INSULIN-RECEPTOR CHARACTERIZATION AND FUNCTION IN BOVINE AORTA ENDOTHELIAL-CELLS - INSULIN DEGRADATION BY A PLASMA-MEMBRANE, PROTEASE-RESISTANT INSULIN-RECEPTOR

The functional significance of the insulin receptor on bovine aorta en dothelial (BAE) cells is not well defined. The insulin receptor expres sed on BAE cells does not mediate insulin hormonal effects and does no t mediate the transcytosis of insulin from the apical to the basolater al domain of the cell monolayer. To assess the role of the insulin rec eptor on BAE cells, the physical characteristics of the BAE cell recep tor were investigated, and the time-dependent interaction of insulin a nd insulin degradation products with BAE cell monolayers was quantitat ed. The BAE cell insulin receptor was found to be highly resistant to the proteolytic action of trypsin, pronase, and proteinase K at either 4-degrees-C or 37-degrees-C. This resistance may permit the receptor to maintain insulin binding capabilities in spite of the high concentr ations of proteases which are normally present in blood. Scatchard ana lysis of cell-surface and total cellular insulin receptor demonstrated dissociation constants similar to values obtained with other cells an d tissues. However, whereas other cells and tissues contain an intrace llular pool of receptor that ranges from 20-40% of the total cellular receptor content, no intracellular population of insulin receptors was detected in BAE cells. Upon incubation of intact BAE cell monolayers with insulin, no endocytosis of cell-surface insulin receptor could be demonstrated. However, insulin degradation by the BAE cells was readi ly quantitated, at a rate of 16.3 fmol/10(6) cells/h at an insulin con centration of 2 nM. This rate of degradation was not inhibited by chlo roquine, which inhibits insulin degradation in fibroblasts, hepatocyte s, and adipocytes, nor by phenylarsine oxide, which inhibits endocytos is. Bacitracin inhibited insulin binding to the cell monolayers and in hibited insulin degradation with identical IC50 values (80 muM). These data suggest that in BAE cells, insulin degradation occurs in the abs ence of receptor-mediated endocytosis and is mediated by binding of in sulin to its receptor. Therefore, it is concluded that the functional role of the insulin receptor expressed in BAE cells is to bind blood-b orne insulin at the plasma membrane of the cell and thereby facilitate the degradation of insulin at the BAE cell plasma membrane. (C) 1993 Wiley-Liss, Inc.