The cytokines that signal through the leukemia inhibitory factor (LIF) receptor are members of the neuropoietic cytokine family and have varied and numerous roles in the nervous system. In this report we have determined the effects of growth factor stimulation on LIF receptor (LIFR) expression and signal transduction in the human neuroblastoma cell line NBFL. We show here that stimulation of NBFL cells with either epidermal growth factor or fibroblast growth factor decreases the level of LIFR in an extracellular signal-regulated kinase (Erk)1/2-dependent manner and that this downregulation is due to an increase in the apparent rate of lysosomal LIFR degradation. Growth factor-induced decreases in LIFR level inhibit both LIF-stimulated phosphorylation of signal transducers and activators of transcription 3 (STAT3) and LIFRmediated gene induction. We also show that Ser1044 of LIFR, which we have previously shown to be phosphorylated by Erk1/2, is required for the inhibitory effects of growth factors. Neurons are exposed to varying combinations and concentrations of growth factors and cytokines that influence their growth, development, differentiation and repair in vivo. These findings demonstrate that LIFR expression and signaling in neuroblastoma cells can be regulated by growth factors that are potent activators of the mitogen activated protein kinase pathway, and thus illustrate a fundamental mechanism that underlies cross-talk between receptor tyrosine kinase and neuropoietic cytokine signaling pathways. Keywords leukemia inhibitory factor; gp130; growth factor; MAP Kinase; cytokine The leukemia inhibitory factor receptor (LIFR), together with glycoprotein 130 (gp130), acts as a signal transducing receptor for a group of related cytokines that include leukemia Address correspondence to: Neil M. Nathanson, Department of Pharmacology, University of Washington, Box 357750, 1959 NE Pacific, Seattle, WA, 98195-7750, Tel. 206-543-9457; Fax. 206-616-4230; E-mail: nathanso@u.washington 1 The abbreviations used in the text are: CaMK, Ca 2+ /calmodulin-dependent kinase; CLC, cardiotrophin-like cytokine; CNTF, ciliary neurotrophic factor; CNTFR, ciliary neurotrophic factor receptor; CT-1, cardiotrophin-1; DMEM, Dulbecco's modified Eagle's medium; DMSO, dimethyl sulfoxide; DTT, dithriotreithol; EGF, epithelial growth factor; ERK, extracellular signal-regulated kinase; FBS, fetal bovine serum; FGF, fibroblast growth factor; G-CSF, granulocyte colony stimulating factor; G-CSFR, G-SCF receptor; gp130, glycoprotein 130; IL, interleukin; JAK, Janus kinase; LIF, leukemia inhibitory factor; LIFR, leukemia inhibitory factor receptor; MAPK, mitogen activated protein kinase; MEK, MAPK kinase; NGF, nerve growth factor; OsM, Oncostatin M; PERK, phosphorylated ERK; PAGE, polyacrylamide gel electrophoresis; PBS, phosphate-buffered saline; PIAS, protein induced by activated STATs; PMA, phorbol 12-myristate 13-acetate; PY-STAT3, Tyr705-phosphorylated STAT3; SDS, sodium dodecyl sulphate; SHP, src homology 2 domain-containing phos...