Insulin-induced redistribution of the insulin-like growth factor II/mannose 6-phosphate receptor in intact rat liver

The ability of acute insulin treatment to elicit a redistribution of the li ver Insulin-like growth factor-II/ mannose 6-phosphate (IGF-II/M6P) recepto r has been studied in rats, using cell fractionation. Injection of insulin (0.4-50 mu g) led to a time- and dose-dependent decrease in IGF-II binding activity in Golgi-endosomal (GE) fractions, along with an increase in activ ity in the plasma membrane (PM) fraction; only receptor number was affected . Quantitative subfractionation of the microsomal fraction on sucrose densi ty gradients showed that ICF-II binding activity distributed similarly to g alactosyrtransferase (a Golgi marker), at slightly higher densities than in vivo internalized I-125-insulin, and at lower densities than 5' nucleotida se and alkaline phosphodiesterase (two plasma membrane markers). Insulin tr eatment led to a slight time-dependent and reversible shift of IGF-II bindi ng activity toward higher densities. Subfractionation of the GE fraction on Percoll gradients showed that IGF-II binding activity was broadly distribu ted, with about 60% at low densities coinciding with galactosyltransferase and early internalized I-125-insulin and with 40% at high densities in the region of late internalized I-125-insulin. Insulin treatment caused a time- dependent and reversible shift of the distribution of IGF-II binding activi ty toward low densities. On SDS-PACE, the size of the affinity-labeled IGF- II/M6P receptor was comparable in GE and PM fractions (about 255 kDa), bur on Western blots receptor size was slightly lower in the latter (245 kDa) t han in the former (255 kDa). insulin treatment did not affect the size, but modified the abundance of the IGF-II/M6P receptor in a manner similar to t hat of IGF-II binding. In vivo chloroquine treatment fully suppressed the c hanges in ICF-II binding activity in liver GE and PM fractions observed in insulin-treated rats. We conclude that insulin elicits a time-dependent and reversible redistribution of liver IGF-II receptors from Golgi elements an d endosomes to the plasma membrane, presumably via early endosomes. (C) 200 0 Wiley-Liss, Inc.