Allosteric Inhibition of Endothelin ETA Receptors by 3,5-Dibromosalicylic Acid

Derivatives of salicylic acid (SA) and benzoic acid prevent endothelin-1 (ET-1) binding to ETA receptors. This study analyzed actions of 30 derivatives of benzoic acid and salicylic acid on 125 I-ET-1 binding to recombinant rat ETA receptors. The most active compounds were 3,5-dibromosalicylic acid (Br2SA, K i = 0.5 mM) and 3,5-diiodosalicylic acid ( K i = 0.3 mM). They were about 50 times more potent than SA and aspirin. Br2SA inhibited equilibrium 125 I-ET-1 binding in an apparently competitive manner. It accelerated 8-fold the dissociation of 125 I-ET-1 receptor complexes and did not modify the second order rate constant of association of 125 I-ET-1 to its receptors. Br2SA also decreased the affinity of ETA receptors for receptor antagonists BQ-123 and bosentan. Br2SA accelerated dissociation of 125 I-ET-1-solubilized ETA receptor complexes and decreased the apparent molecular size of solubilized receptors. Br2SA and 3,5-diiodosalicylic acid inhibited two cellular actions of ET-1: the mobilization of intracellular Ca 2+ stores in isolated cells and contractions of rat aortic rings. They accelerated the relaxing action of BQ-123 and bosentan in ET-1-treated aortic rings. The results suggest the existence of an allosteric modifier site on ETA receptors that recognizes selected derivatives of SA. SA derivatives might be of therapeutic interest to relieve tight ET-1 binding and to favor actions of receptor antagonists.