Possible role of cell surface insulin degrading enzyme in cultured human lymphocytes

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Abstract
The kinetic changes of insulin receptors and cell surface insulin degrading enzyme were examined in Bri-7 cultured human lymphocytes after preincubation with or without insulin. The concentration of cell surface insulin degrading enzyme was determined by immunoenzymatic labeling method using a polyclonal antiserum to insulin degrading enzyme. In Bri-7 cells preincubated with 10−10 to 10−5mol/l insulin for 18h, the surface insulin receptors and insulin degrading enzyme decreased progressively as a function of the concentration of insulin in the preincubation medium. The surface insulin receptors and insulin degrading enzyme of cells preincubated with 10−6mol/l insulin were decreased to 25 and 35% of the control respectively. In Bri-7 cells preincubated with 10−6 mol/l insulin for 30 min to 18 h, the loss of surface insulin degrading enzyme was slightly slower than that of the receptors; however, the curves were essentially parallel to each other. Thus, the treatment of Bri-7 cells with insulin caused down-regulation of insulin receptors in a dose- and time-dependent manner. Cell surface insulin degrading enzyme also decreased simultaneously. A combination of several insulin degradation assays (trichloroacetic acid precipitation, gel filtration and receptor rebinding) demonstrated that cell surface bound insulin remained intact, and that the degradation in Bri-7 cells seemed to be a limiting proteolysis of insulin. Furthermore, by the receptor rebinding method insulin degrading activity in cells after preincubation with 10−6 mol/l insulin (19.6±4.6%) was decreased, although not significantly, as compared with cells after preincubation without insulin (24.6±4.8%). These results suggest a possible hypothesis that cell surface insulin degrading enzyme may be internalized with the insulin-receptor complex, and that it may degrade insulin during the intracellular process.