Hepatic binding and uptake kinetics of epidermal growth factor: studies with isolated rat hepatocytes

Hepatocytes are known to bind and internalize a variety of small molecular weight proteins by a process known as receptor-mediated endocytosis (RME). The purpose of this investigation was to characterize the binding and uptake kinetics of a small protein known to be taken up by the liver by RME, epidermal growth factor (EGF), using suspensions of freshly isolated rat hepatocytes. Rat hepatocytes accumulated 125I-EGF (90pM) in a temperature-dependent fashion. Isolated hepatocytes incubated at 370C with 125I-EGF began to release a TCA-soluble radiolabeled material into the incubation medium with a lag period of 20 min. EGF uptake by isolated hepatocytes was linear for only 60 seconds and displayed saturation kinetics. Hepatocytes incubated at 40C bound, but did not internalize, EGF. Under these conditions, EGF binding was saturable at concentrations above 8 nM. A Scatchard analysis revealed that the average number of receptors per hepatocyte was 7.7 x 104 with a dissociation constant of 2.6 nM. These data demonstrate that freshly isolated hepatocytes are capable of binding, internalizing and metabolizing EGF and thus are a good model to study RME of small molecular weight proteins. 15 references, 5 figures