Receptor-mediated endocytosis in Xenopus oocytes. II. Evidence for two novel mechanisms of hormonal regulation.

Xenopus oocytes exhibit enhanced rates of vitellogenin (VTG) endocytosis following exposure to insulin in vitro and human chorionic gonadotropin in vivo. We investigated this phenomenon using kinetic and steady state analyses and found that the stimulation of VTG uptake was not due to an increase in surface VTG receptors. Instead, both hormones acted by stimulating the specific internalization rate (ke) of the VTG receptor, although by apparently different mechanisms. The stimulation of ke by insulin was most obvious at low levels of receptor occupancy. Insulin also increased the affinity of the VTG receptor for its ligand. Both hormones increased the rate of fluid phase endocytosis, but the magnitude of stimulation was not sufficient to account for the observed increase in VTG uptake. The steady state binding of VTG was biphasic with respect to increasing ligand concentration, primarily due to a nonlinear receptor internalization rate as a function of occupancy. The nonlinear VTG binding was abolished by incubating the oocytes at 0 degree C. Our data are consistent with a model in which there is a cell surface regulatory component that facilitates the internalization of the occupied VTG receptor. Although both human chorionic gonadotropin and insulin stimulate VTG uptake by increasing the net rate of endocytosis, insulin also increases the association of the occupied VTG receptor with this regulatory component.