Regulation of EGF signaling by cell polarity in MDCK kidney epithelial cells

Although the presence of a dominant basolateral sorting signal ensures that the majority of newly synthesized epidermal growth factor (EGF) receptors are delivered directly to the basolateral surface in polarized epithelial c ells, a fraction of the receptors are also delivered to the epical surface. Similar to most basolateral membrane proteins, the EGF receptor has an add itional signal(s) that selectively targets molecules lacking a dominant bas olateral signal to the apical surface. Although the physiological relevance of signal hierarchy is not known, alternative targeting may occur in diffe rent epithelial cell types or during development. The goal of this study, t herefore, was to determine the effect of membrane domain location on EGF re ceptor function, focusing on EGF-induced MAP kinase signaling and DNA synth esis. Whereas ligand responsiveness was restricted to the basolateral domai n in Madin-Darby canine kidney (MDCK) cells expressing a normal complement of receptors, apical ligand was effective if apical receptor density was in creased by overexpression of an exogenous wild-type human gene. Unexpectedl y, cells expressing apically localized, cytoplasmically truncated receptors , which behave as dominant negative mutations in other cell types, were als o responsive to apical EGF. The cytoplasmically truncated molecules appear to have at least two effects: first, to increase the local concentration of ligand at the apical cell surface; and second, to facilitate activation of the relatively few native EGF receptors normally located at the apical sur face. These results indicate that cell context is a critical determinant of receptor mutant protein phenotype. (C) 1999 Wiley-Liss, Inc.