A family of proteins has recently been reported to function in a negative feedback loop to regulate signaling by cytokine receptors via the JAK 1 (Janus kinase)/STAT(signal transducer and activator of transcription) pathway (1-9). The first member of this family to be reported was mouse CIS (cytokine inducible SH2-containing protein) (1). Upon binding of ligand to cytokine receptors, receptor-associated JAKs become activated and phosphorylate tyrosine residues on the membrane distal portion of the receptor (10). Signaling molecules which subsequently bind to these phosphotyrosine containing motifs on the receptor include members of the STAT family. STATs are phosphorylated by cytokine receptor-associated JAKs, form dimers, and travel to the nucleus where they activate transcription. CIS was isolated as a cytokine responsive immediateearly gene in mouse hematopoietic cells (1, 2). CIS mRNA encodes a polypeptide of 257 amino acids that contains an SH2 domain in the middle of the molecule. Expression of CIS in IL-3-dependent hematopoietic cell lines reduced the growth rate of the transformants, suggesting a negative role of CIS in signal transduction. The CIS protein associated with tyrosinephosphorylated erythropoietin (EPO) receptor and the tyrosine-phosphorylated ␤ chain of the IL-3 receptor, presumably by binding of the CIS SH2 domain to phosphotyrosine containing motifs in the receptors. A mutant IL-2 receptor that failed to activate STAT5 could not induce CIS, suggesting that STAT5 was important for cytokine induction of CIS. Indeed, upstream of the transcription initiation site in the CIS promoter are four potential STAT5-binding sites. Expression of STAT5 and the EPO receptor in HEK293 cells conferred EPOdependent activation of the CIS promoter. In these cells, EPOdependent tyrosine phosphorylation of STAT5 was suppressed when CIS was coexpressed. Taken together, these results provide evidence for a negative feedback loop in which CIS is induced by the cytokine and then binds to the cytokine receptor, preventing the activation of STAT by JAKs (1, 2). Subsequently, three CIS-related proteins were described and these proteins have been designated as SSI (STAT-induced STAT inhibitor) or SOCS (suppressor of cytokine signaling) proteins 1-3 (3-7, 9). Together with CIS, SSI/SOCS proteins share a common domain structure consisting of an NH 2 -terminal region of variable length, a central SH2 domain and a COOHterminal motif, termed the SOCS box, of unknown function. In contrast to cytokine receptors which do not have intrinsic tyrosine kinase activity but utilize JAKs for receptor phosphorylation and phosphorylation of downstream signaling molecules such as STATs, the insulin-like growth factor I (IGF-I) receptor is a member of the tyrosine kinase family of growth factor receptors (11,12). The IGF-I receptor is important for cellular growth, differentiation, and inhibition of apoptosis. Binding of IGF-I or IGF-II to the IGF-I receptor results in receptor autophosphorylation. Receptor autophosphorylation amplifies t...

The insulin-like growth factor (IGF) family of peptides, binding proteins, and receptors are ubiquitous and important for normal human growth and development. Modern techniques including specific radioimmunoassays, radioreceptor assays and recombinant DNA technology have improved our understanding of the role of IGFs in growth and development. In addition to enhancing our understanding of normal physiology, these techniques assess changes in these hormones, binding proteins, and receptors in pathologic conditions including growth retardation, acromegaly, malnutrition, diabetes, and malignancy. Further, these studies have led to improvement in the assessment of responses to certain therapies used in the treatment of these diseases and may lead to improvements in these therapies.

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The somatomedin-like peptide multiplication-stimulating activity (MSA) binds specifically to rat serum. The pattern of MSA binding is GH dependent. Specific binding of [125I]iodo-MSA in normal rat serum is primarily in the gamma-globulin region (peak II) on Sephadex G-200, while MSA binding in hypophysectomized (hypox) rat serum is near the albumin region (peak III). This study further characterizes the peak II and peak III somatomedin-binding proteins produced by rat liver cells in culture. [125I]Iodo-MSA binding to normal rat serum is abolished by trypsin pretreatment of rat serum, suggesting that MSA binds to protein components of serum. The only detectable somatomedin activity (measured by [3H]thymidine incorporation into chick embryo fibroblast DNA) in fractions of normal rat serum chromatographed on Sephadex G-200 coincides with peak II binding of [125I]iodo-MSA. In hypox rat serum, the majority of detectable somatomedin activity is in the peak III region. There is complete displacement of the human somatomedins [125I]iodoinsulin-like growth factor I and II and [125I]iodosomatomedin A from the rat serum-binding sites by unlabeled MSA, suggesting that the human somatomedins bind to the same sites as MSA. Treatment of normal rat serum with 1 M acetic acid dissociates somatomedin activity from its binding proteins and converts somatomedin-binding proteins from peak II to peak III. Scatchard analysis of competitive binding data using [125I]iodo-MSA yields a binding affinity that is not appreciably different for either normal or hypox rat sera. The binding capacity of normal or acid-treated normal rat serum for MSA is significantly greater than that for comparably treated hypox rat sera. Although the site of synthesis of somatomedin-binding proteins in vivo is unknown, specific somatomedin-binding proteins are synthesized by two rat liver cell lines in culture. These rat liver cell somatomedin-binding proteins have the same molecular size and the same binding affinity for MSA as the peak III somatomedin-binding protein(s) in rat serum.