Co-culture of Primary Human Mammary Fibroblasts and MCF-7 Cells as an In Vitro Breast Cancer Model

Marjoke Heneweer*,1, Martine Muusse*, Milou Dingemans*, Paul C. de Jong{dagger}, Martin van den Berg* and J. Thomas Sanderson*

* Institute for Risk Assessment Sciences (IRAS), Utrecht University, PO Box 80176, 3508 TD Utrecht, The Netherlands and {dagger} St. Antonius Hospital, PO Box 2500, 3430 EM Nieuwegein, The Netherlands

Received August 31, 2004; accepted November 1, 2004


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Approximately 60% of all breast tumors are estrogen-responsive and chemicals that show estrogenic or anti-estrogenic properties are able to interact with breast tumor growth. In a breast tumor, adipose stromal cells (fibroblasts) surrounding the epithelial tumor contain the aromatase enzyme, which converts androgens into estrogens. Exposure to aromatase inducers can therefore lead to increased estrogen levels and possibly to accelerated breast tumor growth. Subsequently, breast tumor cells synthesize and secrete elevated levels of factors such as prostaglandin E2 (PGE2), interleukin-6 (IL-6), and IL-6 soluble receptor (IL-6sR), which in turn have the ability to stimulate aromatase gene transcription in fibroblasts, establishing a positive feedback loop. In this study, a technique that allows for culturing MCF-7 epithelial breast tumor cells and healthy primary human mammary fibroblasts together in one compartment was developed. To establish the positive feedback loop, the co-culture was exposed to estrogenic compounds. RNA was isolated and reverse-transcriptase polymerase chain reaction (RT-PCR) was performed on the aromatase and pS2 genes. Exposure of the co-culture to estradiol (E2), diethylstilbestrol (DES), and bisphenol-A (BPA), resulted in a three- to seven-fold increase of pS2 transcription levels. Furthermore, pS2 transcription levels increased even more when the aromatase substrate testosterone (20 nM) was present in the co-culture medium. Exposure of the co-culture to the aromatase inducer dexamethasone (DEX) resulted in increased pS2 transcription levels, as well as increased aromatase transcription levels. Simultaneous exposure to DEX and the synthetic anti-estrogen ICI 182,780 almost completely blocked the pS2 response. The aromatase induction response was not altered by ICI 182,780 treatment. Simultaneous exposure to DEX and the non-steroidal aromatase inhibitor fadrozole, abolished the effect of the presence of testosterone in the co-culture medium, but did not result in pS2 gene transcription levels as low as seen after exposure to ICI 182,780. These observations indicate the presence of a positive feedback loop in our co-culture system. This co-culture provides a more sophisticated and sensitive system to detect direct and indirect estrogenic effects of compounds and their possible effects on breast tumor promotion.

Key Words: co-culture; breast tumor promotion; aromatase inducers; human mammary fibroblasts; MCF-7 cells.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Estrogenic effects of chemicals have been the subject of extensive research during the past years. MCF-7 cells have been used to perform classical studies on cell proliferation and estrogen receptor (ER) binding in order to study the estrogenic effects of various compounds including environmental contaminants. In the U.S., companies are obliged to test for estrogenic effects of their new compounds before being allowed to market them. In Japan and Europe, legislation forcing companies to perform tests for estrogenicity is being introduced (Worth and Balls, 2002Go).

Studies on chemicals affecting estrogen synthesis have attracted increasing attention. The aromatase enzyme is responsible for converting androgens into estrogens (Reed and Purohit, 2001Go). Various effects on aromatase activity by flavones (Kellis and Vickery, 1984Go), commonly used pesticides (Heneweer et al., 2004Go; Sanderson et al., 2000Go, 2001Go, 2002Go; Vinggaard et al., 2000Go), imidazole drugs (Ayub and Levell, 1988Go), and organochlorines (Heneweer et al., in pressGo; Letcher et al., 1999Go) have already been described. By enhancing estrogen synthesis, chemicals can exert an indirect estrogenic effect. For example, the widely used herbicide atrazine does not bind to the ER but is capable of increasing aromatase activity approximately three-fold in the human adrenocortical carcinoma cell line H295R (Sanderson et al., 2001Go). Such an increase of aromatase activity can theoretically result in higher estrogen levels leading to an indirect estrogenic effect.

Aromatase (CYP19) comprises of the ubiquitous flavoprotein, NADPH-cytochrome P450 reductase, and a unique cytochrome P450, which is exclusively expressed in estrogen producing cells (Kamat et al., 2002Go). Previous studies have revealed that expression of the aromatase gene is regulated in a species- and tissue specific manner by use of alternate promoter regions (Simpson et al., 2002Go; Singh et al., 1999Go).

Since approximately 60% of all breast tumors are estrogen-responsive (Chen, 1998Go; Richards et al., 2002Go), estrogenic chemicals have the potential to stimulate breast tumor growth. In this case, epithelial breast tumor cells express estrogen receptors to which endogenous and/or exogenous estrogens can bind. Estrogens in a breast tumor are delivered via circulation and formed in the surrounding stromal cells by the aromatase enzyme.

In case of an estrogen-responsive breast tumor, estradiol (E2) that is secreted by the fibroblasts surrounding the tumor can enhance tumor cell growth. In turn, increased numbers of epithelial tumor cells secrete increased amounts of cytokines and growth factors (Imagawa et al., 2002Go), such as interleukin-6 (IL-6) (de Jong, 2001Go), interleukin 6 soluble receptor (IL-6sR) (de Jong et al., 2001Go; Singh et al., 1995Go), and prostaglandin E2 (PGE2) (Richards et al., 2002Go) which, once diffused to fibroblasts, can induce aromatase gene expression in those cells. Cell culture and nude mouse experiments, using aromatase transfected MCF-7 cells, have demonstrated that aromatase expressed in breast cancer cells can play a role in stimulating the growth of tumors. Furthermore, co-culture experiments with aromatase-transfected cells together with untransfected cells showed that estrogen produced by the transfected cells could also stimulate the growth of untransfected cells in a paracrine manner (Sun et al., 1997Go). Abnormal expression of aromatase in fibroblasts surrounding epithelial cells, especially in postmenopausal women, may have a significant influence in breast tumor maintenance and growth. This leads to manifestation of a positive feedback loop that is capable of accelerating estrogen-responsive tumor growth.

The goal of this study was to set up a co-culture system of MCF-7 cells and healthy primary human fibroblasts in order to establish and study the positive feedback loop in vitro. To validate this system, we studied direct and indirect effects of a known aromatase modulator dexamethasone (DEX, synthetic glucocorticoid), the estrogenic compounds estradiol (E2), diethylstilbestrol (DES) and bisphenol A (BPA), the synthetic aromatase inhibitor fadrozole (FAD) and the synthetic anti-estrogen ICI 182,780 on pS2 gene expression in MCF-7 cells and aromatase gene expression in fibroblasts. It was hypothesized that estrogenic compounds directly induce estrogen-related pS2 gene expression in MCF-7 cells. Additionally, estrogenic compounds indirectly induce aromatase expression in primary human fibroblasts, which in the presence of an androgen precursor would result in a synergistic effect on pS2 expression in a co-culture of MCF-7 cells and fibroblasts. Furthermore, it was hypothesized that DEX could directly increase aromatase gene transcription and subsequently cause an indirect increase in pS2 gene expression levels.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Fibroblast isolation, cell culture, and experimental design. Human mammary fibroblasts were isolated from mammary tissue according to previously published methods (Heneweer et al., in pressGo). Tissue was obtained after reduction mammoplasty and pathologically classified as healthy. Patients did not receive any endocrine therapy. Tissue collection was approved by the Medical Ethical Committee (TME/Z-02.09) of the St. Antonius Hospital in Nieuwegein, The Netherlands. Approximately three weeks after isolation, fibroblasts were used to establish a co-culture together with the human mammary carcinoma MCF-7 cells (ATCC# HTB-22). On day 1, fibroblasts were plated at a density of approximately 8 x 104 cells/well in six-well plates in culture medium. Culture medium consisted of RPMI 1640, containing L-glutamine, without phenol red (GibcoBRL 11835-030). The medium was supplemented with 10% heat-inactivated fetal calf serum (FCS), 100 U/l penicilline/100 µg/l streptomycin (GibcoBRL 15140-114) and insulin (10 µg/ml) culture medium. MCF-7 cells were washed with phosphate buffered saline (PBS) and assay medium was added in which heat inactivated FCS was replaced with heat-inactivated, charcoal/dextran-treated FCS (Hyclone, SH30068.03). On day 4, fibroblasts were washed with PBS and MCF-7 cells were trypsinized and seeded on top of the fibroblasts at a density of 8 x 104 cells/well. On day 5, fresh assay medium was added to the co-cultures after which the cells were exposed to 2 µl of test compounds for 72 h (final solvent concentration was 0.1% in medium).

Estradiol (Sigma E2758), dexamethasone (Sigma D1756), ICI 182,780 (Tocris, Bristol, U.K.), and testosterone (Sigma, T1500) were dissolved in 70% ethanol. Diethylstilbestrol (Sigma D4628) and bisphenol A (Sigma I0635) were dissolved in DMSO (Sigma, D8779). Fadrozole (FAD) was kindly provided by Novartis Pharma AG (Basel, Switzerland) and was dissolved in DMSO. It was used as a non-steroidal positive control for aromatase inhibition (Bhatnagar et al., 2001Go; Marty et al., 2001Go).

mRNA isolation and quantification. On day 8, RNA was isolated from the co-cultured cells, using the RNA Instapure System (Eurogentec, Liège, Belgium) according to the enclosed instructions. Obtained RNA was a mixture of RNA from primary human mammary fibroblasts and MCF-7 cells. Purity of RNA was checked by measuring the absorbance ratio at 260/280 nm. RNA was stored at –70°C in aliquots of 50 ng/µl. Primers coding for the estrogen-responsive pS2 gene were used after Lee (Lee et al., 2003Go). A reverse-transcriptase polymerase chain reaction (RT-PCR) (Access, Promega, Madison, WI) was performed with a MgSO4 concentration of 1 mM, annealing temperature of 55°C and 25 cycles. The primer pair and RT-PCR conditions used for aromatase mRNA amplification was described previously (Sanderson et al., 2001Go). Amplification products were 189 bp and 314 bp for pS2 and aromatase, respectively, and were detected using agarose denaturing gel electrophoresis and ethidium bromide staining. Intensity of the stains was quantified using a FluorImager (Amersham Bioscience, Piscataway, NJ). Previous experiments (data not shown) showed that RT-PCR was performed under conditions of linearity to obtain semi-quantitative amplification responses.

Data analysis. In each experiment, each concentration was tested in triplicate. Each experiment was repeated 2–4 times and found to be reproducible. Each graph shows one representative experiment. Human tissue samples were not pooled, therefore each sample served as its own control. For graphs and statistical analyses, Prism 3.0 (GraphPad Software Inc., San Diego, CA) was used. All error bars are presented as standard error of the mean (SEM). To identify statistically significant differences among means, a one-way ANOVA followed by Tukey-Kramer's posteriori-test was performed.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Optimization
Pilot experiments showed that the optimal pS2 and aromatase amplification responses were obtained with an incubation time of 72 h (data not shown). Furthermore, consistent with a previous study (Sanderson et al., 2001Go), our MCF-7 cells did not express aromatase; in turn primary human mammary fibroblasts did not express pS2 (data not shown). Also, DEX did not produce any detectable levels of aromatase gene transcription in MCF-7 cells, nor did estrogens result in detectable pS2 gene transcription in the fibroblasts (data not shown). Exposure of MCF-7 cells to 20 nM of testosterone did not result in a significant increase of pS2 gene transcription (data not shown). Previous studies showed that aromatase activity in primary fibroblasts ranged from 0.005 to 0.09 pmol/h per mg protein in different patients. Also, aromatase gene transcription levels in the fibroblasts differed greatly between batches (Heneweer et al., in pressGo) and varied around the detection limit. As a result we could not consistently use aromatase mRNA transcripts as a parameter in our co-culture experiments, unless aromatase gene transcription was induced by DEX.

Exposure to Estrogenic Compounds
A 72 h exposure of the co-culture to E2 (10–100 pM) resulted in a concentration-dependent increase in pS2 gene transcription. The increase was 8-fold compared to solvent control at the highest E2 concentration. Furthermore, in culture medium containing 20 nM of testosterone, E2 increased pS2 gene transcription even further with a maximum increase of 12-fold compared to solvent control without testosterone (Fig. 1). Aromatase gene transcription was not detectable, probably due to a high detection limit.



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FIG. 1. pS2 gene transcription as % of control (ethanol without testosterone) in co-culture of primary human mammary fibroblasts and MCF-7 cells after 72 h exposure to 0, 30, or 100 pM E2, with (black bars) or without (white bars) 20 nM testosterone. *Significantly different from control (ethanol without testosterone for white bars, ethanol with testosterone for black bars, p < 0.01). #Significantly different from similar E2-treatment without testosterone (p < 0.01). Each bar represents average of triplicate measurements. Error bars represent SEM. Graph shows one representative experiment out of three experiments in tissues from three different samples.

 
A 72 h exposure of the co-culture to the synthetic estrogen DES (30–100 pM) resulted in a concentration-dependent increase in pS2 gene transcription with a maximum of 2.9-fold. When the experiment was performed in culture medium containing 20 nM testosterone, pS2 gene transcription levels were consistently higher than in experiments without testosterone (Fig. 2). Relative values for pS2 gene transcription levels were significantly higher for cells treated with DES and testosterone, than for cells treated with DES in the absence of testosterone. Aromatase gene transcription was not detectable.



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FIG. 2. pS2 gene transcription as % of control (DMSO without testosterone) in co-culture of primary human mammary fibroblasts and MCF-7 cells after 72 h exposure to 0, 30, or 100 pM DES, with (black bars) or without (white bars) 20 nM testosterone. *Significantly different from control (DMSO without testosterone for white bars, DMSO with testosterone for black bars, p < 0.05). #Significantly different from similar DES-treatment without testosterone (p < 0.01). Each bar represents average of triplicate measurements. Error bars represent SEM. Graph shows one representative experiment out of three experiments.

 
A 72 h exposure of the co-culture to the synthetic estrogen BPA (300 nM–1 µM), also resulted in concentration-dependent increase in pS2 gene transcription. The maximum increase was approximately 2.5-fold in the absence of testosterone. In the presence of 20 nM testosterone and 300 nM or 1 µM BPA resulted in a 1.5- or 2.5-fold induction in pS2 gene transcription compared to solvent control, respectively.

As a control experiment, the co-culture was exposed to 30 µM of the aromatase inhibitor FAD in addition to BPA and testosterone, resulting in pS2 gene transcription levels similar to those obtained after exposure to BPA only. Again aromatase gene transcription was not detectable (Fig. 3).



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FIG. 3. pS2 gene transcription as % of control (DMSO without testosterone) in co-culture of primary human mammary fibroblasts and MCF-7 cells after 72 h exposure to 0, 0.3, or 1 µM bisphenol A (BPA, with (black bars) or without (white bars) 20 nM testosterone. Striped bars represent treatment with BPA, 20 nM testosterone, and 30 µM fadrozole. *Significantly different from control (DMSO without testosterone for white bar, DMSO with testosterone for black bars, p < 0.001). #Significantly different from similar BPA-treatment without testosterone (p < 0.05). Each bar represents average of triplicate measurements. $Significantly different from similar BPA treatment with 20 nM testosterone (p < 0.001). Error bars represent SEM. Graph shows one representative experiment out of four experiments.

 
Exposure to Aromatase Inducer Dexamethasone
A 72 h exposure of the co-culture to DEX (30–100 nM) resulted in a concentration-dependent increase in pS2 gene transcription with maximum values of 2-fold and 5-fold for experiments without testosterone and with 20 nM testosterone, respectively (Fig. 4A). Aromatase gene transcription was elevated approximately 3.5-fold for DEX-treatments with and without 20 nM testosterone (Fig. 4B).



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FIG. 4. (A) pS2 gene transcription as % of control (ethanol without testosterone) in co-culture of primary human mammary fibroblasts and MCF-7 cells after 72 h exposure to 0, 30, or 100 nM DEX, with (black bars) or without (white bars) 20 nM testosterone. Striped bars represent treatment with DEX, 20 nM testosterone and 10 nM ICI 182,780. *Significantly different from ethanol with testosterone (p < 0.01). #Significantly different from similar DEX-treatment without testosterone (p < 0.01). $Significantly different from similar DEX-treatment with 20 nM testosterone (p < 0.05). Each bar represents average of triplicate measurements. Error bars represent SEM. (B) Aromatase gene transcription as % of control (ethanol without testosterone) in co-culture of primary human mammary fibroblasts and MCF-7 cells after 72 h exposure to 0, 30, or 100 nM DEX, with (black bars) or without (white bars) 20 nM testosterone. Striped bars represent treatment with DEX, 20 nM testosterone and 10 nM ICI 182,780. *Significantly different from control (ethanol without testosterone for white bar, ethanol with testosterone for black bar (p < 0.05). Each bar represents average of triplicate measurements. Error bars represent SEM.

 
As a control, cells were exposed to 10 nM of the synthetic anti-estrogen ICI 182,780 in addition to DEX and testosterone. When ICI 182,780 was added at a concentration of 10 nM, pS2 gene transcription was almost completely blocked. Aromatase gene transcription in this case appeared lower than in samples with the same DEX- and testosterone treatment but without ICI 182,780 (Fig. 4B), but this decrease was not statistically significant.

When cells in the co-culture were exposed to the synthetic aromatase inhibitor Fadrozole (FAD) in addition to DEX and testosterone, pS2 gene transcription was almost completely blocked, except for the treatments without DEX. Previous experiments have shown that 30 µM FAD blocked aromatase enzyme activity in both H295R and R2C cells (Heneweer et al., 2004Go).


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Preliminary experiments showed that intimate contact between the cell types was required for establishment of the positive feedback loop, which is dependent on intercellular communication by the various estrogens and transcription factors, such as IL-6, IL-6sR, and PGE2. Therefore, we used pS2 gene transcription as a marker for estrogenicity and aromatase gene transcription to assess effects on aromatase. The pS2 gene was originally identified as an estrogen-inducible transcript in MCF-7 cells (Masiakowski et al., 1982Go) and pS2-gene expression is an adequate way to assess estrogenicity in humans (Soto et al., 1997Go).

Regulation of Aromatase Gene Transcription
Aromatase gene transcription is regulated in a tissue-specific manner, using alternate promoter regions. In healthy breast adipose tissue, the predominant aromatase promoter region I.4 is regulated by glucocorticoids and class I cytokines via the Jak1/STAT3 signaling pathway (Purohit and Reed, 2002Go). Previously, we have shown that exposure of fibroblasts to DEX increases promoter I.4 specific aromatase gene transcription and subsequent enzyme acitivity (Heneweer et al., in pressGo).

In breast tumors, it has been found that aromatase promoter regions I.3 and pII become more active. These promoter regions are stimulated by cyclic adenosine monophosphate (cAMP) through the protein kinase A (PKA) and by phorbol esters through the protein kinase C (PKC) phosphorylation pathways. Exposure of human adrenocortical carcinoma H295R cells to PGE2 resulted in elevated levels of promoter I.3 and pII-specific aromatase gene transcription (Heneweer et al., 2004Go). Prostaglandin E2 (PGE2) is a major product secreted by breast tumor epithelial cells (Richards and Brueggemeier, 2003Go) and thought to be the most important stimulator of the I.3/pII pathway (Zhao et al., 1996Go) by stimulating both PKA and PKC phosphorylation pathways. PGE2 interacts with two receptor subtypes in fibroblasts, EP1 and EP2. Upon binding of PGE2 to EP1, the associated G protein stimulates phospholipase C, resulting in formation of second messengers and activation of PKC. Subsequently, a cascade of serine/threonine phosphorylation events is initiated (Richards and Brueggemeier, 2003Go). Upon binding of PGE2 to EP2, the associated G protein stimulates adenylyl cyclase activity, resulting in elevated production of cAMP. Subsequently, the cAMP responsive element (CRE) in promoter regions I.3 and pII binds CRE binding protein (CREB) which is phosphorylated via PKA-mediated pathways (Heijink et al., 2003Go). As a consequence, the stimulation elicited by PGE2 is equivalent to the combined response of forskolin (enhancer of cAMP production, which activates PKA) and phorbol esters (which are activators PKC) (Zhao et al., 1996Go).

Furthermore, PGE2 appears to stimulate IL-6 production in fibroblasts (Singh et al., 1999Go), leading to increased expression of pII/I.3-derived CYP19 transcripts as well as that of I.4. IL-6 was shown only to be present in small quantities (<10 ng/ml) in breast cyst fluids and at these low levels, aromatase activity was not elevated in cultured breast fibroblasts. However, if IL-6sR was added to fibroblasts, aromatase activity was enhancedmarkedly (Miller et al., 1997Go). ER-positive MCF-7 and T47D cells have been shown to secrete IL-6sR after E2 or DEXstimulation in contrast to ER-negative MDA-MB-231 cells (Chiu et al., 1996Go; Singh et al., 1995Go). Based on these studies we suggest that an increased IL-6sR secretion by MCF-7 cells is a possible mechanism explaining the elevated concentration-dependent increase of pS2 gene transcription in our co-culture experiments after exposure to (xeno)estrogens in the presence of testosterone.

Exposure of the Co-culture to Dexamethasone
Addition of DEX to fibroblasts increased aromatase activity significantly (Heneweer et al., in pressGo; Singh et al., 1998Go). Stimulation of aromatase activity by DEX was almost two-fold higher when fibroblasts were co-cultured with MCF-7 cells (Singh et al., 1998Go). The increased stimulation of aromatase activity in these co-cultures is consistent with the possibility of a factor being secreted by MCF-7 cells, which has a paracrine stimulatory effect on aromatase activity in fibroblasts (Singh et al., 1998Go).

In this study, co-cultures exposed to increasing concentrations of DEX had elevated aromatase gene transcription levels, possibly resulting in elevated aromatase activity. Additionally, DEX might have increased IL-6sR secretion by the MCF-7 cells (Singh et al., 1995Go), stimulating gene transcription via promoter region I.4 even further. This confirms observations by Quinn and coworkers, who demonstrated that the culture medium removed from fibroblasts that were exposed to DEX, elicited a pS2 response in MCF-7 cells (Quinn et al., 1999Go).

Aromatase gene transcription was not further elevated by addition of testosterone, which was expected since testosterone is a substrate for the aromatase enzyme and not an inducer of aromatase gene transcription, nor did ICI 182,780 alter aromatase gene transcription. In the presence of testosterone, however, pS2 gene transcription was increased even further. These results indicate that testosterone served as substrate for aromatase in the fibroblasts, resulting in elevated estradiol production which subsequently further increases pS2 gene transcription. ICI 182,780 completely blocked pS2 gene transcription confirming ER-mediated regulation of pS2 expression. Anti-estrogens have been shown to inhibit E2-stimulated IL-6sR secretion by MCF-7 cells (Singh et al., 1995Go), resulting in less pronounced induction of aromatase gene expression (Fig. 6).



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FIG. 6. Schematic representation of proposed positive feedback loop. MCF-7 cells are stimulated by the estrogens, resulting in cell proliferation. MCF-7 cells secrete higher levels of PGE2, IL-6, and IL-6sR through which aromatase gene transcription is elevated. DEX not only induces aromatase gene expression directly via promoter region I.4, but also elevates IL-6sR secretion by MCF-7 cells resulting in increased stimulatory signals for aromatase promoter region I.4 in the fibroblasts. Compounds with estrogenic properties (estradiol [E2], diethylstilbestrol [DES], bisphenol A [BPA]) are able to induce cell proliferation of the MCF-7 cells directly by binding to the ER. Furthermore, they can induce IL-6sR secretion by MCF-7 cells as well, which leads to elevated aromatase gene transcription and subsequent production of E2. Anti-estrogens, such as ICI 182,780 inhibit all actions of estrogens by blocking the ER.

 


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FIG. 5. pS2 gene transcription as % of control (ethanol without testosterone) in co-culture of primary human mammary fibroblasts and MCF-7 cells after 72 h exposure to 0, 30, or 100 nM dexamethasone (DEX), with (black bars) or without (white bars) 20 nM testosterone. Striped bars represent treatment with DEX, 20 nM testosterone and 30 µM fadrozole. *Significantly different from control (ethanol without testosterone for white bars, ethanol with testosterone for black bars, p < 0.001). #Significantly different from similar DEX-treatment without testosterone (p < 0.001). $Significantly different from similar DEX-treatment with 20 nM testosterone (p < 0.001).

 
Exposure to Estrogenic Compounds
MCF-7 cells are stimulated by E2, DES, and BPA, resulting in increased pS2 gene transcription and cell proliferation (Olsen et al., 2003Go; Soto et al., 1995Go, 1997Go).

Although 300 nM of the environmental contaminant BPA did not increase pS2 gene transcription in the absence of testosterone, addition of testosterone to the medium, which is more likely to reflect the in vivo situation, increased pS2 gene transcription by about 1.5-fold at the same BPA concentration. Higher levels of pS2 gene transcription after addition of testosterone to the medium are to be contributed to aromatase enzyme activity in the fibroblasts, since addition of FAD reduces pS2 transcription to the same level as without testosterone, but not to the lower levels that were observed after exposure to the ER-antagonist ICI 182,780.

Compounds with estrogenic properties are able to induce cell proliferation of the MCF-7 cells directly by activation of the ER. Furthermore, they can also induce IL-6sR secretion by MCF-7 cells, which leads to elevated aromatase gene transcription and subsequent synthesis of E2. E2-dependent IL-6sR secretion is also an ER-mediated event, which is completely blocked by the anti-estrogen 4-hydroxytamoxifen (Singh et al., 1995Go).

A greater number of MCF-7 cells secrete higher levels of PGE2, IL-6, and IL-6sR through which aromatase gene transcription is elevated. Various compounds can interfere with the feedback loop (Fig. 6). DEX induces aromatase gene expression directly via promoter region I.4 (Kamat et al., 2002Go) but DEX also elevates IL-6sR secretion by MCF-7 cells (Chiu et al., 1996Go; Singh et al., 1995Go) resulting in increased stimulatory signals for aromatase promoter region I.4 in the adjacent fibroblasts.

Positive Feedback Loop
The consistently higher levels of pS2 gene transcription after exposure of the co-culture to estrogenic compounds in the presence of 20 nM testosterone compared to the same estrogen-treatment without addition of testosterone, suggests the presence of a feedback loop, which is established by intercellular communication between epithelial MCF-7 cells and fibroblasts.

To our knowledge, this is the first study in which the positive feedback loop between human breast cancer cells and primary human mammary fibroblasts has been established. Our observation that estrogenic effects caused by xenoestrogens, such as BPA, are potentiated in a co-culture of both MCF-7 cells and fibroblasts require reassessment of the endocrine disrupting potential of these xenobiotics. Furthermore, compounds that can stimulate aromatase expression and aromatase enzyme activity have to be taken into account when looking for potential endocrine disruptors and possible promoters of already existing breast tumors. These compounds could elicit indirect estrogenic effects and tumor promotion. A co-culture test system of tumor cells and fibroblasts will provide a more realistic and more sensitive test system for direct and indirect estrogenic effects of chemicals.


    NOTES
 

1 To whom correspondence should be addressed. Fax: + 31 30 2535077. E-mail: M.Heneweer{at}iras.uu.nl.


    REFERENCES
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 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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