Insulin binding and processing by H4IIEC3 hepatoma cells: ultrastructural and biochemical evidence for a unique route of internalization and processing.

Biochemical and ultrastructural studies of insulin binding and cellular processing by cultured H4IIEC3 hepatoma cells were performed. Insulin binding and intracellular accumulation were rapid and after 30 min at 37 degrees C, 65% of the total cell-associated 125I-insulin was in an acid-stable compartment. Chloroquine had no significant effect on the amount of total cell-associated insulin or the percentage of insulin in the acid-stable compartment or cell-associated insulin degradation under those conditions, but after 60-min incubations, it slightly decreased the rate of dissociation of internalized hormone. Ultrastructural analysis revealed that monomeric ferritin-insulin (Fm-I) initially bound to single or paired receptors on microvilli. Within 5 min occupied insulin receptors microaggregated and migrated to the intervillous cell surface. During the next 5-10 min occupied receptors aggregated into large clusters on the plasma membrane. Large amounts of insulin were internalized by macropinocytosis and the majority of internalized Fm-I was found in phagosomes. Less than 10% of the membrane-bound insulin was associated with pinocytotic invaginations or coated pits and less than 5% of the total cell-associated insulin was found in lysosomes. Chloroquine had no detectable effect on the amount of Fm-I or its distribution among the intracellular organelles. These studies demonstrated that, compared to previous studies with rat adipocytes or 3T3-L1 adipocytes, insulin interalization and intracellular processing in this hepatoma cell were unique. These differences provide further evidence that insulin binding and processing may be controlled by cell-specific mechanisms and that substantial heterogeneity exists in pathways previously presumed to be similar for all cell types.

Heterogeneous effects of inhibitors of receptor processing on insulin binding and intracellular accumulation in various cell types.

This study utilized uniform incubation conditions and demonstrated heterogeneity in the effects of various agents on 125-insulin binding and intracellular accumulation in five cell types. The cells used in this study included rat adipocytes, H4IIEC3 cultured hepatoma cells, normal human fibroblasts, and 3T3-L1 preadipocytes and adipocytes. Bacitracin increased insulin binding to rat adipocytes but inhibited binding to all other cells. Chloroquine increased total cell-associated insulin in all cells except H4IIEC3 hepatoma cells. Methylamine and dansylcadaverine increased or decreased binding depending on cell type. Similar heterogeneity was found in the intracellular accumulation of insulin. Under control conditions, intracellular insulin at steady state varied from 16 to 52% of the total cell-associated insulin. Bacitracin decreased intracellular accumulation of insulin in 3T3-L1 adipocytes, preadipocytes, and human fibroblasts but increased accumulation in rat adipocytes. Chloroquine increased insulin accumulation in all cell types except H411EC3 hepatoma cells. Both methylamine and dansylcadaverine increased intracellular insulin in rat adipocytes and decreased accumulation in human fibroblasts. These results provide additional evidence of heterogeneity in insulin and/or insulin receptor processing among different cell types.