MULTIPLE REGIONS WITHIN THE CYTOPLASMIC DOMAINS OF THE LEUKEMIA INHIBITORY FACTOR-RECEPTOR AND GP130 COOPERATE IN SIGNAL-TRANSDUCTION IN HEPATIC AND NEURONAL CELLS

The receptor for leukemia inhibitory factor (LIFR), in combination wit h the signal-transducing subunit for interleukin-6-type cytokine recep tors, gp130, and LIF, activates transcription of acute-phase plasma pr otein genes in human and rat hepatoma cells and the vasoactive intesti nal peptide gene in a human neuroblastoma cell line. To identify the r egions within the cytoplasmic domain of LIFR that initiate signal tran sduction independently of gp130, we constructed a chimeric receptor by linking the extracellular domain of the granulocyte colony-stimulatin g factor receptor (G-CSFR) to the transmembrane and cytoplasmic domain of human LIFR. The function of the chimeric receptor protein in trans criptional activation was assessed by G-CSF-mediated stimulation of co transfected cytokine-responsive reporter gene constructs in hepatoma a nd neuroblastoma cells. By using the full-length cytoplasmic domain an d mutants with progressive carboxy-terminal deletions, internal deleti ons, or point mutations, we identified the first 150 amino acid residu es of LIFR as the minimal region necessary for signaling. The signalin g reaction appears to involve a cooperativity between the first 70-ami no-acid region containing the two sequence motifs conserved among hema topoietin receptors (box 1 and box 2) and a critical sequence between residues 141 and 150 (box 3). Analogous analyses of the cytoplasmic do mains of G-CSFR and gp130 indicated similar arrangements of functional domains in these receptor subunits and the requirement of a box 3-rel ated motif for signaling.