Regulation of insulin receptors in erythroid cells

Previous studies using inhibitors have suggested that protein synthesis is necessary for "down-regulation" of insulin receptors. We have tested this hypothesis without the use of inhibitors by studying the ability of cells of the erythroid series to down-regulate their insulin receptors in vitro. The cells tested include mature erythrocytes and reticulocytes from rabbits and Friend erythroleukemia cells (a model for the basophilic erythroblast, a primitive nucleated erythrocyte). All cells were maintained at 37 degrees C for 18 hr +/- insulin (10(-8)M). Cultures were then incubated with phosphate buffered salines (pH 7.0) at 30 degrees C for 40 min to remove bound insulin. Receptors were quantitated by computerized analysis of Scatchard plots of subsequent insulin binding studies. Cells fully capable of both mRNA synthesis and protein synthesis, such as the undifferentiated and differentiated Friend erythroleukemia cell, had reduction of insulin receptors at 60% and 43%, respectively. Reticulocytes, which were capable of protein synthesis but not mRNA synthesis, had decreases of 25%-30% in 8 separate experiments. Mature erythrocytes, capable of neither RNA nor protein synthesis had no significant changes in receptor concentrations. Since mature erythrocytes do not "down-regulate" their insulin receptor concentration, studies of these receptors in erythrocytes of patients should be interpreted with caution.