Sterile tissue injury or infection initiates a local inflammatory response that mobilizes a systemic acute phase reaction resulting in, among other things, the induction of genes encoding the acute phase plasma proteins (APPs). In all vertebrates, a common set of APPs is increased and exerts essential protective functions. Haptoglobin (HP), one of the major APPs, acts as a high-affinity hemoglobin-binding protein and antioxidant. Liver is the major site of HP synthesis; however, regulated, low level expression is also detected in other organs. Induction of the Hp gene is mediated by interleukin-6-type cytokines and is synergistically enhanced by glucocorticoids. Growth stimulation of hepatic cells in vivo or in vitro suppresses the Hp gene-inducing effects of inflammatory cytokines. Receptors for IL-6 cytokines mediate induction of the Hp gene by the transcription factors signal transducer and activator of transcription-3 (STAT3) and CAAT/enhancer binding protein beta (C/EBPbeta), but attenuate the stimulation through co-activated STAT5 and mitogen-activated protein kinases, ERK-1 and ERK-2. The specificity by which the related cytokines, IL-6, oncostatin M, and leukemia inhibitory factor, regulate Hp gene transcription is determined by the profile of the cytokine receptor subunits expressed on the target cells and the relative extents by which these receptors activate the intracellular signaling pathways. The current hypothesis is that HP exerts an anti-inflammatory activity and that by the degree with which HP attenuates the inflammatory process, including the production of IL-6 cytokines, it determines the level and duration of acute phase expression of the Hp gene.

The cytokine receptor subunits gp130, leukemia inhibitory factor receptor ␣ (LIFR␣), and oncostatin M receptor ␤ (OSMR␤) transduce OSM signals that regulate gene expression and cell proliferation. After ligand binding and activation of the Janus protein-tyrosine kinase/STAT and mitogen-activated protein kinase signal transduction pathways, negative feedback processes are recruited. These processes attenuate receptor action by suppression of cytokine signaling and by downregulation of receptor protein expression. This study demonstrates that in human fibroblasts or epithelial cells, OSM first decreases the level of gp130, LIFR␣, and OSMR␤ by ligand-induced receptor degradation and then increases the level of the receptors by enhanced synthesis. The transcriptional induction of gp130 gene by OSM involves STAT3. Various cell lines expressing receptor subunits to the different interleukin-6 class cytokines revealed that only LIFR␣ degradation is promoted by activated ERK and that degradation of gp130, OSMR␤, and a fraction of LIFR␣ involves mechanisms that are separate from signal transduction. These mechanisms include ligand-mediated dimerization, internalization, and endosomal/lysosomal degradation. Proteosomal degradation appears to involve a fraction of receptor subunit proteins that are ubiquitinated independently of ligand binding.Interleukin-6 (IL-6), 1 oncostatin M (OSM), and leukemia inhibitory factor (LIF) are functionally and structurally related and are part of the IL-6 family of cytokines (1-5). Each IL-6 cytokine is recognized by a specific ligand binding receptor subunit. In humans, OSM is exceptional in that it interacts with gp130 and with either LIFR␣ or OSMR␤ to form the high affinity, signaling-competent OSM receptor complex I or II (3, 4). Ligand-induced oligomerization of receptor subunits activates Janus protein-tyrosine kinases (JAKs), which in turn phosphorylate tyrosine residues in the receptor cytoplasmic domain. These phosphorylated tyrosines create docking sites for STAT transcription factors (STAT1, -3, and -5), proteintyrosine phosphatase SHP-2, and linker proteins such as Gab-1, Grb2, or SHC, which propagate the signal to other pathways (ERK 1/2, JNK, phosphatidylinositol 3-kinase; Refs. 1-8). Receptor signaling is manifested by the activation of genes such as acute phase proteins (2) or the cyclin-dependent kinase inhibitor p21 WAF1 , which is primarily activated through STATs (9) and immediate early response genes such as c-fos, c-jun, and egr-1 primarily through ERK 1/2 (6).Signaling by IL-6 cytokine receptors is transient, often restricted temporally and in magnitude by the action of negative regulators. The SH2 domain-containing protein-tyrosine phosphatases SHP1 and -2, through their catalytic function, attenuate the activity of receptor-associated JAKs and consequently lower the induction of STAT-dependent genes (4, 6). The suppressor of cytokine signaling SOCS1 and -3 are rapidly induced by IL-6 cytokines and, through their SH2 domain, interact and deactivate JAKs or gp130 (4,...

Leukemia inhibitory factor (LIF) signals via the heterodimeric receptor complex comprising the LIF receptor ␣ subunit (LIFR␣) and the common signal transducing subunit for interleukin-6 cytokine receptors, gp130. This study demonstrates that in different cell types, the level of LIFR␣ decreases during treatment with LIF or the closely related cytokine oncostatin M (OSM). Moreover, insulin and epidermal growth factor induce a similar LIFR␣ down-regulation. The regulated loss of LIFR␣ is specific since neither gp130 nor OSM receptor ␤ shows a comparable change in turnover. LIFR␣ downregulation correlates with reduced cell responsiveness to LIF. Using protein kinase inhibitors and point mutations in LIFR␣, we demonstrate that LIFR␣ downregulation depends on activation of extracellular signalregulated kinase 1/2 and phosphorylation of the cytoplasmic domain of LIFR␣ at serine 185. This modification appears to promote the endosomal/lysosomal pathway of the LIFR␣. These results suggest that extracellular signal-regulated kinase-activating factors like OSM and growth factors have the potential to lower specifically LIF responsiveness in vivo by regulating LIFR␣ half-life. Leukemia inhibitory factor (LIF)1 is one member of the interleukin (IL)-6-type family of cytokines that also include IL-6, IL-11, oncostatin M (OSM), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1), and neurotrophin-1. LIF is a pleiotropic cytokine that, among other functions, induces differentiation of mouse monocytic leukemia M1 cells, conversion of sympathetic neurons from the adrenergic to cholinergic phenotype, suppresses the differentiation of embryonic stem cells, enhances proliferation of myoblasts, and facilitates endometrial implantation of embryos (reviewed in Refs. 1 and 2).LIF also plays a role in the systemic inflammatory response, activating the hypothalamic-adrenal axis, and inducing the acute phase reaction of the liver (3). In hepatocytes, LIF, similar to other IL-6 cytokines, stimulates the enhanced expression of a set of plasma proteins, termed acute phase proteins (APP) (4). The precise pattern of APP expression is determined by the action of IL-6 cytokines in combination with various other inflammatory mediators, endocrine hormones, and growth factors (5). For instance, during the acute phase reaction, insulin is increased 3-fold (6) and then modulates the cytokine regulation of APP genes (7,8). In myoblasts, IGF-1 also reduces LIF action, apparently by down-regulating LIF receptor number (9).LIFR␣ is a 190-kDa transmembrane protein with low affinity for LIF. In combination with gp130 subunit, it forms the high affinity LIF receptor complex (10, 11). As described in the human system, OSM also uses LIFR␣ and gp130 subunits to form a high affinity OSM receptor complex (then termed OSMR complex type I). CNTF, CT-1, and neurotrophin-1 also utilize LIFR␣ and gp130 subunits (reviewed in Refs. 12 and 13). In addition to the shared LIFR␣/gp130 complex used by either LIF or OSM, a specific OSM receptor complex (type II) has bee...

The related cytokines, interleukin-6 (IL-6), oncostatin M (OSM), and leukemia inhibitory factor (LIF) direct the formation of specific heteromeric receptor complexes to achieve signaling. Each complex includes the common signal-transducing subunit gp130. OSM and LIF also recruit the signaling competent, but structurally distinct OSMR␤ and LIFR␣ subunits, respectively. To test the hypothesis that the particularly prominent cell regulation by OSM is due to signals contributed by OSMR␤, we introduced stable expression of human or mouse OSMR␤ in rat hepatoma cells which have endogenous receptors for IL-6 and LIF, but not OSM. Both mouse and human OSM engaged gp130 with their respective OSMR␤ subunits, but only human OSM also acted through LIFR. Signaling by OSMR␤-containing receptors was characterized by highest activation of STAT5 and ERK, recruitment of the insulin receptor substrate and Jun-N-terminal kinase pathways, and induction of a characteristic pattern of acute phase proteins. Since LIF together with LIFR␣ appear to form a more stable complex with gp130 than OSM with gp130 and OSMR␤, co-activation of LIFR and OSMR resulted in a predominant LIF-like response. These results suggest that signaling by IL-6 cytokines is not identical, and that a hierarchical order of cytokine receptor action exists in which LIFR ranks as dominant member.Proinflammatory cytokines are the principal intercellular mediators of the tissue reaction to trauma and infection (1). Members of the interleukin 6 hematopoietic cytokine family, that include IL-6, 1 leukemia inhibitor factor (LIF), and oncostatin M (OSM), play a particularly prominent role in orchestrating initiation and progression of the inflammatory response and controlling homostatic processes. These roles were identified by the effects of transgenic cytokine expression, the knockout of cytokine genes in mice (2-5), or by in vivo treatments with pharmacological doses of cytokines or activity neutralizing antibodies (6 -11). OSM is produced by activated monocytes and lymphocytes (12, 13) (e.g. at sites of inflammation) and acts locally on stromal cells. Stromal cells in turn respond prominently by enhanced production of IL-6 and LIF (14). IL-6 and LIF enter into circulation and participate in the recruitment of the systemic inflammatory response that includes the acute phase reaction of the liver (1, 15).Each of the IL-6 cytokines is recognized by a specific ligand binding subunit, i.e. IL-6 by IL-6 receptor ␣ (IL-6R␣), LIF by LIFR␣ (note, this subunit has also been called LIFR␤; Ref. 16), and OSM (human) by gp130. These cytokine receptor complexes cooperate with a second, signal-transducing subunit to form a signaling-competent receptor unit (abbreviated as follows: IL-6R consisting of IL-6R␣-gp130; LIFR consisting of LIFR␣-gp130; and OSMR (also defined as type II OSMR) consisting of gp130-OSMR␤ (17, 18)). In human cells, hOSM also engages a type I OSMR consisting of gp130-LIFR␣ that is equivalent to LIFR (19). Ligand-induced receptor subunit interaction results in trans-a...

The related members of the interleukin-6 (IL-6) family of cytokines, leukemia inhibitory factor (LIF), oncostatin M (OSM) and IL-6 are inflammatory mediators that control differentiated cell functions as well as proliferation. The cellular responsiveness to these cytokines is largely determined by the expression of the appropriate receptor proteins. The receptor expression profile for each cell type is established during differentiation and is often altered during oncogenic transformation. Since inhibition of histone deacetylases (HDAC) has the potential to re-activate epigenetically silenced genes, we asked whether inhibition of HDAC enhances the expression of IL-6 cytokine receptors and, thus, increase desirable cytokine responses. We demonstrate that treatment with FR901228 (FR), an HDAC inhibitor, increases the responsiveness to LIF in different cell types, including normal fibroblasts, epithelial cells, macrophages and splenocytes, as well as various tumor cell lines. Depending on the cell type, FR treatment also enhances the responsiveness to OSM and IL-6. These effects involve a transcriptional induction of the cytokine receptor subunits LIFRa, OSMRb, gp130, or the transcription factor STAT3. FR-specific induction of LIFRa occurs independently of de novo protein synthesis and cell proliferation and is mediated in part by the CBP/p300 coactivator. Chromatin immunoprecipitation experiments indicate that the expression of LIFRa and gp130 genes correlates with the level of acetylated histone 3 associated with the receptor promoter regions. The FR-stimulated expression of IL-6-type cytokine receptors in certain tumor cells also provided improved conditions for suppression of cell growth by taking advantage of the growth inhibitory effect of these cytokines.