Kinetic analysis of epidermal growth factor endocytosis in rat hepatocytes. Effects of diabetes.

In the present study we followed the different steps of epidermal growth factor receptor (EGF-R) endocytosis in freshly isolated rat hepatocytes. Hepatocytes exhibit two classes of surface EGF receptors consisting of approximately 5,000 high-affinity sites (Kd = 15 pM) and 166,000 low-affinity sites (Kd = 670 pM). Binding of labeled EGF to hepatocytes permeabilized by digitonin shows that 75% of the total EGF-R are localized at the cell surface. At 37 degrees C, hepatocytes continuously internalized and degraded EGF in spite of a down-regulation of cell surface receptors. The internalization rate constants measured as a function of a range 125I-EGF concentrations (0.01 - 5 nM) involving various degrees of EGF-R occupancy show superimposable curves. This indicates that the specific internalization rate of EGF-R complex is independent of receptor occupancy. Streptozotocin-induced diabetes reduces the number of low-affinity EGF-R to 50,000 and produces a complete loss of high-affinity sites. The dynamics of 125I-EGF endocytosis show that diabetic hepatocytes fail to down-regulate the surface EGF-R efficiently although the constant rate of internalization is not modified. Decreased down-regulation of EGF-R together with enhanced EGF endocytosis suggest a greater efficiency in EGF-R recycling in diabetic rat hepatocytes.