Cytokines are secreted proteins that regulate important cellular responses such as proliferation and differentiation. Key events in cytokine signal transduction are well defined: cytokines induce receptor aggregation, leading to activation of members of the JAK family of cytoplasmic tyrosine kinases. In turn, members of the STAT family of transcription factors are phosphorylated, dimerize and increase the transcription of genes with STAT recognition sites in their promoters. Less is known of how cytokine signal transduction is switched off. We have cloned a complementary DNA encoding a protein SOCS-1, containing an SH2-domain, by its ability to inhibit the macrophage differentiation of M1 cells in response to interleukin-6. Expression of SOCS-1 inhibited both interleukin-6-induced receptor phosphorylation and STAT activation. We have also cloned two relatives of SOCS-1, named SOCS-2 and SOCS-3, which together with the previously described CIS form a new family of proteins. Transcription of all four SOCS genes is increased rapidly in response to interleukin-6, in vitro and in vivo, suggesting they may act in a classic negative feedback loop to regulate cytokine signal transduction.

The four members of the recently identified suppressor of cytokines signaling family (SOCS-1, SOCS-2, SOCS-3, and CIS, where CIS is cytokine-inducible SH2-containing protein) appear, by various means, to negatively regulate cytokine signal transduction. Structurally, the SOCS proteins are composed of an N-terminal region of variable length and amino acid composition, a central SH2 domain, and a previously unrecognized C-terminal motif that we have called the SOCS box. By using the SOCS box amino acid sequence consensus, we have searched DNA databases and have identified a further 16 proteins that contain this motif. These proteins fall into five classes based on the protein motifs found N-terminal of the SOCS box. In addition to four new SOCS proteins (SOCS-4 to SOCS-7) containing an SH2 domain and a SOCS box, we describe three new families of proteins that contain either WD-40 repeats (WSB-1 and -2), SPRY domains (SSB-1 to -3) or ankyrin repeats (ASB-1 to -3) N-terminal of the SOCS box. In addition, we show that a class of small GTPases also contains a SOCS box. The expression of representative members of each class of proteins differs markedly, as does the regulation of expression by cytokines. The function of the WSB, SSB, and ASB protein families remains to be determined.Cytokines act by binding to and inducing dimerization of members of the hemopoietin receptor family expressed on the surface of responsive cells (1). Although the cytoplasmic proteins that then transduce the signal are relatively welldefined and include the Janus kinase (JAK) family of kinases and signal transducers and activators of transcription (STAT) transcription factors (2, 3), the proteins involved in limiting signal transduction are not well characterized.The four known members of the suppressor of cytokine signaling (SOCS) family (CIS, SOCS-1͞SSI-1͞JA B-1, SOCS-2, and SOCS-3, where CIS is cytokine-inducible SH2-containing protein) represent a family of negative regulators of cytokine signal transduction (4-9). The SOCS proteins appear to form part of a classical negative feedback loop that regulates cytokine signal transduction. Transcription of each of the SOCS genes occurs rapidly in vitro and in vivo in response to cytokines, and once produced, the various members of the SOCS family appear to inhibit signaling in different ways. For SOCS-1, inhibition of signal transduction appears to occur by binding to and inhibiting the catalytic activity of members of the JAK family of cytoplasmic kinases (4-6), while CIS appears to act by competing with signaling molecules such as the STATs for binding to phosphorylated receptor cytoplasmic domains (7, 9).The SOCS proteins share structural similarities. Each has an N-terminal region of variable length and highly variable amino acid sequence, a central SH2 domain, and a striking region of C-terminal homology that we designated the SOCS box (4). Given the sequence similarity evident in the SOCS box of the four SOCS proteins and its conserved position at the C terminus of each protein, ...

Suppressor of cytokine signalling-2 (SOCS-2) is a member of the suppressor of cytokine signalling family, a group of related proteins implicated in the negative regulation of cytokine action through inhibition of the Janus kinase (JAK) signal transducers and activators of transcription (STAT) signal-transduction pathway. Here we use mice unable to express SOCS-2 to examine its function in vivo. SOCS-2(-/-) mice grew significantly larger than their wild-type littermates. Increased body weight became evident after weaning and was associated with significantly increased long bone lengths and the proportionate enlargement of most organs. Characteristics of deregulated growth hormone and insulin-like growth factor-I (IGF-I) signalling, including decreased production of major urinary protein, increased local IGF-I production, and collagen accumulation in the dermis, were observed in SOCS-2-deficient mice, indicating that SOCS-2 may have an essential negative regulatory role in the growth hormone/IGF-I pathway.

SOCS-1 was originally identified as an inhibitor of interleukin-6 signal transduction and is a member of a family of proteins (SOCS-1 to SOCS-7 and CIS) that contain an SH2 domain and a conserved carboxyl-terminal SOCS box motif. Mutation studies have established that critical contributions from both the amino-terminal and SH2 domains are essential for SOCS-1 and SOCS-3 to inhibit cytokine signaling. Inhibition of cytokinedependent activation of STAT3 occurred in cells expressing either SOCS-1 or SOCS-3, but unlike SOCS-1, SOCS-3 did not directly interact with or inhibit the activity of JAK kinases. Although the conserved SOCS box motif appeared to be dispensable for SOCS-1 and SOCS-3 action when overexpressed, this domain interacts with elongin proteins and may be important in regulating protein turnover. In gene knockout studies, SOCS-1 -/-mice were born but failed to thrive and died within 3 weeks of age with fatty degeneration of the liver and hemopoietic infiltration of several organs. The thymus in SOCS-1 -/-mice was small, the animals were lymphopenic, and deficiencies in B lymphocytes were evident within hemopoietic organs. We propose that the absence of SOCS-1 in these mice prevents lymphocytes and liver cells from appropriately controlling signals from cytokines with cytotoxic side effects. J. Leukoc. Biol. 66: 588-592; 1999.

An N-ethyl-N-nitrosourea mutagenesis screen in mice was performed to isolate regulators of circulating platelet number. We report here recessive thrombocytopenia and kidney disease in plt1 mice, which is the result of a severe but partial loss-of-function mutation in the gene encoding glycoprotein-N-acetylgalactosamine-3-␤-galactosyltransferase (C1GalT1), an enzyme essential for the synthesis of extended mucin-type O-glycans. Platelet half-life and basic hemostatic parameters were unaffected in plt1͞plt1 mice, and the thrombocytopenia and kidney disease were not attenuated on a lymphocyte-deficient rag1-null background. gpIb␣ and podocalyxin were found to be major underglycosylated proteins in plt1͞plt1 platelets and the kidney, respectively, implying that these are key targets for C1GalT1, appropriate glycosylation of which is essential for platelet production and kidney function. Compromised C1GalT1 activity has been associated with immune-mediated diseases in humans, most notably Tn syndrome and IgA nephropathy. The disease in plt1͞plt1 mice suggests that, in addition to immune-mediated effects, intrinsic C1Gal-T1 deficiency in megakaryocytes and the kidney may contribute to pathology. core 1 galactosyltransferase ͉ N-ethyl-N-nitrosourea mutagenesis ͉ nephropathy ͉ platelet

The pivotal role of receptor tyrosine kinases (RTKs) in regulation of cellular growth and differentiation has triggered considerable interest in the identification of novel members of this ubiquitous protein family. Screening techniques, which were not dependent on function, brought about the isolation of numerous novel RTKs for which the ligands were not known. Many were EPH-like RTKs, which comprise the largest RTK family known to date (1-19).In contrast to the PCR-based approaches used for most other EPH-like RTKs, human EPH-like kinase (HEK) was identified on the cell surface of a human pre-B-cell line with monoclonal antibody (mAb) IIIA4 (2). HEK protein was affinity purified on a mAb IIIA4 column (2), and its amino acid sequence revealed homology with EPH and the predicted amino acid sequences of other EPH-like RTKs [ELK (3), ECK (4), and ERK (5)]. The sequences of the mouse (MEK4) and chicken (CEK4) homologues of HEK have also been reported (6, 7). To date, at least 28 members of the EPH subfamily have been identified in diverse vertebrate species including zeThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

SOCS7 is a member of the suppressor of cytokine signaling (SOCS) family of proteins (SOCS1-SOCS7 and CIS). SOCS proteins are composed of an N-terminal domain of variable length, a central Src homology 2 domain, and a C-terminal SOCS box. Biochemical and genetic studies have revealed that SOCS1, SOCS2, SOCS3, and CIS play an important role in the termination of cytokine and growth factor signaling. However, the biological actions of other SOCS proteins are less well defined. To investigate the physiological role of SOCS7, we have used gene targeting to generate mice that lack expression of the Socs7 gene. Socs7 ؊/؊ mice were born in expected numbers, were fertile, and did not exhibit defects in hematopoiesis or circulating glucose or insulin concentrations. However, Socs7 ؊/؊ mice were 7-10% smaller than their wild-type littermates, and within 15 weeks of age Ϸ50% of the Socs7 ؊/؊ mice died as a result of hydrocephalus that was characterized by cranial distortion, dilation of the ventricular system, reduced thickness of the cerebral cortex, and disorganization of the subcommissural organ. In situ hybridization studies revealed prominent expression of Socs7 in the brain, suggestive of an important functional role of SOCS7 in this organ. SOCS7 is a member of the suppressor of cytokine signaling (SOCS) family, which consists of eight proteins, SOCS1-SOCS7 and CIS. SOCS family members contain an N-terminal domain of variable length, a central Src homology 2 (SH2) domain, and a 40-aa motif at the C terminus termed the SOCS box (1). CIS and SOCS1-SOCS3 are potent feedback inhibitors of cytokines regulated by the Janus family of tyrosine kinases (JAKs) and the signal transducers and activators of transcription (STATs) (1, 2). Comparatively little is known about the actions of SOCS4-SOCS7. At the primary amino acid level, SOCS7 is most homologous to SOCS6, the two proteins exhibiting 56% amino acid identity within the SH2 domains and 53% within the SOCS box. The N-terminal domains of SOCS6 and SOCS7 exceed 350 aa in length, and whereas the SOCS6 N-terminal domain contains no identifiable protein interaction motifs, the SOCS7 N-terminal domain contains a putative nuclear localization signal and six proline-rich regions (3). Homology within the SOCS6 and SOCS7 SH2 domains extends to binding specificity, in that both SH2 domains preferentially bind to phosphopeptides containing a valine in the phosphotyrosine (pY) ϩ1 position and a hydrophobic residue in the pY ϩ2 and pY ϩ3 positions (4). The primary structural similarity and the shared phosphopeptidebinding specificity raise the possibility that SOCS6 and SOCS7 might share at least some biochemical and biological actions.Several lines of evidence suggest that SOCS6 and SOCS7 might regulate insulin signaling. The SOCS6 and SOCS7 SH2 domains bind to insulin receptor substrate 2 (IRS-2), IRS-4, and the p85 subunit of phosphatidylinositol-3 kinase (PI3K), proteins that play an important role in insulin-induced signaling (4). Overexpressed versions of SOCS6 have been reported t...

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