Ectodomain Shedding-Dependent Transactivation of Epidermal Growth Factor Receptors in Response to Insulin-Like Growth Factor Type I

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Abstract
Diverse extracellular stimuli activate the ERK1/2 MAPK cascade by transactivating epidermal growth factor (EGF) receptors. Here, we have examined the role of EGF receptors in IGF-I-stimulated ERK1/2 activation in several cultured cell lines. In human embryonic kidney 293 cells, IGF-I triggered proteolysis of heparin binding (HB)-EGF, increased tyrosine autophosphorylation of EGF receptors, stimulated EGF receptor inhibitor (AG1478)-sensitive ERK1/2 phosphorylation, and promoted EGF receptor endocytosis. In a mixed culture system that employed IGF-I receptor null murine embryo fibroblasts (MEFs) (R− cells) to detect paracrine signals produced by MEFs expressing the human IGF-I receptor (R+ cells), stimulation of R+ cells provoked rapid activation of green fluorescent protein-tagged ERK2 in cocultured R− cells. The R− cell response was abolished by either the broad-spectrum matrix metalloprotease inhibitor batimastat or by AG1478, indicating that it resulted from the proteolytic generation of an EGF receptor ligand from adjacent R+ cells. These data suggest that the paracrine production of EGF receptor ligands leading to EGF receptor transactivation is a general property of IGF-I receptor signaling. In contrast, the contribution of transactivated EGF receptors to IGF-I-stimulated downstream events, such as ERK1/2 activation, varies in a cell type-dependent manner.