The pseudokinase domain is required for suppression of basal activity of Jak2 and Jak3 tyrosine kinases and for cytokine-inducible activation of signal transduction

New targeted therapies such as anti-adhesion molecules, anti-IL-12/23 and anti-Janus kinases are looking toward a more effective treatment of inflammatory bowel disease

Antitumor necrosis factor α agents have dramatically changed the management of inflammatory bowel disease (IBD). However, a significant proportion of patients does not respond or lose response over time. Hence, there is an urgent need for new molecules, with different mechanisms of action, and with a targeted and more effective approach. These new drugs include either small molecules or biological agents. We describe the three most promising classes of molecules in the field of IBD: anti-adhesion, anti-interleukin 12/23 and anti-Janus Kinases therapies.

Cooperating JAK1 and JAK3 mutants increase resistance to JAK inhibitors

The acquisition of growth signal self-sufficiency is 1 of the hallmarks of cancer. We previously reported that the murine interleukin-9-dependent TS1 cell line gives rise to growth factor-independent clones with constitutive activation of the Janus kinase (JAK)- signal transducer and activator of transcription (STAT) pathway. Here, we show that this transforming event results from activating mutations either in JAK1, JAK3, or in both kinases. Transient and stable expression of JAK1 and/or JAK3 mutants showed that each mutant induces STAT activation and that their coexpression further increases this activation. The proliferation of growth factor-independent TS1 clones can be efficiently blocked by JAK inhibitors such as ruxolitinib or CMP6 in short-term assays. However, resistant clones occur upon long-term culture in the presence of inhibitors. Surprisingly, resistance to CMP6 was not caused by the acquisition of secondary mutations in the adenosine triphosphate-binding pocket of the JAK mutant. Indeed, cells that originally showed a JAK1-activating mutation became resistant to inhibitors by acquiring another activating mutation in JAK3, whereas cells that originally showed a JAK3-activating mutation became resistant to inhibitors by acquiring another activating mutation in JAK1. These observations underline the cooperation between JAK1 and JAK3 mutants in T-cell transformation and represent a new mechanism of acquisition of resistance against JAK inhibitors.

The molecular regulation of Janus kinase (JAK) activation

The JAK (Janus kinase) family members serve essential roles as the intracellular signalling effectors of cytokine receptors. This family, comprising JAK1, JAK2, JAK3 and TYK2 (tyrosine kinase 2), was first described more than 20 years ago, but the complexities underlying their activation, regulation and pleiotropic signalling functions are still being explored. Here, we review the current knowledge of their physiological functions and the causative role of activating and inactivating JAK mutations in human diseases, including haemopoietic malignancies, immunodeficiency and inflammatory diseases. At the molecular level, recent studies have greatly advanced our knowledge of the structures and organization of the component FERM (4.1/ezrin/radixin/moesin)-SH2 (Src homology 2), pseudokinase and kinase domains within the JAKs, the mechanism of JAK activation and, in particular, the role of the pseudokinase domain as a suppressor of the adjacent tyrosine kinase domain's catalytic activity. We also review recent advances in our understanding of the mechanisms of negative regulation exerted by the SH2 domain-containing proteins, SOCS (suppressors of cytokine signalling) proteins and LNK. These recent studies highlight the diversity of regulatory mechanisms utilized by the JAK family to maintain signalling fidelity, and suggest alternative therapeutic strategies to complement existing ATP-competitive kinase inhibitors.

Update on Janus kinase antagonists in inflammatory bowel disease

Janus kinase (JAK) inhibitors have emerged as a novel orally administered small-molecule therapy for the treatment of ulcerative colitis and possibly Crohn disease. These molecules are designed to selectively target the activity of specific JAKs and to offer a targeted mechanism of action without risk of immunogenicity. Based on data from clinical trials in rheumatoid arthritis and phase 2 studies in inflammatory bowel disease, tofacitinib and other JAK inhibitors are likely to become a new form of medical therapy for the treatment of inflammatory bowel disease.

Molecular basis for pseudokinase-dependent autoinhibition of JAK2 tyrosine kinase

Janus kinase-2 (JAK2) mediates signaling by various cytokines, including erythropoietin and growth hormone. JAK2 possesses tandem pseudokinase and tyrosine-kinase domains. Mutations in the pseudokinase domain are causally linked to myeloproliferative neoplasms (MPNs) in humans. The structure of the JAK2 tandem kinase domains is unknown, and therefore the molecular bases for pseudokinase-mediated autoinhibition and pathogenic activation remain obscure. Using molecular dynamics simulations of protein-protein docking, we produced a structural model for the autoinhibitory interaction between the JAK2 pseudokinase and kinase domains. A striking feature of our model, which is supported by mutagenesis experiments, is that nearly all of the disease mutations map to the domain interface. The simulations indicate that the kinase domain is stabilized in an inactive state by the pseudokinase domain, and they offer a molecular rationale for the hyperactivity of V617F, the predominant JAK2 MPN mutation.

Mechanism of activation of protein kinase JAK2 by the growth hormone receptor

Signaling from JAK (Janus kinase) protein kinases to STAT (signal transducers and activators of transcription) transcription factors is key to many aspects of biology and medicine, yet the mechanism by which cytokine receptors initiate signaling is enigmatic. We present a complete mechanistic model for activation of receptor-bound JAK2, based on an archetypal cytokine receptor, the growth hormone receptor. For this, we used fluorescence resonance energy transfer to monitor positioning of the JAK2 binding motif in the receptor dimer, substitution of the receptor extracellular domains with Jun zippers to control the position of its transmembrane (TM) helices, atomistic modeling of TM helix movements, and docking of the crystal structures of the JAK2 kinase and its inhibitory pseudokinase domain with an opposing kinase-pseudokinase domain pair. Activation of the receptor dimer induced a separation of its JAK2 binding motifs, driven by a ligand-induced transition from a parallel TM helix pair to a left-handed crossover arrangement. This separation leads to removal of the pseudokinase domain from the kinase domain of the partner JAK2 and pairing of the two kinase domains, facilitating trans-activation. This model may well generalize to other class I cytokine receptors.

Structure of the pseudokinase-kinase domains from protein kinase TYK2 reveals a mechanism for Janus kinase (JAK) autoinhibition

Janus kinases (JAKs) are receptor-associated multidomain tyrosine kinases that act downstream of many cytokines and interferons. JAK kinase activity is regulated by the adjacent pseudokinase domain via an unknown mechanism. Here, we report the 2.8-Å structure of the two-domain pseudokinase-kinase module from the JAK family member TYK2 in its autoinhibited form. We find that the pseudokinase and kinase interact near the kinase active site and that most reported mutations in cancer-associated JAK alleles cluster in or near this interface. Mutation of residues near the TYK2 interface that are analogous to those in cancer-associated JAK alleles, including the V617F and "exon 12" JAK2 mutations, results in increased kinase activity in vitro. These data indicate that JAK pseudokinases are autoinhibitory domains that hold the kinase domain inactive until receptor dimerization stimulates transition to an active state.

Ars Moriendi; the art of dying well - new insights into the molecular pathways of necroptotic cell death

When our time comes to die most people would probably opt for a quick, peaceful and painless exit. But the manner and timing are rarely under our direct control. Hence the Ars moriendi, literally, "The Art of Dying", two texts written in Latin around the 15th century that offered advice on how to die well according to the Christian ideals of the time. In contrast, for individual cells, the death process is frequently under their control and several signaling pathways that cause cell death, including apoptosis, pyroptosis and necroptosis, have been described. Furthermore the manner in which cells die can have good or bad consequences for the organism. In this review we will discuss how cells die via the necroptotic signaling pathway, with emphasis on recent structural work and place this work in a biological context by discussing relevant studies with knock-out animals.

Discovery and development of Janus kinase (JAK) inhibitors for inflammatory diseases

The Janus kinases (JAKs) are a family of intracellular tyrosine kinases that play an essential role in the signaling of numerous cytokines that have been implicated in the pathogenesis of inflammatory diseases. As a consequence, the JAKs have received significant attention in recent years from the pharmaceutical and biotechnology industries as therapeutic targets. Here, we provide a review of the JAK pathways, the structure, function, and activation of the JAK enzymes followed by a detailed look at the JAK inhibitors currently in the clinic or approved for these indications. Finally, a perspective is provided on what the past decade of research with JAK inhibitors for inflammatory indications has taught along with thoughts on what the future may hold in terms of addressing the opportunities and challenges that remain.

New insights into the structure and function of the pseudokinase domain in JAK2

JAK (Janus kinase) 2 plays a critical role in signal transduction through several cytokine receptors. JAKs contain a typical tyrosine kinase domain preceded by a pseudokinase [JH2 (JAK homology 2)] domain which has been considered to be catalytically inactive. Identification of activating mutations in the JH2 domain of JAK2 as the major cause for polycythaemia vera and other MPNs (myeloproliferative neoplasms) demonstrate the critical regulatory function for this domain, but the underlying mechanisms have remained elusive. We have performed biochemical and functional analysis on the JH2 domain of JAK2. The results indicate that JH2 functions as an active protein kinase and phosphorylates two residues in JAK2 (Ser523 and Tyr570) that have been shown previously to be negative regulatory sites for JAK2 activity. The crystal structure of the JAK2 JH2 domain provides an explanation for the functional findings and shows that JH2 adopts a prototypical kinase fold, but binds MgATP through a non-canonical mode. The structure of the most prevalent pathogenic JH2 mutation V617F shows a high level of similarity to wild-type JH2. The most notable structural deviation is observed in the N-lobe αC-helix. The structural and biochemical data together with MD (molecular dynamics) simulations show that the V617F mutation rigidifies the αC-helix, which results in hyperactivation of the JH1 domain through an as yet unidentified mechanism. These results provide structural and functional insights into the normal and pathogenic function of the JH2 domain of JAK2.

JAK inhibition in the myeloproliferative neoplasms: lessons learned from the bench and bedside

The discovery of the JAK2 V617F mutation in the classic BCR-ABL1–negative myeloproliferative neoplasms in 2005 catalyzed a burst of research efforts that have culminated in substantial dividends for patients. Beyond JAK2 V617F, a more detailed picture of the pathobiologic basis for activated JAK-STAT signaling has emerged. In some patients with myelofibrosis (MF), next-generation sequencing technologies have revealed a complex clonal architecture affecting both genetic and epigenetic regulators of cell growth and differentiation. Although these bench-top findings have informed the clinical development of JAK inhibitors in MF, they have also provided scientific context for some of their limitations. The JAK1/JAK2 inhibitor ruxolitinib is approved for treatment of MF in North America and Europe and other lead JAK inhibitors discussed herein (fedratinib [SAR302503], momelotinib [CYT387], and pacritinib [SB1518]), have entered advanced phases of trial investigation. Uniformly, these agents share the ability to reduce spleen size and symptom burden. A major challenge for practitioners is how to optimize dosing of these agents to secure clinically relevant and durable benefits while minimizing myelosuppression. Suboptimal responses have spurred a “return to the bench” to characterize the basis for disease persistence and to inform new avenues of drug therapy.

The pseudokinase MLKL mediates necroptosis via a molecular switch mechanism

Mixed lineage kinase domain-like (MLKL) is a component of the "necrosome," the multiprotein complex that triggers tumor necrosis factor (TNF)-induced cell death by necroptosis. To define the specific role and molecular mechanism of MLKL action, we generated MLKL-deficient mice and solved the crystal structure of MLKL. Although MLKL-deficient mice were viable and displayed no hematopoietic anomalies or other obvious pathology, cells derived from these animals were resistant to TNF-induced necroptosis unless MLKL expression was restored. Structurally, MLKL comprises a four-helical bundle tethered to the pseudokinase domain, which contains an unusual pseudoactive site. Although the pseudokinase domain binds ATP, it is catalytically inactive and its essential nonenzymatic role in necroptotic signaling is induced by receptor-interacting serine-threonine kinase 3 (RIPK3)-mediated phosphorylation. Structure-guided mutation of the MLKL pseudoactive site resulted in constitutive, RIPK3-independent necroptosis, demonstrating that modification of MLKL is essential for propagation of the necroptosis pathway downstream of RIPK3.

Activating Janus kinase pseudokinase domain mutations in myeloproliferative and other blood cancers

The discovery of the highly prevalent activating JAK (Janus kinase) 2 V617F mutation in myeloproliferative neoplasms, and of other pseudokinase domain-activating mutations in JAK2, JAK1 and JAK3 in blood cancers, prompted great interest in understanding how pseudokinase domains regulate kinase domains in JAKs. Recent functional and mutagenesis studies identified residues required for the V617F mutation to induce activation. Several X-ray crystal structures of either kinase or pseudokinase domains including the V617F mutant of JAK2 pseudokinase domains are now available, and a picture has emerged whereby the V617F mutation induces a defined conformational change around helix C of JH (JAK homology) 2. Effects of mutations on JAK2 can be extrapolated to JAK1 and TYK2 (tyrosine kinase 2), whereas JAK3 appears to be different. More structural information of the full-length JAK coupled to cytokine receptors might be required in order to define the structural basis of JH1 activation by JH2 mutants and eventually obtain mutant-specific inhibitors.

Whole-genome sequencing identifies recurrent mutations in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most deadly cancers worldwide and has no effective treatment, yet the molecular basis of hepatocarcinogenesis remains largely unknown. Here we report findings from a whole-genome sequencing (WGS) study of 88 matched HCC tumor/normal pairs, 81 of which are Hepatitis B virus (HBV) positive, seeking to identify genetically altered genes and pathways implicated in HBV-associated HCC. We find beta-catenin to be the most frequently mutated oncogene (15.9%) and TP53 the most frequently mutated tumor suppressor (35.2%). The Wnt/beta-catenin and JAK/STAT pathways, altered in 62.5% and 45.5% of cases, respectively, are likely to act as two major oncogenic drivers in HCC. This study also identifies several prevalent and potentially actionable mutations, including activating mutations of Janus kinase 1 (JAK1), in 9.1% of patients and provides a path toward therapeutic intervention of the disease.

Advances in the discovery of selective JAK inhibitors

In this review, we describe the current knowledge of the biology of the JAKs. The JAK family comprises the four nonreceptor tyrosine kinases JAK1, JAK2, JAK3, and Tyk2, all key players in the signal transduction from cytokine receptors to transcription factor activation. We also review the progresses made towards the optimization of JAK inhibitors and the importance of their selectivity profile. Indeed, the full array of many medicinal chemistry enabling tools (HTS, X-ray crystallography, scaffold morphing, etc.) has been deployed to successfully design molecules that discriminate among JAK family and other kinases. While the first JAK inhibitor was launched in 2011, this review also summarizes the status of several other small-molecule JAK inhibitors currently in development to treat arthritis, psoriasis, organ rejection, and multiple cancer types.

Acylglycerol kinase augments JAK2/STAT3 signaling in esophageal squamous cells

JAK2 activity is tightly controlled through a self-inhibitory effect via its JAK homology domain 2 (JH2), which restricts the strength and duration of JAK2/STAT3 signaling under physiological conditions. Although multiple mutations within JAK2, which abrogate the function of JH2 and sustain JAK2 activation, are widely observed in hematological malignancies, comparable mutations have not been detected in solid tumors. How solid tumor cells override the autoinhibitory effect of the JH2 domain to maintain constitutive activation of JAK2/STAT3 signaling remains puzzling. Herein, we demonstrate that AGK directly interacted with the JH2 domain to relieve inhibition of JAK2 and activate JAK2/STAT3 signaling. Overexpression of AGK sustained constitutive JAK2/STAT3 activation, consequently promoting the cancer stem cell population and augmenting the tumorigenicity of esophageal squamous cell carcinoma (ESCC) cells both in vivo and in vitro. Furthermore, AGK levels significantly correlated with increased STAT3 phosphorylation, poorer disease-free survival, and shorter overall survival in primary ESCC. More importantly, AGK expression was significantly correlated with JAK2/STAT3 hyperactivation in ESCC, as well as in lung and breast cancer. These findings uncover a mechanism for constitutive activation of JAK2/STAT3 signaling in solid tumors and may represent a prognostic biomarker and therapeutic target.

On the regulation and activation of JAK2: a novel hypothetical model

Janus kinase 2 (JAK2) is a protein tyrosine kinase central to a multitude of cellular processes. Here, a novel model of JAK2 regulation and activation is proposed. In the JAK2 dimer, instead of being auto-inhibited by its own JH2 domain, inhibition comes from the JH2 domain of the partnering JAK2 monomer. Upon ligand binding, the receptor undergoes a conformational rotation that is passed to its dimeric partner. The activation is achieved by the rotation of two JAK2 molecules, which relieves the JH1/JH2 inhibitory interface and brings two JH1 domains in proximity for the subsequent trans-phosphorylation event. This hypothetical model is consistent with most of the currently available experimental evidence and warrants further tests. Based on the proposed model, it is possible to rationalize the differential responses of JAK2 signaling involving various receptors and ligands.

IMPLICATIONS

The proposed model of JAK2 regulation and activation is poised to suggest potential alternative drug-discovery strategies that could impact a number of relevant diseases.

Ab initio modeling and experimental assessment of Janus Kinase 2 (JAK2) kinase-pseudokinase complex structure

The Janus Kinase 2 (JAK2) plays essential roles in transmitting signals from multiple cytokine receptors, and constitutive activation of JAK2 results in hematopoietic disorders and oncogenesis. JAK2 kinase activity is negatively regulated by its pseudokinase domain (JH2), where the gain-of-function mutation V617F that causes myeloproliferative neoplasms resides. In the absence of a crystal structure of full-length JAK2, how JH2 inhibits the kinase domain (JH1), and how V617F hyperactivates JAK2 remain elusive. We modeled the JAK2 JH1-JH2 complex structure using a novel informatics-guided protein-protein docking strategy. A detailed JAK2 JH2-mediated auto-inhibition mechanism is proposed, where JH2 traps the activation loop of JH1 in an inactive conformation and blocks the movement of kinase αC helix through critical hydrophobic contacts and extensive electrostatic interactions. These stabilizing interactions are less favorable in JAK2-V617F. Notably, several predicted binding interfacial residues in JH2 were confirmed to hyperactivate JAK2 kinase activity in site-directed mutagenesis and BaF3/EpoR cell transformation studies. Although there may exist other JH2-mediated mechanisms to control JH1, our JH1-JH2 structural model represents a verifiable working hypothesis for further experimental studies to elucidate the role of JH2 in regulating JAK2 in both normal and pathological settings.

Kinase activation in circulating cells: opportunities for biomarkers for diagnosis and therapeutic monitoring

A clinically useful tool to assay phosphorylation-dependent signaling in circulating cells has the potential to provide a wealth of information about a patient's health, including information unavailable by any other method. Patterns of kinase activation, such as the abnormal signaling characteristic of myeloproliferative disorders, may offer highly specific biomarkers for diagnosis or monitoring the efficacy of therapeutics. For assays of kinase activity in circulating leukocytes to be standardized, let alone made practical for the clinic, numerous technical hurdles must be overcome. In this review the current status of analysis of kinase signaling in circulating cells and recent progress in biomarker discovery and validation is discussed. Looking forward, the potential value of signaling patterns as complex biomarkers and the resulting need for future development of robust, multiplexed assays of kinase activation is addressed.

Investigational Janus kinase inhibitors

INTRODUCTION

Dysregulation of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway is central to the pathophysiology of myeloproliferative neoplasms (MPN). Small molecule inhibitors of JAK family members are currently under investigation for the treatment of MPN. Of these, ruxolitinib has received approval for clinical use in myelofibrosis in the United States and Europe.

AREAS COVERED

The clinical results and future development program of major JAK inhibitors, including ruxolitinib, CYT387, SAR302503, lestaurtinib, pacritinib, XL-019, LY2784544, BMS-911453, AZD1480 and NS-018 are reviewed.

EXPERT OPINION

JAK inhibitors are effective in relieving organomegaly (splenomegaly and hepatomegaly) and constitutional symptoms of myelofibrosis and some modulate inflammatory cytokines. However, they have little impact on disease burden and bone marrow fibrosis. The relationship between clinical efficacy, toxicity profile and specificity of JAK family member inhibition (i.e., JAK2 specific vs JAK1/JAK2 active) is poorly defined. Novel resistance mechanisms including heterodimerization of JAK2 with other JAK family members have been described. It is likely that the future lies in the use of rational drug combinations that target multiple signaling pathways.

JAK inhibition in the myeloproliferative neoplasms: lessons learned from the bench and bedside

The discovery of the JAK2 V617F mutation in the classic BCR-ABL1-negative myeloproliferative neoplasms in 2005 catalyzed a burst of research efforts that have culminated in substantial dividends for patients. Beyond JAK2 V617F, a more detailed picture of the pathobiologic basis for activated JAK-STAT signaling has emerged. In some patients with myelofibrosis (MF), next-generation sequencing technologies have revealed a complex clonal architecture affecting both genetic and epigenetic regulators of cell growth and differentiation. Although these bench-top findings have informed the clinical development of JAK inhibitors in MF, they have also provided scientific context for some of their limitations. The JAK1/JAK2 inhibitor ruxolitinib is approved for treatment of MF in North America and Europe and other lead JAK inhibitors discussed herein (fedratinib [SAR302503], momelotinib [CYT387], and pacritinib [SB1518]), have entered advanced phases of trial investigation. Uniformly, these agents share the ability to reduce spleen size and symptom burden. A major challenge for practitioners is how to optimize dosing of these agents to secure clinically relevant and durable benefits while minimizing myelosuppression. Suboptimal responses have spurred a "return to the bench" to characterize the basis for disease persistence and to inform new avenues of drug therapy.

Genetic and epigenetic alterations of myeloproliferative disorders

The classical BCR-ABL negative myeloproliferative neoplasms (MPN) polycythemia vera, essential thrombocythemia, and primary myelofibrosis are clonal hematopoietic disorders characterized by excessive production of terminally differentiated myeloid cells. In MPN patients, the disease can progress to secondary myelofibrosis or acute myeloid leukemia. Clonal hematopoiesis, disease phenotype, and progression are caused by somatically acquired genetic lesions of genes involved in cytokine signaling, RNA splicing, as well as epigenetic regulation. This review provides an overview of point mutations and cytogenetic lesions associated with MPN and addresses the role of these somatic lesions in MPN disease progression.

Small-molecule inhibitors in myeloproliferative neoplasms: are we aiming for the right targets?

The ATP-binding pocket of the kinase domain of JAK2 is the major target of the present treatment of myeloproliferative neoplasms. Several inhibitors of JAK2 that are ATP competitive have been developed, but they do not discriminate between wild-type and mutant JAK2. These inhibitors have been used in myelofibrosis and, for the first time, treatment induced a reduction in spleen size and in constitutional symptoms. However, no dramatic effects on BM fibrosis, allele burden, or peripheral blast numbers were observed. These data indicate that other avenues should be explored that would either target mutant molecules (JAKs or receptors) more specifically and spare wild-type JAK2 or that would address other pathways that contribute to the malignant proliferation. Future success in treating myeloproliferative neoplasms will depend on advances of the understanding of JAK-STAT signaling and also on a better understanding of the disease pathogenesis, especially the role that mutants in spliceosome factors and epigenetic regulators play in the phenotype of the disease and the precise mechanism of fibrosis development.

[Analysis of oncogenic signaling pathway induced by a myeloproliferative neoplasm-associated Janus kinase 2 (JAK2) V617F mutant]

Janus kinase 2 (JAK2) is an essential non-receptor type tyrosine kinase for various cytokine signals. In 2005, a somatic JAK2 mutation (V617F) was found in the majority of myeloproliferative neoplasm (MPN) patients. It has been shown that the V617F mutation caused the constitutive activation of JAK2, exhibiting the cytokine-independent survival and proliferation of Ba/F3 cells. In addition, tumorigenesis was induced after a transplantation of Ba/F3 cells expressing JAK2 V617F mutant in nude mice, suggesting that JAK2 V617F mutant behaves as a potent oncogene product. We found that JAK2 V617F mutant causes aberrant activation of a transcription factor c-Myc, which is critical for the JAK2 V617F mutant-caused oncogenic activities. In the screening of genes which expression was induced by JAK2 V617F mutant, we detected the significant induction of target genes of c-Myc such as Aurora kinase A (Aurka) and ornithine decarboxylase (ODC). Interestingly, JAK2 V617F mutant enhanced resistance to cisplatin (CDDP)-induced DNA damage and ectopic expression of Aurka in Ba/F3 cells exhibited similar resistance to CDDP. Conversely, knockdown and inhibition of Aurka in cells expressing JAK2 V617F mutant abolished the resistance to CDDP, suggesting that Aurka is most likely critical for resistance to DNA damage in cells transformed by JAK2 V617F mutant. In addition, we found that ODC inhibitor, DL-α-difluoromethylornithine (DFMO) prevented the proliferation of the JAK2 V617F mutant-induced transformed cells. Taking these observations together, c-Myc plays an essential role in JAK2 V617F mutant-induced hematopoietic disorder and would be a good target for the treatment of MPN.

Regulation of JAK2 activation by Janus homology 2: evidence from molecular dynamics simulations

Janus kinase 2 (JAK2) is a protein tyrosine kinase implicated in signaling by specific members of the cytokine receptor family. Although it has been established that the JAK2 tyrosine kinase is negatively regulated by the JAK homology 2 (JH2) pseudokinase domain, the underlying mechanism of JH2 mediated regulation remains elusive. To elucidate the regulation of JAK2 kinase, we have built a structural model for the kinase and pseudokinase domains of JAK2. An asymmetric dimer is proposed, in which the kinase domain JH1 occupies a position where it could not be activated. We investigate the dynamic and energetic properties of the dimer by molecular dynamics simulation. JAK2 activation requires the two domains to be dissociated and rearranged in a form such that the JH1 kinase domain can adopt an active conformation. The significance of the above mechanism is emphasized by the finding that the activating V617F mutation destabilizes JH1-JH2 association in the proposed asymmetric dimer. Thus abrogation of the domain-domain interaction seems to be a possible first step for the structural rearrangement of the two domains, resulting in constitutive activation of JAK2 by the V617F mutation.

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.

Disordered signaling in myeloproliferative neoplasms

The human myeloproliferative neoplasms (MPN) have long been associated with abnormal responses to cytokines and activation of signaling pathways, although the exact molecular mechanisms underlying these observations were unknown. This situation altered with the discovery of the JAK2 V617F, which presaged the ongoing description of further mutations predicted to activate canonical signaling pathways in MPN. This article covers the nature of these mutations and summarizes functional experiments in model systems and in human MPN cells to define the signaling pathways altered and how these drive and determine the MPN cellular phenotype. Also discussed are recently described, novel noncanonical signaling pathways to chromatin predicted to alter gene transcription more directly and to also contribute to the MPN phenotype.

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.

Tyrosine 201 is required for constitutive activation of JAK2V617F and efficient induction of myeloproliferative disease in mice

The JAK2V617F mutation has been detected in most cases of Ph-negative myeloproliferative neoplasms (MPNs). The JAK2V617F protein is a constitutively activated tyrosine kinase that leads to transformation of hematopoietic progenitors. Previous studies have shown that several tyrosine residues within JAK2 are phosphorylated on growth factor or cytokine stimulation. However, the role of these tyrosine residues in signaling and transformation mediated by JAK2V617F remains unclear. In this study, we sought to determine the role of tyrosine 201, which is a potential binding site for Src homology 2 domain-containing proteins, in JAK2V617F-induced hematopoietic transformation by introducing a tyrosine-to-phenylalanine point mutation (Y201F) at this site. We observed that the Y201F mutation significantly inhibited cytokine-independent cell growth and induced apoptosis in Ba/F3-EpoR cells expressing JAK2V617F. The Y201F mutation also resulted in significant inhibition of JAK2V617F-mediated transformation of hematopoietic cells. Biochemical analyzes revealed that the Y201F mutation almost completely inhibited constitutive phosphorylation/activation of JAK2V617F. We also show that the Y201 site of JAK2V617F promotes interaction with Stat5 and Shp2, and constitutive activation of downstream signaling pathways. Furthermore, using a BM transduction/transplantation approach, we found that tyrosine 201 plays an important role in the induction of MPNs mediated by JAK2V617F.

Crystal structures of the JAK2 pseudokinase domain and the pathogenic mutant V617F

The protein tyrosine kinase JAK2 mediates signaling through numerous cytokine receptors. JAK2 possesses a pseudokinase domain (JH2) and a tyrosine kinase domain (JH1). Through unknown mechanisms, JH2 regulates the catalytic activity of JH1, and hyperactivating mutations in the JH2 region of human JAK2 cause myeloproliferative neoplasms (MPNs). We showed previously that JAK2 JH2 is, in fact, catalytically active. Here we present crystal structures of human JAK2 JH2, including both wild type and the most prevalent MPN mutant, V617F. The structures reveal that JH2 adopts the fold of a prototypical protein kinase but binds Mg-ATP noncanonically. The structural and biochemical data indicate that the V617F mutation rigidifies α-helix C in the N lobe of JH2, facilitating trans-phosphorylation of JH1. The crystal structures of JH2 afford new opportunities for the design of novel JAK2 therapeutics targeting MPNs.

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.

Alternative TEL-JAK2 fusions associated with T-cell acute lymphoblastic leukemia and atypical chronic myelogenous leukemia dissected in zebrafish

BACKGROUND

Chromosomal translocations resulting in alternative fusions of the human TEL (ETV6) and JAK2 genes have been observed in cases of acute lymphoblastic leukemia and chronic myelogenous leukemia, but a full understanding of their role in disease etiology has remained elusive. In this study potential differences between these alternative TEL-JAK2 fusions, including their lineage specificity, were investigated.

DESIGN AND METHODS

TEL-JAK2 fusion types derived from both T-cell acute lymphoblastic leukemia and atypical chronic myelogenous leukemia were generated using the corresponding zebrafish tel and jak2a genes and placed under the control of either the white blood cell-specific spi1 promoter or the ubiquitously-expressed cytomegalovirus promoter. These constructs were injected into zebrafish embryos and their effects on hematopoiesis examined using a range of molecular approaches. In addition, the functional properties of the alternative fusions were investigated in vitro.

RESULTS

Injection of the T-cell acute lymphoblastic leukemia-derived tel-jak2a significantly perturbed lymphopoiesis with a lesser effect on myelopoiesis in zebrafish embryos. In contrast, injection of the atypical chronic myelogenous leukemia-derived tel-jak2a resulted in significant perturbation of the myeloid compartment. These phenotypes were observed regardless of whether expressed in a white blood cell-specific or ubiquitous manner, with no overt cellular proliferation outside of the hematopoietic cells. Functional studies revealed subtle differences between the alternative forms, with the acute lymphoblastic leukemia variant showing higher activity, but reduced downstream signal transducer and activator of transcription activation and decreased sensitivity to JAK2 inhibition. JAK2 activity was required to mediate the effects of both variants on zebrafish hematopoiesis.

CONCLUSIONS

This study indicates that the molecular structure of alternative TEL-JAK2 fusions likely contributes to the etiology of disease. The data further suggest that this class of oncogene exerts its effects in a cell lineage-specific manner, which may be due to differences in downstream signaling.

The role of pseudokinases in cancer

Kinases play a critical role in regulating many cellular functions including development, differentiation and proliferation. To date, over 518 proteins with kinase activity, comprising ~2-3% of total cellular proteins, have been identified from within the human kinome. Interestingly, approximately 10% of kinases are categorised as pseudokinases since they lack one or more conserved catalytic residues within their kinase domain and were originally thought to have no enzymatic activity. Recently, there has been strong evidence to suggest that some pseudokinsases can not only function as scaffold proteins, but may also possess kinase activity leading to modulation of cell signalling pathways. Altered activity of these pseudokinases can result in impaired cellular function, particularly in malignancies. In this review we are discussing recent evidence that apart from a scaffolding role, pseudokinases also orchestrate cellular processes as active kinases per se in signalling pathways of malignant cells.

Biology and significance of the JAK/STAT signalling pathways

Since its discovery two decades ago, the activation of the Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway by numerous cytokines and growth factors has resulted in it becoming one of the most well-studied intracellular signalling networks. The field has progressed from the identification of the individual components to high-resolution crystal structures of both JAK and STAT, and an understanding of the complexities of the molecular activation and deactivation cycle which results in a diverse, yet highly specific and regulated pattern of transcriptional responses. While there is still more to learn, we now appreciate how disruption and deregulation of this pathway can result in clinical disease and look forward to adoption of the next generation of JAK inhibitors in routine clinical treatment.

Janus kinase 3: the controller and the controlled

Janus kinase (JAK)-signal transducer and activators of transcription (STAT) signaling pathways play crucial roles in lymphopoiesis. In particular, JAK3 has unique functions in the lymphoid system such that JAK3 ablation results in phenotypes resembling severe combined immunodeficiency syndrome. This review focuses on the biochemistry, immunological functions, and clinical significance of JAK3. Compared with other members of the JAK family, the biochemical properties of JAK3 are relatively less well characterized and thus largely inferred from studies of JAK2. Furthermore, new findings concerning the cross-talks between Notch and JAK signaling pathways through ubiquitin-mediated protein degradation are discussed in more detail.

A structure-function perspective of Jak2 mutations and implications for alternate drug design strategies: the road not taken

Jak2 is a non-receptor tyrosine kinase that is involved in the control of cellular growth and proliferation. Due to its significant role in hematopoiesis, Jak2 is a frequent target for mutations in cancer, especially myeloid leukemia, lymphoid leukemia and the myeloproliferative neoplasms (MPN). These mutations are common amongst different populations all over the world and there is a great deal of effort to develop therapeutic drugs for the affected patients. Jak2 mutations, whether they are point, deletion, or gene fusion, most commonly result in constitutive kinase activation. Here, we explore the structure-function relation of various Jak2 mutations identified in cancer and understand how they disrupt Jak2 regulation. Current Jak2 inhibitors target the highly conserved active site in the kinase domain and therefore, these inhibitors may lack specificity. Based on our knowledge regarding structure-function correlations as they pertain to regulation of Jak2 kinase activity, an alternative approach for specific Jak2 targeting could be via allosteric inhibitor design. Successful reports of allosteric inhibitors developed against other kinases provide precedent for the development of Jak2 allosteric inhibitors. Here, we suggest plausible target sites in the Jak2 structure for allosteric inhibition. Such targets include the type II inhibitor pocket and substrate binding site in the kinase domain, the kinase-pseudokinase domain interface, SH2-JH2 linker region and the FERM domain. Thus, future Jak2 inhibitors that target these sites via allosteric mechanisms may provide alternative therapeutic strategies to existing ATP competitive inhibitors.

Targeting JAK2 in the therapy of myeloproliferative neoplasms

INTRODUCTION

Myeloproliferative neoplasms (MPNs) are a group of stem cell diseases, including polycythemia vera, essential thrombocythemia and primary myelofibrosis. Currently, there is no curative therapy for these diseases other than bone marrow transplant; therefore there is an apparent need for palliative treatment. MPNs are frequently associated with activating mutations in JAK2; small-molecule drugs targeting this molecule have entered clinical trials.

AREAS COVERED

In this review novel JAK2 inhibitors are discussed and alternative approaches to inhibiting their transforming potential are highlighted. Current clinical approaches do not only aim at blocking JAK2 activity, but also at reducing its stability and expression are highlighted, including inhibition of heat shock protein 90 (HSP90) and deacetylases (DAC) have the potential to significantly enhance the efficacy of JAK2 inhibitors.

EXPERT OPINION

Preliminary results from clinical trials indicate the feasibility and efficacy of JAK2-targeted approaches. However, JAK2 inhibitor treatment is limited by dose-dependent toxicity and combination treatment might be required. The discovery of JAK2 mutations that cause secondary resistance in vitro would further highlight the need for the development of next-generation JAK2 inhibitors and novel synergistic approaches.

Virulence without catalysis: how can a pseudokinase affect host cell signaling?

A hallmark of the pathogenic lifestyle is the secretion of enzymes and other effectors that dysregulate host signaling. Intriguingly, the most potent virulence locus identified in the intracellular parasite Toxoplasma gondii encodes a family of related catalytically inactive protein kinases, or pseudokinases. Toxoplasma has in its kinome among the highest percentage of pseudokinases among all sequenced organisms, and the majority of these appear to be secreted into the host cell. We posit that the pseudokinase fold represents a particularly well-suited domain for functional diversification, discuss the relevance of gene expansion at these loci, and outline potential mechanisms by which a pseudokinase might affect host signaling.

Jak/STAT pathways in cytokine signaling and myeloproliferative disorders: approaches for targeted therapies

Hematopoiesis is the cumulative result of intricately regulated signaling pathways that are mediated by cytokines and their receptors. Studies conducted over the past 10 to 15 years have revealed that hematopoietic cytokine receptor signaling is largely mediated by a family of tyrosine kinases termed Janus kinases (JAKs) and their downstream transcription factors, termed STATs (signal transducers and activators of transcription). Aberrations in these pathways, such as those caused by the recently identified JAK2(V617F) mutation and translocations of the JAK2 gene, are underlying causes of leukemias and other myeloproliferative disorders. This review discusses the role of JAK/STAT signaling in normal hematopoiesis as well as genetic abnormalities associated with myeloproliferative and myelodisplastic syndromes. This review also summarizes the status of several small molecule JAK2 inhibitors that are currently at various stages of clinical development. Several of these compounds appear to improve the quality of life of patients with myeloproliferative disorders by palliation of disease-related symptoms. However, to date, these agents do not seem to significantly affect bone marrow fibrosis, alter marrow histopathology, reverse cytopenias, reduce red cell transfusion requirements, or significantly reduce allele burden. These results suggest the possibility that additional mutational events might be associated with the development of these neoplasms, and indicate the need for combination therapies as the nature and significance of these additional molecular events is better understood.