KINETIC-ANALYSIS OF EPIDERMAL GROWTH-FACTOR ENDOCYTOSIS IN RAT HEPATOCYTES - EFFECTS OF DIABETES

In the present study we followed the different steps of epidermal grow th factor receptor (EGF-R) endocytosis in freshly isolated rat hepatoc ytes, Hepatocytes exhibit two classes of surface EGF receptors consist ing of approximately 5000 high-affinity sites (Kd = 15 pM) and 166000 low-affinity sites (Kd = 670 pM). Binding of labeled EGF to hepatocyte s permeabilized by digitonin shows that 75% of the total EGF-R are loc alized at the cell surface. At 37 degrees C, hepatocytes continuously internalized and degraded EGF in spite of a down-regulation of cell su rface receptors, The internalization rate constants measured as a func tion of a range I-125-EGF concentrations (0.01 - 5 nM) involving vario us degrees of EGF-R occupancy show superimposable curves. This indicat es that the specific internalization rate of EGF-R complex is independ ent of receptor occupancy, Streptozotocin-induced diabetes reduces the number of low-affinity EGF-R to 50000 and produces a complete loss of high-affinity sites. The dynamics of I-125-EGF endocytosis show that diabetic hepatocytes fail to down-regulate the surface EGF-R efficient ly although the constant rate of internalization is not modified. Decr eased down-regulation of EGF-R together with enhanced EGF endocytosis suggest a greater efficiency in EGF-R recycling in diabetic rat hepato cytes.