Quercetin induces type‐II estrogen‐binding sites in estrogen‐receptor‐negative (MDA‐MB231) and estrogen‐receptor‐positive (MCF‐7) human breast‐cancer cell lines

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Abstract
We show that flavonoids positively regulate type-II estrogen-binding site (type-ll EBS) levels both in MCF-7 (ER-positive) and in MDA-MB231 (ER-negative) breast-cancer cells. Type-ll EBS were measured by a whole-cell assay at 4°C for 2.5 hr using [3H]-estradiol as tracer. In both cell lines the effect of quercetin (Q) was dose-related and already evident after 12 hr of Q treatment. The increase of type-ll EBS levels after Q exposure requires both RNA and protein synthesis, since actinomycin D and cycloheximide completely abolished the stimulatory effect. The ability of flavonoids in inducing type-ll EBS is well correlated with their relative binding affinity for type-ll EBS. The flavonoid-induced enhancement of type-ll EBS levels is accompanied by increased sensitivity of cancer cells to the inhibitory effect of low Q concentrations. Our data suggest that type-ll EBS are ligand-regulated receptors.