Overexpression of a dominant negative form of STAT3 selectively impairs hematopoietic stem cell activity

A novel mesenchymal stem cell-based regimen for acute myeloid leukemia differentiation therapy

Currently the main treatment of acute myeloid leukemia (AML) is chemotherapy combining hematopoietic stem cell transplantation. However, the unbearable side effect of chemotherapy and the high risk of life-threatening infections and disease relapse following hematopoietic stem cell transplantation restrict its application in clinical practice. Thus, there is an urgent need to develop alternative therapeutic tactics with significant efficacy and attenuated adverse effects. Here, we revealed that umbilical cord-derived mesenchymal stem cells (UC-MSC) efficiently induced AML cell differentiation by shuttling the neutrophil elastase (NE)-packaged extracellular vesicles (EVs) into AML cells. Interestingly, the generation and release of NE-packaged EVs could be dramatically increased by vitamin D receptor (VDR) activation in UC-MSC. Chemical activation of VDR by using its agonist 1α,25-dihydroxyvitamin D3 efficiently enhanced the pro-differentiation capacity of UC-MSC and then alleviated malignant burden in AML mouse model. Based on these discoveries, to evade the risk of hypercalcemia, we synthetized and identified sw-22, a novel non-steroidal VDR agonist, which exerted a synergistic pro-differentiation function with UC-MSC on mitigating the progress of AML. Collectively, our findings provided a non-gene editing MSC-based therapeutic regimen to overcome the differentiation blockade in AML.

Transcriptional Activation of Regenerative Hematopoiesis via Vascular Niche Sensing

Transition between activation and quiescence programs in hematopoietic stem and progenitor cells (HSC/HSPCs) is perceived to be governed intrinsically and by microenvironmental co-adaptation. However, HSC programs dictating both transition and adaptability, remain poorly defined. Single cell multiome analysis divulging differential transcriptional activity between distinct HSPC states, indicated for the exclusive absence of Fli-1 motif from quiescent HSCs. We reveal that Fli-1 activity is essential for HSCs during regenerative hematopoiesis. Fli-1 directs activation programs while manipulating cellular sensory and output machineries, enabling HSPCs co-adoptability with a stimulated vascular niche. During regenerative conditions, Fli-1 presets and enables propagation of niche-derived Notch1 signaling. Constitutively induced Notch1 signaling is sufficient to recuperate functional HSC impairments in the absence of Fli-1. Applying FLI-1 modified-mRNA transduction into lethargic adult human mobilized HSPCs, enables their vigorous niche-mediated expansion along with superior engraftment capacities. Thus, decryption of stem cell activation programs offers valuable insights for immune regenerative medicine.

STAT3 protects HSCs from intrinsic interferon signaling and loss of long-term blood-forming activity

STAT3 function in hematopoietic stem and progenitor cells (HSPCs) has been difficult to discern as Stat3 deficiency in the hematopoietic system induces systemic inflammation, which can impact HSPC activity. To address this, we established mixed bone marrow (BM) chimeric mice with CreER-mediated Stat3 deletion in 20% of the hematopoietic compartment. Stat3-deficient HSPCs had impaired hematopoietic activity and failed to undergo expansion in BM in contrast to Stat3-sufficient (CreER) controls. Single-cell RNA sequencing of Lin-ckit+Sca1+ BM cells revealed altered transcriptional responses in Stat3-deficient hematopoietic stem cells (HSCs) and multipotent progenitors, including intrinsic activation of cell cycle, stress response, and interferon signaling pathways. Consistent with their deregulation, Stat3-deficient Lin-ckit+Sca1+ cells accumulated γH2AX over time. Following secondary BM transplantation, Stat3-deficient HSPCs failed to reconstitute peripheral blood effectively, indicating a severe functional defect in the HSC compartment. Our results reveal essential roles for STAT3 in HSCs and suggest the potential for using targeted synthetic lethal approaches with STAT3 inhibition to remove defective or diseased HSPCs.

Acute myeloid leukemia cell-derived extracellular vesicles carrying microRNA-548ac regulate hematopoietic function via the TRIM28/STAT3 pathway

microRNAs (miRNAs or miRs) can be delivered from acute myeloid leukemia (AML) cells to hematopoietic stem cells (HSCs) to regulate hematopoietic function via extracellular vesicles (EVs). In this study, we investigated the roles played by EVs that transport miR-548ac from AML cells in normal hematopoiesis. Bioinformatics analysis demonstrated that miR-548ac was highly expressed in AML-derived EVs. The expression of miR-548ac and TRIM28 and the targeting relationship were identified, and the results demonstrated that the expression of miR-548ac was upregulated in AML cell lines and AML cell-secreted EVs compared with CD34⁺ HSCs. AML-derived EVs targeted CD34⁺ HSCs to induce decreased expression of TRIM28 and downstream activation of STAT3. Exosomal miR-548ac was transferred into CD34⁺ HSCs to target TRIM28. Through gain- and loss-of-function assays, it was observed that the abrogated expression of miR-548ac or STAT3 promoted colony-forming units (CFU), whereas overexpressed miR-548ac repressed CFU, which was rescued by overexpression of TRIM28. Taken together, these results indicated that miR-548ac delivered by AML cell-derived EVs inhibits hematopoiesis via TRIM28-dependent STAT3 activation.

The Dual Role of ROS in Hematological Malignancies: Stem Cell Protection and Cancer Cell Metastasis

BACKGROUND AND OBJECTIVE

Reactive oxygen species (ROS) play crucial role in hematopoiesis, regulation of differentiation, self-renewal, and the balance between quiescence and proliferation of hematopoietic stem cells (HSCs). The HSCs are a small population of undifferentiated cells that reside in the bone marrow (BM) and can undergo self-renewal by giving rise to mature cells.

METHODS

Relevant literature was identified through a PubMed search (2000-2019) of English-language papers using the following terms: reactive oxygen species, hematopoietic stem cell, leukemic stem cell, leukemia and chemotherapy.

RESULTS

HSCs are very sensitive to high levels of ROS and increased production of ROS have been attributed to HSC aging. HSC aging induced by both cell intrinsic and extrinsic factors is linked to impaired HSC self-renewal and regeneration. In addition, the elevated ROS levels might even trigger differentiation of Leukemic stem cells (LSCs) and ROS may be involved in the initiation and progression of hematological malignancies, such as leukemia.

CONCLUSION

Targeting genes involved in ROS in LSCs and HSCs are increasingly being used as a critical target for therapeutic interventions. Appropriate concentration of ROS may be an optimal therapeutic target for treatment of leukemia during chemotherapy, but still more studies are required to better understanding of the of ROS role in blood disorders.

MDH1-mediated malate-aspartate NADH shuttle maintains the activity levels of fetal liver hematopoietic stem cells

The connections between energy metabolism and stemness of hematopoietic stem cells (HSCs) at different developmental stages remain largely unknown. We generated a transgenic mouse line for the genetically encoded NADH/NAD+ sensor (SoNar) and demonstrate that there are 3 distinct fetal liver hematopoietic cell populations according to the ratios of SoNar fluorescence. SoNar-low cells had an enhanced level of mitochondrial respiration but a glycolytic level similar to that of SoNar-high cells. Interestingly, 10% of SoNar-low cells were enriched for 65% of total immunophenotypic fetal liver HSCs (FL-HSCs) and contained approximately fivefold more functional HSCs than their SoNar-high counterparts. SoNar was able to monitor sensitively the dynamic changes of energy metabolism in HSCs both in vitro and in vivo. Mechanistically, STAT3 transactivated MDH1 to sustain the malate-aspartate NADH shuttle activity and HSC self-renewal and differentiation. We reveal an unexpected metabolic program of FL-HSCs and provide a powerful genetic tool for metabolic studies of HSCs or other types of stem cells.

Acute myeloid leukemia cells secrete microRNA-4532-containing exosomes to mediate normal hematopoiesis in hematopoietic stem cells by activating the LDOC1-dependent STAT3 signaling pathway

BACKGROUND

MicroRNA (miR)-containing exosomes released by acute myeloid leukemia (AML) cells can be delivered into hematopoietic progenitor cells to suppress normal hematopoiesis. Herein, our study was performed to evaluate the effect of exosomal miR-4532 secreted by AML cells on hematopoiesis of hematopoietic stem cells.

METHODS

Firstly, differentially expressed miRs related to AML were identified using microarray analysis. Subsequently, AML cell lines were collected, and CD34+ HSCs were isolated from healthy pregnant women. Then, miR-4532 expression was measured in AML cells and AML cell-derived exosomes and CD34+ HSCs, together with evaluation of the targeting relationship between miR-4532 and LDOC1. Then, AML cells were treated with miR-4532 inhibitor, and exosomes were separated from AML cells and co-cultured with CD34+ HSCs. Gain- and loss-function approaches were employed in CD34+ HSCs. Colony-forming units (CFU) and expression of dickkopf-1 (DKK1), a hematopoietic inhibiting factor associated with pathogenesis of AML, were determined in CD34+ HSCs, as well as the extents of JAK2 and STAT3 phosphorylation and LDOC1 expression.

RESULTS

miR-4532 was found to be upregulated in AML cells and AML cell-derived exosomes, while being downregulated in CD34+ HSCs. In addition, exosomes released by AML cells targeted CD34+ HSCs to decrease the expression of CFU and increase the expression of DKK1. miR-4532 was delivered into CD34+ HSCs to target LDOC1 via AML cell-released exosomes. AML cell-derived exosomes containing miR-4532 inhibitor increased CFU but reduced DKK1 in CD34+ HSCs. Inhibition of miR-4532 or JAK2, or ectopic expression of LDOC1 upregulated CFU and downregulated DKK1 expression as well as the extents of JAK2 and STAT3 phosphorylation in CD34+ HSCs.

CONCLUSION

In conclusion, AML cell-derived exosomes carrying miR-4532 repress normal HSC hematopoiesis via activation of the LDOC1-dependent STAT3 signaling pathway.

CD38-Mediated Immunosuppression as a Mechanism of Tumor Cell Escape from PD-1/PD-L1 Blockade

Although treatment with immune checkpoint inhibitors provides promising benefit for patients with cancer, optimal use is encumbered by high resistance rates and requires a thorough understanding of resistance mechanisms. We observed that tumors treated with PD-1/PD-L1 blocking antibodies develop resistance through the upregulation of CD38, which is induced by all-trans retinoic acid and IFNβ in the tumor microenvironment. In vitro and in vivo studies demonstrate that CD38 inhibits CD8+ T-cell function via adenosine receptor signaling and that CD38 or adenosine receptor blockade are effective strategies to overcome the resistance. Large data sets of human tumors reveal expression of CD38 in a subset of tumors with high levels of basal or treatment-induced T-cell infiltration, where immune checkpoint therapies are thought to be most effective. These findings provide a novel mechanism of acquired resistance to immune checkpoint therapy and an opportunity to expand their efficacy in cancer treatment.Significance: CD38 is a major mechanism of acquired resistance to PD-1/PD-L1 blockade, causing CD8+ T-cell suppression. Coinhibition of CD38 and PD-L1 improves antitumor immune response. Biomarker assessment in patient cohorts suggests that a combination strategy is applicable to a large percentage of patients in whom PD-1/PD-L1 blockade is currently indicated. Cancer Discov; 8(9); 1156-75. ©2018 AACR.See related commentary by Mittal et al., p. 1066This article is highlighted in the In This Issue feature, p. 1047.

Genetic rescue of lineage-balanced blood cell production reveals a crucial role for STAT3 antiinflammatory activity in hematopoiesis

Blood cell formation must be appropriately maintained throughout life to provide robust immune function, hemostasis, and oxygen delivery to tissues, and to prevent disorders that result from over- or underproduction of critical lineages. Persistent inflammation deregulates hematopoiesis by damaging hematopoietic stem and progenitor cells (HSPCs), leading to elevated myeloid cell output and eventual bone marrow failure. Nonetheless, antiinflammatory mechanisms that protect the hematopoietic system are understudied. The transcriptional regulator STAT3 has myriad roles in HSPC-derived populations and nonhematopoietic tissues, including a potent antiinflammatory function in differentiated myeloid cells. STAT3 antiinflammatory activity is facilitated by STAT3-mediated transcriptional repression of Ube2n, which encodes the E2 ubiquitin-conjugating enzyme Ubc13 involved in proinflammatory signaling. Here we demonstrate a crucial role for STAT3 antiinflammatory activity in preservation of HSPCs and lineage-balanced hematopoiesis. Conditional Stat3 removal from the hematopoietic system led to depletion of the bone marrow lineage- Sca-1+ c-Kit+ CD150+ CD48- HSPC subset (LSK CD150+ CD48- cells), myeloid-skewed hematopoiesis, and accrual of DNA damage in HSPCs. These responses were accompanied by intrinsic transcriptional alterations in HSPCs, including deregulation of inflammatory, survival and developmental pathways. Concomitant Ube2n/Ubc13 deletion from Stat3-deficient hematopoietic cells enabled lineage-balanced hematopoiesis, mitigated depletion of bone marrow LSK CD150+ CD48- cells, alleviated HSPC DNA damage, and corrected a majority of aberrant transcriptional responses. These results indicate an intrinsic protective role for STAT3 in the hematopoietic system, and suggest that this is mediated by STAT3-dependent restraint of excessive proinflammatory signaling via Ubc13 modulation.

Interleukin-10-Overexpressing Mesenchymal Stromal Cells Induce a Series of Regulatory Effects in the Inflammatory System and Promote the Survival of Endotoxin-Induced Acute Lung Injury in Mice Model

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life-threatening inflammatory conditions with no effective pharmacological treatment. Previous studies suggested that mesenchymal stromal/stem cell (MSC) infusion resulted in better survival in mouse ALI models and presented low toxicity in human subjects. Therefore, in this study, we investigated the possibility of treating a murine model of ALI using MSCs with constant interleukin-10 overexpression (IL-10-MSC) by retroviral infection. ALI in mice was induced by intratracheal lipopolysaccharides (LPS) instillation. After 96 h, 80% of mice receiving IL-10-MSCs survived, whereas the survival rate of the mice receiving other treatments was only 20-50%. Mice receiving IL-10-MSCs also demonstrated significantly less weight loss (p < 0.01), and lower protein level and TNF concentration in the BAL (p < 0.01). Interestingly, IL-10-MSCs given to mice 3 and 1 day before ALI induction still conferred significant protection against ALI. While direct IL-10 transfusion resulted in an intensive, but transient peak in serum IL-10 level, IL-10-MSCs provided a milder, but more persistent increase in serum IL-10 level, together with significantly higher levels of IL-10-producing T cells and B cells, both in the spleen and in the lung. IL-10-MSCs given 3 days before LPS challenge resulted in higher pulmonary infiltration of IL-10-producing T cells and B cells in mice. On average, mice that survived the LPS challenge for 96 h presented higher pulmonary infiltration of IL-10-producing T cells and B cells than mice that deceased within the experimental period. Together, these results demonstrated that IL-10-MSCs offered superior protection against LPS-induced ALI when given before or at the time of ALI induction, and significantly increased the frequencies of IL-10-expressing T cells and B cells. IL-10-MSCs may thus represent a promising new treatment option in ALI/ARDS.

LMO2 is required for TAL1 DNA binding activity and initiation of definitive haematopoiesis at the haemangioblast stage

LMO2 is a bridging factor within a DNA binding complex and is required for definitive haematopoiesis to occur. The developmental stage of the block in haematopoietic specification is not known. We show that Lmo2^−/− mouse embryonic stem cells differentiated to Flk-1⁺ haemangioblasts, but less efficiently to haemogenic endothelium, which only produced primitive haematopoietic progenitors. Genome-wide approaches indicated that LMO2 is required at the haemangioblast stage to position the TAL1/LMO2/LDB1 complex to regulatory elements that are important for the establishment of the haematopoietic developmental program. In the absence of LMO2, the target site recognition of TAL1 is impaired. The lack of LMO2 resulted in altered gene expression levels already at the haemangioblast stage, with transcription factor genes accounting for ∼15% of affected genes. Comparison of Lmo2^−/− with Tal1^−/− Flk-1⁺ cells further showed that TAL1 was required to initiate or sustain Lmo2 expression.

Hematopoietic Stem Cells in Neural-crest Derived Bone Marrow

Hematopoietic stem cells (HSCs) in the endosteum of mesoderm-derived appendicular bones have been extensively studied. Neural crest-derived bones differ from appendicular bones in developmental origin, mode of bone formation and pathological bone resorption. Whether neural crest-derived bones harbor HSCs is elusive. Here, we discovered HSC-like cells in postnatal murine mandible, and benchmarked them with donor-matched, mesoderm-derived femur/tibia HSCs, including clonogenic assay and long-term culture. Mandibular CD34 negative, LSK cells proliferated similarly to appendicular HSCs, and differentiated into all hematopoietic lineages. Mandibular HSCs showed a consistent deficiency in lymphoid differentiation, including significantly fewer CD229 + fractions, PreProB, ProB, PreB and B220 + slgM cells. Remarkably, mandibular HSCs reconstituted irradiated hematopoietic bone marrow in vivo, just as appendicular HSCs. Genomic profiling of osteoblasts from mandibular and femur/tibia bone marrow revealed deficiencies in several HSC niche regulators among mandibular osteoblasts including Cxcl12. Neural crest derived bone harbors HSCs that function similarly to appendicular HSCs but are deficient in the lymphoid lineage. Thus, lymphoid deficiency of mandibular HSCs may be accounted by putative niche regulating genes. HSCs in craniofacial bones have functional implications in homeostasis, osteoclastogenesis, immune functions, tumor metastasis and infections such as osteonecrosis of the jaw.

Transient Tcf3 Gene Repression by TALE-Transcription Factor Targeting

Transplantation of hematopoietic stem and progenitor cells (HSCs) i.e., self-renewing cells that retain multipotentiality, is now a widely performed therapy for many hematopoietic diseases. However, these cells are present in low number and are subject to replicative senescence after extraction; thus, the acquisition of sufficient numbers of cells for transplantation requires donors able to provide repetitive blood samples and/or methods of expanding cell numbers without disturbing cell multipotentiality. Previous studies have shown that HSCs maintain their multipotentiality and self-renewal activity if TCF3 transcription function is blocked under B cell differentiating conditions. Taking advantage of this finding to devise a new approach to HSC expansion in vitro, we constructed an episomal expression vector that specifically targets and transiently represses the TCF3 gene. This consisted of a vector encoding a transcription activator-like effector (TALE) fused to a Krüppel-associated box (KRAB) repressor. We showed that this TALE-KRAB vector repressed expression of an exogenous reporter gene in HEK293 and COS-7 cell lines and, more importantly, efficiently repressed endogenous TCF3 in a human B lymphoma cell line. These findings suggest that this vector can be used to maintain multipotentiality in HSC being subjected to a long-term expansion regimen prior to transplantation.

Catalase inhibits ionizing radiation-induced apoptosis in hematopoietic stem and progenitor cells

Hematologic toxicity is a major cause of mortality in radiation emergency scenarios and a primary side effect concern in patients undergoing chemo-radiotherapy. Therefore, there is a critical need for the development of novel and more effective approaches to manage this side effect. Catalase is a potent antioxidant enzyme that coverts hydrogen peroxide into hydrogen and water. In this study, we evaluated the efficacy of catalase as a protectant against ionizing radiation (IR)-induced toxicity in hematopoietic stem and progenitor cells (HSPCs). The results revealed that catalase treatment markedly inhibits IR-induced apoptosis in murine hematopoietic stem cells and hematopoietic progenitor cells. Subsequent colony-forming cell and cobble-stone area-forming cell assays showed that catalase-treated HSPCs can not only survive irradiation-induced apoptosis but also have higher clonogenic capacity, compared with vehicle-treated cells. Moreover, transplantation of catalase-treated irradiated HSPCs results in high levels of multi-lineage and long-term engraftments, whereas vehicle-treated irradiated HSPCs exhibit very limited hematopoiesis reconstituting capacity. Mechanistically, catalase treatment attenuates IR-induced DNA double-strand breaks and inhibits reactive oxygen species. Unexpectedly, we found that the radioprotective effect of catalase is associated with activation of the signal transducer and activator of transcription 3 (STAT3) signaling pathway and pharmacological inhibition of STAT3 abolishes the protective activity of catalase, suggesting that catalase may protect HSPCs against IR-induced toxicity via promoting STAT3 activation. Collectively, these results demonstrate a previously unrecognized mechanism by which catalase inhibits IR-induced DNA damage and apoptosis in HSPCs.

Spinal cord regeneration

Three theories of regeneration dominate neuroscience today, all purporting to explain why the adult central nervous system (CNS) cannot regenerate. One theory proposes that Nogo, a molecule expressed by myelin, prevents axonal growth. The second theory emphasizes the role of glial scars. The third theory proposes that chondroitin sulfate proteoglycans (CSPGs) prevent axon growth. Blockade of Nogo, CSPG, and their receptors indeed can stop axon growth in vitro and improve functional recovery in animal spinal cord injury (SCI) models. These therapies also increase sprouting of surviving axons and plasticity. However, many investigators have reported regenerating spinal tracts without eliminating Nogo, glial scar, or CSPG. For example, many motor and sensory axons grow spontaneously in contused spinal cords, crossing gliotic tissue and white matter surrounding the injury site. Sensory axons grow long distances in injured dorsal columns after peripheral nerve lesions. Cell transplants and treatments that increase cAMP and neurotrophins stimulate motor and sensory axons to cross glial scars and to grow long distances in white matter. Genetic studies deleting all members of the Nogo family and even the Nogo receptor do not always improve regeneration in mice. A recent study reported that suppressing the phosphatase and tensin homolog (PTEN) gene promotes prolific corticospinal tract regeneration. These findings cannot be explained by the current theories proposing that Nogo and glial scars prevent regeneration. Spinal axons clearly can and will grow through glial scars and Nogo-expressing tissue under some circumstances. The observation that deleting PTEN allows corticospinal tract regeneration indicates that the PTEN/AKT/mTOR pathway regulates axonal growth. Finally, many other factors stimulate spinal axonal growth, including conditioning lesions, cAMP, glycogen synthetase kinase inhibition, and neurotrophins. To explain these disparate regenerative phenomena, I propose that the spinal cord has evolved regenerative mechanisms that are normally suppressed by multiple extrinsic and intrinsic factors but can be activated by injury, mediated by the PTEN/AKT/mTOR, cAMP, and GSK3b pathways, to stimulate neural growth and proliferation.

Molecular integration of HoxB4 and STAT3 for self-renewal of hematopoietic stem cells: a model of molecular convergence for stemness

The upregulation of HoxB4 promotes self-renewal of hematopoietic stem cells (HSCs) without overriding the normal stem cell pool size. A similar enhancement of HSC self-renewal occurs when signal transducer and activator of transcription 3 (STAT3) is activated in HSCs. In this study, to gain insight into the functional organization of individual transcription factors (TFs) that have similar effects on HSCs, we investigated the molecular interplay between HoxB4 and STAT3 in the regulation of HSC self-renewal. We found that while STAT3-C or HoxB4 similarly enhanced the in vitro self-renewal and in vivo repopulating activities of HSCs, simultaneous transduction of both TFs did not have additive effects, indicating their functional redundancy in HSCs. In addition, activation of STAT3 did not cause changes in the expression levels of HoxB4. In contrast, the inhibition of STAT3 activity in HoxB4-overexpressing hematopoietic cells significantly abrogated the enhancing effects of HoxB4, and the upregulation of HoxB4 caused a ligand-independent Tyr-phosphorylation of STAT3. Microarray analysis revealed a significant overlap of the transcriptomes regulated by STAT3 and HoxB4 in undifferentiated hematopoietic cells. Moreover, a gene set enrichment analysis showed significant overlap in the candidate TFs that can recapitulate the transcriptional changes induced by HoxB4 or STAT3. Interestingly, among these common TFs were the pluripotency-related genes Oct-4 and Nanog. These results indicate that tissue-specific TFs regulating HSC self-renewal are functionally organized to play an equivalent role in transcription and provide insights into the functional convergence of multiple entries of TFs toward a conserved transcription program for the stem cell state.

STAT3 signaling controls satellite cell expansion and skeletal muscle repair

The progressive loss of muscle regenerative capacity with age or disease results in part from a decline in the number and function of satellite cells, the direct cellular contributors to muscle repair. However, little is known about the molecular effectors underlying satellite cell impairment and depletion. Elevated levels of inflammatory cytokines, including interleukin-6 (IL-6), are associated with both age-related and muscle-wasting conditions. The levels of STAT3, a downstream effector of IL-6, are also elevated with muscle wasting, and STAT3 has been implicated in the regulation of self-renewal and stem cell fate in several tissues. Here we show that IL-6-activated Stat3 signaling regulates satellite cell behavior, promoting myogenic lineage progression through myogenic differentiation 1 (Myod1) regulation. Conditional ablation of Stat3 in Pax7-expressing satellite cells resulted in their increased expansion during regeneration, but compromised myogenic differentiation prevented the contribution of these cells to regenerating myofibers. In contrast, transient Stat3 inhibition promoted satellite cell expansion and enhanced tissue repair in both aged and dystrophic muscle. The effects of STAT3 inhibition on cell fate and proliferation were conserved in human myoblasts. The results of this study indicate that pharmacological manipulation of STAT3 activity can be used to counteract the functional exhaustion of satellite cells in pathological conditions, thereby maintaining the endogenous regenerative response and ameliorating muscle-wasting diseases.

A medium-throughput analysis of signaling pathways involved in early stages of stem cell reprogramming

The induction of pluripotency from adult cells has enormous potential in regenerative medicine. While initial efforts to study mechanisms and improve efficiency of induced pluripotent stem cell (iPSC) reprogramming focused on the direct roles of transcriptional regulators, increasing evidence indicates that cellular signal transduction pathways can modulate this process. Here, we present a medium-throughput system to study the effect of signaling pathways on the early stages of reprogramming. We generated a set of lentiviral vectors encoding 38 genes that upregulate or downregulate major signal transduction pathways and quantified each signaling factor's effect on reprogramming. This approach confirmed the role of several factors previously implicated in reprogramming, as well as identified several GTPases-factors that to date have not been largely studied in reprogramming-that improve or hinder iPSC reprogramming. In addition, this methodology is useful in determining new targets for enhancing pluripotency reprogramming, lineage reprogramming, and/or cell differentiation.

Combined STAT3 and BCR-ABL1 inhibition induces synthetic lethality in therapy-resistant chronic myeloid leukemia

Mutations in the BCR-ABL1 kinase domain are an established mechanism of tyrosine kinase inhibitor (TKI) resistance in Philadelphia chromosome-positive leukemia, but fail to explain many cases of clinical TKI failure. In contrast, it is largely unknown why some patients fail TKI therapy despite continued suppression of BCR-ABL1 kinase activity, a situation termed BCRABL1 kinase-independent TKI resistance. Here, we identified activation of signal transducer and activator of transcription 3 (STAT3) by extrinsic or intrinsic mechanisms as an essential feature of BCR-ABL1 kinase-independent TKI resistance. By combining synthetic chemistry, in vitro reporter assays, and molecular dynamics-guided rational inhibitor design and high-throughput screening, we discovered BP-5-087, a potent and selective STAT3 SH2 domain inhibitor that reduces STAT3 phosphorylation and nuclear transactivation. Computational simulations, fluorescence polarization assays, and hydrogen-deuterium exchange assays establish direct engagement of STAT3 by BP-5-087 and provide a high-resolution view of the STAT3 SH2 domain/BP-5-087 interface. In primary cells from CML patients with BCR-ABL1 kinase-independent TKI resistance, BP-5-087 (1.0 μM) restored TKI sensitivity to therapy-resistant CML progenitor cells, including leukemic stem cells (LSCs). Our findings implicate STAT3 as a critical signaling node in BCR-ABL1 kinase-independent TKI resistance, and suggest that BP-5-087 has clinical utility for treating malignancies characterized by STAT3 activation.

Constitutive stabilization of hypoxia-inducible factor alpha selectively promotes the self-renewal of mesenchymal progenitors and maintains mesenchymal stromal cells in an undifferentiated state

With the increasing use of culture-expanded mesenchymal stromal cells (MSCs) for cell therapies, factors that regulate the cellular characteristics of MSCs have been of major interest. Oxygen concentration has been shown to influence the functions of MSCs, as well as other normal and malignant stem cells. However, the underlying mechanisms of hypoxic responses and the precise role of hypoxia-inducible factor-1α (Hif-1α), the master regulatory protein of hypoxia, in MSCs remain unclear, due to the limited span of Hif-1α stabilization and the complex network of hypoxic responses. In this study, to further define the significance of Hif-1α in MSC function during their self-renewal and terminal differentiation, we established adult bone marrow (BM)-derived MSCs that are able to sustain high level expression of ubiquitin-resistant Hif-1α during such long-term biological processes. Using this model, we show that the stabilization of Hif-1α proteins exerts a selective influence on colony-forming mesenchymal progenitors promoting their self-renewal and proliferation, without affecting the proliferation of the MSC mass population. Moreover, Hif-1α stabilization in MSCs led to the induction of pluripotent genes (oct-4 and klf-4) and the inhibition of their terminal differentiation into osteogenic and adipogenic lineages. These results provide insights into the previously unrecognized roles of Hif-1α proteins in maintaining the primitive state of primary MSCs and on the cellular heterogeneities in hypoxic responses among MSC populations.

Novel aspects of parenchymal-mesenchymal interactions: from cell types to molecules and beyond

Mesenchymal stem or stromal cells (MSCs) were initially isolated from the bone marrow and received their name on the basis of their ability to differentiate into multiple lineages such as bone, cartilage, fat and muscle. However, more recent studies suggest that MSCs residing in perivascular compartments of the small and large blood vessels play a regulatory function supporting physiologic and pathologic responses of parenchymal cells, which define the functional representation of an organ or tissue. MSCs secrete or express factors that reach neighbouring parenchymal cells via either a paracrine effect or a direct cell-to-cell interaction promoting functional activity, survival and proliferation of the parenchymal cells. Previous concept of 'epithelial-stromal' interactions can now be widened. Given that MSC can also support hematopoietic, neuronal and other non-epithelial parenchymal lineages, terms 'parenchymal-stromal' or 'parenchymal-mesenchymal' interactions may better describe the supportive or 'trophic' functions of MSC. Importantly, in many cases, MSCs specifically provide supportive microenvironment for the most primitive stem or progenitor populations and therefore can play a role as 'stem/progenitor niche' forming cells. So far, regulatory roles of MSCs have been reported in many tissues. In this review article, we summarize the latest studies that focused on the supportive function of MSC. This thread of research leads to a new perspective on the interactions between parenchymal and mesenchymal cells and justifies a principally novel approach for regenerative medicine based on co-application of MSC and parenchymal cell for the most efficient tissue repair.

JAK-STAT signaling in stem cells

Adult stem cells are essential for the regeneration and repair of tissues in an organism. Signals from many different pathways converge to regulate stem cell maintenance and differentiation while preventing overproliferation. Although each population of adult stem cells is unique, common themes arise by comparing the regulation of various stem cell types in an organism or by comparing similar stem cell types across species. The JAK-STAT signaling pathway, identified nearly two decades ago, is now known to be involved in many biological processes including the regulation of stem cells. Studies in Drosophila first implicated JAK-STAT signaling in the control of stem cell maintenance in the male germline stem cell microenvironment, or niche; subsequently it has been shown play a role in other niches in both Drosophila and mammals. In this chapter, we will address the role of JAK-STAT signaling in stem cells in the germline, intestinal, hematopoietic and neuronal niches in Drosophila as well as the hematopoietic and neuronal niches in mammals. We will comment on how the study of JAK-STAT signaling in invertebrate systems has helped to advance our understanding of signaling in vertebrates. In addition to the role of JAK- STAT signaling in stem cell niche homeostasis, we will also discuss the diseases, including cancers, that can arise when this pathway is misregulated.

Stat3 is required to maintain the full differentiation potential of mammary stem cells and the proliferative potential of mammary luminal progenitors

Stat3 has a defined role in mammary gland where it is a critical mediator of cell death during post-lactational regression. On the other hand, Stat3 is required for the self-renewal of embryonic stem cells and is sufficient for the induction of a naïve pluripotent state in epiblast stem cells. Mammary stem cells (MaSCs) have a high capacity for self-renewal and can grow robustly in transplantation experiments in vivo. However, a role for Stat3 in MaSCs has not been investigated. Here we show that depletion of Stat3 from basal cells results in reduced primary transplantation efficiency and diminishes the potential to generate ductal, but not alveolar, outgrowths. In addition, Stat3 is required for maximal proliferation of luminal progenitors.

Stem cell-based delivery of brain-derived neurotrophic factor gene in the rat retina

As an alternative to a viral vector, the application of stem cells to transfer specific genes is under investigation in various organs. Using this strategy may provide more effective method to supply neurotrophic factor to the neurodegenerative diseases caused by neurotrophic factor deprivation. This study investigated the possibility and efficacy of stem cell-based delivery of the brain-derived neurotrophic factor (BDNF) gene to rat retina. Rat BDNF cDNA was transduced into rat bone marrow mesenchymal stem cells (rMSCs) using a retroviral vector. Its incorporation into the experimental rat retina and the expression of BDNF after intravitreal injection or subretinal injection were detected by real-time PCR, western blot analysis, and immunohistochemical staining. For the incorporated rMSCs, retinal-specific marker staining was performed to investigate the changes in morphology and the characteristics of the stem cells. Transduction of the rMSCs by retrovirus was effective, and the transduced rMSCs expressed high levels of the BDNF gene and protein. The subretinal injection of rMSCs produced rMSC migration and incorporation into the rat retina (about 15.7% incorporation rate), and retinal BDNF mRNA and protein expression was increased at 4 weeks after transplantation. When subretinal injection of rMSCs was applied to axotomized rat retina, it significantly increased the expression of BDNF until 4 weeks after transplantation. Some of the transplanted rMSCs exhibited morphological changes, but the retinal-specific marker stain was not sufficient to indicate whether neuronal differentiation had occurred. Using mesenchymal stem cells to deliver the BDNF gene to the retina may provide new treatment for glaucoma.

Mouse hematopoietic cell-targeted STAT3 deletion: stem/progenitor cell defects, mitochondrial dysfunction, ROS overproduction, and a rapid aging-like phenotype

Nuclear transcription factor Stat3 is important for proper regulation of hematopoietic stem cell (HSC) and hematopoietic progenitor cell (HPC) proliferation, survival, and cytokine signaling responses. A new, noncanonical role for Stat3 in mitochondrial function has been discovered recently. However, there is little information on the role(s) of mitochondrial Stat3 in HSC/HPC function, especially potential effects of Stat3/mitochondrial dysregulation in human diseases. We investigated hematopoietic cell-targeted deletion of the STAT3 gene in HSCs/HPCs with a focus on mitochondrial function. We found that STAT3(-/-) mice, which have a very shortened lifespan, dysfunctional/dysregulated mitochondrial function and excessive reactive oxygen species production in HSCs/HPCs that coincides with pronounced defects in function. These animals have a blood phenotype with similarities to premature aging and to human diseases of myelodysplastic syndrome and myeloproliferative neoplasms such as erythroid dysplasia, anemia, excessive myeloproliferation, and lymphomyeloid ratio shifts. We show herein that the lifespan of STAT3(-/-) animals is lengthened by treatment with a reactive oxygen species scavenger, which lessened the severity of the blood phenotype. These data suggest a need for more detailed studies of role(s) of Stat3 in HSC/HPC mitochondrial function in human diseases and raise the idea that mitochondrial Stat3 could be used as a potential therapeutic target.

Role of STAT3 in Transformation and Drug Resistance in CML

Chronic myeloid leukemia (CML) is initially driven by the bcr-abl fusion oncoprotein. The identification of bcr-abl led to the discovery and rapid translation into the clinic of bcr-abl kinase inhibitors. Although, bcr-abl inhibitors are efficacious, experimental evidence indicates that targeting bcr-abl is not sufficient for elimination of minimal residual disease found within the bone marrow (BM). Experimental evidence indicates that the failure to eliminate the leukemic stem cell contributes to persistent minimal residual disease. Thus curative strategies will likely need to focus on strategies where bcr-abl inhibitors are given in combination with agents that specifically target the leukemic stem cell or the leukemic stem cell niche. One potential target to be exploited is the Janus kinase (JAK)/signal transducers and activators of transcription 3 (STAT3) pathway. Recently using STAT3 conditional knock-out mice it was shown that STAT3 is critical for initiating the disease. Interestingly, in the absence of treatment, STAT3 was not shown to be required for maintenance of the disease, suggesting that STAT3 is required only in the tumor initiating stem cell population (Hoelbl et al., 2010). In the context of the BM microenvironment, STAT3 is activated in a bcr-abl independent manner by the cytokine milieu. Activation of JAK/STAT3 was shown to contribute to cell survival even in the event of complete inhibition of bcr-abl activity within the BM compartment. Taken together, these studies suggest that JAK/STAT3 is an attractive therapeutic target for developing strategies for targeting the JAK-STAT3 pathway in combination with bcr-abl kinase inhibitors and may represent a viable strategy for eliminating or reducing minimal residual disease located in the BM in CML.

Critical requirement for Stat5 in a mouse model of polycythemia vera

The JAK2V617F mutation has been identified in most cases of Ph-negative myeloproliferative neoplasms (MPNs) including polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). Expression of JAK2V617F results in constitutive activation of multiple signaling molecules/pathways. However, the key signaling downstream of JAK2V617F required for transformation and induction of MPNs remains elusive. Using a mouse genetic strategy, we show here that Stat5 is absolutely required for the pathogenesis of PV induced by Jak2V617F. Whereas expression of Jak2V617F in mice resulted in all the features of human PV, including an increase in red blood cells, hemoglobin, hematocrit, white blood cells, platelets, and splenomegaly, deletion of Stat5 in the Jak2V617F knockin mice normalized all the blood parameters and the spleen size. Furthermore, deletion of Stat5 completely abrogated erythropoietin (Epo)-independent erythroid colony formation evoked by Jak2V617F, a hallmark feature of PV. Re-expression of Stat5 in Stat5-deficient Jak2V617F knockin mice completely rescued the defects in transformation of hematopoietic progenitors and the PV phenotype. Together, these results indicate a critical function for Stat5 in the pathogenesis of PV. These findings also provide strong support for the development of Stat5 inhibitors as targeted therapies for the treatment of PV and other JAK2V617F-positive MPNs.

Absolute requirement for STAT3 function in small-intestine crypt stem cell survival

The transcription factor signal transducer and activator of transcription 3 (STAT3) is frequently activated in human cancers. Interestingly, STAT3 also maintains the pluripotency and self-renewal of murine embryonic stem cells, and several tissue stem cell types. To investigate whether STAT3 also maintains the small-intestine crypt stem cell, we conditionally inactivated a Floxed Stat3 allele (Stat3(fl)) in murine small-intestine crypt stem cells. Following Cre recombinase expression, apoptosis increased in Stat3(fl/-) experimental crypts relative to Stat3(wt/-) controls before declining. Control Stat3(wt/-) mice carrying a Flox-STOP LacZ reporter transgene stably expressed LacZ after Cre induction. In contrast, Stat3(fl/-) intestine LacZ expression initially increased modestly, before declining to background levels. Quantitative PCRs revealed a similar transient in recombined Stat3(fl) allele levels. Long-term bromodeoxyuridine labelling directly demonstrated that functional STAT3 is required for +4 to +6 region label-retaining small-intestine stem cell survival. Rapid clearance of recombined Stat3(fl/-) cells involves apoptosis potentially induced by elevated c-Myc in non-recombined cells and involves elevated p53 expression and caspase 3 activation. Intriguingly, Stat3(fl/-) intestine recombination triggered dramatically upregulated polycomb transcriptional repressor Bmi1 - potentially accelerating recombined crypt repopulation. In summary, STAT3 activity is absolutely required for small-intestine crypt stem cell survival at both the +4 to +6 label-retaining and crypt base columnar cell locations.

Stem cell gene expression programs influence clinical outcome in human leukemia

Xenograft studies indicate that some solid tumors and leukemias are organized as cellular hierarchies sustained by cancer stem cells (CSCs). Despite the promise of the CSC model, its relevance in humans remains uncertain. Here we show that acute myeloid leukemia (AML) follows a CSC model on the basis of sorting multiple populations from each of 16 primary human AML samples and identifying which contain leukemia stem cells (LSCs) using a sensitive xenograft assay. Analysis of gene expression from all functionally validated populations yielded an LSC-specific signature. Similarly, a hematopoietic stem cell (HSC) gene signature was established. Bioinformatic analysis identified a core transcriptional program shared by LSCs and HSCs, revealing the molecular machinery underlying 'stemness' properties. Both stem cell programs were highly significant independent predictors of patient survival and were found in existing prognostic signatures. Thus, determinants of stemness influence the clinical outcome of AML, establishing that LSCs are clinically relevant and not artifacts of xenotransplantation.

A Novel Function of Interleukin-10 Promoting Self-Renewal of Hematopoietic Stem Cells

Self-renewal of hematopoietic stem cells (HSCs) is key to their reconstituting ability, but the factors regulating the process remain poorly understood. Here, we show that Interleukin-10 (IL-10), a pleiotropic immune modulating cytokine, can also play a role in regulating HSC self-renewal. First, a quantitative decrease of primitive hematopoietic cell populations, but not more matured cells, was observed in the bone marrows of IL-10 disrupted mice as determined by long-term in vitro cultures or in vivo competitive repopulation assays. In contrast, normal HSCs from 5-fluorouracil treated marrows cultured on the IL-10 secreting stroma displayed an enhanced repopulating activity compared with cells grown on control stroma, with ninefold higher numbers of donor-derived HSCs in the reconstituted recipient marrows. Moreover, limiting dilution transplantation assay demonstrated that exogenous addition of IL-10 in the stroma-free cultures of purified Lin- Sca-1+ c-kit+ cells caused three- to fourfold higher frequencies of HSCs in the 5-day short-term culture without indirect inhibitory effect of IL-10 on tumor necrosis factor-alpha or interferon-gamma secretion. Interestingly, primitive hematopoietic cells, including Lin- Sca-1+ c-kit+ or side population cells, expressed the surface receptor for IL-10, and microenvironmental production of IL-10 was sharply increased in the osteoblasts lining the trabecular regions of the radiation-stressed marrow but not in the steady-state marrows. These results show that IL-10 may be a ligand that can stimulate self-renewal of HSCs to promote their regeneration in addition to being a ligand for immune regulation. Disclosure of potential conflicts of interest is found at the end of this article.

Leukemia inhibitory factor enhances formation of germ cell colonies in neonatal mouse testis culture

Spermatogonial stem cells continuously divide in the testis to support spermatogenesis throughout the life of adult male animals. Although very few spermatogonial stem cells are present in vivo, we recently succeeded in expanding these cells in vitro. Germ cells from postnatal testes were able to proliferate in the presence of several types of cytokines, and they formed uniquely shaped colonies of spermatogonia (germline stem or GS cells). These cells reinitiated normal spermatogenesis when transplanted into seminiferous tubules. However, much remains unknown about the contributions of cytokines to successful stem cell culture. In the present study, we examined the role of leukemia inhibitory factor (LIF) in GS cell culture. We found that the addition of LIF to newborn testis cell culture enhances the formation of germ cell colonies. Ciliary neurotrophic factor, but not oncostatin M, had the same effect, although they both bind to the IL-6ST (gp130) receptor. On the other hand, GS cells could be established from pup or adult testes in the absence of LIF. No phenotypic or functional difference was found between GS cells established from different stages, and normal offspring were born from pup-derived GS cells that had been maintained in the absence of LIF, indicating that LIF per se is not involved in the self-renewal of GS cells. These results demonstrate that LIF is useful in the initiation of GS cell culture and suggest that LIF or a related cytokine is involved in the maturation of gonocytes into spermatogonia.

BCR-ABL activates STAT3 via JAK and MEK pathways in human cells

Chronic myeloid leukaemia (CML) is characterised by a progression from a chronic towards an acute phase. We previously reported that signal transducer and activator of transcription 3 (STAT3), a major oncogenic signalling protein, is the target of p210-BCR-ABL in a murine embryonic stem (ES) cell model and in primary CD34+ CML cells. This activation was associated with inhibition of differentiation in ES cells. The present study found that BCR-ABL greatly phosphorylated STAT3 Ser727 residue and, to a lesser extent, Tyr705 residue in BCR-ABL-expressing cell lines (UT7-p210, MO7E-p210, and K562) and in primary CD34+ CML cells. Using BCR-ABL mutants, it was shown that BCR-ABL tyrosine kinase activity and its Tyr177 residue were necessary for STAT3 Ser727 phosphorylation. Constitutive STAT3 Tyr705 phosphorylation was associated with constitutive phosphorylation of Janus kinase (JAK)1 and JAK2, and was inhibited by the JAK inhibitor AG490, suggesting the involvement of JAK proteins in this process. Specific MEK [mitogen-activated protein (MAP) kinase/extracellular signal-regulated kinase (ERK) kinase] inhibitors PD98056 and UO126, as well as the use of a dominant-negative form of MEK1 abrogated STAT3 Ser727 phosphorylation, suggesting involvement of MAP-Kinase/Erk pathway. Inhibition of BCR-ABL with imatinib mesylate led to a dose-dependent downregulation of total STAT3 protein and mRNA, suggesting that BCR-ABL is involved in the transcriptional regulation of STAT3. Targeting JAK, MEK and STAT3 pathways could therefore be of therapeutic value, especially in advanced stage CML.

Unique effects of Stat3 on the early phase of hematopoietic stem cell regeneration

Self-renewal of hematopoietic stem cells (HSCs) is key to their reconstituting ability, but the signaling pathways that regulate this process remain poorly understood. Here we show that transduction of adult mouse bone marrow cells with a constitutively activated form of Stat3 (Stat3-C) increased their regenerative activity in lethally irradiated recipients. Conversely, transduction of these cells with a dominant-negative form of Stat3 suppressed their regenerative activity. Serial transplantation and clonal tracking of the HSC progeny regenerated in vivo from STAT3-C-transduced HSCs demonstrated that the major effect of forced expression of STAT3-C was to enhance HSC self-renewal during the initial phase of hematopoietic recovery. This acquired potential for enhanced self-renewal divisions then became latent, but was reactivated when the cells were transferred to new irradiated recipients. Increased levels of activated STAT3 were also found to be associated with greater preservation of primitive hematopoietic cells in short-term cultures. These results indicate a novel biphasic regulation of HSC self-renewal in vivo in which activated STAT3 promotes HSC self-renewal under stimulated, but not homeostatic, conditions. STAT3 may thus be an important regulator of hematopoietic regeneration and a novel target for HSC engineering.

Regulation of matrilysin expression in endothelium by fibroblast growth factor-2

Matrilysin (MMP7) is a secreted matrix metalloproteinase, which contributes to angiogenesis by breaking down basement membranes. We show that the angiogenic factor FGF-2 induces MMP7 expression in human endothelial cells. The promoter contains a Lef/Tcf consensus sequence, but using wildtype or Lef/Tcf-mutated promoter constructs, FGF-2-induced MMP7 reporter activity is independent from Lef/Tcf sites. Instead, we show that overexpression of a dominant negative Stat3 mutant reduces FGF-2-mediated MMP7 promoter activity. However, Stat3 does not bind to the MMP7 promoter, but activates MMP7 gene expression indirectly via AP-1. This is confirmed by MMP7 promoter constructs with mutated AP-1 sites which did not respond to FGF-2 and by siRNAs against Stat1 and Stat3, which repressed FGF-2-induced MMP7 protein expression. In conclusion, we show that FGF-2-induced MMP7 expression in endothelium depends on AP-1 and FGF-2 signaling to AP-1 involves a Stat1/3-dependent pathway.

Selective activation of STAT5 unveils its role in stem cell self-renewal in normal and leukemic hematopoiesis

Although the concept of a leukemic stem cell system has recently been well accepted, its nature and the underlying molecular mechanisms remain obscure. Constitutive activation of signal transducers and activators of transcription 3 (STAT3) and STAT5 is frequently detected in various hematopoietic tumors. To evaluate their role in normal and leukemic stem cells, we took advantage of constitutively active STAT mutants to activate STAT signaling selectively in hematopoietic stem cells (HSCs). Activation of STAT5 in CD34- c-Kit+ Sca-1+ lineage marker- (CD34- KSL) HSCs led to a drastic expansion of multipotential progenitors and promoted HSC self-renewal ex vivo. In sharp contrast, STAT3 was demonstrated to be dispensable for the HSC maintenance in vivo, and its activation facilitated lineage commitment of HSCs in vitro. In a mouse model of myeloproliferative disease (MPD), sustained STAT5 activation in CD34- KSL HSCs but not in CD34+ KSL multipotential progenitors induced fatal MPD, indicating that the capacity of STAT5 to promote self-renewal of hematopoietic stem cells is crucial to MPD development. Our findings collectively establish a specific role for STAT5 in self-renewal of normal as well as leukemic stem cells.

The threshold of gp130-dependent STAT3 signaling is critical for normal regulation of hematopoiesis

The interleukin-6 (IL-6) cytokine family plays an important role in regulating cellular responses during hematopoiesis. We report here that mice homozygous for a knock-in mutation in the IL-6 cytokine family receptor signaling subunit glycoprotein (gp) 130 (gp130(Y757F/Y757F)) that leads to gp130-dependent signal transducers and activators of transcription (STAT) 1/3 hyperactivation develop a broad spectrum of hematopoietic abnormalities, including splenomegaly, lymphadenopathy, and thrombocytosis. To determine whether STAT3 hyperactivation was responsible for the perturbed hematopoiesis in gp130(Y757F/Y757F) mice, we generated gp130(Y757F/Y757F) mice on a Stat3 heterozygous (Stat3(+/-)) background to specifically reduce gp130-dependent activation of STAT3, but not STAT1. Normal hematopoiesis was observed in gp130(Y757F/Y757F):Stat3(+/-) bone marrow and spleen, with no evidence of the splenomegaly and thrombocytosis displayed by gp130(Y757F/Y757F) mice. The perturbed cellular composition of thymus and lymph nodes in gp130(Y757F/Y757F) mice was also alleviated in gp130(Y757F/Y757F): Stat3(+/-) mice. Furthermore, we show that hematopoietic cells from gp130(Y757F/Y757F) mice exhibited increased survival and proliferation in response to IL-6 family cytokines. Collectively, these data provide genetic evidence that gp130-dependent STAT3 hyperactivation during hematopoiesis has pathological consequences affecting multiple organs, and therefore identify the threshold of STAT3 signaling elicited by IL-6 family cytokines as a critical determinant for hematopoietic homeostasis.

BCL-6 negatively regulates macrophage proliferation by suppressing autocrine IL-6 production

The transcription repressor BCL-6 is known to play critical roles in B-cell lymphomagenesis, germinal center formation, and balanced Th1/Th2 differentiation. In macrophages, although BCL-6 has also been shown to regulate the expression of several chemokine genes, its function in other aspects of macrophage biology has not been studied. In addition, the precise role of BCL-6 in cell proliferation is poorly understood in general. Here we report that BCL-6(-/-) macrophages hyperproliferate due to an accelerated G(1)/S transition accompanied by increased cyclin D2 and c-myc and decreased expression of p27. Crucial to this enhanced proliferation is spontaneous interleukin 6 (IL-6) production and signal transducer and activator of transcription 3 (STAT3) activation in BCL-6(-/-) macrophages. In colony-forming assays, BCL- 6(-/-) bone marrow progenitor cells form spontaneous macrophage colonies that can be inhibited by anti-IL-6 antibodies. Gene expression studies demonstrate that BCL-6 binds to several sequence motifs scattered in the IL-6 locus and can repress IL-6 transcription both in 293T cells and in macrophages. In conclusion, our results indicate that BCL-6 negatively regulates proliferation of the monocytic/macrophage lineage by suppressing an autocrine IL-6/STAT3-mediated gene expression program. Our work also suggests that BCL-6 prevents abnormal Th2 differentiation by suppressing basal level IL-6 production in antigen-presenting cells (APCs).

Transcription factor Gfi1 regulates self-renewal and engraftment of hematopoietic stem cells

The generation of all blood cells depends on the ability of hematopoietic stem cells (HSCs) for self-renewal and multilineage differentiation. We show here that the transcription factor Gfi1 is expressed in HSCs and in more mature cells such as common lymphoid progenitors (CLPs) and granulo/monocytic progenitors, but is absent in common myeloid progenitors and megakaryocyte/erythroid progenitors. When Gfi1 is deleted in mice, HSC frequencies are significantly reduced and CLPs all but disappear from the bone marrow. This specific requirement of Gfi1 for the maintenance of HSC numbers is cell autonomous. Transplantation of Gfi1-deficient bone marrow results in a compromised radioprotection and lower numbers of colony forming units in the spleen of wild-type recipients. Strikingly, Gfi1-/- bone marrow cells are severely impaired in competitive long-term reconstituting abilities after transplantation and show a surprisingly high proportion of actively cycling HSCs, suggesting that Gfi1 restrains proliferation of HSCs and thereby regulates their self-renewal and long-term engraftment abilities.

The origin of hematopoietic cell type diversity

The hematopoietic system remains robust with regards to extrinsic perturbations, in sharp contrast with the stochastic behavior of hematopoeitic stem cells (HSCs) at the single cell level, suggesting that stability may be achieved within a stem cell system that undergoes constant self-renewal, commitment to differentiation and generates cell type diversification. Converging evidence at the interface of cellular, molecular and numerical studies suggests that diversity is generated by the chaotic dynamics of transcription factor networks within a cell and of the combination of growth factors and cytokines in the environment, both involving cooperation and competition. Current evidence indicates that HSCs are primed for multilineage gene expression. A subtle shift in transcription factor dosage is sufficient to perturb this equilibrium and to drive lineage commitment that involves a resolution of complexity at the molecular level and a transition towards less chaotic behavior. This dynamical instability establishes a state of responsiveness to extrinsic signals. Evolutionary conserved environmental cues that drive pattern formation or migratory behavior during embryonic development operate in the adult to influence the decision between self-renewal and differentiation in HSCs, as exemplified by the role of Notch1, Wnt proteins, BMPs and VEGF. In contrast, a network of cytokines uniquely present in mammalians influences later developmental stages, from progenitors with more restricted potentials (tri-, bi- or unipotent) to mature functional cells. These cytokines have co-opted the ancient Jak-STAT pathway but also appear to trigger lineage-affiliated transcription factors, thus linking environmental signaling to cell fate decisions.

ABC transporter activities of murine hematopoietic stem cells vary according to their developmental and activation status

Primitive hematopoietic cells from several species are known to efflux both Hoechst 33342 and Rhodamine-123. We now show that murine hematopoietic stem cells (HSCs) defined by long-term multilineage repopulation assays efflux both dyes variably according to their developmental or activation status. In day 14.5 murine fetal liver, very few HSCs efflux Hoechst 33342 efficiently, and they are thus not detected as “side population” (SP) cells. HSCs in mouse fetal liver also fail to efflux Rhodamine-123. Both of these features are retained by most of the HSCs present until 4 weeks after birth but are reversed by 8 weeks of age or after a new HSC population is regenerated in adult mice that receive transplants with murine fetal liver cells. Activation of adult HSCs in vivo following 5-fluorouracil treatment, or in vitro with cytokines, induces variable losses in Rhodamine-123 and Hoechst 33342 efflux activities, and HSCs from mdr-1a/1b-/- mice show a dramatic decrease in Rhodamine-123 efflux ability. Thus, the Rhodamine-123 and Hoechst 33342 efflux properties of murine HSCs fluctuate in the same fashion as a number of other HSC markers, suggesting these are regulated by a common control mechanism that operates independently of that regulating the regenerative function of HSCs. (Blood. 2004;103:4487-4495)

Different in vivo repopulating activities of purified hematopoietic stem cells before and after being stimulated to divide in vitro with the same kinetics

OBJECTIVE

The Hoechst 33342-effluxing side population (SP) of adult mouse bone marrow (BM) contains most of the hematopoietic stem cells (HSCs). Here we measured the HSC content of specific subsets of SP cells and then used a highly HSC-enriched fraction to investigate the effect of different growth factors on the initial rate of HSC proliferation in vitro and the accompanying maintenance (or loss) of HSCs in the first-division progeny.

MATERIALS AND METHODS

Staining with Rhodamine-123 (Rho) was used to subfractionate lineage marker-negative (lin-) SP cells. Cells were assayed for HSCs by examining their ability to generate sustained (>4 months) multi-lineage lympho-myeloid clones in irradiated hosts. Cultures of single lin- Rho- SP cells were used to monitor growth factor effects on HSC proliferation and function.

RESULTS

More than 40% of mice injected with single lin- Rho- SP cells showed long-term lympho-myeloid reconstitution. Some clones peaked within 8 weeks but others developed more slowly apparently unrelated to the pattern of lineage representation. 3/3 clones tested repopulated secondary mice. Either Steel factor+interleukin-11 (+/- flt3-ligand) or Steel factor+thrombopoietin stimulated at least 75% of single lin- Rho- SP cells to divide in vitro with the same synchronous kinetics. However, in the first cocktail, the frequency of HSCs among the first-division doublets was preserved but in the latter it was greatly diminished.

CONCLUSION

Exogenous growth factors can differentially affect the ability of HSCs to execute a self-renewal division within a single cell cycle even when the kinetics of proliferation are the same.