Decreased binding of epidermal growth factor to BALB/c 3T3 mutant cells defective in glycoprotein synthesis

Abstract

THE surface of animal cells contains a variety of complex macromolecules, some of which modulate cell growth, adhesion and motility. A number of peptide hormones and growth factors influence cellular proliferation and macromolecular synthesis with binding to surface proteins of responsive cells as the first phase of their action1–4. The structure and identity of these receptor molecules is a topic of widespread interest. Epidermal growth factor (EGF) is a low molecular weight peptide (MW 6,000) capable of stimulating DNA synthesis and proliferation in epidermal cells5, human fibroblasts6, and mouse embryo fibroblasts (3T3)7. It has also been reported to stimulate the synthesis of a major cell surface glycoprotein (CSP)8 and hyaluronic acid9 in 3T3 cells and human fibroblasts maintained in low serum. These effects occur at extremely low concentrations of EGF, in the ng ml−1 range, and the first step in this process is the specific binding to surface receptors, followed by rapid internalisation and degradation10. Pouyssegur and Pastan11 have isolated a nontransformed mutant (AD6) of BALB/c 3T3 cells that has a dramatic decrease in cell surface carbohydrates due to a specific, but partial block in the acetylation of N-acetylglucosamine-6-phosphate. This early defect in the biosynthesis of amino sugars leads to incomplete glycosylation of glycoproteins and to decreased exposure of glycoproteins on the cell surface12. Culturing the cells in the presence of N-acetylglucosamine (GlcNAc) bypasses the enzymatic block and results in a restoration of altered surface components13. We report here that the mutant AD6 cells show a dramatic decrease in EGF binding, whereas their ability to bind insulin is normal. Our data are consistent with the notion that the receptor for EGF is a glycoprotein.