immunosuppressive agents has improved graft and patient survival remarkably in early post-transplant stages, but long-term outcomes are frequently unsatisfactory due to the development of chronic graft rejection, which ultimately leads to transplant failure. Moreover, prolonged immunosuppression entails severe side effects that severely impact patient survival and quality of life. The achievement of tolerance, i.e., stable graft function without the need for immunosuppression, is considered the Holy Grail of the field of solid organ transplantation. However, spontaneous tolerance in solid allograft recipients is a rare and unpredictable event. Several strategies that include peri-transplant administration of non-hematopoietic immunomodulatory cells can safely and effectively induce tolerance in pre-clinical models of solid organ transplantation. Mesenchymal stromal cells (MSC), non-hematopoietic cells that can be obtained from several adult and fetal tissues, are among the most promising candidates. In this review, we will focus on current pre-clinical evidence of the immunomodulatory effect of MSC in solid organ transplantation, and discuss the available evidence of their safety and efficacy in clinical trials.

Cadherins, Selectins, and Integrins in CAM-DR in Leukemia

The interaction between leukemia cells and the bone microenvironment is known to provide drug resistance in leukemia cells. This phenomenon, called cell adhesion-mediated drug resistance (CAM-DR), has been demonstrated in many subsets of leukemia including B- and T-acute lymphoblastic leukemia (B- and T-ALL) and acute myeloid leukemia (AML). Cell adhesion molecules (CAMs) are surface molecules that allow cell-cell or cell-extracellular matrix (ECM) adhesion. CAMs not only recognize ligands for binding but also initiate the intracellular signaling pathways that are associated with cell proliferation, survival, and drug resistance upon binding to their ligands. Cadherins, selectins, and integrins are well-known cell adhesion molecules that allow binding to neighboring cells, ECM proteins, and soluble factors. The expression of cadherin, selectin, and integrin correlates with the increased drug resistance of leukemia cells. This paper will review the role of cadherins, selectins, and integrins in CAM-DR and the results of clinical trials targeting these molecules.

B-cell lymphoma-2 inhibition and resistance in acute myeloid leukemia

Spurred by better understanding of disease biology, improvements in molecular diagnostics, and the development of targeted therapies, the treatment of acute myeloid leukemia (AML) has undergone significant evolution in recent years. Arguably, the most exciting shift has come from the success of treatment with the B-cell lymphoma-2 inhibitor venetoclax. When given in combination with a hypomethylating agent or low dose cytarabine, venetoclax demonstrates high response rates, some of which are durable. In spite of this, relapses after venetoclax treatment are common, and much interest exists in elucidating the mechanisms of resistance to the drug. Alterations in leukemic stem cell metabolism have been identified as a possible escape route, and clinical trials focusing on targeting metabolism in AML are ongoing. This review article highlights current research regarding venetoclax treatment and resistance in AML with a focus on cellular metabolism.

Location First: Targeting Acute Myeloid Leukemia Within Its Niche

Despite extensive research and development of new treatments, acute myeloid leukemia (AML)-backbone therapy has remained essentially unchanged over the last decades and is frequently associated with poor outcomes. Eradicating the leukemic stem cells (LSCs) is the ultimate challenge in the treatment of AML. Emerging evidence suggests that AML remodels the bone marrow (BM) niche into a leukemia-permissive microenvironment while suppressing normal hematopoiesis. The mechanism of stromal-mediated protection of leukemic cells in the BM is complex and involves many adhesion molecules, chemokines, and cytokines. Targeting these factors may represent a valuable approach to complement existing therapies and overcome microenvironment-mediated drug resistance. Some strategies for dislodging LSCs and leukemic blasts from their protective niche have already been tested in patients and are in different phases of the process of clinical development. Other strategies, such as targeting the stromal cells remodeling processes, remain at pre-clinical stages. Development of humanized xenograft mouse models, which overcome the mismatch between human leukemia cells and the mouse BM niche, is required to generate physiologically relevant, patient-specific human niches in mice that can be used to unravel the role of human AML microenvironment and to carry out preclinical studies for the development of new targeted therapies.

Gap Junctions in the Bone Marrow Lympho-Hematopoietic Stem Cell Niche, Leukemia Progression, and Chemoresistance

Abstract: The crosstalk between hematopoietic stem cells (HSC) and bone marrow (BM) microenvironment is critical for homeostasis and hematopoietic regeneration in response to blood formation emergencies after injury, and has been associated with leukemia transformation and progression. Intercellular signals by the BM stromal cells in the form of cell-bound or secreted factors, or by physical interaction, regulate HSC localization, maintenance, and differentiation within increasingly defined BM HSC niches. Gap junctions (GJ) are comprised of arrays of membrane embedded channels formed by connexin proteins, and control crucial signaling functions, including the transfer of ions, small metabolites, and organelles to adjacent cells which affect intracellular mechanisms of signaling and autophagy. This review will discuss the role of GJ in both normal and leukemic hematopoiesis, and highlight some of the most novel approaches that may improve the efficacy of cytotoxic drugs. Connexin GJ channels exert both cell-intrinsic and cell-extrinsic effects on HSC and BM stromal cells, involved in regenerative hematopoiesis after myelosuppression, and represent an alternative system of cell communication through a combination of electrical and metabolic coupling as well as organelle transfer in the HSC niche. GJ intercellular communication (GJIC) in the HSC niche improves cellular bioenergetics, and rejuvenates damaged recipient cells. Unfortunately, they can also support leukemia proliferation and survival by creating leukemic niches that provide GJIC dependent energy sources and facilitate chemoresistance and relapse. The emergence of new strategies to disrupt self-reinforcing malignant niches and intercellular organelle exchange in leukemic niches, while at the same time conserving normal hematopoietic GJIC function, could synergize the effect of chemotherapy drugs in eradicating minimal residual disease. An improved understanding of the molecular basis of connexin regulation in normal and leukemic hematopoiesis is warranted for the re-establishment of normal hematopoiesis after chemotherapy.

Exosomes derived from human bone marrow mesenchymal stem cells transfer miR-222-3p to suppress acute myeloid leukemia cell proliferation by targeting IRF2/INPP4B

AIM

This study aims to explore the role and mechanism of exosomes derived from human bone marrow mesenchymal stem cells (hBM-MSCs-Exo) in regulating proliferation and apoptosis of acute myeloid leukemia (AML) cell line THP-1.

METHODS

hBM-MSCs-Exo was isolated by ultra-centrifugation and administered into THP-1 cells to elucidate the effects of exosomes in THP-1 cells. Cell proliferation and apoptosis were examined by CCK-8 assay and flow cytometry, respectively. The expression of miR-222-3p, IRF2, and INPP4B were measured by qRT-PCR and western blot. The interaction between miR-222-3p and IRF2 was analyzed by luciferase reporter assay.

RESULTS

Lower cell viability rate, higher apoptosis ratio, higher miR-222-3p expression, and lower IRF1/INPP4B expression were observed in THP-1 cells exposed to BM-MSCs-Exo. The proliferation-inhibitory and pro-apoptotic effects of BM-MSCs-Exo on THP-1 cells were markedly compromised when miR-222-3p expression in BM-MSCs-Exo was inhibited. Furthermore, miR-222-3p directly targeted IRF2 and negatively regulated IRF2/INPP4B signaling in THP-1 cells. Moreover, overexpression of either IRF2 or INPP4B counteracted the proliferation-inhibitory and pro-apoptotic effects mediated by BM-MSCs-Exo.

CONCLUSION

BM-MSCs delivered miR-222-3p via exosomes to inhibit cell proliferation and promote cell apoptosis by targeting IRF2 and negatively regulating IRF2/INPP4B signaling in THP-1 cells.

Bone Marrow-Derived Mesenchymal Stromal Cells: A Novel Target to Optimize Hematopoietic Stem Cell Transplantation Protocols in Hematological Malignancies and Rare Genetic Disorders

: Mesenchymal stromal cells (MSCs) are crucial elements in the bone marrow (BM) niche where they provide physical support and secrete soluble factors to control and maintain hematopoietic stem progenitor cells (HSPCs). Given their role in the BM niche and HSPC support, MSCs have been employed in the clinical setting to expand ex-vivo HSPCs, as well as to facilitate HSPC engraftment in vivo. Specific alterations in the mesenchymal compartment have been described in hematological malignancies, as well as in rare genetic disorders, diseases that are amenable to allogeneic hematopoietic stem cell transplantation (HSCT), and ex-vivo HSPC-gene therapy (HSC-GT). Dissecting the in vivo function of human MSCs and studying their biological and functional properties in these diseases is a critical requirement to optimize transplantation outcomes. In this review, the role of MSCs in the orchestration of the BM niche will be revised, and alterations in the mesenchymal compartment in specific disorders will be discussed, focusing on the need to correct and restore a proper microenvironment to ameliorate transplantation procedures, and more in general disease outcomes.