Expression of the adipocyte phenotype by differentiating 3T3-LI preadipocytes occurs upon exposure of the cells to insulin. Differentiation-linked changes in 125I-labeled insulin binding to 3T3-LI cells were monitored and compared with those in nondifferentiating 3T3-C2 controls treated similarly. Without chronic insulin treatment, 3T3-LI cells failed to express the adipocyte phenotype but maintained a level of 25 000-35 000 insulin-binding sites per cell. Treatment of 3T3-LI cells with insulin resulted in an initial suppression of insulin binding followed by a 12-fold increase that paralleled the appearance of differentiated cells. A maximum of 170,000 insulin-binding sites per cell was attained for a population in which >75% of the cells had differentiated. The increase of insulin receptor level appears to be differentiation-dependent and is not a general response of cells to the culture conditions. 3T3-C2 cells maintained in the presence of insulin for 30 days exhibited the undifferentiated phenotype and suppressed levels of insulin binding (35,000 sites per cell). The binding capacity of 3T3-LI cells for epidermal growth factor remained unchanged between 25,000 and 40,000 sites per cell and was independent of the state of differentiation. Thus, induction by insulin of differentiation-linked expression in 3T3-LI cells results in receptor-specific changes. Insulin receptors increase in number but epidermal growth factor receptors remain constant.
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