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Cutting Edge |


,
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* Integrated Program in Molecular, Cellular, and Biophysical Studies and Departments of
Microbiology and
Medicine, Columbia College of Physicians and Surgeons, New York, NY 10032; and
Department of Immunology, Erasmus University, Rotterdam, The Netherlands
| Abstract |
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| Introduction |
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In addition to protein tyrosine phosphatase recruitment, another
mechanism in the regulation of cytokine signaling involves members of
the suppressor of cytokine signaling (SOCS) gene family
(9, 10, 11). SOCS proteins have all been shown to inhibit
cytokine signaling by interfering with the JAK-STAT pathway
(12). The existence of parallels between cytokine receptor
and TCR-mediated signaling has led to interest in the potential
involvement of SOCS family members in TCR-mediated signaling.
Cytokine-induced Src 2 homology-containing protein (CIS) has been shown
to be induced by TCR stimulation and to enhance TCR-mediated
mitogen-activated protein kinase activation, possibly through its
interaction with protein kinase C-
(13). Although the
effects of SOCS-1 on TCR signaling remain unclear, SOCS-1 has recently
been shown to inhibit the activation of NFAT in 293T cells expressing
CD8 and Syk (14). Whether SOCS-3 plays a role in the
regulation of TCR-mediated signaling has not previously been reported.
In this report, we show that SOCS-3 is induced in T cells on TCR
ligation and can inhibit TCR-mediated activation of the IL-2 promoter.
Furthermore, SOCS-3 is shown to interact with calcineurin and inhibit
the activation of NFATp in response to calcium signaling. Our results
suggest a role for SOCS-3 in the regulation of NFATp activity in T
cells.
| Materials and Methods |
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Isolation and culture of primary CD4+ and Jurkat T cells were performed as previously described (15). Jurkat cells were transfected by electroporating Transfected cells were selected by resistance to G418 (Life Technologies, Gaithersburg, MD). 293T cells were transfected by the calcium phosphate method.
Intracellular cytoplasmic staining
Staining for intracellular PE-conjugated rat anti-IL-2 (BD PharMingen, San Diego, CA) was performed as previously described (16).
EMSA
The probe used was derived from the distal NFAT-binding site from the human IL-2 promoter; its sequence is 5'-GGAGGAAAAACTGTTTCATACAGAAGGCGT-3'. Double-stranded oligonucleotides were prepared and labeled by filling in recessed ends with Klenow enzyme (New England Biolabs, Beverly, MA). EMSA was performed as described previously (16, 17).
Immunoprecipitation, Western blotting, and Northern analysis
Cell extracts were made as described (18). Extracts
were precleared with normal mouse IgG or normal rabbit IgG before
incubation with NFATp antisera, hemagglutinin (HA) antisera, or
SOCS-3 antisera (Santa Cruz Biotechnology, Santa Cruz, CA). Antisera
used in Western blotting included NFATp, HA, PP-2B
(calcineurin),
SOCS-3, (Santa Cruz Biotechnology), and Xpress (Invitrogen, San Diego,
CA). Northern blotting was performed as described (16)
using a SOCS-3 cDNA fragment.
GST precipitations
Plasmid-encoding GST-SOCS-3 fusion protein was constructed by cloning SOCS-3 into the BamHI site of pGEX-3X (Amersham Pharmacia Biotech, Piscataway, NJ). GST and GST-SOCS-3 proteins were generated as described (19). Precipitation experiments were performed using beads coupled to 15 µg GST or GST-SOCS-3.
Retroviral infections
Infections were performed as previously described (20).
| Results |
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Several cytokines can induce SOCS-3 mRNA (9, 21). To
examine the effects of TCR stimulation on SOCS-3 expression, RNA from
purified murine T cells stimulated with anti-CD3 was examined.
SOCS-3 mRNA levels were increased by stimulation with anti-CD3,
reaching peak levels after 8 h of stimulation and remaining
elevated for at least 24 h (Fig. 1
).
SOCS-3 induction by TCR ligation is less rapid and of much greater
duration than induction by cytokines.
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To investigate the effects of SOCS-3 expression on signaling
downstream of the TCR, Jurkat cells were cotransfected with a
luciferase reporter driven by the proximal IL-2 promoter and either
SOCS-3 or a vector control. SOCS-3 inhibited the activation of the IL-2
promoter by PMA and ionomycin (Fig. 2
A). Further cotransfection
experiments using a luciferase reporter driven by three copies of the
distal NFAT/AP-1 site from the IL-2 promoter revealed the ability of
SOCS-3 to inhibit transcriptional activation driven by this element
(Fig. 2
B). In contrast, SOCS-3 had no effect on the
PMA/ionomycin induced activation of a luciferase reporter driven by
multimerized AP-1 binding sites (Fig. 2
C) or a multimerized
NF-
B binding site (data not shown). The results of these transient
transfection experiments suggest that SOCS-3 suppresses NFAT-dependent
transcriptional activation. Furthermore, the ability of SOCS-3 to block
reporter activity induced by PMA and ionomycin suggests that it acts
downstream of the initial tyrosine kinase activity of lck
and ZAP70.
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To further study the effects of SOCS-3 on NFAT function, Jurkat
cells constitutively expressing SOCS-3 were generated. Jurkat cells
expressing SOCS-3 were deficient in their ability to produce IL-2 as
compared with Jurkat cells transfected with vector alone when
stimulated with PMA and ionomycin (Fig. 3
A). When Jurkat cells
expressing SOCS-3 were stimulated with anti-CD3, the pattern of
tyrosine phosphorylation seen in whole cell extracts
was similar to that observed in cells transfected with empty vector
(data not shown), again suggesting that SOCS-3 does not interfere with
the activation of tyrosine kinases through the TCR. To determine
whether the overexpression of SOCS-3 altered NFAT activation in these
cells, NFAT was investigated directly by EMSA. Nuclear extracts from
stimulated Jurkat cells expressing SOCS-3 show reduced binding to the
distal NFAT site in the IL-2 promoter. These data demonstrate that
overexpression of SOCS-3 can inhibit NFAT activation and IL-2
production in Jurkat cells.
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Interaction of SOCS-3 with calcineurin
The observation that SOCS-3 inhibited NFATp
dephosphorylation induced by ionomycin raised the
possibility that SOCS-3 could interact with NFATp or calcineurin. To
determine whether SOCS-3 interacts with these molecules, whole cell
extracts from 293T cells transfected with calcineurin, NFATp, or empty
vector were incubated with agarose beads coupled to GST or a GST-SOCS-3
fusion protein. Although there is a slight interaction between
calcineurin and beads coupled to GST, a much stronger interaction was
observed between calcineurin and beads coupled to GST-SOCS-3 (Fig. 4
A). The association of
calcineurin with beads coupled to GST-SOCS-3 was also much stronger
than that seen between NFATp and beads coupled to GST-SOCS-3 (Fig. 4
A). The reverse interaction of SOCS-3 with calcineurin was
tested by transfecting 293T cells with combinations of SOCS-3,
calcineurin, and empty vector. SOCS-3 coimmunoprecipitated with
calcineurin only in cells transfected with both genes (Fig. 4
B). Furthermore, we investigated the interaction between
SOCS-3 and calcineurin in vivo. Primary murine splenocytes were
stimulated with plate-bound Abs to CD3 and CD28 for 48 h to induce
SOCS-3, followed by lysis and immunoprecipitation. We found that SOCS-3
interacts with calcineurin A
in stimulated primary T cells.
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Given that both the interaction of SOCS3 with calcineurin and the
initial observation of SOCS-3 mRNA modulation by anti-CD3 was made
in primary murine T cells, the ability of SOCS-3 to alter
NFAT-dependent responses in these cells was examined.
CD4+ T cells isolated from lymph nodes of C57BL/6
mice were infected with retroviruses encoding either SOCS-3 and green
fluorescent protein (GFP) or GFP alone. Three days after infection, the
cells were stimulated for 4 h with PMA/ionomycin and analyzed for
IL-2 production by intracellular cytoplasmic staining. Although
PMA/ionomycin induced a marked activation of IL-2 synthesis in cells
expressing GFP alone, this activation was greatly reduced in cells also
expressing SOCS-3 (Fig. 5
). The failure of T
cells expressing SOCS-3 to synthesize IL-2 resembles that seen in T
cells from transgenic mice expressing a dominant negative NFAT
(24).
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| Discussion |
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The ability of SOCS-3 to associate with calcineurin leads to the question of whether SOCS-3 affects the phosphatase activity of calcineurin. Calcineurin phosphatase activity toward an in vitro labeled peptide from the RII subunit of protein kinase A (26) was similar in extracts from Jurkat cells stably transfected with SOCS-3 and extracts from Jurkat cells transfected with empty vector (data not shown). Further phosphatase assays using extracts from 293T cells transfected with calcineurin demonstrated that recombinant GST-SOCS-3 does not inhibit the dephosphorylation of in vitro labeled peptide by calcineurin (data not shown). This inability of SOCS-3 to block calcineurin phosphatase activity toward an in vitro labeled peptide substrate leaves open the question of the mechanism of SOCS-3 in the inhibition of NFATp activation. One possibility is that SOCS-3 alters the interaction between NFATp and calcineurin by competing with NFATp for a binding site on calcineurin. It is also possible that SOCS-3 simultaneously associates with calcineurin and a serine/threonine kinase, bringing a kinase capable of phosphorylating NFATp to the NFATp-calcineurin complex. In support of this possibility, SOCS-3 has been shown to interact with the Pim family of serine/threonine kinases (B. Vuong and P. Rothman, unpublished data), and Pim family kinases can phosphorylate NFAT in vitro (27, 28). In the case of cytokine signaling, CIS, SOCS-1, and SOCS-3 are all negative regulators of JAK-STAT signaling (12). In contrast, CIS, SOCS-1, and SOCS-3 have divergent effects on TCR-mediated signals.
The observed increases in SOCS-3 mRNA levels in T cells activated through the TCR suggest the involvement of SOCS-3 in a classical feedback loop of TCR-mediated signal transduction. A recent report showing the ability of SOCS-3 to inhibit IL-2 signaling in lymphocytes adds an interesting dimension to the loop, because IL-2 cooperates with TCR-mediated signals to stimulate T cell proliferation (29). SOCS-3 may have a dual role in regulating T cell responses, inhibiting the activation of both NFAT by calcineurin and JAK1 by the IL-2 receptor.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Paul B. Rothman, Department of Medicine, Columbia University, 630 West 168th Street, New York, NY 10032. E-mail address: pbr3{at}columbia.edu ![]()
3 Abbreviations used in this paper: SHP, Src homology 2-containing protein tyrosine phosphatase; HA, hemagglutinin; JAK, Janus kinase; SOCS, suppressor of cytokine signaling; CIS, cytokine-induced Src homology 2-containing protein; GFP, green fluorescent protein. ![]()
Received for publication October 12, 2001. Accepted for publication February 28, 2002.
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