Secretory pathways in endothelin synthesis.

Endothelin (ET) is a potent vasoconstrictor peptide that is generated via unique processing of a low activity precursor, big ET-1 by endothelin converting enzymes (ECEs). ET has a physiological role in the maintenance of basal tone in humans (Haynes & Webb, 1994; Haynes, 1995; Haynes et al., 1995), but may also have a role in the pathophysiology of cardiovascular diseases, including atherosclerosis, coronary vasospasm and congestive heart disease (Kurihara et al., 1989; Bacon et al., 1996; Cohn, 1996). The ET system is therefore a potential therapeutic target in the e€ective management of these diseases. There are two current strategies being pursued to attenuate adverse haemodynamic e€ects and the migration and proliferation of vascular smooth muscle cells by ET. These include the use of antagonists to receptors that mediate responses to ET and the use of selective inhibitors to ECE. The pathways involved in big ET-1 processing and ET transport are only now being elucidated and these ®ndings will be useful in predicting the characteristics of inhibitors that best suit inhibition of ECE. It will be important to determine whether ECE is expressed on the cell surface and/or intracellularly to decide whether inhibitors are required to penetrate the plasma membrane. This article reviews the secretory pathways involved in ET transport and the subcellular processing of big ET-1 by ECE. ECEs are membrane-bound proteases with structural homology to neutral endopeptidase 24.11 (NEP) and Kell blood group protein (for review, see Opgenorth et al., 1992; Turner, 1993; Turner & Murphy, 1996). The cDNA sequences of two converting enzymes, ECE-1 and ECE-2 have been reported (Schmidt et al., 1994; Emoto & Yanagisawa, 1995; Shimada et al., 1995; Valdenaire et al., 1995; Yorimitsu et al., 1995). The enzymes, which have 59% overall homology, are membrane bound phosphoramidon-sensitive metalloproteases with speci®city for big ET-1. ECE-1 appears to be the predominant endothelin converting enzyme in humans (Emoto & Yanagisawa, 1995). Two isoforms of ECE-1 (ECE-1a and ECE-1b) are encoded by a single gene and di€er only in their cytoplasmic N-terminal domains. Studies on a soluble construct of ECE-1 (Korth et al., 1997), and molecular modelling experiments (Sansom et al., 1995) reveal that the putative extracellular domain contains the catalytic site for ECE activity. A third converting enzyme, ECE-3 has recently been puri®ed from bovine iris microsomes (Hasegawa et al., 1998) and this enzyme has speci®city for big ET-3. With the exception of studies exploiting selective antisera, the precise identity of the enzyme catalyzing conversion of big ET in tissues in functional studies is not yet known and therefore is referred to as ECE activity.