Endothelin (ET) is a 21-amino-acid peptide that is a highly potent vasoconstrictor. The three isoforms of endothelin 21 are termed endothelins 1, 2, and 3 (ET-1, ET-2, and ET-3). The structures of the endothelins and their corresponding proendothelins are presented in Figure 1A/B/C. Three separate ET genes have been identified; each codes for a separate and distinct endothelin. After transcription of the ET-1 gene, the mature mRNA codes for a prepro-ET-1 of 212 amino acids. Removal of the amino-terminal 20-amino-acid leader sequence generates a 192 amino acid protein that is an intermediate form of ET-1. Processing of this “intermediate” ET-1 by a pair of dibasic specific endopeptidases (at Lys51-Arg52 and Arg91-Arg92) generates the 38-amino acid polypeptide pro-ET-1 (amino acid residues 53–73) which is secreted; it is sometimes referred to as “big” ET-1 (Figure 1B). Pro-ET-1 represents the secreted form of endothelin; further processing occurs outside the endothelin cell by the action of an endothelin-converting enzyme (ECE). The sites of ECE cleavage of the proendothelins are between residues #73 and #74 which are shown in Figure 1B; this generates mature ET-1. The same processing occurs for ET-2 and ET-3. Pro-ET-1 has only 1% of the biological activity of mature ET-1.
Fig1. The endothelin gene, their hormones and receptor families and receptor signal transduction events in the vasculature resulting in a reduction of blood pressure. (A) Processing of the endothelin prepro ET-1 mRNA. The human genome has three separate genes which individually code for three endothelin amino acid sequences; they are ET-1, ET-2, and ET-3. The ET-1 prepro endothelial-1 has 212 amino acids, the “big” ET-1 has 38 amino acids, and the mature ET-1 has 21 amino acids. (B) Isoforms of endothelins. There are three separate biologically active isoforms of endothelin which are all about the same pro-size; they are ET-1 (90 amino acids, [aa], blue color), ET-2 (89 aa, red color), and ET-3 (93 aa, green). Panel B shows the amino acid sequence for ET-1, ET-2, and ET-3. The reference amino acid sequence is ET-1; all the residues are colored blue. ET-2 has eight amino acids residues different from that of ET-1; each are marked by the red color. ET-3 has 16 aa residues (colored green) that are different from ET-1. The vertical arrow indicates the site of proteolytic cleavage by the endothelin-converting enzyme (ECE), which generates the three mature, biologically active endothelins. The two solid lines at the left end loops indicate the presence of their two disulfide linkages for each of the three endothelins. Also shown in the bottom row is the amino acid sequence of the structurally homologous sarafotoxin b, which is present in the venom of the burrowing Egyptian asp, Atractaspesis engaddensis. The sarafotoxin b is a potent vasoconstrictor in cerebral arteries of some mammals where it is employed to protect the asp. (C) Schematic presentation of four families of endothelin receptors. At present there are at least four known endothelin receptors; they are designated as ETA, ETB1, ETB2, and ETC, all of which are G protein-coupled receptors. Ligand binding of the endothelins to their appropriate receptor(s) results in the activation of the receptor and stimulates an increase of the concentration of the intracellular-free calcium. Activation of the ETA receptor leads to vasoconstriction whereas activation of the ETB1 receptor leads to vasodilation. The biological function of the receptor ETC is not yet clear. (D) Biological responses mediated by endothelins in the presence of smooth muscle. In Panel D, the two endothelial producing cells are colored green while the two adjacent endothelial responding smooth muscle cells are colored tan. The endothelial cell on the left has the capability to positively respond to ten hormones (see left rectangle +) and to stimulate gene transcription and production of ET-1 for secretion. This panel is described in more detail in section III.F.3. Abbreviations: Ca2+, cytosolic calcium; NO, nitric oxide; PLC, phospholipase C; PKC, protein kinase C; DAG, diacylglycerol; IP3; inositol triphosphate; ECE, endothelium converting enzyme; PT, pertussis toxin; PLD; phospholipase D; TK, tyrosine kinase.
