Insulin receptors internalize by a rapid, saturable pathway requiring receptor autophosphorylation and an intact juxtamembrane region.
The effect of receptor occupancy on insulin receptor endocytosis was examined in CHO cells expressing normal human insulin receptors (CHO/IR), autophosphorylation- and internalization-deficient receptors (CHO/IRA1018), and receptors which undergo autophosphorylation but lack a sequence required for internalization (CHO/IR delta 960). The rate of [125I]insulin internalization in CHO/IR cells at 37 degrees C was rapid at physiological concentrations, but decreased markedly in the presence of increasing unlabeled insulin (ED50 = 1-3 nM insulin, or 75,000 occupied receptors/cell). In contrast, [125I]insulin internalization by CHO/IRA1018 and CHO/IR delta 960 cells was slow and was not inhibited by unlabeled insulin. At saturating insulin concentrations, the rate of internalization by wild-type and mutant receptors was similar. Moreover, depletion of intracellular potassium, which has been shown to disrupt coated pit formation, inhibited the rapid internalization of [125I]insulin at physiological insulin concentrations by CHO/IR cells, but had little or no effect on [125I]insulin uptake by CHO/IR delta 960 and CHO/IRA1018 cells or wild-type cells at high insulin concentrations. These data suggest that the insulin-stimulated entry of the insulin receptor into a rapid, coated pit-mediated internalization pathway is saturable and requires receptor autophosphorylation and an intact juxtamembrane region. Furthermore, CHO cells also contain a constitutive nonsaturable pathway which does not require receptor autophosphorylation or an intact juxtamembrane region; this second pathway is unaffected by depletion of intracellular potassium, and therefore may be independent of coated pits. Our data suggest that the ligand-stimulated internalization of the insulin receptor may require specific saturable interactions between the receptor and components of the endocytic system.
Backer
J M
JM
Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts.
Shoelson
S E
SE
Haring
E
E
White
M F
MF
D000818:Animals; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism*; D003034:Coated Pits, Cell-Membrane / Q000378:metabolism; D006224:Cricetinae; D004705:Endocytosis; D006801:Humans; D007328:Insulin / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D007335:Insulin-Like Growth Factor II / Q000378:metabolism; D007700:Kinetics; D009154:Mutation; D010766:Phosphorylation; D011972:Receptor, Insulin / Q000235:genetics / Q000378:metabolism*
D007328:Insulin; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D011972:Receptor, Insulin