Phytoestrogens and breast cancer prevention: possible mechanisms of action

Potential utility of natural products as regulators of breast cancer-associated aromatase promoters

Aromatase, the key enzyme in estrogen biosynthesis, converts androstenedione to estrone and testosterone to estradiol. The enzyme is expressed in various tissues such as ovary, placenta, bone, brain, skin, and adipose tissue. Aromatase enzyme is encoded by a single gene CYP 19A1 and its expression is controlled by tissue-specific promoters. Aromatase mRNA is primarily transcribed from promoter I.4 in normal breast tissue and physiological levels of aromatase are found in breast adipose stromal fibroblasts. Under the conditions of breast cancer, as a result of the activation of a distinct set of aromatase promoters (I.3, II, and I.7) aromatase expression is enhanced leading to local overproduction of estrogen that promotes breast cancer. Aromatase is considered as a potential target for endocrine treatment of breast cancer but due to nonspecific reduction of aromatase activity in other tissues, aromatase inhibitors (AIs) are associated with undesirable side effects such as bone loss, and abnormal lipid metabolism. Inhibition of aromatase expression by inactivating breast tumor-specific aromatase promoters can selectively block estrogen production at the tumor site. Although several synthetic chemical compounds and nuclear receptor ligands are known to inhibit the activity of the tumor-specific aromatase promoters, further development of more specific and efficacious drugs without adverse effects is still warranted. Plants are rich in chemopreventive agents that have a great potential to be used in chemotherapy for hormone dependent breast cancer which could serve as a source for natural AIs. In this brief review, we summarize the studies on phytochemicals such as biochanin A, genistein, quercetin, isoliquiritigenin, resveratrol, and grape seed extracts related to their effect on the activation of breast cancer-associated aromatase promoters and discuss their aromatase inhibitory potential to be used as safer chemotherapeutic agents for specific hormone-dependent breast cancer.

Lactogens and estrogens in breast cancer chemoresistance

Tumor resistance to chemotherapy in advanced breast cancer is a major impediment to treatment success. Resistance can be induced by the drugs themselves or result from the action of internal factors. The role of hormones in chemoresistance has received little attention. This article focuses on two classes of hormones: lactogens and estrogens. Lactogens include prolactin, growth hormone and placental lactogen, all of which can activate the prolactin receptor. Estrogens include endogenous steroids and nonsteroidal compounds from the environment termed endocrine disruptors, all of which can activate 'classical' estrogen receptors (ERα and ERβ), as well as other types of receptors. Both lactogens and estrogens antagonize cytotoxicity of multiple chemotherapeutic agents through complementary mechanisms. The implications of chemoresistance by these hormones to patients with breast cancer, and the potential benefits of developing combinatorial anti-lactogen/anti-estrogen treatment regimens, are discussed.

Glycine soya diet synergistically enhances the suppressive effect of tamoxifen and inhibits tamoxifen-promoted hepatocarcinogenesis in 7,12-dimethylbenz[α]anthracene-induced rat mammary tumor model

There is increasing interest in phytoestrogens as potential alternatives to synthetic selective estrogen receptor modulators (SERMs) in the prevention and therapy of breast cancer. The present study is aimed at determining whether dietary glycine soya (Glycine max seeds; GS), which is rich in phytoestrogens, can enhance the anti breast cancer efficacy of the SERM tamoxifen (TAM) and the effect of TAM and GS, either alone or in combination, on DMBA-initiated hepatocarcinogenesis in rat. For determination of enhancing effect, rats bearing palpable 7, 12-dimethylbenz[α] anthracene (DMBA)-induced mammary tumors were treated with TAM (10 mg kg(-1)/day) while being fed AIN-93G diet with or without added GS (3×10(4) mg kg(-1)), and the tumor growth was monitored up to 5 weeks of treatment. For determining the effect on hepatocarcinogenesis, DMBA-initiated rats were exposed to TAM and dietary GS as above for 6 weeks during promotion stage in a medium-term bioassay, and the development of placental form of glutathione-S-transferase (GST-P)-expressing preneoplastic liver lesions was quantified. Exposure to both TAM and dietary GS enhanced the anti tumor efficacy of TAM via a combination of tumor cell apoptosis (determined by TUNEL) and inhibition of tumor cell proliferation (determined by PCNA immunostaining) and suppressed the growth of GST-P-positive liver lesions. The findings show that dietary GS enhances the therapeutic efficacy of TAM against mammary tumors and minimizes TAM's hepatocarcinogenesis promotion potential.

Endocrine disrupting chemicals targeting estrogen receptor signaling: identification and mechanisms of action

Many endocrine disrupting chemicals (EDCs) adversely impact estrogen signaling by interacting with two estrogen receptors (ERs): ERα and ERβ. Though the receptors have similar ligand binding and DNA binding domains, ERα and ERβ have some unique properties in terms of ligand selectivity and target gene regulation. EDCs that target ER signaling can modify genomic and nongenomic ER activity through direct interactions with ERs, indirectly through transcription factors such as the aryl hydrocarbon receptor (AhR), or through modulation of metabolic enzymes that are critical for normal estrogen synthesis and metabolism. Many EDCs act through multiple mechanisms as exemplified by chemicals that bind both AhR and ER, such as 3-methylcholanthrene. Other EDCs that target ER signaling include phytoestrogens, bisphenolics, and organochlorine pesticides, and many alter normal ER signaling through multiple mechanisms. EDCs can also display tissue-selective ER agonist and antagonist activities similar to selective estrogen receptor modulators (SERMs) designed for pharmaceutical use. Thus, biological effects of EDCs need to be carefully interpreted because EDCs can act through complex tissue-selective modulation of ERs and other signaling pathways in vivo. Current requirements by the U.S. Environmental Protection Agency require some in vitro and cell-based assays to identify EDCs that target ER signaling through direct and metabolic mechanisms. Additional assays may be useful screens for identifying EDCs that act through alternative mechanisms prior to further in vivo study.

Meta-analyses of lignans and enterolignans in relation to breast cancer risk

BACKGROUND

Epidemiologic studies that examined whether lignans, the most important class of phytoestrogens in the Western diet, protect against breast cancer have yielded inconsistent results.

OBJECTIVE

In this study, we conducted meta-analyses on the association between lignans and breast cancer risk.

DESIGN

We performed a systematic MEDLINE search to identify epidemiologic studies published between 1997 and August 2009. We calculated pooled risk estimates (REs) for total lignan exposure, dietary lignan intake, enterolignan exposure, and blood or urine concentrations of enterolactone and according to menopausal and estrogen receptor (ER) status of tumors.

RESULTS

We included 21 studies (11 prospective cohort studies and 10 case-control studies) in the meta-analyses. Lignan exposure was not associated with an overall breast cancer risk (RE: 0.92; 95% CI: 0.81, 1.02; P for heterogeneity = 0.004). However, in postmenopausal women, high lignan intake was associated with a significant reduced risk of breast cancer (13 studies; RE: 0.86; 95% CI: 0.78, 0.94; P for heterogeneity = 0.32). Breast cancer risk was also inversely associated with enterolignan exposure (4 studies; RE: 0.84; 95% CI: 0.71, 0.97) but not with blood or urine enterolactone concentrations. The associations were not significantly different between ER-status subgroups (6 studies).

CONCLUSIONS

High lignan exposure may be associated with a reduced breast cancer risk in postmenopausal women. Additional work is warranted to clarify the association between lignan exposure and breast cancer risk.

Chemopreventive potential of genistein and daidzein in combination during 7,12-dimethylbenz[a]anthracene (DMBA) induced mammary carcinogenesis in Sprague-Dawley rats

The chemopreventive potential of two major soy isoflavones, genistein and daidzein, in mammary carcinogenesis remains enigmatic. The aim of the present study was to investigate the chemopreventive potential of orally administered genistein, daidzein and genistein+daidzein in 7,12-dimethylbenz[a]anthracene (DMBA) induced mammary carcinogenesis in Sprague-Dawley rats. The chemopreventive potential was assessed by monitoring the tumor incidence and tumor volume as well as by analyzing the status of biochemical markers (17beta-estradiol (E2)), enzymatic and non-enzymatic antioxidants and phase I and phase II detoxification enzymes) during DMBA-induced mammary carcinogenesis. A single subcutaneous injection of DMBA (25 mg rat(-1)) in the mammary gland developed mammary carcinoma in female Sprague-Dawley rats. Oral administration of genistein (20 mg kg(-1) b.wt.), daidzein (20 mg kg(-1) b.wt.) and genistein+daidzein (20 mg+20 mg kg(-1) b.wt.) to DMBA treated rats significantly prevented the tumor incidence and tumor volume as well as brought back the status of above said biochemical variables. Genistein and daidzein in combination have shown pronounced chemopreventive potential than either as genistein or daidzein alone. The present study revealed the chemopreventive potential of genistein+daidzein in combination during DMBA induced mammary carcinogenesis. The chemopreventive potential of genistein+daidzein is probably due to their antilipid peroxidative efficacy and modulatory effect on phase I and phase II detoxification cascade during DMBA induced mammary carcinogenesis.

Therapeutic effect of centchroman alone and in combination with glycine soya on 7,12-dimethylbenz[alpha]anthracene-induced breast tumor in rat

Centchroman is a non-steroidal oral contraceptive and has been found to be a candidate drug for breast cancer exhibiting partial to complete remission of lesions in 40.5% of breast cancer patients. The therapeutic efficacy of centchroman was monitored alone and together with glycine soya on growth of 7,12-dimethylbenz[alpha]anthracene-induced breast tumor in rat. The tumor regression was monitored at different doses of centchroman alone ranging from 0 to 10 mg kg(-1) and with glycine soya from 1x10(4) to 5x10(4) mg kg(-1) per day until 5weeks treatment. An optimum tumor treatment opus was established with varying treatment parameters including doses of therapeutic agents and treatment period. The tumors were found to be static with a strong anti-estrogenic effect. Overall our study shows that both centchroman and glycine soya alone and jointly combat with breast cancer.

Disposition of soy isoflavones in normal human breast tissue

BACKGROUND

Despite decades of research on the relation between soy and breast cancer, questions regarding the absorption, metabolism, and distribution of isoflavones in breast tissue largely remain unanswered.

OBJECTIVE

We evaluated the potential health effects of isoflavone consumption on normal breast tissue; isoflavone concentrations, metabolites, and biodistribution were investigated and compared with 17beta-estradiol exposure.

DESIGN

In this dietary intervention study, healthy women were randomly allocated to a soy milk (n = 11; 16.98-mg genistein and 5.40-mg daidzein aglycone equivalents per dose), soy supplement (n = 10; 5.27-mg genistein and 17.56-mg daidzein aglycone equivalents per dose), or control (n = 10) group. After a run-in period > or = 4 d, 3 doses of soy milk or soy supplements were taken daily for 5 d before an esthetic breast reduction. Blood and breast biopsies were collected during surgery and analyzed with liquid chromatography-tandem mass spectrometry.

RESULTS

After soy administration, genistein and total daidzein concentrations, which were expressed as aglycone equivalents, ranged from 135.1 to 2831 nmol/L and 105.1 to 1397 nmol/L, respectively, in hydrolyzed serum and from 92.33 to 493.8 pmol/g and 22.15 to 770.8 pmol/g, respectively, in hydrolyzed breast tissue. The major metabolites identified in nonhydrolyzed samples were genistein-7-O-glucuronide and daidzein-7-O-glucuronide, with an overall glucuronidation of 98%. Total isoflavones showed a breast adipose/glandular tissue distribution of 40:60, and their mean (+/-SEM) derived 17beta-estradiol equivalents toward estrogen receptor beta were 21 +/- 4-fold and 40 +/- 10-fold higher than the 17beta-estradiol concentrations in adipose (0.283 +/- 0.089 pmol/g, P < 0.001) and glandular (0.246 +/- 0.091 pmol/g, P = 0.001) fractions, respectively.

CONCLUSION

After intake of soy milk and soy supplements, isoflavones reach exposure levels in breast tissue at which potential health effects may occur.

trans-Resveratrol as a neuroprotectant

Epidemiological evidence indicates that nutritionally-derived polyphenols such as resveratrol (RES) have neuroprotective properties. Administration of RES to culture media protects a wide variety of neuronal cell types from stress-induced death. Dietary supplementation of RES can ameliorate neuronal damage and death resulting from both acute and chronic stresses in rodents. The specific molecular mechanisms by which RES acts at the cellular level remain incompletely understood. However, many experimental data indicate that RES reduces or prevents the occurrence of oxidative damage. Here we discuss possible mechanisms by which RES might exert protection against oxidative damage and cell death. Evidence suggesting that RES’s chemical antioxidant potential is not sufficient explanation for its effects is discussed. Putative biological activities, including interactions with estrogen receptors and sirtuins are critically discussed. We provide a synthesis of how RES’s phytoestrogenic properties might mediate the neuronal stress resistance underlying its observed neuroprotective properties.

Flavonoids from the heartwood of the Thai medicinal plant Dalbergia parviflora and their effects on estrogenic-responsive human breast cancer cells

From the heartwood of Dalbergia parviflora, eight new compounds, khrinones A (1), B (2), C (3), D (4), and E (5), isodarparvinol B (6), dalparvin (7), and (3S)-sativanone (22), along with 32 known compounds, have been isolated and characterized as 17 isoflavones, nine isoflavanones, five flavanones, six isoflavans, and three miscellaneous substances. Isolates were evaluated for their cell proliferation stimulatory activity against the MCF-7 and T47D human breast cancer cell lines, and their luciferase inductive effects using luciferase transiently transfected MCF-7/luc and T47D/luc cells were also determined. Isoflavones such as genistein (10), biochanin A (11), tectorigenin (12), and 2'-methoxyformononetin (13) stimulated the proliferation of both cells, and concentrations of lower than 1 muM of these compounds showed equivalent activity to 10 pM of estradiol (E2). The new isoflavanone (22) also showed activity against both cell types, although it was weaker than that of the corresponding isoflavone (2'-methoxyformononetin, 13). Two optically active isoflavanones (22 and 24: (3S)-violanone) stimulated the proliferation of both cell lines at lower concentrations than three racemates (21: vestitone, 23: 7,3'-dihydroxy-4'-methoxyisoflavanone, and 25: 3-O-methylviolanone). Bowdichione (20), an isoflavone with a quinone structure in its B-ring, showed activity against only one cell line associated with MCF-7 in these assays.

The use of herbal preparations to alleviate climacteric disorders and risk of postmenopausal breast cancer in a German case-control study

BACKGROUND

The use of herbal preparations (HEP) to alleviate climacteric disorders is expected to increase as women seek alternatives to menopausal hormone therapy to avoid the associated breast cancer risk. Data are sparse on the long-term effects of HEP containing phytoestrogens and black cohosh on breast cancer risk.

METHODS

Within a German case-control study, associations between patterns of HEP use and incident breast cancer were investigated in 10,121 postmenopausal women (3,464 cases, 6,657 controls). Information on HEP use was collected in face-to-face interviews supported by a list of brand names. Multivariate logistic and polytomous regression analyses were done.

FINDINGS

Ever use of HEP (9.9%) was inversely associated with invasive breast cancer [odds ratio (OR), 0.74; 95% confidence interval (CI), 0.63-0.87] in a dose-dependent manner (OR, 0.96 per year of use; P = 0.03). Classes of HEP did not differ significantly (P(heterogeneity) = 0.81). Risks for invasive ductal (OR, 0.72; 95% CI, 0.60-0.87) and combined lobular/mixed/tubular tumors (OR, 0.76; 95% CI, 0.58-1.01) were similarly reduced by any HEP use but not for in situ carcinomas (1.34; 95% CI, 0.86-2.09). There were no substantial differences in associations of HEP use by estrogen receptor status (ER(+) OR, 0.74; 95% CI, 0.62-0.89; ER- OR, 0.68, 95% CI, 0.50-0.93) and progesterone receptor status of the tumor.

INTERPRETATION

Our findings support the hypothesis that HEP use protects from invasive breast cancer in postmenopausal women. Among conceivable modes of action, those independent of estrogen receptor-mediated pathways seem to be involved (i.e., cytotoxicity, apoptosis).

Comparative study on the nuclear hormone receptor activity of various phytochemicals and their metabolites by reporter gene assays using Chinese hamster ovary cells

Phytochemicals are naturally present in a wide variety of plants, and have been suggested to exert a number of effects beneficial to human health. Several phytochemicals possess estrogenic activity through estrogen receptor alpha (ERalpha) and ERbeta, and are, therefore, termed phytoestrogens. In this study, we examined whether various phytochemicals have agonistic and/or antagonistic activity against six human nuclear receptors (ERalpha, ERbeta, androgen receptor (AR), glucocorticoid receptor (GR), thyroid hormone receptor alpha(1) (TRalpha(1)) and TRbeta(1)) by in vitro reporter gene assays using Chinese hamster ovary cells. Of the 31 phytochemicals tested, including flavonoids, isoflavonoids, coumestan, lignans, catechins and their metabolites, 20 compounds showed estrogenic activity via ERalpha and/or ERbeta, and we ranked these phytochemicals according to their estrogenic potency via ERalpha and ERbeta. As a result, coumestrol and genistein strongly activated ERalpha and ERbeta at very low concentrations of <1x10(-10) M. Most phytochemicals showing estrogenic activity also exhibited agonistic activity against ERbeta at lower concentrations than those for ERalpha, and two typical isoflavones, genistein and daidzein, in particular, showed a potent preference for ERbeta. Further, we found that baicalein has ERbeta antagonistic activity, and two compounds, enterolacton and O-desmethylangolensin, have AR antagonistic activity. Nevertheless, none of tested compounds showed AR agonistic activity together with GR, TRalpha(1) and TRbeta(1) agonistic/antagonistic activity. These results suggest that various phytochemicals or their metabolites preferentially interact with ERalpha/beta among the six nuclear hormone receptors tested, and that the ERbeta agonistic activity, in particular, of these compounds may be associated with various beneficial effects on human health.

Selective estrogen receptor modulators and phytoestrogens

Scientific achievements in the last two decades have revolutionized the treatment and prevention of breast cancer. This is mainly because of targeted therapies and a better understanding of the relationship between estrogen, its receptor, and breast cancer. One of these discoveries is the use of synthetic selective estrogen modulators (SERMs) such as tamoxifen in the treatment strategy for estrogen receptor (ER)-positive breast cancer. Hundreds of thousands of lives have been saved because of this advance. Not only is tamoxifen used in the treatment strategy for patients who have breast cancer, but also for prevention in high-risk premenopausal women. Another synthetic SERM, raloxifene, which was initially used to prevent osteoporosis, is also as effective as tamoxifen for prevention in high-risk postmenopausal women. In certain regions of the world, particularly in Asia, a low incidence of breast cancer has been observed. These women have diets that are high in soy and low in fat, unlike the Western diet. Interest in the protective effects of soy derivatives has led to the research of phytoestrogens and metabolites of soy that are described by some as natural SERMs. As a result, many clinical questions have been raised as to whether phytoestrogens, which are also found in other natural foods, can protect against breast cancer. This article reviews the development and role of the more common SERMs, tamoxifen and raloxifene. In addition, this paper will also highlight the emerging studies on phytoestrogens and their similarity and dissimilarity to SERMs.