Characterization of the TGF-β1 signaling abnormalities in the Gata1low mouse model of myelofibrosis

Transcription factor 3 is dysregulated in megakaryocytes in myelofibrosis

Transcription factor 3 (TCF3) is a DNA transcription factor that modulates megakaryocyte development. Although abnormal TCF3 expression has been identified in a range of hematological malignancies, to date, it has not been investigated in myelofibrosis (MF). MF is a Philadelphia-negative myeloproliferative neoplasm (MPN) that can arise de novo or progress from essential thrombocythemia [ET] and polycythemia vera [PV] and where dysfunctional megakaryocytes have a role in driving the fibrotic progression. We aimed to examine whether TCF3 is dysregulated in megakaryocytes in MPN, and specifically in MF. We first assessed TCF3 protein expression in megakaryocytes using an immunohistochemical approach analyses and showed that TCF3 was reduced in MF compared with ET and PV. Further, the TCF3-negative megakaryocytes were primarily located near trabecular bone and had the typical "MF-like" morphology as described by the WHO. Genomic analysis of isolated megakaryocytes showed three mutations, all predicted to result in a loss of function, in patients with MF; none were seen in megakaryocytes isolated from ET or PV marrow samples. We then progressed to transcriptomic sequencing of platelets which showed loss of TCF3 in MF. These proteomic, genomic and transcriptomic analyses appear to indicate that TCF3 is downregulated in megakaryocytes in MF. This infers aberrations in megakaryopoiesis occur in this progressive phase of MPN. Further exploration of this pathway could provide insights into TCF3 and the evolution of fibrosis and potentially lead to new preventative therapeutic targets.

Cellular carcinogenesis in preleukemic conditions:drivers and defenses

Acute myeloid leukemia (AML) arises from preleukemic conditions. We have investigated the pathogenesis of typical preleukemia, myeloproliferative neoplasms, and clonal hematopoiesis. Hematopoietic stem cells in both preleukemic conditions harbor recurrent driver mutations; additional mutation provokes further malignant transformation, leading to AML onset. Although genetic alterations are defined as the main cause of malignant transformation, non-genetic factors are also involved in disease progression. In this review, we focus on a non-histone chromatin protein, high mobility group AT-hook2 (HMGA2), and a physiological p53 inhibitor, murine double minute X (MDMX). HMGA2 is mainly overexpressed by dysregulation of microRNAs or mutations in polycomb components, and provokes expansion of preleukemic clones through stem cell signature disruption. MDMX is overexpressed by altered splicing balance in myeloid malignancies. MDMX induces leukemic transformation from preleukemia via suppression of p53 and p53-independent activation of WNT/β-catenin signaling. We also discuss how these non-genetic factors can be targeted for leukemia prevention therapy.

Identification of the Hub Genes Linked to Lead (IV)-Induced Spleen Toxicity Using the Rat Model

Exposure to lead (Pb) has harmful effects on the organs of both humans and animals, particularly the spleen. However, the precise mechanisms through which Pb (IV) exposure leads to spleen toxicity remain unclear. Hence, this study aimed to identify the key genes and signaling pathways involved in spleen toxicity caused by Pb (IV) incubation. We obtained the dataset GSE59925 from the Gene Expression Omnibus, which included spleen samples treated with lead tetraacetate (PbAc4) as well as control samples on the 1st and 5th day. Through differential expression analysis, we identified 607 and 704 differentially expressed genes (DEGs) in the spleens on the 1st and 5th day following PbAc4 treatment, respectively, with 245 overlapping DEGs between the two time points. Gene ontology analysis revealed that the commonly shared DEGs were primarily involved in signal transduction, drug response, cell proliferation, adhesion, and migration. Pathway analysis indicated that the common DEGs were primarily associated with MAPK, TNF, cAMP, Hippo, and TGF-β signaling pathways. Furthermore, we identified the hub genes such as CXCL10, PARP1, APOE, and VDR contributing to PbAc4-induced spleen toxicity. This study enhances our understanding of the molecular mechanisms underlying Pb (IV) toxicity in the spleen.

SRSF2 mutation reduces polycythemia and impairs hematopoietic progenitor functions in JAK2V617F-driven myeloproliferative neoplasm

SRSF2 mutations are found in association with JAK2V617F in myeloproliferative neoplasms (MPN), most frequently in myelofibrosis (MF). However, the contribution of SRSF2 mutation in JAK2V617F-driven MPN remains elusive. To investigate the consequences of SRSF2P95H and JAK2V617F mutations in MPN, we generated Cre-inducible Srsf2P95H/+Jak2V617F/+ knock-in mice. We show that co-expression of Srsf2P95H mutant reduced red blood cell, neutrophil, and platelet counts, attenuated splenomegaly but did not induce bone marrow fibrosis in Jak2V617F/+ mice. Furthermore, co-expression of Srsf2P95H diminished the competitiveness of Jak2V617F mutant hematopoietic stem/progenitor cells. We found that Srsf2P95H mutant reduced the TGF-β levels but increased the expression of S100A8 and S100A9 in Jak2V617F/+ mice. Furthermore, enforced expression of S100A9 in Jak2V617F/+ mice bone marrow significantly reduced the red blood cell, hemoglobin, and hematocrit levels. Overall, these data suggest that concurrent expression of Srsf2P95H and Jak2V617F mutants reduces erythropoiesis but does not promote the development of bone marrow fibrosis in mice.

GATA1-defective immune-megakaryocytes as possible drivers of idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive fibrotic lung disorder with limited therapeutic options. Insufficient understanding of driver mutations and poor fidelity of currently available animal models has limited the development of effective therapies. Since GATA1 deficient megakaryocytes sustain myelofibrosis, we hypothesized that they may also induce fibrosis in lungs. We discovered that lungs from IPF patients and Gata1low mice contain numerous GATA1negative immune-poised megakaryocytes that, in mice, have defective RNA-seq profiling and increased TGF-β1, CXCL1 and P-selectin content. With age, Gata1low mice develop fibrosis in lungs. Development of lung fibrosis in this model is prevented by P-selectin deletion and rescued by P-selectin, TGF-β1 or CXCL1 inhibition. Mechanistically, P-selectin inhibition decreases TGF-β1 and CXCL1 content and increases GATA1positive megakaryocytes while TGF-β1 or CXCL1 inhibition decreased CXCL1 only. In conclusion, Gata1low mice are a novel genetic-driven model for IPF and provide a link between abnormal immune-megakaryocytes and lung fibrosis.

Functionally Relevant Cytokine/Receptor Axes in Myelofibrosis

Dysregulated inflammatory signaling is a key feature of myeloproliferative neoplasms (MPNs), most notably of myelofibrosis (MF). Indeed, MF is considered the prototype of onco-inflammatory hematologic cancers. While increased levels of circulatory and bone marrow cytokines are a well-established feature of all MPNs, a very recent body of literature is intriguingly pinpointing the selective overexpression of cytokine receptors by MF hematopoietic stem and progenitor cells (HSPCs), which, by contrast, are nearly absent or scarcely expressed in essential thrombocythemia (ET) or polycythemia vera (PV) cells. This new evidence suggests that MF CD34+ cells are uniquely capable of sensing inflammation, and that activation of specific cytokine signaling axes may contribute to the peculiar aggressive phenotype and biological behavior of this disorder. In this review, we will cover the main cytokine systems peculiarly activated in MF and how cytokine receptor targeting is shaping a novel therapeutic avenue in this disease.

Inhibition of CXCR1/2 reduces the emperipolesis between neutrophils and megakaryocytes in the Gata1low model of myelofibrosis

Emperipolesis between neutrophils and megakaryocytes was first identified by transmission electron microscopy. Although rare under steady-state conditions, its frequency greatly increases in myelofibrosis, the most severe of myeloproliferative neoplasms, in which it is believed to contribute to increasing the transforming growth factor (TGF)-β microenvironmental bioavailability responsible for fibrosis. To date, the challenge of performing studies by transmission electron microscopy has hampered the study of factors that drive the pathological emperipolesis observed in myelofibrosis. We established a user-friendly confocal microscopy method that detects emperipolesis by staining with CD42b, specifically expressed on megakaryocytes, coupled with antibodies that recognize the neutrophils (Ly6b or neutrophil elastase antibody). With such an approach, we first confirmed that the bone marrow from patients with myelofibrosis and from Gata1low mice, a model of myelofibrosis, contains great numbers of neutrophils and megakaryocytes in emperipolesis. Both in patients and Gata1low mice, the emperipolesed megakaryocytes were surrounded by high numbers of neutrophils, suggesting that neutrophil chemotaxis precedes the actual emperipolesis event. Because neutrophil chemotaxis is driven by CXCL1, the murine equivalent of human interleukin 8 that is expressed at high levels by malignant megakaryocytes, we tested the hypothesis that neutrophil/megakaryocyte emperipolesis could be reduced by reparixin, an inhibitor of CXCR1/CXCR2. Indeed, the treatment greatly reduced both neutrophil chemotaxis and their emperipolesis with the megakaryocytes in treated mice. Because treatment with reparixin was previously reported to reduce both TGF-β content and marrow fibrosis, these results identify neutrophil/megakaryocyte emperipolesis as the cellular interaction that links interleukin 8 to TGF-β abnormalities in the pathobiology of marrow fibrosis.

Therapeutic activity of GARP:TGF-β1 blockade in murine primary myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm characterized by the clonal expansion of myeloid cells, notably megakaryocytes (MKs), and an aberrant cytokine production leading to bone marrow (BM) fibrosis and insufficiency. Current treatment options are limited. TGF-β1, a profibrotic and immunosuppressive cytokine, is involved in PMF pathogenesis. While all cell types secrete inactive, latent TGF-β1, only a few activate the cytokine via cell type-specific mechanisms. The cellular source of the active TGF-β1 implicated in PMF is not known. Transmembrane protein GARP binds and activates latent TGF-β1 on the surface of regulatory T lymphocytes (Tregs) and MKs or platelets. Here, we found an increased expression of GARP in the BM and spleen of mice with PMF and tested the therapeutic potential of a monoclonal antibody (mAb) that blocks TGF-β1 activation by GARP-expressing cells. GARP:TGF-β1 blockade reduced not only fibrosis but also the clonal expansion of transformed cells. Using mice carrying a genetic deletion of Garp in either Tregs or MKs, we found that the therapeutic effects of GARP:TGF-β1 blockade in PMF imply targeting GARP on Tregs. These therapeutic effects, accompanied by increased IFN-γ signals in the spleen, were lost upon CD8 T-cell depletion. Our results suggest that the selective blockade of TGF-β1 activation by GARP-expressing Tregs increases a CD8 T-cell-mediated immune reaction that limits transformed cell expansion, providing a novel approach that could be tested to treat patients with myeloproliferative neoplasms.

Preclinical studies on the use of a P-selectin-blocking monoclonal antibody to halt progression of myelofibrosis in the Gata1low mouse model

The bone marrow (BM) and spleen from patients with myelofibrosis (MF), as well as those from the Gata1low mouse model of the disease contain increased number of abnormal megakaryocytes. These cells express high levels of the adhesion receptor P-selectin on their surface, which triggers a pathologic neutrophil emperipolesis, leading to increased bioavailability of transforming growth factor-β (TGF-β) in the microenvironment and disease progression. With age, Gata1low mice develop a phenotype similar to that of patients with MF, which is the most severe of the Philadelphia-negative myeloproliferative neoplasms. We previously demonstrated that Gata1low mice lacking the P-selectin gene do not develop MF. In the current study, we tested the hypothesis that pharmacologic inhibition of P-selectin may normalize the phenotype of Gata1low mice that have already developed MF. To test this hypothesis, we have investigated the phenotype expressed by aged Gata1low mice treated with the antimouse monoclonal antibody RB40.34, alone and also in combination with ruxolitinib. The results indicated that RB40.34 in combination with ruxolitinib normalizes the phenotype of Gata1low mice with limited toxicity by reducing fibrosis and the content of TGF-β and CXCL1 (two drivers of fibrosis in this model) in the BM and spleen and by restoring hematopoiesis in the BM and the architecture of the spleen. In conclusion, we provide preclinical evidence that treatment with an antibody against P-selectin in combination with ruxolitinib may be more effective than ruxolitinib alone to treat MF in patients.

The role of inflammation in hematopoiesis and bone marrow failure: What can we learn from mouse models?

Hematopoiesis is a remarkable system that plays an important role in not only immune cell function, but also in nutrient transport, hemostasis and wound healing among other functions. Under inflammatory conditions, steady-state hematopoiesis switches to emergency myelopoiesis to give rise to the effector cell types necessary to fight the acute insult. Sustained or aberrant exposure to inflammatory signals has detrimental effects on the hematopoietic system, leading to increased proliferation, DNA damage, different forms of cell death (i.e., apoptosis, pyroptosis and necroptosis) and bone marrow microenvironment modifications. Together, all these changes can cause premature loss of hematopoiesis function. Especially in individuals with inherited bone marrow failure syndromes or immune-mediated aplastic anemia, chronic inflammatory signals may thus aggravate cytopenias and accelerate disease progression. However, the understanding of the inflammation roles in bone marrow failure remains limited. In this review, we summarize the different mechanisms found in mouse models regarding to inflammatory bone marrow failure and discuss implications for future research and clinical practice.

The Hematopoietic Function of Medicinal Wine Maoji Jiu Revealed in Blood Deficiency Model Rats

Maoji Jiu (MJ), a medicinal wine, has been used commonly by the Chinese to enrich and nourish the blood. In this study, the aim is to examine the hematopoietic function of MJ and investigate its hematopoietic regulation mechanism. Thirty-six female Sprague-Dawley rats (200 ± 20 g) were randomly divided into six groups with six rats in each group. The blood deficiency model was induced by injecting hypodermically with N-acetylphenylhydrazine (APH) and injecting intraperitoneally with cyclophosphamide (CTX), and treatment drugs were given by oral gavage twice a day for continuous 10 days from the start of the experiments. The administration of MJ improved the levels of white blood cells (WBCs), red blood cells (RBCs), hemoglobin (HGB), and hematocrit (HCT) in the blood deficiency model rats. Hematopoietic effect involves regulating the antioxidant activity in the liver and the levels of Bcl-2, Bax, erythropoietin (EPO), transforming growth factor-beta-1 (TGF-β1), and macrophage colony-stimulating factor (M-CSF) mRNA in spleen tissues to enhance extramedullary hematopoiesis. This study suggests that MJ has a beneficial effect on blood deficiency model rats.

Platelets and osteoblasts: secretome connections

Megakaryocyte hyperplasia associated with myeloproliferative neoplasms commonly leads to abnormal bone tissue deposition in the bone marrow, known as osteosclerosis. In this study, we aimed to synthesize the known proteomics literature describing factors released by megakaryocytes and platelets and to examine if any of the secreted factors have a known ability to stimulate the bone-forming cells, osteoblasts. Using a systematic search of Medline, we identified 77 articles reporting on factors secreted by platelets and megakaryocytes. After a full-text screening and analysis of the studies, we selected seven papers that reported proteomics data for factors secreted by platelets from healthy individuals. From 60 proteins reported in at least two studies, we focused on 23 that contained a putative signal peptide, which we searched for a potential osteoblast-stimulatory function. From nine proteins with a positive effect on osteoblast formation and function, two extracellular matrix (ECM) proteins, secreted protein acidic and rich in cysteine (SPARC) and tissue inhibitor of metalloproteinase-1 (TIMP1), and three cellular proteins with known extracellular function, the 70-kDa heat shock protein (HSP70), thymosin-β4 (TB4), and super dismutase (SOD), were identified as hypothetical candidate molecules to be examined as potential mediators in mouse models of osteomyelofibrosis. Thus, careful analysis of prior literature can be beneficial in assisting the planning of future experimental studies.

Thrombocytopenia in Patients With Myelofibrosis: Pathogenesis, Prevalence, Prognostic Impact, and Treatment

Myelofibrosis (MF) is a clonal hematopoietic stem cell neoplasm, characterized by pathologic myeloproliferation associated with inflammatory and pro-angiogenic cytokine release, that results in functional compromise of the bone marrow. Thrombocytopenia is a disease-related feature of MF, which portends a poor prognosis impacting overall survival (OS) and leukemia free survival. Thrombocytopenia in MF has multiple causes including ineffective hematopoiesis, splenic sequestration, and treatment-related effects. Presently, allogeneic hematopoietic stem cell transplantation (HSCT) remains the only curable treatment for MF, which, unfortunately, is only a viable option for a minority of patients. All other currently available therapies are either focused on improving cytopenias or the alleviating systemic symptoms and burdensome splenomegaly. While JAK2 inhibitors have moved to the forefront of MF therapy, available JAK inhibitors are advised against in patients with severe thrombocytopenia (platelets < 50 × 10⁹/L). In this review, we describe the pathogenesis, prevalence, and prognostic significance of thrombocytopenia in MF. We also explore the value and limitations of treatments directed at addressing cytopenias, splenomegaly and symptom burden, and those with potential disease modification. We conclude by proposing a treatment algorithm for patients with MF and severe thrombocytopenia.

Megakaryocytes Are Regulators of the Tumor Microenvironment and Malignant Hematopoietic Progenitor Cells in Myelofibrosis

Megakaryocytes (MKs) are multifunctional hematopoietic cells that produce platelets, serve as components of bone marrow (BM) niches that support the development of hematopoietic stem and progenitor cell (HSPC) and provide inflammatory signals. MKs can dynamically change their activities during homeostasis and following stress, thereby regulating hematopoietic stem cell (HSC) function. Myelofibrosis (MF) is a progressive chronic myeloproliferative neoplasm (MPN) characterized by hyperactivation of JAK/STAT signaling and MK hyperplasia, which is associated with an aberrant inflammatory signature. Since JAK1/2 inhibitor alone is incapable of depleting the malignant HSC clones or reversing BM fibrosis, the identification of mechanisms that cooperate with MF JAK/STAT signaling to promote disease progression might help in developing combination therapies to modify disease outcomes. Chronic inflammation and MK hyperplasia result in an abnormal release of TGFβ1, which plays a critical role in the pathobiology of MF by contributing to the development of BM fibrosis. Dysregulated TGFβ signaling can also alter the hematopoietic microenvironment supporting the predominance of MF-HSCs and enhance the quiescence of the reservoir of wild-type HSCs. Upregulation of TGFβ1 levels is a relatively late event in MF, while during the early pre-fibrotic stage of MF the alarmin S100A8/S100A9 heterocomplex promotes pro-inflammatory responses and sustains the progression of MF-HSCs. In this review, we will discuss the recent advances in our understanding of the roles of abnormal megakaryopoiesis, and the altered microenvironment in MF progression and the development of novel combined targeted therapies to disrupt the aberrant interplay between MKs, the BM microenvironment and malignant HSCs which would potentially limit the expansion of MF-HSC clones.

TGF-β signaling in myeloproliferative neoplasms contributes to myelofibrosis without disrupting the hematopoietic niche

Myeloproliferative neoplasms (MPNs) are associated with significant alterations in the bone marrow microenvironment that include decreased expression of key niche factors and myelofibrosis. Here, we explored the contribution of TGF-β to these alterations by abrogating TGF-β signaling in bone marrow mesenchymal stromal cells. Loss of TGF-β signaling in Osx-Cre-targeted MSCs prevented the development of myelofibrosis in both MPLW515L and Jak2V617F models of MPNs. In contrast, despite the absence of myelofibrosis, loss of TGF-β signaling in mesenchymal stromal cells did not rescue the defective hematopoietic niche induced by MPLW515L, as evidenced by decreased bone marrow cellularity, hematopoietic stem/progenitor cell number, and Cxcl12 and Kitlg expression, and the presence of splenic extramedullary hematopoiesis. Induction of myelofibrosis by MPLW515L was intact in Osx-Cre Smad4fl/fl recipients, demonstrating that SMAD4-independent TGF-β signaling mediates the myelofibrosis phenotype. Indeed, treatment with a c-Jun N-terminal kinase (JNK) inhibitor prevented the development of myelofibrosis induced by MPLW515L. Together, these data show that JNK-dependent TGF-β signaling in mesenchymal stromal cells is responsible for the development of myelofibrosis but not hematopoietic niche disruption in MPNs, suggesting that the signals that regulate niche gene expression in bone marrow mesenchymal stromal cells are distinct from those that induce a fibrogenic program.

TGF-β signaling in myeloproliferative neoplasms contributes to myelofibrosis without disrupting the hematopoietic niche

Myeloproliferative neoplasms (MPNs) are associated with significant alterations in the bone marrow microenvironment that include decreased expression of key niche factors and myelofibrosis. Here, we explored the contribution of TGF-β to these alterations by abrogating TGF-β signaling in bone marrow mesenchymal stromal cells. Loss of TGF-β signaling in Osx-Cre-targeted MSCs prevented the development of myelofibrosis in both MPLW515L and Jak2V617F models of MPNs. In contrast, despite the absence of myelofibrosis, loss of TGF-β signaling in mesenchymal stromal cells did not rescue the defective hematopoietic niche induced by MPLW515L, as evidenced by decreased bone marrow cellularity, hematopoietic stem/progenitor cell number, and Cxcl12 and Kitlg expression, and the presence of splenic extramedullary hematopoiesis. Induction of myelofibrosis by MPLW515L was intact in Osx-Cre Smad4fl/fl recipients, demonstrating that SMAD4-independent TGF-β signaling mediates the myelofibrosis phenotype. Indeed, treatment with a c-Jun N-terminal kinase (JNK) inhibitor prevented the development of myelofibrosis induced by MPLW515L. Together, these data show that JNK-dependent TGF-β signaling in mesenchymal stromal cells is responsible for the development of myelofibrosis but not hematopoietic niche disruption in MPNs, suggesting that the signals that regulate niche gene expression in bone marrow mesenchymal stromal cells are distinct from those that induce a fibrogenic program.

GLI1 activates pro-fibrotic pathways in myelofibrosis fibrocytes

Bone marrow (BM) fibrosis was thought to be induced exclusively by mesenchymal stromal cells (MSCs). However, we and others found that neoplastic fibrocytes induce BM fibrosis in myelofibrosis (MF). Because glioma-associated oncogene-1 (GLI1), an effector of the Hedgehog pathway, plays a role in the induction of BM fibrosis, we wondered whether GLI1 affects fibrocyte-induced BM fibrosis in MF. Multiplexed fluorescence immunohistochemistry analysis of MF patients' BM detected high levels of GLI1 in MF fibrocytes compared to MSCs or normal fibrocytes. Immunostaining, RNA in situ hybridization, gene expression analysis, and western immunoblotting detected high levels of GLI1 and GLI1-induced matrix metalloproteases (MMP) 2 and 9 in MF patients BM-derived cultured fibrocytes. Similarly, MF patients' BM-derived GLI1+ fibrocytes were found in BMs and spleens of MF xenograft mice. GLI1 silencing reduced the levels of MMP2/9, phosphorylated SMAD2/3, and procollagen-I, and knockdown or inhibition of GLI1 decreased fibrocyte formation and induced apoptosis of both fibrocytes and fibrocyte progenitors. Because Janus kinase (JAK)2-induced STAT3 is constitutively activated in MF and because STAT3 induces GLI1 expression, we sought to determine whether STAT3 activates GLI1 in MF fibrocytes. Imaging analysis detected phosphotyrosine STAT3 in MF patients' BM fibrocytes, and transfection of fibrocytes with STAT3-siRNA or treatment with a JAK1/2 inhibitor ruxolitinib reduced GLI1 and MMP2/9 levels. Chromatin immunoprecipitation and a luciferase assay revealed that STAT3 induced the expression of the GLI1 gene in both MF BM fibrocytes and fibrocyte progenitors. Together, our data suggest that STAT3-activated GLI1 contributes to the induction of BM fibrosis in MF.

The CXCR1/CXCR2 Inhibitor Reparixin Alters the Development of Myelofibrosis in the Gata1 low Mice

A major role for human (h)CXCL8 (interleukin-8) in the pathobiology of myelofibrosis (MF) has been suggested by observations indicating that MF megakaryocytes express increased levels of hCXCL8 and that plasma levels of this cytokine in MF patients are predictive of poor patient outcomes. Here, we demonstrate that, in addition to high levels of TGF-β, the megakaryocytes from the bone marrow of the Gata1 low mouse model of myelofibrosis express high levels of murine (m)CXCL1, the murine equivalent of hCXCL8, and its receptors CXCR1 and CXCR2. Treatment with the CXCR1/R2 inhibitor, Reparixin in aged-matched Gata1 low mice demonstrated reductions in bone marrow and splenic fibrosis. Of note, the levels of fibrosis detected using two independent methods (Gomori and reticulin staining) were inversely correlated with plasma levels of Reparixin. Immunostaining of marrow sections indicated that the bone marrow from the Reparixin-treated group expressed lower levels of TGF-β1 than those expressed by the bone marrow from vehicle-treated mice while the levels of mCXCL1, and expression of CXCR1 and CXCR2, were similar to that of vehicle-treated mice. Moreover, immunofluorescence analyses performed on bone marrow sections from Gata1 low mice indicated that treatment with Reparixin induced expression of GATA1 while reducing expression of collagen III in megakaryocytes. These data suggest that in Gata1low mice, Reparixin reduces fibrosis by reducing TGF-β1 and collagen III expression while increasing GATA1 in megakaryocytes. Our results provide a preclinical rationale for further evaluation of this drug alone and in combination with current JAK inhibitor therapy for the treatment of patients with myelofibrosis.

Genetic ablation of Pim1 or pharmacologic inhibition with TP-3654 ameliorates myelofibrosis in murine models

Myelofibrosis (MF) is the deadliest form of myeloproliferative neoplasm (MPN). The JAK inhibitor Ruxolitinib can reduce constitutional symptoms but it does not substantially improve bone marrow fibrosis. Pim1 expression is significantly elevated in MPN/MF hematopoietic progenitors. Here, we show that genetic ablation of Pim1 blocked the development of myelofibrosis induced by Jak2V617F and MPLW515L. Pharmacologic inhibition of Pim1 with a second-generation Pim kinase inhibitor TP-3654 significantly reduced leukocytosis and splenomegaly, and attenuated bone marrow fibrosis in Jak2V617F and MPLW515L mouse models of MF. Combined treatment of TP-3654 and Ruxolitinib resulted in greater reduction of spleen size, normalization of blood leukocyte counts and abrogation of bone marrow fibrosis in murine models of MF. TP-3654 treatment also preferentially inhibited Jak2V617F mutant hematopoietic progenitors in mice. Mechanistically, we show that TP-3654 treatment significantly inhibits mTORC1, MYC and TGF-β signaling in Jak2V617F mutant hematopoietic cells and diminishes the expression of fibrotic markers in the bone marrow. Collectively, our results suggest that Pim1 plays an important role in the pathogenesis of MF, and inhibition of Pim1 with TP-3654 might be useful for treatment of MF.

Tanshinone IIA: New Perspective on the Anti-Tumor Mechanism of A Traditional Natural Medicine

The search for natural and efficacious antineoplastic drugs, with minimal toxicity and side effects, is an important part of antitumor drug research and development. Tanshinone IIA is the most evaluated lipophilic active component of Salvia miltiorrhiza. Tanshinone IIA is a path-breaking traditional drug applied in cardiovascular treatment. It has also been found that tanshinone IIA plays an important role in the digestive, respiratory and circulatory systems, as well as in other tumor diseases. Tanshinone IIA significantly inhibits the proliferation of several types of tumors, blocks the cell cycle, induces apoptosis and autophagic death, in addition to inhibiting cell migration and invasion. Among these, the regulation of tumor-cell apoptosis signaling pathways is the key breakthrough point in several modes of antitumor therapy. The PI3K/AKT/MTOR signaling pathway and the JNK pathway are the key pathways for tanshinone IIA to induce tumor cell apoptosis. In addition to glycolysis, reactive oxygen species and signal transduction all play an active role with the participation of tanshinone IIA. Endogenous apoptosis is considered the main mechanism of tumor apoptosis induced by tanshinone IIA. Multiple pathways and targets play a role in the process of endogenous apoptosis. Tanshinone IIA can protect chemotherapy drugs, which is mainly reflected in the protection of the side effects of chemotherapy drugs, such as neurotoxicity and inhibition of the hematopoietic system. Tanshinone IIA also has a certain regulatory effect on tumor angiogenesis, which is mainly manifested in the control of hypoxia. Our findings indicated that tanshinone IIA is an effective treatment agent in the cardiovascular field and plays a significant role in antitumor therapeutics. This paper reviews the pharmacological potential and inhibitory effect of tanshinone IIA on cancer. It is greatly anticipated that tanshinone IIA will be employed as an adjuvant in the treatment of various cancers.

Resident Self-Tissue of Proinflammatory Cytokines Rather than Their Systemic Levels Correlates with Development of Myelofibrosis in Gata1low Mice

Serum levels of inflammatory cytokines are currently investigated as prognosis markers in myelofibrosis, the most severe Philadelphia-negative myeloproliferative neoplasm. We tested this hypothesis in the Gata1low model of myelofibrosis. Gata1low mice, and age-matched wild-type littermates, were analyzed before and after disease onset. We assessed cytokine serum levels by Luminex-bead-assay and ELISA, frequency and cytokine content of stromal cells by flow cytometry, and immunohistochemistry and bone marrow (BM) localization of GFP-tagged hematopoietic stem cells (HSC) by confocal microscopy. Differences in serum levels of 32 inflammatory-cytokines between prefibrotic and fibrotic Gata1low mice and their wild-type littermates were modest. However, BM from fibrotic Gata1low mice contained higher levels of lipocalin-2, CXCL1, and TGF-β1 than wild-type BM. Although frequencies of endothelial cells, mesenchymal cells, osteoblasts, and megakaryocytes were higher than normal in Gata1low BM, the cells which expressed these cytokines the most were malignant megakaryocytes. This increased bioavailability of proinflammatory cytokines was associated with altered HSC localization: Gata1low HSC were localized in the femur diaphysis in areas surrounded by microvessels, neo-bones, and megakaryocytes, while wild-type HSC were localized in the femur epiphysis around adipocytes. In conclusion, bioavailability of inflammatory cytokines in BM, rather than blood levels, possibly by reshaping the HSC niche, correlates with myelofibrosis in Gata1low mice.

Megakaryocyte-driven changes in bone health: lessons from mouse models of myelofibrosis and related disorders

Over the years, numerous studies demonstrated reciprocal communications between processes of bone marrow hematopoiesis and bone remodeling. Megakaryocytes, rare bone marrow cells responsible for platelet production, were demonstrated to be involved in bone homeostasis. Myelofibrosis, characterized by an increase in pleomorphic megakaryocytes in the bone marrow, commonly leads to the development of osteosclerosis. In vivo, an increase in megakaryocyte number was shown to result in osteosclerosis in GATA-1^low, NF-E2^-/-, TPO^high, Mpll^f/f;PF4cre, Lnk^-/-, Mpig6b^-/-, Mpig6b^fl/fl;Gp1ba-Cr^+/KI, Pt-vWD mouse models. In vitro, megakaryocytes stimulate osteoblast proliferation and have variable effects on osteoclast proliferation and activity through soluble factors and direct cell-cell communications. Intriguingly, new studies revealed that the ability of megakaryocytes to communicate with bone cells is affected by the age and sex of animals. This mini-review summarises changes seen in bone architecture and bone cell function in mouse models with an elevated number of megakaryocytes and the effects megakaryocytes have on osteoblasts and osteoclasts in vitro, and discusses potential molecular players that can mediate these effects.

A Novel Megakaryocyte Subpopulation Poised to Exert the Function of HSC Niche as Possible Driver of Myelofibrosis

Careful morphological investigations, coupled with experimental hematology studies in animal models and in in vitro human cultures, have identified that platelets are released in the circulation by mature megakaryocytes generated by hematopoietic stem cells by giving rise to lineage-restricted progenitor cells and then to morphologically recognizable megakaryocyte precursors, which undergo a process of terminal maturation. Advances in single cell profilings are revolutionizing the process of megakaryocytopoiesis as we have known it up to now. They identify that, in addition to megakaryocytes responsible for producing platelets, hematopoietic stem cells may generate megakaryocytes, which exert either immune functions in the lung or niche functions in organs that undergo tissue repair. Furthermore, it has been discovered that, in addition to hematopoietic stem cells, during ontogeny, and possibly in adult life, megakaryocytes may be generated by a subclass of specialized endothelial precursors. These concepts shed new light on the etiology of myelofibrosis, the most severe of the Philadelphia negative myeloproliferative neoplasms, and possibly other disorders. This perspective will summarize these novel concepts in thrombopoiesis and discuss how they provide a framework to reconciliate some of the puzzling data published so far on the etiology of myelofibrosis and their implications for the therapy of this disease.

Role of β1 integrin in thrombocytopoiesis

Thrombocytopoiesis is a complex process beginning at the level of hematopoietic stem cells, which ultimately generate megakaryocytes, large marrow cells with a distinctive morphology, and then, through a process of terminal maturation, megakaryocytes shed thousands of platelets into the circulation. This process is controlled by intrinsic and extrinsic factors. Emerging data indicate that an important intrinsic control on the late stages of thrombopoiesis is exerted by integrins, a family of transmembrane receptors composed of one α and one β subunit. One β subunit expressed by megakaryocytes is the β1 integrin, the role of which in the regulation of platelet formation is beginning to be clarified. Here, we review recent data indicating that activation of β1 integrin by outside-in and inside-out signaling regulates the interaction of megakaryocytes with the endosteal niche, which triggers their maturation, while its inactivation by galactosylation determines the migration of these cells to the perivascular niche, where they complete their terminal maturation and release platelets in the bloodstream. Furthermore, β1 integrin mediates the activation of transforming growth factor β (TGF-β), a protein produced by megakaryocytes that may act in an autocrine fashion to halt their maturation and affect the composition of their surrounding extracellular matrix. These findings suggest that β1 integrin could be a therapeutic target for inherited and acquired disorders of platelet production.

Myeloproliferative Disorders and its Effect on Bone Homeostasis: The Role of Megakaryocytes

Myeloproliferative Neoplasms (MPNs) are a heterogeneous group of chronic hematological diseases that arise from the clonal expansion of abnormal hematopoietic stem cells, of which Polycythemia Vera (PV), Essential Thrombocythemia (ET), and Primary Myelofibrosis (PMF) have been extensively reviewed in context of clonal expansion, fibrosis and other phenotypes. Here, we review current knowledge on the influence of different forms of MPN on bone health. Studies implicated various degrees of effect of different forms of MPN on bone density, and on osteoblast proliferation and differentiation, using murine models and human data. The majority of studies show that bone volume is generally increased in PMF patients, whereas it is slightly decreased or not altered in ET and PV patients, although possible differences between male and female phenotypes were not fully explored in most MPN forms. Osteosclerosis seen in PMF patients is a serious complication that can lead to bone marrow failure, and the loss of bone reported in some ET and PV patients can lead to osteoporotic fractures. Some MPN forms are associated with increased number of megakaryocytes (MKs), and several of the MK-associated factors in MPN are known to affect bone development. Here, we review known mechanisms involved in these processes, with focus on the role of MKs and secreted factors. Understanding MPN-associated changes in bone health could improve early intervention and treatment of this side effect of the pathology.

CDK6 Is a Therapeutic Target in Myelofibrosis

Myelofibrosis (myelofibrosis) is a deadly blood neoplasia with the worst prognosis among myeloproliferative neoplasms (MPN). The JAK2 inhibitors ruxolitinib and fedratinib have been approved for treatment of myelofibrosis, but they do not offer significant improvement of bone marrow fibrosis. CDK6 expression is significantly elevated in MPN/myelofibrosis hematopoietic progenitor cells. In this study, we investigated the efficacy of CDK4/6 inhibitor palbociclib alone or in combination with ruxolitinib in Jak2V617F and MPLW515L murine models of myelofibrosis. Treatment with palbociclib alone significantly reduced leukocytosis and splenomegaly and inhibited bone marrow fibrosis in Jak2V617F and MPLW515L mouse models of myelofibrosis. Combined treatment of palbociclib and ruxolitinib resulted in normalization of peripheral blood leukocyte counts, marked reduction of spleen size, and abrogation of bone marrow fibrosis in murine models of myelofibrosis. Palbociclib treatment also preferentially inhibited Jak2V617F mutant hematopoietic progenitors in mice. Mechanistically, treatment with palbociclib or depletion of CDK6 inhibited Aurora kinase, NF-κB, and TGFβ signaling pathways in Jak2V617F mutant hematopoietic cells and attenuated expression of fibrotic markers in the bone marrow. Overall, these data suggest that palbociclib in combination with ruxolitinib may have therapeutic potential for treatment of myelofibrosis and support the clinical investigation of this drug combination in patients with myelofibrosis. SIGNIFICANCE: These findings demonstrate that CDK6 inhibitor palbociclib in combination with ruxolitinib ameliorates myelofibrosis, suggesting this drug combination could be an effective therapeutic strategy against this devastating blood disorder.

TGFβ1 protein trap AVID200 beneficially affects hematopoiesis and bone marrow fibrosis in myelofibrosis

Myelofibrosis (MF) is a progressive chronic myeloproliferative neoplasm characterized by hyperactivation of JAK/STAT signaling and dysregulation of the transcription factor GATA1 in megakaryocytes (MKs). TGF-β plays a pivotal role in the pathobiology of MF by promoting BM fibrosis and collagen deposition and by enhancing the dormancy of normal hematopoietic stem cells (HSCs). In this study, we show that MF-MKs elaborated significantly greater levels of TGF-β1 than TGF-β2 and TGF-β3 to a varying degree, and we evaluated the ability of AVID200, a potent TGF-β1/TGF-β3 protein trap, to block the excessive TGF-β signaling. Treatment of human mesenchymal stromal cells with AVID200 significantly reduced their proliferation, decreased phosphorylation of SMAD2, and interfered with the ability of TGF-β1 to induce collagen expression. Moreover, treatment of MF mononuclear cells with AVID200 led to increased numbers of progenitor cells (PCs) with WT JAK2 rather than mutated JAK2V617F. This effect of AVID200 on MF PCs was attributed to its ability to block TGF-β1–induced p57^Kip2 expression and SMAD2 activation, thereby allowing normal rather than MF PCs to preferentially proliferate and form hematopoietic colonies. To assess the in vivo effects of AVID200, Gata1^lo mice, a murine model of MF, were treated with AVID200, resulting in the reduction in BM fibrosis and an increase in BM cellularity. AVID200 treatment also increased the frequency and numbers of murine progenitor cells as well as short-term and long-term HSCs. Collectively, these data provide the rationale for TGF-β1 blockade, with AVID200 as a therapeutic strategy for patients with MF.

Lessons from mouse models of MPN

Over the past decades, a variety of MPN mouse models have been developed to express in HSC the main mutations identified in patients: JAK2^V617F, CALRdel52 or ins5 and MPL^W515L. These models mimic quite faithfully human PV or ET with their natural evolutions into MF and their hemostasis complications, demonstrating the driver function of these mutations in MPN. Here, we review these models and show how they have improved our general understanding of MPN regarding (1) the mechanisms of fibrosis, thrombosis/hemorrhages and disease initiation, (2) the roles of additional mutations and signaling pathways in disease progression and (3) the preclinical development of novel therapies. We also address controversial results between these models and remind how these models may differ from human MPN onset and also how basically mice are not humans, encouraging caution when one draw lessons from mice to humans. Furthermore, the contribution of germline genetic predisposition, HSC and niche aging, metabolic, oxidative, replicative or genotoxic stress, inflammation, immune escape and additional mutations need to be considered in further investigations to encompass the full complexity of human MPN in mice.

The Role of Megakaryocytes in Myelofibrosis

Megakaryocytes give rise to platelets, which have a wide variety of functions in coagulation, immune response, inflammation, and tissue repair. Dysregulation of megakaryocytes is a key feature of in the myeloproliferative neoplasms, especially myelofibrosis. Megakaryocytes are among the main drivers of myelofibrosis by promoting myeloproliferation and bone marrow fibrosis. In vivo targeting of megakaryocytes by genetic and pharmacologic approaches ameliorates the disease, underscoring the important role of megakaryocytes in myeloproliferative neoplasms. Here we review the current knowledge of the function of megakaryocytes in the JAK2, CALR, and MPL-mutant myeloproliferative neoplasms.

Current Clinical Investigations in Myelofibrosis

The US Food and Drug Administration (FDA) approval of Janus kinase 2 inhibitors, ruxolitinib and fedratinib for the treatment of intermediate-2 or high-risk primary or secondary myelofibrosis (MF) has revolutionized the management of MF. Nevertheless, these drugs do not reliably alter the natural history of disease. Burgeoning understanding of the molecular pathogenesis and the bone marrow microenvironment in MF has galvanized the development of targeted therapeutics. This review provides insight into the novel therapies under clinical evaluation.

Inhibition of TGFβ improves hematopoietic stem cell niche and ameliorates cancer-related anemia

Background

Cancer cachexia is a wasting syndrome that is quite common in terminal-stage cancer patients. Cancer-related anemia is one of the main features of cancer cachexia and mostly results in a poor prognosis. The disadvantages of the current therapies are obvious, but few new treatments have been developed because the pathological mechanism remains unclear.

Methods

C57BL/6 mice were subcutaneously injected with Lewis lung carcinoma cells to generate a cancer-related anemia model. The treated group received daily intraperitoneal injections of SB505124. Blood parameters were determined with a routine blood counting analyzer. Erythroid cells and hematopoietic stem/progenitor cells were analyzed by flow cytometry. The microarchitecture changes of the femurs were determined by micro-computed tomography scans. Smad2/3 phosphorylation was analyzed by immunofluorescence and Western blotting. The changes in the hematopoietic stem cell niche were revealed by qPCR analysis of both fibrosis-related genes and hematopoietic genes, fibroblastic colony-forming unit assays, and lineage differentiation of mesenchymal stromal cells.

Results

The mouse model exhibited hematopoietic suppression, marked by a decrease of erythrocytes in the peripheral blood, as well as an increase of immature erythroblasts and reduced differentiation of multipotent progenitors in the bone marrow. The ratio of bone volume/total volume, trabecular number, and cortical wall thickness all appeared to decrease, and the increased osteoclast number has led to the release of latent TGFβ and TGFβ signaling over-activation. Excessive TGFβ deteriorated the hematopoietic stem cell niche, inducing fibrosis of the bone marrow as well as the transition of mesenchymal stromal cells. Treatment with SB505124, a small-molecule inhibitor of TGFβ signaling, significantly attenuated the symptoms of cancer-related anemia in this model, as evidenced by the increase of erythrocytes in the peripheral blood and the normalized proportion of erythroblast cell clusters. Meanwhile, hindered hematopoiesis and deteriorated hematopoietic stem cell niche were also shown to be restored with SB505124 treatment.

Conclusion

This study investigated the role of TGFβ released by bone remodeling in the progression of cancer-related anemia and revealed a potential therapeutic approach for relieving defects in hematopoiesis.

Genetics and Pathogenetic Role of Inflammasomes in Philadelphia Negative Chronic Myeloproliferative Neoplasms: A Narrative Review

The last decade has been very important for the quantity of preclinical information obtained regarding chronic myeloproliferative neoplasms (MPNs) and the following will be dedicated to the translational implications of the new biological acquisitions. The overcoming of the mechanistic model of clonal evolution and the entry of chronic inflammation and dysimmunity into the new model are the elements on which to base a part of future therapeutic strategies. The innate immune system plays a major role in this context. Protagonists of the initiation and regulation of many pathological aspects, from cytokine storms to fibrosis, the NLRP3 and AIM2 inflammasomes guide and condition the natural history of the disease. For this reason, MPNs share many biological and clinical aspects with non-neoplastic diseases, such as autoimmune disorders. Finally, cardiovascular risk and disturbances in iron metabolism and myelopoiesis are also closely linked to the role of inflammasomes. Although targeted therapies are already being tested, an increase in knowledge on the subject is desirable and potentially translates into better care for patients with MPNs.

Novel Concepts of Treatment for Patients with Myelofibrosis and Related Neoplasms

Simple Summary

Myelofibrosis (MF) is an advanced form of a group of rare, related bone marrow cancers termed myeloproliferative neoplasms (MPNs). Some patients develop myelofibrosis from the outset, while in others, it occurs as a complication of the more indolent MPNs, polycythemia vera (PV) or essential thrombocythemia (ET). Patients with PV or ET who require drug treatment are typically treated with the chemotherapy drug hydroxyurea, while in MF, the targeted therapies termed Janus kinase (JAK) inhibitors form the mainstay of treatment. However, these and other drugs (e.g., interferons) have important limitations. No drug has been shown to reliably prevent the progression of PV or ET to MF or transformation of MPNs to acute myeloid leukemia. In PV, it is not conclusively known if JAK inhibitors reduce the risk of blood clots, and in MF, these drugs do not improve low blood counts. New approaches to treating MF and related MPNs are, therefore, necessary.

Abstract

Janus kinase (JAK) inhibition forms the cornerstone of the treatment of myelofibrosis (MF), and the JAK inhibitor ruxolitinib is often used as a second-line agent in patients with polycythemia vera (PV) who fail hydroxyurea (HU). In addition, ruxolitinib continues to be studied in patients with essential thrombocythemia (ET). The benefits of JAK inhibition in terms of splenomegaly and symptoms in patients with MF are undeniable, and ruxolitinib prolongs the survival of persons with higher risk MF. Despite this, however, “disease-modifying” effects of JAK inhibitors in MF, i.e., bone marrow fibrosis and mutant allele burden reduction, are limited. Similarly, in HU-resistant/intolerant PV, while ruxolitinib provides excellent control of the hematocrit, symptoms and splenomegaly, reduction in the rate of thromboembolic events has not been convincingly demonstrated. Furthermore, JAK inhibitors do not prevent disease evolution to MF or acute myeloid leukemia (AML). Frontline cytoreductive therapy for PV generally comprises HU and interferons, which have their own limitations. Numerous novel agents, representing diverse mechanisms of action, are in development for the treatment of these three classic myeloproliferative neoplasms (MPNs). JAK inhibitor-based combinations, all of which are currently under study for MF, have been covered elsewhere in this issue. In this article, we focus on agents that have been studied as monotherapy in patients with MF, generally after JAK inhibitor resistance/intolerance, as well as several novel compounds in development for PV/ET.

Cytokine Profiling in Myeloproliferative Neoplasms: Overview on Phenotype Correlation, Outcome Prediction, and Role of Genetic Variants

Among hematologic malignancies, the classic Philadelphia-negative chronic myeloproliferative neoplasms (MPNs) are considered a model of inflammation-related cancer development. In this context, the use of immune-modulating agents has recently expanded the MPN therapeutic scenario. Cytokines are key mediators of an auto-amplifying, detrimental cross-talk between the MPN clone and the tumor microenvironment represented by immune, stromal, and endothelial cells. This review focuses on recent advances in cytokine-profiling of MPN patients, analyzing different expression patterns among the three main Philadelphia-negative (Ph-negative) MPNs, as well as correlations with disease molecular profile, phenotype, progression, and outcome. The role of the megakaryocytic clone as the main source of cytokines, particularly in myelofibrosis, is also reviewed. Finally, we report emerging intriguing evidence on the contribution of host genetic variants to the chronic pro-inflammatory state that typifies MPNs.

Cytokine Profiling in Myeloproliferative Neoplasms: Overview on Phenotype Correlation, Outcome Prediction, and Role of Genetic Variants

Among hematologic malignancies, the classic Philadelphia-negative chronic myeloproliferative neoplasms (MPNs) are considered a model of inflammation-related cancer development. In this context, the use of immune-modulating agents has recently expanded the MPN therapeutic scenario. Cytokines are key mediators of an auto-amplifying, detrimental cross-talk between the MPN clone and the tumor microenvironment represented by immune, stromal, and endothelial cells. This review focuses on recent advances in cytokine-profiling of MPN patients, analyzing different expression patterns among the three main Philadelphia-negative (Ph-negative) MPNs, as well as correlations with disease molecular profile, phenotype, progression, and outcome. The role of the megakaryocytic clone as the main source of cytokines, particularly in myelofibrosis, is also reviewed. Finally, we report emerging intriguing evidence on the contribution of host genetic variants to the chronic pro-inflammatory state that typifies MPNs.

Murine Models of Myelofibrosis

Myelofibrosis (MF) is subtype of myeloproliferative neoplasm (MPN) characterized by a relatively poor prognosis in patients. Understanding the factors that drive MF pathogenesis is crucial to identifying novel therapeutic approaches with the potential to improve patient care. Driver mutations in three main genes (janus kinase 2 (JAK2), calreticulin (CALR), and myeloproliferative leukemia virus oncogene (MPL)) are recurrently mutated in MPN and are sufficient to engender MPN using animal models. Interestingly, animal studies have shown that the underlying molecular mutation and the acquisition of additional genetic lesions is associated with MF outcome and transition from early stage MPN such as essential thrombocythemia (ET) and polycythemia vera (PV) to secondary MF. In this issue, we review murine models that have contributed to a better characterization of MF pathobiology and identification of new therapeutic opportunities in MPN.

Haematopoietic stem cell activity and interactions with the niche

The haematopoietic stem cell (HSC) microenvironment in the bone marrow, termed the niche, ensures haematopoietic homeostasis by controlling the proliferation, self-renewal, differentiation and migration of HSCs and progenitor cells at steady state and in response to emergencies and injury. Improved methods for HSC isolation, driven by advances in single-cell and molecular technologies, have led to a better understanding of their behaviour, heterogeneity and lineage fate and of the niche cells and signals that regulate their function. Niche regulatory signals can be in the form of cell-bound or secreted factors and other local physical cues. A combination of technological advances in bone marrow imaging and genetic manipulation of crucial regulatory factors has enabled the identification of several candidate cell types regulating the niche, including both non-haematopoietic (for example, perivascular mesenchymal stem and endothelial cells) and HSC-derived (for example, megakaryocytes, macrophages and regulatory T cells), with better topographical understanding of HSC localization in the bone marrow. Here, we review advances in our understanding of HSC regulation by niches during homeostasis, ageing and cancer, and we discuss their implications for the development of therapies to rejuvenate aged HSCs or niches or to disrupt self-reinforcing malignant niches.

Novel targets to cure primary myelofibrosis from studies on Gata1low mice

In 2002, we discovered that mice carrying the hypomorphic Gata1^low mutation that reduces expression of the transcription factor GATA1 in megakaryocytes (Gata1^low mice) develop myelofibrosis, a phenotype that recapitulates the features of primary myelofibrosis (PMF), the most severe of the Philadelphia-negative myeloproliferative neoplasms (MPNs). At that time, this discovery had a great impact on the field because mutations driving the development of PMF had yet to be discovered. Later studies identified that PMF, as the others MPNs, is associated with mutations activating the thrombopoietin/JAK2 axis raising great hope that JAK inhibitors may be effective to treat the disease. Unfortunately, ruxolitinib, the JAK1/2 inhibitor approved by FDA and EMEA for PMF, ameliorates symptoms but does not improve the natural course of the disease, and the cure of PMF is still an unmet clinical need. Although GATA1 is not mutated in PMF, reduced GATA1 content in megakaryocytes as a consequence of ribosomal deficiency is a hallmark of myelofibrosis (both in humans and mouse models) and, in fact, a driving event in the disease. Conversely, mice carrying the hypomorphic Gata1^low mutation express an activated TPO/JAK2 pathway and partially respond to JAK inhibitors in a fashion similar to PMF patients (reduction of spleen size but limited improvement of the natural history of the disease). These observations cross-validated Gata1^low mice as a bona fide animal model for PMF and prompted the use of this model to identify abnormalities that might be targeted to cure the disease. We will summarize here data generated in Gata1^low mice indicating that the TGF-β/P-selectin axis is abnormal in PMF and represents a novel target for its treatment.

Shared and Tissue-Specific Expression Signatures between Bone Marrow from Primary Myelofibrosis and Essential Thrombocythemia

Megakaryocytes have been implicated in the micro-environmental abnormalities associated with fibrosis and hematopoietic failure in the bone marrow (BM) of primary myelofibrosis (PMF) patients, the Philadelphia-negative myeloproliferative neoplasm (MPN) associated with the poorest prognosis. To identify possible therapeutic targets for restoring BM functions in PMF, we compared the expression profiling of PMF BM with that of BM from essential thrombocytopenia (ET), a fibrosis-free MPN also associated with BM megakaryocyte hyperplasia. The signature of PMF BM was also compared with published signatures associated with liver and lung fibrosis. Gene set enrichment analysis (GSEA) identified distinctive differences between the expression profiles of PMF and ET. Notch, K-Ras, IL-8, and apoptosis pathways were altered the most in PMF as compared with controls. By contrast, cholesterol homeostasis, unfolded protein response, and hypoxia were the pathways found altered to the greatest degree in ET compared with control specimens. BM from PMF expressed a noncanonical transforming growth factor β (TGF-β) signature, which included activation of ID1, JUN, GADD45b, and genes with binding motifs for the JUN transcriptional complex AP1. By contrast, the expression of ID1 and GADD45b was not altered and there was a modest signal for JUN activation in ET. The similarities among PMF, liver fibrosis, and lung fibrosis were modest and included activation of integrin-α9 and tropomyosin-α1 between PMF and liver fibrosis, and of ectoderm-neural cortex protein 1 and FRAS1-related extracellular matrix protein 1 between PMF and lung fibrosis, but not TGF-β. These data identify TGF-β as a potential target for micro-environmental therapy in PMF.

Advances in potential treatment options for myeloproliferative neoplasm associated myelofibrosis

INTRODUCTION

The Janus kinase (JAK)1/2 inhibitor ruxolitinib provides rapid, sustained and often dramatic benefits to patients with myelofibrosis, inducing spleen shrinkage and ameliorating symptoms, and improves survival. However, the drug has little effect on the underlying bone marrow fibrosis or on mutant allele burden, and clinical resistance eventually develops. Furthermore, ruxolitinib-induced cytopenias can be challenging in everyday practice.

AREAS COVERED

The developmental therapeutics landscape in MF is discussed. This includes potential partners for ruxolitinib being developed with an aim to improve cytopenias, or to enhance its disease-modifying effects. The development of other JAK inhibitors with efficacy post-ruxolitinib or other unique attributes is being pursued in earnest. Agents with novel mechanisms of action are being studied in patients whose disease responds sub-optimally to, is refractory to or progresses after ruxolitinib.

EXPERT OPINION

The JAK inhibitors fedratinib, pacritinib and momelotinib are clearly active, and it is expected that one or more of these will become licensed in the future. The activin receptor ligand traps are promising as treatments for anemia. Imetelstat has shown interesting activity post-ruxolitinib, and azactidine may be a useful partner for ruxolitinib in some patients. Appropriately, multiple pre-clinical and clinical leads are being pursued in this difficult therapeutic area.

Novel strategies for the treatment of myelofibrosis driven by recent advances in understanding the role of the microenvironment in its etiology

Myelofibrosis is the advanced stage of the Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), characterized by systemic inflammation, hematopoietic failure in the bone marrow, and development of extramedullary hematopoiesis, mainly in the spleen. The only potentially curative therapy for this disease is hematopoietic stem cell transplantation, an option that may be offered only to those patients with a compatible donor and with an age and functional status that may face its toxicity. By contrast, with the Philadelphia-positive MPNs that can be dramatically modified by inhibitors of the novel BCR-ABL fusion-protein generated by its genetic lesion, the identification of the molecular lesions that lead to the development of myelofibrosis has not yet translated into a treatment that can modify the natural history of the disease. Therefore, the cure of myelofibrosis remains an unmet clinical need. However, the excitement raised by the discovery of the genetic lesions has inspired additional studies aimed at elucidating the mechanisms driving these neoplasms towards their final stage. These studies have generated the feeling that the cure of myelofibrosis will require targeting both the malignant stem cell clone and its supportive microenvironment. We will summarize here some of the biochemical alterations recently identified in MPNs and the novel therapeutic approaches currently under investigation inspired by these discoveries.

EDA fibronectin-TLR4 axis sustains megakaryocyte expansion and inflammation in bone marrow fibrosis

The fibronectin EDA isoform sustains bone marrow fibrosis, binding to TLR4 on megakaryocytes and inducing NF-κB activation and IL-6 release. In primary myelofibrosis patients, the bone marrow fibrosis correlates with increased levels of fibronectin EDA isoform in plasma.

The fibronectin EDA isoform (EDA FN) is instrumental in fibrogenesis but, to date, its expression and function in bone marrow (BM) fibrosis have not been explored. We found that mice constitutively expressing the EDA domain (EIIIA^+/+), but not EDA knockout mice, are more prone to develop BM fibrosis upon treatment with the thrombopoietin (TPO) mimetic romiplostim (TPO^high). Mechanistically, EDA FN binds to TLR4 and sustains progenitor cell proliferation and megakaryopoiesis in a TPO-independent fashion, inducing LPS-like responses, such as NF-κB activation and release of profibrotic IL-6. Pharmacological inhibition of TLR4 or TLR4 deletion in TPO^high mice abrogated Mk hyperplasia, BM fibrosis, IL-6 release, extramedullary hematopoiesis, and splenomegaly. Finally, developing a novel ELISA assay, we analyzed samples from patients affected by primary myelofibrosis (PMF), a well-known pathological situation caused by altered TPO signaling, and found that the EDA FN is increased in plasma and BM biopsies of PMF patients as compared with healthy controls, correlating with fibrotic phase.

Megakaryocyte Contribution to Bone Marrow Fibrosis: many Arrows in the Quiver

In Primary Myelofibrosis (PMF), megakaryocyte dysplasia/hyperplasia determines the release of inflammatory cytokines that, in turn, stimulate stromal cells and induce bone marrow fibrosis. The pathogenic mechanism and the cells responsible for progression to bone marrow fibrosis in PMF are not completely understood. This review article aims to provide an overview of the crucial role of megakaryocytes in myelofibrosis by discussing the role and the altered secretion of megakaryocyte-derived soluble factors, enzymes and extracellular matrices that are known to induce bone marrow fibrosis.

Serum of myeloproliferative neoplasms stimulates hematopoietic stem and progenitor cells

Background

Myeloproliferative neoplasms (MPN)—such as polycythemia vera (PV), essential thrombocythemia (ET), and myelofibrosis (MF)—are typically diseases of the elderly caused by acquired somatic mutations. However, it is largely unknown how the malignant clone interferes with normal hematopoiesis. In this study, we analyzed if serum of MPN patients comprises soluble factors that impact on hematopoietic stem and progenitor cells (HPCs).

Methods

CD34⁺ HPCs were cultured in medium supplemented with serum samples of PV, ET, or MF patients, or healthy controls. The impact on proliferation, maintenance of immature hematopoietic surface markers, and colony forming unit (CFU) potential was systematically analyzed. In addition, we compared serum of healthy young (<25 years) and elderly donors (>50 years) to determine how normal aging impacts on the hematopoiesis-supportive function of serum.

Results

Serum from MF, PV and ET patients significantly increased proliferation as compared to controls. In addition, serum from MF and ET patients attenuated the loss of a primitive immunophenotype during in vitro culture. The CFU counts were significantly higher if HPCs were cultured with serum of MPN patients as compared to controls. Furthermore, serum of healthy young versus old donors did not evoke significant differences in proliferation or immunophenotype of HPCs, whereas the CFU frequency was significantly increased by serum from elderly patients.

Conclusion

Our results indicate that serum derived from patients with MPN comprises activating feedback signals that stimulate the HPCs–and this stimulatory signal may result in a viscous circle that further accelerates development of the disease.

Regulation of Malignant Hematopoiesis by Bone Marrow Microenvironment

Hematopoietic stem cells (HSCs) that give rise to all kinds of hematopoietic lineage cells on various demands throughout life are maintained in a specialized microenvironment called “niche” in the bone marrow (BM). Defining niche cells and unveiling its function have been the subject of intense study, and it is becoming increasingly clear how niche cells regulate HSCs in normal hematopoiesis. Leukemia stem cells (LSCs), which are able to produce leukemic cells and maintain leukemic clones, are assumed to share common features with healthy HSCs. Accumulating evidence suggests that LSCs reside in a specialized BM microenvironment; moreover, LSCs could control and rebuild the microenvironment to enhance their progression and survival. This article discusses the recent advances in our knowledge of the microenvironment supporting malignant hematopoiesis, including LSC niche.

The identification of fibrosis-driving myofibroblast precursors reveals new therapeutic avenues in myelofibrosis

Myofibroblasts are fibrosis-driving cells and are well characterized in solid organ fibrosis, but their role and cellular origin in bone marrow fibrosis remains obscure. Recent work has demonstrated that Gli1⁺ and LepR⁺ mesenchymal stromal cells (MSCs) are progenitors of fibrosis-causing myofibroblasts in the bone marrow. Genetic ablation of Gli1⁺ MSCs or pharmacologic targeting of hedgehog (Hh)-Gli signaling ameliorated fibrosis in mouse models of myelofibrosis (MF). Moreover, pharmacologic or genetic intervention in platelet-derived growth factor receptor α (Pdgfrα) signaling in Lepr⁺ stromal cells suppressed their expansion and ameliorated MF. Improved understanding of cellular and molecular mechanisms in the hematopoietic stem cell niche that govern the transition of MSCs to myofibroblasts and myofibroblast expansion in MF has led to new paradigms in the pathogenesis and treatment of MF. Here, we highlight the central role of malignant hematopoietic clone-derived megakaryocytes in reprogramming the hematopoietic stem cell niche in MF with potential detrimental consequences for hematopoietic reconstitution after allogenic stem cell transplantation, so far the only therapeutic approach in MF considered to be curative. We and others have reported that targeting Hh-Gli signaling is a therapeutic strategy in solid organ fibrosis. Data indicate that targeting Gli proteins directly inhibits Gli1⁺ cell proliferation and myofibroblast differentiation, which results in reduced fibrosis severity and improved organ function. Although canonical Hh inhibition (eg, smoothened [Smo] inhibition) failed to improve pulmonary fibrosis, kidney fibrosis, or MF, the direct inhibition of Gli proteins ameliorated fibrosis. Therefore, targeting Gli proteins directly might be an interesting and novel therapeutic approach in MF.

Bone Marrow Micro-Environment in Normal and Deranged Hematopoiesis: Opportunities for Regenerative Medicine and Therapies

Various cell types cooperate to create a highly organized and dynamic micro-environmental niche in the bone marrow. Over the past several years, the field has increasingly recognized the critical roles of the interplay between bone marrow environment and hematopoietic cells in normal and deranged hematopoiesis. These advances rely on several new technologies that have allowed us to characterize the identity and roles of these niches in great detail. Here, we review the progress of the last several years, list some of the outstanding questions in the field and propose ways to target the diseased environment to better treat hematologic diseases. Understanding the extrinsic regulation by the niche will help boost hematopoiesis for regenerative medicine. Based on natural development of hematologic malignancies, we propose that combinatory targeting the niche and hematopoietic intrinsic mechanisms in early stages of hematopoietic malignancies may help eliminate minimal residual disease and have the highest efficacy.

GATA1 insufficiencies in primary myelofibrosis and other hematopoietic disorders: consequences for therapy

INTRODUCTION

GATA1, the founding member of a family of transcription factors, plays important roles in the development of hematopoietic cells of several lineages. Although loss of GATA1 has been known to impair hematopoiesis in animal models for nearly 25 years, the link between GATA1 defects and human blood diseases has only recently been realized. Areas covered: Here the current understanding of the functions of GATA1 in normal hematopoiesis and how it is altered in disease is reviewed. GATA1 is indispensable mainly for erythroid and megakaryocyte differentiation. In erythroid cells, GATA1 regulates early stages of differentiation, and its deficiency results in apoptosis. In megakaryocytes, GATA1 controls terminal maturation and its deficiency induces proliferation. GATA1 alterations are often found in diseases involving these two lineages, such as congenital erythroid and/or megakaryocyte deficiencies, including Diamond Blackfan Anemia (DBA), and acquired neoplasms, such as acute megakaryocytic leukemia (AMKL) and the myeloproliferative neoplasms (MPNs). Expert commentary: Since the first discovery of GATA1 mutations in AMKL, the number of diseases that are associated with impaired GATA1 function has increased to include DBA and MPNs. With respect to the latter, we are only just now appreciating the link between enhanced JAK/STAT signaling, GATA1 deficiency and disease pathogenesis.

A vicious interplay between genetic and environmental insults in the etiology of blood cancers

Over the years, the etiology of cancer has been attributed alternatively to genetic and environmental insults. According to the genetic hypothesis, cancer cells arise from the acquisition of mutations that dysregulate the intrinsic mechanisms controlling normal cell growth and survival. In contrast, the environmental hypothesis holds that cancer can be caused by multiple extrinsic challenges that alter normal tissue homeostasis, but may not necessarily involve mutations. It is, however, quite possible that these two mechanisms are not mutually exclusive. According to this third hypothesis, environmental challenges, by mechanisms still poorly understood, may act by imposing a selection pressure that confers a proliferative advantage on cells carrying specific driver mutations, leading to disease initiation. The authors of a brief report published in this journal (Exp Hematol. 2017;56:1-6) tested this third hypothesis experimentally and provide new evidence that chronic inflammation, by increasing tumor necrosis factor (TNF)-α, may positively select malignant hematopoietic stem cells (HSCs) carrying loss-of-function TET2 mutations that switch the TNF-α signaling responses to activate proliferation rather than inducing quiescence. Furthermore, these mutations skew hematopoietic differentiation toward the myelomonocytic lineage and the output of macrophages producing TNF-α constitutively, promoting further environment-independent expansion of the malignant HSCs. These findings support a model in which the formation of a malignant population is triggered by a vicious interplay between genetic (TET2 mutations) and environmental (inflammation) insults, suggesting the need for strategies that target both the malignant cells and their environment.