Acknowledgments The authors thank Dr Vladimir Poltoratsky
The authors thank Dr. Vladimir Poltoratsky and Zhihui Xiao for their help with the in vitro FRET assay. Support for this research was provided by Saint John’s University.
Introduction We predicted the existence of endothelium-derived contractile factors (EDCFs) in 1982 . In 1988, one of these EDCFs was identified as the 21-amino-acid bicyclic peptide endothelin-1 (ET1), which causes sustained and long-lasting vasoconstrictor and vasopressor effects . The endothelin system in mammals includes three 21-amino-acid bicyclic signaling peptides (ET1, ET2 and ET3), the enzymes involved in their synthesis and degradation (endothelin-converting enzymes and, to a lesser extent, chymase and neutral endopetidase) and two G protein-coupled receptors (GPCRs, ETA and ETB) 3, 4, 5. These GPCRs are involved in embryonic development and cardiovascular homeostasis, and in the pathogenesis of cardiovascular and renal diseases, diabetes, cancers and chronic pain 6, 7, 8, 9, 10. Prolonged ETA stimulation causes vasospasm, inflammation, oxidative stress, and cell growth and proliferation. In blood vessels, endothelial ETBs counteract these deleterious effects of ETA and scavenge ET1 from the circulation 12, 13. There is ongoing debate on the therapeutic effects of selective ETA versus mixed ETA and ETB antagonists . Here we focus on the peculiar molecular pharmacology of ETA. Based on its amino 3463 mg sequence, ETA is a class A GPCR 5, 14. Compared with the well-known rhodopsin, β-adrenoceptor, adenosine and muscarinic members of this family , it displays atypical pharmacological properties. Most notably, agonist-induced effects of ETA stimulation persist for several hours and are little affected by desensitization, tachyphylaxis or tolerance 2, 4, 16. In this article, we discuss the mechanisms and consequences of long-lasting ET1-induced ETA-mediated effects.
GPCRs are flexible seven-transmembrane (7TM) proteins that can isomerize between inactive and active conformations. Binding of the endogenous agonist to its orthosteric binding site on the GPCR promotes transition of the receptor from an inactive to an active state that can interact with intracellular proteins involved in signal transduction  such as GTP-binding regulatory proteins and arrestins. Activated ETA can bind both low-molecular-weight monomeric G proteins (Ras and Rho) and heterotrimeric G proteins. Several subtypes of Gα subunits can be activated. This promiscuity explains stimulation of various signal transduction pathways. These include calcium influx, phospholipase Cβ, phospholipase D, protein kinase C (PKC), mitogen-activated protein kinases (MAPKs), Ca2+/calmodulin-dependent kinases and, at a higher level of complexity, stimulation of vasospasm, oxidative stress, inflammation and cellular proliferation . Whether Gβγ subunits contribute to signaling (as is the case for several other GPCRs ) has not been investigated yet. ETA can be phosphorylated by G-receptor kinases (GRKs) and subsequently bind arrestins 20, 21, 22. Arrestin-mediated signaling, which is increasingly recognized for other GPCRs , has also been demonstrated for ETA. It was originally proposed that activated G proteins, GRK-mediated phosphorylation and arrestin binding cause desensitization, tachyphylaxis, internalization and tolerance of GPCRs . However, because ETA-mediated vasopressor responses persist for a long time, chronically activated ETA seems to be little affected by these negative feedback mechanisms. This might involve tight binding of endothelins to ETA4, 16. The molecular mechanisms of ongoing signaling by activated ETA, however, remain to be established. Candidates include (i) inhibition of GRK activity by nitrosylation , (ii) G-protein-independent arrestin-mediated signaling  and (iii) rapid recycling of internalized ETA in tissue and in in vivo systems where the long-lasting effects of ET1 are most prominent.