Ciliary neurotrophic factor (
CNTF) associates with an alpha subunit (
CNTFRalpha) of the receptor complex to initiate signal transduction by facilitating heterodimerization of the
gp130 transducing protein and the
leukemia inhibitory factor receptor (LIFR) beta.
CNTFRalpha is anchored to the membrane by a
glycosylphosphatidylinositol linkage; however, a soluble form of the alpha subunit can still bind
CNTF to recruit the signal transducing components of the receptor complex. In the present study we show that
alanine substitution for residues Thr268 and Asp269 of the
CNTFRalpha subunit results in a mutated receptor subunit (R3), which can bind
CNTF with an affinity similar to that of the wild type
CNTFRalpha but, when expressed as a soluble receptor subunit, lowers the binding of
CNTF to its tripartite receptor. In addition, CNTFR3alpha inhibits the proliferation of the TF1 hematopoietic cell line triggered by
CNTF plus soluble wild type
CNTFRalpha but not by
IL-6 or
oncostatin M. Similarly, CNTFR3alpha specifically antagonizes the induction of gp130 and LIFRbeta
tyrosine phosphorylation observed in response to
CNTF and wild type soluble
CNTFRalpha in the HepG2
hepatoma cell line, as well as the subsequent events leading to
haptoglobin synthesis. Positions 268 and 269 of
CNTFRalpha appear to be critical for its interaction with gp130 and LIFRbeta, whereby
alanine substitution of the residues at these positions results in antagonism of the
CNTF-induced response.