Akt activation through the phosphorylation of erythropoietin receptor at tyrosine 479 is required for myeloproliferative disorder-associated JAK2 V617F mutant-induced cellular transformation

Non-canonical Hedgehog signaling mediates profibrotic hematopoiesis-stroma crosstalk in myeloproliferative neoplasms

The role of hematopoietic Hedgehog signaling in myeloproliferative neoplasms (MPNs) remains incompletely understood despite data suggesting that Hedgehog (Hh) pathway inhibitors have therapeutic activity in patients. We aim to systematically interrogate the role of canonical vs. non-canonical Hh signaling in MPNs. We show that Gli1 protein levels in patient peripheral blood mononuclear cells (PBMCs) mark fibrotic progression and that, in murine MPN models, absence of hematopoietic Gli1, but not Gli2 or Smo, significantly reduces MPN phenotype and fibrosis, indicating that GLI1 in the MPN clone can be activated in a non-canonical fashion. Additionally, we establish that hematopoietic Gli1 has a significant effect on stromal cells, mediated through a druggable MIF-CD74 axis. These data highlight the complex interplay between alterations in the MPN clone and activation of stromal cells and indicate that Gli1 represents a promising therapeutic target in MPNs, particularly that Hh signaling is dispensable for normal hematopoiesis.

The indispensable role of the RNA helicase DDX5 in tumorigenesis induced by the myeloproliferative neoplasm-associated JAK2V617F mutant

A point mutation (V617F) in the Janus kinase 2 (JAK2) gene results in the production of disorderly activated tyrosine kinase, which causes myeloproliferative neoplasms (MPN). We herein demonstrated that the RNA helicase DDX5 was highly expressed at the mRNA and protein levels through the activation of signal transducer and activator of transcription 5 (STAT5) in Ba/F3 cells expressing a JAK2V617F mutant and erythropoietin receptor (V617F/EpoR cells) and MPN patient-derived HEL cells. A treatment with the JAK1/2 inhibitor, ruxolitinib and STAT5 inhibitor, pimozide significantly inhibited DDX5 mRNA expression and enhanced the degradation of DDX5 in these cells, suggesting that the JAK2V617F mutant positively regulates DDX5 mRNA expression and DDX5 protein stability by activating STAT5. The knockdown of DDX5 specifically inhibited the activation of mechanistic target of rapamycin (mTOR) in V617F/EpoR cells and HEL cells and significantly suppressed the proliferation of these cells. Furthermore, the knockdown of DDX5 markedly suppressed tumorigenesis, splenomegaly, and liver hypertrophy caused by an inoculation of V617F/EpoR cells in nude mice. Collectively, these results revealed that JAK2V617F exhibits transforming activity by inducing the expression of DDX5 in a STAT5-dependent manner, indicating the potential of the JAK2V617F/STAT5/DDX5 axis as a therapeutic target in the treatment of MPN.

JAK2-V617F is a negative regulation factor of SHIP1 protein and thus influences the AKT signaling pathway in patients with Myeloproliferative Neoplasm (MPN)

BACKGROUND

Myeloproliferative neoplasms (MPN) are a group of chronic haematological disorders. At the molecular level of MPN cells, the gain-of-function mutation V617F of the Janus kinase 2 (JAK2) leads to a constitutive activation of the downstream signaling cascade and is a conventional criteria for diagnosis. Here, the functional role of the tumor suppressor SHIP1 (SH2 domain containing inositol-5 phosphatase 1) in the pathogenesis of MPNs was investigated.

METHODS

Primary blood samples of MPN-patients were analysed using Western Blot technique regarding the level of SHIP1 expression. Moreover, SHIP1 and SHIP1-mutations were lentivirally transduced in the JAK2-V617F-positive UKE-1 cell line and expression was monitored over time. In addition, we examined SHIP1 reconstitution by inhibition of JAK2-V617F. Furthermore, we transfected SHIP1-expressing cells with a JAK2-V617F respectively a BCR-ABL construct and investigated changes in SHIP1 expression.

RESULTS

Four out of five MPN-patient samples showed a loss or a reduction in SHIP1 expression. We identified JAK2 as a negative regulator of SHIP1 expression in MPN cells and inhibition of JAK2-V617F implicates a reconstituted SHIP1 expression. This is significant because SHIP1 negatively regulates the AKT signaling pathway and in consequence the reconstitution of SHIP1 expression leads to a decreased cell growth. Moreover, we examined the impact of SHIP1 and patient-derived SHIP1-mutations on AKT phosphorylation and show the benefit of a combined therapy in MPN cells with inhibitors of the AKT/mTOR pathway.

CONCLUSION

In summary, the data suggest that SHIP1 may play a role during the development of MPNs and could be the basis for establishing a targeted therapy.

Tyrosine-phosphorylated SOCS3 negatively regulates cellular transformation mediated by the myeloproliferative neoplasm-associated JAK2 V617F mutant

In the majority of myeloproliferative neoplasms (MPNs) patients, a point mutation, V617F has been found in Janus kinase 2 (JAK2) gene, and this JAK2 mutant provoked aberrant signaling pathway. In the current study, we found that suppressor of cytokine signaling proteins 3 (SOCS3) possessed the tumor suppressive activity against the JAK2 V617F mutant-provoked cellular transformation. The knockdown of SOCS3 increased the expression level of the JAK2 V617F mutant, which enhanced the activation of signaling mediators, including signal transducer and activator of transcription 3 and 5 (STAT3, STAT5) and extracellular signal-regulated kinase (ERK), and also increased of the proliferation rate and tumorigenesis activity of Ba/F3 cells expressing the JAK2 V617F mutant and erythropoietin receptor (EpoR). In contrast, the enforced expression of SOCS3 significantly inhibited the JAK2 V617F mutant-induced activation of downstream signaling molecules, cell proliferation, and tumorigenesis by down-regulating the expression level of the JAK2 V617F mutant. SOCS3 interacted with the JAK2V617F mutant through its SH2 domain and was phosphorylated at Tyr-204 and Tyr-221 in its SOCS box by the JAK2V617F mutant. SOCS3 mutants carrying a mutation in the SH2 domain (R71E) and a substitution at Tyr-221 (Y221F) failed to exert inhibitory effects on JAK2V617F mutant-induced cellular transformation and tumorigenesis. Collectively, these results imply that SOCS3 plays a negative role in the JAK2 V617F mutant-induced oncogenic signaling pathway through its SH2 domain and the phosphorylation of Tyr-221 in its SOCS box.

IL-6 stimulation of DNA replication is JAK1/2 mediated in cross-talk with hyperactivated ERK1/2 signaling

Myeloproliferative neoplasms (MPNs) are developing resistance to therapy by JAK1/2 inhibitor ruxolitinib. To explore the mechanism of ruxolitinib's limited effect, we examined the JAK1/2 mediated induction of proliferation related ERK1/2 and AKT signaling by proinflammatory interleukin-6 (IL-6) in MPN granulocytes and JAK2V617F mutated human erythroleukemia (HEL) cells. We found that JAK1/2 or JAK2 inhibition prevented the IL-6 activation of STAT3 and AKT pathways in polycythemia vera and HEL cells. Further, we showed that these inhibitors also blocked the IL-6 activation of the AKT pathway in primary myelofibrosis (PMF). Only JAK1/2 inhibitor ruxolitinib largely activated ERK1/2 signaling in essential thrombocythemia and PMF (up to 4.6 fold), with a more prominent activation in JAK2V617F positive granulocytes. Regarding a cell cycle, we found that IL-6 reduction of HEL cells percentage in G2M phase was reversed by ruxolitinib (2.6 fold). Moreover, ruxolitinib potentiated apoptosis of PMF granulocytes (1.6 fold). Regarding DNA replication, we found that ruxolitinib prevented the IL-6 augmentation of MPN granulocytes frequency in the S phase of the cell cycle (up to 2.9 fold). The inflammatory stimulation induces a cross-talk between the proliferation linked pathways, where JAK1/2 inhibition is compensated by the activation of the ERK1/2 pathway during IL-6 stimulation of DNA replication.

Therapeutic time window for the effects of erythropoietin on astrogliosis and neurite outgrowth in an in vitro model of spinal cord injury

BACKGROUND

The objective of this study was to investigate the underlying molecular mechanisms and the therapeutic time window for preventing astrogliosis with erythropoietin (EPO) treatment after in vitro modeled spinal cord injury (SCI).

METHODS

Cultured rat spinal cord astrocytes were treated with kainate and scratching to generate an in vitro model of SCI. EPO (100U/mL or 300U/mL) was added immediately or 2, 4, or 8 hours after injury. Some cultures were also treated with AG490, an inhibitor of the EPO-EPO receptor (EpoR) pathway mediator Janus kinase 2 (JAK2). To evaluate neurite extension, rat embryonic spinal cord neurons were seeded onto astrocyte cultures and treated with EPO immediately after injury in the presence or absence of anti-EpoR antibody.

RESULTS

EPO treatment at up to 8 hours after injury reduced the expression of axonal growth inhibiting molecules (glial fibrillary acidic protein, vimentin, and chondroitin sulfate proteoglycan), cytoskeletal regulatory proteins (Rho-associated protein kinase and ephephrin A4), and proinflammatory cytokines (tumor necrosis factor-alpha, transforming growth factor-beta, and phosphorylated-Smad3) in a dosedependent manner (P < .001). Most effects peaked with EPO treatment 2-4hours after injury. Additionally, EPO treatment up to 4 hours after injury promoted expression of the EpoR (>2-fold) and JAK2 (>3-fold) in a dose-dependent manner (P < .001), whereas co-treatment with AG490 precluded these effects (P < .001). EPO treatment up to 4hours after injury also enhanced axonal b-III tubulin-immunoreactivity (>12-fold), and this effect was precluded by co-treatment with an anti-EpoR antibody (P < .001).

CONCLUSIONS

EPO treatment within 8 hours after injury reduced astrogliosis, and EPO treatment within 4 hours promoted neurite outgrowth. EPO therapy immediately after spinal cord injury may regulate glia to generate an environment permissive of axonal regeneration.

Developmental Therapeutics in Myeloproliferative Neoplasms

The unprecedented success of the Janus kinase (JAK) 1/2 inhibitor ruxolitinib in myelofibrosis (MF) provided much-needed impetus for clinical drug development for the Philadelphia chromosome-negative myeloproliferative neoplasms. The survival benefit conferred by this agent, along with its marked efficacy with regard to spleen volume and symptom reduction, have made ruxolitinib the cornerstone of drug therapy in MF. However, there remain significant unmet needs in the treatment of patients with MF, and many novel classes of agents continue to be investigated in efforts to build on the progress made with ruxolitinib. These include inhibitors of histone deacetylases (HDACs) and DNA methyltransferases, phosphatidylinositol-3-kinase isoforms, heat shock protein 90, cyclin-dependent kinases 4/6, and Hedgehog signaling, among others. In parallel, other JAK inhibitors with potential for less myelosuppression or even improvement of anemia, greater selectivity for JAK1 or JAK2, and the ability to overcome JAK inhibitor persistence are in various stages of development. First-in-class agents such as the activin receptor IIA ligand trap sotatercept (for anemia of MF), the telomerase inhibitor imetelstat, and the antifibrotic agent PRM-151 (recombinant human pentraxin-2) are also in clinical trials. In polycythemia vera, a novel interferon administered every 2 weeks is being developed for front-line therapy in high-risk individuals, and inhibitors of human double minute 2 (HDM2) have shown promise in preclinical studies, as have HDAC inhibitors such as givinostat (both in the laboratory and in the clinic). Ruxolitinib is approved for second-line therapy of polycythemia vera and is being developed for essential thrombocythemia.

Mechanisms of Resistance to JAK2 Inhibitors in Myeloproliferative Neoplasms

Myeloproliferative neoplasms are driven by activated JAK2 signaling due to somatic mutations in JAK2, the thrombopoietin receptor MPL or the chaperone calreticulin in hematopoietic stem/progenitor cells. JAK2 inhibitors have been developed, but despite clinical benefits, they do not signficantly reduce the mutant clone. Loss of response to JAK2 inhibitors occurs and several mechanisms of resistance, genetic and functional, have been identified. Resistance mutations have not been reported in MPN patients suggesting incomplete target inhibition. Alternative targeting of JAK2 by HSP90 inhibitors or type II JAK2 inhibition overcomes resistance to current JAK2 inhibitors. Additional combined therapy approaches are currently being evaluated.

The multi-site docking protein Gab1 is constitutively phosphorylated independent from its recruitment to the plasma membrane in Jak2-V617F-positive cells and mediates proliferation of human erythroleukaemia cells

The constitutively active Janus kinase 2 mutant Jak2-V617F is responsible for cytokine-independent growth of hematopoietic cells and the development of myeloproliferative neoplasms, such as polycythaemia vera and essential thrombocythaemia. Cells expressing Jak2-V617F exhibit constitutive STAT, MAPK, and PI3K signalling, and constitutive association of the multi-site docking protein Gab1 to PIP3 at the plasma membrane. Here, we demonstrate the crucial role of Gab1 for the proliferation of Jak2-V617F-positive human erythroleukaemia (HEL) cells. In Jak2-V617F-expressing cells Gab1 is constitutively phosphorylated by Erk1/2 on serine residue 552, which regulates binding to PIP3. Additionally, Gab1 is constitutively phosphorylated on tyrosine residue 627. Tyrosine 627 is a SHP2 binding site and required for Gab1-dependent Erk1/2 activation. As previously shown, Jak2-V617F-dependent Erk1/2 and PI3K activation act synergistically on the proliferation of Jak2-V617F-positive cells. Here, we examined whether constitutive membrane association of Gab1 explains cytokine-independent Gab1 phosphorylation in Jak2-V617F-expressing cells. Although we could demonstrate Jak2-V617F-dependent constitutive serine 552 and tyrosine 627 phosphorylation of Gab1, interestingly, both phosphorylations do not require binding of Gab1 to PIP3 at the plasma membrane. Instead, we observed a constitutive interaction of Gab1 with the erythropoietin receptor in Jak2-V617F-expressing cells, which depends on Janus kinase activity. Thus, constitutive Gab1-dependent signalling in Jak2-V617F-expressing cells does not occur due to the constitutive association of Gab1 with PIP3 at the plasma membrane.

IRS2 silencing increases apoptosis and potentiates the effects of ruxolitinib in JAK2V617F-positive myeloproliferative neoplasms

The recurrent V617F mutation in JAK2 (JAK2^V617F) has emerged as the primary contributor to the pathogenesis of myeloproliferative neoplasms (MPN). However, the lack of complete response in most patients treated with the JAK1/2 inhibitor, ruxolitinib, indicates the need for identifying pathways that cooperate with JAK2. Activated JAK2 was found to be associated with the insulin receptor substrate 2 (IRS2) in non-hematological cells. We identified JAK2/IRS2 binding in JAK2^V617F HEL cells, but not in the JAK2^WT U937 cell line. In HEL cells, IRS2 silencing decreased STAT5 phosphorylation, reduced cell viability and increased apoptosis; these effects were enhanced when IRS2 silencing was combined with ruxolitinib. In U937 cells, IRS2 silencing neither reduced cell viability nor induced apoptosis. IRS1/2 pharmacological inhibition in primary MPN samples reduced cell viability in JAK2^V617F-positive but not JAK2^WT specimens; combination with ruxolitinib had additive effects. IRS2 expression was significantly higher in CD34⁺ cells from essential thrombocythemia patients compared to healthy donors, and in JAK2^V617F MPN patients when compared to JAK2^WT. Our data indicate that IRS2 is a binding partner of JAK2^V617F in MPN. IRS2 contributes to increased cell viability and reduced apoptosis in JAK2-mutated cells. Combined pharmacological inhibition of IRS2 and JAK2 may have a potential clinical application in MPN.

Investigational histone deacetylase inhibitors (HDACi) in myeloproliferative neoplasms

INTRODUCTION

The Philadelphia chromosome negative myeloproliferative neoplasms (MPN) mainly comprise polycythemia vera (PV), essential thrombocythemia (ET) and myelofibrosis (MF, primary or post-PV/ET). Therapy in PV and ET focuses on minimizing thrombosis and bleeding risk, while in MF, prolongation of survival is an important goal. Different cytoreductive agents are employed in high risk PV and ET, while the JAK inhibtior ruxolitinib is the cornerstone of therapy in MF. Histone deacetylase inhibitors (HDACi) are pleiotropic agents with diverse epigenetic and non-epigenetic actions, selectively in transformed cells. A number of HDACi have been or are being investigated in MPN. Areas covered: The mechanisms of action of HDACI in neoplastic cells are summarized, and the preclinical rationale and data supporting their development in MPN specifically examined, particularly their synergism with JAK inhibitors. Major findings of clinical trials of HDACi, both alone and in combination with ruxolitinib, in MPN are then discussed, with particular attention to their toxicities and disease-modifying effects. Expert opinion: HDACi are clearly active in MPN, and there is good preclinical rationale for this. Their combination with ruxolitinib in MF is promising, but the long-term tolerability of these agents is an important concern. Further development in PV or ET appears unlikely.

Designing a small molecule erythropoietin mimetic

Erythropoietin (EPO) is a protein made by the kidneys in response to low red blood cell count that is secreted into the bloodstream and binds to a receptor on hematopoietic stem cells in the bone marrow inducing them to become new red blood cells. EPO made with recombinant DNA technology was brought to market in the 1980s to treat anemia caused by kidney disease and cancer chemotherapy. Because EPO infusion was able to replace blood transfusions in many cases, it rapidly became a multibillion dollar per year drug and as the first biologic created with recombinant technology it launched the biotech industry. For many years intense research was focused on creating a small molecule orally available EPO mimetic. The Robert Wood Johnson (RWJ) group seemed to definitively establish that only large peptides with a minimum of 60 residues could replace EPO, as anything less was not a full agonist. An intense study of the published work led me to hypothesize that the size of the mimetic is not the real issue, but the symmetry making and breaking of the EPO receptor induced by the ligand is the key to activating the stem cells. This analysis meant that residues in the binding site of the receptor deemed absolutely essential for ligand binding and activation from mutagenesis experiments, were probably not really that important. My fundamental hypotheses were: (a) the symmetric state of the homodimeric receptor is the most stable state and thus must be the off-state, (b) a highly localized binding site exists at a pivot point where the two halves of the receptor meet, (c) small molecules can be created that have high potency for this site that will be competitive with EPO and thus can displace the protein-protein interaction, (d) small symmetric molecules will stabilize the symmetric off-state of the receptor, and (e) a key asymmetry in the small molecule will stabilize a mirror image asymmetry in the receptor resulting in the stabilization of the on-state and proliferation of the stem cells into red blood cells. Researchers at Amgen published a co-crystal structure of EPO bound to the EPO receptor, which has a beautiful twofold symmetry-it was argued that this is the active state of the receptor. Activating the EPO receptor with EPO induces an almost instantaneous shutdown mechanism to sharply curtail any proliferative signal transduction, and thus, my hypotheses lead to the conclusion that the Amgen co-crystal is actually the state after receptor downregulation and thus an off-state. To put these hypotheses to the test, my computational method of Simulated Annealing of Chemical Potential was run using the co-crystal created at RWJ, which is the receptor trapped in a partial agonist state. The simulations predicted a previously unknown high affinity binding site at the pivot point where the two halves of the dimeric receptor meet, and detailed analysis of the fragment patterns led to the prediction of a molecule less than 300 MW that is basically twofold symmetric with a chiral center on one side and not the other. Thus, to the degree that computer simulations can be taken seriously, these results support my hypotheses on small molecule receptor activation. When this small molecule was synthesized and tested it indeed induced human hematopoietic stems cells to become red blood cells. When the predicted chiral center of this molecule was removed eliminating its one asymmetric feature, the resulting molecule was an antagonist-it could potently displace hot EPO but could no longer induce stem cell proliferation and differentiation. These results provided strong support for my theories on how to create potent small molecule EPO agonists and were used to launch the new company Locus Pharmaceuticals. These molecules, however, required significant chemical changes in order to make them stable in other in vitro assays and to be in vivo active, but these alterations had to be done in a way that maintained the symmetry-asymmetry considerations that led to the creation of an in vitro active molecule. The combination of changing functional groups to enable good pharmacokinetics, while not changing the key intrinsic symmetry properties were never seriously pursued at Locus and the program died. Investigations into how red blood cells are created have occupied many prominent researchers for the entire twentieth century. In the second half of the century EPO was discovered and by the end of the century it became a blockbuster commercial product that launched the biotech revolution.

JAK kinase targeting in hematologic malignancies: a sinuous pathway from identification of genetic alterations towards clinical indications

Constitutive JAK-STAT pathway activation occurs in most myeloproliferative neoplasms as well as in a significant proportion of other hematologic malignancies, and is frequently a marker of poor prognosis. The underlying molecular alterations are heterogeneous as they include activating mutations in distinct components (cytokine receptor, JAK, STAT), overexpression (cytokine receptor, JAK) or rare JAK2 fusion proteins. In some cases, concomitant loss of negative regulators contributes to pathogenesis by further boosting the activation of the cascade. Exploiting the signaling bottleneck provided by the limited number of JAK kinases is an attractive therapeutic strategy for hematologic neoplasms driven by constitutive JAK-STAT pathway activation. However, given the conserved nature of the kinase domain among family members and the interrelated roles of JAK kinases in many physiological processes, including hematopoiesis and immunity, broad usage of JAK inhibitors in hematology is challenged by their narrow therapeutic window. Novel therapies are, therefore, needed. The development of more selective inhibitors is a questionable strategy as such inhibitors might abrogate the beneficial contribution of alleviating the cancer-related pro-inflammatory microenvironment and raise selective pressure to a threshold that allows the emergence of malignant subclones harboring drug-resistant mutations. In contrast, synergistic combinations of JAK inhibitors with drugs targeting cascades that work in concert with JAK-STAT pathway appear to be promising therapeutic alternatives to JAK inhibitors as monotherapies.

[Study of cytokine signaling: the quest for immunomodulatory drugs interacting with cytokine production and activity]

I have been engaged in research and education in the fields of immunology and biochemistry at a medical college and college of pharmacy for 40 years. The original reasons why I began studying cytokines and some of the interests that have motivated me to continue working in the field of cytokine research are described: 1) the roles of cytokines in various immunological and inflammatory diseases (e.g., chemokines in bacterial infections and inflammatory diseases, particularly the role of interleukin-5 and eotaxins in eosinophilia); 2) the role of focal adhesion kinase in antiapoptosis and metastasis of melanoma; 3) recent findings on the role of JAK2/STAT pathways, particularly how JAK2V617F mutation induces dysregulated proliferation and tumorigenesis; and 4) the interactions of various chemical compounds and natural products in cytokine gene activation and signaling. Previous discoveries and published findings by my research group are described, along with comments and discussion pertaining to recent developments in the field.

Effect of chemical modification on the ability of pyrrolidinium fullerene to induce apoptosis of cells transformed by JAK2 V617F mutant

JAK2 V617F mutant, a gene responsible for human myeloproliferative neoplasms (MPNs), causes not only cellular transformation but also resistance to various anti-cancer drugs. We previously reported that pyrrolidinium fullerene markedly induced the apoptosis of JAK2 V617F mutant-induced transformed cells through the reduction of apoptosis signal-regulating kinase 1 (ASK1), following inhibition of the c-Jun N-terminal kinase (JNK) pathway. In the current study, we found that the replacement of the 2-hydrogen atom (H) or N-methyl group (CH3) by the butyl group (C4C9) caused the more than 3-fold potent cytotoxic effects on cells transformed by the JAK2 V617F mutant. Strikingly, these chemical modification of pyrrolidinium fullerene resulted in more marked reduction of ASK1 protein and a more potent inhibitory effect on the JNK signaling cascade. On the other hand, when modified with a longer alkyl group, the derivatives lacked their cytotoxicity. These observations clearly indicate that the modification of pyrrolidinium fullerene with a suitable length of alkyl group such as butyl group enhances its apoptotic effect through inhibition of the ASK1-MKK4/7-JNK pathway.

Molecular pathways: molecular basis for sensitivity and resistance to JAK kinase inhibitors

Janus-activated kinases (JAK) are the mediators of a variety of cytokine signals via their cognate receptors that result in activation of intracellular signaling pathways. Alterations in JAK1, JAK2, JAK3, and TYK2 signaling contribute to different disease states, and dysregulated JAK-STAT signaling is associated with hematologic malignancies, autoimmune disorders, and immune-deficient conditions. Genetic alterations of JAK2 occur in the majority of patients with myeloproliferative neoplasms and occur in a subset of patients with acute leukemias. JAK-mediated signaling critically relies on STAT transcription factors, and on activation of the MAPK and PI3K/Akt signaling axes. Hyperactive JAK at the apex of these potent oncogenic signaling pathways therefore represents an important target for small-molecule kinase inhibitors in different disease states. The JAK1/2 inhibitor ruxolitinib and the JAK3 inhibitor tofacitinib were recently approved for the treatment of myelofibrosis and rheumatoid arthritis, respectively, and additional ATP-competitive JAK inhibitors are in clinical development. Although these agents show clinical activity, the ability of these JAK inhibitors to induce clinical/molecular remissions in hematologic malignancies seems limited and resistance upon chronic drug exposure is seen. Alternative modes of targeting JAK2 such as allosteric kinase inhibition or HSP90 inhibition are under evaluation, as is the use of histone deacetylase inhibitors. Combination therapy approaches integrating inhibition of STAT, PI3K/Akt, and MAPK pathways with JAK kinase inhibitors might be critical to overcome malignancies characterized by dysregulated JAK signaling.

Combination treatment for myeloproliferative neoplasms using JAK and pan-class I PI3K inhibitors

Current JAK2 inhibitors used for myeloproliferative neoplasms (MPN) treatment are not specific enough to selectively suppress aberrant JAK2 signalling and preserve physiological JAK2 signalling. We tested whether combining a JAK2 inhibitor with a series of serine threonine kinase inhibitors, targeting nine signalling pathways and already used in clinical trials, synergized in inhibiting growth of haematopoietic cells expressing mutant and wild-type forms of JAK2 (V617F) or thrombopoietin receptor (W515L). Out of 15 kinase inhibitors, the ZSTK474 phosphatydylinositol-3'-kinase (PI3K) inhibitor molecule showed strong synergic inhibition by Chou and Talalay analysis with JAK2 and JAK2/JAK1 inhibitors. Other pan-class I, but not gamma or delta specific PI3K inhibitors, also synergized with JAK2 inhibitors. Synergy was not observed in Bcr-Abl transformed cells. The best JAK2/JAK1 and PI3K inhibitor combination pair (ruxolitinib and GDC0941) reduces spleen weight in nude mice inoculated with Ba/F3 cells expressing TpoR and JAK2 V617F. It also exerted strong inhibitory effects on erythropoietin-independent erythroid colonies from MPN patients and JAK2 V617F knock-in mice, where at certain doses, a preferential inhibition of JAK2 V617F mutated progenitors was detected. Our data support the use of a combination of JAK2 and pan-class I PI3K inhibitors in the treatment of MPNs.

JAK2 mutants (e.g., JAK2V617F) and their importance as drug targets in myeloproliferative neoplasms

The Janus kinase 2 (JAK2) mutant V617F and other JAK mutants are found in patients with myeloproliferative neoplasms and leukemias. Due to their involvement in neoplasia and inflammatory disorders, Janus kinases are promising targets for kinase inhibitor therapy. Several small-molecule compounds are evaluated in clinical trials for myelofibrosis, and ruxolitinib (INCB018424, Jakafi®) was the first Janus kinase inhibitor to receive clinical approval. In this review we provide an overview of JAK2V617F signaling and its inhibition by small-molecule kinase inhibitors. In addition, myeloproliferative neoplasms are discussed regarding the role of JAK2V617F and other mutant proteins of possible relevance. We further give an overview about treatment options with special emphasis on possible combination therapies.

JAK2V617F activates Lu/BCAM-mediated red cell adhesion in polycythemia vera through an EpoR-independent Rap1/Akt pathway

Polycythemia vera (PV) is characterized by an increased RBC mass, spontaneous erythroid colony formation, and the JAK2V617F mutation. PV is associated with a high risk of mesenteric and cerebral thrombosis. PV RBC adhesion to endothelial laminin is increased and mediated by phosphorylated erythroid Lu/BCAM. In the present work, we investigated the mechanism responsible for Lu/BCAM phosphorylation in the presence of JAK2V617F using HEL and BaF3 cell lines as well as RBCs from patients with PV. High levels of Rap1-GTP were found in HEL and BaF3 cells expressing JAK2V617F compared with BaF3 cells with wild-type JAK2. This finding was associated with increased Akt activity, Lu/BCAM phosphorylation, and cell adhesion to laminin that were inhibited by the dominant-negative Rap1S17N or by the specific Rap1 inhibitor GGTI-298. Surprisingly, knocking-down EpoR in HEL cells did not alter Akt activity or cell adhesion to laminin. Our findings reveal a novel EpoR-independent Rap1/Akt signaling pathway that is activated by JAK2V617F in circulating PV RBCs and responsible for Lu/BCAM activation. This new characteristic of JAK2V617F could play a critical role in initiating abnormal interactions among circulating and endothelial cells in patients with PV.

Critical roles of Myc-ODC axis in the cellular transformation induced by myeloproliferative neoplasm-associated JAK2 V617F mutant

The acquired mutation (V617F) of Janus kinase 2 (JAK2) is observed in the majority of patients with myeloproliferative neoplasms (MPNs). In the screening of genes whose expression was induced by JAK2 (V617F), we found the significant induction of c-Myc mRNA expression mediated by STAT5 activation. Interestingly, GSK-3β was inactivated in transformed Ba/F3 cells by JAK2 (V617F), and this enhanced the protein expression of c-Myc. The enforced expression of c-Myc accelerated cell proliferation but failed to inhibit apoptotic cell death caused by growth factor deprivation; however, the inhibition of GSK-3β completely inhibited the apoptosis of cells expressing c-Myc. Strikingly, c-Myc T58A mutant exhibited higher proliferative activity in a growth-factor-independent manner; however, this mutant failed to induce apoptosis. In addition, knockdown of c-Myc significantly inhibited the proliferation of transformed cells by JAK2 (V617F), suggesting that c-Myc plays an important role in oncogenic activity of JAK2 (V617F). Furthermore, JAK2 (V617F) induced the expression of a target gene of c-Myc, ornithine decarboxylase (ODC), known as the rate-limiting enzyme in polyamine biosynthesis. An ODC inhibitor, difluoromethylornithine (DFMO), prevented the proliferation of transformed cells by JAK2 (V617F). Importantly, administration of DFMO effectively delayed tumor formation in nude mice inoculated with transformed cells by JAK2 (V617F), resulting in prolonged survival; therefore, ODC expression through c-Myc is a critical step for JAK2 (V617F)-induced transformation and DFMO could be used as effective therapy for MPNs.

JAK2 inhibitors in the treatment of myeloproliferative neoplasms

INTRODUCTION

Dysregulation of JAK-STAT signaling is a pathogenetic hallmark of myeloproliferative neoplasms (MPNs) arising from several distinct molecular aberrations, including mutations in JAK2, the thrombopoietin receptor (MPL), mutations in negative regulators of JAK-STAT signaling, such as lymphocyte-specific adapter protein (SH2B3), and epigenetic dysregulation as seen with Suppressor of Cytokine Signaling (SOCS) proteins. In addition, growth factor/cytokine stimulatory events activate JAK-STAT signaling independent of mutations.

AREAS COVERED

The various mutations and molecular events activating JAK-STAT signaling in MPNs are reviewed. Detailed inhibitory kinase profiles of the currently developed JAK inhibitors are presented. Clinical trial results for currently developed JAK targeting agents are comprehensively summarized. The limitations of JAK-STAT targeting in MPNs, as well as potential rational combination therapies with JAK2 inhibitors, are discussed.

EXPERT OPINION

Aberrant JAK-STAT signaling is an underlying theme in the pathogenesis of MPNs. While JAK2 inhibitors are active in JAK2V617F and wild-type JAK2 MPNs, JAK2V617F mutation-specific or JAK2-selective inhibitors may possess unique clinical attributes. Complimentary targeting of parallel pathways operating in MPNs may offer novel therapeutic approaches in combination with JAK inhibition. Understanding the intricacies of JAK-STAT pathway activation, including growth factor/cytokine-driven signaling, will open new avenues for therapeutic intervention at known and novel molecular vulnerabilities of MPNs.

Genotype-phenotype interactions in the myeloproliferative neoplasms

The chronic myeloproliferative neoplasms (MPNs) are clonal disorders characterized by overproduction of mature myeloid cells. They share associations with molecular abnormalities such as the JAK2V617F mutation but are distinguished by important phenotypic differences. This review first considers the factors that may influence phenotype in JAK2-mutated MPNs, especially polycythemia vera (PV) and essential thrombocythemia (ET), and then discusses the mutations implicated in JAK2-negative MPNs such as in MPL and epigenetic regulators. Current evidence supports a model where ET and PV are disorders of relatively low genetic complexity, whereas evolution to myelofibrosis or blast-phase disease reflects accumulation of a higher mutation burden.

Fullerene derivative prevents cellular transformation induced by JAK2 V617F mutant through inhibiting c-Jun N-terminal kinase pathway

The constitutively activated mutation (V617F) of tyrosine kinase Janus kinase 2 (JAK2) is found in the majority of patients with myeloproliferative neoplasms (MPNs). The development of a novel chemical compound to suppress JAK2 V617F mutant-induced onset of MPNs and clarification of the signaling cascade downstream of JAK2 V617F mutant will provide clues to treat MPNs. Here we found that a water-soluble pyrrolidinium fullerene derivative, C(60)-bis (N, N-dimethylpyrrolidinium iodide), markedly induced apoptosis of JAK2 V617F mutant-induced transformed cells through a novel mechanism, inhibiting c-Jun N-terminal kinase (JNK) activation pathway but not generation of reactive oxygen species (ROS). Pyrrolidinium fullerene derivative significantly reduced the protein expression level of apoptosis signal-regulating kinase 1 (ASK1), one of the mitogen-activated protein kinase kinase kinases (MAPKKK), resulting in the inhibition of upstream molecules of JNK, mitogen-activated protein kinase kinase 4 (MKK4) and mitogen-activated protein kinase kinase 7 (MKK7). Strikingly, the knockdown of ASK1 enhanced the sensitivity to pyrrolidinium fullerene derivative-induced apoptosis, and the treatment with a JNK inhibitor, SP600125, also induced apoptosis of the transformed cells by JAK2 V617F mutant. Furthermore, administration of both SP600125 and pyrrolidinium fullerene derivative markedly inhibited JAK2 V617F mutant-induced tumorigenesis in nude mice. Taking these findings together, JAK2 V617F mutant-induced JNK signaling pathway is an attractive target for MPN therapy, and pyrrolidinium fullerene derivative is now considered a candidate potent drug for MPNs.

Janus kinase deregulation in leukemia and lymphoma

Genetic alterations affecting members of the Janus kinase (JAK) family have been discovered in a wide array of cancers and are particularly prominent in hematological malignancies. In this review, we focus on the role of such lesions in both myeloid and lymphoid tumors. Oncogenic JAK molecules can activate a myriad of canonical downstream signaling pathways as well as directly interact with chromatin in noncanonical processes, the interplay of which results in a plethora of diverse biological consequences. Deciphering these complexities is shedding unexpected light on fundamental cellular mechanisms and will also be important for improved diagnosis, identification of new therapeutic targets, and the development of stratified approaches to therapy.