Dual pathways for the intracellular processing of insulin. Relationship between retroendocytosis of intact hormone and the recycling of insulin receptors.

Adipocytes process insulin through either of two pathways: a retroendocytotic pathway that culminates in the release of intact insulin, and a degradative pathway that terminates in the intracellular catabolism and release of degraded ligand. Mechanistically, these pathways were found to differ in several ways. First, temporal differences were found in the rate at which intact and degraded products were extruded. After 125I-insulin was preloaded into the cell interior, intact ligand was completely released during the first 10 min (t 1/2 = 2 min), whereas degraded insulin was released at a much slower rate over 1 h (t 1/2 greater than 8 min). Secondly, it was found that chloroquine profoundly inhibited the insulin degradative pathway, resulting in the intracellular accumulation of intact ligand and a reduction in the release of degraded products. In contrast, however, chloroquine was without effect on the retroendocytotic processing of insulin. Based on the known actions of chloroquine, it appears that retroendocytosis of insulin does not involve vesicular acidification or dissociation of the insulin-receptor complex and that insulin is most likely carried to the cell exterior in the same vesicles (either receptor-bound or free) as those mediating recycling receptors. Interestingly, accumulation of undergraded insulin within chloroquine-treated cells did not result in the release of additional intact ligand, suggesting that once insulin enters the degradative compartment it is committed to catabolism and cannot exit the cell through the retroendocytotic pathway. A third difference was revealed by the finding that extracellular unlabeled insulin (100 ng/ml) markedly accelerated the rate at which preloaded 125I-insulin was released from adipocytes (t 1/2 of 3 min versus 7 min in controls cells). Analysis of the composition of the released products revealed that extracellular insulin rapidly augmented (over 10 min) in a dose-dependent manner (5-200 ng/ml) the amount of insulin released intact (from 25 to 38% of preloaded counts; insulin ED50 = 10 ng/ml). Although extracellular insulin had no effect on the early extrusion of degraded insulin, the release of catabolized products was reduced at later times. The interpretation of these results is that the rate or amount of incoming insulin-receptor complexes can effect a sorting process (prior to bifurcation) such that a proportion of insulin is shunted from the slower degradative pathway to the more rapid retroendocytotic pathway.(ABSTRACT TRUNCATED AT 400 WORDS)