Estrogenic procarcinogenic effects of piceatannol (PIC) contrast reports about anticarcinogenic activities of PIC. To explain this contradiction, we investigated PIC in estrogen-dependent MCF-7 breast cancer cells and elucidated those cellular mechanisms that correlated with the observed cell effects induced by PIC. Low PIC concentrations (50 nM) induced c-Myc that depended on progesterone receptor (PR) and estrogen receptor (ER). PR-mediated c-Myc induction by PIC was independent of nuclear PR activity but depended on mitogen-activated protein kinase (MAPK) signaling and was associated with an acceleration of cancer cell proliferation. In contrast, 25 mu M PIC inhibited deoxynucleotide triphosphate synthesis, activated Chk2 and p38-MAPK and this was accompanied by an attenuation of cancer cell growth. Apoptosis was most probably inhibited due to activation of Akt; however, high PIC concentrations (> 100 mu M) permitted apoptosis-like cell death in consequence to disruption of orchestrated mitotic signaling. The presented results show for the first time that nanomolar PIC concentrations signal through PR and Erk1/2 and provide a mechanistic explanation why moderate wine consumption-but not other alcoholic beverages-increases the breast cancer risk in women. In contrast, higher PIC concentrations in the micromolar range are considered for adjuvant anticancer therapeutic concepts.