Biosynthesis of the Epidermal Growth Factor Receptor in Human Squamous Cell Carcinoma Lines: Secretion of the Truncated Receptor is not Common to Epidermal Growth Factor Receptor-Hyperproducing Cells

The biosynthesis of the EGF receptor was examined in the epidermoid carcinoma cell line A431 and five novel cell lines from human squamous cell carcinomas possessing high numbers of EGF receptors. Newly synthesized EGF receptors were visualized by labeling with [35S]methionine and immunoprecipitation with a monoclonal anti-EGF receptor antibody. In addition, the processing of the EGF receptor and its intracellular transport was analyzed by distinguishing cell surface receptors from intracellular receptors and by treating cells with inhibitors such as tunicamycin, monensin and brefeldin A. These analyses revealed that in all the tumor cell lines the EGF receptor is synthesized as a glycosylated protein of Mr 160, 000 which is converted to the receptor of Mr 170, 000 through posttranslational glycosylation. The receptors of Mr 160, 000 and 170, 000 appeared to possess high mannose type oligosaccharide chains because endoglycosidase H treatment reduced their molecular weights by approximately 30, 000. A431 was the only tumor cell line studied that secreted the truncated EGF receptor of Mr 110, 000. In A431 cells, the truncated EGF receptor was generated from a protein of Mr 60, 000 through tunicamycin-and monensin-sensitive glycosylation. A431 cells treated with monensin secreted the truncated receptor as a Mr 95, 000 form.