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

Maintenance of hematopoietic stem cells by tyrosine-unphosphorylated STAT5 and JAK inhibition

ABSTRACT

Adult hematopoietic stem cells (HSCs) are responsible for the lifelong production of blood and immune cells, a process regulated by extracellular cues, including cytokines. Many cytokines signal through the conserved Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway in which tyrosine-phosphorylated STATs (pSTATs) function as transcription factors. STAT5 is a pivotal downstream mediator of several cytokines known to regulate hematopoiesis, but its function in the HSC compartment remains poorly understood. In this study, we show that STAT5-deficient HSCs exhibit an unusual phenotype, including reduced multilineage repopulation and self-renewal, combined with reduced exit from quiescence and increased differentiation. This was driven not only by the loss of canonical pSTAT5 signaling, but also by the loss of distinct transcriptional functions mediated by STAT5 that lack canonical tyrosine phosphorylation (uSTAT5). Consistent with this concept, expression of an unphosphorylatable STAT5 mutant constrained wild-type HSC differentiation, promoted their maintenance, and upregulated transcriptional programs associated with quiescence and stemness. The JAK1/2 inhibitor, ruxolitinib, which increased the uSTAT5:pSTAT5 ratio, had similar effects on murine HSC function; it constrained HSC differentiation and proliferation, promoted HSC maintenance, and upregulated transcriptional programs associated with stemness. Ruxolitinib also enhanced serial replating of normal human hematopoietic stem and progenitor cells (HSPCs), calreticulin-mutant murine HSCs, and HSPCs obtained from patients with myelofibrosis. Our results therefore reveal a previously unrecognized interplay between pSTAT5 and uSTAT5 in the control of HSC function and highlight JAK inhibition as a potential strategy for enhancing HSC function during ex vivo culture. Increased levels of uSTAT5 may also contribute to the failure of JAK inhibitors to eradicate myeloproliferative neoplasms.

Age-associated myeloid malignancies - the role of STAT3 and STAT5 in myelodysplastic syndrome and acute myeloid leukemia

In the last few decades, the increasing human life expectancy has led to the inflation of the elderly population and consequently the escalation of age-related disorders. Biological aging has been associated with the accumulation of somatic mutations in the Hematopoietic Stem Cell (HSC) compartment, providing a fitness advantage to the HSCs leading to clonal hematopoiesis, that includes non-malignant and malignant conditions (i.e. Clonal Hematopoiesis of Indeterminate Potential, Myelodysplastic Syndrome and Acute Myeloid Leukemia). The Janus Kinase-Signal Transducer and Activator of Transcription (JAK-STAT) pathway is a key player in both normal and malignant hematopoiesis. STATs, particularly STAT3 and STAT5, are greatly implicated in normal hematopoiesis, immunity, inflammation, leukemia, and aging. Here, the pleiotropic functions of JAK-STAT pathway in age-associated hematopoietic defects and of STAT3 and STAT5 in normal hematopoiesis, leukemia, and inflammaging are reviewed. Even though great progress has been made in deciphering the role of STATs, further research is required to provide a deeper understanding of the molecular mechanisms of leukemogenesis, as well as novel biomarkers and therapeutic targets for improved management of age-related disorders.

[Ruxolitinib combined with venetoclax and azacitidine in the treatment of refractory T-ALL patients with JAK1, JAK3, and STAT5B gene mutations: a case report and literature review]

Refractory acute T-lymphoblastic leukemia (T-ALL), which is characterized by a low sensitivity to conventional induction therapy and poor prognosis, poses significant challenges during treatment. This study reported a case of refractory T-ALL patient with mutations in the JAK1, JAK3, and STAT5B genes from Nanjing University's Gulou Hospital. Following an unsuccessful course of standard VDLP regimen chemotherapy, the treatment was modified to include ruxolitinib in combination with venetoclax and azacitidine. Subsequent to this therapy, the patient achieved bone marrow minimal residual disease (MRD) negativity. Notably, pleural effusion and mediastinal mass significantly improved the post-chest cavity infusion of dexamethasone combined with etoposide at the same stage. The patient also underwent allogeneic hematopoietic stem cell transplantation upon achieving bone marrow remission and was followed up until January 2024. Ruxolitinib combined with venetoclax and azacytidine has shown promising efficacy and safety in treating refractory T-ALL harboring the JAK1, JAK3, and STAT5B mutations, providing a novel therapeutic approach for such patients.

CD38 promotes hematopoietic stem cell dormancy

A subpopulation of deeply quiescent, so-called dormant hematopoietic stem cells (dHSCs) resides at the top of the hematopoietic hierarchy and serves as a reserve pool for HSCs. The state of dormancy protects the HSC pool from exhaustion throughout life; however, excessive dormancy may prevent an efficient response to hematological stresses. Despite the significance of dHSCs, the mechanisms maintaining their dormancy remain elusive. Here, we identify CD38 as a novel and broadly applicable surface marker for the enrichment of murine dHSCs. We demonstrate that cyclic adenosine diphosphate ribose (cADPR), the product of CD38 cyclase activity, regulates the expression of the transcription factor c-Fos by increasing the release of Ca2+ from the endoplasmic reticulum (ER). Subsequently, we uncover that c-Fos induces the expression of the cell cycle inhibitor p57Kip2 to drive HSC dormancy. Moreover, we found that CD38 ecto-enzymatic activity at the neighboring CD38-positive cells can promote human HSC quiescence. Together, CD38/cADPR/Ca2+/c-Fos/p57Kip2 axis maintains HSC dormancy. Pharmacological manipulations of this pathway can provide new strategies to improve the success of stem cell transplantation and blood regeneration after injury or disease.

Distinct roles of hematopoietic cytokines in the regulation of leukemia stem cells in murine MLL-AF9 leukemia

Lymphoid-primed multipotent progenitor (LMPP)-like and granulocyte-monocyte progenitor (GMP)-like leukemia stem cells (LSCs) co-exist in the blood of most patients with acute myeloid leukemia (AML). Complete elimination of both types of LSCs is required to cure AML. Using an MLL-AF9-induced murine AML model, we studied the role of hematopoietic cytokines in the survival of LMPP- and GMP-like LSCs. We found that SCF or FLT3L promotes the survival of LMPP-like LSCs by stimulating Stat5-mediated Mcl1 expression, whereas interleukin-3 (IL-3) or IL-6 induces the survival of GMP-like LSCs by stimulating Stat3/nuclear factor κB (NF-κB)-mediated Bcl2 expression. Functional study demonstrated that, compared to AML cells cultured in IL-3 and IL-6 medium, AML cells in SCF- or Flt3L-only culture are highly clonogenic in in vitro culture and are highly leukemogenic in vivo. Our study suggests that co-inhibition of both STAT5-MCL1 and STAT3/NF-κB-BCL2 signaling might represent an improved treatment strategy against AML, specifically AML cases with a monocytic phenotype and/or FLT3 mutations.

The Progress of Small Molecule Targeting BCR-ABL in the Treatment of Chronic Myeloid Leukemia

Chronic myelogenous leukemia (CML) is a malignant myeloproliferative disease. According to the American Cancer Society's 2021 cancer data report, new cases of CML account for about 15% of all leukemias. CML is generally divided into three stages: chronic phase, accelerated phase, and blast phase. Nearly 90% of patients are diagnosed as a chronic phase. Allogeneic stem cell transplantation and chemotherapeutic drugs, such as interferon IFN-α were used as the earliest treatments for CML. However, they could generate obvious side effects, and scientists had to seek new treatments for CML. A new era of targeted therapy for CML began with the introduction of imatinib, the first-generation BCR-ABL kinase inhibitor. However, the ensuing drug resistance and mutant strains led by T315I limited the further use of imatinib. With the continuous advancement of research, tyrosine kinase inhibitors (TKI) and BCR-ABL protein degraders with novel structures and therapeutic mechanisms have been discovered. From biological macromolecules to classical target protein inhibitors, a growing number of compounds are being developed to treat chronic myelogenous leukemia. In this review, we focus on summarizing the current situation of a series of candidate small-molecule drugs in CML therapy, including TKIs and BCR-ABL protein degrader. The examples provided herein describe the pharmacology activity of small-molecule drugs. These drugs will provide new enlightenment for future treatment directions.

Microbial ligand-independent regulation of lymphopoiesis by NOD1

Aberrant differentiation of progenitor cells in the hematopoietic system is known to severely impact host immune responsiveness. Here we demonstrate that NOD1, a cytosolic innate sensor of bacterial peptidoglycan, also functions in murine hematopoietic cells as a major regulator of both the generation and differentiation of lymphoid progenitors as well as peripheral T lymphocyte homeostasis. We further show that NOD1 mediates these functions by facilitating STAT5 signaling downstream of hematopoietic cytokines. In steady-state, loss of NOD1 resulted in a modest but significant decrease in numbers of mature T, B and natural killer cells. During systemic protozoan infection this defect was markedly enhanced, leading to host mortality. Lack of functional NOD1 also impaired T cell-dependent anti-tumor immunity while preventing colitis. These findings reveal that, in addition to its classical role as a bacterial ligand receptor, NOD1 plays an important function in regulating adaptive immunity through interaction with a major host cytokine signaling pathway.

STAT5 promotes PD-L1 expression by facilitating histone lactylation to drive immunosuppression in acute myeloid leukemia

Immunotherapy is a revolutionized therapeutic strategy for tumor treatment attributing to the rapid development of genomics and immunology, and immune checkpoint inhibitors have successfully achieved responses in numbers of tumor types, including hematopoietic malignancy. However, acute myeloid leukemia (AML) is a heterogeneous disease and there is still a lack of systematic demonstration to apply immunotherapy in AML based on PD-1/PD-L1 blockage. Thus, the identification of molecules that drive tumor immunosuppression and stratify patients according to the benefit from immune checkpoint inhibitors is urgently needed. Here, we reported that STAT5 was highly expressed in the AML cohort and activated the promoter of glycolytic genes to promote glycolysis in AML cells. As a result, the increased-lactate accumulation promoted E3BP nuclear translocation and facilitated histone lactylation, ultimately inducing PD-L1 transcription. Immune checkpoint inhibitor could block the interaction of PD-1/PD-L1 and reactive CD8+ T cells in the microenvironment when co-culture with STAT5 constitutively activated AML cells. Clinically, lactate accumulation in bone marrow was positively correlated with STAT5 as well as PD-L1 expression in newly diagnosed AML patients. Therefore, we have illustrated a STAT5-lactate-PD-L1 network in AML progression, which demonstrates that AML patients with STAT5 induced-exuberant glycolysis and lactate accumulation may be benefited from PD-1/PD-L-1-based immunotherapy.

E3 ubiquitin ligase on the biological properties of hematopoietic stem cell

Hematopoietic stem cells are a group of heterogeneity cells with the potential to differentiate into various types of mature blood cells. Their basic biological properties include quiescence, self-renewal, multilineage differentiation, and homing ability, with the homing of exogenous hematopoietic stem cells after transplantation becoming a new focus, while the first three properties share some similarity in mechanism due to connectivity. In various complex mechanisms, the role of E3 ubiquitin ligases in hematopoietic homeostasis and malignant transformation is receiving increasing attention. As a unique part, E3 ubiquitin ligases play an important role in physiological regulation mechanism of posttranslational modification. In this review, we focus on the recent progress of the crucial role of E3 ubiquitin ligases that target specific proteins for ubiquitination to regulate biological properties of hematopoietic stem cells. Additionally, this paper deals with E3 ubiquitin ligases that affect the biological properties through aging and summarizes the relevant applications of targeting E3 ligases in hematopoietic malignancies. We present some ideas on the clinical application of E3 ubiquitin ligase to regulate hematopoietic stem cells and also believe that it is meaningful to study the upstream signal of these E3 ubiquitin ligases because hematopoietic stem cell dysfunction is caused by deficiency of some E3 ligases.

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.

Partial STAT5 signaling is sufficient for CD4+ T cell priming but not memory formation

Signal transducer and activator of transcription 5 (STAT5) plays an important role in regulating gene expression in response to cytokines of the common (γc) chain family. In this capacity, STAT5 promotes CD8⁺ effector and memory T cell survival and regulatory T cell development. However, its function in conventional CD4⁺ T cells is less clear. In this study, the requirement of intact STAT5 signaling for CD4⁺ effector and memory T cell generation and maintenance was investigated by using DO11.10 TCR transgenic T cells that are genetically deficient in STAT5A or B, as well as by transducing DO11 T cells with a dominant-negative STAT5 to temporally block STAT5 function. We found that the presence of STAT5A or B alone was sufficient for primary CD4⁺ effector T cell generation, but not for establishing a long-lived memory cell population. Similarly, blocking STAT5 signaling during priming did not prevent initial T cell activation, but inhibited the generation of memory cells. Surprisingly, blocking STAT5 post-priming did not impact the long-term survival of CD4⁺ memory T cells in vivo. Mechanistically, intact STAT5B, but not STAT5A, was required for IL-7Rα re-expression in activated T cells, which is an important cytokine receptor for CD4⁺ memory generation. These data show that fully functional STAT5 is essential to deliver an early, non-redundant signal for memory programming during the primary CD4⁺ T cell response, while partial STAT5 signaling is sufficient for effector differentiation. Our results have implications for the precise use of STAT5 inhibitors to timely inhibit memory T cell responses.

Host T cell dedifferentiation effects drive HIV-1 latency stability

The development of therapies to eliminate the latent HIV-1 reservoir is hampered by our incomplete understanding of the biomolecular mechanism governing HIV-1 latency. To further complicate matters, recent single cell RNA-seq studies reported extensive heterogeneity between latently HIV-1-infected primary T cells, implying that latent HIV-1 infection can persist in greatly differing host cell environments. We here show that transcriptomic heterogeneity is also found between latently infected T cell lines, which allowed us to study the underlying mechanisms of intercell heterogeneity at high signal resolution. Latently infected T cells exhibited a de-differentiated phenotype, characterized by the loss of T cell-specific markers and gene regulation profiles reminiscent of hematopoietic stem cells (HSC). These changes had functional consequences. As reported for stem cells, latently HIV-1 infected T cells efficiently forced lentiviral superinfections into a latent state and favored glycolysis. As a result, metabolic reprogramming or cell re-differentiation destabilized latent infection. Guided by these findings, data-mining of single cell RNA-seq data of latently HIV-1 infected primary T cells from patients revealed the presence of similar dedifferentiation motifs. >20% of the highly detectable genes that were differentially regulated in latently infected cells were associated with hematopoietic lineage development (e.g. HUWE1, IRF4, PRDM1, BATF3, TOX, ID2, IKZF3, CDK6) or were hematopoietic markers (SRGN; hematopoietic proteoglycan core protein). The data add to evidence that the biomolecular phenotype of latently HIV-1 infected cells differs from normal T cells and strategies to address their differential phenotype need to be considered in the design of therapeutic cure interventions. IMPORTANCE HIV-1 persists in a latent reservoir in memory CD4 T cells for the lifetime of a patient. Understanding the biomolecular mechanisms used by the host cells to suppress viral expression will provide essential insights required to develop curative therapeutic interventions. Unfortunately, our current understanding of these control mechanisms is still limited. By studying gene expression profiles, we demonstrated that latently HIV-1-infected T cells have a de-differentiated T cell phenotype. Software-based data integration allowed for the identification of drug targets that would re-differentiate viral host cells and, in extension, destabilize latent HIV-1 infection events. The importance of the presented data lies within the clear demonstration that HIV-1 latency is a host cell phenomenon. As such, therapeutic strategies must first restore proper host cell functionality to accomplish efficient HIV-1 reactivation.

JAK-STAT in Early Hematopoiesis and Leukemia

Hematopoietic stem cells (HSCs) produce all the terminally differentiated blood cells and are controlled by extracellular signals from the microenvironment, the bone marrow (BM) niche, as well as intrinsic cell signals. Intrinsic signals include the tightly controlled action of signaling pathways, as the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway. Activation of JAK-STAT leads to phosphorylation of members of the STAT family to regulate proliferation, survival, and self-renewal of HSCs. Mutations in components of the JAK-STAT pathway are linked with defects in HSCs and hematologic malignancies. Accumulating mutations in HSCs and aging contribute to leukemia transformation. Here an overview of hematopoiesis, and the role of the JAK-STAT pathway in HSCs and in the promotion of leukemic transformation is presented. Therapeutic targeting of JAK-STAT and clinical implications of the existing research findings are also discussed.

A centric view of JAK/STAT5 in intestinal homeostasis, infection, and inflammation

Cytokines, growth factors or hormones take action through the JAK/STAT5 signaling pathway, which plays a critical role in regulating the intestinal response to infection and inflammation. However, the way in which STAT5 regulates intestinal epithelial compartment is largely ignored due to the lack of genetic tools for proper exploration and because the two STAT5 transcription factors (STAT5A and STAT5B) have some redundant but also distinct functions. In this review article, by focusing on STAT5 functions in the intestinal undifferentiated and differentiated epithelia, we discuss major advances of the growth factor/cytokine-JAK/STAT5 research in view of intestinal mucosal inflammation and immunity. We highlight the gap in the research of the intestinal STAT5 signaling to anticipate the gastrointestinal explorative insights. Furthermore, we address the critical questions to illuminate how STAT5 signaling influences intestinal epithelial cell differentiation and stem cell regeneration during homeostasis and injury. Overall, our article provides a centric view of the relevance of the relationship between chronic inflammatory diseases and JAK/STAT5 pathway and it also gives an example of how chronic infection and inflammation pirate STAT5 signaling to worsen intestinal injuries. Importantly, our review suggests how to protect a wound healing from gastrointestinal diseases by modulating intestinal STAT5.

SYK inhibition targets acute myeloid leukemia stem cells by blocking their oxidative metabolism

Spleen tyrosine kinase (SYK) is an important oncogene and signaling mediator activated by cell surface receptors crucial for acute myeloid leukemia (AML) maintenance and progression. Genetic or pharmacologic inhibition of SYK in AML cells leads to increased differentiation, reduced proliferation, and cellular apoptosis. Herein, we addressed the consequences of SYK inhibition to leukemia stem-cell (LSC) function and assessed SYK-associated pathways in AML cell biology. Using gain-of-function MEK kinase mutant and constitutively active STAT5A, we demonstrate that R406, the active metabolite of a small-molecule SYK inhibitor fostamatinib, induces differentiation and blocks clonogenic potential of AML cells through the MEK/ERK1/2 pathway and STAT5A transcription factor, respectively. Pharmacological inhibition of SYK with R406 reduced LSC compartment defined as CD34⁺CD38⁻CD123⁺ and CD34⁺CD38⁻CD25⁺ in vitro, and decreased viability of LSCs identified by a low abundance of reactive oxygen species. Primary leukemic blasts treated ex vivo with R406 exhibited lower engraftment potential when xenotransplanted to immunodeficient NSG/J mice. Mechanistically, these effects are mediated by disturbed mitochondrial biogenesis and suppression of oxidative metabolism (OXPHOS) in LSCs. These mechanisms appear to be partially dependent on inhibition of STAT5 and its target gene MYC, a well-defined inducer of mitochondrial biogenesis. In addition, inhibition of SYK increases the sensitivity of LSCs to cytarabine (AraC), a standard of AML induction therapy. Taken together, our findings indicate that SYK fosters OXPHOS and participates in metabolic reprogramming of AML LSCs in a mechanism that at least partially involves STAT5, and that SYK inhibition targets LSCs in AML. Since active SYK is expressed in a majority of AML patients and confers inferior prognosis, the combination of SYK inhibitors with standard chemotherapeutics such as AraC constitutes a new therapeutic modality that should be evaluated in future clinical trials.

Altering chromatin methylation patterns and the transcriptional network involved in regulation of hematopoietic stem cell fate

Hematopoietic stem cells (HSCs) are quiescent cells with self-renewal capacity and potential multilineage development. Various molecular regulatory mechanisms such as epigenetic modifications and transcription factor (TF) networks play crucial roles in establishing a balance between self-renewal and differentiation of HSCs. Histone/DNA methylations are important epigenetic modifications involved in transcriptional regulation of specific lineage HSCs via controlling chromatin structure and accessibility of DNA. Also, TFs contribute to either facilitation or inhibition of gene expression through binding to enhancer or promoter regions of DNA. As a result, epigenetic factors and TFs regulate the activation or repression of HSCs genes, playing a central role in normal hematopoiesis. Given the importance of histone/DNA methylation and TFs in gene expression regulation, their aberrations, including changes in HSCs-related methylation of histone/DNA and TFs (e.g., CCAAT-enhancer-binding protein α, phosphatase and tensin homolog deleted on the chromosome 10, Runt-related transcription factor 1, signal transducers and activators of transcription, and RAS family proteins) could disrupt HSCs fate. Herewith, we summarize how dysregulations in the expression of genes related to self-renewal, proliferation, and differentiation of HSCs caused by changes in epigenetic modifications and transcriptional networks lead to clonal expansion and leukemic transformation.

Pharmacological Inhibition of Oncogenic STAT3 and STAT5 Signaling in Hematopoietic Cancers

Signal Transducer and Activator of Transcription (STAT) 3 and 5 are important effectors of cellular transformation, and aberrant STAT3 and STAT5 signaling have been demonstrated in hematopoietic cancers. STAT3 and STAT5 are common targets for different tyrosine kinase oncogenes (TKOs). In addition, STAT3 and STAT5 proteins were shown to contain activating mutations in some rare but aggressive leukemias/lymphomas. Both proteins also contribute to drug resistance in hematopoietic malignancies and are now well recognized as major targets in cancer treatment. The development of inhibitors targeting STAT3 and STAT5 has been the subject of intense investigations during the last decade. This review summarizes the current knowledge of oncogenic STAT3 and STAT5 functions in hematopoietic cancers as well as advances in preclinical and clinical development of pharmacological inhibitors.

Constitutive Activation of the Canonical NF-κB Pathway Leads to Bone Marrow Failure and Induction of Erythroid Signature in Hematopoietic Stem Cells

Constitutive activation of the canonical NF-κB pathway has been associated with a variety of human pathologies. However, molecular mechanisms through which canonical NF-κB affects hematopoiesis remain elusive. Here, we demonstrate that deregulated canonical NF-κB signals in hematopoietic stem cells (HSCs) cause a complete depletion of HSC pool, pancytopenia, bone marrow failure, and premature death. Constitutive activation of IKK2 in HSCs leads to impaired quiescence and loss of function. Gene set enrichment analysis (GSEA) identified an induction of “erythroid signature” in HSCs with augmented NF-κB activity. Mechanistic studies indicated a reduction of thrombopoietin (TPO)-mediated signals and its downstream target p57 in HSCs, due to reduced c-MpI expression in a cell-intrinsic manner. Molecular studies established Klf1 as a key suppressor of c-MpI in HSPCs with increased NF-κB. In essence, these studies identified a previously unknown mechanism through which exaggerated canonical NF-κB signals affect HSCs and cause pathophysiology.

Nakagawa and Rathinam demonstrate that constitutive activation of IKK2 in HSCs causes a complete depletion of the HSC pool and impairs HSC functions due to a loss of “sternness” signature and an induction of erythroid signature.

Distinct effects of ruxolitinib and interferon-alpha on murine JAK2V617F myeloproliferative neoplasm hematopoietic stem cell populations

JAK2V617F is the most common mutation in patients with BCR-ABL negative myeloproliferative neoplasms (MPNs). The eradication of JAK2V617F hematopoietic stem cells (HSCs) is critical for achieving molecular remissions and cure. We investigate the distinct effects of two therapies, ruxolitinib (JAK1/2 inhibitor) and interferon-alpha (IFN-α), on the disease-initiating HSC population. Whereas ruxolitinib inhibits Stat5 activation in erythroid progenitor populations, it fails to inhibit this same pathway in HSCs. In contrast, IFN-α has direct effects on HSCs. Furthermore, STAT1 phosphorylation and pathway activation is greater after IFN-α stimulation in Jak2V617F murine HSCs with increased induction of reactive oxygen species, DNA damage and reduction in quiescence after chronic IFN-α treatment. Interestingly, ruxolitinib does not block IFN-α induced reactive oxygen species and DNA damage in Jak2V617F murine HSCs in vivo. This work provides a mechanistic rationale informing how pegylated IFN-α reduces JAK2V617F allelic burden in the clinical setting and may inform future clinical efforts to combine ruxolitinib with pegylated IFN-α in patients with MPN.

STAT5A and STAT5B-Twins with Different Personalities in Hematopoiesis and Leukemia

The transcription factors STAT5A and STAT5B have essential roles in survival and proliferation of hematopoietic cells-which have been considered largely redundant. Mutations of upstream kinases, copy number gains, or activating mutations in STAT5A, or more frequently in STAT5B, cause altered hematopoiesis and cancer. Interfering with their activity by pharmacological intervention is an up-and-coming therapeutic avenue. Precision medicine requests detailed knowledge of STAT5A's and STAT5B's individual functions. Recent evidence highlights the privileged role for STAT5B over STAT5A in normal and malignant hematopoiesis. Here, we provide an overview on their individual functions within the hematopoietic system.

IL-32γ suppresses lung cancer stem cell growth via inhibition of ITGAV-mediated STAT5 pathway

The cancer stem cells (CSCs) are thought to be responsible for cancer initiation, recurrence, and metastasis via a multifactorial process. IL-32γ has been known to inhibit several tumor developments. However, the role of IL-32γ in CSCs is unknown. The role of IL-32γ on tumor development was assessed in IL-32γ transgenic (Tg) mice allograft and xenograft model. In the in vitro assay, we analyzed CSC growth and apoptosis in cells with IL-32γ overexpression by cell viability assay and tumor-sphere formation assay. In addition, expression of cell proliferation, apoptosis markers, and signaling molecules was determined by western blot analysis. IL-32γ suppressed CD133+ CSC-induced allograft model in IL-32γ Tg mice and xenograft model. Tumor-sphere formation and cell viability assay revealed a greater inhibition of CSC proliferation and antineoplastic activity of IL-32γ in CD133+ CSCs as compared with normal cancer cells. The inhibitory effects of IL-32γ on tumor development were associated with inhibition of the STAT5 pathway. In addition, inhibition of STAT5 increased cleavage of caspase-3, but suppressed CD133 expression and colony formation. Web-based gene network analysis showed that IL-32 is correlated with ITGAV, an integrin gene. Our result revealed that knockdown of ITGAV by siRNA inhibited the phosphorylation of STAT5. Moreover, we identified that ITGAV overexpression reversed the effect of IL-32γ on phosphorylation of STAT5 and the expression of CD133. Our results demonstrate that IL-32γ negatively regulates CD133+ CSC proliferation and tumor development and suggest that IL-32γ has great potential for use in the treatment of cancer progression.

Asrij/OCIAD1 suppresses CSN5-mediated p53 degradation and maintains mouse hematopoietic stem cell quiescence

Inactivation of the tumor suppressor p53 is essential for unrestrained growth of cancers. However, only 11% of hematological malignancies have mutant p53. Mechanisms that cause wild-type p53 dysfunction and promote leukemia are inadequately deciphered. The stem cell protein Asrij/OCIAD1 is misexpressed in several human hematological malignancies and implicated in the p53 pathway and DNA damage response. However, Asrij function in vertebrate hematopoiesis remains unknown. We generated the first asrij null (knockout [KO]) mice and show that they are viable and fertile with no gross abnormalities. However, by 6 months, they exhibit increased peripheral blood cell counts, splenomegaly, and an expansion of bone marrow hematopoietic stem cells (HSCs) with higher myeloid output. HSCs lacking Asrij are less quiescent and more proliferative with higher repopulation potential as observed from serial transplantation studies. However, stressing KO mice with sublethal γ irradiation or multiple injections of 5-fluorouracil results in reduced survival and rapid depletion of hematopoietic stem/progenitor cells (HSPCs) by driving them into proliferative exhaustion. Molecular and biochemical analyses revealed increased polyubiquitinated protein levels, Akt/STAT5 activation and COP9 signalosome subunit 5 (CSN5)-mediated p53 ubiquitination, and degradation in KO HSPCs. Further, we show that Asrij sequesters CSN5 via its conserved OCIA domain, thereby preventing p53 degradation. In agreement, Nutlin-3 treatment of KO mice restored p53 levels and reduced high HSPC frequencies. Thus, we provide a new mouse model resembling myeloproliferative disease and identify a posttranslational regulator of wild-type p53 essential for maintaining HSC quiescence that could be a potential target for pharmacological intervention.

Oncogenic N-Ras and Tet2 haploinsufficiency collaborate to dysregulate hematopoietic stem and progenitor cells

Concurrent genetic lesions exist in a majority of patients with hematologic malignancies. Among these, somatic mutations that activate RAS oncogenes and inactivate the epigenetic modifier ten-eleven translocation 2 (TET2) frequently co-occur in human chronic myelomonocytic leukemias (CMMLs) and acute myeloid leukemias, suggesting a cooperativity in malignant transformation. To test this, we applied a conditional murine model that endogenously expressed oncogenic Nras^G12D and monoallelic loss of Tet2 and explored the collaborative role specifically within hematopoietic stem and progenitor cells (HSPCs) at disease initiation. We demonstrate that the 2 mutations collaborated to accelerate a transplantable CMML-like disease in vivo, with an overall shortened survival and increased disease penetrance compared with single mutants. At preleukemic stage, N-Ras^G12D and Tet2 haploinsufficiency together induced balanced hematopoietic stem cell (HSC) proliferation and enhanced competitiveness. Nras^G12D/+/Tet2^+/- HSCs displayed increased self-renewal in primary and secondary transplantations, with significantly higher reconstitution than single mutants. Strikingly, the 2 mutations together conferred long-term reconstitution and self-renewal potential to multipotent progenitors, a pool of cells that usually have limited self-renewal compared with HSCs. Moreover, HSPCs from Nras^G12D/+/Tet2^+/- mice displayed increased cytokine sensitivity in response to thrombopoietin. Therefore, our studies establish a novel tractable CMML model and provide insights into how dysregulated signaling pathways and epigenetic modifiers collaborate to modulate HSPC function and promote leukemogenesis.

Drosophila Jak/STAT Signaling: Regulation and Relevance in Human Cancer and Metastasis

Over the past three-decades, Janus kinase (Jak) and signal transducer and activator of transcription (STAT) signaling has emerged as a paradigm to understand the involvement of signal transduction in development and disease pathology. At the molecular level, cytokines and interleukins steer Jak/STAT signaling to transcriptional regulation of target genes, which are involved in cell differentiation, migration, and proliferation. Jak/STAT signaling is involved in various types of blood cell disorders and cancers in humans, and its activation is associated with carcinomas that are more invasive or likely to become metastatic. Despite immense information regarding Jak/STAT regulation, the signaling network has numerous missing links, which is slowing the progress towards developing drug therapies. In mammals, many components act in this cascade, with substantial cross-talk with other signaling pathways. In Drosophila, there are fewer pathway components, which has enabled significant discoveries regarding well-conserved regulatory mechanisms. Work across species illustrates the relevance of these regulators in humans. In this review, we showcase fundamental Jak/STAT regulation mechanisms in blood cells, stem cells, and cell motility. We examine the functional relevance of key conserved regulators from Drosophila to human cancer stem cells and metastasis. Finally, we spotlight less characterized regulators of Drosophila Jak/STAT signaling, which stand as promising candidates to be investigated in cancer biology. These comparisons illustrate the value of using Drosophila as a model for uncovering the roles of Jak/STAT signaling and the molecular means by which the pathway is controlled.

Suppression of m6A reader Ythdf2 promotes hematopoietic stem cell expansion

Transplantation of hematopoietic stem cells (HSCs) from human umbilical cord blood (hUCB) holds great promise for treating a broad spectrum of hematological disorders including cancer. However, the limited number of HSCs in a single hUCB unit restricts its widespread use. Although extensive efforts have led to multiple methods for ex vivo expansion of human HSCs by targeting single molecules or pathways, it remains unknown whether it is possible to simultaneously manipulate the large number of targets essential for stem cell self-renewal. Recent studies indicate that N⁶-methyladenosine (m⁶A) modulates the expression of a group of mRNAs critical for stem cell-fate determination by influencing their stability. Among several m⁶A readers, YTHDF2 is recognized as promoting targeted mRNA decay. However, the physiological functions of YTHDF2 in adult stem cells are unknown. Here we show that following the conditional knockout (KO) of mouse Ythdf2 the numbers of functional HSC were increased without skewing lineage differentiation or leading to hematopoietic malignancies. Furthermore, knockdown (KD) of human YTHDF2 led to more than a 10-fold increase in the ex vivo expansion of hUCB HSCs, a fivefold increase in colony-forming units (CFUs), and more than an eightfold increase in functional hUCB HSCs in the secondary serial of a limiting dilution transplantation assay. Mapping of m⁶A in RNAs from mouse hematopoietic stem and progenitor cells (HSPCs) as well as from hUCB HSCs revealed its enrichment in mRNAs encoding transcription factors critical for stem cell self-renewal. These m⁶A-marked mRNAs were recognized by Ythdf2 and underwent decay. In Ythdf2 KO HSPCs and YTHDF2 KD hUCB HSCs, these mRNAs were stabilized, facilitating HSC expansion. Knocking down one of YTHDF2's key targets, Tal1 mRNA, partially rescued the phenotype. Our study provides the first demonstration of the function of YTHDF2 in adult stem cell maintenance and identifies its important role in regulating HSC ex vivo expansion by regulating the stability of multiple mRNAs critical for HSC self-renewal, thus identifying potential for future clinical applications.

The Role of Interferon-Gamma in Hematopoietic Stem Cell Development, Homeostasis, and Disease

Purpose of Review

Interferon-gamma (IFN-γ) is a pro-inflammatory cytokine that participates in the regulation of hematopoietic stem cells (HSC) during development and under homeostatic conditions. IFN-γ also plays a key pathogenic role in several diseases that affect hematopoiesis including aplastic anemia, hemophagocytic lymphohistiocytosis, and cirrhosis of the liver.

Recent Findings

Studies have shown that increased IFN-γ negatively affects HSC homeostasis, skewing HSC towards differentiation over self-renewal and eventually causing exhaustion of the HSC compartment.

Summary

Here, we explore the mechanisms by which IFN-γ regulates HSC in both normal and pathological conditions. We focus on the role of IFN-γ signaling in HSC fate decisions, and the transcriptional changes it elicits. Elucidating the mechanisms through which IFN-γ regulates HSCs may lead to new therapeutic options to prevent or treat adverse hematologic effects of the many diseases to which IFN-γ contributes.

Designing Motif-Engineered Receptors To Elucidate Signaling Molecules Important for Proliferation of Hematopoietic Stem Cells

The understanding of signaling events is critical for attaining long-term expansion of hematopoietic stem cells ex vivo. In this study, we aim to analyze the contribution of multiple signaling molecules in proliferation of hematopoietic stem cells. To this end, we design a bottom-up engineered receptor with multiple tyrosine motifs, which can recruit multiple signaling molecules of interest. This is followed by a top-down approach, where one of the multiple tyrosine motifs in the bottom-up engineered receptor is functionally knocked out by tyrosine-to-phenylalanine mutation. The combination of these two approaches demonstrates the importance of Shc in cooperation with STAT3 or STAT5 in the proliferation of hematopoietic stem cells. The platform developed herein may be applied for analyzing other cells and/or other cell fate regulation systems.

Transcriptional Regulation of Emergency Granulopoiesis in Leukemia

Neutropenic conditions are prevalent in leukemia patients and are often associated with increased susceptibility to infections. In fact, emergency granulopoiesis (EG), a process regulating neutrophil homeostasis in inflammatory conditions and infections, may occur improperly in leukemic conditions, leading to reduced neutrophil counts. Unfortunately, the mechanisms central to dysfunctional EG remain understudied in both leukemia patients and leukemic mouse models. However, despite no direct studies on EG response in leukemia are reported, recently certain transcription factors (TFs) have been found to function at the crossroads of leukemia and EG. In this review, we present an update on TFs that can potentially govern the fate of EG in leukemia. Transcriptional control of Fanconi DNA repair pathway genes is also highlighted, as well as the newly discovered role of Fanconi proteins in innate immune response and EG. Identifying the TFs regulating EG in leukemia and dissecting their underlying mechanisms may facilitate the discovery of therapeutic drugs for the treatment of neutropenia.

Modern management of phagocyte defects

Phagocytic neutrophil granulocytes are among the first immune cells active at sites of infection, forming an important first-line defense against invading microorganisms. Congenital immune defects concerning these phagocytes may be due to reduced neutrophil numbers or function. Management of affected patients depends on the type and severity of disease. Here, we provide an overview of causes and treatment of diseases associated with congenital neutropenia, as well as defects of the phagocytic respiratory burst.

Acquired expression of CblQ367P in mice induces dysplastic myelopoiesis mimicking chronic myelomonocytic leukemia

Chronic myelomonocytic leukemia (CMML) is a hematological malignancy characterized by uncontrolled proliferation of dysplastic myelomonocytes and frequent progression to acute myeloid leukemia (AML). We identified mutations in the Cbl gene, which encodes a negative regulator of cytokine signaling, in a subset of CMML patients. To investigate the contribution of mutant Cbl in CMML pathogenesis, we generated conditional knockin mice for Cbl that express wild-type Cbl in a steady state and inducibly express Cbl^Q367P , a CMML-associated Cbl mutant. Cbl^Q367P mice exhibited sustained proliferation of myelomonocytes, multilineage dysplasia, and splenomegaly, which are the hallmarks of CMML. The phosphatidylinositol 3-kinase (PI3K)-AKT and JAK-STAT pathways were constitutively activated in Cbl^Q367P hematopoietic stem cells, which promoted cell cycle progression and enhanced chemokine-chemokine receptor activity. Gem, a gene encoding a GTPase that is upregulated by Cbl^Q367P , enhanced hematopoietic stem cell activity and induced myeloid cell proliferation. In addition, Evi1, a gene encoding a transcription factor, was found to cooperate with Cbl^Q367P and progress CMML to AML. Furthermore, targeted inhibition for the PI3K-AKT and JAK-STAT pathways efficiently suppressed the proliferative activity of Cbl^Q367P -bearing CMML cells. Our findings provide insights into the molecular mechanisms underlying mutant Cbl-induced CMML and propose a possible molecular targeting therapy for mutant Cbl-carrying CMML patients.

Metabolism and the Control of Cell Fate Decisions and Stem Cell Renewal

Although the stem cells of various tissues remain in the quiescent state to maintain their undifferentiated state, they also undergo cell divisions as required, and if necessary, even a single stem cell is able to provide for lifelong tissue homeostasis. Stem cell populations are precisely controlled by the balance between their symmetric and asymmetric divisions, with their division patterns determined by whether the daughter cells involved retain their self-renewal capacities. Recent studies have reported that metabolic pathways and the distribution of mitochondria are regulators of the division balance of stem cells and that metabolic defects can shift division balance toward symmetric commitment, which leads to stem cell exhaustion. It has also been observed that in asymmetric division, old mitochondria, which are central metabolic organelles, are segregated to the daughter cell fated to cell differentiation, whereas in symmetric division, young and old mitochondria are equally distributed between both daughter cells. Thus, metabolism and mitochondrial biology play important roles in stem cell fate decisions. As these decisions directly affect tissue homeostasis, understanding their regulatory mechanisms in the context of cellular metabolism is critical.

Cytokine-induced megakaryocytic differentiation is regulated by genome-wide loss of a uSTAT transcriptional program

Metazoan development is regulated by transcriptional networks, which must respond to extracellular cues including cytokines. The JAK/STAT pathway is a highly conserved regulatory module, activated by many cytokines, in which tyrosine-phosphorylated STATs (pSTATs) function as transcription factors. However, the mechanisms by which STAT activation modulates lineage-affiliated transcriptional programs are unclear. We demonstrate that in the absence of thrombopoietin (TPO), tyrosine-unphosphorylated STAT5 (uSTAT5) is present in the nucleus where it colocalizes with CTCF and represses a megakaryocytic transcriptional program. TPO-mediated phosphorylation of STAT5 triggers its genome-wide relocation to STAT consensus sites with two distinct transcriptional consequences, loss of a uSTAT5 program that restrains megakaryocytic differentiation and activation of a canonical pSTAT5-driven program which includes regulators of apoptosis and proliferation. Transcriptional repression by uSTAT5 reflects restricted access of the megakaryocytic transcription factor ERG to target genes. These results identify a previously unrecognized mechanism of cytokine-mediated differentiation.

STAT5-regulated microRNA-193b controls haematopoietic stem and progenitor cell expansion by modulating cytokine receptor signalling

Haematopoietic stem cells (HSCs) require the right composition of microRNAs (miR) for proper life-long balanced blood regeneration. Here we show a regulatory circuit that prevents excessive HSC self-renewal by upregulation of miR-193b upon self-renewal promoting thrombopoietin (TPO)-MPL-STAT5 signalling. In turn, miR-193b restricts cytokine signalling, by targeting the receptor tyrosine kinase c-KIT. We generated a miR-193b knockout mouse model to unravel the physiological function of miR-193b in haematopoiesis. MiR-193b^−/− mice show a selective gradual enrichment of functional HSCs, which are fully competent in multilineage blood reconstitution upon transplantation. The absence of miR-193b causes an accelerated expansion of HSCs, without altering cell cycle or survival, but by decelerating differentiation. Conversely, ectopic miR-193b expression restricts long-term repopulating HSC expansion and blood reconstitution. MiR-193b-deficient haematopoietic stem and progenitor cells exhibit increased basal and cytokine-induced STAT5 and AKT signalling. This STAT5-induced microRNA provides a negative feedback for excessive signalling to restrict uncontrolled HSC expansion.

MicroRNAs regulate haematopoietic stem cell (HSC) development to ensure the correct generation of blood cells. Haetscher et al. show in mice that miR-193b controls the life-long self-renewal ability of HSCs via AKT and STAT5 pathways, with loss of miR-193b accelerating HSC expansion and reducing differentiation.

CXCL12/CXCR4 axis in the pathogenesis of acute lymphoblastic leukemia (ALL): a possible therapeutic target

Acute lymphoblastic leukemia (ALL) is the commonest childhood malignancy, accounting for approximately 80 % of leukemia in the pediatric group, and its etiology is unknown. This neoplasia is characterized by male predominance, high-risk features and poor outcome, mainly in recurrence patients and adults. In recent years, advances in the success of childhood ALL treatment were verified, and the rate of cure is over 80 % of individuals. However, there is a considerable scope for improving therapeutic outcome in this neoplasia. Improvements in ALL therapy might readily be achieved by developing additional biomarkers that can predict and refine prognosis in patients with ALL. In normal hematopoietic cells, cytokines provide the stimulus for proliferation, survival, self-renewal, differentiation and functional activation. Abnormalities of cytokines are characteristic in all forms of leukemia, including ALL. The stromal cell-derived factor-1 (SDF-1 or CXCL12) is a member of the CXC chemokine family that binds to CXC chemokine receptor 4 (CXCR4). The CXCL12/CXCR4 axis appears to play a role in dissemination of solid tumors and hematopoietic diseases. Understanding the mechanisms by which ALL cells are disseminated will provide additional information to expand therapeutic approach. Therefore, this review summarizes information relating to ALL cell biology, focusing specifically in a cytokine receptor important axis, CXCL12/CXCR4, that may have implications for novel treatment strategies to improve life expectancy of patients with this neoplasia.

SOCS2 Controls Proliferation and Stemness of Hematopoietic Cells under Stress Conditions and Its Deregulation Marks Unfavorable Acute Leukemias

Hematopoietic stem cells (HSC) promptly adapt hematopoiesis to stress conditions, such as infection and cancer, replenishing bone marrow-derived circulating populations, while preserving the stem cell reservoir. SOCS2, a feedback inhibitor of JAK-STAT pathways, is expressed in most primitive HSC and is upregulated in response to STAT5-inducing cytokines. We demonstrate that Socs2 deficiency unleashes HSC proliferation in vitro, sustaining STAT5 phosphorylation in response to IL3, thrombopoietin, and GM-CSF. In vivo, SOCS2 deficiency leads to unrestricted myelopoietic response to 5-fluorouracil (5-FU) and, in turn, induces exhaustion of long-term HSC function along serial bone marrow transplantations. The emerging role of SOCS2 in HSC under stress conditions prompted the investigation of malignant hematopoiesis. High levels of SOCS2 characterize unfavorable subsets of acute myeloid and lymphoblastic leukemias, such as those with MLL and BCR/ABL abnormalities, and correlate with the enrichment of genes belonging to hematopoietic and leukemic stemness signatures. In this setting, SOCS2 and its correlated genes are part of regulatory networks fronted by IKZF1/Ikaros and MEF2C, two transcriptional regulators involved in normal and leukemic hematopoiesis that have never been linked to SOCS2. Accordingly, a comparison of murine wt and Socs2(-/-) HSC gene expression in response to 5-FU revealed a significant overlap with the molecular programs that correlate with SOCS2 expression in leukemias, particularly with the oncogenic pathways and with the IKZF1/Ikaros and MEF2C-predicted targets. Lentiviral gene transduction of murine hematopoietic precursors with Mef2c, but not with Ikzf1, induces Socs2 upregulation, unveiling a direct control exerted by Mef2c over Socs2 expression.

Suppression of cancer relapse and metastasis by inhibiting cancer stemness

Partial or even complete cancer regression can be achieved in some patients with current cancer treatments. However, such initial responses are almost always followed by relapse, with the recurrent cancer being resistant to further treatments. The discovery of therapeutic approaches that counteract relapse is, therefore, essential for advancing cancer medicine. Cancer cells are extremely heterogeneous, even in each individual patient, in terms of their malignant potential, drug sensitivity, and their potential to metastasize and cause relapse. Indeed, hypermalignant cancer cells, termed cancer stem cells or stemness-high cancer cells, that are highly tumorigenic and metastatic have been isolated from cancer patients with a variety of tumor types. Moreover, such stemness-high cancer cells are resistant to conventional chemotherapy and radiation. Here we show that BBI608, a small molecule identified by its ability to inhibit gene transcription driven by Stat3 and cancer stemness properties, can inhibit stemness gene expression and block spherogenesis of or kill stemness-high cancer cells isolated from a variety of cancer types. Moreover, cancer relapse and metastasis were effectively blocked by BBI608 in mice. These data demonstrate targeting cancer stemness as a novel approach to develop the next generation of cancer therapeutics to suppress cancer relapse and metastasis.

Activated STAT5 confers resistance to intestinal injury by increasing intestinal stem cell proliferation and regeneration

Intestinal epithelial stem cells (IESCs) control the intestinal homeostatic response to inflammation and regeneration. The underlying mechanisms are unclear. Cytokine-STAT5 signaling regulates intestinal epithelial homeostasis and responses to injury. We link STAT5 signaling to IESC replenishment upon injury by depletion or activation of Stat5 transcription factor. We found that depletion of Stat5 led to deregulation of IESC marker expression and decreased LGR5(+) IESC proliferation. STAT5-deficient mice exhibited worse intestinal histology and impaired crypt regeneration after γ-irradiation. We generated a transgenic mouse model with inducible expression of constitutively active Stat5. In contrast to Stat5 depletion, activation of STAT5 increased IESC proliferation, accelerated crypt regeneration, and conferred resistance to intestinal injury. Furthermore, ectopic activation of STAT5 in mouse or human stem cells promoted LGR5(+) IESC self-renewal. Accordingly, STAT5 promotes IESC proliferation and regeneration to mitigate intestinal inflammation. STAT5 is a functional therapeutic target to improve the IESC regenerative response to gut injury.

Inhibition of STAT5: a therapeutic option in BCR-ABL1-driven leukemia

The two transcription factors STAT5A and STAT5B are central signaling molecules in leukemias driven by Abelson fusion tyrosine kinases and they fulfill all criteria of drug targets. STAT5A and STAT5B display unique nuclear shuttling mechanisms and they have a key role in resistance of leukemic cells against treatment with tyrosine kinase inhibitors (TKI). Moreover, STAT5A and STAT5B promote survival of leukemic stem cells. We here discuss the possibility of targeting up-stream kinases with TKI, direct STAT5 inhibition via SH2 domain obstruction and blocking nuclear translocation of STAT5. All discussed options will result in a stop of STAT5 transport to the nucleus to block STAT5-mediated transcriptional activity. In summary, recently described shuttling functions of STAT5 are discussed as potentially druggable pathways in leukemias.

PIM inhibitors target CD25-positive AML cells through concomitant suppression of STAT5 activation and degradation of MYC oncogene

CD25 is a predictive biomarker for sensitivity to PIM inhibitors in AML cells. PIM inhibitors may prolong overall/relapse-free survival through attenuating STAT5 activation and destabilizing MYC in CD25+ AML cells.

STAT5-mediated self-renewal of normal hematopoietic and leukemic stem cells

The level of transcription factor activity critically regulates cell fate decisions such as hematopoietic stem cell self-renewal and differentiation. The balance between hematopoietic stem cell self-renewal and differentiation needs to be tightly controlled, as a shift toward differentiation might exhaust the stem cell pool, while a shift toward self-renewal might mark the onset of leukemic transformation. A number of transcription factors have been proposed to be critically involved in governing stem cell fate and lineage commitment, such as Hox transcription factors, c-Myc, Notch1, β-catenin, C/ebpα, Pu.1 and STAT5. It is therefore no surprise that dysregulation of these transcription factors can also contribute to the development of leukemias. This review will discuss the role of STAT5 in both normal and leukemic hematopoietic stem cells as well as mechanisms by which STAT5 might contribute to the development of human leukemias.

STAT transcription factors in normal and cancer stem cells

Signal transducer and activator of transcription proteins (STATs) play vital roles in the regulation of cellular proliferation and survival in normal hematopoietic cells, including hematopoietic stem cells. However, aberrant activation of STATs is commonly observed in a number of hematologic malignancies, and recent studies indicate that targeting of STATs may have therapeutic benefit in these diseases. Additional studies have provided greater understanding of the cells responsible for leukemia initiation, referred to as leukemia stem cells. Emerging evidence indicates that STATs are important in maintaining leukemia stem cells and represent a promising target for eradication of this dangerous cell population. Here we summarize what is known about normal hematopoietic stem cells and the origin of leukemic stem cells. We further describe the roles of STAT proteins in these cell populations, as well as current progress toward the development of novel agents and strategies for targeting the STAT proteins.

Bortezomib inhibits STAT5-dependent degradation of LEF-1, inducing granulocytic differentiation in congenital neutropenia CD34(+) cells

The transcription factor lymphoid enhancer-binding factor 1 (LEF-1), which plays a definitive role in granulocyte colony-stimulating factor (G-CSF) receptor-triggered granulopoiesis, is downregulated in granulocytic progenitors of severe congenital neutropenia (CN) patients. However, the exact mechanism of LEF-1 downregulation is unclear. CN patients are responsive to therapeutically high doses of G-CSF and are at increased risk of developing acute myeloid leukemia. The normal expression of LEF-1 in monocytes and lymphocytes, whose differentiation is unaffected in CN, suggests the presence of a granulopoiesis-specific mechanism downstream of G-CSF receptor signaling that leads to LEF-1 downregulation. Signal transducer and activator of transcription 5 (STAT5) is activated by G-CSF and is hyperactivated in acute myeloid leukemia. Here, we investigated the effects of activated STAT5 on LEF-1 expression and functions in hematopoietic progenitor cells. We demonstrated that constitutively active STAT5a (caSTAT5a) inhibited LEF-1-dependent autoregulation of the LEF-1 gene promoter by binding to the LEF-1 protein, recruiting Nemo-like kinase and the E3 ubiquitin-ligase NARF to LEF-1, leading to LEF-1 ubiquitination and a reduction in LEF-1 protein levels. The proteasome inhibitor bortezomib reversed the defective G-CSF-triggered granulocytic differentiation of CD34(+) cells from CN patients in vitro, an effect that was accompanied by restoration of LEF-1 protein levels and LEF-1 messenger RNA autoregulation. Taken together, our data define a novel mechanism of LEF-1 downregulation in CN patients via enhanced ubiquitination and degradation of LEF-1 protein by hyperactivated STAT5.

In vivo leukemogenic potential of an interleukin 7 receptor α chain mutant in hematopoietic stem and progenitor cells

Gain-of function mutation of IL7Rα induces lymphoid leukemia as well as myeloproliferative disease. In vivo oncogenicity of mutant IL7Rα is influenced by the differentiation stage at which it occurs.

STAT5 in hematopoietic stem cell biology and transplantation

Signal transducer and activator of transcription 5 (STAT5) regulates normal lympho-myeloid development through activation downstream of early-acting cytokines, their receptors, and Janus kinases (JAKs). Despite a general understanding of the role of STAT5 in hematopoietic stem cell (HSC) proliferation, survival, and self-renewal, the transcriptional targets and mechanisms of gene regulation that control multi-lineage engraftment following transplantation for the most part remain to be understood. In this review, we focus on the role of STAT5 in HSC transplantation and recent developments toward identifying the relevant downstream target genes and their role as part of a pleiotropic STAT5 mediated signaling response.

Thrombopoietin promotes NHEJ DNA repair in hematopoietic stem cells through specific activation of Erk and NF-κB pathways and their target, IEX-1

Loss of hematopoietic stem cell (HSC) function and increased risk of developing hematopoietic malignancies are severe and concerning complications of anticancer radiotherapy and chemotherapy. We have previously shown that thrombopoietin (TPO), a critical HSC regulator, ensures HSC chromosomal integrity and function in response to γ-irradiation by regulating their DNA-damage response. TPO directly affects the double-strand break (DSB) repair machinery through increased DNA-protein kinase (DNA-PK) phosphorylation and nonhomologous end-joining (NHEJ) repair efficiency and fidelity. This effect is not shared by other HSC growth factors, suggesting that TPO triggers a specific signal in HSCs facilitating DNA-PK activation upon DNA damage. The discovery of these unique signaling pathways will provide a means of enhancing TPO-desirable effects on HSCs and improving the safety of anticancer DNA agents. We show here that TPO specifically triggers Erk and nuclear factor κB (NF-κB) pathways in mouse hematopoietic stem and progenitor cells (HSPCs). Both of these pathways are required for a TPO-mediated increase in DSB repair. They cooperate to induce and activate the early stress-response gene, Iex-1 (ier3), upon DNA damage. Iex-1 forms a complex with pERK and the catalytic subunit of DNA-PK, which is necessary and sufficient to promote TPO-increased DNA-PK activation and NHEJ DSB repair in both mouse and human HSPCs.

Identification of a STAT5 target gene, Dpf3, provides novel insights in chronic lymphocytic leukemia

STAT5 controls essential cellular functions and is encoded by two genes, Stat5a and Stat5b. To provide insight to the mechanisms linking hematologic malignancy to STAT5 activation/regulation of target genes, we identified STAT5 target genes and focused on Dpf3 gene, which encodes for an epigenetic factor. Dpf3 expression was induced upon IL-3 stimulation in Ba/F3 cells, while strong binding of both STAT5a and STAT5b was detected in its promoter. Reduced expression of Dpf3 was detected in Ba/F3 cells with Stat5a and Stat5b knock-down, suggesting that this gene is positively regulated by STAT5, upon IL-3 stimulation. Furthermore, this gene was significantly up-regulated in CLL patients, where DPF3 gene/protein up-regulation and strong STAT5 binding to the DPF3 promoter, correlated with increased STAT5 activation, mainly in non-malignant myeloid cells (granulocytes). Our findings provide insights in the STAT5 dependent transcriptional regulation of Dpf3, and demonstrate for the first time increased STAT5 activation in granulocytes of CLL patients. Novel routes of investigation are opened to facilitate the understanding of the role of STAT5 activation in the communication between non-malignant myeloid and malignant B-cells, and the functions of STAT5 target genes networks in CLL biology.

Novel mechanism of tumor suppression by polarity gene discs large 1 (DLG1) revealed in a murine model of pediatric B-ALL

Drosophila melanogaster discs large (dlg) is an essential tumor suppressor gene (TSG) controlling epithelial cell growth and polarity of the fly imaginal discs in pupal development. A mammalian ortholog, Dlg1, is involved in embryonic urogenital morphogenesis, postsynaptic densities in neurons, and immune synapses in lymphocytes. However, a potential role for Dlg1 as a mammalian TSG is unknown. Here, we present evidence that loss of Dlg1 confers strong predisposition to the development of malignancies in a murine model of pediatric B-cell acute lymphoblastic leukemia (B-ALL). Using mice with conditionally deleted Dlg1 alleles, we identify a novel "pre-leukemic" stage of developmentally arrested early B-lineage cells marked by preeminent c-Myc expression. Mechanistically, we show that in B-lineage progenitors Dlg1 interacts with and stabilizes the PTEN protein, regulating its half-life and steady-state abundance. The loss of Dlg1 does not affect the level of PTEN mRNAs but results in a dramatic decrease in PTEN protein, leading to excessive phosphoinositide 3-kinase signaling and proliferation. Our data suggest a novel model of tumor suppression by a PDZ domain-containing polarity gene in hematopoietic cancers.