The myeloproliferative disorder–associated JAK2 V617F mutant escapes negative regulation by suppressor of cytokine signaling 3

Abstract

The somatic JAK2 valine-to-phenylalanine (V617F) mutation has been detected in up to 90% of patients with polycythemia and in a sizeable proportion of patients with other myeloproliferative disorders such as essential thrombocythemia and idiopathic myelofibrosis. Suppressor of cytokine signaling 3 (SOCS3) is known to be a strong negative regulator of erythropoietin (EPO) signaling through interaction with both the EPO receptor (EPOR) and JAK2. We report here that JAK2 V617F cannot be regulated and that its activation is actually potentiated in the presence of SOCS3. Instead of acting as a suppressor, SOCS3 enhanced the proliferation of cells expressing both JAK2 V617F and EPOR. Additionally, although SOCS1 and SOCS2 are degraded in the presence of JAK2 V617F, turnover of SOCS3 is inhibited by the JAK2 mutant kinase and this correlated with marked tyrosine phosphorylation of SOCS3 protein. We also observed constitutive tyrosine phosphorylation of SOCS3 in peripheral blood mononuclear cells (PBMCs) derived from patients homozygous for the JAK2 V617F mutant. These findings suggest that the JAK2 V617F has overcome normal SOCS regulation by hyperphosphorylating SOCS3, rendering it unable to inhibit the mutant kinase. Thus, JAK2 V617F may even exploit SOCS3 to potentiate its myeloproliferative capacity. IntroductionThe somatic valine-to-phenylalanine (V617F) mutation in JAK2 has been associated with a variety of myeloproliferative disorders (MPD), including polycythemia vera (PV), essential thrombocythemia (ET), and idiopathic myelofibrosis (IMF). [1][2][3][4][5] In wild-type JAKs the JH2 domain inhibits the JH1 kinase domain through interactions at 2 interfaces, with the region containing V617 being predicted to preserve the inactive conformation of the activation loop. 6 The V617F mutation might alter this conformation and perhaps stabilize the activation loop in an active state, or it may prevent access of other proteins to the catalytic domain. The V617 residue of JAK2 is conserved in the JH2 domain of JAK1 and TYK2, whereas in JAK3 it is replaced by methionine. Like JAK2 V617F, analogous mutations in JAK1 or TYK2 also results in their constitutive activation. 7 Janus kinases require the JH2 pseudokinase domain for normal physiologic activation of the JH1 catalytic domain. Therefore, it seems that the V617F mutation may disrupt the putative inhibition of the catalytic domain by the pseudokinase domain and create a constitutively activated kinase. However, the Janus kinases are also potently regulated by the suppressor of cytokine signaling (SOCS) proteins that are thought to bind to the JH1 catalytic loop and target the kinases for degradation. Whether SOCS can regulate the JAK2 V617F mutant has not been explored. 8 SOCS1 and SOCS3 bind to the catalytic groove of JAK2 via their kinase inhibitory region (KIR) to inhibit catalytic activity. 8 Both of these SOCS proteins can also target TEL-JAK2 and wild-type JAK2 for ubiquitination and degradation via their SOCS box ECS ubiquitin E3 ligase interaction motif. 9,10 A...

SOCS-mediated downregulation of mutant Jak2 (V617F, T875N and K539L) counteracts cytokine-independent signaling

“…Moreover, SOCS1 and SOCS3 dose dependently reduce Jak2-V617F phosphorylation and protein levels (Supplementary Figure 3c). Our data are in contrast to the results of a previous study reporting that SOCS3 fails to inhibit Jak2-V617F and potentiates both its phosphorylation and expression levels (Hookham et al, 2007). We therefore tested the specificity of the observed inhibition of Jak2-V617F by SOCS3.…”

Epigenetic regulation of signal transducer and activator of transcription 3 in acute myeloid leukemia

“…Interestingly, SOCS1 mRNA expression was significantly higher in JAK2 V617F samples compared to the control group [82]. Furthermore, SOCS3 enhanced the proliferation of cells expressing JAK2 V617F [83]. This suggests that, as in CML, the expected compensatory feedback mechanism failed in JAK2 V617F diseases.…”

Methylated Alteration of SHP1 Complements Mutation of JAK2 Tyrosine Kinase in Patients with Myeloproliferative Neoplasm

“…SOCS3 was inactivated by DNA methylation in more than 40% MPN (Capello et al, 2008). Moreover, hyperphosphorylaiton of SOCS3 caused by JAK2V617F mutation rendered it unable to inhibit the mutant kinase (Hookham et al, 2007). Inactivation of SHP1 by DNA methylation may enhance the response of tumour cells to cytokines and may explain the wellknown hypersensitivity to haematopoietic growth factors of MPN.…”