Resveratrol is considered be a phytoestrogen, based on its structural similarity to diethylstilbestrol, a synthetic estrogen. It can bind to both alpha- and beta-estrogen receptors, and activates estrogen receptor-dependent transcription in human breast cancer cells. Despite a number of studies performed using both hormone-sensitive and hormone-resistant breast cancer cells, the estrogen-modulatory effects of resveratrol remain controversial (
Le Corre et al., 2005). In some cell types, such as estrogen receptor (ER) positive MCF-7 and T47D cells, resveratrol acts as a superagonist, whereas in others, it produces activation equal to or less than that of estradiol (
Gehm et al., 1997). In other studies, Clement
et al. reported Fas/Fas ligand-mediated growth inhibition of T47D by resveratrol (
Clement et al., 1998). Resveratrol (> 1 μM) inhibited the growth of MCF-7 cells by antagonizing the growth-stimulatory effect of 17-beta-estradiol (E2) in a dose-dependent manner (
Lu and Serrero, 1999), as well as the highly invasive MDA-MB-435 cells (
Hsieh et al., 1999). In the absence of E2, resveratrol was observed to carry out mixed estrogen agonist/antagonist activities in some mammary cancer cell lines, but in the presence of E2, resveratrol (1 nM) was shown to function as an anti-estrogen (
Bhat et al., 2001). Resveratrol was also shown to inhibit the proliferation of the estrogen-receptor negative human breast carcinoma cell line, MDA-MB-468, by inhibiting the levels of autocrine growth stimulators, transforming growth factor-alpha (TGF-α), PC cell-derived growth factor, and insulin-like growth factor I receptor, and increasing the growth inhibitor TGF-β2 (
Serrero and Lu, 2001).
One implication of these varied
in vitro results is that the chemopreventive effects of resveratrol likely to be very complex. In fact, in addition to its antioxidant scavenging of free radicals and modulating ER activity (
Magee and Rowland, 2004), resveratrol can interfere with an ERα-associated PI3K pathway, following a process that could be independent of the nuclear functions of the ERα (
Pozo-Guisado et al., 2004), and acts as an agonist for the cAMP/kinase-A system (
El-Mowafy and Alkhalaf, 2003). It also promotes the accumulation of growth inhibitory/pro-apoptotic ceramide (
Scarlatti et al., 2003) and induction of quinone reductase (QR, a phase II detoxification enzyme) (
Bianco et al., 2005), and induces caspase-independent apoptosis through Bcl-2 downregulation (
Pozo-Guisado et al., 2005). It has been shown to suppress Src tyrosine kinase activity (
Kotha et al., 2006a), nitric oxide generation, and the NFκB pathway (
Bhat and Pezzuto, 2002).
In adult rats, resveratrol inhibited the formation of estrogen-dependent preneoplastic ductal lesions induced by DMBA in these mammary glands (IC50 = 3.2 μM) and reduced MNU-induced mammary tumorigenesis when administered by gavage, suggesting the potential beneficial effects of resveratrol against mammary tumorigenesis (
Bhat et al., 2001). Furthermore, dietary resveratrol (10 ppm) in Sprague Dawley rats produced striking reductions in the incidence (45%; p < 0.05), multiplicity (55%; P < 0.001), and extended the latent period of tumor induction by DMBA in a mammary carcinogenesis protocol. This was associated with decreased COX-2 and matrix metalloprotease-9 expression and suppression of NFκB activation (
Banerjee et al., 2002). Recently, lifetime administration of resveratrol in the diet (1 g/kg) to rats reportedly reduced susceptibility to mammary cancer in DMBA-induced mammary carcinogenesis, which correlated with a significant reduction in proliferative cells and an increase in apoptotic cells in mammary terminal ductal structures, and more differentiated lobular structures (
Whitsett et al., 2006).
Resveratrol supplementation (1 mg/l; 4 μg/mouse) delayed spontaneous mammary tumor development and reduced metastasizing capacity in HER-2/new overexpressing transgenic mice, which develop multiple mammary tumors at an early age. This anti-cancer effect was associated with the downregulation of HER-2/neu (
Provinciali et al., 2005). In human breast cancer xenografts, lower tumor growth, decreased angiogenesis, and an increased apoptotic index was seen in MDA-MB-231 tumors in resveratrol-treated (25 mg/kg/day) nude mice (
Garvin et al., 2006). In one study, however, no growth-inhibitory effect was observed in highly metastatic 4T1 murine mammary cancer cells when resveratrol was administered intraperitoneally (1–5 mg/kg daily for 23 days, starting at the time of tumor inoculation), although the growth of these cells was inhibited in
in vitro studies (
Bove et al., 2002). The differences in these xenograft experiments remain largely unexplained, but it may be possibly be due to inadequate dosing.
The available experimental evidence regarding breast cancer risk and consumption of estrogenic chemicals during critical periods of development indicates that the timing and level of exposure to estrogenic chemicals are likely to be important risk factors. A short treatment of pre-pubertal female Sprague-Dawley rats with high-dose resveratrol (100 mg/kg) enhanced MNU-induced mammary carcinogenesis in an estrogen-free environment, as reflected by significant increases in the incidence and multiplicity of mammary tumors (
Sato et al., 2003). These data suggest that pre-pubertal resveratrol exposure affects endocrine function. Although the precise mechanism by which resveratrol accelerates the occurrence of mammary cancer in these mice requires further investigation, an increase in the number of TEBs (terminal end buds) and reduction of their differentiation into alveolar buds has been shown to play a critical role in increasing the risk of breast cancer (
Russo and Russo, 1987;
Sato et al., 2003). It is unclear whether a high dose of resveratrol mediates this process. Although the dose of 100 mg/kg/day of resveratrol represents 5,000 times the amount that is consumed by a person drinking one glass of red wine a day (
Juan et al., 2002), these studies point to the need for further investigation to determine whether the potential use of resveratrol and/or other dietary consumption of phytoestrogens as a chemopreventive agent for breast cancer is safe in younger, high-risk populations.