CROSS-TALK AMONG TYROSINE KINASE RECEPTORS IN PC12 CELLS - DESENSITIZATION OF MITOGENIC EPIDERMAL GROWTH-FACTOR RECEPTORS BY THE NEUROTROPHIC FACTORS, NERVE GROWTH-FACTOR AND BASIC FIBROBLAST GROWTH-FACTOR

We have studied the effects of nerve growth factor (NGF) and basic fib roblast growth factor (bFGF) on epidermal growth factor (EGF) binding to PC12 cells. We show that NGF and bFGF rapidly induce a reduction in I-125-EGF binding to PC12 cells in a dose-dependent manner. This decr ease amounts to 50% for NGF and 35% for bFGF. Both factors appear to a ct through a protein kinase C(PKC)-independent pathway, because their effect persists in PKC-downregulated PC12 cells. Scatchard analysis in dicates that NGF and bFGF decrease the number of high affinity EGF bin ding sites. In addition to their effect on EGF binding, NGF and bFGF a ctivate in intact PC12 cells one or several serine/threonine kinases l eading to EGF receptor threonine phosphorylation. Using an in vitro ph osphorylation system, we show that NGF- or bFGF-activated extracellula r regulated kinase 1 (ERK1) is able to phosphorylate a kinase-deficien t EGF receptor. Phosphoamino acid analysis indicates that this phospho rylation occurs mainly on threonine residues. Furthermore, two compara ble phosphopeptides are observed in the EGF receptor, phosphorylated e ither in vivo after NGF treatment or in a cell-free system by NGF-acti vated ERK1. Finally, a good correlation was found between the time cou rses of ERK1 activation and I-125-EGF binding inhibition after NGF or bFGF treatment. In conclusion, in PC12 cells the NGF- and bFGF-stimula ted ERK1 appears to be involved in the induction of the threonine phos phorylation of the EGF receptor and the decrease in the number of high affinity EGF binding sites.