The SOCS Box of SOCS-1 Accelerates Ubiquitin-dependent Proteolysis of TEL-JAK2

Abstract

Fusion of the TEL gene on 12p13 to the JAK2 tyrosine kinase gene on 9p24 has been found in human leukemia. TEL-mediated oligomerization of JAK2 results in constitutive activation of the tyrosine kinase (JH1) domain and confers cytokine-independent proliferation on interleukin-3-dependent Ba/F3 cells. Forced expression of the JAK inhibitor gene SOCS1/JAB/SSI-1 induced apoptosis of TEL-JAK2-transformed Ba/F3 cells. This suppression of TEL-JAK2 activity was dependent on SOCS box-mediated proteasomal degradation of TEL-JAK2 rather than on kinase inhibition. Degradation of JAK2 depended on its phosphorylation and its high affinity binding with SOCS1 through the kinase inhibitory region and the SH2 domain. It has been demonstrated that von Hippel-Lindau disease (VHL) tumor-suppressor gene product possesses the SOCS box that forms a complex with Elongin B and C and Cullin-2, and it functions as a ubiquitin ligase. The SOCS box of SOCS1/JAB has also been shown to interact with Elongins; however, ubiquitin ligase activity has not been demonstrated. We found that the SOCS box interacted with Cullin-2 and promoted ubiquitination of TEL-JAK2. Furthermore, overexpression of dominant negative Cullin-2 suppressed SOCS1-dependent TEL-JAK2 degradation. Our study demonstrates the substrate-specific E3 ubiquitinligase-like activity of SOCS1 for activated JAK2 and may provide a novel strategy for the suppression of oncogenic tyrosine kinases.Cytokines induce the activation of the JAK family tyrosine kinases (JAKs) 1 and the subsequent recruitment of various signaling proteins to the receptor complex, including the STAT family of transcription factors. Constitutive activation of the JAK/STAT pathway has been found in many leukemic cell lines, including cells transformed with Bcr-Abl (1, 2), as well as in human T-cell lymphotrophic virus-1-transformed T cells (3,4). A constitutively activated form of STAT5 conferred factorindependent growth on Ba/F3 cells (5), and that of STAT3 has also been shown to function as an oncogene (6). Moreover, a constitutively activated JAK kinase generated by chromosome translocation between the TEL gene on 12p13 and the JAK2 gene on 9p24 has been shown to be associated with human leukemia (7,8). TEL, a subset of the ETS family of transcription factors, contains a conserved oligomerization domain, known as the PNT domain, in the N-terminal region. Like other TEL-tyrosine kinase fusion proteins such as the TEL-PDGF receptor ␤ chain and TEL-Abl, the JAK2 tyrosine kinase domain is constitutively activated by oligomerization mediated by the PNT domain. Stable expression of TEL-JAK2 confers factor-independent growth on IL-3-dependent Ba/F3 cells and induces myeloproliferative and T-cell lymphoproliferative diseases in mice (9).The JAK/STAT pathway is regulated by several mechanisms, including dephosphorylation by protein phosphatases and degradation by the ubiquitin/proteasome system (see review; Yasukawa et al.(10)). The CIS family (also referred to as the SOCS or SSI family) has been shown to play an impo...

The tumor suppressor activity of SOCS-1

“…This observation is similar to our previous report that SOCS-1 destabilizes the steady state expression of VAV and onco-VAV through a process of ubiquitination and proteasomal degradation . Three recent reports describe the promotion of TEL-JAK2 ubiquitination and degradation by SOCS-1 (Frantsve et al, 2001;Kamizono et al, 2001;Monni et al, 2001). Collectively, these data support the hypothesis SOCS-1 links target proteins such as VAV or TEL-JAK to the machinery of ubiquitination and degradation (Tyers and Rottapel, 1999).…”

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

“…Taken together, our results suggest that constitutively active Jak2 mutants are regulated by SOCS proteins at the level of kinase inhibition and by regulation of the expression levels of the mutant kinases. A similar mechanism was proposed for TEL-Jak2, for which a SOCS1-dependent inhibition of kinase activity and proteasomal degradation was reported (Frantsve et al, 2001;Kamizono et al, 2001).…”

“…The members of this protein family contain a central SH2 domain, as well as a C-terminal domain called SOCS-Box, which is required for proteasomal degradation of SOCS-binding partners (De Sepulveda et al, 2000;Frantsve et al, 2001;Kamizono et al, 2001;Rui et al, 2002;Ungureanu et al, 2002). Also, both SOCS1 and SOCS3 have been shown to interact directly with Jaks and thereby inhibit the phosphorylation of cytokine receptors, STATs and the Jaks themselves.…”

Increased cytoplasmic levels of CIS, SOCS1, SOCS2, or SOCS3 are required for nuclear translocation

“…For example, they interact with the Jak, Stat, or other receptor tyrosine kinases via their SH2 domain in order to block signaling [2,10]. SOCS proteins also interact with E3 ubiquitin ligase, which is involved in proteasome-mediated degradation of proteins [11,12]. However, recent studies showed that SOCS members with a long N-terminal region localize to the nucleus.…”