Cytokines play an important role in vertebrate homeostasis by controlling the growth and differentiation of target cells. Binding of these extracellular ligands to transmembrane receptors at the surface of cells initiates signal transduction pathways that influence several cellular processes. The components of the downstream signal pathways that transduce these signals have been the focus of intense study in recent years. Interaction of a cytokine with its receptor results in receptor aggregation and activation of accessory protein kinase proteins called Janus kinases (JAKs). Activation of JAK tyrosine kinases results in the phosphorylation of specific residues in the intracellular domain of the cytokine receptor that, in turn, act as docking sites for proteins that initiate multiple signal transduction cascades, including phosphatidylinositol 3-kinase (PI3K), ras, phospholipase C-γ, and the signal transducers and activators of transcription (STATs; O'Shea et al., 2002). An important component of the intracellular effector cascades are the negative regulatory molecules that serve to limit the response of the cell to the cytokine signal (Krebs and Hilton, 2001). Three classes of signal modulators that inhibit the cytokine-mediated signaling pathways have been identified: the protein tyrosine phosphatases, the protein inhibitors of activated STATs (PIAS) and the suppressor of cytokine signaling (SOCS) proteins.
There are eight known members of the SOCS family (SOCS1–7 and CIS; Krebs and Hilton, 2001). The SOCS family is defined by the presence of a Src-homology 2 domain (binds proteins at the sites of phosphorylated tyrosine residues) and a SOCS box (a 40 amino acid domain in the carboxyl terminal domain). The SOCS proteins act in a negative-feedback loop to suppress signal transduction from cytokine receptors by at least two distinct mechanisms. SOCS1, for example, binds to JAK proteins and inhibits their kinase activity. CIS uses a different mechanism to inhibit cytokine signaling, it directly associates with the cytokine receptor and competes with STAT proteins for activated binding sites. SOCS3 also binds to the cytokine receptor but blocks Janus kinase activity by binding JAK-proximal sites on the activated receptor. The SOCS proteins are induced by cytokine receptor signaling pathways. For example, SOCS3, the focus of this study, is induced by a variety of cytokine signals, including interleukin (IL) -2, IL-3, IL-4, IL-6, IL-10, interferon-γ (IFN-γ), erythropoietin (EPO), prolactin, granulocyte/macrophage colony stimulating factor (GM-CSF), G-CSF, leukemia inhibitory factor (LIF), growth hormone (GH), and lipopolysaccharide. The induction of SOCS expression by cytokine signaling is thought to be mediated, in part, by the binding of STAT proteins to the proximal promoter regions of the SOCS genes. SOCS proteins can bind and inhibit multiple cytokine receptors and their associated molecules, thus providing potential crosstalk for multiple cytokine signaling pathways. For example, SOCS3 binds and inhibits signaling from Lck, gp130, GH receptor, EPO receptor, leptin receptor, insulin-like growth factor-1 receptor, PYK2, and the fibroblast growth factor receptor. Targeted gene disruption experiments in mice have shown that SOCS3 is essential for embryonic survival (Marine et al., 1999; Roberts et al., 2001).
The isolation of Xenopus cytokine receptors has been hampered by the low degree of sequence homology with mammalian counterparts. We sought to identify intracellular components of the cytokine signaling pathways. In this study, we present the cloning and initial characterization of the Xenopus SOCS3 (xSOCS3) protein. To our surprise, in situ hybridization studies revealed that xSOCS3 is rapidly induced at the site of epithelial wounds. The induction of SOCS3 by epithelial wounding is transient, with maximal expression being reached 1 hr after trauma and diminishing thereafter.