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Originally published online as doi:10.1189/jlb.0104041 on April 23, 2004

Published online before print April 23, 2004
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(Journal of Leukocyte Biology. 2004;76:237-244.)
© 2004 by Society for Leukocyte Biology

Distinct activities of suppressor of cytokine signaling (SOCS) proteins and involvement of the SOCS box in controlling G-CSF signaling

Gert-Jan M. van de Geijn, Judith Gits and Ivo P. Touw1

Institute of Hematology, Erasmus University Medical Center, Rotterdam, The Netherlands

1Correspondence: Erasmus Medical Center, Institute of Hematology, Room Ee 1330c, Dr Molewaterplein 50, 3015 GE Rotterdam, The Netherlands. E-mail: i.touw{at}erasmusmc.nl


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ABSTRACT
 
Granulocyte-colony stimulating factor (G-CSF) induces proliferation of myeloid progenitor cells and controls their differentiation into mature neutrophils. Signal transducer and activator of transcription (STAT) proteins STAT3 and STAT5 are activated by G-CSF and play distinct roles in neutrophil development. Suppressor of cytokine signaling (SOCS) proteins are induced by STATs and inhibit signaling through various negative-feedback mechanisms. SOCS proteins can compete with docking of signaling substrates to receptors, interfere with Janus tyrosine kinase activity, and target proteins for proteasomal degradation. The latter process is mediated through the conserved C-terminal SOCS box. We determined the role of various SOCS proteins in controlling G-CSF responses and investigated the involvement of the SOCS box therein. We show that SOCS1 and SOCS3, but not CIS and SOCS2, inhibited G-CSF-induced STAT activation in human embryo kidney 293 cells. In myeloid 32D cells, SOCS1 and SOCS3 are induced by G-CSF. However, relative to interleukin-3-containing cultures, during G-CSF-induced neutrophilic differentiation, SOCS3 expression was further elevated, while SOCS1 levels remained constant. SOCS box deletion mutants of SOCS1 and SOCS3 were severely hampered in their abilities to inhibit STAT activation and to efficiently suppress colony formation by primary myeloid progenitors in response to G-CSF. These data demonstrate the importance of the SOCS box for the inhibitory effects of SOCS proteins on G-CSF signaling and show that among the different SOCS family members, SOCS3 is the major negative regulator of G-CSF responses during neutrophilic differentiation.

Key Words: signal transduction • STAT • granulopoiesis • negative feedback


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INTRODUCTION
 
Most hematopoietic growth factors and cytokines exert their action via membrane receptors of the hematopoietin superfamily. Upon ligand binding, these receptors activate Janus tyrosine kinases (Jaks) and subsequently, one or more members of the signal transducer and activator of transcription (STAT) protein family [1 ]. The expression of several suppressor of cytokine signaling (SOCS) proteins, i.e., CIS, SOCS1, and SOCS3, is under the direct transcriptional control of STATs [2 3 4 5 6 7 ]. SOCS proteins, characterized by their Src homology (SH)2 domain and their C-terminal SOCS box [8 , 9 ], act in a classical negative-feedback loop to inhibit signaling from a variety of hematopoietic growth factors and cytokines including Epo, interleukin (IL)-2, IL-3, and granulocyte-colony stimulating factor (G-CSF) [10 11 12 13 ]. SOCS proteins can inhibit signaling via multiple mechanisms. They may out-compete other signaling substrates for recruitment to a receptor [14 ]. Alternatively, SOCS1 and SOCS3 can directly inhibit Jak kinase activity using their extended SH2 subdomains and their kinase inhibitory regions (KIR), two functional domains lacking in other family members [15 16 17 ]. SOCS1 has a high affinity for direct binding to Jaks [18 ], whereas SOCS3 needs recruitment to receptor tyrosines for efficient inhibition [11 , 12 , 19 20 21 ]. A third proposed mechanism of inhibition by SOCS proteins involves recruitment to the SOCS box of elongins B and C, which form part of an E3 ubiquitin ligase complex, leading to subsequent proteasomal degradation of signaling substrates [22 , 23 ]. The SOCS box may also regulate the stability of the SOCS proteins themselves [23 ]. Indeed, SOCS1, -2, and -3 are highly unstable proteins, suggesting that active degradation is important for the regulation of SOCS protein levels [24 ]. It was shown that the SOCS box is involved in proteasomal targeting of SOCS proteins [23 , 25 ]. However, the roles of the SOCS box and elongin binding in SOCS protein degradation remain controversial. Several reports have suggested that these interactions may actually lead to SOCS protein stabilization [22 , 26 27 28 29 ].

G-CSF controls survival, proliferation, and differentiation of myeloid progenitor cells via multiple signaling mechanisms activated by the G-CSF receptor (G-CSF-R). Among the different STAT family members, STAT3 and STAT5 are most prominently activated by the G-CSF-R. STAT5 contributes to G-CSF-induced proliferation and survival of G-CSF-R-transduced Ba/F3 cells [30 ]. STAT3 has been suggested to play a role in the regulation of the G1 arrest required for neutrophilic differentiation but not in the execution of the differentiation process itself [31 ]. Studies in conditional STAT3 knockout mice showed that removal of STAT3 resulted in neutrophilia, supporting the notion that STAT3 is not essential for neutrophilic differentiation in vivo but is required for maintaining appropriately balanced neutrophil production [7 ]. G-CSF failed to up-regulate SOCS3 transcripts in STAT3-deficient bone marrow cells, suggesting that STAT3-induced SOCS3 is a major negative regulator of G-CSF-controlled neutrophil production [7 ]. Recent studies in mice lacking SOCS3 in their hematopoietic cells or neutrophils also pointed toward a role for SOCS3 in controlling G-CSF-induced neutrophil formation [32 , 33 ]. However, the mechanism by which SOCS3 inhibits G-CSF-induced granulopoiesis was not addressed in these studies. Generation of mice deficient for different members of the SOCS family has revealed important functions of the SOCS proteins in controlling signaling by multiple cytokines in hematopoietic and nonhematopoietic cells [34 35 36 37 38 39 ]. In addition to SOCS3, G-CSF induces the expression of SOCS1, SOCS2, and CIS in hematopoietic cells [12 , 40 ]. The ability of these additional SOCS proteins to suppress G-CSF-R signaling has not been investigated.

Here, we report that SOCS1 and SOCS3 inhibit G-CSF-induced STAT3 and STAT5 activation, whereas CIS and SOCS2 do not. Rather, SOCS2 appeared to exert an enhancing effect on activation of STAT3. In addition, we show that transcription of SOCS3 is induced during G-CSF-stimulated neutrophilic differentiation, while SOCS1 remains present at a relatively low and constant level. Finally, using SOCS box deletion mutants (SOCSdbox), we demonstrate that the SOCS boxes of SOCS1 and SOCS3 are important for efficient inhibition of STAT activation and for inhibition of G-CSF-induced colony formation of primary bone marrow cells. These data suggest for the first time an important role for the SOCS box in SOCS-mediated inhibition of G-CSF-controlled granulopoiesis.


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MATERIALS AND METHODS
 
Constructs
Expression constructs of myc-tagged CIS, SOCS1, SOCS2, and SOCS3 in pcDNA3 were provided by Yoshimura and co-workers [18 ]. The SOCSdbox mutants were made by introduction of a stop codon before the SOCS box in the SOCS1 and SOCS3 constructs using a site-directed mutagenesis kit, according to the manufacturer’s instructions (Stratagene, La Jolla, CA). Primers used were for SOCS1: FmutSOCS1, 5'-cccgctgcgctagcgccgcgtg, and RmutSOCS1, 5'-cacgcggcgctagcgcagcggg; for SOCS3: FmutSOCS3, 5'-ctactccgggggctagaagatccc, and Rmut SOCS3, 5'-gggatcttctagcccccggagtag. All SOCS constructs were cloned into the BamHI and SnaBI sites of retroviral vector pBabe [41 ], and correct orientation of inserts was verified by nucleotide sequencing. Human G-CSF-R wild-type (WT) in the pBabe vector has been described before [13 ].

Cells and RNA isolation
Generation and culture of 32D cells stably expressing the G-CSF-R (32D/WT) have been described previously [31 ]. For short-term stimulation experiments, the cells were washed twice with Hanks’ balanced saline solution (HBBS) and were starved in RPMI at a concentration of 1 million cells/ml for 4 h and stimulated with 100 ng/ml G-CSF for the indicated times. For growth on G-CSF, cells were washed twice with HBBS to remove IL-3 and cultured in the presence of 10 ng/ml G-CSF for multiple days. At the indicated time-points, cells were harvested, resuspended in TRIzol® (Invitrogen, Breda, The Netherlands), snap-frozen, and stored at –80°C. RNA was isolated according to manufacturer’s instructions. To remove genomic DNA, 5 µg RNA was treated with 10 U DNase I (Stratagene) in DNase buffer (40 mM Tris-HCl, pH 7.5, 6 mM MgCl2, 2 mM CaCl2) for 1 h at 37°C.

Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR)
For generation of cDNA, 1 µg RNA was denatured at 65°C for 5 min, followed by 10 min on ice. First-strand buffer (250 mM Tris-HCl, pH 8.3, 375 mM KCl, 15 mM MgCl2), deoxy-unspecified nucleoside 5'-triphosphates (dNTPs; 1 mM final concentration), dithiothreitol (1 mM final concentration), 40 U RNasin, 4 µg random hexamers (Amersham Pharmacia, Uppsala, Sweden), and 200 U Superscript II RT (Invitrogen) were added, and the reaction was incubated at 42°C for 2 h. Dilutions of cDNA used for PCR amplification for SOCS1, SOCS3, and RNase inhibitor were 1:10, 1:30, and 1:60, respectively. Primers used for amplification of SOCS1 were FTMSOCS1, 5'-tggtagcacgcaaccaggtg, and RTMSOCS1, 5'-tggcgaggacgaagacgag. For SOCS3, FTMSOCS3, 5'-tcaagaccttcagctccaa, and RTMSOCS3, 5'-tcttgacgctcaacgtgaag, were used. Primers for murine RNase inhibitor were: forward, 5'-tccagtgtgagcagctgag, and reverse, 5'-tgcaggcactgaagcacca. Taqman technology (Model 7900 sequence detector, PE Applied Biosystems, Foster City, CA) was used for quantitative real-time PCR. The reactions were performed in a volume of 25 µl of a mixture containing 2 µl of the respective cDNA dilution, primers at 5 µM and 12.5 µl 2x SYBR Green PCR master mix (PE Applied Biosystems) containing Amplitaq Gold® DNA polymerase, reaction buffer, dNTP mix with uridine 5'-triphosphate, and the double-stranded DNA-specific fluorescence dye SYBR Green I. The PCR program used was 1 cycle of 2 min at 50°C, 1 cycle of 10 min at 95°C, 45 cycles of denaturation for 15 s at 95°C, annealing for 30 s at 62°C, and extension for 30 s at 62°C. Samples were tested in duplicate, and the average values of the threshold cycle (Ct) were used for quantification. To quantify the relative expression of SOCS1 and SOCS3, the Ct values were normalized for endogenous reference ({Delta}Ct=CtSOCS–CtRNase inhibitor) and compared with a calibrator using the {Delta}{Delta}Ct method (Ct=CtSample–CtCalibrator). As calibrator, we used the expression in 32D cells deprived of growth factors and serum for 4 h (see Fig. 2A ) or the expression in 32D cells grown on IL-3 (see Fig. 2B ).



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  		Figure 2. G-CSF stimulation of myeloid cells induces transient SOCS1 mRNA expression, whereas SOCS3 is induced during G-CSF-mediated neutrophilic differentiation. 32D cells expressing the WT-G-CSF-R were deprived of growth factors for 4 h and stimulated with G-CSF for the indicated times (A) or cultured in the presence of G-CSF for 3 days (B). SOCS1 (solid bars) and SOCS3 (shaded bars) mRNA expression levels were determined by quantitative RT-PCR and were normalized using expression of RNase inhibitor. SOCS levels were expressed relative to growth factor-deprived 32D cells (A) or 32D cells cultured under proliferation conditions in IL-3-containing medium (B).

Luciferase assays
Luciferase assays were performed as described previously [13 ]. In short, human embryo kidney (HEK) 293 cells were transfected by CaPO4 precipitation [42 ] with a mixture of the following plasmids: pME18S-STAT5 to obtain a robust STAT5 luciferase signal, a ß-casein-derived STAT5 luciferase reporter plasmid, pRSVLacZ, different amounts of pcDNA3 with myc-tagged SOCS or empty pcDNA3 (Invitrogen), and pBabe with WT-G-CSF receptor. For STAT3 luciferase experiments, pME18S-STAT5 was replaced by empty pcDNA3, and an m67-derived STAT3 luciferase reporter was used [43 ]. Twenty-four hours after transfection, the medium was replaced by serum-free medium [Dulbecco’s modified Eagle’s medium (DMEM)+0.1% bovine serum albumin (BSA)]. The next day, the cells were stimulated with G-CSF for 6 h and lysed, and luciferase activity was measured using Steady-Glo reagents (Promega, Madison, WI). In parallel, the transfection efficiency was determined using lacZ staining. Luciferase activity levels were corrected for transfection efficiency using ß-galactosidase expression levels. All experiments were performed in triplicate.

Western blotting
Lysate from the luciferase assay was used in parallel for expression analysis of myc-tagged SOCS proteins by Western blot. Antibodies used for detection were mouse anti-myc (9E10, Santa Cruz Biotechnology, Santa Cruz, CA) and goat anti-actin (Santa Cruz Biotechnology).

Virus production and spot-blot analysis
Phoenix E virus producer cells (a gift from Garry Nolan, Department of Molecular Pharmacology, Stanford Unviersity School of Medicine, Stanford, CA) were transfected with pBabe-SOCS constructs by CaPO4 precipitation. Supernatants containing high-titer, helper-free recombinant viruses were harvested from 80% confluent producer cells grown for 16–20 h in DMEM medium (with 10% fetal calf serum and penicillin/streptomycin) and were passed through a 45-µm filter. To determine titers of pBabe-SOCS and pBabe-enhanced green fluorescent protein (EGFP) viruses, the virus particles were spun down by ultracentrifugation at 30,000 rpm (Beckman, Mijdrecht, The Netherlands), and viral RNA was extracted with phenol (pH=4.0) and spot-blotted on nitrocellulose filters. This blot was hybridized with a pBabe-specific probe (SV40 fragment, BamHI-HindIII digest).

Infection of bone marrow progenitors with SOCS constructs
Hematopoietic cells were harvested from the femurs and tibiae of Fvb/N or 129 SV mice [44 ]. Prestimulation and retroviral transduction of the bone marrow cells with pBabe SOCS expression constructs was performed as described before [13 ]. After the transduction procedure, bone marrow cells were plated in duplo at densities of 0.5 and 2.0 x 105 cells per ml methyl cellulose containing medium supplemented with 30% fetal bovine serum, 1% BSA, 0.1 mM 2-mercaptoethanol, 2 mM L-glutamine, with or without 1.5 µg/ml puromycin (Sigma, Zwijndrecht, The Netherlands), and G-CSF (100 ng/ml). Colonies were counted on day 7 of culture. Three independent experiments were performed with fresh virus supernatant in all cases.


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RESULTS
 
SOCS1 and SOCS3 inhibit G-CSF-induced STAT activation
CIS, SOCS1, SOCS2, and SOCS3 are the major SOCS proteins expressed in hematopoietic cells [12 , 40 ]. We first assessed which of these proteins affected G-CSF-induced STAT5 and STAT3 activity. SOCS1 and -3 inhibited G-CSF-induced STAT5 luciferase activity in a dose-dependent manner. In contrast, CIS and SOCS2 did not significantly affect STAT5 activity, even at the highest levels of expression (Fig. 1A ). Similarly, SOCS1 and SOCS3 inhibited STAT3 luciferase reporter activity (Fig. 1B) , and CIS and SOCS2 had no inhibitory effects. Rather, SOCS2 and to a lesser extent, CIS appeared to stimulate G-CSF-induced STAT3 activation.



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  		Figure 1. G-CSF-induced activation of STAT5 and STAT3 is inhibited by SOCS1 and SOCS3 but not by CIS and SOCS2. HEK 293 cells were transiently transfected with WT-G-CSF-R, STAT5, and lacZ expression constructs in combination with a STAT5 luciferase (luc.) reporter (A) or WT-G-CSF-R and lacZ expression constructs combined with a STAT3 luciferase reporter (B) and different amounts of CIS (x), SOCS1 ({blacktriangleup}), SOCS2 ({diamondsuit}), or SOCS3 ({blacksquare}) DNA. G-CSF-induced luciferase activity was corrected for transfection efficiency using lacZ staining and set at 100% in the absence of SOCS. Data shown are mean ± SEM of at least two independent experiments with triplicate measurements. Lysate of the luciferase assay was also used for Western blotting to detect expression of myc-tagged SOCS proteins.

G-CSF differentially induces SOCS1 and SOCS3 gene expression
SOCS1 and SOCS3 may be part of a G-CSF-induced negative-feedback loop activated during neutrophilic differentiation. To address this, we tested whether and to what extent G-CSF controls the expression of SOCS1 and -3 during short-term stimulation with G-CSF and upon G-CSF-induced neutrophilic differentiation of 32D/WT cells. Stimulation with G-CSF results in a transient up-regulation of SOCS1 mRNA, whereas SOCS3 expression is more robust (Fig. 2A ). In addition, SOCS3 expression is induced over several days during differentiation on G-CSF, while SOCS1 levels were comparable with values of 32D/WT cells cultured under proliferation conditions in the presence of IL-3 (Fig. 2B) . These results establish that SOCS3 is the most prominent SOCS member involved in the negative feedback during G-CSF-induced neutrophilic differentiation of myeloid cells.

Role of the SOCS box in suppressing STAT3 and STAT5 activation
SOCS1 and SOCS3 are thought to inhibit cytokine signaling by interfering with the phosphorylation of downstream signaling substrates of Jak kinases via their KIR [16 , 17 ]. An alternative mechanism by which SOCS proteins may attenuate signaling is by targeting critical signaling molecules for proteasomal degradation, mediated via the SOCS box. To determine the involvement of the SOCS box in the inhibitory effects of SOCS1 and SOCS3, we performed luciferase reporter assays to test the activity of SOCS mutants lacking the SOCS box (SOCS1dbox and SOCS3dbox) on G-CSF-R-induced activation of STAT5 and STAT3. Loss of the SOCS box of SOCS1 reduced inhibition of STAT5 activation (Fig. 3A ). In contrast, inhibition of STAT3 activity remained essentially intact upon deletion of the SOCS box (Fig. 3B) . SOCS3dbox did not inhibit STAT5 reporter activity at all, clearly showing that the SOCS box is essential for the inhibitory effects of SOCS3 on STAT5 (Fig. 3D) . Suppression of STAT3 activation by SOCS3 was also drastically reduced by deletion of the SOCS box, although the effect was somewhat less pronounced than for STAT5 (Fig. 3E) . Western blot analysis revealed that upon transfection of comparable amounts of expression plasmids, WT-SOCS protein levels were consistently higher than levels of SOCSdbox mutants (Fig. 3C and 3F) . Upon titrating down the amounts of transfected SOCS plasmids, protein expression of the SOCSdbox mutants decreased more steeply than that of WT-SOCS1 and -SOCS3. These latter observations suggest that removal of the SOCS box reduces protein stability of SOCS1 and SOCS3. However, the amounts of SOCSdbox protein at the highest concentrations tested were comparable with amounts of full-length SOCS1 and SOCS3, which already gave complete inhibition of STAT5 signaling. On this basis, we conclude that the reduced inhibition by SOCSdbox mutants is not merely the result of decreased expression levels of the protein but involves an intrinsic loss of its inhibitory capacity as a result of the removal of the SOCS box.



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  		Figure 3. The SOCS boxes of SOCS1 and SOCS3 are important for inhibition of STAT5 activation and to a lesser extent, for inhibition of STAT3 activation by G-CSF. STAT5 (A and D) and STAT3 (B and E) luciferase (luc) reporter assays with SOCS1 (•), SOCS1dbox ({blacktriangleup}), SOCS3 ({blacksquare}), and SOCS3dbox ({diamondsuit}) were performed as described in Figure 1 . Average ± SEM out of at least three independent experiments is shown. In parallel, Western blot analysis of myc-tagged SOCS proteins and actin as a control for equal loading was performed. Blots from representative experiments are shown (C and F).

The SOCS box of SOCS1 and SOCS3 contributes to inhibition of G-CSF-induced colony formation
To determine the contribution of the SOCS box to the inhibitory effects of SOCS1 and SOCS3 on G-CSF-induced proliferation and differentiation of primary hematopoietic cells, we introduced SOCS1 and SOCS3 and their SOCSdbox mutants in mouse bone marrow progenitor cells by retroviral gene transfer. To assure that comparable viral titers were used for the various infections, viral RNA was isolated from the supernatants and quantified using spot-blot analysis with a vector-specific cDNA probe. As shown in Figure 4A , the viral RNA contents of the various supernatants were comparable, indicating that the titers of the different retroviral vectors were similar. Introduction of SOCS1 and SOCS3 dramatically reduced G-CSF-induced colony numbers and size compared with bone marrow cells transduced with EGFP-containing control vector (Fig. 4B and 4C) . The absence of the SOCS box resulted in a significant relief of the inhibitory effects of SOCS1 and SOCS3 on the numbers and size of CFU-G colonies. Thus, the SOCS box is also important for SOCS-mediated suppression of G-CSF responses in primary myeloid progenitor cells.



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  		Figure 4. The SOCS box of SOCS1 and SOCS3 is required for efficient inhibition of G-CSF-induced colony formation of bone marrow cells. (A) RNA spot-blot analysis of supernatants containing pBabe retrovirus, demonstrating that titers used for infection were comparable. (B) Colony-forming unit (CFU)-G assay of primary bone marrow progenitor cells following infection with pBabe EGFP and SOCS constructs. Cells were plated in methylcellulose medium containing G-CSF (100 ng/ml) and puromycin (1.5 µg/ml). Data shown are mean of at least two independent experiments + SEM. (C) Photomicrographs of representative examples of G-CSF colonies of transduced bone marrow cells (original magnification: x50), showing reduced colony size of SOCS-transduced but not SOCSdbox-transduced cells compared with control (EGFP-transduced) cells.


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DISCUSSION
 
In this study, we investigated the role of the SOCS protein family members CIS, SOCS1, SOCS2, and SOCS3 in G-CSF signaling. We showed that SOCS1 and -3 inhibited G-CSF-induced STAT3 as well as STAT5 activation, and no inhibition by CIS and SOCS2 was detected. Additionally, we have demonstrated for the first time a major role for the SOCS box in the inhibition of G-CSF-R signaling by SOCS1 and SOCS3, in reporter assays and in primary bone marrow progenitors. Upon the introduction of comparable amounts of DNA, the expression of SOCSdbox mutants was consistently lower than that of WT-SOCS proteins (Fig. 3) . This supports a role for the SOCS box in stabilizing SOCS protein levels. Similar conclusions were reported for SOCS1 and SOCS3 in different model systems [26 , 29 ]. However, based on protein expression data (Fig. 3C and 3F) , we consider it unlikely that prevention of SOCS degradation is the only contribution of the SOCS box to the inhibition of signaling.

We observed that there are considerable differences between the abilities of SOCS1dbox and SOCS3dbox to inhibit G-CSF-induced activation of STAT3 and STAT5 (Fig. 3) . Although we currently have no explanation for these findings, we anticipate that differences in recruitment mechanisms of SOCS1 and SOCS3 and STAT3 and STAT5 to the activated G-CSF-R determine the outcome of deleting the SOCS box on G-CSF signaling. It is clear that SOCS1dbox is a more potent inhibitor than SOCS3dbox in both luciferase readouts. Conceivably, because of its high-affinity binding to Jaks, SOCS1dbox is able to recruit to Jaks and to suppress signaling with the KIR independently of the SOCS box, even at reduced expression levels. In addition, we showed that STAT3 activation is more sensitive to inhibition by SOCS1dbox and SOCS3dbox than G-CSF-induced STAT5 activation. There are at least two explanations for this. STAT3 activation depends to a significant extent on recruitment to tyrosines 704 and 744 of the G-CSF-R, which first requires tyrosine phosphorylation of the receptor. This multistep process might be more sensitive for inhibition than STAT5 activation, which is mediated through direct recruitment to Jak itself [45 ]. Reports on other cytokine receptors corroborate this by demonstrating that the SOCS box is dispensable for inhibition of STATs that require recruitment to receptor tyrosines for their activation [15 , 16 ]. In addition, the difference between the down-regulatory effects of SOCS1dbox and SOCS3dbox on STAT3 versus STAT5 may relate to differential involvement of regulatory proteins that are sensitive to proteasomal targeting.

The turnover rate of STAT3 protein is low, and phospho-STAT3 protein levels are not affected by proteasomal inhibition, suggesting that proteasomal degradation is not a major down-regulatory mechanism for STAT3 [24 , 46 ]. Phosphorylated STAT5 protein, on the other hand, is stabilized by proteasomal inhibitors. As ubiquitination or degradation of STAT5 protein was not detected, it was proposed that a STAT5 phosphatase is present, which is kept inactive by an unidentified protein [46 ]. Upon ubiquitination and proteasomal degradation of this protein, the phosphatase would become active and dephosphorylate STAT5 [46 ]. We have recently shown that G-CSF induces binding of the protein SH2-containing tyrosine phosphatase-2 (SHP-2) to STAT5, which depends on the presence of Tyr729 of the G-CSF-R, a major docking site for SHP-2 (van de Geijn et al., unpublished results). In view of the data showing that SHP-2 functions as a STAT5 phosphatase [47 48 49 ], these observations may point to a scenario in which recruitment of SHP-2 contributes to STAT5 down-regulation. Although the exact molecular features remain to be elucidated, these findings are consistent with the model proposed by Wang et al. [46 ], implicating a major role for proteasomal targeting in down-regulation of STAT5 by the degradation of a phosphatase inhibitory activity.

It is striking that although inhibition by SOCS3dbox in the luciferase assays was reduced compared with the effects of SOCS1dbox, SOCS3dbox still inhibited G-CSF-induced colony formation to some extent, whereas SOCS1dbox did not. Although the reason for this difference is not clear, this may reflect inhibition of effector mechanisms other than STATs that are involved in colony formation and does not necessarily reflect direct effects of SOCS3dbox specific for G-CSF signaling.

We observed that SOCS2 (and to a lesser extent, CIS) dose-dependently stimulated G-CSF-induced STAT3 activation (Fig. 1B) . We currently have no explanation for this stimulatory effect of SOCS2. A stimulatory role for SOCS2 has also been reported for other cytokine receptors. For instance, SOCS2 stimulated the activation of STAT5 by the growth hormone receptor and the prolactin receptor [50 51 52 ]. It is interesting that the murine SOCS2 gene (cish2) was recently identified as a frequent, common retrovirus integration site in a screen for novel leukemia genes. The virus integrations occurred 5' of the cish2 gene, which predictively results in the aberrant expression of SOCS2 transcripts [53 ]. In addition, SOCS2 is up-regulated in chronic myeloid leukemia (CML) cells in accelerated phase and in cell lines expressing the chimeric oncoprotein Bcr-Abl characteristic of CML [54 ]. These data suggest that SOCS2 can act as a positive regulator of cytokine signaling and therefore might be considered an oncoprotein rather than a negative regulator of growth, at least in hematopoietic cells.

In conclusion, we demonstrated in this paper that SOCS1 and SOCS3, but not the other SOCS family members expressed in hematopoietic cells, are capable of inhibiting G-CSF-induced STAT activation. During G-CSF-induced neutrophilic differentiation, SOCS3 mRNA is up-regulated, whereas SOCS1 levels are comparable with levels during proliferation on IL-3, suggesting that SOCS3 is the most important SOCS family member for negative feedback during neutrophil development. Our studies have unveiled a major role for the SOCS box in the inhibition of G-CSF-induced colony formation of primary hematopoietic progenitor cells, suggesting a role for proteasomal degradation mediated via SOCS1 and SOCS3 in the down-regulation of G-CSF responses during myeloid cell proliferation and differentiation in a physiological context.


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ACKNOWLEDGEMENTS
 
This work is supported by the Dutch Cancer Society "Koningin Wilhelmina Fonds". We thank Dr. Joanna Prasher for critical reading and helpful suggestions regarding the manuscript, Dr. Akihiko Yoshimura for providing SOCS expression plasmids, and Karola van Rooyen for help with preparation of the figures.

Received January 26, 2004; revised February 23, 2004; accepted March 4, 2004.


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