No abstract

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...

Mast cells play critical roles in the regulation of acute and chronic inflammations. Apoptosis is one of the mechanisms that limit and resolve inflammatory responses. Mast cell survival can be controlled by growth factors and activation of the IgE-receptor FceRI. Members of the Bcl-2 protein family are critical regulators of apoptosis and our study provides evidence that the proapoptotic BH3-only family member Bim is essential for growth factor deprivation-induced mast cell apoptosis and that Bim levels increase upon FceRI activation. Bim deficiency or Bcl-2 overexpression delayed or even prevented cytokine withdrawal-induced mast cell apoptosis in culture. The prosurvival protein Bcl-X L and the proapoptotic Bim were both induced upon FceRI activation. These results suggest that Bim and possibly also other BH3-only proteins control growth factor withdrawal-induced mast cell apoptosis and that the fate of mast cells upon FceRI activation depends on the relative levels of pro-and antiapoptotic Bcl-2 family members.

A panel of six naïve 14-residue random peptide libraries displayed polyvalently on M13 phage was pooled and sorted against human leukemia inhibitory factor (LIF). After four rounds of selection, a single large family of peptides with the consensus sequence XCXXXXG(A/S)(D/E)(W/F)WXCF was found to bind specifically to LIF. Peptides within this family did not bind related members of the interleukin-6 family of cytokines, nor to murine LIF that has 80% sequence identity with human LIF. A representative peptide from this family was synthesized and found to bind to LIF with an affinity of approximately 300 nM. The phage-displayed form of this peptide was able to compete with the LIF receptor alpha chain (LIFR) for binding to LIF; however, the free synthetic peptide was unable to inhibit LIF-LIFR binding or inhibit LIF bioactivity in vitro. Using a panel of human/murine chimeric LIF molecules, the peptide-binding site on LIF was mapped to a groove located between the B and the C helices of the LIF structure, which is distinct from the surfaces involved in binding to receptor. To mimic the effect of the phage particle and convert the free peptide into an antagonist of LIFR binding, a 40 kDa poly(ethylene glycol) (PEG) moiety was conjugated to the synthetic LIF-binding peptide. This PEG-peptide conjugate was found to be both an antagonist of LIF-LIFR binding and of LIF signaling in engineered Ba/F3 cells expressing LIFR and the gp130 coreceptor.