Signaling pathways recruited by the cardiotrophin-like cytokine/cytokine-like factor-1 composite cytokine - Specific requirement of the membrane-bound form of ciliary neurotrophic factor receptor alpha component

Ciliary neurotrophic factor (CNTF) is a cytokine sup porting the differenti ation and survival of a number of neural cell types. Its receptor complex c onsists of a ligand-binding component, CNTF receptor (CNTFR), associated wi th two signaling receptor components, gp130 and leukemia inhibitory factor receptor (LIFR), Striking phenotypic differences between CNTF- and CNTFR-de ficient mice suggest that CNTFR serves as a receptor for a second developme ntally important ligand, We recently demonstrated that cardiotrophin-like c ytokine (CLC) associates with the soluble orphan receptor cytokine-like fac tor-1 (CLF) to form a heterodimeric cytokine that displayed activities only on cells expressing the tripartite CNTF receptor on their surface. In this present study we examined the membrane binding of the CLC/CLF composite cy tokine and observed a preferential interaction of the cytokine with the CNT FR subunit. Signaling pathways recruited by the CLC/CLF complex in human ne uroblastoma cell lines were also analyzed in detail. The results obtained s howed an activation of Janus kinases (JAK1, JAK2, and TYK2) leading to a ty rosine phosphorylation of the gp130 and LIFR, The phosphorylated signaling receptors served in turn as docking proteins for signal transducing molecul es such as STAT3 and SHP-2, In vitro analysis revealed that the gp130-LIFR pathway could also stimulate the phosphatidylinositol 3-kinase and the mito gen-activated protein kinase pathways. In contrast to that reported before for CNTF, soluble CNTFR failed to promote the action CLC/CLF, and an absolu te requirement of the membrane form of CNTFR was required to generate a fun ctional response to the composite cytokine. This study reinforces the funct ional similarity between CNTF and the CLC/CLF composite cytokine defining t he second ligand for CNTFR.