Janus Kinase 2 Signaling Research Articles

HSP90 Inhibition Targets JAK2 and Is Highly Effective in CRLF2-Rearranged Acute Lymphoblastic Leukemia

Abstract 576Enzymatic inhibitors of Janus kinase 2 (JAK2) are in clinical development for the treatment of B-cell acute lymphoblastic leukemia (B-ALL) with rearrangements of the cytokine receptor subunit CRLF2, myeloproliferative neoplasms, and other tumors with constitutive JAK2 signaling. In addition, JAK2 is a client of heat shock protein 90 (HSP90) and HSP90 inhibition results in JAK2 degradation. To explore the utility of blocking JAK2 in CRLF2-rearranged B-ALL, we exposed the MHH-CALL4 and MUTZ-5 cell lines, which both have CRLF2/IGH rearrangements and activating JAK2 mutations to a panel of JAK2 inhibitors (JAK inhibitor-1, INCB18424, tofacitinib, NVP-BSK805, NVP-BVB808, TG101348) and the HSP90 inhibitor NVP-AUY922. Both MUTZ-5 and MHH-CALL4 were highly sensitive to AUY922 (GI50, 25–26 nM), with 50- to >1,000-fold superior potency compared with the panel of JAK2 enzymatic inhibitors. AUY922 and the structurally divergent HSP90 inhibitors 17-AAG, PU-H71 and HSP990 were all potently active against a panel of Ba/F3 lines dependent on CRLF2 and JAK2 signaling (GI50, 1–11 nM). Treatment of MUTZ-5 and MHH-CALL4 cells with JAK inhibitor-1 reduced but did not eliminate phospho (P-) STAT5 and P-ERK1/2 in both lines but promoted increases in P-AKT in MUTZ-5 and P-JAK2 in MHH-CALL4. In contrast, AUY922 treatment more extensively reduced or eliminated phosphorylation of all the targets. The combination of AUY922+JAK inhibitor-1 had little or no additional effect on target phosphorylation compared with AUY922 alone and pairwise dose-response studies with isobologram analysis failed to identify synergistic effects. We performed transcriptional profiling on MUTZ-5 and MHH-CALL cells treated with either vehicle (DMSO), JAK inhibitor-1, AUY922 or JAK inhibitor-1+AUY922. Unsupervised hierarchical clustering distinguished samples treated with AUY922 (or combination) from those treated with JAK inhibitor-1 or vehicle. To formally assess whether AUY922 targets the same genes as JAK inhibitor-1, we defined a ‘JAK inhibitor signature’ from the top/bottom 250 most differentially expressed genes following treatment with JAK inhibitor-1. Using gene set enrichment analysis (GSEA), the ‘JAK inhibitor signature’ was highly enriched upon treatment with AUY922 (p=0.003). GSEA also demonstrated that STAT5A signatures were enriched upon treatment with JAK inhibitor-1, AUY922, or JAK inhibitor-1+AUY922. To identify additional targets of HSP90 inhibition beyond the inhibition of JAK2, we used the C3 database from the MsigDB compendium to perform a transcription factor binding site enrichment analysis on the most differentially expressed genes between JAK inhibitor-1 and AUY922. The top 5 hits were all heat-shock factors (HSF, FDR<0.05). GSEA revealed that an HSF1 signature was only enriched upon treatment with AUY922 or AUY922+JAKinh-1, but not after JAKinh-1 alone. To extend our findings to in vivo treatment of human B-ALL, we established primary B-ALL xenografts from CRLF2-rearranged, patient-derived bone marrow samples in NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice. To stringently assay established disease in vivo, we waited until bone marrow leukemia burden exceeded 30% and then initiated treatment with BVB808 50mg/kg twice daily, AUY922 50mg/kg thrice weekly, BVB808+AUY922 or vehicle. After 5 days of treatment, spleens from mice treated with vehicle or BVB808 had nearly complete effacement by B-ALL, while AUY922 or BVB808+AUY922 treatment resulted in visible islands of hematopoiesis. Based on immunohistochemistry, mice receiving AUY922 or BVB808+AUY922, but not BVB808 or vehicle, had nearly complete loss of P-STAT5 as well as upregulation of the pharmacodynamic marker HSP70. Immunoblotting of spleens from treated mice demonstrated reductions in P-STAT5, P-JAK2, and total JAK2 in AUY922- or BVB808+AUY922-treated mice. In contrast, treatment with single-agent BVB808 only modestly suppressed P-STAT5. Treatment with either BVB808 or AUY922 prolonged overall survival compared to vehicle (p=0.01 for both xenografts). Treatment with AUY922 further prolonged overall survival compared to BVB808 (p<0.01 for both xenografts), while the combination of BVB808 and AUY922 had no additional benefit compared to AUY922 alone. In conclusion, HSP90 is a promising therapeutic target in CRLF2-rearranged B-ALL and merits clinical evaluation. Disclosures:Romanet:Novartis Pharma AG: Employment. Murakami:Novartis Pharma AG: Employment. Sallan:Enzon Pharmaceuticals: Honoraria. Kung:Novartis Pharmaceuticals: Consultancy, Research Funding. Radimerski:Novartis Pharma AG: Employment. Weinstock:Novartis: Consultancy, Research Funding.

Genetic Resistance to JAK2 Enzymatic Inhibitors Is Overcome by HSP90 Inhibition

Abstract 62Mutation within the kinase domain of tyrosine kinases is a common mechanisms of resistance to enzymatic inhibitors. Inhibitors of janus kinase 2 (JAK2) are under evaluation in patients with myeloproliferative neoplasms (MPNs), B-cell acute lymphoblastic leukemia (B-ALL) with rearrangements of the cytokine receptor subunit CRLF2, and other tumors with constitutive JAK2 signaling. To identify resistance mutations in JAK2, we randomly mutagenized human JAK2 R683G, which is observed in approximately half of CRLF2-rearranged B-ALL. We transduced the mutagenized JAK2 cDNA library into murine Ba/F3 cells that express CRLF2. Expression of CRLF2 and JAK2 R683G confers IL3 independent growth in Ba/F3 cells. The transduced population was selected in the JAK2-selective inhibitor NVP-BVB808 in the absence of IL3. Multiple BVB808-resistant clones were recovered that harbored either E864K, Y931C or G935R mutations in JAK2. Alignment of homologous regions of the JAK2 kinase domain (JH1) with ABL1 demonstrated that the three mutations are located in regions homologous to imatinib resistance hotspots in ABL1. Codons Y931 and G935 are within the hinge region of the kinase domain. Based on structural modeling, Y931C is likely to inhibit substrate binding. E864K is located in the middle of b3 following the P-loop in the N-lobe and may modify the structure and flexibility of the preceding P-loop, thus destabilizing the conformation required for inhibitor binding. We expressed JAK2 V617F alleles harboring Y931C, G935R or E864K in Ba/F3-EPOR cells and exposed the cells to the JAK2 enzymatic inhibitors JAK inhibitor-1, NVP-BSK805, TG101348, tofacitinib (formerly tasocitnib), ruxolitinib (formerly INCB18424) and BVB808. All three mutations conferred 2- to >10-fold resistance against BVB808, NVP-BSK805, TG101348, ruxolitinib and JAK inhibitor-1. Y931C and E864K but not G935R conferred resistance to tofactinib. Modeling of G935R indicated that a 935R side-chain would occlude the hydrophobic channel of the ATP-binding pocket. As a consequence, this mutation would decrease the binding affinity of compounds occupying the hydrophobic channel like JAK inhibitor-1 or BSK805, but not affect the potency of tofactinib, which does not bind in this region. Mutation of G935 to arginine, histidine or glutamine reduced the inhibitory effects of JAK inhibitor-1, but not tofacitinib, on JAK2 kinase domain activity. None of the codon 935 mutations had significant effects on Km or Vmaxin vitro. BVB808 treatment partially reduced activation state-specific phosphorylation of STAT5 in Ba/F3-EPOR/JAK2 V617F cells but not in Ba/F3-EPOR/JAK2 V617F/G935R or G935H cells. JAK2 is a known client of HSP90, and HSP90 inhibitors promote the degradation of both wild-type and mutant JAK2. We hypothesized that resistance mutations within the JAK2 kinase domain would not affect JAK2 degradation induced by HSP90 inhibitors. We assayed the cytotoxicity of the resorcinylic isoxazole amide NVP-AUY922 and the benzoquinone ansamycin 17-AAG in Ba/F3 cells that express the erythropoietin receptor (EPOR) and JAK2 V617F, which is observed in more than half of MPNs. Mutation of JAK2 V617F to include E864K, Y931C or G935R did not affect sensitivity to either AUY922 or 17-AAG. In fact, AUY922 was more active against cells harboring G935R (GI50, 3.87 nM) or E864K (GI50, 6.14 nM) compared to cells with no resistance mutation (GI50, 14.7 nM; p<0.05). Both HSP90 inhibitors had similar potency in Ba/F3-CRLF2 cells expressing the resistance mutations in cis with R683G. Treatment of both lines with AUY922 at levels achievable in vivo reduced P-JAK2, P-STAT5, and total JAK2 regardless of mutations that conferred resistance to enzymatic JAK2 inhibitors. Thus, HSP90 inhibitors maintain activity in JAK2-dependent cells with resistance mutations in JAK2. Treatment of JAK2-dependent cancers with HSP90 inhibitors is an attractive treatment strategy either up-front or upon the selection of resistance to JAK2 enzymatic inhibitors. Disclosures:Gaul:Novartis: Employment. Vangrevelinghe:Novartis: Employment. De Pover:Novartis: Employment. Regnier:Novartis: Employment. Erdmann:Novartis: Employment. Hofmann:Novartis: Employment. Eck:Novartis: Consultancy, Research Funding. Kung:Novartis Pharmaceuticals: Consultancy, Research Funding. Radimerski:Novartis Pharma AG: Employment. Weinstock:Novartis: Consultancy, Research Funding.

Efficacy of SAR302503, a JAK2 Inhibitor, in the Treatment of a Primary Xenograft Model of Human Acute Myeloid Leukemia,

Abstract 3624Small molecule inhibitors targeting somatic mutations of Janus kinase 2 (JAK2) have demonstrated effectiveness in clinical trials for treatment of myeloproliferative disorders. Activated JAK2 signaling has been reported in some acute myeloid leukemia (AML) samples even though the frequency of JAK2 mutations is relatively rare in AML. Whether JAK2 inhibitors are effective in AML is not clear. Since many myeloid malignancies including AML are organized as cellular hierarchies driven by leukemia stem cells (LSC) at the apex, it is also unknown whether LSCs are sensitive to JAK2 inhibition. We report that SAR302503 (SAR503, Sanofi, Cambridge, MA), an orally administered small molecule inhibitor of JAK2, shows therapeutic efficacy in a xenograft model of human AML established by intrafemoral injection of primary human AML cells into anti-NK treated irradiated NOD.SCID mice. Drug pharmacokinetic studies confirm that SAR503 exhibits good bioavailability in NOD.SCID mice. Starting 2 weeks post transplantation to permit establishment of an AML graft, mice were orally gavaged twice a day with 60 mg/kg SAR503 or vehicle alone (0.5% methylcellulose) for 14 consecutive days. In 5 of 7 AML samples, treated mice exhibited significantly lower engraftment (3 to 18 fold; p < 0.05) in the injected femur compared to control mice. For 4 samples, there was also a significantly reduced level of engraftment (2 to 19 fold; p < 0.05) in non-injected bones, indicating that JAK2 inhibition affected the migratory ability of AML cells. The observed heterogeneous drug response (5 responders and 2 non-responders) correlated well with phosphoflow analysis showing that AML samples that responded to JAK2 inhibition in vivo had high basal and marked reduction in the STAT signaling pathway after in vitro treatment, whereas non-responding samples did not. Serial transplantation studies are ongoing to evaluate the effect of JAK2 inhibition on LSCs in treated primary mice. To evaluate whether AML samples that did not respond to JAK2 inhibition alone would respond to combination therapy, we treated engrafted mice with SAR503 plus cytarabine, a standard chemotherapeutic drug used in AML. Combination therapy of one non-responding sample resulted in a significantly reduced leukemic burden (2.3%) compared to mice treated with SAR503 alone (85.8%) or cytarabine alone (16.8%; p < 0.05 versus combination therapy). Our results demonstrate the potential of SAR503 to target AML cells and AML LSCs across a cross section of primary AML samples. Our pilot studies warrant a much larger scale evaluation of AML samples to identify responders and non-responders along with associated proteomic and genomic biomarkers of drug response. The approach we have taken, which focuses on large-scale analysis of primary samples using state-of-the-art xenograft assays, offers a new paradigm for preclinical drug development to identify both novel agents that effectively target LSCs and the patient populations most likely to benefit from targeted treatment. Disclosures:Off Label Use: We describe using SAR302503 to treat AML in a mouse model.

Elevated Serum IL-10 Levels in Patients with Diffuse Large B Cell Lymphoma: A Mechanism of Aberrant JAK2 Kinase Activation

Abstract 960Diffuse large B cell lymphoma (DLBCL) is an aggressive lymphoma whose survival depends on various signaling pathways one of which is Signal transducer and activator of transcription 3 (STAT3)/Janus kinase 2 (JAK2). We have demonstrated that JAK2 is constitutively activated at the auto-phosphorylation site in many cases of DLBCL. The most common mechanism causing abnormal JAK2 activation is through dysregulated cytokine signaling. Cytokines are deregulated in several hematological malignancies and may play a role in tumor cell growth through receptor-mediated and ligand dependent activation of the JAK2 kinase. In this study we investigated the role of cytokine signaling in JAK2 activation in DLBCL and use of a novel JAK2 inhibitor (JAK2i) to inhibit IL-10 induced JAK/STAT signaling.We studied serum cytokines from 70 patients with new untreated DLBCL who participated in a clinical trial in the North Central Cancer Treatment Group (Micallef, IN et al Blood 2011). Compared to control sera, patients with DLBCL had higher levels of several JAK2 kinase related cytokines (IL-2, IL-6, IL-10, and EGF). IL-10 and IL-6 were significantly higher in DLBCL patients vs controls with p values >0.03 and >0.001, respectively. We next examined whether DLBCL cell lines produced IL-10. The phospho-JAK2 (pJAK2) positive DLBCL cell lines Ly10, DHL2 and HBL1 produced more IL-10 (40–80 pg/ml) than the pJAK2 negative cell lines Ly1, Ly19 and DHL6 (0–5 pg/ml). Analysis of the surface expression of IL-10 receptors (IL-10R) determined that most p-JAK2 positive DLBCL cells express either IL-10Ra or IL-10Rb or both. IL-10 had no effect on DLBCL cell survival in vitro; however, it did promote their proliferation. Only IL-10 (but not the other elevated cytokines) was specifically able to activate JAK2 and STAT3 phosphorylation in a subgroup of DLBCL cell lines. IL-10 was not able to activate JAK1, STAT1 and STAT5, which suggests a specific role of IL-10 in the JAK2 pathway. Moreover, neutralizing antibody to IL-10 inhibited IL-10-induced JAK2 and STAT3 tyrosine phosphorylation. We studied the effect of SAR302503 (SAR503, Sanofi, Cambridge, MA), a selective JAK2i currently in clinical trial for myelofibrosis on constitutive JAK2 signaling in DLBCL cells. SAR503 was able to inhibit JAK2 and STAT3 phosphorylation in a dose and time dependent manner in a variety of DLBCL cell lines and patient samples. JAK2 inhibition with SAR2503 caused a dose-dependent inhibitory effect on survival of pJAK2 positive DLBCL cells not observed in pJAK2 negative cells. Activation of STAT3 signaling up-regulates the expression of various genes involved in cell survival and proliferation such as bcl-xl, bcl-2, mcl-1 and c-myc. We observed a dose-dependent decrease in c-myc protein and mRNA levels in Ly3 and DHL2 cells with SAR503 treatment; however, SAR503 did not effect expression of bcl2, mcl-1 and bcl-xl proteins in DLBCL cells. Interestingly, JAK2 inhibition with SAR503 inhibited the autocrine secretion of IL-10 in a dose-dependent manner. Next, we determined if higher pre-treatment serum IL-10 correlates with the overall survival of 81 DLBCL patients. IL-10 by ELISA demonstrated that IL-10 levels were high in 51% (41/81) of patients (median, 57.7pg/ml; range, 26.1–503.7), and low in 49% (40/81) (median, 15.9 pg/ml; range, 0–25.9). Using a cut-off value of 26.1pg/ml, patients with a high serum IL-10 level had a significantly worse event-free survival (p=0.01). Clinical correlation of serum IL-10 with disease parameters showed that the IL-10 level was correlated with elevated serum LDH (p= 0.0085) and IPI score (p=0.01); there was no correlation with the number of extranodal sites, age, or B symptoms. DLBCL tumors from 40 patients were classified into germinal center B-cell type (GCB) and non-GCB type using the Hans algorithm. There was a clear trend towards a higher serum IL-10 in non-GCB DLBCL patients (p <0.06).In summary, our data identifies mechanisms of JAK2 kinase activation in DLBCL and supports the hypothesis that the IL-10-JAK2-cmyc pathway is activated in patients with DLBCL. Finally our data provide a mechanistic basis of selecting and targeting DLBCL tumor cells with high IL-10 levels and/or constitutive JAK2 activity with potent and novel JAK2 inhibitors such as SAR503. Disclosures:No relevant conflicts of interest to declare.

β Common receptor integrates the erythropoietin signaling in activation of endothelial nitric oxide synthase

Erythropoietin (EPO), the key hormone for erythropoiesis, also increases nitric oxide (NO) bioavailability in endothelial cells (ECs), yet the definitive mechanisms are not fully understood. Increasing evidence has demonstrated that β common receptor (βCR) plays a crucial role in EPO-mediated non-hematopoietic effects. We investigated the role of βCR in EPO-induced endothelial NO synthase (eNOS) activation in bovine aortic ECs (BAECs) and the molecular mechanisms involved. Results of confocal microscopy and immunoprecipitation analyses revealed that βCR was colocalized and interacted with EPO receptor (EPOR) in ECs. Inhibition of βCR or EPOR by neutralizing antibodies or small interfering RNA abolished the EPO-induced NO production. Additionally, blockage of βCR abrogated the EPO-induced increase in the phosphorylation of eNOS, Akt, Src, or Janus kinase 2 (JAK2). Immunoprecipitation analysis revealed that treatment with EPO increased the interaction between βCR and eNOS, which was suppressed by inhibition of Src, JAK2, or Akt signaling with specific pharmacological inhibitors. Furthermore, EPO-induced EC proliferation, migration, and tube formation were blocked by pretreatment with βCR antibody and Src, JAK2, or PI3K/Akt inhibitors. Moreover, in vivo experiments showed that EPO increased the level of phosphorylated eNOS, Src, JAK2, and Akt, as well as βCR-eNOS association in aortas and promoted the angiogenesis in Matrigel plug, which was diminished by βCR or EPOR neutralizing antibodies. Our findings suggest that βCR may play an integrative role in the EPO signaling-mediated activation of eNOS in ECs.

Increased basal intracellular signaling patterns do not correlate with JAK2 genotype in human myeloproliferative neoplasms

AbstractMyeloproliferative neoplasms (MPNs) are associated with recurrent activating mutations of signaling proteins such as Janus kinase 2 (JAK2). However, the actual downstream signaling events and how these alter myeloid homeostasis are poorly understood. We developed an assay to measure basal levels of phosphorylated signaling intermediates by flow cytometry during myeloid differentiation in MPN patients. Our study provides the first systematic demonstration of specific signaling events and their comparison with disease phenotype and JAK2 mutation status. We demonstrate increased basal signaling in MPN patients, which occurs in both early and later stages of myeloid differentiation. In addition, the pattern of signaling is not correlated with JAK2 mutation status and signaling intensity is poorly correlated with mutant JAK2 allele burden. In contrast, signaling differences are detected between different MPN disease phenotypes. Finally, we demonstrate that signaling can be inhibited by a JAK2-selective small molecule, but that this inhibition is not JAK2 V617F specific, because MPN patients with mutant JAK2, wild-type JAK2, and control patients were inhibited to a similar degree. Our data suggest that, in addition to JAK2 mutations, other factors contribute significantly to the MPN phenotype, results that are relevant to both the pathogenesis and therapy of MPN.

Abstract 3570: SB1518, a novel JAK2/FLT3 inhibitor for the treatment of myeloid malignancies

Abstract Introduction: Janus kinase 2 (JAK2) and fms-related tyrosine kinase 3 (FLT3) are important tyrosine kinases in hematopoietic progenitor cell differentiation and proliferation. Aberrant signaling of JAK2 or FLT3 is implicated in various myeloid malignancies. SB1518, a new chemical entity, is an inhibitor of both JAK2 (IC50 = 22 nM) and its JAK2V617F mutant (IC50 = 19 nM) as well as FLT3 (IC50 = 22 nM) and its mutant FLT3D835Y (IC50 = 6 nM). It is currently being evaluated in Phase I and II clinical trials for advanced hematologic malignancies and is showing promising therapeutic effects. Here we present a pharmacological evaluation of SB1518 in in vitro and in vivo models of myeloid disease that provide a rationale for its selection for clinical development. Methods: The inhibitory effect of SB1518 on JAK2, FLT3 and STAT signaling was evaluated by Western-blot analysis in cell lines with different JAK2/FLT3 mutation status. Cell proliferation assays using the CellTiterGlo® assay were performed on a panel of different cancer cell lines and primary cells from patients. Selected cells were treated with SB1518 to determine effects on apoptosis (Caspase 3/7 activation) and cell cycle (propidium iodide stain). SET-2 cells (a megakaryoblastic cell line derived from an ET-patient bearing JAK2V617F) and MOLM-13 cells (an AML cell line bearing FLT3-ITD) were used to establish subcutaneous tumors in nude mice to test the anti-tumor efficacy of SB1518 after oral daily dosing. Results: SB1518 inhibits phosphorylation of JAK2 at Y221, FLT3 and of STAT proteins and shows anti-proliferative activity in cell lines driven by mutant or wt JAK2 or FLT3-ITD (IC50 range of 47-348 nM), leading to cell cycle arrest (in G0/1) and induction of apoptosis. Cells bearing JAK2 or FLT-3 mutations or genetic alterations leading to high expression of JAK2, are the most sensitive to the anti-proliferative effects of SB1518, as are primary cells isolated from PV or AML patients. In these cells SB1518 induces cell cycle arrest and apoptosis. SB1518 has favorable physicochemical properties and pharmacokinetics allowing oral dosing. It significantly blocks tumor growth in a subcutaneous model of megakaryoblastic AML (SET-2 cells, TGI = 61% at 150 mg/kg b.i.d.). In a model of late stage, aggressive AML (MOLM-13), SB1518 not only blocked tumor growth (TGI = 81%, at 150 mg/kg b.i.d.), but also significantly reduced lung metastasis. In both models SB1518 was very well tolerated and did not induce cytopenia. Conclusions: SB1518, a novel, orally bio-available small molecule inhibitor of JAK2/FLT3, shows promising activities in various hematological cancer cell lines as well as primary cells from patients. It is efficacious in mouse xenograft models of FLT-3 dependent MOLM-13 cells and JAK2-dependent SET-2 cells. SB1518 is currently undergoing evaluation in Phase 2 clinical trials in myelofibrosis (post-PV, post-ET and primary) and has shown encouraging clinical activity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3570. doi:10.1158/1538-7445.AM2011-3570

Leptin signaling and apoptotic effects in human prostate cancer cell lines

Prostate cancer (PCa) progression is often associated with transactivation of the androgen receptor (AR) by endogenous hormones/growth factors. One such factor affecting growth, proliferation, and apoptostis (pro-/anti-) in various cancers is the adipokine leptin. This research studied leptin-induced signaling and apoptosis in androgen sensitive (LNCaP, PC3/AR) and insensitive (PC3, DU145) PCa cell lines. Signaling was studied by immunoblotting in cells overexpressing leptin receptors (LRb), Janus kinase 2 (JAK2), and kinase negative-HER2-YFP cDNAs. Apoptosis was measured by immunoblotting of apoptotic proteins and by Hoechst staining of condensed DNA. Leptin rapidly induced activation of JAK2, STAT3, and MAPK (ERK1/2) signaling cascades; it may also induce HER2 transactivation via leptin-induced phospho-JAK2. Leptin was then shown to exert clear pro-apoptotic effects, increasing levels of caspase 3, cleavage of its substrate, poly (ADP-ribose) polymerase (PARP) to cleaved PARP(89) , levels of CK 18, a cytoskeletal protein formed during apoptosis, and DNA condensation. Kinase inhibitors indicated that leptin-induced apoptosis is probably mediated by balanced activation of JAK2/STAT3, p38 MAPK, and PKC pathways in PCa cells. A human leptin mutein LRb antagonist, L39A/D40A/F41A, fully inhibited leptin-induced phosphorylation of JAK2, ERK1/2, and Akt/PKB, and partially abrogated effects on apoptotic proteins. In LNCaP and PC3/AR cells, leptin increased AR protein levels in correlation with raised apoptotic markers. Thus, AR may mediate, at least partly, the leptin-induced apoptotic response. Leptin can clearly induce apoptosis in human PCa cell lines. These findings could lead to development of new leptin agonists with enhanced pro-apoptotic effects and targeted for use in human PCa.

From Vaquez to Dameshek through JAK…2 much for polycythemia vera to be feared?

Clinical practice in modern medicine is a gift that comes from afar. It comes sometimes from a lucky discovery, or sometimes from brave attempt chasing what at that time seemed crazy theories. Anyway, the gift we currently enjoy is the result of careful observation at first, and then like a sculptor with his chisel that free from marble an hidden figure, doctors and researchers carve during the years the features of many diseases. In this number of the Internal and Emergency Medicine Journal, Landolfi and his co-workers [1] reviewed the principal characteristics of polycythemia vera (PV), starting from the history of the first published description by Luis Henry Vaquez in 1892 [2]. Since this first description something other than plethora and cyanosis were soon identified: the high risk of major thrombotic events potentially leading to death, both in arterial district (acute myocardial infarction, stroke or transient ischemic attack, TIA), and in venous district (deep vein thrombosis, pulmonary embolism, and splanchnic thrombosis). The mortality rate of PV, as reassessed in the ECLAP study [3] is about 3.7/100 patients/year due not only to the vascular complications but also for a possible evolution to malignancy. The history of PV went through the recognition of an absolute increase of blood cells, due to an abnormal erythropoiesis as the result of a pluripotential stem cell being erythropoietin hypersensitive and independent [4]. The related clinical condition must be differentiated from secondary erythrocytosis and this exercise has challenged haematologist for decades. So, like industrious sculptors, haematologist has learnt to use diagnostic criteria like chisels to carve for each patient the hidden truth of primary or secondary hyperglobulia. But not all sculptors are the same. Amedeo Modigliani, the great painter and sculptor used to say that he could see at a glance what was inside a piece of marble, and that he just had to free it. In the same way, Dameshek [5] in 1951 described his vision ‘‘from outside’’ and speculated that PV has the same origin of essential thrombocythemia (ET), chronic idiopathic myelofibrosis (IMF) and chronic myeloid leukemia (CML). What was really inside, and Dameshek had seen, was proved more than 50 years later with the discovery of the mutation at position 617 in the JH2 domain of the Janus Kinase 2 (JAK2) cytoplasmic tyrosine kinase [6–9]. This gainof-function mutation, recognizable in almost 90% of PV patients, is acquired and present in clonal hemopoietic stem cells and responsible for the activation of Epo-signaling pathway, leading to hypersensitivity and uncontrolled growth of cells. The JAK2V617F mutation is also found in about 50% of patients with ET and IMF, in atypical myeloproliferative disorders, but it has been described also in some patients with CML and in patients with portal vein thrombosis but without hematological abnormalities. From these observation, recent theories and hypothesis have been explained, such as different intensity of JAK2 signaling, presence of pre-JAK2 event that will result in different myeloproliferative disorders, different host modifier polymorphisms [10], just to cite some of them. M. Carpenedo (&) E. M. Pogliani Hematology and Bone Marrow Transplantation Unit, San Gerardo Hospital, University of Milan Bicocca, Via Pergolesi 33, 20035 Monza, Italy e-mail: mcarpenedo@katamail.com

Blockade of Janus kinase-2 signaling ameliorates mouse liver damage due to ischemia and reperfusion

Janus kinase/signal transducers and activators of transcription (JAK/STAT) signaling is one of the major pathways for cytokine signal transduction. However, the role of the JAK/STAT pathway in liver ischemia/reperfusion is not clear. This study focuses on Janus kinase-2 (JAK2), which functions upstream of signal transducer and activator of transcription 1 (STAT1) in JAK/STAT, and its role in the mechanism of liver ischemia/reperfusion injury (IRI). Partial warm ischemia was produced in the hepatic lobes of C57BL/6 mice for 90 minutes, and this was followed by 6 hours of reperfusion. Mice were treated with a JAK2 inhibitor (tyrphostin AG490; 40 mg/kg intraperitoneally) or vehicle 60 minutes prior to ischemic insult. JAK2 blockade resulted in a significant reduction of hepatocyte apoptosis and liver injury. Macrophage and neutrophil infiltration, as assessed by immunohistochemistry, was markedly decreased in AG490-treated livers in comparison with controls. The expression of pro-inflammatory cytokines [tumor necrosis factor alpha, interleukin 6 (IL-6), and IL-1beta] and chemokines [chemokine (C-X-C motif) ligand 10 (CXCL-10) and CXCL-2] was also significantly reduced in the AG490-treated group in comparison with controls. AG490-treated livers showed fewer cells positive for terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling and reduced cleaved caspase-3 protein expression in parallel with increased B-cell lymphoma extra large expression. We employed AG490 (75 mM) in primary bone marrow-derived macrophage (BMM) and hepatoma cell (CRL1830) cultures, which were both stimulated with lipopolysaccharide (LPS; 10 ng/mL). In BMM cultures, AG490 depressed otherwise LPS-induced pro-inflammatory gene expression programs (IL-6, IL-12p40, IL-1beta, CXCL-10, and inducible nitric oxide synthase). In hepatoma cells, AG490 reduced cleaved caspase-3 expression. Moreover, JAK2 blockade inhibited STAT1 and STAT3 phosphorylation. This is the first report documenting that JAK2 signaling is essential in the pathophysiology of liver IRI, as its selective blockage ameliorated the disease process and protected livers from inflammation and apoptosis.

A JAK2 Interdomain Linker Relays Epo Receptor Engagement Signals to Kinase Activation

JAK2 (Janus kinase 2) is essential for cytokine receptor signaling, and several lines of evidence support a causal role of an activating JAK2 mutation in myeloproliferative disorders. JAK2 activity is autoinhibited by its pseudokinase domain in the basal state, and the inhibition is released by cytokine stimulation; how engagement of the cognate receptor triggers this release is unknown. From a functional screen for gain-of-function JAK2 mutations, we discovered 13 missense mutations, nine in the pseudokinase domain and four in the Src homology 2 (SH2)-pseudokinase domain linker. These mutations identified determinants for autoinhibition and inducible activation in JAK2. Two of the mutants, K539I and N622I, resulted in erythrocytosis in mice. Scanning mutagenesis of the SH2-pseudokinase domain linker indicated that its N-terminal part was essential for interaction of JAK2 with the Epo receptor, whereas certain mutations in the C-terminal region conferred constitutive activation. We further showed that substitutions for Glu(543)-Asp(544) in this linker or Leu(611), Arg(683), or Phe(694) in the hinge proximal region of the pseudokinase domain resulted in activated JAK2 mutants that could not be further stimulated by Epo. These results suggest that the SH2-pseudokinase domain linker acts as a switch that relays cytokine engagement to JAK2 activation by flexing the pseudokinase domain hinge.

Activation of Growth Hormone Receptors by Growth Hormone and Growth Hormone Antagonist Dimers: Insights into Receptor Triggering

GH binds dimerized GH receptors (GHRs) to form a trimolecular complex and induces downstream signaling events. The mechanism by which GH binding converts the inactive predimerized GHR to its active signaling conformation is uncertain. GH has no axis of symmetry. Its interaction with GHR is mediated by two asymmetric binding sites on GH, each with distinct affinity. Site 1 is of high affinity and is thought to mediate the first binding step. Mutation of binding site 2 (as in the human GH mutant, G120R) disrupts the second binding but leaves site 1 binding intact. G120R is a GH antagonist; it binds only one GHR and thus fails to signal, and it prevents productive GHR binding by normal GH. We previously demonstrated that prolactin receptor signaling was achieved by a dimeric version of a prolactin antagonist. We now employ assays of cellular signaling and receptor conformational changes to examine whether GH molecules harboring two site 1 regions can trigger GHR activation. We used recombinantly produced GH-GH and G120R-G120R dimers in which monomers in tandem are connected by a short linker peptide. Rabbit GHR-expressing human fibrosarcoma cells (C14) were treated with GH, G120R, GH-GH, or G120R-G120R. As expected, GH and GH-GH, but not G120R, induced GHR disulfide linkage, as assessed by anti-GHR blotting of cell extracts resolved by SDS-PAGE under nonreducing conditions. Disulfide linkage of GHRs reflects attainment of the active signaling conformation. Likewise, GH and GH-GH, but not G120R, caused Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) activation. Notably, G120R-G120R, despite its lack of an intact site 2 in either dimer partner, also promoted GHR disulfide linkage and JAK2 and STAT5 activation, albeit less potently than either GH or GH-GH. Time-course responses of the three agonists were similar in terms of JAK2 and STAT5 activation. Pretreatment of cells with our conformation-sensitive inhibitory monoclonal antibody, anti-GHR ext-mAb, prevented ligand-induced receptor activation for all three agonists. GHR was also rendered less immunoprecipitable by anti-GHR ext-mAb after treatment with these agonists. These results are important in that they indicate that a ligand with two intact binding sites 1 causes GHR to adopt similar conformational changes as does GH and thus triggers activation of JAK2 and downstream signaling. Furthermore, we infer that there is substantial flexibility in the GHR extracellular domain, such that it productively accommodates GH dimers that are much larger than GH.

Janus kinase-2 signaling mediates apoptosis in rat cardiomyocytes

We tested the hypothesis that activation Jak2, which is prominently involved in the up-regulation of the renin-angiotensin system (RAS), constitutes a focal point in relaying signals triggered by a Angiotensin II (Ang II) and hypoxia/reoxygenation separately to cause an enhanced susceptibility of cardiac myocyte to apoptotic cell death. Ang II-treated adult cardiomyocytes in culture exhibited an increased level of apoptosis that accompanied activation of pro-apoptotic as well as anti-apoptotic signaling pathways. We observed increased phosphorylation of Jak2 kinase, Stat1, JNK, with increased expression of Bax protein, followed by an increase in caspase-1 and caspase-3 activity. Activation of these pro-apoptotic pathways was blocked by the Jak2 pharmacological inhibitor, Tyrphostin AG490. We also observed an increase in phosphorylation of cardioprotective pathway components, namely S6 ribosomal protein, and heat shock protein 27 (HSP27). Likewise, the oxidative stress, via the hypoxia/reoxygenation treatment of rat adult cardiomyocytes, produced apoptosis that was dependent upon activation of Jak2. The apoptotic response was not only reduced by Losartan, an inverse agonist of the AT1, receptor, but by treatment with AG490 as well. Taken together, these observations provide clear evidence in favor of Jak2 signaling as mediator of the apoptotic response in cardiomyocytes. However, there was a concomitant induction of cytoprotective signaling that presumably provides a negative feed-back to the deleterious effects of the agonist.

Tyrosine 813 is a site of JAK2 autophosphorylation critical for activation of JAK2 by SH2-B beta.

The tyrosine kinase Janus kinase 2 (JAK2) binds to the majority of the known members of the cytokine family of receptors. Ligand-receptor binding leads to activation of the associated JAK2 molecules, resulting in rapid autophosphorylation of multiple tyrosines within JAK2. Phosphotyrosines can then serve as docking sites for downstream JAK2 signaling molecules. Despite the importance of these phosphotyrosines in JAK2 function, only a few sites and binding partners have been identified. Using two-dimensional phosphopeptide mapping and a phosphospecific antibody, we identified tyrosine 813 as a site of JAK2 autophosphorylation of overexpressed JAK2 and endogenous JAK2 activated by growth hormone. Tyrosine 813 is contained within a YXXL sequence motif associated with several other identified JAK2 phosphorylation sites. We show that phosphorylation of tyrosine 813 is required for the SH2 domain-containing adapter protein SH2-B beta to bind JAK2 and to enhance the activity of JAK2 and STAT5B. The homologous tyrosine in JAK3, tyrosine 785, is autophosphorylated in response to interleukin-2 stimulation and is required for SH2-B beta to bind JAK3. Taken together these data strongly suggest that tyrosine 813 is a site of autophosphorylation in JAK2 and is the SH2-B beta-binding site within JAK2 that is required for SH2-B beta to enhance activation of JAK2.

Angiotensin II Blocks Nicotine-Mediated Neuroprotection against β-Amyloid (1-42) via Activation of the Tyrosine Phosphatase SHP-1

We showed recently that nicotine activates the growth-promoting enzyme Janus kinase 2 (JAK2) in PC12 cells and that preincubation of these cells with the JAK2-specific inhibitor AG-490 blocked the nicotine-induced neuroprotection against beta-amyloid (1-42) [Abeta (1-42)]. These results provided direct evidence for linkage between JAK2 and the alpha7 nicotinic acetylcholine receptor-induced neuroprotection in PC12 cells. We also showed that preincubation with angiotensin II (Ang II), functioning via the angiotensin II type 2 (AT2) receptor, blocked both the nicotine-induced activation of JAK2 and its neuroprotection against Abeta (1-42). Recently growth-inhibitory effects of the AT2 receptor have been reported to be mediated by the activation of protein tyrosine phosphatases (PTPases) and that AT2 receptor stimulation is associated with a rapid activation of the PTPase SHP-1 (the cytoplasmic tyrosine phosphatase that contains Src homology 2 domains), a negative regulator of JAK2 signaling. Therefore, the potential biological significance of AT2 receptor-induced effects on both the nicotine-induced activation of JAK2 and its neuroprotection against Abeta (1-42) led us to investigate whether SHP-1 activation could be involved in this process. We found that Ang II induced the activation of SHP-1 and that an antisense against SHP-1 not only augmented the nicotine-induced tyrosine phosphorylation of JAK2 but also blocked the Ang II neutralization of the nicotine-induced neuroprotection. These results demonstrate that nicotine-induced tyrosine phosphorylation of JAK2 and neuroprotection against Abeta (1-42) in PC12 cells are blocked by Ang II via AT2 receptor-induced activation of SHP-1.

Janus kinase 2 determinants for growth hormone receptor association, surface assembly, and signaling.

GH signaling depends on functional interaction of the GH receptor (GHR) and the cytoplasmic tyrosine kinase, Janus kinase 2 (JAK2), which possesses a C-terminal kinase domain, a catalytically inactive pseudokinase domain just N-terminal to the kinase domain, and an N-terminal half shown by us and others to harbor elements for GHR association. Computational analyses indicate that JAKs contain in their N termini ( approximately 450 residues) divergent FERM domains. FERM domains (or subdomains within them) in JAKS may be important for associations with cytokine receptors. For some cytokine receptors, JAK interaction may be required for receptor surface expression. We previously demonstrated that a JAK2 mutant devoid of its N-terminal 239 residues (JAK2-Delta1-239) did not associate with GHR and could not mediate GH- induced signaling. In this report we employ a JAK2-deficient cell line to further define N-terminal JAK2 regions required for physical and functional association with the GHR. We also examine whether JAK2 expression affects cell surface expression of the GHR. Our results suggest that FERM motifs play an important role in the interaction of GHR and JAK2. While JAK2 expression is not required for detectable surface GHR expression, an increased JAK2 level increases the fraction of GHRs that achieves resistance to deglycosylation by endoglycosidase H, suggesting that the GHR-JAK2 association may enhance either the receptor's efficiency of maturation or its stability. Further, we report evidence for the existence of a novel GH-inducible functional interaction between JAK2 molecules that may be important in the mechanism of GH-triggered JAK2 signaling.

Effect of Angiotensin II on Gene Expression of cGMP-Specific Phosphodiesterases

Background and Objectives:Angiotensin II (Ang II)opposes the actions of nitric oxide (NO). A balance between Ang II and NO is critical for the maintenance of normal vessel tone, and is dependent on the intracellular cGMP level. We examined the effect of Ang II treatment on cGMP-hydrolyzing phosphodiesterases (PDEs), such as PDE1 and PDE5, the two major cGMP-hydrolyzing PDEs present in rat aortic vascular smooth muscle cells (VSMCs). We also investigated whether the Ang II signaling pathway affects the expression of the cGMP-hydrolyzing PDEs gene. Materials and Methods:VSMCs from SpragueDawley rats were incubated. Relative quantitative RT-PCR was performed for quantitation of PDE1A1, PDE5A1 mRNA expression a following in vitro Ang II treatment, and also done after pretreatment with inhibitors of the Ang II type1 (AT1) receptor, protein kinase C (PKC), mitogen activated protein kinase kinase (MEK) 1/2 and janus kinase 2 (JAK2) to investigate the effect of the Ang II signaling pathway on PDE1A1, PDE5A1 gene expression. Western blot analyses were performed to evaluate the changes of PDE1A1 and PDE5A1 protein level following treatment with Ang II. Results:Ang II (400 nM) increased the PDE1A1 and the PDE5A1 mRNA expression by 2.4 and 2.3-fold, respectively, 1 hour after treatment. Ang II also increased the protein level 2 hours after treatment. Inductions of PDE1A1 and PDE5A1 mRNA were blocked by the AT1 receptor inhibitors, PKC, MEK 1/2 and JAK2. Conclusion:In vitro Ang II treatment upregulates the PDE1A1, PDE5A1 gene expressions and the protein levels. The PKC, MEK1/2 and JAK2 signaling pathways were essential for the Ang II-mediated PDEs gene regulation. These findings may suggest that Ang II antagonizes NO actions through the upregulation of cGMP-hydrolyzing PDEs gene expressions. (Korean Circulation J 2003;33(2):130-138)

Termination of growth hormone pulse-induced STAT5b signaling.

STAT5b (signal transducer and activator of transcription 5b) is a key mediator of the effects of plasma GH pulses on male-specific liver gene expression. STAT5b is activated in liver cells in vivo by physiological pulses of GH and then is rapidly deactivated. Investigation of the cellular events involved in this activation/deactivation cycle using the rat liver cell line CWSV-1 established that a brief exposure to GH and the associated activation of JAK2 (Janus kinase 2) tyrosine kinase activity are both necessary and sufficient to initiate all of the downstream steps associated with STAT5b activation by tyrosine phosphorylation and the subsequent deactivation of both JAK2 kinase and STAT5b. JAK2 signaling to STAT5b at the conclusion of a GH pulse could be sustained by the protein synthesis inhibitor cycloheximide or by the proteasome inhibitor MG132, indicating that termination of this JAK2-catalyzed STAT activation loop requires synthesis of a labile or GH-inducible protein factor and is facilitated by the proteasome pathway. This factor may be a phosphotyrosine phosphatase, since the phosphatase inhibitor pervanadate both sustained GH pulse-induced JAK2 signaling to STAT5b and blocked the rapid deactivation of phosphorylated STAT5b (t(1/2) = 8.8 +/- 0.9 min) seen in its absence. Finally, the serine kinase inhibitor H7 blocked down-regulation of JAK2 signaling to STAT5b in a manner that enabled cells to respond to a subsequent GH pulse without the need for the approximately 3-h interpulse interval normally required for full recovery of GH pulse responsiveness. Termination of GH pulse-induced STAT5b signaling is thus a complex process that involves multiple biochemical events. These are proposed to include the down-regulation of JAK2 signaling to STAT5b via a cycloheximide- and H7-sensitive step, proteasome-dependent degradation of a key component or regulatory factor, and dephosphorylation leading to deactivation of the receptor-kinase signaling complex and its STAT5b substrate via the action of a phosphotyrosine phosphatase.