Epidermal growth factor stimulates fluid phase endocytosis in human fibroblasts through a signal generated at the cell surface

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Abstract
We have investigated the stimulation of fluid phase endocytosis by epidermal growth factor (EGF) in normal human fibroblasts using 125I‐labeled polyvinylpyrrolidone (125I‐PVP) as a fluid phase marker. We found that EGF initially induced a thereefold increase in the rate of 125I‐PVP uptake. This initial burst of fluid uptake terminated within 10 min. Thereafter, the rate of fluie uptake in EGF‐treated cells was approximately 40% higher than in control cells. To identify the cellular site of EGF action in stimulating fluid phase endocytosis, we examined the kinetics of the induction of this response as well as the kinetics of cell surface binding and internalization of 125I‐EGF. Although there was no detectable lag between binding of EGF to the cell surface and its internalization, the kinetics of the two processes were quite different. Significantly, the kinetics of induction of 125I‐PVP uptake matched the kinetics of binding of 125I‐EGF to its cell surface receptors, indicating that the signal for the increase in fluid phase endocytosis is generated at the cell surface. To determine if EGF‐stimulated fluid phase endocytosis was related to EGF‐stimulated endocytosis of its own receptor, we compared the EGF dose dependency and time course of the two processes. Although the stimulated endocytosis of the EGF receptor was not saturable with respect to the concentration of EGF used, the stimulation of fluid phase endocytosis was half maximal at an EGF concentration of 1 ng/ml and saturated at a concentration of 5 ng/ml. Also, the stimulation of fluid phase endocytosis was sevenfold greater initially after adding EGF than after a 30‐min continuous incubation with the hormone, whereas the enhanced clearance of the EGF receptor did not change during this time period. We conclude that the EGF‐stimulated increase in fluid phase endocytosis is not directly coupled to EGF‐stimulated endocytosis of its own receptor but instead to a separate signal generated at the cell surface.