Lysosomal Degradation of Receptor-bound 125I-labeled Insulin by Rat Adipocytes: Its Characterization and Dissociation from the Short-term Biologic Effects of Insulin

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Abstract
In this study we used chloroquine to characterize the internalization and lysosomal degradation of receptorbound 125I-insulin by rat adipocytes and to determine the role of lysosomal processing of insulin in the shortterm biologic effects of the hormone. Chloroquine inhibited the degradation of 125I-insulin bound to adipocytes by both association and dissociation experiments. In the former experiments, chloroquine caused a time- and concentration-dependent increase in specifically bound insulin owing to an increase in intact insulin and a decrease in degradation products, as determined by trichloroacetic acid precipitability and gel chromatographic analysis of material extracted from the cells. In the dissociation experiments, 50 μM chloroquine decreased the rate of degradation by two thirds, as reflected in the release of degradation products from the cells. Chloroquine had no effect on the amount of 125I-insulin specifically bound to or degraded by isolated plasma membranes, on the degradation of 125I-insulin by proteases in the incubation medium, or on the endocytotic uptake of receptorbound insulin. Quantitative electron microscopy, using monomeric ferritin-insulin, showed 50 μM chloroquine doubled the number of lysosomal structures containing ferritin. These findings are consistent with an inhibition by chloroquine of lysosomal degradation of internalized receptor-bound insulin. Chloroquine, at these same concentrations, had no effect on the ability of insulin to stimulate glucose transport and oxidation or to inhibit epinephrine-stimulated lipolysis. In these studies, we show that lysosomal degradation of internalized receptor-bound insulin is not necessary for insulin to cause short-term biologic effects in the adipocyte.

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