title: Insulin elicits a redistribution of transferrin receptors in 3T3-L1 adipocytes through an increase in the rate constant for receptor externalization.
abstract: Incubation of 3T3-L1 adipocytes with insulin at 37 degrees C resulted in a 2-fold increase in specific binding of transferrin to cell-surface receptors, as measured by a subsequent incubation of cells at 4 degrees C with 125I-transferrin. The insulin concentration required for half-maximal effect was 10 nM, and the half-time for insulin action was 40 s. By comparison, insulin stimulated hexose transport in 3T3-L1 adipocytes with a half-maximal effect at 8 nM and a half-time of 105 s. Scatchard analysis of 125I-transferrin binding to cells at 4 degrees C showed that the insulin-induced increase in transferrin receptor binding was due to an increase in the number of surface transferrin receptors. When cells were incubated for 2 h at 37 degrees C with 125I-transferrin to achieve steady-state binding and then exposed to insulin, there was a 1.7-fold increase in surface-bound transferrin (acid-sensitive) and a corresponding decrease in intracellularly bound transferrin (acid-insensitive). Thus, insulin elicits translocation of intracellular transferrin receptors to the plasma membrane. Concomitant with the 2-fold increase in surface receptors in response to insulin, there was a 2-fold increase in the rate of 59Fe3+ uptake from 59Fe3+-loaded transferrin. The rate of externalization of the intracellular 125I-transferrin-receptor complex at 37 degrees C was determined for basal and insulin-treated cells. Insulin increased the first-order rate constant for this process 1.7-fold. The effect of insulin on the rate of externalization is sufficient to account for the increase in surface transferrin receptors.
authors.forename: L I
authors.initials: LI
authors.lastname: Tanner
authors.forename: G E
authors.initials: GE
authors.lastname: Lienhard
mesh_terms: D000273:Adipose Tissue / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D003847:Deoxyglucose / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D007328:Insulin / Q000494:pharmacology*; D007700:Kinetics; D008433:Mathematics; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000378:metabolism; D011990:Receptors, Transferrin / Q000378:metabolism*; D013997:Time Factors
chemical_list: D007328:Insulin; D009004:Monosaccharide Transport Proteins; D011990:Receptors, Transferrin; D003847:Deoxyglucose