Transformation of NIH3T3 fibroblasts by an expression vector for the human epidermal growth factor precursor

Epidermal growth factor (EGF) and transforming growth factor alpha (TGF alpha) bind to a common cell surface receptor that mediates their diverse biological activities. NIH3T3 fibroblasts transfected with either full-length EGF precursor (preproEGF) or proTGF alpha cDNA displayed distinct patterns of growth in culture. PreproEGF induced focal transformation, and transfectants grew in a chemically defined medium (CDM) at low cell density in the absence of added EGF. In contrast, TGF alpha failed to cause focal transformation, and transfectants grew in CDM in the absence of added growth factors only when seeded at high cell density. The 53 amino acid EGF portion of the preproEGF translation product was essential for its effects. These results indicate that constitutive expression of preproEGF is sufficient to establish autocrine growth of NIH3T3 expressing low levels of EGF receptors. At high cell density, where paracrine as well as autocrine effects of these growth factors would be evident, TGF alpha transfectants displayed at least as high or higher levels of EGF receptor (EGFR) tyrosine phosphorylation than preproEGF transfectants. Since quantitative levels of ligand expression did not account for differences in their transforming properties, preproEGF must be more efficient than proTGF alpha in binding and/or activating EGF receptors in an autocrine manner.