Induction of an intracellular mitogenic messenger by epidermal growth factor

This paper explores the pathway from nuclear quiescence to mitogenesis. It describes an in vitro assay for an activator of DNA replication induced by epidermal growth factor (EGF) in responsive cells. Cytoplasmic extracts from EGF-treated 3T3 were found to contain substances that can stimulate DNA synthesis in isolated nuclei from spleen cells of adult frogs. Extracts from untreated resting 3T3 cells lack this activity, and EGF itself is incapable of stimulating, DNA synthesis in these cell-free systems. The extract-induced stimulation of 3H-dTTP incorporation into nuclear DNA is ATP-dependent and requires the presence of the 4 deoxyribonucleoside triphosphates, suggesting the occurrence of replication rather than repair synthesis. This cell-free assay has been used to obtain some initial insights into the mechanism of induction and biochemical characterization of the intermediate in EGF action. Half-maximal induction of the active intracellular substance is achieved at about 0.08nM EGF, a concentration that correlates well with the concentration required for half-maximal mitogenesis. Studies on the biochemical characteristics of this active substance strongly suggest that the activity is associated with a protein. The activity is nondialyzable and sensitive to trypsin and heat. Sucrose-gradient centrifugation of the extract revealed 3 peaks of activity with molecular weights of 46,000, 110,000, and 270,000 daltons (sedimentation coefficients: 3 7S, 6.6S, and 12S, respectively). These results indicate that receptor-EGF interaction at the cell surface leads to the intracellular generation of proteins that are capable of stimulating quiescent nuclei into activity.