Characterization of receptors for ciliary neurotrophic factor on rat hippocampal astrocytes

We have identified by Scatchard analysis both high (124 pM, 14.4 x106 sites/micrograms protein, 7600 sites/cell) and low (1.6 nM, 7.7x106 sites/micrograms protein, 4100 sites/cell) affinity receptors for [125I]-rat ciliary neurotrophic factor (rCNTF) on astrocytes. Ligand competition studies showed that the binding of [125I]-rCNTF was effectively competed by rCNTF and human CNTF, but not by hLIF, mIL-6 or mIL-1B. Three proteins specifically crossed-linked to [125I]-rCNTF, with the molecular weights of 190, 100, and 43 kDa, were immunoprecipitated by anti-rCNTF antibodies. Anti-LIFR or anti-gp130 antibodies immunoprecipitated the 100 and the 190 kDa proteins. CNTF induced the tyrosine phosphorylation of LIFR and gp130, as well as of proteins with the molecular weights of 88/91 and 42 kDa. The phosphorylation of the 88/91 kDa protein(s) was inhibited by pretreating the cells with staurosporine, 12-myristate 13-acetate phorbol (PMA), W7, chlorpromazine, or the intracellular Ca+2 chelator BAPTA/AM. In contrast, CNTF and PMA acted synergistically to induce the phosphorylation of two proteins with the molecular weights of 42 and 44 kDa. At later time points following CNTF treatment, c-fos messenger RNA and protein levels were increased. Collectively, these data indicate that hippocampal astrocytes express high-affinity, biologically functional receptor complexes for CNTF.