A regulating role of the JAK2 FERM domain in hyperactivation of JAK2(V617F)

Essential contribution of the JAK/STAT pathway to carcinogenesis, lytic infection of herpesviruses and pathogenesis of COVID‑19 (Review)

The intracellular pathway of Janus kinase/signal transducer and activator of transcription (JAK/STAT) and modification of nucleosome histone marks regulate the expression of proinflammatory mediators, playing an essential role in carcinogenesis, antiviral immunity and the interaction of host proteins with Herpesviral particles. The pathway has also been suggested to play a vital role in the clinical course of the acute infection caused by severe acute respiratory syndrome coronavirus type 2 (SARS‑CoV‑2; known as coronavirus infection‑2019), a novel human coronavirus initially identified in the central Chinese city Wuhan towards the end of 2019, which evolved into a pandemic affecting nearly two million people worldwide. The infection mainly manifests as fever, cough, myalgia and pulmonary involvement, while it also attacks multiple viscera, such as the liver. The pathogenesis is characterized by a cytokine storm, with an overproduction of proinflammatory mediators. Innate and adaptive host immunity against the viral pathogen is exerted by various effectors and is regulated by different signaling pathways notably the JAK/STAT. The elucidation of the underlying mechanism of the regulation of mediating factors expressed in the viral infection would assist diagnosis and antiviral targeting therapy, which will help overcome the infection caused by SARS‑CoV‑2.

Molecular landscape of the JAK2 gene in chronic myeloproliferative neoplasm patients from the state of Amazonas, Brazil

JAK2V617F (dbSNP: rs77375493) is the most frequent and most-studied variant in BCR::ABL1 negative myeloproliferative neoplasms and in the JAK2 gene. The present study aimed to molecularly characterize variants in the complete coding region of the JAK2 gene in patients with BCR::ABL1 negative chronic myeloproliferative neoplasms. The study included 97 patients with BCR::ABL1 negative myeloproliferative neoplasms, including polycythemia vera (n=38), essential thrombocythemia (n=55), and myelofibrosis (n=04). Molecular evaluation was performed using conventional PCR and Sanger sequencing to detect variants in the complete coding region of the JAK2 gene. The presence of missense variants in the JAK2 gene including rs907414891, rs2230723, rs77375493 (JAK2V617F), and rs41316003 were identified. The coexistence of variants was detected in polycythemia vera and essential thrombocythemia. Thus, individuals with high JAK2V617F variant allele frequency (≥50% VAF) presented more thrombo-hemorrhagic events and manifestations of splenomegaly compared with those with low JAK2V617F variant allele frequency (<50% VAF). In conclusion, individuals with BCR::ABL1 negative neoplasms can display >1 variant in the JAK2 gene, especially rs2230722, rs2230724, and rs77375493 variants, and those with high JAK2V617F VAF show alterations in the clinical-laboratory profile compared with those with low JAK2V617F VAF.

The Contribution of JAK2 46/1 Haplotype in the Predisposition to Myeloproliferative Neoplasms

Haplotype 46/1 (GGCC) consists of a set of genetic variations distributed along chromosome 9p.24.1, which extend from the Janus Kinase 2 gene to Insulin like 4. Marked by four jointly inherited variants (rs3780367, rs10974944, rs12343867, and rs1159782), this haplotype has a strong association with the development of BCR-ABL1-negative myeloproliferative neoplasms (MPNs) because it precedes the acquisition of the JAK2V617F variant, a common genetic alteration in individuals with these hematological malignancies. It is also described as one of the factors that increases the risk of familial MPNs by more than five times, 46/1 is associated with events related to inflammatory dysregulation, splenomegaly, splanchnic vein thrombosis, Budd–Chiari syndrome, increases in RBC count, platelets, leukocytes, hematocrit, and hemoglobin, which are characteristic of MPNs, as well as other findings that are still being elucidated and which are of great interest for the etiopathological understanding of these hematological neoplasms. Considering these factors, the present review aims to describe the main findings and discussions involving the 46/1 haplotype, and highlights the molecular and immunological aspects and their relevance as a tool for clinical practice and investigation of familial cases.

JAK2 Alterations in Acute Lymphoblastic Leukemia: Molecular Insights for Superior Precision Medicine Strategies

Acute lymphoblastic leukemia (ALL) is the most common pediatric cancer, arising from immature lymphocytes that show uncontrolled proliferation and arrested differentiation. Genomic alterations affecting Janus kinase 2 (JAK2) correlate with some of the poorest outcomes within the Philadelphia-like subtype of ALL. Given the success of kinase inhibitors in the treatment of chronic myeloid leukemia, the discovery of activating JAK2 point mutations and JAK2 fusion genes in ALL, was a breakthrough for potential targeted therapies. However, the molecular mechanisms by which these alterations activate JAK2 and promote downstream signaling is poorly understood. Furthermore, as clinical data regarding the limitations of approved JAK inhibitors in myeloproliferative disorders matures, there is a growing awareness of the need for alternative precision medicine approaches for specific JAK2 lesions. This review focuses on the molecular mechanisms behind ALL-associated JAK2 mutations and JAK2 fusion genes, known and potential causes of JAK-inhibitor resistance, and how JAK2 alterations could be targeted using alternative and novel rationally designed therapies to guide precision medicine approaches for these high-risk subtypes of ALL.

Baricitinib: From Rheumatoid Arthritis to COVID-19

Baricitinib is a JAK1/2 inhibitor that was first approved for treating moderate to severe rheumatoid arthritis (RA) but that later showed considerable efficacy in the control of exaggerated inflammatory responses that occur in a wide range of diseases. There is a growing body of evidence, obtained from clinical trials and case reports, demonstrating clinical and paraclinical improvement in patients following administration of baricitinib including RA, systemic lupus erythematosus, psoriasis, atopic dermatitis, alopecia areata, interferon‐mediated autoinflammatory diseases, graft‐versus‐host disease, diabetic kidney disease, and, recently, coronavirus disease‐19. However, despite overall encouraging results, many adverse effects have been observed in baricitinib‐treated patients, ranging from simple infections to increased risk of malignancies, particularly in long‐term use. The significant efficacy of baricitinib, versus the probable adverse effects, urge further investigation before establishing it as a part of standard therapeutic protocols. Here, we have provided a review of the studies that have used baricitinib for treating various inflammatory disorders and summarized the advantages and disadvantages of its administration.

Janus Kinases in Leukemia

Janus kinases (JAKs) transduce signals from dozens of extracellular cytokines and function as critical regulators of cell growth, differentiation, gene expression, and immune responses. Deregulation of JAK/STAT signaling is a central component in several human diseases including various types of leukemia and other malignancies and autoimmune diseases. Different types of leukemia harbor genomic aberrations in all four JAKs (JAK1, JAK2, JAK3, and TYK2), most of which are activating somatic mutations and less frequently translocations resulting in constitutively active JAK fusion proteins. JAKs have become important therapeutic targets and currently, six JAK inhibitors have been approved by the FDA for the treatment of both autoimmune diseases and hematological malignancies. However, the efficacy of the current drugs is not optimal and the full potential of JAK modulators in leukemia is yet to be harnessed. This review discusses the deregulation of JAK-STAT signaling that underlie the pathogenesis of leukemia, i.e., mutations and other mechanisms causing hyperactive cytokine signaling, as well as JAK inhibitors used in clinic and under clinical development.

Nanoparticles Targeting STATs in Cancer Therapy

Over the past decades, an increase in the incidence rate of cancer has been witnessed. Although many efforts have been made to manage and treat this life threatening condition, it is still one of the leading causes of death worldwide. Therefore, scientists have attempted to target molecular signaling pathways involved in cancer initiation and metastasis. It has been shown that signal transducers and activator of transcription (STAT) contributes to the progression of cancer cells. This important signaling pathway is associated with a number of biological processes including cell cycle, differentiation, proliferation and apoptosis. It appears that dysregulation of the STAT signaling pathway promotes the migration, viability and malignancy of various tumor cells. Hence, there have been many attempts to target the STAT signaling pathway. However, it seems that currently applied therapeutics may not be able to effectively modulate the STAT signaling pathway and suffer from a variety of drawbacks such as low bioavailability and lack of specific tumor targeting. In the present review, we demonstrate how nanocarriers can be successfully applied for encapsulation of STAT modulators in cancer therapy.

Pathogenic and Therapeutic Relevance of JAK/STAT Signaling in Systemic Lupus Erythematosus: Integration of Distinct Inflammatory Pathways and the Prospect of Their Inhibition with an Oral Agent

Four Janus kinases (JAKs) (JAK1, JAK2, JAK3, TYK2) and seven signal transducers and activators of transcription (STATs) (STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B, STAT6) mediate the signal transduction of more than 50 cytokines and growth factors in many different cell types. Located intracellularly and downstream of cytokine receptors, JAKs integrate and balance the actions of various signaling pathways. With distinct panels of STAT-sensitive genes in different tissues, this highly heterogeneous system has broad in vivo functions playing a crucial role in the immune system. Thus, the JAK/STAT pathway is critical for resisting infection, maintaining immune tolerance, and enforcing barrier functions and immune surveillance against cancer. Breakdowns of this system and/or increased signal transduction may lead to autoimmunity and other diseases. Accordingly, the recent development and approval of the first small synthetic molecules targeting JAK molecules have opened new therapeutic avenues of potentially broad therapeutic relevance. Extensive data are now available regarding the JAK/STAT pathway in rheumatoid arthritis. Dysregulation of the cytokines is also a hallmark of systemic lupus erythematosus (SLE), and targeting the JAK/STAT proteins allows simultaneous suppression of multiple cytokines. Evidence from in vitro studies and animal models supports a pivotal role also in the pathogenesis of cutaneous lupus and SLE. This has important therapeutic implications, given the current paucity of targeted therapies especially in the latter. Herein, we summarize the currently available literature in experimental SLE, which has led to the recent promising Phase II clinical trial of a JAK inhibitor.

Polyphosphoinositide-Binding Domains: Insights from Peripheral Membrane and Lipid-Transfer Proteins

Within eukaryotic cells, biochemical reactions need to be organized on the surface of membrane compartments that use distinct lipid constituents to dynamically modulate the functions of integral proteins or influence the selective recruitment of peripheral membrane effectors. As a result of these complex interactions, a variety of human pathologies can be traced back to improper communication between proteins and membrane surfaces; either due to mutations that directly alter protein structure or as a result of changes in membrane lipid composition. Among the known structural lipids found in cellular membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the membrane-anchored precursor of low-abundance regulatory lipids, the polyphosphoinositides (PPIn), which have restricted distributions within specific subcellular compartments. The ability of PPIn lipids to function as signaling platforms relies on both non-specific electrostatic interactions and the selective stereospecific recognition of PPIn headgroups by specialized protein folds. In this chapter, we will attempt to summarize the structural diversity of modular PPIn-interacting domains that facilitate the reversible recruitment and conformational regulation of peripheral membrane proteins. Outside of protein folds capable of capturing PPIn headgroups at the membrane interface, recent studies detailing the selective binding and bilayer extraction of PPIn species by unique functional domains within specific families of lipid-transfer proteins will also be highlighted. Overall, this overview will help to outline the fundamental physiochemical mechanisms that facilitate localized interactions between PPIn lipids and the wide-variety of PPIn-binding proteins that are essential for the coordinate regulation of cellular metabolism and membrane dynamics.

Effects of JAK2 V556F mutation on the JAK2's activity, structural stability and the transformation of Ba/F3 cells

Although roles of somatic JAK2 mutations in clonally myeloproliferative neoplasms (MPNs) are well established, roles of germline JAK2 mutations in the pathogenesis of MPNs remain unclear. Recently, a novel activating, germline JAK2 F556V mutation was identified and involved in the pathogenesis of MPNs, but, its pathogenesis mechanism was still unknown. In this study, homology models of JAK2 demonstrated that F556 located between two threonine residues which interacted with ATP phosphate groups by hydrogen bonds, Thr555 with the γ-phosphate and Thr557 with the β-phosphate in the active site of JAK2's JH2 domain. Moreover, the hydrogen bond between Thr557 and Arg715 played vital roles in sustaining the structural conformation of JH2's active site and JH1-JH2 domains' interactions. When F556 was replaced by other amino acids except Trp, the hydrogen bond, JH2 domain's structural conformation and JH1-JH2 domains' interactions disrupted for changing the helix between β2 and β3 strands which finally caused JAK2 activation. Mechanistic and functional studies showed that JAK2 F556V mutation disrupted JAK2 JH2 domain's activity, caused JAK2-STAT5 pathway activation and promoted the proliferation of BaF3 cells. Thus, our results herein may provide clues to understand the pathogenesis mechanism of JAK2 F556V mutation in the MPNs.

Mechanistic Insights into Regulation of JAK2 Tyrosine Kinase

JAK2 is a member of the Janus kinase (JAKs) family of non-receptor protein tyrosine kinases, which includes JAK1-3 and TYK2. JAKs serve as the cytoplasmic signaling components of cytokine receptors and are activated through cytokine-mediated trans-phosphorylation, which leads to receptor phosphorylation and recruitment and phosphorylation of signal transducer and activator of transcription (STAT) proteins. JAKs are unique among tyrosine kinases in that they possess a pseudokinase domain, which is just upstream of the C-terminal tyrosine kinase domain. A wealth of biochemical and clinical data have established that the pseudokinase domain of JAKs is crucial for maintaining a low basal (absence of cytokine) level of tyrosine kinase activity. In particular, gain-of-function mutations in the JAK genes, most frequently, V617F in the pseudokinase domain of JAK2, have been mapped in patients with blood disorders, including myeloproliferative neoplasms and leukemias. Recent structural and biochemical studies have begun to decipher the molecular mechanisms that maintain the basal, low-activity state of JAKs and that, via mutation, lead to constitutive activity and disease. This review will examine these mechanisms and describe how this knowledge could potentially inform drug development efforts aimed at obtaining a mutant (V617F)-selective inhibitor of JAK2.

Crystal Structure of the FERM-SH2 Module of Human Jak2

Jak-family tyrosine kinases mediate signaling from diverse cytokine receptors. Binding of Jaks to their cognate receptors is mediated by their N-terminal region, which contains FERM and SH2 domains. Here we describe the crystal structure of the FERM-SH2 region of Jak2 at 3.0Å resolution. The structure reveals that these domains and their flanking linker segments interact intimately to form an integrated structural module. The Jak2 FERM-SH2 structure closely resembles that recently described for Tyk2, another member of the Jak family. While the overall architecture and interdomain orientations are preserved between Jak2 and Tyk2, we identify residues in the putative receptor-binding groove that differ between the two and may contribute to the specificity of receptor recognition. Analysis of Jak mutations that are reported to disrupt receptor binding reveals that they lie in the hydrophobic core of the FERM domain, and are thus expected to compromise the structural integrity of the FERM-SH2 unit. Similarly, analysis of mutations in Jak3 that are associated with severe combined immunodeficiency suggests that they compromise Jak3 function by destabilizing the FERM-SH2 structure.

JAK2 tyrosine kinase phosphorylates and is negatively regulated by centrosomal protein Ninein

JAK2 is a cytoplasmic tyrosine kinase critical for cytokine signaling. In this study, we have identified a novel centrosome-associated complex containing ninein and JAK2. We have found that active JAK2 localizes around the mother centrioles, where it partly colocalizes with ninein, a protein involved in microtubule (MT) nucleation and anchoring. We demonstrated that JAK2 is an important regulator of centrosome function. Depletion of JAK2 or use of JAK2-null cells causes defects in MT anchoring and increased numbers of cells with mitotic defects; however, MT nucleation is unaffected. We showed that JAK2 directly phosphorylates the N terminus of ninein while the C terminus of ninein inhibits JAK2 kinase activity in vitro. Overexpressed wild-type (WT) or C-terminal (amino acids 1179 to 1931) ninein inhibits JAK2. This ninein-dependent inhibition of JAK2 significantly decreases prolactin- and interferon gamma (IFN-γ)-induced tyrosyl phosphorylation of STAT1 and STAT5. Downregulation of ninein enhances JAK2 activation. These results indicate that JAK2 is a novel member of centrosome-associated complex and that this localization regulates both centrosomal function and JAK2 kinase activity, thus controlling cytokine-activated molecular pathways.

The JH2 domain and SH2-JH2 linker regulate JAK2 activity: A detailed kinetic analysis of wild type and V617F mutant kinase domains

JAK2 tyrosine kinase regulates many cellular functions. Its activity is controlled by the pseudokinase (JH2) domain by still poorly understood mechanisms. The V617F mutation in the pseudokinase domain activates JAK2 and causes myeloproliferative neoplasms. We conducted a detailed kinetic analysis of recombinant JAK2 tyrosine kinase domain (JH1) and wild-type and V617F tandem kinase (JH1JH2) domains using peptide microarrays to define the functions of the kinase domains. The results show that i) JAK2 follows a random Bi-Bi reaction mechanism ii) JH2 domain restrains the activity of the JH1 domain by reducing the affinity for ATP and ATP competitive inhibitors iii) V617F decreases affinity for ATP but increases catalytic activity compared to wild-type and iv) the SH2-JH2 linker region participates in controlling activity by reducing the affinity for ATP.

Loss of p53 enhances NF-κB-dependent lamellipodia formation

Tumor suppressor p53 prevents tumorigenesis and tumor growth by suppressing the activation of several transcription factors, including nuclear factor-κB (NF-κB) and STAT3. On the other hand, p53 stimulates actin cytoskeleton remodeling and integrin-related signaling cascades. Here, we examined the p53-mediated link between regulation of the actin cytoskeleton and activation of NF-κB and STAT3 in MCF-7 cells and mouse embryonic fibroblasts (MEFs). In the absence of p53, STAT3 was constitutively activated. This activation was attenuated by depleting the expression of p65, a component of NF-κB. Integrin β3 expression and lamellipodia formation were also downregulated by NF-κB depletion. Inhibition of integrin αvβ3, Rac1 or Arp2/3, which diminished lamellipodia formation, suppressed STAT3 activation induced by p53 depletion. These results suggest that loss of p53 leads to STAT3 activation via NF-κB-dependent lamellipodia formation. Our study proposes a novel role for p53 in modulating the actin cytoskeleton through suppression of NF-κB, which restricts STAT3 activation.

Mechanistic insights into activation and SOCS3-mediated inhibition of myeloproliferative neoplasm-associated JAK2 mutants from biochemical and structural analyses

JAK2 (Janus kinase 2) initiates the intracellular signalling cascade downstream of cell surface receptor activation by cognate haemopoietic cytokines, including erythropoietin and thrombopoietin. The pseudokinase domain (JH2) of JAK2 negatively regulates the catalytic activity of the adjacent tyrosine kinase domain (JH1) and mutations within the pseudokinase domain underlie human myeloproliferative neoplasms, including polycythaemia vera and essential thrombocytosis. To date, the mechanism of JH2-mediated inhibition of JH1 kinase activation as well as the susceptibility of pathological mutant JAK2 to inhibition by the physiological negative regulator SOCS3 (suppressor of cytokine signalling 3) have remained unclear. In the present study, using recombinant purified JAK2JH1-JH2 proteins, we demonstrate that, when activated, wild-type and myeloproliferative neoplasm-associated mutants of JAK2 exhibit comparable enzymatic activity and inhibition by SOCS3 in in vitro kinase assays. SAXS (small-angle X-ray scattering) showed that JAK2JH1-JH2 exists in an elongated configuration in solution with no evidence for interaction between JH1 and JH2 domains in cis. Collectively, these data are consistent with a model in which JAK2's pseudokinase domain does not influence the activity of JAK2 once it has been activated. Our data indicate that, in the absence of the N-terminal FERM domain and thus cytokine receptor association, the wild-type and pathological mutants of JAK2 are enzymatically equivalent and equally susceptible to inhibition by SOCS3.

The effects of R683S (G) genetic mutations on the JAK2 activity, structure and stability

Janus kinase 2 (JAK2) is an important mediator of cytokine receptor signaling and plays key roles in the hematopoietic and immune response. The acquired JAK2 R683S (G) mutations are presumed to be a biomarker for B-cell acute lymphoblastic leukemia (B-ALL). However, how these mutations leading to the B-ALL is still unclear. The crystal structure of JAK2 JH2 domain suggests that the residue R683 locating in the linker between the N and C lobes of JH2 domain is important for keeping the compact structure, activity and structural stability of this domain. Mutations R683S, R683G and R683E significantly increase JAK2 activity and decrease its structural stability. While the R683K and R683H mutations almost have no effects on the JAK2 activity and structural stability. Furthermore, the spectroscopic experiments imply that mutations R683S, R683G and R683E impair the structure of JAK2 JH2 domain, and lead JAK2 to partially unfolded state. It may be this partially unfolded state that caused JAK2 R683S (G) constitutive activation. This study provides clues in understanding the mechanism of JAK2 R683S (G) mutations caused B-ALL.

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.

Disruption of E627 and R683 interaction is responsible for B-cell acute lymphoblastic leukemia caused by JAK2 R683G(S) mutations

Janus kinase 2 (JAK2) is an important mediator of cytokine receptor signaling and plays key roles in hematopoietic and immune responses. The acquired JAK2 R683G(S) somatic mutations are detected in 15% of patients with B-cell acute lymphoblastic leukemia (B-ALL) and are presumed to be a biomarker for B-ALL. However, how JAK2 R683G(S) mutations lead to B-ALL is still unclear. Our results indicated that the E627 and R683 interaction played a vital role in JAK2 autoinhibition. Mutations (R683S, R683G and E627A) disrupting this interaction led to JAK2 constitutive activation, while mutations (R683K, E627D) restoring this interaction decreased its activity. Furthermore, spectroscopy experiments implied that disruption of the E627 and R683 interaction abolished JH1/JH2 domain interactions and forced the JH1 domain into the open, active conformation. Mutations abolishing this interaction promoted the proliferation of Ba/F3 cells. The results herein may provide clues to understanding the mechanism of JAK2 R683G(S) mutation-associated B-ALL.

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.

Suppression of cytokine signaling by SOCS3: characterization of the mode of inhibition and the basis of its specificity

Janus kinases (JAKs) are key effectors in controlling immune responses and maintaining hematopoiesis. SOCS3 (suppressor of cytokine signaling-3) is a major regulator of JAK signaling and here we investigate the molecular basis of its mechanism of action. We found that SOCS3 bound and directly inhibited the catalytic domains of JAK1, JAK2, and TYK2 but not JAK3 via an evolutionarily conserved motif unique to JAKs. Mutation of this motif led to the formation of an active kinase that could not be inhibited by SOCS3. Surprisingly, we found that SOCS3 simultaneously bound JAK and the cytokine receptor to which it is attached, revealing how specificity is generated in SOCS action and explaining why SOCS3 inhibits only a subset of cytokines. Importantly, SOCS3 inhibited JAKs via a noncompetitive mechanism, making it a template for the development of specific and effective inhibitors to treat JAK-based immune and proliferative diseases.

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.

Identification of novel SAR properties of the Jak2 small molecule inhibitor G6: significance of the para-hydroxyl orientation

In this study, we analyzed the structure-activity relationship properties of the small molecule Jak2 inhibitor G6. We synthesized a set of derivatives containing the native para-hydroxyl structure or an alternative meta-hydroxyl structure and examined their Jak2 inhibitory properties. We found that the para-hydroxyl derivative known as NB15 had excellent Jak2 inhibitory properties in silico, in vitro, and ex vivo when compared with meta-hydroxyl derivatives. These results indicate that NB15 is a potent derivative of the Jak2 inhibitor G6, and that maintaining the para-hydroxyl orientation of G6 is critical for its Jak2 inhibitory potential.

A Non-ATP-Competitive Dual Inhibitor of JAK2 and BCR-ABL Kinases: Elucidation of a Novel Therapeutic Spectrum Based on Substrate Competitive Inhibition

Here we report the discovery of ON044580, an α-benzoyl styryl benzyl sulfide that possesses potent inhibitory activity against two unrelated kinases, JAK2 and BCR-ABL, and exhibits cytotoxicity to human tumor cells derived from chronic myelogenous leukemia (CML) and myelodysplasia (MDS) patients or cells harboring a mutant JAK2 kinase. This novel spectrum of activity is explained by the non-ATP-competitive inhibition of JAK2 and BCR-ABL kinases. ON044580 inhibits mutant JAK2 kinase and the proliferation of JAK2(V617F)-positive leukemic cells and blocks the IL-3-mediated phosphorylation of JAK2 and STAT5. Interestingly, this compound also directly inhibits the kinase activity of both wild-type and imatinib-resistant (T315I) forms of the BCR-ABL kinase. Finally, ON044580 effectively induces apoptosis of imatinib-resistant CML patient cells. The apparently unrelated JAK2 and BCR-ABL kinases share a common substrate, STAT5, and such substrate competitive inhibitors represent an alternative therapeutic strategy for development of new inhibitors. The novel mechanism of kinase inhibition exhibited by ON044580 renders it effective against mutant forms of kinases such as the BCR-ABL(T315I) and JAK2(V617F). Importantly, ON044580 selectively reduces the number of aneuploid cells in primary bone marrow samples from monosomy 7 MDS patients, suggesting another regulatory cascade amenable to this agent in these aberrant cells. Data presented suggest that this compound could have multiple therapeutic applications including monosomy 7 MDS, imatinib-resistant CML, and myeloproliferative neoplasms that develop resistance to ATP-competitive agents.

MUC16 induced rapid G2/M transition via interactions with JAK2 for increased proliferation and anti-apoptosis in breast cancer cells

MUC16/CA125 is a tumor marker currently used in clinics for the follow-up of patients with ovarian cancer. However, MUC16 expression is not entirely restricted to ovarian malignancies and has been reported in other cancers including breast cancer. Although it is well established as a biomarker, function of MUC16 in cancer remains to be elucidated. In the present study, we investigated the role of MUC16 in breast cancer and its underlying mechanisms. Interestingly, our results showed that MUC16 is overexpressed in breast cancer tissues whereas not expressed in non-neoplastic ducts. Further, stable knockdown of MUC16 in breast cancer cells (MDA MB 231 and HBL100) resulted in significant decrease in the rate of cell growth, tumorigenicity and increased apoptosis. In search of a mechanism for breast cancer cell proliferation we found that MUC16 interacts with the ezrin/radixin/moesin domain-containing protein of Janus kinase (JAK2) as demonstrated by the reciprocal immunoprecipitation method. These interactions mediate phosphorylation of STAT3 (Tyr705), which might be a potential mechanism for MUC16-induced proliferation of breast cancer cells by a subsequent co-transactivation of transcription factor c-Jun. Furthermore, silencing of MUC16 induced G2/M arrest in breast cancer cells through downregulation of Cyclin B1 and decreased phosphorylation of Aurora kinase A. This in turn led to enhanced apoptosis in the MUC16-knockdown breast cancer cells through Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated extrinsic apoptotic pathway with the help of c-Jun N-terminal kinase signaling. Collectively, our results suggest that MUC16 has a dual role in breast cancer cell proliferation by interacting with JAK2 and by inhibiting the apoptotic process through downregulation of TRAIL.

Phosphorylation of Y372 is critical for Jak2 tyrosine kinase activation

Jak2 tyrosine kinase plays an important role in cytokine mediated signal transduction. There are 49 tyrosine residues in Jak2 and phosphorylation of some of these are known to play important roles in the regulation of Jak2 kinase activity. Here, using mass spectrometry, we identified tyrosine residues Y372 and Y373 as novel sites of Jak2 phosphorylation. Mutation of Y372 to F (Y372F) significantly inhibited Jak2 phosphorylation, including that of Y1007, whereas the Jak2-Y373F mutant displayed only modest reduction in phosphorylation. Relative to Jak2-WT, the ability of Jak2-Y372F to bind to and phosphorylate STAT1 was decreased, resulting in reduced Jak2-mediated downstream gene transcription. While the Y372F mutation had no effect on receptor-independent, hydrogen peroxide-mediated Jak2 activation, it impaired interferon-gamma (IFNγ) and epidermal growth factor (EGF)-dependent Jak2 activation. Interestingly however, the Y372F mutant exhibited normal receptor binding properties. Finally, co-expression of SH2-Bβ only partially restored the activation of the Jak2-Y372F mutant suggesting that the mechanism whereby phosphorylation of Y372 is important for Jak2 activation is via dimerization. As such, our results indicate that Y372 plays a critical yet differential role in Jak2 activation and function via a mechanism involving Jak2 dimerization and stabilization of the active conformation.

Transforming JAK1 mutations exhibit differential signalling, FERM domain requirements and growth responses to interferon-γ

Recent work has highlighted roles for JAK (Janus kinase) family members in haemopoietic diseases. Although sequencing efforts have uncovered transforming JAK1 mutations in acute leukaemia, they have also identified non-transforming JAK1 mutations. Thus with limited knowledge of the mechanisms of JAK1 activation by mutation, sequencing may not readily identify transforming mutations. Therefore we sought to further understand the repertoire of transforming mutations of JAK1. We identified seven randomly generated transforming JAK1 mutations, including V658L and a deletion of amino acids 629-630 in the pseudokinase domain, as well as L910P, F938S, P960S, K1026E and Y1035C within the kinase domain. These mutations led to differential signalling activation, but exhibited similar transforming abilities, in BaF3 cells. Interestingly, these properties did not always correlate with JAK1 activation-loop phosphorylation. We also identified a JAK1 mutant that did not require a functional FERM (4.1/ezrin/radixin/moesin) domain for transformation. Although we isolated a mutation of JAK1 at residue Val658, which is found mutated in acute leukaemia patients, most of the mutations we identified are within the kinase domain and have yet to be identified in patients. Interestingly, compared with cells expressing JAK1-V658F, cells expressing these mutants had higher STAT1 (signal transducer and activator of transcription 1) phosphorylation and were more sensitive to interferon-γ-mediated growth inhibition. The differential STAT1 activation and interferon-sensitivity of JAK1 mutants may contribute to the determination of which specific JAK1 mutations ultimately contribute to disease and thus are identified in patients. Our characterization of these novel mutations contributes to a better understanding of mutational activation of JAK1.

The constitutive activation of Jak2-V617F is mediated by a π stacking mechanism involving phenylalanines 595 and 617

Somatic mutations in the Jak2 allele that lead to constitutive kinase activation of the protein have been identified in human disease conditions such as the myeloproliferative neoplasms (MPNs). The most common mutation in these patients is a V617F substitution mutation, which is believed to play a causative role in the MPN pathogenesis. As such, identifying the molecular basis for the constitutive activation of Jak2-V617F is important for understanding its clinical implications and potential treatment. Here, we hypothesized that conversion of residue 617 from Val to Phe resulted in the formation of novel π stacking interactions with neighboring Phe residues. To test this, we first examined the Jak2 structure via molecular modeling and identified a potential π stacking interaction between F594, F595, and F617. Disruption of this interaction through site-directed mutagenesis impaired Jak2 autophosphorylation, Jak2-dependent gene transcription, and in vitro kinase activity of the Jak2-V617F protein. Further, substitution of F594 and F595 with Trp did not affect Jak2 function significantly, but replacement with charged residues did, showing the importance of aromaticity and hydropathy index conservation at these positions. Using molecular dynamics (MD) simulations, we found that the π stacking interaction between residues 595 and 617 in the Jak2-V617F protein was of much greater energy and conformed to the properties of π stacking, relative to the Jak2-WT or Jak2-V617F/F594A/F595A. In summary, we have identified a π stacking interaction between F595 and F617 that is specific to and is critical for the constitutive activation of Jak2-V617F.

JAK2 exon 14 deletion in patients with chronic myeloproliferative neoplasms

Background

The JAK2 V617F mutation in exon 14 is the most common mutation in chronic myeloproliferative neoplasms (MPNs); deletion of the entire exon 14 is rarely detected. In our previous study of >10,000 samples from patients with suspected MPNs tested for JAK2 mutations by reverse transcription-PCR (RT-PCR) with direct sequencing, complete deletion of exon 14 (Δexon14) constituted <1% of JAK2 mutations. This appears to be an alternative splicing mutation, not detectable with DNA-based testing.

Methodology/Principal Findings

We investigated the possibility that MPN patients may express the JAK2 Δexon14 at low levels (<15% of total transcript) not routinely detectable by RT-PCR with direct sequencing. Using a sensitive RT-PCR–based fluorescent fragment analysis method to quantify JAK2 Δexon14 mRNA expression relative to wild-type, we tested 61 patients with confirmed MPNs, 183 with suspected MPNs (93 V617F-positive, 90 V617F-negative), and 46 healthy control subjects. The Δexon14 variant was detected in 9 of the 61 (15%) confirmed MPN patients, accounting for 3.96% to 33.85% (mean  = 12.04%) of total JAK2 transcript. This variant was also detected in 51 of the 183 patients with suspected MPNs (27%), including 20 of the 93 (22%) with V617F (mean [range] expression  = 5.41% [2.13%–26.22%]) and 31 of the 90 (34%) without V617F (mean [range] expression  = 3.88% [2.08%–12.22%]). Immunoprecipitation studies demonstrated that patients expressing Δexon14 mRNA expressed a corresponding truncated JAK2 protein. The Δexon14 variant was not detected in the 46 control subjects.

Conclusions/Significance

These data suggest that expression of the JAK2 Δexon14 splice variant, leading to a truncated JAK2 protein, is common in patients with MPNs. This alternatively spliced transcript appears to be more frequent in MPN patients without V617F mutation, in whom it might contribute to leukemogenesis. This mutation is missed if DNA rather than RNA is used for testing.