As shown in Physique?8C and D, anti-hETBR polyclonal antibodies recognized both CHO-hETBR cells and melanoma cells similarly. the native form of human ETBR (hETBR). Rendomab-B1 is the first-reported mAb that behaves as a potent antagonist of hETBR. It recognizes an original extracellular conformational epitope around the receptor, distinct from the endothelin-1 (ET-1) binding site. Rendomab-B1 not only blocks ET-1-induced calcium signaling pathway and triggers rapid receptor internalization on recombinant hETBR-expressing cells, but also exerts pharmacological activities on human vascular endothelial cells, reducing both cell viability and ET-1-induced hETBR synthesis. In addition, binding experiments using rendomab-B1 on different melanoma cell lines reveal the structural and functional heterogeneity of hETBR expressed at the surface of these cancer cells, strongly suggesting the presence of tumor-specific receptors. Collectively, our results underscore the value of rendomab-B1 for research, therapeutic and diagnostic applications dealing with hETBR. Keywords: endothelin B receptor, monoclonal antibody, antagonist, genetic immunization, GPCRs, melanoma Introduction The endothelin family is composed of three identified isopeptides termed ET-1, ET-2 and ET-3. Closantel Each peptide is usually characterized by a 21-amino-acid primary sequence, two intramolecular disulfide bridges and strong sequence similarities with cardiotoxic peptides (the sarafotoxins) found in the venom of the snake Atractaspis engaddensis.1 In humans, endothelins are produced by various organs2-4 following a complex biosynthesis pathway requiring the cleavage of pro-peptides (the big-endothelins) by endothelin-converting enzymes to obtain mature and physiologically active endothelins. Among the three endothelins, ET-1 is the most abundant isoform and is mainly generated within the vascular wall. Once produced, the endothelins exert their biological action in a paracrine or autocrine fashion and intervene in a wide range of physiological functions such as vascular tone homeostasis,5-7 neural crest development,8 ovarian cycle,9 cell proliferation, angiogenesis and inflammation.10 To mediate their numerous physiological effects, endothelins activate two distinct G protein-coupled receptors: ETA receptor (ETAR) and ETB receptor (ETBR). ETBR equally binds all three endothelin isoforms, whereas Closantel ETAR shows a higher affinity for ET-1 and ET-2 than for ET-3. Both receptors present a quasi-ubiquitous expression pattern, but ETAR predominates on vascular easy muscle cells and cardiomyocytes while ETBR is particularly abundant on vascular endothelial cells. Endothelins and Closantel their receptors (i.e., the endothelin axis) have been implicated in a large variety of diseases.4 Vascular endothelial ETBR notably is involved in the two most prevalent diseases in humans, i.e., cardiovascular disorders and cancers. Overexpression or overstimulation of endothelial ETBR promotes atherosclerotic lesions, tissue fibrosis and atheroma plaque development.11 In the field of oncology too, the pathological role of endothelial ETBR has been particularly documented in recent years since it has been reported that ETBR located in the tumor-surrounding vascular endothelium is implicated in: (1) cancer cell growth (by increasing neoangiogenesis12); (2) invasiveness and metastatic dissemination (by promoting macrophage homing to tumors, which release extracellular matrix-degrading metalloproteinases13,14); and (3) tumor escape from immune surveillance (by largely reducing cytotoxic T cell homing to tumors)15,16 Besides this deleterious role played by endothelial ETBR in any kind of cancer, it has also long been reported that some tumor cells themselves can overexpress ETBR, which contributes to their development and aggressiveness. 17 This has been exhibited essentially for melanoma,18,19 but also for glioblastomas, 20 and bone and lung cancers.21 In this context, we decided to develop new tools not only to gather more information on human ETBR (hETBR) cytochemical distribution, structure and roles under both normal and pathological conditions, but also, ideally, to block this receptor, i.e., to exert antagonist activity when hETBR is usually associated with vascular disease and cancer, for example. Currently, the only way to investigate the structural-pharmacological properties of ETBR is to use small chemical brokers, displaying either antagonist (bosentan, BQ-788) or agonist (IRL-1620, sarafotoxins) properties. These small molecules are very useful but they generally lack specificity (e.g., bosentan is JAG1 usually a dual ETAR/ETBR antagonist), give relatively little information concerning receptor structure, are not adapted for Closantel imaging and often remain ineffective for some therapeutic applications. Consequently, we focused on the development of monoclonal antibodies (mAbs), which have emerged as very attractive alternatives to conventional chemicals to study the pharmacology.