Activation of the signal transducer gp130 by interleukin-11 and interleukin-6 is mediated by similar molecular interactions

Increased expression of the interleukin-11 receptor and evidence of STAT3 activation in prostate carcinoma

Previous investigations have shown that interleukin-6, a member of the JAK-STAT activating family of cytokines, plays an important role in prostate carcinoma. Here we demonstrate the co-expression of another member of this cytokine family, interleukin-11 (IL-11), and components of its receptor (interleukin-11 receptor; IL-11R), ie, IL-11Ralpha (involved in ligand recognition), and gp130 (involved in signal transduction) in cultured normal and malignant prostate-derived epithelial cell lines. In the DU-145 prostate carcinoma cell line, rhIL-11 stimulates a transient and dose-dependent increase in the tyrosine 705-phosphorylated, active form of STAT3 (STAT3 P-Tyr705), involved in the downstream signaling of IL-11R and other members of the gp130-dependent receptors. The ability of IL-11 to activate STAT3 in prostate-derived cells may be mechanistically important, given recent data suggesting that constitutively activated STAT3 may be associated with the malignant phenotype. In 51 human primary tissues derived from normal prostate, benign prostatic hyperplasia, and prostate carcinomas, IL-11Ralpha and gp130 were commonly expressed, with a statistically significant elevation in the expression of IL-11Ralpha in prostate carcinoma. Also, the tyrosine-phosphorylated, activated form of STAT3 was observed more prominently in the nuclei of cells residing in malignant glands compared to those in nonmalignant samples. Thus, the IL-11 receptor system is up-regulated in prostate carcinoma, and may be one part of a cytokine network that maintains STAT3 in its activated form in these tissues.

Interleukin-11 signals through the formation of a hexameric receptor complex

Interleukin-11 (IL-11) is a member of the gp130 family of cytokines. These cytokines drive the assembly of multisubunit receptor complexes, all of which contain at least one molecule of the transmembrane signaling receptor gp130. IL-11 has been shown to induce gp130-dependent signaling through the formation of a high affinity complex with the IL-11 receptor (IL-11R) and gp130. Site-directed mutagenesis studies have identified three distinct receptor binding sites of IL-11, which enable it to form this high affinity receptor complex. Here we present data from immunoprecipitation experiments, using differentially tagged forms of ligand and soluble receptor components, which show that multiple copies of IL-11, IL-11R, and gp130 are present in the receptor complex. Furthermore, it is demonstrated that sites II and III of IL-11 are independent gp130 binding epitopes and that both are essential for gp130 dimerization. We also show that a stable high affinity complex of IL-11, IL-11R, and gp130 can be resolved by nondenaturing polyacrylamide gel electrophoresis, and its composition verified by second dimension denaturing polyacrylamide gel electrophoresis. Results indicate that the three receptor binding sites of IL-11 and the Ig-like domain of gp130 are all essential for this stable receptor complex to be formed. We therefore propose that IL-11 forms a hexameric receptor complex composed of two molecules each of IL-11, IL-11R, and gp130.

Monoclonal antibodies against the human interleukin-11 receptor alpha-chain (IL-11Ralpha) and their use in studies of human mononuclear cells

A panel of 14 hybridoma cell lines secreting monoclonal antibodies against the human interleukin-11 receptor alpha chain (hIL-11Ralpha) was obtained using two different approaches. Two antibodies were raised against peptides of the N- and C-terminal sequences, respectively, of the extracellular part of the hIL-11Ralpha. Another group of 12 antibodies was generated against a hybrid protein consisting of the extracellular part of the hIL-11Ralpha fused to mature full-length human IL-2. All these antibodies recognized native hIL-11Ralpha and most also recognized the denatured receptor on immunoblots after SDS-PAGE. Four different epitopes were identified on the extracellular part of the hIL-11Ralpha. One epitope, defined by the E27 antibody, is located at the N-terminus and the other three epitopes are clustered in the membrane-proximal, C-terminal region. The antibodies defining epitopes I and II recognized membrane-bound hIL-11Ralpha expressed in gp130/hIL-11Ralpha-co-transfected Ba/F3 cells. The E27 antibody cross-reacted with murine IL-11Ralpha, in agreement with the fact that the N-terminal region is highly conserved between species. The other 13 antibodies all recognized a region between amino acids 319 and 363, which is the membrane-proximal part of the hIL-11Ralpha. This region, which is less conserved between mouse and human, is shown here to be an immunodominant region. Anti-IL-11Ralpha monoclonal antibodies, which have not been described previously enabled us to explore the expression and tissue distribution of IL-11Ralpha on human peripheral blood mononuclear cells and cell lines. The antibodies provide powerful tools for the study of the regulation and function of the receptor.

Leukemia-inhibitory factor-neuroimmune modulator of endocrine function

Leukemia-inhibitory factor (LIF) is a pleiotropic cytokine expressed by multiple tissue types. The LIF receptor shares a common gp130 receptor subunit with the IL-6 cytokine superfamily. LIF signaling is mediated mainly by JAK-STAT (janus-kinase-signal transducer and activator of transcription) pathways and is abrogated by the SOCS (suppressor-of cytokine signaling) and PIAS (protein inhibitors of activated STAT) proteins. In addition to classic hematopoietic and neuronal actions, LIF plays a critical role in several endocrine functions including the utero-placental unit, the hypothalamo-pituitary-adrenal axis, bone cell metabolism, energy homeostasis, and hormonally responsive tumors. This paper reviews recent advances in our understanding of molecular mechanisms regulating LIF expression and action and also provides a systemic overview of LIF-mediated endocrine regulation. Local and systemic LIF serve to integrate multiple developmental and functional cell signals, culminating in maintaining appropriate hormonal and metabolic homeostasis. LIF thus functions as a critical molecular interface between the neuroimmune and endocrine systems.

Importance of the membrane-proximal extracellular domains for activation of the signal transducer glycoprotein 130

The transmembrane glycoprotein gp130 is the common signal transducing receptor subunit of the IL-6-type cytokines. The gp130 extracellular part is predicted to consist of six individual domains. Whereas the role of the three membrane-distal domains (D1-D3) in binding of IL-6 and IL-11 is well established, the function of the membrane-proximal domains (D4-D6) is unclear. Mapping of a neutralizing mAb to the membrane-proximal part of gp130 suggests a functional role of D4-D6 in receptor activation. Individual deletion of these three domains differentially interferes with ligand binding of the soluble and membrane-bound receptors. All deletion mutants do not signal in response to IL-6 and IL-11. The deletion mutants Delta4 and, to a lesser extent, Delta6 are still activated by agonistic monoclonal gp130 Abs, whereas the deletion mutant Delta5 does not respond. Because membrane-bound Delta5 binds IL-6/soluble IL-6R as does wild-type gp130, but does not transduce a signal in response to various stimuli, this domain plays a prominent role in coupling of ligand binding and signal transduction. Replacement of the fifth domain of gp130 by the corresponding domain of the homologous G-CSF receptor leads to constitutive activation of the chimera upon overexpression in COS-7 cells. In HepG2 cells this mutant responds to IL-6 comparable to wild-type gp130. Our findings suggest a functional role of the membrane-proximal domains of gp130 in receptor activation. Thus, within the hematopoietic receptor family the mechanism of receptor activation critically depends on the architecture of the receptor ectodomain.

Definition of receptor binding sites on human interleukin-11 by molecular modeling-guided mutagenesis

Interleukin-11 (IL-11) belongs to the interleukin-6 (IL-6)-type subfamily of long-chain helical cytokines including IL-6, ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), oncostatin M, and cardiotrophin-1, which all share the glycoprotein gp130 as a signal transducing receptor component. IL-11 acts on cells expressing gp130 and the IL-11 receptor (IL-11R) alpha-subunit (IL-11Ralpha). The structural epitopes of IL-11 required for the recruitment of the individual receptor subunits have not yet been defined. Based on the structure of CNTF, a three-dimensional model of human IL-11 was built. Using this model, 10 surface exposed amino acid residues of IL-11 were selected for mutagenesis using analogies to the well-characterized receptor recruitment sites of IL-6, CNTF, and LIF. The respective mutants of human IL-11 were expressed as soluble fusion proteins in bacteria. Their biological activities were determined on HepG2 and Ba/F3-130-11alpha cells. Several mutants with substantially decreased bioactivity and one hyperagonistic mutant were identified and further analyzed with regard to recruitment of IL-11Ralpha and gp130. The low-activity mutant I171D still binds IL-11Ralpha but fails to recruit gp130, whereas the hyperagonistic variant R135E more efficiently engages the IL-11R subunits. The low-activity mutants R190E and L194D failed to bind to IL-11Ralpha. These findings reveal a common mechanism of receptor recruitment in the family of IL-6-type cytokines and offer considerable perspectives for the rational design of IL-11 antagonists and hyperagonists.

A fusion protein of interleukin-11 and soluble interleukin-11 receptor acts as a superagonist on cells expressing gp130

Interleukin-11 is a hematopoietic cytokine that signals via the signal transducer gp130. Although gp130 is ubiquitously expressed, interleukine-11 responsiveness is restricted to cells that express the interleukine-11 receptor alpha-subunit. The interleukine-11 receptor alpha-subunit can be functionally replaced by its soluble form indicating that the transmembrane and cytoplasmic parts are not required for signal transduction. Here, we show that a recombinant fusion protein of a fragment of the human interleukine-11 receptor alpha-subunit ectodomain linked to human interleukine-11 acts as a superagonist on cells expressing gp130 but lacking the membrane-bound interleukine-11 receptor alpha-subunit. It induces acute phase protein synthesis in hepatoma cells and efficiently promotes proliferation of Ba/F3 cells stably, transfected with gp130. In these bioassays, the fusion protein of a fragment of the human interleukine-11 receptor alpha-subunit ectodomain linked to human interleukine-11 is 50 times more potent than the combination of interleukine-11 and the soluble interleukine-11 receptor alpha-subunit. Thus, our findings support the concept that covalent fusion of two soluble proteins required for receptor activation dramatically increases their bioactivity.

Interleukin-11 stimulates proopiomelanocortin gene expression and adrenocorticotropin secretion in corticotroph cells: evidence for a redundant cytokine network in the hypothalamo-pituitary-adrenal axis

We recently characterized leukemia inhibitory factor (LIF) as an important modulator of hypothalamo-pituitary-adrenal (HPA) axis activity. We now describe the role of interleukin (IL)-11, another member of the IL-6 cytokine family, in the neuro-immuno-endocrine modulation of the HPA axis. In murine hypothalamus, pituitary and corticotroph AtT-20 cells, IL-11 messenger RNA (mRNA) was detectable by RT-PCR only, whereas IL-11R mRNA transcripts were demonstrated by Northern blot. Using RT-PCR, IL-11 and IL-11R gene expression were also detected in normal human pituitaries, as well as in corticotropic and nonfunctioning pituitary adenomas. Incubation of AtT-20 cells for 24 h with 10(-9) M IL-11 stimulated ACTH secretion 1.4 +/- 0.1-fold (P < 0.01), whereas LIF at the same concentration caused a 1.5 +/- 0.1-fold increase (P < 0.001). POMC mRNA expression was induced by IL-11 (0.5 x 10(-9) M) and LIF (0.5 x 10(-9) M) 1.5 +/- 0.18-fold (P < 0.05) and 1.7 +/- 0.13-fold (P < 0.01), respectively. POMC promoter activity, assayed by a -706/+64 rat POMC promoter-luciferase construct, was stimulated by 0.5 x 10(-9) M IL-11 (1.9 +/- 0.06-fold; P < 0.001) and 5 mM Bu2cAMP (7.1 +/- 0.52-fold, P < 0.001), and combined treatment of IL-11 plus Bu2cAMP caused a synergistic 11.7+/-0.71-fold increase ofluciferase activity (P < 0.001 vs. Bu2cAMP alone). Gene expression of SOCS-3, an intracellular inhibitor of cytokine action, peaked as early as 60 min after incubation with IL-11 (0.5 x 10(-9) M) and was induced 3.5-fold. In comparison to mock-transfected AtT-20 cells (AtT-20M), stable overexpression of SOCS-3 (AtT-20S) resulted in significant inhibition of ACTH secretion induced by IL-11 alone (1.5 +/- 0.09 vs. 1.1 +/- 0.04-fold induction, P < 0.01) and IL-11 plus Bu2cAMP (2.1 +/- 0.21 vs. 1.5 +/- 0.06-fold, P < 0.05), but not by Bu2cAMP alone (1.5 +/- 0.12 vs. 1.4 +/- 0.06). In summary, human and murine pituitary express IL-11 and IL-11R transcripts. In murine corticotroph AtT-20 cells, IL- 11 induces POMC gene transcription and ACTH secretion. IL-11 induction of SOCS-3 indicates an intracellular negative feedback control of cytokine-induced POMC expression and ACTH secretion. Thus, IL-11 regulates the HPA axis similarly to LIF, providing further evidence for a redundant cytokine network in the neuro-immuno-endocrine regulation of the HPA axis.

Activation of the Signal Transducer Glycoprotein 130 by Both IL-6 and IL-11 Requires Two Distinct Binding Epitopes

The coordination and regulation of immune responses are primarily mediated by cytokines that bind to specific cell surface receptors. Glycoprotein 130 (gp130) belongs to the family of class I cytokine receptors and is the common signal-transducing receptor subunit shared by the so-called IL-6 type cytokines (IL-6, IL-11, ciliary neurotrophic factor, leukemia inhibitory factor, oncostatin M, and cardiotrophin-1). The inflammatory cytokines IL-6 and IL-11 induce gp130 homodimerization after binding to their specific α receptors, which leads to the activation of the Janus kinase/STAT signal transduction pathway. A molecular model of IL-6/IL-6R/gp130, which is based on the structure of the growth hormone/growth hormone receptor complex, allowed the selection of several amino acids located in the cytokine-binding module of gp130 for mutagenesis. The mutants were analyzed with regard to IL-6- or IL-11-induced STAT activation and ligand binding. It was found that Y190 and F191 are essential for the interaction of gp130 with IL-6 as well as IL-11, suggesting a common mode of recognition of helical cytokines by class I cytokine receptors. Furthermore, the requirement of the gp130 N-terminal Ig-like domain for ligand binding and signal transduction was demonstrated by the use of deletion mutants. Thus, besides the observed analogy to the growth hormone/growth hormone receptor complex, there is a substantial difference in the mechanism of receptor engagement by cytokines that signal via gp130.

The signal transducer gp130: solution structure of the carboxy-terminal domain of the cytokine receptor homology region

The transmembrane glycoprotein gp130 is the common signal transducing receptor subunit of the interleukin-6-type cytokines. It is a member of the cytokine-receptor superfamily predicted to consist of six domains in its extracellular part. The second and third domain constitute the cytokine-binding module defined by a set of four conserved cysteines and a WSXWS motif, respectively. The three-dimensional structure of the carboxy-terminal domain of this region was determined by multidimensional NMR. The domain consists of seven beta-strands constituting a fibronectin type III-like topology. The structure reveals that the WSDWS motif of gp130 is part of an extended tryptophan/arginine zipper which modulates the conformation of the CD loop.

Interleukin-6-type cytokine signalling through the gp130/Jak/STAT pathway

The family of cytokines signalling through the common receptor subunit gp130 comprises interleukin (IL)-6, IL-11, leukaemia inhibitory factor, oncostatin M, ciliary neurotrophic factor and cardiotrophin-1. These so-called IL-6-type cytokines play an important role in the regulation of complex cellular processes such as gene activation, proliferation and differentiation. The current knowledge on the signal-transduction mechanisms of these cytokines from the plasma membrane to the nucleus is reviewed. In particular, we focus on the assembly of receptor complexes after ligand binding, the activation of receptor-associated kinases of the Janus family, and the recruitment and phosphorylation of transcription factors of the STAT family, which dimerize, translocate to the nucleus, and bind to enhancer elements of respective target genes leading to transcriptional activation. The important players in the signalling pathway, namely the cytokines and the receptor components, the Janus kinases Jak1, Jak2 and Tyk2, the signal transducers and activators of transcription STAT1 and STAT3 and the tyrosine phosphatase SHP2 [SH2 (Src homology 2) domain-containing tyrosine phosphatase] are introduced and their structural/functional properties are discussed. Furthermore, we review various mechanisms involved in the termination of the IL-6-type cytokine signalling, namely the action of tyrosine phosphatases, proteasome, Jak kinase inhibitors SOCS (suppressor of cytokine signalling), protein inhibitors of activated STATs (PIAS), and internalization of the cytokine receptors via gp130. Although all IL-6-type cytokines signal through the gp130/Jak/STAT pathway, the comparison of their physiological properties shows that they elicit not only similar, but also distinct, biological responses. This is reflected in the different phenotypes of IL-6-type-cytokine knock-out animals.