The leukemia inhibitory factor receptor (LIF-R) is activated not only by LIF, but also by cardiotrophin-1, ciliary neurotrophic factor with its receptor, and oncostatin M (OSM). Each of these cytokines induces the hetero-oligomerization of LIF-R with gp130, a signaltransducing subunit shared with interleukin-6 and interleukin-11. The introduction of mutations into human LIF that reduced the affinity for gp130 while retaining affinity for LIF-R has generated antagonists for LIF. In the current study, a LIF antagonist that was free of detectable agonistic activity was tested for antagonism against the family of LIF-R ligands. On cells that express LIF-R and gp130, all LIF-R ligands were antagonized. On cells that also express OSM receptor, OSM was not antagonized, demonstrating that the antagonist is specific for LIF-R. Ligand-triggered tyrosine phosphorylation of both LIF-R and gp130 was blocked by the antagonist. The antagonist is therefore likely to work by preventing receptor oligomerization.Functional overlap among cytokines often derives from shared receptor components. Interleukin-6 (IL-6), 1 IL-11, leukemia inhibitory factor (LIF), cardiotrophin-1 (CT-1), ciliary neurotrophic factor (CNTF), and OSM (oncostatin M) have biological activities in common, and all require gp130 as a signal-transducing subunit (for review see Ref. 1). LIF (2-6), CT-1 (7), CNTF (8,9), and OSM (6, 10, 11) activate gp130 in complexes with LIF receptor (LIF-R). LIF-R is a transmembrane signaling subunit that is structurally related to gp130 and belongs to the same hematopoietin receptor family (4). Ligand binding drives the heterodimerization of LIF-R and gp130, which results in inter alia activation and tyrosine phosphorylation of the Jak-Tyk cytoplasmic tyrosine kinases (12,13). The Jaks in turn phosphorylate tyrosine residues in the cytoplasmic domains of both LIF-R and gp130 (8), allowing the recruitment of substrates with Src homology 2 domains including STAT3 and protein tyrosine phosphatase PTP1D (14 -17) (for review see Refs. 18 and 19). CNTF weakly activates LIF-R-gp130, becoming a potent agonist once first bound to a nonsignal-transducing subunit, CNTFR␣. In addition to LIF-Rgp130, OSM also activates a receptor complex unique to OSM made up of . OSM-R has recently been identified as a transmembrane signaling subunit related to . Thus, LIF-R participates in a subset of gp130-mediated responses that includes all known LIF, CT-1, and CNTF responses, as well as some responses to OSM.LIF-R is moderately expressed in testis, eye, skeletal muscle, ovary, uterus, thymus, brain, and fat and is highly expressed in liver and placenta (21). Activation of LIF-R-gp130 complexes regulates the differentiation and proliferation of a variety of cell lines as well as influencing the behavior of cultured neurons, hepatocytes, and adipocytes (for review see Ref. 22). Gene knockout experiments have demonstrated roles for LIF-R in development. LIF-RϪ/Ϫ and CNTFR␣Ϫ/Ϫ mice show a substantial loss of motor neurons (23,24). LIF-RϪ/Ϫ mice have also be...