title: An analysis of the relationship between the cellular distribution and the rate of turnover for the separate classes of unoccupied, noncovalently occupied, and covalently occupied insulin receptor.
abstract: To further investigate insulin's role in regulating the turnover of insulin receptor during down-regulation in 3T3-L1 adipocytes, the relationship between the cellular distribution and turnover of unoccupied, noncovalently occupied, and covalently occupied receptor was examined. At steady-state 12% of the unoccupied receptors and 46% of covalently occupied receptors are intracellular. The apparent first-order rate constant (Kapp) for turnover of the total pool of covalently occupied receptors (0.16 h-1) is 3.8-fold higher than that for unoccupied receptors (0.042 h-1). When unlabeled insulin is added, identical values for both Kapp (0.10 h-1) and distribution (26% internal) are measured for noncovalently and covalently occupied receptors. The rate constant (Kdeg), describing the relative sensitivity of internalized receptor to degradation, is identical (0.36-0.41 h-1) for unoccupied, noncovalently occupied, and permanently occupied pools of internal receptor. Mechanisms for down-regulation postulating: (a) an occupancy-dependent alteration in the conformation of internal receptor increasing receptor sensitivity to internal proteases, (b) a preferential sorting of internal occupied receptor to degradative pathways, or (c) induction of intracellular proteases by insulin, would all reflect a substantial change in Kdeg for occupied receptor and thus are unlikely mechanisms by which insulin increases the rate of receptor turnover. The turnover of insulin receptor in 3T3-L1 adipocytes is regulated primarily by its intracellular concentration and not by the state of occupancy of internalized receptor.
authors.affiliation: Department of Biochemistry, University of Texas Southwestern Medical School, Dallas 75235.
authors.forename: D K
authors.initials: DK
authors.lastname: Reed
authors.forename: C
authors.initials: C
authors.lastname: Newton
authors.forename: M
authors.initials: M
authors.lastname: Fraga
authors.forename: K
authors.initials: K
authors.lastname: Glastad
authors.forename: A
authors.initials: A
authors.lastname: Bagheri
authors.forename: S
authors.initials: S
authors.lastname: Harris
authors.forename: B C
authors.initials: BC
authors.lastname: Reed
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D007328:Insulin / Q000494:pharmacology; D007700:Kinetics; D051379:Mice; D008954:Models, Biological; D011972:Receptor, Insulin / Q000187:drug effects / Q000378:metabolism*
chemical_list: D007328:Insulin; D011972:Receptor, Insulin