Insulin receptor processing in hepatoma cells: some steps involved in both basal receptor turnover and...

Insulin receptor processing was studied in cultured Zajdela hepatoma cells (ZHC). The basal receptor turnover was estimated in the presence of tunicamycin (TM), which inhibited the insertion into plasma membranes of newly synthesized underglycosylated receptors. After a lag phase of 4 h, the surface receptor number decreased, with a t 1/2 of 7 h, for up to 24 h. This process was markedly slowed down when cells were either briefly preincubated with dansylcadaverine, chloroquine, or cycloheximide or treated for 24 h with TM. The effects of these agents on the insulin-induced receptor down-regulation process was then tested. When cells were treated with chloroquine or dansylcadaverine or placed in calcium-free medium, this process was impeded; similarly, it was inhibited by actinomycin D or cycloheximide, but was not affected by TM after a brief incubation. However, after a 24-h treatment with TM, it disappeared, although receptors remained functional when testing insulin's action upon glycogen synthesis. These results indicate that receptor degradation, both basal and activated by insulin leading to the down-regulated state, was altered under similar experimental conditions. The effects of dansylcadaverine, chloroquine, or the absence of calcium reveal that endocytotic pathways were involved in these processes. The results obtained when mRNA, protein, or glycoprotein synthesis was inhibited indicate that cellular glycoprotein(s) and short-lived protein(s) were necessary for the receptor processing in both cases. These data led us to postulate that basal and insulin-activated receptor degradation may occur in the same way within the cell.