Functionally active fusion protein of the novel composite cytokine CLC/soluble CNTF receptor

The heterodimeric cytokine composed of the soluble ciliary neurotrophic factor receptor (sCNTFR) and the IL-6 family member cardiotrophin-like cytokine (CLC) was recently identified as a new ligand for gp130–leukemia inhibitory factor receptor (LIFR) complex [Plun-Favreau, H., Elson, G., Chabbert, M., Froger, J., deLapeyriere, O., Lelievre, E., Guillet, C., Hermann, J., Gauchat, J. F., Gascan, H. & Chevalier, S. (2001) EMBO J.20, 1692–1703]. This heterodimer shows overlapping biological properties with LIF. Although CLC contains a putative signal peptide and therefore should enter into the classical secretory pathway, the protein has been shown to be retained within transfected mammalian cells, unless coexpressed with either sCNTFR or cytokine like factor (CLF) [Elson, G. C., Lelievre, E., Guillet, C., Chevalier, S., Plun-Favreau, H., Froger, J., Suard, I., de Coignac, A. B., Delneste, Y., Bonnefoy, J. Y., Gauchat, J. F. & Gascan, H. (2000) Nat. Neurosci.3, 867–872]. In the present study, we demonstrate that a fusion protein comprising CLC covalently coupled through a glycine/serine linker to sCNTFR (CC–FP) is efficiently secreted from transfected mammalian cells. CC–FP shows enhanced activities in respect to the CLC/sCNTFR native complex, on a number of cells expressing gp130 and LIFR on their surface. In addition, CC–FP is able to compete with CNTF for cell binding, indicating that both cytokines share binding epitope(s) expressed by their receptor complex. Analysis of the downstream signaling events revealed the recruitment by CC–FP of the signal transducer and activator of transcription (STAT)-3, Akt and mitogen-activated protein (MAP) kinase pathways. The monomeric bioactive CLC/sCNTFR fusion protein is therefore a powerful tool to study the biological role of the recently described cytokine CLC.