Abbreviations
Abbreviations
CAM: chicken egg chorioallantoic membrane
COX-2: cyclooxygenase-2
Cdk: cyclin-dependent kinase
ELN: enterolactone
HMR: hydroxymatairesinol
NSAID: nonsteroidal anti-inflammatory drugs
NFκB: nuclear factor kappa B
OPN: osteopontin
PSA: prostate specific antigen
SOD1: superoxide dismutase-1
TPA: 12-O-tetradecanoylphorbol 13-acetate
VEGF: vascular growth factor
Introduction
Introduction
In many patients with breast cancer and prostate cancer, the tumors are hormonally
dependent [1], [2]. This has led to anticancer therapeutic approaches that involve antagonists to estrogens
(the use of tamoxifen in breast cancer) [3], or to inhibitors of estrogen/androgen synthesis (aromatase inhibitors in breast
cancer [4] and diethylstilbestrol and finasteride in prostate cancer [5]. Peri- and postmenopausal women have also been treated with estrogens to prevent
bone and cognition loss and to improve cardiovascular risk [6]. The Women's Health Trial (randomized, placebo-controlled clinical trial) revealed
that, contrary to expectation, the presumed benefit of the estrogens on cardiovascular
risk was minimal or absent [7]. This created a concern that the estrogen therapy, while still beneficial regarding
bone loss, increases breast cancer risk. The latter has been estimated as being a
1–2 % increase in risk per year of use of estrogen therapy [8]. This has promoted the search for selective estrogen receptor modulators (SERMs)
– compounds that provide benefit by promoting good bone health, but which do not act
as estrogens on estrogen-sensitive tumors [9]. Raloxifene is an example of an SERM that was generated by chemical synthesis [10]. However, investigators have also pursued the concept that plant estrogens (phytoestrogens)
that are part of the diet may have a role as natural SERMs [10], [11].
Besides their property of being estrogen receptor agonists, phytoestrogens have several
other biological mechanisms of action. They include being protein tyrosine kinase
inhibitors (e.g., the epidermal growth factor receptor) [12], antioxidants [13], inhibitors of tumor necrosis factor alpha [14], inhibitors of 3β- and 17β-hydroxysteroid dehydrogenase activities [15], [16] and inhibitors of aromatase mRNA expression and activity [16], and agonists of the peroxisome proliferator activated-receptor alpha and gamma
(PPAR) [17], [18]. These other properties would be expected to differentiate phytoestrogens from physiological
estrogens. Indeed, they have been reported to inhibit cell cycle events and prevent
cancer in different preclinical models. In addition, epidemiological data have revealed
that populations with a high intake of phytoestrogens in the diet have a very much
lower incidence of breast and prostate cancer [19], [20]. Since they can be tolerated at oral doses 16 times the maximum intake from the
diet [21], [22], clinical investigations are underway to evaluate the effectiveness of phytoestrogens
in the treatment of breast and prostate cancers. The present review will focus on
roles of phytoestrogens in cell cycle regulation, inflammation, metastasis, and survival
as they relate to the ongoing clinical trials. The literature search was performed
with the PubMed search engine using the following criteria: manuscript publication
years 2002–2009, and combinations of search terms: phytoestrogens, soy isoflavones,
cancer, mechanism of action in cultured cell lines, various animal models, cancer
therapy, survivorship and clinical experiments. If the manuscripts were not immediately
available, reprints were requested from the corresponding authors. The principal exclusion
criterion was for those manuscripts that added little or no new data to the existing
information. In such cases, the original manuscripts were used for this study – this
expanded our search time span beyond the initial 7–8 years guideline. The current
ongoing trials on phytoestrogens were searched from the National Institutes of Health
website to attain the most current information on such studies.
Chemistry and Dietary Forms of Phytoestrogens
Chemistry and Dietary Forms of Phytoestrogens
Phytoestrogens are members of the polyphenols that are widely distributed throughout
the plant kingdom. They are a subclass of the polyphenols that have structural similarity
to the endogenous hormone 17β-estradiol, and bind to estrogen receptors. They may be broadly classified into four
classes – isoflavones, lignans, coumestans and resorcyclic acid lactones or myco-estrogens.
As their name suggests, the latter are a class of phytoestrogens that are produced
by mold found on cereal crops. This review will address isoflavones ([Fig. 1]) and lignans ([Fig. 2]), as most research is focused on these two classes and very little is known about
the other two classes. The main dietary sources of isoflavones include legumes such
as soybean, kala channa, mung bean (green lentil), red lentils and red clover. Daidzein
and genistein are the two major isoflavones present in soy. Lignans are present mainly
in flaxseed, seaweed, whole grains, legumes, oil seeds, fruits and vegetables. The
two main lignans in food include secoisolariciresinol diglucoside and matairesinol,
which are also the precursors of the mammalian lignans enterodiol and enterolactone.
The phytoestrogen content of grain/cereal crops may be affected by genetic, environmental
factors, as well as quantification methods used [23], [24]. Intestinal bacteria play an essential role in the absorption and metabolism of
both isoflavones and lignans. The detailed background on absorption and metabolism
of different isoflavones and lignans is beyond the scope of this review – see [25], [26], [27]. For a detailed list of the isoflavone content of food items, please see the United
States Department of Agriculture online database [28].
Fig. 1 Chemical structures of isoflavones. Daidzein (R1 = H, R2 = OH, R3 = H, R4 = H), genistein (R1 = OH, R2 = OH, R3 = H, R4 = H), glycitein (R1 = H, R2 = OH, R3 = OCH3, R4 = H), factor 2 (R1 = H, R2 = OH, R3 = OH, R4 = H), formononetin (R1 = H, R2 = OCH3, R3 = H, R4 = H), biochanin A (R1 = OH, R2 = OCH3, R3 = H, R4 = H), orobol (R1 = OH, R2 = OH, R3 = H, R4 = OH).
Fig. 2 Chemical structures of lignans. Plant lignans secoisolariciresinol (A) and matairesinol (B) and mammalian lignans enterodiol (C) and enterolactone (D).
Doses and Metabolism of Phytoestrogens
Doses and Metabolism of Phytoestrogens
In evaluating the potential uses(s) of phytoestrogens, careful consideration should
be given to the doses or concentrations used in preclinical experiments (cultured
cell lines and animal models) and in clinical trials. When isoflavones are administered
orally, intestinal cells are exposed to high phytoestrogen concentrations (20–100 µM).
Although the isoflavones are well absorbed in the small intestine [29], they are re-excreted into bile and hence the intestine as β-glucuronides. These are not well absorbed from the intestine until bacterial β-glucuronidases hydrolyze them in the large bowel. Cell culture experiments using
intestinal cancer cells at high phytoestrogen concentrations are therefore relevant
to in vivo situations. In contrast, cancer cells in organs at peripheral sites are exposed to
phytoestrogen concentrations that are less than 1 µM, most of which are inactive β-glucuronide conjugates. When phytoestrogens are administered intravenously or intraperitoneally,
the unconjugated (active) phytoestrogens can persist for longer periods of time and
at higher concentrations. Phytoestrogen concentrations are nonetheless high (1–50 µM
depending on dose) in two other non-gastrointestinal body fluids, in the urine [30] and prostatic fluid [31]. Since rodents have a high rate of metabolism, typically tenfold higher oral doses
are required to achieve plasma phytoestrogen concentrations comparable to those in
humans. Therefore, a daily dose of 50–100 mg isoflavones (0.7–1.4 mg/kg body weight)
in humans would be equivalent to a daily dose of 7–14 mg/kg body weight in rodent
models. Also note that giving a rat 1 mg of genistein per day is equivalent to ∼ 4 mg/kg
body weight (assuming a mean body weight of 250 g). In the mouse a 1 mg dose of genistein
is equivalent to 25–50 mg/kg body weight (for mouse body weights of 20–40 g).
Effects of Phytoestrogens on the Cell Cycle
Effects of Phytoestrogens on the Cell Cycle
The cell cycle consists of four phases – G1, S, G2 and M. The progression through
each phase is both ordered and controlled by various regulatory signaling molecules,
and disruption in the regulation can result in neoplastic growth or cancer [32]. There are specific complexes for each phase and formation of cyclins and cyclin-dependent
kinase (cdk) complexes and their degradation is essential for cell cycle progression.
A number of studies have described an important role of phytoestrogens in regulating
the cell cycle ([Table 1]). In a highly metastatic bladder cancer cell line (253J B–V), genistein has been
shown to inhibit cell growth by inducing cell cycle arrest at the G2/M transition,
and significantly reduced the expression of cell cycle regulators cyclin B1 and Cdk-1
[33]. The similar effect of genistein on G2/M arrest was observed in the cervical cancer
cell line ME180 that contains integrated HPV-16 and HPV-18 [34]. In prostate cancer PC-3 cells, treatment with genistein, radiation, or as a combined
treatment showed G2/M phase cell cycle arrest, and increased apoptosis in the combined
treatment [35]. To understand the influence of the combined treatment on cell cycle progression,
Western blot analysis of cell cycle regulatory molecule expression revealed a significant
downregulation of cyclin B1 and upregulation of inhibitory molecule P21WAF1 for genistein and the combined treatment. The monotherapy with either radiation or
genistein increased the P21WAF1 expression. However, radiation compared to all other treatments significantly increased
cyclin B1 protein, suggesting that the combination treatment is needed to achieve
optimum benefit than either treatment alone [12]. Soy isoflavones have growth inhibitory effects on breast, prostate, hepatic, pancreatic,
cervical, and renal cell lines. Genistein (100 µg/mL; 370 µM) caused dose- and time-dependent
inhibition of (SMRT R-1, R-2, R-3 and R-4) renal carcinoma cell lines [36]. Highly metastatic bladder cancer cells (253J B–V) treated with a range of various
isoflavone concentrations (10–50 µM) for 48 hours showed growth inhibition effects,
with genistein being the most effective even at the lowest dose of 10 µM [33].
Table 1 Effects of phytoestrogen treatment on the cell cycle.
|
Nature of isoflavone
|
Concentration
|
Cell type
|
Biological change
|
Reference
|
|
Genistein, glycitein, and daidzein
|
0, 10, 25, and 50 µmol/L each
|
human bladder cancer cell line 253JB‐V
|
a) genistein, compared to other isoflavones, significantly inhibited the cell growth
b) cell cycle arrest at G2/M transition
|
Singh et al. 2006 [33]
|
|
Genistein
|
2.5 to 40 µmol/L
|
cervical cancer cell lines CaSki and ME180
|
a) dose-dependent inhibition of both cell lines
b) cell cycle arrest at G2/M transition in ME180 cell line only
|
Yashar et al. 2005 [34]
|
|
Genistein alone or combined with radiation
|
15 µmol/L genistein alone or 3 Gy radiation alone or 15 µmol/L genistein + 3 Gy radiation
|
prostate cancer cell line PC-3
human breast cancer cell line BR231
human renal cancer cell lines KCI-18 and RC-2
|
genistein combined with radiation compared to either treatment alone
a) significantly inhibited cell growth in all cell lines
b) significantly greater G2/M cell cycle arrest
c) downregulation of cyclin B1
d) upregulation of P21WAF1
pretreatment of cells with 30 µmol/L genistein followed by 3 Gy radiation significantly
decreased NF‐kB DNA binding
|
Raffoul et al. 2006 [35]
|
|
Genistein
|
0 to 370 µmol/L
|
human renal carcinoma cell lines SMKT R-1, R-2, R-3, and R-4
|
dose-dependent inhibition of growth and the 100 µg/L (370 µmol/L) dose resulted in
a time-dependent inhibition in all four cell lines
|
Sasamura et al. 2002 [36]
|
Role of Phytoestrogens in Inflammation
Role of Phytoestrogens in Inflammation
Inflammation is a biological response of vascular tissue and is important for immune
response and wound healing [37]. However, in cancer, inflammation can create a microenvironment around the tumor,
resulting in the attraction of chemokines, cytokines, selectins, and tumor-associated
macrophages. Instead of their normal function of phagocytosis of tumor cells, they
promote cancer progression, matrix degradation, invasion and metastasis [38], [39]. The use of anti-inflammatory drugs has been shown to significantly reduce the colorectal
reoccurrence in patients with previous history of colorectal cancer [40]. The role of nonsteroidal anti-inflammatory drugs (NSAID) is well documented in
lowering cancer incidence [41], [42], [43]. The mechanism of action of NSAIDs on inflammation is through the inactivation of
cyclooxygenase-2 (COX-2), a key enzyme for the prostaglandin production that plays
an important role in inflammation, but the risk of gastrointestinal bleeding remains
the major side effect of such treatment. In a recent study, red clover extract showed
an anti-inflammatory effect comparable to that of hydrocortisone treatment in an in vivo chicken egg chorioallantoic membrane assay (CAM) [44].
Topical treatment with genistein alone or with capsaicin (a bioactive compound from
peppers) in Sprague-Dawley rats, pre-treated with 100 µM 12-O-tetradecanoylphorbol 13-acetate (TPA) to induce an inflammatory response, effectively
inhibited COX-2 and all three mitogen-activated protein kinases – pJNK, pERK, and
pp38 [44]. Similar effects of genistein alone or in combination with capsaicin were observed
in MCF-7 breast cancer cells [45]. In another study C57BL/6 mice were fed either a soy-free control diet or 3 g/kg
soy diet (total aglycone content of 298 mg/g in the diet) for 4 weeks, and were then
injected with either 0.2 µg/kg body weight lipopolysaccharide (endotoxin) or vehicle.
The soy-fed group showed inhibition of the endotoxin by maintaining the glutathione
levels [46]. The expression of inflammation-induced genes, metallothionein and manganese superoxide
dismutase, were significantly lower in the soy-fed group ([Table 2]). The study revealed the dose-dependent inhibitory effects of genistein on IL-6
secretion in human intestinal Caco-2 cells. In a recent study of a rat model of inflammatory
bowel disease, trinitrobenzene sulfonic acid-induced colitis in Wistar rats, a 100 mg/kg
body weight oral dose of genistein significantly inhibited myeloperoxidase and COX-2
[47]. More recently, a case control study of gastric cancer patients revealed that a
genetic polymorphism of the anti-inflammatory IL-10 gene and low soy intake were associated
with an increased risk of gastric cancer compared to no polymorphism and high soy
intake [48].
Table 2 Effects of phytoestrogen treatment in animal models of cancer.
|
Nature of isoflavone
|
Amount
|
Animal model
|
Biological response
|
Reference
|
|
Genistein or genistein with capsaicin
|
topical treatment with 25 µmol/L genistein or capsaicin alone or combined in the rats,
and 50 µmol/L each alone or together in the cell culture
|
Sprague-Dawley rats treated with 100 nmol/L TPA MCF-7 breast cancer cells treated
with 20 nmol/L TPA
|
inhibition of COX-2, pJNK, pERK, and pp38
|
Hwang et al. 2009 [45]
|
|
Novasoy diet
|
diet supplemented with 0.3 % Novasoy
|
male C57BL/6 mice fed the experimental diet followed with 0.2 µg/kg body weight endotoxin
injection
|
Novasoy treatment resulted in inhibition of the endotoxin.
|
Paradkar et al. 2004 [46]
|
|
Genistein
|
100 mg (370 µmol)/kg body weight administered orally
|
TNBS-induced chronic colitis in Wistar rats
|
significant reduction in COX-2 and myeloperoxidase
|
Seibel et al. 2009 [47]
|
|
Genistein
|
human renal carcinoma cells (RCC) pretreated with 100 µg/mL (370 µmol/L) genistein
for 12 h or genistein 100 µg/mL (370 µmol/L) put in Millipore filter chamber with
or without genistein pretreated RCC
|
C57BL/6 mice with human renal carcinoma cells injected in dorsal air sac
|
No effect of genistein pretreatment on vascular volume. Genistein in Millipore without
genistein pretreated RCC decreased vascular volume by 48.9 %. Genistein in Millipore
with genistein pretreated RCC decreased vascular volume by 56.4 %.
|
Sasamura et al. 2004 [53]
|
|
Genistein or soybean-based diet
|
in vitro 1–100 µmol/L genistein
|
in vitro B16F0 melanoma F3II mammary carcinoma cell lines
|
no cytotoxicity with 100 µmol/L in both cell lines
|
Farina et al. 2006 [54]
|
|
|
in vivo 15 mg (55.5 µmol)/kg body weight genistein injection or soybean-based diet
|
in vivo Balb/c and C57BL/6 mice injected with B16 or F3II cells
|
1–50 µmol/L, and 20–50 µmol/L genistein significantly reduced tumor cell migration
in F3II and B16F0 cells, respectively. Both treatments reduced angiogenesis in mice.
|
|
|
Isoflavone mixture or genistein followed with 5 Gy photon radiation
|
50 mg/kg body weight isoflavone mixture 21.5 mg (80 µmol)/kg body weight genistein
|
male nu/nu mice with established prostate tumors with PC-3 cells implants
|
Isoflavone mixture with radiation inhibited 86 % of prostate tumor growth. This inhibition
was similar to that of genistein followed by radiation.
|
Raffoul et al. 2007 [61]
|
In the rat models of inflammation, soy and soy isoflavone treatment have shown promising
results of COX-2 inhibition, without any side effects. In a 16-week soymilk supplementation
study among US postmenopausal women, there was a significant reduction in inflammatory
markers [49]. Randomized placebo-controlled trials are needed to further evaluate the anti-inflammatory
effects of various phytoestrogens.
Role of Phytoestrogens in Angiogenesis
Role of Phytoestrogens in Angiogenesis
Angiogenesis is the process by which new blood vessels are formed from the existing
vasculature. It is essential for normal physiological processes such as growth and
wound healing, and is crucial in tumor progression [50], [51]. Vascular endothelial growth factor (VEGF) is a potent stimulator of angiogenesis.
Antiangiogenic drugs aim to inhibit tumor vascularization. The soy isoflavones genistein,
daidzein, orobol (3′-hydroxygenistein) and factor 2 (6,7,4′-trihydroxyisoflavone)
([Fig. 1]) significantly inhibit angiogenesis in the CAM assay [52]. Densitometric evaluation of the blood vessel surfaces showed that, compared to
negative controls, genistein and daidzein reduced angiogenesis by 75 % and 48 %, respectively.
A recent study demonstrated that the soy isoflavones daidzein and genistein, as well
as red clover extract, exerted antiangiogenic effects comparable to that of thalidomide
(a potent antiangiogenic) in the CAM assay [44]. Although the CAM assay and the corneal assay are highly sensitive assays to detect
new vasculature, there is a large difference between these assays and an actual tumor
environment. To address this issue, Sasamura et al. conducted a study with renal cell
carcinoma cells injected in the female C57BL6 mice dorsal air sac, and treated them
in situ with 100 µg/mL (370 µM) genistein which caused a 50 % reduction in new vasculature
compared with the vehicle [53]. As noted by the authors, this interesting result would have greater importance
if genistein was antiangiogenic when using much lower concentrations (1–10 µM), consistent
with therapeutic doses.
Hormone-independent mammary tumor cells (F3II) and melanoma cells (B16F0) treated
with a very high concentration (100 µM) of genistein showed no cytotoxicity, and a
lower concentration (1–50 µM in F3II cells and 20–50 µM in B16F0 cells) showed a significant
reduction in tumor cell migration. The tumor cells were injected intradermally in
C57BL/6 and BALB/c mice, which were treated either with a soy-based diet or a daily
dose of 15 mg (55.6 µmol)/kg body weight genistein. Both treatments significantly
reduced the new vasculature as compared to the control group [54]. In human breast cancer xenografted BALB/c nu/nu mice, treatment with a phytoestrogen
supplemented diet as 10 % ground flaxseed, or injected with the lignans enterodiol
and enterolactone with 15 mg (50 µmol)/kg body weight showed a significant decrease
in tumor growth, extracellular VEGF, and tumor vasculature as compared with controls
[55].
Phytoestrogens exert antiangiogenic effects through the inhibition of VEGF. Since
the antiangiogenic effect of genistein is stronger as compared to other isoflavones,
it is not known whether or not other phytoestrogens will have the similar effects
on angiogenesis and if their mode of action will be through VEGF or some other mechanism.
Role of Phytoestrogens in Reducing Metastases
Role of Phytoestrogens in Reducing Metastases
Metastasis is a multistep process by which tumor cells detach from the growing tumor
mass, and migrate to form secondary tumors in distant tissues and organs of the body
[56]. A large proportion of cancer mortality is due to deaths from metastases rather
than from primary tumors. Phytoestrogens have shown promising results in this area
of cancer therapy. Hydroxymatairesinol (HMR), a plant lignan and its metabolite enterolactone
(ELN), a mammalian lignan, inhibit cell proliferation and invasion in rat hepatoma
cells (AH109A) at ≥ 50 µM and > 200 µM, respectively. A similar significant inhibitory
effect was observed with serum from rats fed HMR [15 mg (40 µmol)/100 g body weight]
in an AH109A cell culture [57]. Many studies have reported the benefit of phytoestrogens to enhance the efficacy
of cancer chemotherapeutics. A 10-week treatment with either soy phytochemical concentrate
or genistin (the β-glucoside form of genistein present in soy) in SCID mice xenografted with human bladder
tumor cells showed a significant 52 % and 54 % tumor reduction, respectively. Both
treatments significantly reduced lung metastasis; however, no effect was observed
for lymph node metastasis [33]. BALB/c mice transplanted with undifferentiated human pancreatic carcinoma cells
with great metastatic potential (MIA PaCa-2) and treated with a daily 1.3 mg intraperitoneal
dose of genistein significantly inhibited distant metastasis, and improved survival
as compared to the controls [58]. In a bone metastatic prostate cancer mouse (SCID) model, where mice were injected
with the human PC3 cancer cells, a combined treatment of genistein with the chemotherapeutic
docetaxel showed significant inhibition of tumor growth, downregulation of MMP9, and
significant inhibition in invasion, compared to monotherapy with either compound alone
[59]. In a postsurgical orthotopic breast cancer model where mammary fat pads of nude
mice were injected with human ductal carcinoma cells (MDA‐MD‐435/HAL) with high lung
metastatic potential, genistein treatment [750 µg/g of diet – an approximate daily
dose of 45 mg (180 µmol)/kg body weight] for 70 days resulted in a significant inhibition
in lung metastasis. Adjuvant treatment with genistein along with primary tumor resection
using the above cancer model revealed a 100-fold reduction in metastatic burden [60]. In both experiments genistein treatment significantly reduced cell proliferation
and increased apoptosis within the lung metastasis.
Two studies where orthotopic prostate cancer bearing mice showed spontaneous metastasis
to the lymph nodes upon treatment with genistein [oral dose of 5 mg (18.5 µmol)/day]
alone as compared to genistein combined with radiation (5 Gy photons as single dose)
have raised the question about its safety [61], [62]. However, the median survival was similar in control and genistein-treated mice,
and genistein treatment had no detrimental effects on survival. Nonetheless, genistein
combined with radiation exerted a significant increase in survival which was greater
than that of radiation alone. Genistein treatment was initiated 13 and 14 days post-tumor
cell implantation, and there may be little or no effect when metastasis may have already
started. Moreover, the sample size was fairly small (6–7 mice), and the lymph node
weight was reported only in the short treatment group, with no data reported from
the long-term genistein treatment group [62]. The investigators repeated the experiment and treated the same tumor model with
1 mg/day soy isoflavone mixture (genistein, daidzein, and glycitein) or a lower dose
of pure genistein 0.43 mg (1.6 µmol)/day, after the tumor xenograft was established.
The treatment with a soy isoflavone mixture alone had no stimulatory effect on the
lymph node weight, and there was no significant effect for genistein alone. Genistein
or a soy isoflavone mixture as a combined therapy with radiation significantly inhibited
metastasis [61]. Similarly, in a study of a human prostate cancer mouse BALB/c model, treatment
with either 100 mg (370 µmol) or 250 mg (926 µmol)/kg of diet of genistein started
prior to the cancer xenograft showed a significant reduction in lung metastasis, and
no difference was observed for lymph node weight compared to control [63]. HMR treatment (0.15 % of diet) for 14 days in Donryu rats (4 weeks of age) bearing
hepatoma resulted in significantly lower tumor weights and smaller tumors, and the
rats had no metastasis. Similar effects of ELN treatment (0.001 % and 0.01 %) were
observed on tumor weights and metastasis [57].
There are several ongoing phase I, II, and III clinical trials on phytoestrogens as
adjuvant therapy in cancer patients undergoing surgery, radiation, or hormonal manipulations
for evaluation of the safety, and effectiveness ([Table 3]) [64].
Table 3 Ongoing clinical trials of phytoestrogens in cancer patients.
|
Currently ongoing Study
|
Phase
|
Condition
|
Intervention
|
ClinicalTrials.gov identifier
|
|
Flaxseed and/or anastrozole in treating postmenopausal women undergoing surgery for
newly diagnosed stage I or II breast cancer
|
pilot study
|
newly diagnosed primary, invasive breast cancer
estrogen receptor-positive tumor
|
flaxseed, anastrozole, or placebo
|
NCT00635908
|
|
Genistein in treating patients undergoing external-beam radiation therapy for bone
metastases
|
phase I; phase II
|
histology confirmed malignant solid tumor, including any of the following: breast
cancer, kidney cancer, lung cancer, prostate cancer, or melanoma
|
genistein, radiation therapy, and quality of life assessment
|
NCT00769990
|
|
Vitamin D and soy supplements in treating patients with recurrent prostate cancer
|
phase II
|
histology confirmed adenocarcinoma of the prostate
|
cholecalciferol genistein, and soy isoflavones
|
NCT00499408
|
|
Genistein in treating patients with pancreatic cancer that can be removed by surgery
|
phase II
|
pancreatic adenocarcinoma
|
genistein, and therapeutic conventional surgery
|
NCT00882765
|
|
Genistein in patients undergoing surgery for bladder cancer
|
phase II
|
bladder cancer
|
genistein versus placebo
|
NCT00118040
|
|
Clinical trial of purified isoflavones in prostate cancer: comparing safety and effectiveness
|
phase II
|
localized prostate cancer
|
purified isoflavones
|
NCT01036321
|
|
Randomized study of soy protein/isoflavones and venlafaxine on vasomotor symptoms
in patints with prostate cancer undergoing hormonal manipulations
|
phase III
|
histology confirmed prostate cancer
|
soy protein/isoflavone powder, venlafaxine, or placebo
|
NCT00354432
|
In summary, phytoestrogens in animal models of breast, prostate, bladder, and pancreatic
cancer have shown significant inhibition of distant metastases. Phytoestrogen therapy
along with primary treatment either with docetaxel or tumor resection showed significant
improvement in metastasis inhibition. The animal data are encouraging, and phase I
and II clinical trials are ongoing in this area.
Phytoestrogens Enhance the Efficacy of Cancer Therapy
Phytoestrogens Enhance the Efficacy of Cancer Therapy
Overcoming drug resistance is one of the challenges for anticancer therapies, as with
time cancers develop resistance to chemotherapeutics, which is in part due to the
activation of nuclear factor kappa B (NFκB). Many antitumor drugs act as stimulators
for NFκB activation and DNA binding that is responsible for the drug resistance [65]. NFκB plays an important role in cell survival by protecting cells from apoptosis.
Prostate cancer cells (PC-3) treated with a low-dose 3 Gy photon radiation exhibited
significantly increased NFκB‐DNA binding. However, pretreatment with a 30 µM genistein
dose for 24 hours prior to the irradiation significantly reduced the NFκB‐DNA binding
by 66 % [35]. Genistein treatment in a number of cancer cell lines including prostate, breast,
renal cell carcinoma, as an adjuvant therapy followed by radiation, showed a greater
effect than either therapy alone. Pretreatment with 15–30 µM genistein in prostate,
breast, lung, and pancreatic cancer cell lines significantly inhibits NFκB DNA binding
and enhances the action of antitumor drugs by greater inhibition of cell proliferation
and induction of apoptosis [66]. Genistein regulates the Akt signaling pathway [66]. Akt plays a critical role in cell survival and apoptosis, and regulates the NFκB
pathway [67], suggesting dual role of genistein on NFκB.
In a metastatic prostate cancer model, PC-3 cancer cells implanted in the prostate
of male nude mice, isoflavone therapy (50 mg/kg body weight/day) or an equivalent
genistein dose [(0.43 mg (1.6 µmol)/day)] along with radiation significantly inhibited
prostate tumor growth compared with control, or either therapy alone. Although isoflavone
treatment alone significantly inhibited the tumor growth [61], another study utilized the prostate cancer orthotopic model and demonstrated that
a 4-week treatment with 5 mg/day genistein given in combination with 5 Gy photon radiation
resulted in a greater magnitude of tumor inhibition, and a significant increase in
survival than either therapy administered alone [62]. In contrast, a high 5 mg (18.5 µmol)/day dose of genistein to treat orthotopic
renal cell carcinoma in nude mice resulted in a nonsignificant increase in tumor weight,
and showed no effect to radiation at a dose of 8 Gy. However, when radiation and genistein
were given together, there was a significant inhibition of kidney tumor growth compared
to control or either therapy alone [68]. Nonetheless, the Shanghai Breast Cancer Study reported an adverse outcome with
radiotherapy compared to no radiotherapy in a population known to consume high amounts
of soy food. However, the results may be skewed by the fact that radiation therapy
was given to the most advanced staged breast cancer patients and no data were reported
on dose and frequency of radiation [69].
In summary, phytoestrogens play an important role in NFκB regulation. Animal studies
support the benefit of a combined regimen in cancer therapy as compared to radiation
alone on tumor growth. Randomized clinical studies are needed to evaluate the extent
of their chemotherapeutic potential in cancer patients.
Randomized Clinical Trials of Phytoestrogens in Cancer Patients
Randomized Clinical Trials of Phytoestrogens in Cancer Patients
High phytoestrogen intake is well documented among Asian women, and the epidemiological
evidence for the long-term intake of such compounds has shown them to be both safe
and protective against many diseases including cancer. There has been an open question
whether or not phytoestrogens are safe to be used among cancer patients [70], [71]. Little is known about phytoestrogen intake among cancer patients. A recent cross-sectional
study of 100 women treated for breast cancer at the Cancer Treatment Centers of America
reported that 65 % of patients have had some soy food with the mean daily intake of
11.6 mg (43 µmol) genistein and 7.4 mg (29 µmol) daidzein. The isoflavone dose is
equivalent to a daily intake of Πcup of tofu, and is higher than the usual intake
of isoflavones among non-Asian women [72], and the lack of evidence on potential harm suggests that moderation may be the
key to their use [73].
A study where 20 prostate cancer patients were treated with purified genistein [300 mg
(1.11 mmol)/day] for 28 days and with 600 mg (2.22 mmol)/day thereafter for another
56 days, showed no genotoxic effects, no changes in the micronuclei number and no
chromosomal damage were observed when compared with pre-treatment status [74]. Recently, a randomized, double-blind, placebo study demonstrated no effect of a
12-week treatment with 20 g soy protein powder containing a high dose of phytoestrogens
(160 mg total soy isoflavones) on cognition, sleep quality, sexual function and quality
of life, in prostate cancer patients undergoing androgen deprivation therapy [75]. The investigators reported that soy protein or milk powder (placebo) was mixed
in beverages. However, no data were reported on dietary intake; it is not known whether
or not the subjects in both arms had any soy isoflavone intake through diet or supplementation
prior to or during the study. The small sample size (16 subjects in the placebo, and
17 in the intervention arm) and absence of any measures of isoflavone and their metabolite
levels in body fluids (urine or blood) makes it difficult to interpret the results.
However, the high dose was well tolerated and there were no reports of any adverse
effects from the intervention among prostate cancer patients. Similarly, no detrimental
effects of soy powder treatment were observed in a pilot study with 18 prostate cancer
patients receiving a two-step intervention to reduce fat intake to less than 15 %
of daily calories, followed by soy powder supplementation. In the median follow-up
time of 10.5 (range 3–45) months, only 2 subjects successfully attained the fat calorie
reduction of the study goals; also, the length of the soy supplementation was not
clear, whether or not it was carried throughout the follow-up time. There was no information
on the wide follow-up range of 3 months to 45 months, and no information on how many
subjects were followed-up for short- or long-term periods [76]. In another study, 100 patients with isolated high-grade prostatic intraepithelial
neoplasia were supplemented with a 100 mg soy isoflavones along with 200 µg selenium
and 60 mg vitamin E for 6 months. Two subgroups in the study population were identified;
the larger group (68 % of subjects) with a significant decrease in prostate specific
antigen (PSA) levels from baseline, and only 16 % men with diagnosed prostate cancer.
A small proportion of subjects had elevated PSA levels after the intervention, and
25 % of the men in this group were diagnosed with the cancer. However, in the absence
of a control group, it is not clear whether the PSA levels changed due to the phytoestrogens
or the underlying disease progression over time [77]. A phase II, randomized, placebo-controlled trial of 80 mg/day isoflavone treatment
among localized prostate cancer patients for 12 weeks did not find any significant
effects on serum hormone binding globulin, total estradiol, or testosterone [78], [79]. Nonetheless, the isoflavone dose resulted in a significant increase in serum isoflavone
levels among the intervention arm without any cytotoxicity or harmful effects [78], [79].
A randomized, placebo-controlled, double-blind study of flaxseed supplementation for
a month in postmenopausal breast cancer patients showed a significant reduction in
tumor cell proliferation, c-erb B2 expression, and increased apoptosis [80]. The study used whole wheat flour for the placebo group, which is known to have
a high lignan content [23], [24]. The effect size may have been stronger if the placebo were a true placebo (no lignans).
In a crossover design 7 postmenopausal breast cancer survivors were given supplementation
with 138 mg soy isoflavones per day for 24 days, and following a two-week washout
period, the subjects were crossed over to the placebo group. The subjects were blinded
to the intervention and the order of the treatment was random. Changes in the superoxide
dismutase (SOD1) enzyme were measured as a surrogate marker of antioxidant activity
of isoflavones. The treatment significantly increased erythrocyte SOD1, and the increased
levels were associated with a lower risk for breast cancer recurrence [81] ([Table 4]).
Table 4 Clinical trials of phytoestrogens.
|
Clinical stage of the cancer patients
|
Sample size
|
Nature of phytoestrogen treatment
|
Dosage
|
Length of treatment
|
Side effects
|
Results
|
Limitations
|
Reference
|
|
Stages B, C, or D Prostate neoplasia
|
N = 20
|
purified genistein
|
a) 300 mg (1.11 mmol)/d followed by b) 600 mg (2.22 mmol)/d
|
a. 28 days
b. 56 days
|
not reported in the study
|
no genotoxic effects observed
|
The goal of the study was to evaluate safety and there were no data reported on efficacy
of the treatment.
|
Miltyk et al. 2003 [74]
|
|
Prostate cancer (clinical stage not reported)
|
N = 33, placebo 16, treatment 17
|
20 gm revival soy protein
whole milk protein as placebo
|
64 mg (237 µmol) genistein, 63 mg (248 µmol) daidzein, and 34 mg (120 µmol) glycitein
20 gm of protein/d
|
12 weeks
|
none
|
no benefit on cognition, sleep quality, sexual function or quality of life
|
small sample size, no dietary data, did not measure biomarkers; the length of treatment
may not be sufficient to observe any differences
|
Sharma et al. 2009 [75]
|
|
Prostate cancer (no stage reported)
|
N = 17
|
soy powder, in addition, Vit E, selenium and multivitamin
|
70.39 mg (261 µmol) genistein, 36.87 mg (145 µmol) daidzein, and 6.70 mg (24 µmol)
glycitein, 400 IU/d vitamin E; 200 µg/d selenium
|
not specified and 2 months
|
none
|
no significant decrease in PSA levels
|
small sample size, no specified length of soy treatment
|
Spentzos et al. 2003 [76]
|
|
High-grade prostatic intraepithelial neoplasia
|
N = 100
|
prevalon tablet; in addition, 100 µg selenium, 30 mg vitamin E
|
42 mg (156 µmol) genistein, 35.2 mg (139 µmol) daidzein and 22.8 mg (80 µmol) glycitein
200 µg/d selenium, and 60 mg/d vitamin E
|
6 months
|
not reported in the study
|
Two subgroups were identified: one group with decreasing or stable PSA with 25 % risk
of finding prostate cancer, second group with increasing PSA and 52.2 % risk of finding
prostate cancer.
|
A high drop-out rate; at the 6-month follow-up the study had only 58 subjects; it
remains questionable whether or not PSA is a good marker for prostate cancer risk
and in this study if the levels change due to phytoestrogen treatment or simply the
disease progression over time.
|
Joniau et al. 2007 [77]
|
|
Localized prostate cancer
|
N = 53, placebo 28, treatment 25
|
prevastein HC pills (soy based isoflavone concentrate), in addition, multivitamin
|
80 mg/day (2 pills). Isoflavone content of the pills not provided
|
12 weeks
|
Two subjects dropped out due to grade 1 to 2 adverse events (mild to moderate side
effects including GI symptoms such as gas, bloating, loss of appetite, dyspepsia,
and diarrhea)
grade I event reported by 5 subjects in treatment and 7 in placebo;
no clinical toxicity.
|
no significant effects on serum hormone binding globulin, total estradiol, or testosterone
|
|
Kumar et al. 2007 [78], [79]
|
|
Primary breast cancer
|
N = 32, placebo 13, flaxseed 19
|
flaxseed supplemented muffin
|
25 g/day flaxseed
|
13–55 days
|
|
significantly reduced cell proliferation, and c-erbB2 expression, increased apoptosis
|
A wide treatment range; lack of a true placebo group (whole wheat flour was given
to the placebo group that is known to have considerable amount of lignans).
|
Thompson et al. 2005 [80]
|
|
Breast cancer survivors
|
N = 7
|
soy isoflavone concentrate) and placebo
|
15 mg (55.5 µmol) genistin + genistein, 81 mg (319 µmol) daidzein + daidzein, and
42 mg (148 µmol) glycitin + glycitein
|
crossover study – soy isoflavones and placebo for 24 days each with 2 weeks wash-out
time between the two
|
none reported
|
significant increase in erythrocyte SOD1
|
short intervention time and small sample size
|
diSilvestro et al. 2005 [81]
|
Role of Phytoestrogens on Survival
Role of Phytoestrogens on Survival
In transgenic mice with prostatic adenocarcinoma, genistein treatment at a dose of
250 (0.93 mmol) or 500 mg (1.85 mmol)/kg diet for 28 weeks inhibited poorly differentiated
cancer, improved survival, and significantly reduced osteopontin (OPN, extracellular
matrix protein secreted by the macrophages) compared to the mice fed a genistein-free
diet [82], [83]. Similarly, genistein given as a preventive treatment (in mice 4–20 weeks of age)
prior to tumor initiation reduced OPN dose-dependently, but when genistein treatment
was started on 12-week-old mice, a lower concentration increased OPN levels [84]. The elevated level of OPN is associated with tumor progression and increased metastatic
potential. A recent study of targeting OPN by anti-OPN antibody in nude mice bearing
highly metastatic breast cancer demonstrated the increased tumor latency and inhibited
lung metastases [85]. There is an inverse relation between OPN levels and survival among prostate cancer
patients [86].
Only a little is known about the effect of phytoestrogens on cancer survival in patients.
We were able to locate three studies of such a nature. The first study was conducted
among women from Long Island, California, USA; all-cause mortality significantly decreased
for subjects in the highest vs. lowest quintile of isoflavone intake among both postmenopausal
and premenopausal women. There was a modest reduction in breast cancer mortality [87]. The other was the follow-up study from the Shanghai breast cancer cohort study
where the total soy food intake was analyzed as soy protein or isoflavones, and showed
no association with breast cancer survival. However, there were no data on the type
of chemotherapeutics used, and the distribution of tamoxifen use among the study population
[69]. The null association at least assures no harm associated with the use of soy in
breast cancer patients. However, more recently, the Shanghai breast survival study,
a large population-based follow-up cohort of breast cancer survivors in China, showed
that soy protein or isoflavone intake was significantly associated with decreases
in cancer recurrence and mortality. The inverse association was observed regardless
of estrogen receptor or tamoxifen users and menopausal status, suggesting their safe
use among breast cancer patients [88]. This result is tempered by previous findings that adolescent exposure to soy (as
tofu consumption) is conditional for the lowered adult risk of breast cancer [89], [90]. Therefore, while benefits of soy with respect to lowered recurrence and prolonged
survival occurred in these Chinese women, it cannot be assumed that this result will
apply to groups who did not have early life exposure to soy and its isoflavones.
Conclusions
Conclusions
There is evidence for the role of phytoestrogens in cell cycle arrest, antiangiogenic
potential, antimetastatic potential and in enhancing radiotherapy treatment. In the
rodent cancer models phytoestrogens significantly inhibit tumor growth, invasion and
metastasis ([Fig. 3]). Phytoestrogens have shown to be beneficial in reducing the resistance to anticancer
drugs through regulating NFκB. The objective measures of phytoestrogen intake compliance
are lacking in most of the clinical trials and wide ranges (80–600 mg/day) of doses
have been used among cancer patients. However, the lack of side effects at the highest
dose [600 mg (2.22 mmol)/day] of pure genistein at least assures their safe use among
this population. More recently, high phytoestrogen intake has been shown to reduce
cancer recurrence and mortality among breast cancer survivors. However, the minimum
effective dose and whether it is going to differ according to the stage and type of
cancer are yet to be determined. The individual differences in metabolism may play
another important role in determining the effective dose. It is not known whether
these effects of phytoestrogens would vary by obesity status and degree of obeseness.
The use of phytoestrogens in cancer therapy holds potential for further research.
Fig. 3 Summary of the targets of phytoestrogens. The majority of the effects were identified
in experiments using the isoflavone genistein. The lignans have qualitatively different
effects to the isoflavones. The majority of the effects were negative in nature. However,
genistein stimulates the proliferation of estrogen receptor-positive cells at low
(< 1 µM) concentrations. The boldness of the arrows indicates the strength of the
data as determined by this review.
Acknowledgements
Acknowledgements
We would like to thank Maria Johnson, Xingsheng Li, and Daniel Smith for their help
with the proofreading of this manuscript. Research on phytoestrogens and cancer is
funded in part by grants from the National Cancer Institute (U54 CA100949 and R21
AT04661, S. Barnes, PI). M. Virk-Baker and T. Nagy are supported in part by the Cancer
Prevention and Control Training Program (R25 CA047888). The opinions expressed herein
are those of the authors and not necessarily those of the NIH or any other organization
with which the authors are affiliated.
