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/IR_A1018), and receptors which undergo autophosphorylation but lack a sequence required for internalization (CHO/IR_Δ960)). The rate of [¹²⁵I]insulin internalization in CHO/IR cells at 37°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/IR_A1018 and CHO/IR_Δ960 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_Δ960 and CHO/IR_A1018 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.