Kinetic studies of insulin degrading enzyme in cultured human lymphocytes

Abstract

After insulin binds to its receptors, insulin-receptor complexes are internalized by absorptive endocytosis, and then insulin seems to be degraded in the intracellular sites. Although the degradation of insulin has been extensively studied, the sites and enzymes of intracellular degradation have still not been appeared. In order to clarify this problem, following experiments were performed. The effects on insulin degradation of the various inhibitors and anti-IDE serum were investigated in isolated rat hepatocytes and Bri-7 human cultured lymphocytes. N-ethylmaleimide and bacitracin, inhibitors which inhibit the activity of insulin-degrading enzyme (IDE), and anti-IDE serum were decreased insulin degradation in Bri-7 cells which does not contain lysosomal pathway. IDE mainly exists in the cytosol fraction, but also on the surface of various cell types. The kinetic changes of insulin receptors and cell surface IDE was determined in Bri-7 cells after preincubation with or without insulin. The concentration of cell surface IDE was determined by immunoenzymatic labeling method using anti-IDE serum. Bri-7 cells were preincubated with 10(-6) M insulin for 30 min to 18 h. Fifty percent of the receptors were lost at 6 h after the preincubation, and level of the receptors achieved a steady state at 18 h. Although the loss of surface IDE was slightly slower than that of receptors, the curves were essentially parallel to each other. Thus, the loss of cell surface receptors and IDE was directly related to the preincubation time. Furthermore, the recovery of decreased surface receptors and IDE was needed for 36 and 72 h, respectively. The alpha-subunit of insulin receptor (135 K) and IDE (110 K) were assessed by cross-linking of 125I-insulin to the plasma membrane and the cytosol of Bri-7 cells, respectively. Cell surface insulin receptor was decreased, whereas cytosolic IDE was increased in Bri-7 cells incubated with insulin. Thus, it is likely that both cell surface and cytosolic IDE, acting either individually or in concert, constitute a physiological mechanism by which the cellular response to insulin is terminated. These results suggest that IDE may play a major role in insulin degradation in the intact cell. Moreover, one possible mechanism of intracellular insulin degradation is that cell surface IDE may be internalized with the insulin receptor complex and may degrade insulin during the intracellular process.