A proposal that EGF action is mediated through enhanced internalization of EGF receptors is modified t o account for more recent evidence. EGF receptors turn over at a rapid rate, and the maintenance of a steady state of EGF receptors on the cell surface is provided through a rapid synthesis of EGF receptors, balancing their removal. This rapid turnover of unoccupied receptors may arise through their internalization and proteolysis in the lysosomes, in much the same way as receptors are internalized and degraded when exposed to EGF, which enhances internalization. This provides a dilemma for the endocytic activation concept, since slight enhancement of receptor internalization gives rise t o a strong hormone response. This problem may be solved by the observation that EGF induces a change in its receptor, exposing an otherwise unavailable site for proteolytic cleavage. This hormone-dependent modification of receptors may be the critical step in the induction of responses to EGF and other hormones that are internalized with their receptors. Both platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) are shown to downregulate EGF receptors, though transiently, placing still more stringent requirements on the specificity by which hormones might act through endocytic activation of their receptors.
Key words: direct labeling of EGF receptors, transient down-regulation of EGF receptors, platelet derived growth factor, receptor proteolysis Three properties of murine EGF [ I ] have led several laboratories to select it for studies on the biochemical mechanism of polypeptide hormone-induced stimulation of cell proliferation: 1) EGF can be readily purified by a simple procedure [2] ; 2) EGF can be radioiodinated t o high specific activity and retain its biological activity [ 3 ] ; and 3) many cultured cell lines elicit a mitogenic response t o EGF (for review see [4] ). The mitogenic activity of polypeptide hormones is usually studied in growth-arrested cell monolayers, maintained in culture with a minimal concentration of serum, which is itself rich in mitogens Addition of nanomolar concentrations of EGF t o a monolayer of responsive cells -eg, murine 3T3 cellsinduces a sequence of events that ultimately leads t o DNA replication.
EGF gives rise t o these events after binding specifically to high-affinity surface receptors