Mice deficient in SOCS2 display an excessive growth phenotype characterized by a 30-50% increase in mature body size. Here we show that the SOCS2 -/-phenotype is dependent upon the presence of endogenous growth hormone (GH) and that treatment with exogenous GH induced excessive growth in mice lacking both endogenous GH and SOCS2. This was reflected in terms of overall body weight, body and bone lengths, and the weight of internal organs and tissues. A heightened response to GH was also measured by examining GH-responsive genes expressed in the liver after exogenous GH administration. To further understand the link between SOCS2 and the GH-signaling cascade, we investigated the nature of these interactions using structure/function and biochemical interaction studies. Analysis of the 3 structural motifs of the SOCS2 molecule revealed that each plays a crucial role in SOCS2 function, with the conserved SOCS-box motif being essential for all inhibitory function. SOCS2 was found to bind 2 phosphorylated tyrosines on the GH receptor, and mutational analysis of these amino acids showed that both were essential for SOCS2 function. Together, the data provide clear evidence that SOCS2 is a negative regulator of GH signaling.

Suppressor of cytokine signaling (SOCS)-2 is a member of a family of intracellular proteins implicated in the negative regulation of cytokine signaling. The generation of SOCS-2-deficient mice, which grow to one and a half times the size of their wild-type littermates, suggests that SOCS-2 may attenuate growth hormone (GH)signaling. In vitro studies indicate that, while SOCS-2 can inhibit GH action at low concentrations, at higher concentrations it may potentiate signaling. To determine whether a similar enhancement of signaling is observed in vivo or alternatively whether increased SOCS-2 levels repress growth in vivo, we generated and analyzed transgenic mice that overexpress SOCS-2 from a human ubiquitin C promoter. These mice are not growth-deficient and are, in fact, significantly larger than wild-type mice. The overexpressed SOCS-2 was found to bind to endogenous GH receptors in a number of mouse organs, while phosphopeptide binding studies with recombinant SOCS-2 defined phosphorylated tyrosine 595 on the GH receptor as the site of interaction. Together, the data implicate SOCS-2 as having dual effects on GH signaling in vivo.The suppressor of cytokine signaling (SOCS) 1 proteins are a family of eight SH2 domain-containing proteins, comprising cytokine-inducible SH2 domain-containing protein (CIS) and SOCS-1-7. Studies in many laboratories have implicated SOCS proteins in the attenuation of cytokine action through inhibition of the Janus kinase (JAK)/signal transducer and activators of transcription (STAT) signal transduction pathway. SOCS proteins operate as part of a classical negative feedback loop, in which activation of cytokine signaling leads to their expression. Once produced, SOCS proteins bind to key components of the signaling apparatus to prevent further signal transduction and possibly target them for degradation via a conserved C-terminal motif, called the SOCS Box, that recruits ubiquitin ligases (reviewed in Refs. 1-3).While in vitro studies have suggested that SOCS proteins may be promiscuous in their activity, gene deletion studies in mice have highlighted their importance in a limited number of signaling pathways. SOCS-1 is a key regulator of interferon ␥ signaling, T-cell homeostasis, and lactation (4 -6), while SOCS-3 is thought to play a crucial role in placental function (7). CIS-deficient mice are reported to have no phenotype, although CIS transgenic mice display growth retardation and defects in mammary development which are accompanied by reductions in STAT5 phosphorylation (8). Interestingly, this phenotype has similarities to those observed in STAT5a-and STAT5b-deficient mice (9 -11).SOCS-2-deficient animals exhibit accelerated post-natal growth resulting in a 30 -50% increase in body weight by 12 weeks of age, significant increases in bone and body lengths, thickening of the skin due to collagen deposition, and increases in internal organ size (12). This phenotype has striking similarities to those of insulin-like growth factor (IGF)-I and growth hormone (GH) transgenic mic...

Carbohydrate modification of proteins includes N-linked and O-linked glycosylation, proteoglycan formation, glycosylphosphatidylinositol anchor synthesis, and O-GlcNAc modification. Each of these modifications requires the sugar nucleotide UDP-GlcNAc, which is produced via the hexosamine biosynthesis pathway. A key step in this pathway is the interconversion of GlcNAc-6-phosphate (GlcNAc-6-P) and GlcNAc-1-P, catalyzed by phosphoglucomutase 3 (Pgm3). In this paper, we describe two hypomorphic alleles of mouse Pgm3 and show there are specific physiological consequences of a graded reduction in Pgm3 activity and global UDP-GlcNAc levels. Whereas mice lacking Pgm3 die prior to implantation, animals with less severe reductions in enzyme activity are sterile, exhibit changes in pancreatic architecture, and are anemic, leukopenic, and thrombocytopenic. These phenotypes are accompanied by specific rather than wholesale changes in protein glycosylation, suggesting that while universally required, the functions of certain proteins and, as a consequence, certain cell types are especially sensitive to reductions in Pgm3 activity.

Mice deficient in SOCS2 display an excessive growth phenotype characterized by a 30-50% increase in mature body size. Here we show that the SOCS2 -/-phenotype is dependent upon the presence of endogenous growth hormone (GH) and that treatment with exogenous GH induced excessive growth in mice lacking both endogenous GH and SOCS2. This was reflected in terms of overall body weight, body and bone lengths, and the weight of internal organs and tissues. A heightened response to GH was also measured by examining GH-responsive genes expressed in the liver after exogenous GH administration. To further understand the link between SOCS2 and the GH-signaling cascade, we investigated the nature of these interactions using structure/function and biochemical interaction studies. Analysis of the 3 structural motifs of the SOCS2 molecule revealed that each plays a crucial role in SOCS2 function, with the conserved SOCS-box motif being essential for all inhibitory function. SOCS2 was found to bind 2 phosphorylated tyrosines on the GH receptor, and mutational analysis of these amino acids showed that both were essential for SOCS2 function. Together, the data provide clear evidence that SOCS2 is a negative regulator of GH signaling.