Rapid stimulation of amyloid precursor protein release by epidermal growth factor: role of protein kinase C

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Abstract
The amyloid precursor protein (APP) of Alzheimer's disease is a transmembrane protein that is cleaved by an uncharacterized enzyme known as α-secretase within its extracellular/intraluminal domain after the activation of guanine nucleotide-binding protein-coupled receptors linked to phosphoinositide hydrolysis. The secretory process results in the release of large soluble derivatives of APP (APPs), and, when elicited by muscarinic receptor activation, exhibits both protein kinase C (PKC)-dependent and tyrosine phosphorylation-dependent components [Slack, Breu, Petryniak, Srivastava and Wurtman (1995) J. Biol. Chem. 270, 8337–8344]. In this report we examine the regulation of the release of APPs by epidermal growth factor (EGF) receptors, which possess intrinsic tyrosine kinase activity, and are coupled to a variety of effectors including phosphoinositide-specific phospholipase Cγ. In A431 cells, EGF caused time-dependent and dose-dependent increases in the formation of inositol phosphates in cultures prelabelled with myo-[3H]inositol, and in the release of APPs into the culture medium; the two responses exhibited similar time courses and EC50 values for EGF. Concomitant with these effects, there were concentration-dependent (3–300 ng/ml) increases in the phosphorylation of tyrosine residues in several proteins, including the EGF receptor itself. The specific PKC antagonist GF 109203X decreased the effect of EGF by approx. 35% at a concentration that abolished the stimulation of the release of APPs by the PKC activator PMA. Tyrphostin AG 1478, an inhibitor of EGF receptor tyrosine kinase, abolished the EGF-induced release of APPs. These results demonstrate that in A431 cells, activation of the EGF receptor stimulates α-secretase activity by a mechanism that is partly dependent on PKC activity.

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