Resveratrol, a hydroxystilbene found in grapes and wine, has previously been shown to be a non-
flavonoid phytoestrogen, and to act as an
estrogen receptor (ER) superagonist in MCF-7 cells transiently transfected with
estrogen-responsive reporter constructs. Several additional hydroxystilbenes, including
diethylstilbestrol (DES) and
piceatannol, were tested, and all showed ER agonism or partial agonism, but superagonism was specific to
resveratrol. Moreover, superagonism was observed in cells carrying a stably integrated reporter gene, indicating that this phenomenon is not a result of transient transfection. To examine the role of the transcriptional activation function (AF) domains of
ERalpha in
resveratrol agonism, we compared the effects of
resveratrol and
estradiol (E2) on expression of exogenous reporter genes and an endogenous
estrogen-regulated gene (
TGFalpha) in MDA-MB-231 cells stably transfected with wild-type (wt)
ERalpha or mutants with deleted or mutated AF domains. In reporter gene assays, cells expressing wtERalpha showed a superagonistic response to
resveratrol. Deletion of AF-1 or mutation of
AF-2 attenuated the effect of
resveratrol disproportionately compared to that of E2, while deletion of
AF-2 abrogated the response to both
ligands. In
TGFalpha expression assays,
resveratrol acted as a full agonist in cells expressing wtERalpha. Deletion of AF-1 attenuated stimulation by E2 more severely than that by
resveratrol, as did deletion of
AF-2. In contrast, mutation of
AF-2 left both
ligands with a limited ability to induced
TGFalpha expression. In summary, the effect of modifying or deleting AF domains depends strongly on the
ligand and the target gene.