Beta(2)-adrenergic receptor down-regulation. Evidence for a pathway that does not require endocytosis.
Sustained activation of most G protein-coupled receptors causes a time-dependent reduction of receptor density in intact cells. This phenomenon, known as down-regulation, is believed to depend on a ligand-promoted change of receptor sorting from the default endosome-plasma membrane recycling pathway to the endosome-lysosome degradation pathway. This model is based on previous studies of epidermal growth factor (EGF) receptor degradation and implies that receptors need to be endocytosed to be down-regulated. In stable clones of L cells expressing beta(2)-adrenergic receptors (beta(2)ARs), sustained agonist treatment caused a time-dependant decrease in both beta(2)AR binding sites and immuno-detectable receptor. Blocking beta(2)AR endocytosis with chemical treatments or by expressing a dominant negative mutant of dynamin could not prevent this phenomenon. Specific blockers of the two main intracellular degradation pathways, lysosomal and proteasome-associated, were ineffective in preventing beta(2)AR down-regulation. Further evidence for an endocytosis-independent pathway of beta(2)AR down-regulation was provided by studies in A431 cells, a cell line expressing both endogenous beta(2)AR and EGF receptors. In these cells, inhibition of endocytosis and inactivation of the lysosomal degradation pathway did not block beta(2)AR down-regulation, whereas EGF degradation was inhibited. These data indicate that, contrary to what is currently postulated, receptor endocytosis is not a necessary prerequisite for beta(2)AR down-regulation and that the inactivation of beta(2)ARs, leading to a reduction in binding sites, may occur at the plasma membrane.
Jockers
R
R
Immuno-Pharmacologie Moléculaire, UPR 415 of CNRS, 75014 Paris, France.
Angers
S
S
Da Silva
A
A
Benaroch
P
P
Strosberg
A D
AD
Bouvier
M
M
Marullo
S
S
D058666:Adrenergic beta-2 Receptor Agonists; D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015536:Down-Regulation; D034281:Dynamins; D004705:Endocytosis / Q000187:drug effects; D004815:Epidermal Growth Factor / Q000378:metabolism; D020558:GTP Phosphohydrolases / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology; D007739:L Cells; D008247:Lysosomes / Q000378:metabolism; D051379:Mice; D008856:Microscopy, Fluorescence; D009097:Multienzyme Complexes / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D018343:Receptors, Adrenergic, beta-2 / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015842:Serine Proteinase Inhibitors / Q000494:pharmacology; D014452:Ubiquitins / Q000378:metabolism
D058666:Adrenergic beta-2 Receptor Agonists; D009097:Multienzyme Complexes; D018343:Receptors, Adrenergic, beta-2; D011993:Recombinant Fusion Proteins; D015842:Serine Proteinase Inhibitors; D014452:Ubiquitins; D004815:Epidermal Growth Factor; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D020558:GTP Phosphohydrolases; D034281:Dynamins; D007545:Isoproterenol