Selective reduction of JAK2V617F-dependent cell growth by siRNA/shRNA and its reversal by cytokines

Cholesterol and Its Metabolites in Tumor Growth: Therapeutic Potential of Statins in Cancer Treatment

Cholesterol is essential for cell function and viability. It is a component of the plasma membrane and lipid rafts and is a precursor for bile acids, steroid hormones, and Vitamin D. As a ligand for estrogen-related receptor alpha (ESRRA), cholesterol becomes a signaling molecule. Furthermore, cholesterol-derived oxysterols activate liver X receptors (LXRs) or estrogen receptors (ERs). Several studies performed in cancer cells reveal that cholesterol synthesis is enhanced compared to normal cells. Additionally, high serum cholesterol levels are associated with increased risk for many cancers, but thus far, clinical trials with 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) have had mixed results. Statins inhibit cholesterol synthesis within cells through the inhibition of HMG-CoA reductase, the rate-limiting enzyme in the mevalonate and cholesterol synthetic pathway. Many downstream products of mevalonate have a role in cell proliferation, since they are required for maintenance of membrane integrity; signaling, as some proteins to be active must undergo prenylation; protein synthesis, as isopentenyladenine is an essential substrate for the modification of certain tRNAs; and cell-cycle progression. In this review starting from recent acquired findings on the role that cholesterol and its metabolites fulfill in the contest of cancer cells, we discuss the results of studies focused to investigate the use of statins in order to prevent cancer growth and metastasis.

Mediator Kinase Phosphorylation of STAT1 S727 Promotes Growth of Neoplasms With JAK-STAT Activation

Constitutive JAK-STAT signaling drives the proliferation of most myeloproliferative neoplasms (MPN) and a subset of acute myeloid leukemia (AML), but persistence emerges with chronic exposure to JAK inhibitors. MPN and post-MPN AML are dependent on tyrosine phosphorylation of STATs, but the role of serine STAT1 phosphorylation remains unclear. We previously demonstrated that Mediator kinase inhibitor cortistatin A (CA) reduced proliferation of JAK2-mutant AML in vitro and in vivo and also suppressed CDK8-dependent phosphorylation of STAT1 at serine 727. Here we report that phosphorylation of STAT1 S727 promotes the proliferation of AML cells with JAK-STAT pathway activation. Inhibition of serine phosphorylation by CA promotes growth arrest and differentiation, inhibits colony formation in MPN patient samples and reduces allele burden in MPN mouse models. These results reveal that STAT1 pS727 regulates growth and differentiation in JAK-STAT activated neoplasms and suggest that Mediator kinase inhibition represents a therapeutic strategy to regulate JAK-STAT signaling.

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CDK8/19 inhibitor cortistatin A synergizes with FDA-approved JAK1/2 ruxolitinib and inhibits ruxolitinib-persistent cells.

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CDK8/19 phosphorylation of STAT1 S727 promotes growth and suppresses differentiation.

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Cortistatin A upregulates expression of STAT1 pS727- and SE-associated genes.

Previously, it was known that cancer cells with activated JAK-STAT signaling are driven by oncogenic actions of JAK2 and tyrosine-phosphorylated STAT3 and STAT5. The FDA-approved JAK inhibitor ruxolitinib targets these dependencies, but significant challenges remain in the clinic, especially for leukemia patients. We show here that JAK2-mutant leukemia cells that become resistant to ruxolitinib are sensitive to CDK8/19 inhibitor CA and that CA synergizes with ruxolitinib, indicating that CDK8/19 inhibitors may be an effective therapeutic strategy for these cancers. Further, our studies provide insights into the mechanistic role of STAT1 serine phosphorylation by CDK8/19 in JAK2-activated leukemia.

IL-33 signaling contributes to the pathogenesis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are characterized by the clonal expansion of one or more myeloid cell lineage. In most cases, proliferation of the malignant clone is ascribed to defined genetic alterations. MPNs are also associated with aberrant expression and activity of multiple cytokines; however, the mechanisms by which these cytokines contribute to disease pathogenesis are poorly understood. Here, we reveal a non-redundant role for steady-state IL-33 in supporting dysregulated myelopoiesis in a murine model of MPN. Genetic ablation of the IL-33 signaling pathway was sufficient and necessary to restore normal hematopoiesis and abrogate MPN-like disease in animals lacking the inositol phosphatase SHIP. Stromal cell-derived IL-33 stimulated the secretion of cytokines and growth factors by myeloid and non-hematopoietic cells of the BM, resulting in myeloproliferation in SHIP-deficient animals. Additionally, in the transgenic JAK2V617F model, the onset of MPN was delayed in animals lacking IL-33 in radio-resistant cells. In human BM, we detected increased numbers of IL-33-expressing cells, specifically in biopsies from MPN patients. Exogenous IL-33 promoted cytokine production and colony formation by primary CD34+ MPN stem/progenitor cells from patients. Moreover, IL-33 improved the survival of JAK2V617F-positive cell lines. Together, these data indicate a central role for IL-33 signaling in the pathogenesis of MPNs.

Emerging EPO and EPO receptor regulators and signal transducers

As essential mediators of red cell production, erythropoietin (EPO) and its cell surface receptor (EPO receptor [EPOR]) have been intensely studied. Early investigations defined basic mechanisms for hypoxia-inducible factor induction of EPO expression, and within erythroid progenitors EPOR engagement of canonical Janus kinase 2/signal transducer and activator of transcription 5 (JAK2/STAT5), rat sarcoma/mitogen-activated protein kinase/extracellular signal-regulated kinase (RAS/MEK/ERK), and phosphatidylinositol 3-kinase (PI3K) pathways. Contemporary genetic, bioinformatic, and proteomic approaches continue to uncover new clinically relevant modulators of EPO and EPOR expression, and EPO's biological effects. This Spotlight review highlights such factors and their emerging roles during erythropoiesis and anemia.

Heterozygous and homozygous JAK2(V617F) states modeled by induced pluripotent stem cells from myeloproliferative neoplasm patients

JAK2^V617F is the predominant mutation in myeloproliferative neoplasms (MPN). Modeling MPN in a human context might be helpful for the screening of molecules targeting JAK2 and its intracellular signaling. We describe here the derivation of induced pluripotent stem (iPS) cell lines from 2 polycythemia vera patients carrying a heterozygous and a homozygous mutated JAK2^V617F, respectively. In the patient with homozygous JAK2^V617F, additional ASXL1 mutation and chromosome 20 allowed partial delineation of the clonal architecture and assignation of the cellular origin of the derived iPS cell lines. The marked difference in the response to erythropoietin (EPO) between homozygous and heterozygous cell lines correlated with the constitutive activation level of signaling pathways. Strikingly, heterozygous iPS cells showed thrombopoietin (TPO)-independent formation of megakaryocytic colonies, but not EPO-independent erythroid colony formation. JAK2, PI3K and HSP90 inhibitors were able to block spontaneous and EPO-induced growth of erythroid colonies from GPA⁺CD41⁺ cells derived from iPS cells. Altogether, this study brings the proof of concept that iPS can be used for studying MPN pathogenesis, clonal architecture, and drug efficacy.

JAK2-V617F-mediated signalling is dependent on lipid rafts and statins inhibit JAK2-V617F-dependent cell growth

Aberrant JAK2 signalling plays an important role in the aetiology of myeloproliferative neoplasms (MPNs). JAK2 inhibitors, however, do not readily eliminate neoplastic MPN cells and thus do not induce patient remission. Further understanding JAK2 signalling in MPNs may uncover novel avenues for therapeutic intervention. Recent work has suggested a potential role for cellular cholesterol in the activation of JAK2 by the erythropoietin receptor and in the development of an MPN-like disorder in mice. Our study demonstrates for the first time that the MPN-associated JAK2-V617F kinase localizes to lipid rafts and that JAK2-V617F-dependent signalling is inhibited by lipid raft disrupting agents, which target membrane cholesterol, a critical component of rafts. We also show for the first time that statins, 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase inhibitors, widely used to treat hypercholesterolaemia, induce apoptosis and inhibit JAK2-V617F-dependent cell growth. These cells are more sensitive to statin treatment than non-JAK2-V617F-dependent cells. Importantly, statin treatment inhibited erythropoietin-independent erythroid colony formation of primary cells from MPN patients, but had no effect on erythroid colony formation from healthy individuals. Our study is the first to demonstrate that JAK2-V617F signalling is dependent on lipid rafts and that statins may be effective in a potential therapeutic approach for MPNs.

Therapy with JAK2 inhibitors for myeloproliferative neoplasms

The development of JAK2 inhibitors followed the discovery of activating mutation of JAK2 (JAK2V617F) in patients with classic Philadelphia-negative myeloproliferative neoplasms (Ph-negative MPN). It is now known that mutations activating the JAK-STAT pathway are ubiquitous in Ph-negative MPN, and that the deregulated JAK-STAT pathway plays a central role in the pathogenesis of these disorders. JAK2 inhibitors thus are effective in patients both with and without the JAK2V617F mutation. This article reviews the rationale for using JAK2 inhibitors in Ph-negative MPN, and the results of more recent clinical trials with these drugs.

Thrombosis in myeloproliferative neoplasms with JAK2V617F mutation

Myeloproliferative neoplasms (MPNs) are clonal hematopoietic stem cell disorders and are characterized by advanced proliferation and survival advantage. These abnormalities are considered to derive from the point mutation in exon 14 of the Janus kinase 2 genes (JAK2 V617F). The thrombosis rate and the high prevalence of JAK2V617F in patients with MPN suggest that there is an association between the 2 in MPN. Apart from the mutation, other variables are documented to cause endothelial dysfunction and involve in thrombotic tendency. Endothelial progenitor cells differentiated from hematopoietic stem cell in patients with JAK2V617F MPN play an indispensable role in initiating and modulating neoangiogenesis. Although a risk-oriented therapeutic approach has been applied to MPN treatments, the further study on pathogenesis of MPN may provide more novel preventions and therapies for MPN.

New pieces of a puzzle: the current biological picture of MPN

Over the last years, we have witnessed significant improvement in our ability to elucidate the genetic events, which contribute to the pathogenesis of acute and chronic leukemias, and also in patients with myeloproliferative neoplasms (MPN). However, despite significant insight into the role of specific mutations, including the JAK2V617F mutation, in MPN pathogenesis, the precise mechanisms by which specific disease alleles contribute to leukemic transformation in MPN remain elusive. Here we review recent studies aimed at understanding the role of downstream signaling pathways in MPN initiation and phenotype, and discuss how these studies have begun to lead to novel insights with biologic, clinical, and therapeutic relevance.

Modulation of activation-loop phosphorylation by JAK inhibitors is binding mode dependent

Janus kinase (JAK) inhibitors are being developed for the treatment of rheumatoid arthritis, psoriasis, myeloproliferative neoplasms, and leukemias. Most of these drugs target the ATP-binding pocket and stabilize the active conformation of the JAK kinases. This type I binding mode can lead to an increase in JAK activation loop phosphorylation, despite blockade of kinase function. Here we report that stabilizing the inactive state via type II inhibition acts in the opposite manner, leading to a loss of activation loop phosphorylation. We used X-ray crystallography to corroborate the binding mode and report for the first time the crystal structure of the JAK2 kinase domain in an inactive conformation. Importantly, JAK inhibitor-induced activation loop phosphorylation requires receptor interaction, as well as intact kinase and pseudokinase domains. Hence, depending on the respective conformation stabilized by a JAK inhibitor, hyperphosphorylation of the activation loop may or may not be elicited.

JAK2 inhibitors: are they the solution?

The discovery of the JAK2V617F mutation in patients with Philadelphia-negative myeloproliferative neoplasms (Ph-negative MPN) started the era of targeted therapy for these diseases. Until now, patients had few treatment options available, which usually were restricted to hydroxyurea, interferon preparations, and chemotherapy in more aggressive cases. JAK2 inhibitors have been developed over the past 5 years, and the results of the first clinical trials with JAK2 inhibitors for patients with myelofibrosis were recently published. Current research results suggest that JAK2 inhibitors have a potential to decrease disease burden and its activity, as manifested by a decrease in splenomegaly and improvement in systemic disease-related symptoms, but they do not seem to be able to eradicate the malignant clone. However, JAK2 inhibitors help patients regardless of their mutation status, because patients without JAK2V617F mutation benefit to the same extent as patients with JAK2V617F mutation. A greater understanding of the pathophysiology of MPNs is needed before we can cure myelofibrosis with drug therapy. Currently, several new JAK2 inhibitors are in clinical trials for patients with myelofibrosis, and clinical trials for patients with polycythemia vera and essential thrombocythemia have also started. We review recent data on JAK2 inhibitors for the management of patients with Ph-negative MPNs.

The dominant negative β isoform of the glucocorticoid receptor is uniquely expressed in erythroid cells expanded from polycythemia vera patients

Glucocorticoid receptor (GR) agonists increase erythropoiesis in vivo and in vitro. To clarify the effect of the dominant negative GRβ isoform (unable to bind STAT-5) on erythropoiesis, erythroblast (EB) expansion cultures of mononuclear cells from 18 healthy (nondiseased) donors (NDs) and 16 patients with polycythemia vera (PV) were studied. GRβ was expressed in all PV EBs but only in EBs from 1 ND. The A3669G polymorphism, which stabilizes GRβ mRNA, had greater frequency in PV (55%; n = 22; P = .0028) and myelofibrosis (35%; n = 20) patients than in NDs (9%; n = 22) or patients with essential thrombocythemia (6%; n = 15). Dexamethasone stimulation of ND cultures increased the number of immature EBs characterized by low GATA1 and β-globin expression, but PV cultures generated great numbers of immature EBs with low levels of GATA1 and β-globin irrespective of dexamethasone stimulation. In ND EBs, STAT-5 was not phosphorylated after dexamethasone and erythropoietin treatment and did not form transcriptionally active complexes with GRα, whereas in PV EBs, STAT-5 was constitutively phosphorylated, but the formation of GR/STAT-5 complexes was prevented by expression of GRβ. These data indicate that GRβ expression and the presence of A3669G likely contribute to development of erythrocytosis in PV and provide a potential target for identification of novel therapeutic agents.

JAK2 inhibitors: what's the true therapeutic potential?

Physicians treating patients with the classic Philadelphia-negative myeloproliferative neoplasms (Ph-negative MPNs) (polycythemia vera [PV], essential thrombocythemia [ET] and primary myelofibrosis [PMF]) traditionally had few therapeutic drugs available. Spurred by the discovery of activating mutation of the JAK2 tyrosine kinase (JAK2 V617F mutation) in patients with Ph-negative MPNs several years ago, several JAK2 inhibitors were synthesized and are currently undergoing clinical trials in patients with PMF, PV and ET. Initial results from these studies have shown that these drugs can markedly reduce spleen size and alleviate constitutional symptoms, increase weight and improve exercise capacity in MF patients, thus improve quality of their life, which is significant clinical benefit. In ET and PV JAK2 inhibitor therapy may efficiently control blood cell count, as well as improve splenomegaly and control disease related symptoms. JAK2 inhibitors are a novel class of agents with promising results for treating patients with MF, PV and ET. In this article we will review the current evidence regarding the role of JAK2 mutations in the pathogenesis of Ph-negative MPNs and summarize results from the most recent clinical trials with JAK2 inhibitors in these disorders. JAK2 inhibitors are a novel class of agents with promising results for treating patients with MF, PV and ET.

Preclinical characterization of the selective JAK1/2 inhibitor INCB018424: therapeutic implications for the treatment of myeloproliferative neoplasms

Constitutive JAK2 activation in hematopoietic cells by the JAK2V617F mutation recapitulates myeloproliferative neoplasm (MPN) phenotypes in mice, establishing JAK2 inhibition as a potential therapeutic strategy. Although most polycythemia vera patients carry the JAK2V617F mutation, half of those with essential thrombocythemia or primary myelofibrosis do not, suggesting alternative mechanisms for constitutive JAK-STAT signaling in MPNs. Most patients with primary myelofibrosis have elevated levels of JAK-dependent proinflammatory cytokines (eg, interleukin-6) consistent with our observation of JAK1 hyperactivation. Accordingly, we evaluated the effectiveness of selective JAK1/2 inhibition in experimental models relevant to MPNs and report on the effects of INCB018424, the first potent, selective, oral JAK1/JAK2 inhibitor to enter the clinic. INCB018424 inhibited interleukin-6 signaling (50% inhibitory concentration [IC(50)] = 281nM), and proliferation of JAK2V617F(+) Ba/F3 cells (IC(50) = 127nM). In primary cultures, INCB018424 preferentially suppressed erythroid progenitor colony formation from JAK2V617F(+) polycythemia vera patients (IC(50) = 67nM) versus healthy donors (IC(50) > 400nM). In a mouse model of JAK2V617F(+) MPN, oral INCB018424 markedly reduced splenomegaly and circulating levels of inflammatory cytokines, and preferentially eliminated neoplastic cells, resulting in significantly prolonged survival without myelosuppressive or immunosuppressive effects. Preliminary clinical results support these preclinical data and establish INCB018424 as a promising oral agent for the treatment of MPNs.

The current status and the future of JAK2 inhibitors for the treatment of myeloproliferative diseases

Janus kinases (JAKs) are critical components of cytokine signaling pathways which regulate immunity, inflammation, hematopoiesis, growth, and development. The recent discovery of JAK2-activating mutations as a causal event in the majority of patients with Philadelphia chromosome negative (Ph-) myeloproliferative disorders (MPDs) prompted many pharmaceutical companies to develop JAK2-selective inhibitors for the treatment of MPDs. JAK2 inhibitors effectively reduce JAK2-driven phosphorylation of signal transducer and activator of transcription 5, and cell proliferation and cell survival in JAK2-activated cells in vitro and in vivo. Most inhibitors are currently being evaluated in patients with one form of MPD, myelofibrosis. Patients treated with these inhibitors experienced a rapid reduction of splenomegaly, significant improvement of constitutional symptoms, and increased daily activity with few adverse events. A partial reduction of JAK2V617F disease burden during the treatment with JAK2 inhibitors was also observed. The inhibitors appear to have a therapeutic benefit in the treatment of these disorders. The results of ongoing clinical trials will allow further evaluation of clinical benefits and safety of these compounds. In this review, the authors summarize the status of JAK2 inhibitors in development and discuss their benefits and challenges.

Combined inhibition of Janus kinase 1/2 for the treatment of JAK2V617F-driven neoplasms: selective effects on mutant cells and improvements in measures of disease severity

PURPOSE

Deregulation of the Janus kinase-signal transducers and activators of transcription (JAK-STAT) pathway is a hallmark for the Philadelphia chromosome-negative myeloproliferative diseases polycythemia vera, essential thrombocythemia, and primary myelofibrosis. We tested the efficacy of a selective JAK1/2 inhibitor in cellular and in vivo models of JAK2-driven malignancy.

EXPERIMENTAL DESIGN

A novel inhibitor of JAK1/2 was characterized using kinase assays. Cellular effects of this compound were measured in cell lines bearing the JAK2V617F or JAK1V658F mutation, and its antiproliferative activity against primary polycythemiavera patient cells was determined using clonogenic assays. Antineoplastic activity in vivo was determined using a JAK2V617F-driven xenograft model, and effects of the compound on survival, organomegaly, body weight, and disease-associated inflammatory markers were measured.

RESULTS

INCB16562 potently inhibited proliferation of cell lines and primary cells from PV patients carrying the JAK2V617F or JAK1V658F mutation by blocking JAK-STAT signaling and inducing apoptosis. In vivo, INCB16562 reduced malignant cell burden, reversed splenomegaly and normalized splenic architecture, improved body weight gains, and extended survival in a model of JAK2V617F-driven hematologic malignancy. Moreover, these mice suffered from markedly elevated levels of inflammatory cytokines, similar to advanced myeloproliferative disease patients, which was reversed upon treatment.

CONCLUSIONS

These data showed that administration of the dual JAK1/2 inhibitor INCB16562 reduces malignant cell burden, normalizes spleen size and architecture, suppresses inflammatory cytokines, improves weight gain, and extends survival in a rodent model of JAK2V617F-driven hematologic malignancy. Thus, selective inhibitors of JAK1 and JAK2 represent a novel therapy for the patients with myeloproliferative diseases and other neoplasms associated with JAK dysregulation.

Constitutive JunB expression, associated with the JAK2 V617F mutation, stimulates proliferation of the erythroid lineage

The JAK2 V617F mutation, present in the majority of polycythemia vera (PV) patients, causes constitutive activation of JAK2 and seems to be responsible for the PV phenotype. However, the transcriptional changes triggered by the mutation have not yet been totally characterized. In this study, we performed a large-scale gene expression study using serial analysis of gene expression in bone marrow cells of a newly diagnosed PV patient harboring the JAK2 V617F mutation and in normal bone marrow cells of healthy donors. JUNB was one of the genes upregulated in PV, and we confirmed, by quantitative real-time PCR, an overexpression of JUNB in hematopoietic cells of other JAK2 V617F PV patients. Using Ba/F3-EPOR cell lines and primary human erythroblast cultures, we found that JUNB was transcriptionally induced after erythropoietin addition and that JAK2 V617F constitutively induced JunB protein expression. Furthermore, JUNB knockdown reduced not only the growth of Ba/F3 cells by inducing apoptosis, but also the clonogenic and proliferative potential of human erythroid progenitors. These results establish a role for JunB in normal erythropoiesis and indicate that JunB may play a major role in the development of JAK2 V617F myeloproliferative disorders.