The effect of inhibitors of insulin processing on generation of insulin intermediate products from human fibroblast as detected by high performance liquid chromatography (HPLC)

Insulin degradation in vivo: a high-performance liquid chromatographic analysis

The metabolism of insulin in vivo was investigated using an isocratic reversed-phase high-performance liquid chromatographic (RP-HPLC) method. After intravenous injection of A14-[125I]insulin into normals, eight labelled insulin derivatives were found in plasma (peaks 1-8). Two of them (peaks 1 and 7) showed an elution pattern identical with those of reference [125I]monoiodotyrosine and intact A14-[125I]insulin, respectively. Of the other six peaks, five (2-6) eluted before and one (peak 8) after insulin. This pattern was highly reproducible in terms of capacity factors and peak heights. Radioactivity separated by RP-HPLC was further characterized for its trichloroacetic acid precipitability and immunoprecipitability. Fractions corresponding to peaks 4-6 and 8, which showed an immunoprecipitability higher than 50%, were pooled in order to obtain sufficient radioactivity and were found to be insulin separated by Sephadex G-50 chromatography, containing in its structure, after sulphitolysis, intact A-chain and to be partially rebindable to monocyte insulin receptors. These data demonstrate that in blood, products of insulin metabolism circulate which retain a part of the immunological and biological properties of the hormone. These products are clearly separated from one another and from intact insulin by RP-HPLC, suggesting that the appropriate use of this technique may allow a further and more accurate qualitative and quantitative characterization of in vivo insulin metabolism in physiological and pathological conditions.

Endocytotic uptake, processing, and retroendocytosis of human biosynthetic proinsulin by rat fibroblasts transfected with the human insulin receptor gene

The cellular itinerary and processing of insulin and proinsulin were studied to elucidate possible mechanisms for the observed in vivo differences in the biologic half-lives of these two hormones. A rat fibroblast cell line transfected with a normal human insulin receptor gene was used. Due to gene amplification, the cells express large numbers of receptors and are ideal for studying a ligand, such as proinsulin, that has a low affinity for the insulin receptor. Competitive binding at 4 degrees C showed that the concentration of unlabeled insulin and proinsulin that is needed to displace 50% of tracer insulin or proinsulin was 0.85-0.95 nM and 140-150 nM, respectively. Binding to surface receptors and internalization occur at rates that are four to five times faster in cells incubated with insulin compared with proinsulin. Chloroquine led to an increase in cell-associated radioactivity of approximately 1.4-fold in cells incubated with insulin or proinsulin, but inhibited the appearance of degraded insulin by 54% and degraded proinsulin by only 10%. To study the fate of internalized ligand, cells were incubated with insulin and proinsulin until steady state binding occurred. Surface bound ligand was removed by an acid wash and the remaining cell-associated radioactivity represented internalized ligand. Cells were then reincubated in 37 degrees C buffer and the cell-associated radioactivity and radioactivity released into the medium were analyzed by TCA precipitation, Sephadex G-50, and HPLC. The results demonstrated that proinsulin more readily bypasses the intracellular degradative machinery and is therefore released intact from the cell via the retroendocytotic pathway. These results may help to explain the prolonged metabolic clearance rate and biologic responsiveness of proinsulin in vivo.

Insulin-degrading enzyme is capable of degrading receptor-bound insulin

In the investigation of the intracellular sites of insulin degradation, it might be important whether receptor-bound insulin could be a substrate for insulin-degrading enzyme (IDE). Insulin receptor and IDE were purified from rat liver using a wheat germ agglutinin column and monoclonal anti-IDE antibody affinity column, respectively. [125I]insulin-receptor complex was incubated with various amounts of IDE at 0 degree C in the presence of disuccinimidyl suberate and analyzed by reduced 7.5% SDS-PAGE and autoradiography. With increasing amounts of IDE, the radioactivity of 135 kd band (insulin receptor alpha-subunit) decreased, whereas that of 110 kd band (IDE) appeared then gradually increased, suggesting that IDE could bind to receptor-bound insulin. During incubation of insulin-receptor complex with IDE at 37 degrees C, about half of the [125I]insulin was dissociated from the complex. However, the time course of [125I]insulin degradation in this incubation was essentially identical to that of free [125I]insulin degradation. Cross-linked, non-dissociable receptor-bound [125I]insulin was also degraded by IDE. Rebinding studies to IM-9 cells showed that the receptor binding activity of dissociated [125I]insulin from insulin-receptor complex incubated with IDE was significantly (p less than 0.001) decreased as compared with that without the enzyme. These results, therefore, show that IDE could recognize and degrade receptor-bound insulin, and suggest that IDE may be involved in insulin metabolism during receptor-mediated endocytosis through the degradation of receptor-bound insulin in early neutral vesicles before their internal pH is acidified.

Intermediate peptides of insulin degradation in liver and cultured hepatocytes of rats

1. Bestatin, a microbial aminopeptidase inhibitor, induced accumulation of low-molecular weight intermediate peptides of insulin degradation in liver of rats in vivo and in primary cultured rat hepatocytes. However, bestatin did not affect the association and internalization of the hormone into hepatic cells. 2. Results of the HPLC analyses showed that the intermediate peptides of insulin degradation are small ones and specifically accumulate only in the presence of bestatin. 3. The above results, together with those employing other protease inhibitors, show that cytosolic bestatin-sensitive protease(s), trypsin-like protease(s) and thiol protease(s) play an important role in the intracellular degradation process of insulin.

Cycle of VIP in the human transformed colonic epithelial cells (HT-29) in culture

The kinetics of VIP processing in different compartments (medium, plasma membrane and intracellular) by HT-29 cells was studied using direct biochemical methods and a homogeneous monoiodinated VIP. The compartmental radioactivity was characterized by HPLC fractionation and specific receptor binding. VIP once bound to the cell surface remains intact and is rapidly and maximally internalized (less than 10 min) at 37 degrees C. Then two different processes occur: (1) release of degradation products 125I and monoiodinated tyrosine in the medium; (2) VIP remains intact in the cells representing 67.2 +/- 4.7% of total radioactivity up to 90 min. The overall processing of VIP is time- and temperature-dependent and maximal internalization of VIP with minimal medium release is observed at 20 degrees C. Our results demonstrate a receptor mediated internalization of VIP and that at least two intracellular pathways may exist in the cycle of VIP. One is associated with a complete degradation of VIP detected in the extracellular medium and is optimal at 37 degrees C. The other results in the presence of intact intracellular VIP and is optimal at 20 degrees C.