Regulation of expression and function of neurokine receptors

Leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) are cytokines which signal through receptor complexes that include the subunits gp130 and LIF receptor (LIFR), but CNTF also requires the non-signal transducing CNTF receptor (CNTFR) for binding. We show here in IMR-32 neuronal cells endogenously expressing these receptors that CNTFR, but not gp130 or LIFR, is found in detergent-resistant lipid rafts. Stimulation of these cells with CNTF resulted in a rapid translocation of gp130 and LIFR into detergent-resistant lipid rafts while stimulation with LIF did not. Raft disruption by cholesterol depletion of cell membranes blocked the CNTF-induced translocation of LIFR and gp130. While cholesterol depletion did not inhibit STAT3 phosphorylation by either cytokine stimulation, it strongly inhibited both CNTF- and LIF-mediated phosphorylation of Erk1/2 and Akt. Intriguingly, the data presented here suggest a possible mechanism whereby cytokines that signal through CNTFR could generate signals in an environment distinct from those elicited by LIF.; In the second part of this work, we have determined the effects of growth factor stimulation on LIFR expression and signal transduction in the human neuronal 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 LIFR-mediated gene induction. We also found 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 neuronal 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.