Bone marrow endosteal mesenchymal progenitors depend on HIF factors for maintenance and regulation of hematopoiesis

Bioengineered niches that recreate physiological extracellular matrix organisation to support long-term haematopoietic stem cells

Long-term reconstituting haematopoietic stem cells (LT-HSCs) are used to treat blood disorders via stem cell transplantation. The very low abundance of LT-HSCs and their rapid differentiation during in vitro culture hinders their clinical utility. Previous developments using stromal feeder layers, defined media cocktails, and bioengineering have enabled HSC expansion in culture, but of mostly short-term HSCs and progenitor populations at the expense of naive LT-HSCs. Here, we report the creation of a bioengineered LT-HSC maintenance niche that recreates physiological extracellular matrix organisation, using soft collagen type-I hydrogels to drive nestin expression in perivascular stromal cells (PerSCs). We demonstrate that nestin, which is expressed by HSC-supportive bone marrow stromal cells, is cytoprotective and, via regulation of metabolism, is important for HIF-1α expression in PerSCs. When CD34+ve HSCs were added to the bioengineered niches comprising nestin/HIF-1α expressing PerSCs, LT-HSC numbers were maintained with normal clonal and in vivo reconstitution potential, without media supplementation. We provide proof-of-concept that our bioengineered niches can support the survival of CRISPR edited HSCs. Successful editing of LT-HSCs ex vivo can have potential impact on the treatment of blood disorders.

Co-administration of human MSC overexpressing HIF-1α increases human CD34+ cell engraftment in vivo

Background

Poor graft function or graft failure after allogeneic stem cell transplantation is an unmet medical need, in which mesenchymal stromal cells (MSC) constitute an attractive potential therapeutic approach. Hypoxia-inducible factor-1α (HIF-1α) overexpression in MSC (HIF-MSC) potentiates the angiogenic and immunomodulatory properties of these cells, so we hypothesized that co-transplantation of MSC-HIF with CD34⁺ human cord blood cells would also enhance hematopoietic stem cell engraftment and function both in vitro and in vivo.

Methods

Human MSC were obtained from dental pulp. Lentiviral overexpression of HIF-1α was performed transducing cells with pWPI-green fluorescent protein (GFP) (MSC WT) or pWPI-HIF-1α-GFP (HIF-MSC) expression vectors. Human cord blood CD34⁺ cells were co-cultured with MSC WT or HIF-MSC (4:1) for 72 h. Then, viability (Annexin V and 7-AAD), cell cycle, ROS expression and immunophenotyping of key molecules involved in engraftment (CXCR4, CD34, ITGA4, c-KIT) were evaluated by flow cytometry in CD34⁺ cells. In addition, CD34⁺ cells clonal expansion was analyzed by clonogenic assays. Finally, in vivo engraftment was measured by flow cytometry 4-weeks after CD34⁺ cell transplantation with or without intrabone MSC WT or HIF-MSC in NOD/SCID mice.

Results

We did not observe significant differences in viability, cell cycle and ROS expression between CD34⁺ cells co-cultured with MSC WT or HIF-MSC. Nevertheless, a significant increase in CD34, CXCR4 and ITGA4 expression (p = 0.009; p = 0.001; p = 0.013, respectively) was observed in CD34⁺ cells co-cultured with HIF-MSC compared to MSC WT. In addition, CD34⁺ cells cultured with HIF-MSC displayed a higher CFU-GM clonogenic potential than those cultured with MSC WT (p = 0.048). We also observed a significant increase in CD34⁺ cells engraftment ability when they were co-transplanted with HIF-MSC compared to CD34⁺ co-transplanted with MSC WT (p = 0.016) or alone (p = 0.015) in both the injected and contralateral femurs (p = 0.024, p = 0.008 respectively).

Conclusions

Co-transplantation of human CD34⁺ cells with HIF-MSC enhances cell engraftment in vivo. This is probably due to the ability of HIF-MSC to increase clonogenic capacity of hematopoietic cells and to induce the expression of adhesion molecules involved in graft survival in the hematopoietic niche.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02669-z.

Characterization of human bone marrow niches with metabolome and transcriptome profiling

Bone marrow (BM) niches are special microenvironments that work in harmony with each other for the regulation and maintenance of hematopoiesis. Niche investigations have thus far been limited to various model organisms and animal studies; therefore, little is known about different niches in healthy humans. In this study, a special harvesting method for the collection of BM from two different anatomical regions in the iliac crest of humans was used to investigate the presence of different niches in BM. Additionally, metabolomic and transcriptomic profiles were compiled using comparative 'omics' technologies, and the main cellular pathways and corresponding transcripts and metabolites were identified. As a result, we found that the energy metabolism between the regions was different. This study provides basic broad data for regenerative medicine in terms of the design of the appropriate microenvironment for in vitro hematopoietic niche modeling, and identifies the normal reference values that can be compared in hematological disease.

Understanding of the crosstalk between normal residual hematopoietic stem cells and the leukemic niche in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease, yet clinically most patients present with pancytopenia resulting from bone marrow failure, predisposing them to life-threatening infections and bleeding. The mechanisms by which AML mediates hematopoietic suppression is not well known. Indeed, much effort has so far been focused on how AML remodels the bone marrow niche to make it a more permissive environment, with less focus on how the remodeled niche affects normal hematopoietic cells. In this perspective, we present evidence of the key role of the bone marrow niche in suppressing hematopoietic stem cells (HSCs) during leukemic progression and provide perspectives on how future research on this topic may be exploited to provide treatments for one of the key complications of AML.

Mesenchymal niche remodeling impairs hematopoiesis via stanniocalcin 1 in acute myeloid leukemia

Acute myeloid leukemia (AML) disrupts the generation of normal blood cells, predisposing patients to hemorrhage, anemia, and infections. Differentiation and proliferation of residual normal hematopoietic stem and progenitor cells (HSPCs) are impeded in AML-infiltrated bone marrow (BM). The underlying mechanisms and interactions of residual hematopoietic stem cells (HSCs) within the leukemic niche are poorly understood, especially in the human context. To mimic AML infiltration and dissect the cellular crosstalk in human BM, we established humanized ex vivo and in vivo niche models comprising AML cells, normal HSPCs, and mesenchymal stromal cells (MSCs). Both models replicated the suppression of phenotypically defined HSPC differentiation without affecting their viability. As occurs in AML patients, the majority of HSPCs were quiescent and showed enrichment of functional HSCs. HSPC suppression was largely dependent on secreted factors produced by transcriptionally remodeled MSCs. Secretome analysis and functional validation revealed MSC-derived stanniocalcin 1 (STC1) and its transcriptional regulator HIF-1α as limiting factors for HSPC proliferation. Abrogation of either STC1 or HIF-1α alleviated HSPC suppression by AML. This study provides a humanized model to study the crosstalk among HSPCs, leukemia, and their MSC niche, and a molecular mechanism whereby AML impairs normal hematopoiesis by remodeling the mesenchymal niche.

Mesenchymal Progenitors Derived from Different Locations in Long Bones Display Diverse Characteristics

Mesenchymal progenitors within bone marrow have multiple differentiation potential and play an essential role in the maintenance of adult skeleton homeostasis. Mesenchymal progenitors located in bone regions other than the bone marrow also display bone-forming properties. However, owing to the differences in each distinct microenvironment, the mesenchymal characteristics of skeletal progenitor cells within different regions of long bones may show some differences. In order to clearly elucidate these differences, we performed a comparative study on mesenchymal progenitors from different regions of long bones. Here, we isolated mesenchymal progenitors from the periosteum, endosteum, and bone marrow of rat long bones. The three groups exhibited similar cellular morphologies and expressed the typical surface markers associated with mesenchymal stem cells. Interestingly, after cell proliferation assays and bidirectional differentiation analysis, periosteal mesenchymal progenitors showed a higher proliferative ability and adipogenic differentiation potential. In contrast, endosteal mesenchymal progenitors were more prone to osteogenic differentiation. Using in vitro osteoclast culture systems, conditioned media from different mesenchymal progenitor cultures were used to induce osteoclastic differentiation. Osteoclast formation was found to be significantly promoted by the secretion of RANKL and IL-6 by endosteal progenitors. Overall, our results provide strong evidence for the importance of selecting the appropriate source of skeletal progenitors for applications in future skeleton regeneration therapies.

HIF-α factors as potential therapeutic targets in leukemia

INTRODUCTION

Hypoxia-inducible transcription factors have been identified as regulators of adaptive responses to hypoxia. Over the past 20 years, more than 8000 papers have described their increasingly complex role and regulation in cancer. Presently, it is recognized that hypoxia-inducible factors (HIFs) are regulated by oxygen-dependent and oxygen-independent mechanisms in cancer development; the list of their targets has increased to include more than 500 genes involved in most hallmarks of cancer. Areas covered: Most literature describes the function of HIF factors in solid tumors; however, in the past 10 years, evidence has steadily accumulated to indicate that HIFs are implicated in hematological malignancies. This review summarizes our current understanding of the function and regulation of HIF factors in hematopoiesis and leukemia. Moreover, we provide an update on pharmacological inhibitors of this pathway that have shown promising therapeutic effects in clinical trials or leukemia pre-clinical models. Expert opinion: The inhibition of the function of HIF factors may provide an interesting approach for treating leukemia. We posit that before moving into the clinic, we should (i) fully characterize the outcome of HIF inhibition in specific leukemia contexts (ii) test the possibility of combining HIF-targeting strategies with cytotoxic compounds and (iii) consider patient selection to increase therapeutic efficacy.

A non-cell-autonomous role for Pml in the maintenance of leukemia from the niche

Disease recurrence after therapy, due to the persistence of resistant leukemic cells, represents a fundamental problem in the treatment of leukemia. Elucidating the mechanisms responsible for the maintenance of leukemic cells, before and after treatment, is therefore critical to identify curative modalities. It has become increasingly clear that cell-autonomous mechanisms are not solely responsible for leukemia maintenance. Here, we report a role for Pml in mesenchymal stem cells (MSCs) in supporting leukemic cells of both CML and AML. Mechanistically, we show that Pml regulates pro-inflammatory cytokines within MSCs, and that this function is critical in sustaining CML-KLS and AML ckit+ leukemic cells non-cell autonomously.

Decisive differences in the bone repair processes of the metaphysis and diaphysis in young mice

Fractures are common traumatic injuries that mainly occur in the metaphyses of long bones such as the proximal humerus, distal radius, and proximal femur. However, most studies of fracture repair processes have focused on the diaphyseal region. In this study, we compared the bone repair processes of the metaphysis and the diaphysis of the mouse tibia. Bone apertures were formed in the tibial metaphysis and diaphysis. At indicated times after surgery, samples were collected, and the healing process was investigated using micro-computed tomography, as well as histological, immunohistochemical, and mRNA expression analyses. In the metaphysis, cartilage formation was not detected on the periosteal side. The bone aperture was filled with newly formed bone produced from bone marrow at day 7. In the case of the diaphysis, cartilage was formed around the aperture at day 4 and sequentially replaced by bone on the periosteal side. The bone aperture was filled with newly formed bone at day 14. In the bone marrow, expression of the osteogenic markers such as alkaline phosphatase, osteocalcin, and type I collagen, appeared earlier with metaphyseal injury than with diaphyseal injury. The mRNA expression of chondrogenesis markers was markedly upregulated in the diaphysis compared with that in the metaphysis on the periosteal side. These results indicate differences in the bone repair processes of the two regions, suggesting functional heterogeneity of the periosteum and bone marrow mesenchymal cells in response to bone fractures.

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Fractures occur mainly in the metaphyses of long bones, but most studies of fracture repair have focused on the diaphysis.

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We compared the bone repair processes of the metaphysis and the diaphysis of the mouse tibia.

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In the metaphysis, cartilage was not detected by histology and newly formed bone was produced from the bone marrow on day 7.

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In the diaphysis, cartilage was formed on day 4 and replaced with newly formed bone by day 14.

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Expression of osteogenic and chondrogenic markers also differed between the two regions of bone during the repair process.

Notch2 Signaling Regulates the Proliferation of Murine Bone Marrow-Derived Mesenchymal Stem/Stromal Cells via c-Myc Expression

Mesenchymal stem/stromal cells (MSCs) reside in the bone marrow and maintain their stemness under hypoxic conditions. However, the mechanism underlying the effects of hypoxia on MSCs remains to be elucidated. This study attempted to uncover the signaling pathway of MSC proliferation. Under low-oxygen culture conditions, MSCs maintained their proliferation and differentiation abilities for a long term. The Notch2 receptor was up-regulated in MSCs under hypoxic conditions. Notch2-knockdown (Notch2-KD) MSCs lost their cellular proliferation ability and showed reduced gene expression of hypoxia-inducible transcription factor (HIF)-1α, HIF-2α, and c-Myc. Overexpression of the c-Myc gene in Notch2-KD MSCs allowed the cells to regain their proliferation capacity. These results suggested that Notch2 signaling is linked to c-Myc expression and plays a key role in the regulation of MSC proliferation. Our findings provide important knowledge for elucidating the self-replication competence of MSCs in the bone marrow microenvironment.

Conditioned Medium from Placental Mesenchymal Stem Cells Reduces Oxidative Stress during the Cryopreservation of Ex Vivo Expanded Umbilical Cord Blood Cells

Background

The limited cell dose in umbilical cord blood (UCB) necessitates ex vivo expansion of UCB. Further, the effective cryopreservation of these expanded cells is important in widening their use in the clinics. During cryopreservation, cells experience oxidative stress due to the generation of reactive oxygen species (ROS). Conditioned medium from mesenchymal stem cells (MSCs-CM) has been shown to alleviate the oxidative stress during wound healing, Alzheimer’s disease and ischemic disease. This premise prompted us to investigate the influence of MSCs-CM during cryopreservation of expanded UCB cells.

Methodology/Principle findings

CM-was collected from cord/placental MSCs(C-MSCs-CM, P-MSC-CM). UCB CD34⁺cells were expanded as suspension cultures in serum free medium containing cytokines for 10 days. Cells were frozen with/without C-MSCs-CM and or P-MSCs-CM in the conventional freezing medium containing 20%FCS +10%DMSO using a programmable freezer and stored in liquid nitrogen. Upon revival, cells frozen with MSCs-CM were found to be superior to cells frozen in conventional medium in terms of viability, CD34⁺content and clonogenecity. Priming of revived cells for 48 hrs with MSCs-CM further improved their transplantation ability, as compared to those cultured without MSCs-CM. P-MSCs-CM radically reduced the oxidative stress in cryopreserved cells, resulting in better post thaw functionality of CD34⁺ cells than with C-MSCs-CM. The observed cryoprotective effect of MSCs-CM was primarily due to anti-oxidative and anti-apoptotic properties of the MSCs-CM and not because of the exosomes secreted by them.

Conclusions/Significance

Our data suggest that MSCs-CM can serve as a valuable additive to the freezing or the priming medium for expanded UCB cells, which would increase their clinical applicability.

24-Hydroxycholesterol participates in pancreatic neuroendocrine tumor development

Cells in the tumor microenvironment may be reprogrammed by tumor-derived metabolites. Cholesterol-oxidized products, namely oxysterols, have been shown to favor tumor growth directly by promoting tumor cell growth and indirectly by dampening antitumor immune responses. However, the cellular and molecular mechanisms governing oxysterol generation within tumor microenvironments remain elusive. We recently showed that tumor-derived oxysterols recruit neutrophils endowed with protumoral activities, such as neoangiogenesis. Here, we show that hypoxia inducible factor-1a (HIF-1α) controls the overexpression of the enzyme Cyp46a1, which generates the oxysterol 24-hydroxycholesterol (24S-HC) in a pancreatic neuroendocrine tumor (pNET) model commonly used to study neoangiogenesis. The activation of the HIF-1α-24S-HC axis ultimately leads to the induction of the angiogenic switch through the positioning of proangiogenic neutrophils in proximity to Cyp46a1⁺ islets. Pharmacologic blockade or genetic inactivation of oxysterols controls pNET tumorigenesis by dampening the 24S-HC-neutrophil axis. Finally, we show that in some human pNET samples Cyp46a1 transcripts are overexpressed, which correlate with the HIF-1α target VEGF and with tumor diameter. This study reveals a layer in the angiogenic switch of pNETs and identifies a therapeutic target for pNET patients.

Adult hematopoietic stem cells lacking Hif-1α self-renew normally

The hematopoietic stem cell (HSC) pool is maintained under hypoxic conditions within the bone marrow microenvironment. Cellular responses to hypoxia are largely mediated by the hypoxia-inducible factors, Hif-1 and Hif-2. The oxygen-regulated α subunits of Hif-1 and Hif-2 (namely, Hif-1α and Hif-2α) form dimers with their stably expressed β subunits and control the transcription of downstream hypoxia-responsive genes to facilitate adaptation to low oxygen tension. An initial study concluded that Hif-1α is essential for HSC maintenance, whereby Hif-1α-deficient HSCs lost their ability to self-renew in serial transplantation assays. In another study, we demonstrated that Hif-2α is dispensable for cell-autonomous HSC maintenance, both under steady-state conditions and following transplantation. Given these unexpected findings, we set out to revisit the role of Hif-1α in cell-autonomous HSC functions. Here we demonstrate that inducible acute deletion of Hif-1α has no impact on HSC survival. Notably, unstressed HSCs lacking Hif-1α efficiently self-renew and sustain long-term multilineage hematopoiesis upon serial transplantation. Finally, Hif-1α-deficient HSCs recover normally after hematopoietic injury induced by serial administration of 5-fluorouracil. We therefore conclude that despite the hypoxic nature of the bone marrow microenvironment, Hif-1α is dispensable for cell-autonomous HSC maintenance.

Hypoxia regulates the hematopoietic stem cell niche

Bone marrow, the site of hematopoiesis throughout adulthood, is a physiologically hypoxic organ. Thus, various biological oxygen sensors and their signaling cascades play a pivotal role in hematopoietic systems in the bone marrow under both physiologic and pathologic conditions. Hypoxia-inducible factors (HIFs) are hypoxic stress sensor proteins that are stabilized under homeostatic or stress-induced hypoxia. In the hypoxic bone marrow, HIFs play crucial roles in hematopoietic stem cells (HSCs) and in the cells of the HSC niche. The signals downstream of the HIFs maintain HSC quiescence, survival, and metabolic homeostasis through both cell-autonomous and non-cell-autonomous mechanisms. Leukemic stem cells (LSCs) hijack these delicate hypoxia-sensing mechanisms to sustain their self-renewal potential, promoting disease progression and drug resistance even under normoxic conditions. This review focuses on HIF-mediated oxygen-sensing mechanisms of adult HSCs and LSCs and their niche cells in the hypoxic bone marrow.

Differential ability of MSCs isolated from placenta and cord as feeders for supporting ex vivo expansion of umbilical cord blood derived CD34(+) cells

INTRODUCTION

Ex vivo expansion of umbilical cord blood (UCB) is attempted to increase cell numbers to overcome the limitation of cell dose. Presently, suspension cultures or feeder mediated co-cultures are performed for expansion of hematopoietic stem cells (HSCs). Mesenchymal stem cells (MSCs) have proved to be efficient feeders for the maintenance of HSCs. Here, we have established MSCs-HSCs co-culture system with MSCs isolated from less invasive and ethically acceptable sources like umbilical cord tissue (C-MSCs) and placenta (P-MSCs). MSCs derived from these tissues are often compared with bone marrow derived MSCs (BM-MSCs) which are considered as a gold standard. However, so far none of the studies have directly compared C-MSCs with P-MSCs as feeders for ex vivo expansion of HSCs. Thus, we for the first time performed a systematic comparison of hematopoietic supportive capability of C and P-MSCs using paired samples.

METHODS

UCB-derived CD34(+) cells were isolated and co-cultured on irradiated C and P-MSCs for 10 days. C-MSCs and P-MSCs were isolated from the same donor. The cultures comprised of serum-free medium supplemented with 25 ng/ml each of SCF, TPO, Flt-3 L and IL-6. After 10 days cells were collected and analyzed for phenotype and functionality.

RESULTS

C-MSCs and P-MSCs were found to be morphologically and phenotypically similar but exhibited differential ability to support ex vivo hematopoiesis. Cells expanded on P-MSCs showed higher percentage of primitive cells (CD34(+)CD38(-)), CFU (Colony forming unit) content and LTC-IC (Long term culture initiating cells) ability. CD34(+) cells expanded on P-MSCs also exhibited better in vitro adhesion to fibronectin and migration towards SDF-1α and enhanced NOD/SCID repopulation ability, as compared to those grown on C-MSCs. P-MSCs were found to be closer to BM-MSCs in their ability to expand HSCs. P-MSCs supported expansion of functionally superior HSCs by virtue of reduction in apoptosis of primitive HSCs, higher Wnt and Notch activity, HGF secretion and cell-cell contact. On the other hand, C-MSCs facilitated expansion of progenitors (CD34(+)CD38(+)) and differentiated (CD34(-)CD38(+)) cells by secretion of IL1-α, β, MCP-2, 3 and MIP-3α.

CONCLUSIONS

P-MSCs were found to be better feeders for ex vivo maintenance of primitive HSCs with higher engraftment potential than the cells expanded with C-MSCs as feeders.

Hypoxic niche-mediated regeneration of hematopoiesis in the engraftment window is dominantly affected by oxygen tension in the milieu

The bone marrow (BM) microenvironment or the hematopoietic stem cell (HSC) niche is normally hypoxic, which maintains HSC quiescence. Paradoxically, transplanted HSCs rapidly proliferate in this niche. Pretransplant myelosuppression results in a substantial rise in oxygen levels in the marrow microenvironment due to reduced cellularity and consequent low oxygen consumption. Therefore, it may be construed that the rapid proliferation of the engrafted HSCs in the BM niche is facilitated by the transiently elevated oxygen tension in this milieu during the “engraftment window.” To determine whether oxygen tension dominantly affects the regeneration of hematopoiesis in the BM niche, we created an “oxygen-independent hypoxic niche” by treating BM-derived mesenchymal stromal cells (BMSCs) with a hypoxia-mimetic compound, cobalt chloride (CoCl₂) and cocultured them with BM-derived HSC-enriched cells under normoxic conditions (HSCs; CoCl₂-cocultures). Cocultures with untreated BMSCs incubated under normoxia (control- cocultures) or hypoxia (1% O₂; hypoxic-cocultures) were used as comparators. Biochemical analyses showed that though, both CoCl₂ and hypoxia evoked comparable signals in the BMSCs, the regeneration of hematopoiesis in their respective cocultures was radically different. The CoCl₂-BMSCs supported robust hematopoiesis, while the hypoxic-BMSCs exerted strong inhibition. The hematopoiesis-supportive ability of CoCl₂-BMSCs was abrogated if the CoCl₂-cocultures were incubated under hypoxia, demonstrating that the prevalent oxygen tension in the milieu dominantly affects the outcome of the HSC-BM niche interactions. Our data suggest that pharmacologically delaying the reestablishment of hypoxia in the BM may boost post-transplant regeneration of hematopoiesis.

Progenitor cell dysfunctions underlie some diabetic complications

Stem cells and progenitor cells are integral to tissue homeostasis and repair. They contribute to health through their ability to self-renew and commit to specialized effector cells. Recently, defects in a variety of progenitor cell populations have been described in both preclinical and human diabetes. These deficits affect multiple aspects of stem cell biology, including quiescence, renewal, and differentiation, as well as homing, cytokine production, and neovascularization, through mechanisms that are still unclear. More important, stem cell aberrations resulting from diabetes have direct implications on tissue function and seem to persist even after return to normoglycemia. Understanding how diabetes alters stem cell signaling and homeostasis is critical for understanding the complex pathophysiology of many diabetic complications. Moreover, the success of cell-based therapies will depend on a more comprehensive understanding of these deficiencies. This review has three goals: to analyze stem cell pathways dysregulated during diabetes, to highlight the effects of hyperglycemic memory on stem cells, and to define ways of using stem cell therapy to overcome diabetic complications.

Synergistic Leukemia Eradication by Combined Treatment with Retinoic Acid and HIF Inhibition by EZN-2208 (PEG-SN38) in Preclinical Models of PML-RARα and PLZF-RARα-Driven Leukemia

PURPOSE

Retinoic acid-arsenic trioxide (ATRA-ATO) combination therapy is the current standard of care for patients with acute promyelocytic leukemia (APL) carrying the oncogenic fusion protein PML-RARα. Despite the high cure rates obtained with this drug combination, resistance to arsenic is recently emerging. Moreover, patients with APL carrying the PLZF-RARα fusion protein are partially resistant to ATRA treatment. Hypoxia-inducible factor-1α (HIF-1α) activation has been recently reported in APL, and EZN-2208 (PEG-SN38) is a compound with HIF-1α inhibitory function currently tested in clinical trials. This study investigates the effect of EZN-2208 in different preclinical APL models, either alone or in combination with ATRA.

EXPERIMENTAL DESIGN

Efficacy of EZN-2208 in APL was measured in vitro by assessing expression of HIF-1α target genes, cell migration, clonogenicity, and differentiation, vis a vis the cytotoxic and cytostatic effects of this compound. In vivo, EZN-2208 was used in mouse models of APL driven by PML-RARα or PLZF-RARα, either alone or in combination with ATRA.

RESULTS

Treatment of APL cell lines with noncytotoxic doses of EZN-2208 causes dose-dependent downregulation of HIF-1α bona fide target genes and affects cell migration and clonogenicity in methylcellulose. In vivo, EZN-2208 impairs leukemia progression and prolongs mice survival in APL mouse models. More importantly, when used in combination with ATRA, EZN-2208 synergizes in debulking leukemia and eradicating leukemia-initiating cells.

CONCLUSIONS

Our preclinical data suggest that the combination ATRA-EZN-2208 may be tested to treat patients with APL who develop resistance to ATO or patients carrying the PLZF-RARα fusion protein.

Effect of fatty acids on human bone marrow mesenchymal stem cell energy metabolism and survival

Successful stem cell therapy requires the optimal proliferation, engraftment, and differentiation of stem cells into the desired cell lineage of tissues. However, stem cell therapy clinical trials to date have had limited success, suggesting that a better understanding of stem cell biology is needed. This includes a better understanding of stem cell energy metabolism because of the importance of energy metabolism in stem cell proliferation and differentiation. We report here the first direct evidence that human bone marrow mesenchymal stem cell (BMMSC) energy metabolism is highly glycolytic with low rates of mitochondrial oxidative metabolism. The contribution of glycolysis to ATP production is greater than 97% in undifferentiated BMMSCs, while glucose and fatty acid oxidation combined only contribute 3% of ATP production. We also assessed the effect of physiological levels of fatty acids on human BMMSC survival and energy metabolism. We found that the saturated fatty acid palmitate induces BMMSC apoptosis and decreases proliferation, an effect prevented by the unsaturated fatty acid oleate. Interestingly, chronic exposure of human BMMSCs to physiological levels of palmitate (for 24 hr) reduces palmitate oxidation rates. This decrease in palmitate oxidation is prevented by chronic exposure of the BMMSCs to oleate. These results suggest that reducing saturated fatty acid oxidation can decrease human BMMSC proliferation and cause cell death. These results also suggest that saturated fatty acids may be involved in the long-term impairment of BMMSC survival in vivo.

Making sense of hematopoietic stem cell niches

The hematopoietic stem cell (HSC) niche commonly refers to the pairing of hematopoietic and mesenchymal cell populations that regulate HSC self-renewal, differentiation, and proliferation. Anatomic localization of the niche is a dynamic unit from the developmental stage that allows proliferating HSCs to expand before they reach the bone marrow where they adopt a quiescent phenotype that protects their integrity and functions. Recent studies have sought to clarify the complexity behind the HSC niche by assessing the contributions of specific cell populations to HSC maintenance. In particular, perivascular microenvironments in the bone marrow confer distinct vascular niches that regulate HSC quiescence and the supply of lineage-committed progenitors. Here, we review recent data on the cellular constituents and molecular mechanisms involved in the communication between HSCs and putative niches.

A genetic platform to model sarcomagenesis from primary adult mesenchymal stem cells

The regulatory factors governing adult mesenchymal stem cell (MSC) physiology and their tumorigenic potential are still largely unknown, which substantially delays the identification of effective therapeutic approaches for the treatment of aggressive and lethal forms of MSC-derived mesenchymal tumors, such as undifferentiated sarcomas. Here, we have developed a novel platform to screen and quickly identify genes and pathways responsible for adult MSC transformation, modeled undifferentiated sarcoma in vivo, and, ultimately, tested the efficacy of targeting the identified oncopathways. Importantly, by taking advantage of this new platform, we demonstrate the key role of an aberrant LRF-DLK1-SOX9 pathway in the pathogenesis of undifferentiated sarcoma, with important therapeutic implications.

SIGNIFICANCE

The paucity of therapeutic options for the treatment of sarcoma calls for a rapid and effective preclinical assessment of new therapeutic modalities. We have here developed a new platform to deconstruct the molecular genetics underlying the pathogenesis of sarcoma and to evaluate in vivo the efficacy of novel targeted therapies.