Ligand-induced protein tyrosine kinase activity in living cells coexpressing intact EGF receptors and receptors with an extensive cytosolic deletion

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 cells, a fraction of the receptors are also delivered to the apical surface. Similar to most basolateral membrane proteins, the EGF receptor has an additional signal(s) that selectively targets molecules lacking a dominant basolateral signal to the apical surface. Although the physiological relevance of signal hierarchy is not known, alternative targeting may occur in different epithelial cell types or during development. The goal of this study, therefore, was to determine the effect of membrane domain location on EGF receptor function, focusing on EGF-induced MAP kinase signaling and DNA synthesis. Whereas ligand responsiveness was restricted to the basolateral domain in Madin-Darby canine kidney (MDCK) cells expressing a normal complement of receptors, apical ligand was effective if apical receptor density was increased by overexpression of an exogenous wild-type human gene. Unexpectedly, cells expressing apically localized, cytoplasmically truncated receptors, which behave as dominant negative mutations in other cell types, were also 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 surface. These results indicate that cell context is a critical determinant of receptor mutant protein phenotype.

Identification of an intracellular domain of the EGF receptor required for high-affinity binding of EGF

Although all EGF receptors in EGF receptor-expressing cells are molecularly identical, they can be subdivided in two different classes that have either a high or a low affinity for EGF. Specifically the high-affinity class is associated with filamentous actin. To determine whether the interaction of the EGF receptor with actin induces its high-affinity state, we studied EGF-binding properties of an EGF receptor mutant that lacks the actin-binding site. Interestingly, we found that cells expressing this mutant receptor still display both high- and low-affinity classes of EGF receptors, indicating that the actin-binding domain does not determine the high-affinity binding state. By further mutational analysis we identified a receptor domain, within the tyrosine kinase domain, that regulates the affinity for EGF.

Ligand-independent dimerization of oncogenic v-erbB products involves covalent interactions

Mutant v-erbB products of avian c-erbB1 have previously been used to correlate structural domains of the receptor encoded by this proto-oncogene with tissue-specific transformation potential. In these studies, deletion of the ligand-binding domain of the receptor has been shown to be required for transformation of erythroblasts, fibroblasts, and endothelial cells. It has, therefore, been postulated that deletion of this domain results in an allosteric change in the receptor analogous to the ligand-bound state of the epidermal growth factor receptor; i.e., it induces a receptor conformation that is constitutively active with respect to mitogenic signaling. While oncogenic v-erbB products have been shown to be expressed on the cell surface of both fibroblasts and erythroblasts, no comprehensive analysis of the oligomeric potential of these products has been conducted. Since the first event known to follow epidermal growth factor binding to its receptor is oligomerization, and receptor dimerization has been correlated with mitogenic signaling, we have carefully analyzed the ability of several v-erbB products to oligomerize in the three target cell types transformed by these oncogenes. In this report, we demonstrate the v-erbB products can efficiently homodimerize in all three target tissues, that this dimerization is ligand independent and occurs at the cell surface, and that there is no apparent correlation between v-erbB dimerization and transformation of avian fibroblasts. Furthermore, both oncogenic and nononcogenic v-erbB products can heterodimerize with the native c-erbB1 product in chicken embryo fibroblasts, suggesting that heterodimerization between v-erB and native c-erbB1 is not sufficient to result in c-erbB1-mediated sarcomagenesis.

Cleavage of the epidermal growth factor receptor by a membrane-bound leupeptin-sensitive protease active in nonionic detergent lysates of senescent but not young human diploid fibroblasts

Numerous studies suggest that epidermal growth factor (EGF) signaling is impaired in nonproliferating senescent human diploid fibroblasts downstream of receptor binding. One possible explanation for these results is that senescent cells possess unique enzymatic activities capable of regulating functional levels of the EGF receptor. To test that hypothesis, nonionic detergent lysates of young and senescent cells were compared for proteolytic activity directed towards the EGF receptor, and a protease that cleaves the 170 kDa EGF receptor was identified in lysates from senescent but not young cells. Although studies presented here were carried out with WI-38 cells, our data indicate that other senescent fibroblasts possess a similar activity. The degradation product immunoprecipitated by a monoclonal antibody specific for an EGF receptor exocytosolic epitope had an approximate molecular weight of 100,000. This product was also detected following cell surface labeling with 125I, and by cross-linking 125I-EGF to intact cells with disuccinimidyl suberate. The proteolytic activity in senescent cell lysates was specifically inhibited by leupeptin and did not require divalent cations; it was also inactivated by aprotic solvents such as dimethylsulfoxide (DMSO) or ethylene carbonate. Interestingly, this protease was not active during ligand-induced intracellular processing of the EGF receptor, suggesting that it does not normally function in endocytic or lysosomal compartments. The susceptibility of the protease to inactivation by cell surface trypsinization is consistent with a plasma membrane localization. Since EGF receptor cleavage is not observed unless senescent cells are solubilized with nonionic detergents, it seems likely that the protease is confined to specialized regions of the plasma membrane. Whether or not the EGF receptor is a physiologic target for this protease is unclear. Its expression at the cell surface is nevertheless significant, since it suggests there are mechanisms for regulating membrane-bound proteins, or biologically active peptides in the extracellular space, in senescent cells that are either absent or inactive in young cells.