Transcriptional Down-regulation of Epidermal Growth Factor (EGF) Receptors by Nerve Growth Factor (NGF) in PC12 Cells

Nerve growth factor (NGF) treatment causes a profound down-regulation of epidermal growth factor (EGF) receptors (EGFR) during the neuronal differentiation of PC12 cells. This process was characterized by a progressive decrease in EGFR level, as measured by I-125-EGF binding and Scatchard analysis, tyrosine phosphorylation, Western blotting, and bio-imaging using EGF-labeled with a near-infrared probe. Differentiation of the cells with NGF for 5-7 days produces a 95 % reduction in the amount of S-35-methionine-labeled EGFR. This down-regulation does not occur in PC12-nnr5 cells, which lack the TrkA NGF receptor but is reconstituted in these cells upon their stable transfection with TrkA. The process of NGF-induced EGFR down-regulation was inhibited by K252a, a TrkA antagonist and by anti-TrkA antibodies but not by Thx-B, a blocker of the interaction of NGF with p75(NTR) receptors. NGF-induced (heterologous) down-regulation, but not EGF-induced (homologous) down-regulation of EGFR, was blocked in Ras-deficient PC12 cells. NGF treatment for 5-7 days of PC12 cells, grown in suspension or in 3D collagen gels, induces down-regulation of EGFR independent of neurite outgrowth. The messenger RNA (mRNA) for EGFR decreased in a comparable fashion. This process was correlated temporally with a decrease in the transcription of the EGFR gene. Treatment with NGF also increased the cellular content of GCF(2), a putative inhibitory transcription factor of the EGFR gene. The temporal increase in GCF(2), like the decrease in the EGFR mRNA, was not seen in TrkA deficient PC12 cells nor in cells expressing dominant-negative Ras. The results suggest that NGF-induced down-regulation of the EGFR is under transcriptional control, is TrkA and Ras-dependent, may involve transcriptional repression by GCF(2), and independent of mechanisms that lead to NGF-induced neurite outgrowth in PC12cells. This heterologous down-regulation of EGFR would appear to be an efficient mean of desensitizing the neuron to proliferative stimuli, thereby representing a safety latch for initiating and sustaining NGF-induced neuronal differentiation.