Disease relevance of JAK2

• Mice deficient in TYK2 developed Abelson-induced B lymphoid leukemia/lymphoma as well as TEL-JAK2-induced T lymphoid leukemia with a higher incidence and shortened latency compared with WT controls [1].
• Also, JAK2 was found to be constitutively associated with p95Vav in vivo when expressed at high levels in insect cells using baculovirus vectors [2].
• A recurrent somatic activating mutation in the nonreceptor tyrosine kinase JAK2 (JAK2V617F) occurs in the majority of patients with the myeloproliferative disorders polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia, and, less commonly, chronic myelomonocytic leukemia [3].
• Recently, rearrangements of the gene encoding the tyrosine Janus kinase 2 (JAK2) have been reported in human leukemias indicating a direct JAK-signal transduction and activator of transcription (STAT)-mediated leukemic process [4].
• We show here that a t(9;12)(p24;p13) in a case of early pre-B acute lymphoid leukemia and a t(9;15;12)(p24;q15;p13) in atypical chronic myelogenous leukemia in transformation involve the ETV6 gene at 12p13 and the JAK2 gene at 9p24 [5].
• Similar screening of six additional cases - three each with idiopathic erythrocytosis (IE) or otherwise unexplained erythrocytosis (UE) - did not reveal either JAK2V617F or JAK2 exon 12 mutations [6].
• This strategy led to the identification of a novel JAK2-acquired mutation in a patient with Down syndrome (DS) with B-cell precursor acute lymphoblastic leukemia (BCP-ALL) [7].
• JAK2 V617F mutation contributed to hemoglobin, aquagenic pruritus, and platelet count variability, whereas homozygous mutation was independently associated with higher white blood cell count, larger spleen size, and greater need for cytoreductive therapies [8].
• This study indicates that patients carrying the JAK2 (617V>F) mutation have higher risk of developing pregnancy complications [9].
• No significant association of JAK2 617V>F homozygosity with thrombosis risk was observed in PV [10].
• These results suggest that, for PV, erythrocytosis can occur through two mechanisms: terminal erythroid amplification triggered by JAK2 617V>F homozygosity, and a 2-step process including the upstream amplification of heterozygous cells that may involve additional molecular events [11].
• A genetic predisposition to acquisition of different JAK2 mutations is inherited in families with myeloproliferative disorders [12].
• As expected, WP1066 inhibited the proliferation of peripheral blood hematopoietic progenitors of patients with polycythemia vera carrying the JAK2 V617F mutation in a dose-dependent manner [13].
• Induction of the JAK2-V617F transgene with the interferon-inducible MxCre resulted in expression of JAK2-V617F approximately equal to wild-type Jak2 and a PV-like phenotype with increased hemoglobin, thrombocytosis, and neutrophilia [14].

High impact information on JAK2

• Briefly, binding of GH to GH receptor induces receptor dimerization and activation of the tyrosine kinase JAK2 [15].
• This growth suppression was caused by apoptosis and was reproduced by AG490, a specific, chemical JAK2 inhibitor that reversed constitutive phosphorylation of STAT3 in SOCS-1 inactivated cells [16].
• Moreover, the restoration of SOCS-1 suppressed both growth rate and anchorage-independent growth of cells in which SOCS-1 was methylation-silenced and JAK2 was constitutively activated [16].
• The data provided demonstrate that Jak2 has pivotal functions for signal transduction of a set of cytokine receptors required in definitive erythropoiesis [17].
• Jak2 deficiency defines an essential developmental checkpoint in definitive hematopoiesis [17].

Chemical compound and disease context of JAK2

• AG490 (JAK2 specific) induced apoptosis in DU145 but not in NRP-154 prostate cancer lines, whereas piceatannol (JAK1 specific) induced apoptosis in NRP-154 but not in DU145 cells [18].
• Hyperglycemia activates JAK2 signaling pathway in human failing myocytes via angiotensin II-mediated oxidative stress [19].
• BACKGROUND: The V617F mutation, which causes the substitution of phenylalanine for valine at position 617 of the Janus kinase (JAK) 2 gene (JAK2), is often present in patients with polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis [20].
• Activation of the Jak2-Stat5 signaling pathway in Nb2 lymphoma cells by an anti-apoptotic agent, aurintricarboxylic acid [21].
• A point mutation in the Janus kinase 2 exchanging a valine for a phenylalanine at position 617 (JAK2 V617F) was found in 65% to 97% of polycythemia vera (PV) patients, as well as in approximately 50% of essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF) patients [22].

Biological context of JAK2

• The crystal structure of the extracellular domain of EPOR in its unliganded form at 2.4 angstrom resolution has revealed a dimer in which the individual membrane-spanning and intracellular domains would be too far apart to permit phosphorylation by JAK2 [23].
• CONCLUSIONS: GH inhibits CCh-induced chloride secretion via a JAK2-dependent mechanism involving transactivation of EGFr and consequent recruitment of ERK1/2 [24].
• E(2) suppressed GH-induced JAK2 phosphorylation, an effect attenuated by actinomycin D, suggesting a requirement for gene expression [25].
• Taken together, these data indicate that JAK2 likely plays a key role in TPO-mediated signal transduction [26].
• The carboxyl terminus of SH2-Bbeta (SH2-Bbetac), which contains the SH2 domain, specifically interacts with kinase-active, tyrosyl-phosphorylated JAK2 but not kinase-inactive, unphosphorylated JAK2 in the yeast two-hybrid system [27].

Anatomical context of JAK2

• Moreover, lysates from mesangial cells treated with JAK2 inhibitor, retained significant STAT1alpha activity [28].
• We have recently demonstrated that stimulation of human T and natural killer cells with IL-12 induces tyrosine phosphorylation of the Janus family tyrosine kinase JAK2 and Tyk2, implicating these kinases in the immediate biochemical response to IL-12 [29].
• Constitutive activation of JAK2 also was detected in the factor-independent cell lines, but not in Mo7e cells without cytokine exposure [30].
• Interleukin-5 signaling in human eosinophils involves JAK2 tyrosine kinase and Stat1 alpha [31].
• Chemotaxis assays showed that inhibition of JAK2 diminished SDF-1alpha-induced migration in both CTS cells and CD34(+) human bone marrow progenitor cells [32].

Associations of JAK2 with chemical compounds

• Murine JAK2 has a total of 49 tyrosines which, if phosphorylated, could serve as docking sites for Src homology 2 (SH2) or phosphotyrosine binding domain-containing signaling molecules [27].
• Detailed study of the C-terminal truncated cytoplasmic domain of hIL-5Ralpha revealed that the cytoplasmic stretch at position 346-387, containing the proline-rich region, is necessary for JAK2 binding [33].
• These data suggest that phosphorylation of either serine or tyrosine residues of the EPOR can enhance the association of the receptor with JAK2, possibly increasing the sensitivity to EPO [34].
• PKC inhibitors or LY294002 did not affect membrane expression of the EpoR, the association of JAK2 with the EpoR, or the in vitro kinase activity of JAK2 [35].
• Verapamil inhibited neither [3H]thymidine incorporation nor JAK2 phosphorylation stimulated by hGH, whereas a tyrosine kinase inhibitor, lavendustin A, blocked the mitogenic effect [36].

Physical interactions of JAK2

• In 1993, GH receptor (GHR) was first observed to bind to the tyrosine kinase JAK2 [37].
• RESULTS: We showed that endogenous CIS3 bound to JAK2 in intact cells [38].
• The signaling molecules that are recruited and activated by the GHR-JAK2 complex include signal transducers and activators of transcription (Stat) factors, the adapter protein Shc, and the insulin receptor substrates (IRSs) 1 and 2 [39].
• Phosphorylated JAK2 was primarily bound to a short 106kDa leptin receptor isoform and to a lesser extent to a 210kDa molecule [40].
• Only after 60 min of this treatment did we observe tyrosine phosphorylation of Jak2 and p91 and assembly of the transcription factor complex FcRF gamma that binds to the promoter of the fcgr1 gene [41].

Enzymatic interactions of JAK2

• MGF-Stat5 can be phosphorylated and activated in its DNA binding activity by Jak2 [42].
• Furthermore, within seconds of SDF-1alpha activation, the CXCR4 receptor becomes tyrosine phosphorylated through the activation and association with the receptor of JAK2 and JAK3 kinases [43].
• Dissociation of Janus kinase 2 and signal transducer and activator of transcription 5 activation after treatment of Nb2 cells with a molecular mimic of phosphorylated prolactin [44].
• We show that JAK2 kinase is rapidly phosphorylated and activated in response to IL-13 [45].
• We report here that JAK1 and JAK2 tyrosine kinases are tyrosine phosphorylated in response to G-CSF induction [46].

Regulatory relationships of JAK2

• The results also demonstrate that OSM induces direct interaction of JAK2 kinase with Grb2, an SH2/SH3 domain containing adaptor protein [47].
• Receptors for interleukin (IL)-4 do not associate with the common gamma chain, and IL-4 induces the phosphorylation of JAK2 tyrosine kinase in human colon carcinoma cells [48].
• PRL enhanced JAK2 phosphorylation in peripheral blood mononuclear cells (PBMC) but not in granulocytes [49].
• Therefore, CIS3 and JAB inhibit JAK2 tyrosine kinase activity by an essentially similar mechanism [38].
• Epoetin-alpha treatment induced prominent JAK2 phosphorylation and enhanced cell invasion [50].

Other interactions of JAK2

• Our results also indicate therapeutic strategies for the treatment of HCC including use of SOCS-1 in gene therapy and inhibition of JAK2 by small molecules, such as AG490 [16].
• JAK2 and Tyk2 although activated cannot, in the absence of JAK1, efficiently mediate activation of STATs 1 and 3 [51].
• IL-3 treatment increases JAK2 activation and STAT5A and STAT5B tyrosine and serine phosphorylation and leads to cell cycle progression in adherent cells [52].
• Thus, EPOR and JAK2 association seems to be important for EPO responsiveness in CTLL-2 cells [53].
• Hence, these results suggest that JAK2 is required for CXCR4 receptor-mediated signaling that regulates cytoskeletal proteins and cell migration through PI3-kinase pathways in hematopoietic progenitor cells [32].

Analytical, diagnostic and therapeutic context of JAK2

• SH2-Bbetac also binds to immunoprecipitated wild-type but not kinase-inactive JAK2 in a far Western blot [27].
• Hence, H-2g signals through JAK2 and its downstream signal transducers STAT3, Erk1/2, and phosphatidylinositol 3-kinase result in ICAM-1 expression and cell adhesion [54].
• By RT-PCR, we confirmed the fusion of 3' part of JAK2 with the 5' part of PCM1 [55].
• Identification of the proteins recruited to the GH receptor-JAK2 complex and dissection of the signaling pathways that are subsequently activated will ultimately provide a basis for understanding GH action at the molecular level [56].
• Western blot analysis of purified nuclear extracts revealed the presence of immunoreactive JAK1 at 130 kDa and immunoreactive JAK2 at 128 kDa [57].

References