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Leukemia inhibitory factor (LIF) is a member of the cytokine family of growth factors. It has been shown to exert a variety of actions on a diverse range of cell types, including neuronal, bone, and hemopoietic cells (Hilton, 1992, Trends Biochem. Sci., 17:72-76). In many of these cell types, studies have indicated the presence of specific receptors for LIF (Godard et al., 1982, J. Biol. Chem., 267: 3214-3222; Hilton et al., Proc. Natl. Acad. Sci. USA, 85:5971-5975; Hilton and Nicola, 1992, J. Biol. Chem., 267:10238-10247.). The mechanism by which these receptors act is believed to involve tyrosine phosphorylation and the signal transducing receptor component gp130. We have previously shown that LIF is capable of inducing both human and murine myoblasts to proliferate in culture (Austin et al., 1992, J. Neurol. Sci., 112:185-191). We now report that LIF binds specifically to receptors on the surface of myoblasts, with an equilibrium dissociation constant of 400 pM and the number of receptors per cell varies with cell density. Binding competition studies showed that LIF binding to these receptor sites was not competed for by a number of other growth factors which stimulate myoblast proliferation including basic fibroblast growth factor (bFGF), transforming growth factor-alpha (TGF alpha), insulin-like growth factor 1 (IGF-1), and interleukin-6 (IL-6). There was a time and concentration-dependent down-regulation of receptor numbers following preincubation of myoblasts with LIF. The processing of these receptors subsequent to binding, involves as a first step, internalization and degradation by the myoblast. LIF appeared to stimulate myoblast proliferation rather than cell survival.

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