Diethylstilbestrol and Other Nonsteroidal Estrogens: Novel Class of Store-Operated Calcium Channel Modulators

Background: Compounds with the stilbene pharmacophore and other nonsteroidal estrogens have previously been shown to inhibit thrombin-induced elevation of intracellular free calcium ([Ca2+](i)) in human platelets. Thrombin elevates [Ca2+](i) in platelets predominantly by activating a store-operated Ca2+ entry (SOCE) mechanism, probably involving STIM1 and Orai1 although other components may be involved. Methods: Human platelets were loaded with the Ca2+ sensitive indicator fura-2, various concentrations of stilbene compounds and other nonsteroidal estrogens were added to the platelets, and thrombin was added to elevate [Ca2+](i). The degree of inhibition by each compound was determined at the peak increase in [Ca2+](i) induced by thrombin. Results: The additional compounds that were examined in the present study were analogs of diethylstilbestrol (DES), namely trans-resveratrol, hexestrol, tetrahydrochrysene (THC), indenestrol, isoflavones, flavones, and flavanones. DES, indenestrols, and substituted THC diols had the highest inhibitory activity. Dietary polyphenols were less active, and isoflavones were more active than flavones. Glycosides of flavones, flavanones, and isoflavones were inactive. Equol (a product of isoflavone metabolism) had low activity. Among the compounds with a stilbene moiety the presence of unsubstituted phenyl hydroxyls in the para-position were required for activity. Esterification of hydroxyls and bulky substituents at a hydroxyl group diminished or abolished activity. Presence of the ethyl side chains enhanced activity, and shortening or removal of these side chains was detrimental to activity. Presence of the conjugated double bound was necessary for activity. Reduction of the double bond (in fused rings such as equol, dihydrogenistein, indanestrol, or in open chain stilbene derivatives) or repositioning of this double bond outside the stilbene moiety was detrimental to activity, because phenyl rings are not coplanar and have to be at a certain angle to each other. Conclusion: DES likely represents the pharmacophore of this group of nonsteroidal estrogens as an inhibitor of SOCE in platelets.