Insulin receptor recycling in vascular endothelial cells. Regulation by insulin and phorbol ester

Endothelial cell insulin receptors mediate the transcytosis of insulin from luminal to abluminal cell surface. We have investigated the kinetics of insulin receptor translocation by immunoprecipitation of radiolabeled receptors at various times before and after trypsin treatment of intact endothelial cells. Insulin receptors were constitutively internalized with t1/2 = 18 +/- 2 min and were recycled to the cell surface. Insulin stimulated receptor internalization and externalization rates 2.6- and 2.4-fold, respectively. Changes in cell-surface binding of 125I-insulin were consistent with the receptor translocation rates observed in surface-labeling experiments. Phorbol myristate acetate (PMA) treatment increased the rate of insulin-stimulated receptor externalization 1.7-fold. PMA treatment increased the constitutive externalization rate 3.5-fold without affecting the constitutive internalization rate, suggesting that recycling might occur via a mobilization of receptors from intracellular sites in a manner independent of internalization rate. Analysis of the intracellular distribution of receptors by 125I-insulin binding and immunogold electron microscopy revealed that less than one-third of the total insulin receptor pool resided on the cell surface. In summary, endothelial cell insulin receptors are constitutively recycled, and internalization and externalization rates are increased by receptor occupancy and PMA treatment.