N. Bhowmick et al., THE ENDOTHELIN SUBTYPE-A RECEPTOR UNDERGOES AGONIST-MEDIATED AND ANTAGONIST-MEDIATED INTERNALIZATION IN THE ABSENCE OF SIGNALING, Endocrinology, 139(7), 1998, pp. 3185-3192
The potent vasoconstrictor and mitogen to smooth muscle cells, endothe
lin-1(ET-1), acts via two distinct G protein-coupled receptors, subtyp
e A (ETAR) and subtype B, that are coupled primarily to the G(q)-phosp
holipase C signaling pathway. It is known that ET-1 binding to ETAR pr
omotes internalization, with subsequent degradation of at least a port
ion of the bound ligand. To investigate whether signaling is required
for endocytosis, we developed stably transfected lines of human embryo
nic kidney 293 cells expressing wild-type ETAR and a receptor chimera
(ETARC) in which the C-terminal cytoplasmic tail to ETAR was replaced
with that of the lutropin receptor, another G protein-coupled receptor
, but one which signals through the G(s)-adenylyl cyclase pathway. ETA
RC binds ET-1 like ETAR, but is deficient in signaling. Using a combin
ed concanavalin A/sucrose gradient centrifugation technique to separat
e plasma membranes from other cellular membranes, we found that [I-125
] ET-1 is rapidly internalized into ETAR-expressing cells at 37 C (t(1
/2) for internalization = 5 min; endocytic rate constant = 0.1 min(-1)
); ETARC-expressing cells also internalize [I-125]ET-1, albeit at a so
mewhat slower rate than wild-type receptor (t(1/2) for internalization
= 15 min; endocytic rate constant = 0.03 min(-1)). Using immunofluore
scence confocal microscopy and an antibody developed to the N-terminal
region of ETAR, qualitatively similar results were obtained. In addit
ion, it was found using confocal microscopy that the ETAR-selective an
tagonist, BQ123, also promoted rapid internalization in cells expressi
ng ETAR. These results establish that inositol 1,4,5-trisphosphate sig
naling is not required for ligand-mediated internalization of ETAR and
suggest that a receptor conformational change is necessary. Moreover,
the finding that BQ123 promotes ETAR internalization is novel and has
potentially important implications in its clinical use.