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1 Medical and Research Services, WJB Dorn Veterans Medical Center, Columbia, South Carolina 29201, USA
2 Department of Medicine, Medical Library Building, Suite 316, School of Medicine, University of South Carolina School of Medicine, Columbia, South Carolina 29208, USA
3 Department of Cell Biology and Biochemistry, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
(Requests for offprints should be addressed to T Lin at Department of Medicine, University of South Carolina; Email: lin{at}med.sc.edu)
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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is present in the Leydig cells and ERß is detected exclusively in the Sertoli cells (Saunders et al. 1998, Pelletier et al. 2000). Estradiol treatment suppressed progesterone synthesis (Freeman 1985), but stimulated proliferation of mouse Leydig tumor cells (Sato et al. 1987, DuMond et al. 2001). Furthermore, mice with a genetic disruption of ER ( estrogen receptor knockout (ERKO)) appeared infertile, had lower sperm counts and elevated serum testosterone levels (Eddy et al. 1996), and were not responsive to the estrogen benzoate-mediated decreases in both serum luteinizing hormone (LH) and androgen levels (Akingbemi et al. 2003). In addition, cytochrome P450 17-hydroxylase (C17), 17ß-hydroxysteroid dehydrogenase (17ß-HSD), and StAR mRNAs were higher in ERKO mice than in wild-type mice (Akingbemi et al. 2003). This finding suggested that estrogen and ER-mediated signals regulate Leydig cell steroidogenesis. Phytoestrogens such as genistein, resveratrol, and quercetin are widely distributed in human and animal diet and have chemopreventive properties against estrogen-responsive diseases, including inhibition of tumor cell growth (Setchell & Cassidy 1999, Miodini et al. 1999, Mitchell et al. 1999, Xing et al. 2001), lowering serum cholesterol, and prevention of bone loss in rodents (Mizutani et al. 2000, Nakajima et al. 2001, Wattel et al. 2003). Genistein is the most potent estrogenic compound in soy and soy products. Resveratrol is a potent antioxidant in grapes and red wines. Quercetin is found in apples, onions, and vegetables. Importantly, the exposure of young animals to phytoestrogens has potential impact on male and female gonadal functions. For example, a dietary (Weber et al. 2001, Wisniewski et al. 2003, Svechnikov et al. 2005) or s.c. (Ohno et al. 2003) administration of male rats with genistein resulted in lower serum testosterone levels and demasculinization. Exposure of resveratrol to female rats led to the development of ovarian hypertrophy and to shorter estrous cycle (Henry & Witt 2002). All of these phenomena are believed to be due to the perturbation on gonadal steroidogenesis by the phytoestrogens. In fact, steroidogenic enzymes such as cytochrome P450 side-chain cleavage complex (P450scc) and 17ß-HSD in the gonad and adrenal (Le Bail 2002, Ohno et al. 2002, Supornsilchai et al. 2005, Svechnikov et al. 2005) were altered by phytoestrogen treatments. As yet, reports for phytoestrogen-mediated effects on steroidogenic acute regulatory (StAR) protein expression are limited. Therefore, we focused on phytoestrogen-mediated effects on StAR gene expression and steroidogenesis.
In the gonad, the initial step of steroid synthesis depends on the delivery of cholesterol from cytosol into the mitochondria by the StAR protein (Clark et al. 1994, 1997), which is the rate-limiting step in steroidogenesis. The expression of StAR is especially abundant in testis, ovary, and adrenal glands (Sugawara et al. 1995, Clark et al. 1997), and its importance in steroidogenesis was clearly shown by the virtual absence of steroid synthesis in StAR knockout mice (Caron et al. 1997b) and in patients with congenital lipoid adrenal hyperplasia caused by mutations in the StAR gene (Lin et al. 1995, Bose et al. 1996). The expression of the StAR gene is regulated by numerous factors (Manna & Stocco 2005) and have been shown to be suppressed by 17ß-estradiol in MA-10 cells (Houk et al. 2004). Apart from their estrogenic effects, genistein (Sargeant et al. 1993) and quercetin (Lloyd et al. 1995) are potent tyrosine kinase inhibitors and have effects in the alteration of cytoplasm Ca2+ level. Resveratrol has been characterized as a nonspecific cyclooxygenase (COX) inhibitor (Murias et al. 2004). The aim of the present study was to compare the effects of quercetin, resveratrol, and genistein on StAR gene expression and steroid hormone synthesis in MA-10 cells, in which progesterone is the main steroid product due to the lack of P450c17 expression.
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Materials and Methods |
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ICI 182,780 was purchased from Tocris (Ellisville, MO, USA). EGTA, mibefradil, verapamil, BK4866, sodium orthovana-date, and 8-bromo-adenosine-cAMP (8-Br-cAMP) were obtained from Sigma. DNase I, Murine Moloney Leukemia Virus (M-MLV) reverse transcriptase, Dual Luciferase Assay System, pRL-SV40 Renilla luciferase vector, and CellTiter 96 Aqueous Non-Radioactive Cell Proliferation Assay kits were purchased from Promega. Progesterone Enzyme Immunoassay kits were obtained from Cayman (Ann Arbor, MI, USA). Bicinchoninic Acid Protein Assay kits were purchased from Pierce Biochemicals (Rockford, IL, USA). The Geneport transfection reagent was obtained from Gene Therapy System, Inc. (San Diego, CA, USA). Trizol reagent, horse serum, and Waymouth’s MB572/1 culture medium were obtained from Invitrogen Life Technologies, Inc.
Cell line and culture condition
The MA-10 mouse Leydig tumor cell line (Ascoli 1981) was a generous gift from Dr Mario Ascoli (University of Iowa, Iowa City, IA). MA-10 cells were grown in complete Waymouth’s MB572/1 medium containing 2.2 g sodium bicarbonate, 100 U/ml penicillin/streptomycin, and 15% horse serum in a 37°C and 5% CO2 incubator.
Progesterone assay
MA-10 cells (5 x 104/ml) were seeded into each well of 96-well plates. On the next day, the cells were incubated with 1, 10, 25, or 50 µM quercetin, resveratrol, or genistein. After 24 h, the cells were washed with Ca2+-free phosphate buffer saline twice and replaced with assay medium (Waymouth MB572/1 containing 0.1% BSA (pH 7.4)) containing 0.2 mM 8-Br-cAMP for 4 h. The media were collected for the measurement of progesterone. Steroids were extracted by mixing 1 vol medium with 10 vol diethyl ether in 12 x 75 mm glass tubes. The steroid extracts were evaporated in a 34 °C water bath in aventilated chemical hood. The amount of progesterone in the extracts was measured according to the protocol accompanying the Progesterone Enzyme Immunoassay Kit. Ten microliters of radio immunoprecipitation assay (RIPA) lysis buffer (50 mM Tris–HCl (pH 7.5), 150 mM NaCl, 1 mM EGTA, 1% NP-40, 0.25% sodium deoxycholate, 2 µg/ml aprotinin, 1 µg/ml leupeptin, and 100 µg/ml phenyl methyl sulfonyl fluoride (PMSF)) were added to each well containing cell monolayer and the protein concentration was determined using a Bicinchoninic Acid Protein Assay Kit.
StAR promoter–luciferase assay
MA-10 cells (1 x 105/ml) were seeded into each well of six-well culture plates for 2 days until they were 50–60% confluent. The cells in each well were transfected with 2.0 µg mouse StAR promoter (P-966)–luciferase reporter gene (Caron et al. 1997a), 0.2 µg pRL-SV40 Renilla luciferase vector, and 15 µl Gene-PORTER transfection reagent in serum-free Waymouth’s MB752/1 medium for 5 h. Cells were replaced with complete Waymouth’s MB752/1 medium and incubated at 37 °C, 5% CO2. On the next day, the cells were incubated with fresh Waymouth’s MB752/1 medium containing 0.5, 1, 10, 25, or 50 µM quercetin, resveratrol, or genistein for another 24 h. The cells were then treated with or without 0.2 mM 8-Br-cAMP for 6 h and the lysate was harvested using Passive Lysis Buffer (Promega). The luciferase activity of the cell lysate was determined according to the manufacturer’s protocol of Dual-Luciferase Reporter Assay System. The light intensity of each sample was measured in an EG&G Berthold LB 9507 Luminometer (Berthold, Bad Wildband, German). The StAR P-996 luciferase activity was normalized to that of the Renilla luciferase vector in order to compensate for the variation in transfection efficiency. Each treatment group consisted of triplicate cultures, and each experiment was repeated at least thrice.
Cell proliferation assay
MA-10 cells (2.5 x 104/ml) were seeded into each well of 96-well plates. After 24 h, the cells were treated with 1, 10, 25, or 50 µM quercetin, resveratrol, or genistein. Cell growth was measured from days 0 to 5 with the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) assay according to the manufacturer’s protocol. Absorbance was recorded at 490 nm using a Bio-Rad Benchmark microplate reader (Bio-Rad). Each treatment contained four replicates, and this experiment was performed at least thrice.
Real-time reverse transcription-PCR
MA-10 cells were treated with 50 µM quercetin, resveratrol, or genistein and 0.2 mM 8-Br-cAMP for 24 h. Total RNA was prepared using Trizol reagent according to the manufacturer’s protocol. RNA samples were incubated with RQ1 DNase at 37 °C for 30 min and then inactivated at 65 °C for 10 min. First strand cDNA was generated using 5 µg DNase-treated RNAs and M-MLV reverse transcriptase at 42 °C for 1 h. The first strand cDNA was used for real-time PCR amplification reactions that were carried out using the MyIQ system (Bio-Rad). Reactions were initiated with hot start at 94 °C for 5 min, followed by 48 cycles at 94 °C for 30 s, 60 °C for 30 s, and 72 °C for 1 min with an extension of 5 min. The StAR gene was amplified using the primer pairs: forward: 5'-TTCA-AGCTGTGTGCTGG GAGCTCCTA-3' and reverse: 5'-TTAACACTGGGCC TCAGAGGCAGGGCTGG-3' which gave an amplification product of 247 bp. Primer S14 ribosomal protein was amplified using the forward: 5'-TTTGGTGTCTGCCACATCTTTG-3' and reverse: 5'-ATGGGGGTGACATCCTCAAT-3' sequences which yielded an amplification product of 323 bp (Mamchaoui et al. 2002). Reactions were performed in triplicate, and the average threshold cycle (CT) was used in subsequent calculations to determine the StAR mRNA levels using the 
CT method of MyIQ system (Bio-Rad). The mean relative values ± S.E.M. from three independent experiments were determined.
Statistical analysis
All datawere analyzed byone-way ANOVA followed by Fisher’s least significant difference test using the StatView 4.5 program (Abacus Concepts, Inc., Berkley, CA, USA) fitted for the Macintosh computer. The results were expressed as the mean ± S.E.M., and P 
0.05 was considered statistically significant.
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Results |
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In the present study, progesterone secretion of MA-10 cells in response to cAMP stimulation was decreased by genistein or resveratrol at concentrations higher than 25 µM (Fig. 1
). On the other hand, quercetin increased progesterone synthesis (Fig. 1). These results suggested that there are differential effects of phytoestrogens on MA-10 cell steroidogenesis.
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We next evaluated the effects of genistein, resveratrol, and quercetin on StAR promoter activities in transiently transfected MA-10 cells. StAR promoter activity was inhibited by treatment with either genistein or resveratrol at concentrations higher than 10 or 25 µM respectively (Fig. 2), when compared with cells treated with cAMP only. On the other hand, StAR promoter activity was enhanced by quercetin (Fig. 2). Cyclic AMP-stimulated StAR mRNA levels in MA-10 cells were decreased by treatment with 50 µM genistein or resveratrol, but were increased by quercetin (Fig. 3). These results indicated that genistein and resveratrol have inhibitory effects, while quercetin has a stimulatory effect on MA-10 cell steroidogenesis.
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It has not been reported whether genistein, resveratrol, or quercetin affects MA-10 cell proliferation. In the present study, genistein inhibited MA-10 cell proliferation in a dose-dependent manner (Fig. 4A), while resveratrol or quercetin had no effect. The proliferation rate between days 1 and 5 in cells treated with genistein exhibited a gradual decline when compared with those treated with 0.1% dimethyl sulfoxide (DMSO). However, this phenomenon was not observed following treatment with either resveratrol or quercetin (Fig. 4B). During the first 48 h, incubation with 50 µM genistein did not show any obvious inhibition on MA-10 cell growth and its cytotoxicity became discernable after 72 h in the cultures.
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To investigate the role of ER in quercetin-stimulated steroid hormone synthesis, ICI 182,780, a potent ER antagonist (Wakeling et al. 1991), was used to block endogenous ER activity in MA-10 cells. ICI 182,780 had no effect on cAMP-induced progesterone secretion, but reduced quercetin-plus cAMP-stimulated progesterone secretion by 31% (Fig. 5A). The effect of ICI 182,780 on quercetin-stimulated StAR reporter activity was next investigated. ICI 182,780 did not alter StAR promoter activity when cells were treated with cAMP alone or with cAMP plus quercetin (Fig. 5B). If activated ER has an inhibitory effect on MA-10 cell steroidogenesis, ER antagonists should increase MA-10 cell steroidogenesis. This result in the present study indicated that the effects of quercetin are not mediated by ER and are at a step beyond StAR.
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Steroid hormone synthesis in the gonads and adrenal glands is dependent on the trophic hormones and modulation by Ca2+ and various cytokines (Huhtaniemi & Toppari 1995, Rossier 1997). Whether extracellular Ca2+ plays a role in quercetin-stimulated steroid hormone synthesis remains to be determined. In the presence of cAMP, EGTA decreased progesterone production and attenuated quercetin-mediated increase in progesterone secretion by 28% (Fig. 6A). EGTA also reduced cAMP- and cAMP- plus quercetin-stimulated StAR promoter activities (Fig. 6B). These results indicated that extracellular Ca2+ is required for quercetin-stimulated steroidogenesis in MA-10 cells. Voltage-gated Ca2+ channels and calcium-release-activated Ca2+(CRAC) channels provide the major routes for Ca2+ to enter the cells in response to a variety of stimulations (Rossier 1997). We sought to determine if blockade of Ca2+ channels affects quercetin-mediated responses in MA-10 cells. We utilized mibefradil (Massie 1998) andverapamil (Atlas & Adler 1981) to block T- and L-type voltage-gated Ca2+ channel. Treatment of MA-10 cells with 10 µM of either mibefradil or verapamil had no effect on cAMP-stimulated progesterone synthesis, but significantly reduced quercetin-plus cAMP-stimulated progesterone secretion (Fig. 7A). However, neither mibefradil nor verapamil had an inhibitory effect on quercetin-plus cAMP-stimulated StAR promoter activity (Fig. 7B). Together, these results supported the view that extracellular Ca2+influx is required for quercetin-plus cAMP-stimulated steroidogenesis in MA-10 cells and that Ca2+acts at a step beyond StAR.
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Calcium leakage through the plasma membrane (PM) Ca2+ pump or uptake by intracellular Ca2+ stores is essential for maintaining the homeostatic environment of the cytosol after elevation of cytosolic Ca2+ levels (Strehler & Treiman 2004). Vanadate and quercetin have been reported to inhibit the parathyroid hormone-stimulated PM Ca2+ pump activity in osteoblasts (Lloyd et al. 1995). We investigated the effect of sodium orthovanadate on MA-10 cell steroidogenesis. Treatment with 10 µM vanadate inhibited progesterone secretion by 28% and quercetin-stimulated progesterone secretion by 36% (Fig. 8A). In the parallel experiments, vanadate had no effect on cAMP-stimulated StAR promoter, but significantly inhibited quercetin-plus cAMP-stimulated StAR promoter (Fig. 8B). These results indicate that Ca2+ pump activity is required for quercetin-stimulated pro-gesterone synthesis.
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We next examined the effect of ICI 182,780, EGTA, or vanadate on quercetin-induced StAR mRNA expression in the MA-10 cells. We observed that vanadate alone significantly reduced cAMP-stimulated StAR mRNA expression, while EGTA or ICI 182,780 had no effect. Vanadate and EGTA reduced quercetin-plus cAMP-increased StAR mRNA level by 50 or 36% respectively, while ICI 182,780 had no effect (Fig. 9).
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Discussion |
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Feeding rats with a phytoestrogen- or genistein-rich diet resulted in lower serum testosterone levels, but had no obvious effect on testicular StAR protein levels when compared with those fed with a regular diet (Weber et al. 2001, Svechnikov et al. 2005). Serum genistein levels were approximately 1.5 µM in rats (Weber et al. 2001) and 10 µM in men (Busby et al. 2002) after consumption with a phytoestrogen-rich diet. However, it was not clear whether such concentrations of genistein are sufficient to suppress StAR gene expression. In the present study, we treated MA-10 cells with 0.5–50 µM genistein, resveratrol, and quercetin. We found that genistein and resveratrol in concentrations higher than 25 µM decreased cAMP-stimulated progesterone secretion through down-regulation of StAR expression, while quercetin had an opposite effect. Our results are comparable with the previous report that genistein had an inhibitory effect on steroid hormone synthesis in adrenal glomerulosa (Bodart et al. 1995, Mesiano et al. 1999, Aptel et al. 1999, Ohno et al. 2002), Leydig (Svechnikov et al. 2005), and ovarian cells (Lacey et al. 2005). Moreover, genistein, resveratrol, and quercetin have been shown to suppress the proliferation of several tumor cells (Miodini et al. 1999, Mitchell et al. 1999, Setchell & Cassidy 1999, Xing et al. 2001). Since our assays were performed by the incubation with genistein for less than 30 h and 50 µM genistein had no obvious inhibitory effect on MA-10 cell proliferation during the first 48 h in the cultures (Fig. 4B
), therefore it is unlikely that genistein-mediated decreases on steroidogenesis and StAR gene expression were due to its cytotoxicity. On the other hand, we cannot rule out the possibility that genistein-mediated cytotoxicity has an impact on steroidogenesis in the animals receiving a long-term exposure to the higher doses of genistein. Feeding rats with resveratrol has been shown to decrease serum corticosterone levels (Supornsilchai et al. 2005) and our results showed that resveratrol decreased progesterone production in MA-10 cells. Resveratrol-mediated decrease in StAR gene expression is probably due to its estrogenic effect rather than its inhibitory effect on COX activities (Murias et al. 2004), because StAR expression in the MA-10 cells was increased by COX-2 inhibitors (Wang et al. 2003). Together, we conclude that genistein and resveratrol play a negative role in steroidogenesis. In contrast, we report for the first time that quercetin has a stimulatory effect on steroid production in MA-10 cells through up-regulation of StAR expression. In fact, quercetin alone was sufficient to induce a 2.5-fold increase in basal progesterone secretion in MA-10 cells (data not shown). Previously, quercetin was reported to have no suppressive effect on dibutyryl cAMP-stimulated cortisol secretion in H295R cells (Ohno et al. 2002), supporting our present finding.
Genistein, resveratrol, and quercetin are classified as phytoestrogens because of their capability to bind to both ER and ERß and to activate the transcription of several estrogen-responsive genes in vitro (Kuiper et al. 1998, Bowers et al. 2000). The hierarchy of the binding affinity for both ER subtypes is genistein > quercetin 
resveratrol when compared with 17ß-estradiol. The differences in the binding affinity for ER and ERß among these three phytoestrogens are unlikely to explain the discrepancy in quercetin-induced progesterone production in MA-10 cells. In the present study, ICI 182,780 did not affect cAMP-induced progesterone production but inhibited cAMP-plus quercetin-induced progesterone production. Our result is in agreement with the previous report that ICI 182,780 did not increase LH-stimulated testosterone production in isolated primary rat Leydig cells (Akingbemi et al. 2003). ICI 182,780 suppressed cAMP- plus quercetin-increased progesterone production without affecting StAR promoter activity and StAR mRNA expression, suggesting that the effect of ICI 182,780 is at a step beyond StAR.
Many factors such as trophic hormones, cytokines, Ca2+, kinases, and phosphatases have been reported to affect StAR gene expression (Manna & Stocco 2005). Previously, genistein was reported to decrease AngII-stimulated aldosterone synthesis by diminishing Ca2+ influx in adrenal glomerulosa cells (Aptel et al. 1999). In the present study, EGTA significantly decreased quercetin-plus cAMP-stimulated StAR transcriptional activity and StAR mRNA expression that attenuated progesterone production in MA-10 cells, suggesting that Ca2+signal is involved in quercetin-stimulated steroidogenesis. Verapamil significantly reduced human chorionic gonadotropin (hCG)-plus potassium (K+)-stimulated progesterone synthesis and StAR mRNA levels in mLTC-1 mouse Leydig tumor cells (Manna et al. 1999). In the presence of quercetin plus cAMP stimulation, verapamil as well as mibefradil significantly reduced progesterone secretion, but fail to decrease StAR promoter activity and StAR mRNA expression in MA-10 cells. These results indicate that the major effect of Ca2+ influx through voltage-gated Ca2+ channels may contribute to quercetin-stimulated increases in steroidogenesis and its effect is at a step beyond StAR.
In the present report, vanadate suppressed cAMP- and cAMP-plus quercetin-stimulated progesterone secretion in MA-10 cells, implicating that cellular signals associated with vanadate may play a role in regulating quercetin-stimulated steroidogenesis. Vanadate did not alter StAR promoter activity, but reduced StAR mRNA expression when MA-10 cells were treated with cAMP, suggesting that there is a post-transcriptional regulation on StAR gene expression by vanadate. On the other hand, vanadate inhibited StAR promoter activity and StAR mRNA expression when the cells were treated with cAMP plus quercetin, suggesting that vanadate-sensitive cellular factors are responsible for quercetin effects on steroidogenesis. Vanadate is a well-known inhibitor for phosphoprotein phosphatase (PP) and alkaline phosphatase (ALP) (Gordon 1991) and is able to enhance the sensitivity of mouse Leydig cell proliferation in response to 17ß-estradiol stimulation (Sato et al. 1987). Recently, PP was reported to play a role in regulating cAMP-mediated increase in StAR protein expression and steroidogenesis in mouse Y1 adrenocortical cells (Jones et al. 2000), while PPs are also involved in the elevation of mitochondria Ca2+ level that may be associated with K+-stimulated aldosterone synthesis in H295R cells (Lalevee et al. 2003).
In conclusion, phytoestrogens have differential effects on MA-10 cell steroidogenesis. Genistein and resveratrol have a negative role, while quercetin has a stimulatory effect on steroidogenesis. The effects of quercetin are complex and may depend on the non-ER-mediated events and Ca2+ in MA-10 cells.
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Acknowledgements |
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References |
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Received in final form 26 November 2006
Accepted 19 December 2006
Made available online as an Accepted Preprint 3 January 2007
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denotes significant increase or decrease when compared with the cells treated with cAMP only (P < 0.05 or 0.01 respectively). 
Denotes significant increase when compared with the cells without cAMP treatment (P < 0.001). 
Denotes significant difference when compared with the cells treated with cAMP plus genistein or resveratrol (P < 0.01).
