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1 Center for Vascular Biology, Department of Cell Biology, University of Connecticut Health Center, Farmington, Connecticut; 2 Division of Nephrology and Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, Tennessee; and 3 Department of Biological Chemistry, University of California, Los Angeles, California
Requests for reprints: Timothy Hla, Center for Vascular Biology, Department of Cell Biology, University of Connecticut School of Medicine, 263 Farmington Avenue, Farmington, CT 06030-3501. Phone: 860-679-4128; Fax: 860-679-1269; E-mail: hla{at}nso2.uchc.edu.
| Abstract |
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| Introduction |
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| Materials and Methods |
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Animals. MMTV-COX-2 transgenic mice (4) in FVB/N background were crossed with Ep2/ mice in the C57BL/6J background (6). The resulting MMTV-COX-2 Ep2+/ mice in the (FVB/N;C57BL/6) background were backcrossed with FVB/N mice three to five times. The resulting MMTV-COX-2 Ep2+/ female mice were crossed with Ep2+/ male mice in same strain yielding the three experimental groups: MMTV-COX-2 Ep2+/+, MMTV-COX-2 Ep2+/, and MMTV-COX-2 Ep2/. Some females are kept as virgins for 3 months and dissected for the precocious development study. All other females underwent three rounds of pregnancies. Pups were removed after 4 days postpartum. After 1 month of weaning, mammary glands were dissected for experiments.
Histology and immunohistochemistry. Mammary glands were isolated and mammary gland 4 was used for whole mount analysis and mammary gland 9 was embedded in paraffin for H&E and immunohistochemistry as described (5). After blocking with goat serum (COX-2 and EP2) or rabbit serum (amphiregulin), anti-COX-2 (Cayman Chemical), anti-EP2 (Cayman Chemical), or anti-amphiregulin (R&D Systems, Minneapolis, MN) was incubated and followed by the Vectastain avidin-biotin complex reagent and 3,3'-diaminobenzidine for color development as described (5).
Cyclic AMP measurement. Mammary glands (100 mg) were homogenized in radioimmunoprecipitation assay buffer (RIPA: 0.1% SDS, 0.5% sodium deoxycholate, 1% NP40, 1 mmol/L sodium orthovanadate, 50 mmol/L ß-glycerophosphate, and 1x protease inhibitor cocktail). Samples were centrifuged 14,000 x g for 10 minutes, and 50 µL of supernatant were used for cAMP measurement by a RIA (Amersham Biosciences, Piscataway, NJ). cAMP amount was quantitated after normalization of protein concentration measured by Bradford assay.
RNA purification and reverse transcription-PCR analysis. Total RNA was isolated from mammary gland tissues as well as mammary tumor cells and quantitative reverse transcription-PCR was done as described (7). Primers for amphiregulin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are as follows. Amphiregulin sense 5'-GGTCTTAGGCTCAGGCCATTA-3' and Amphiregulin antisense 5'-CGCTTATGGTGGAAACCTCTC-3'. GAPDH sense 5'-CAACTACATGGTCTACATGTTCCAGTATG-3' and GAPDH antisense 5'-TGACCCGTTTGGCTCCA-3'.
Western blot analysis. Mammary tumor cells were lysed with RIPA buffer and Western blot analysis was carried out as described (4). Incubation of primary antibodies for amphiregulin (1:500, R&D Systems) or ß-actin (1:10,000, Sigma, St. Louis, MO) was followed by incubation of secondary antibodies, respectively, anti-goat or anti-mouse (1:5,000, ICN Amersham Pharmacia) conjugated to horseradish peroxidase and visualized by an enhanced chemiluminescence system (Amersham Pharmacia).
| Results |
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Reduced expression of amphiregulin in the EP2 null MMTV-COX-2 mammary glands. Epidermal growth factor receptor (EGFR) ligands are potent regulators of mammary gland development and tumorigenesis (8). We hypothesized that PGE2 signaling via the EP2 receptor might regulate the EGF/EGFR system and thereby induce mammary gland hyperplasia. Therefore, we examined the expression of EGF-like ligands in these tissues (9). Whereas the level of EGF mRNA was relatively similar in both groups, the expression of amphiregulin mRNA was markedly reduced in MMTV-COX-2 Ep2/ compared with MMTV-COX-2 Ep2+/ mammary glands obtained from multiparous mice (Fig. 2A). Consistently, amphiregulin-expressing cells were markedly reduced in MMTV-COX-2 Ep2/ mammary glands. In contrast, amphiregulin was strongly expressed in most ductal and alveolar epithelial compartment of MMTV-COX-2 Ep2+/ mammary glands (Fig. 2B). These data suggest that EP2 signaling in the mammary gland regulates amphiregulin expression.
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50% in MMTV-COX-2 Ep2/ mammary glands when compared with MMTV-COX-2 Ep2+/ mammary glands (Fig. 3B). We next tested if inhibition of PKA signaling affects EP2-dependent induction of amphiregulin. Treatment with H-89, a PKA inhibitor, potently blocked the induction of amphiregulin mRNA by CAY10399in EP2-expressing cells (Fig. 3C). Similar changes in amphiregulin protein expression were observed (Fig. 3D). These data indicate that EP2/Gs/cAMP/PKA signaling pathway is directly involved in the induction of amphiregulin.
| Discussion |
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We recently showed that overexpression of COX-2 in the mammary gland of CD1 transgenic mice resulted in tumorigenic transformation (4). This effect was only seen in multiparous female mice, was inhibited by indomethacin and Celecoxib, and was seen in CD1 and FVB/N background but not in the C57Bl/6 background (4, 5).4 These data suggest that enzymatic function of COX-2 cooperate with other events to induce tumorigenesis and that genetic modifier genes can dramatically alter the effect. Molecular mechanisms involved are important to be delineated as this information has the potential to be rapidly applied to cancer prevention and/or treatment.
In this study, we focused on the EP2 receptor, a Gs-coupled receptor for PGE2. Deletion of the Ep2 gene in mice led to defects in ovulation, embryonic implantation, and salt-sensitive hypertension (6). However, viability of the Ep2/ mice are generally uncompromised. EP2 receptor is expressed in mammary epithelial cells, its expression is induced during mammary gland development (during pregnancy), and is highly expressed in mammary tumor epithelial cells (5). Because COX-2 is also expressed in the mammary epithelial cells of the MMTV-COX-2 mice (4), we hypothesized that autocrine signaling of EP2 receptor may be involved in the tumorigenic phenotype. Indeed, the development of mammary hyperplasia, a precursor lesion for invasive mammary tumors, is strongly suppressed in Ep2/ mice. Similarly, precocious development of mammary gland was also inhibited in Ep2/ mice. These data suggest that PGE2/EP2 signaling within the mammary epithelial compartment are involved in cellular proliferation and hyperplasia. Interestingly, we did not observe significant changes in microvessel density or vascular endothelial growth factor expression. This is probably due to the fact that EP2 is expressed in the epithelial cell compartment and therefore does not regulate stromal events such as angiogenesis. For example, the EP4 receptor is expressed in the mammary stromal cells (5) and may be involved in the regulation of angiogenic response and tumorigenesis. Indeed, both EP2 and EP4 receptors are important for different aspects of intestinal tumorigenesis (16).
PGE2 signaling intersects with EGF signaling in the context of cancer. For example, PGE2 transactivates the EGF receptors in colon cancer cells in vitro (17). Moreover, PGE2 treatment of colon cancer cells induces the expression of amphiregulin, an EGF family member ligand implicated in mammary cancer development (11). In that study, it was shown that the cAMP/PKA pathway is important in the transcriptional induction of amphiregulin. As COX-2 inhibitors delay cancer development in HER-2/neu (EGFR) model of mammary cancer (18), we investigated the expression patterns of EGF-ligand family members in Ep2/ and wild-type mice. Our results clearly show that EP2 receptor controls the expression of amphiregulin in the mammary epithelial cells in vivo. Amphiregulin was shown to regulate ductal morphogenesis as a mitogen in mice (9). In addition, retroviral overexpression of amphiregulin into mammary glands induces hyperplasia in vivo (19), suggesting an important role of amphiregulin in premalignant mammary hyperplasia. Our data imply that COX-2derived PGE2/EP2 signaling may influence the EGF signaling system by up-regulating the expression of amphiregulin.
The novel mechanism elucidated in this study (i.e., EP2/Gs/cAMP/amphiregulin pathway in the induction of mammary hyperplasia) adds to our knowledge of the role of the COX-2 pathway in mammary cancer development. EP2 receptor inhibitors may be useful in the prevention and/or treatment of breast cancer.
| Acknowledgments |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
| Footnotes |
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Received 1/27/05. Revised 3/ 9/05. Accepted 3/26/05.
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