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Identification and Localization of the Extracellular Calcium-Sensing Receptor in Human Breast¹

Department of Surgery (I.C., M.E.K., D.I.S.), Endocrine-Hypertension Division and Department of Medicine (N.C., O.K., R.R.B., E.M.B.), Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts 02115

Address all correspondence and requests for reprints to: Edward M. Brown, M.D., Endocrine-Hypertension Division, Brigham and Women’s Hospital, 221 Longwood Avenue, Boston, Massachusetts 02115. E-mail: embrown{at}bics.bwh.harvard.edu

	Abstract

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The extracellular calcium (Ca²⁺_o)-sensing receptor (CaR) plays a critical role in maintaining Ca²⁺_o homeostasis in mammals by virtue of its presence in parathyroid gland and kidney. The breast is well recognized as a Ca²⁺-handling organ, and the effects of altering Ca²⁺_o on the proliferation of breast epithelial cells are well documented. To date there are no data regarding the expression and localization of CaR in breast tissue. In the present study, we assessed the distribution of CaR messenger ribonucleic acid (mRNA) and protein in normal and fibrocystic human breast tissue as well as in ductal carcinoma of the breast using RT-PCR, Northern analysis, and immunohistochemistry with CaR-specific antisera. In all tissues, RT-PCR performed using sense and antisense primers based on the sequence of the human parathyroid CaR complementary DNA amplified a product of the size expected (425 bp) for genuine CaR transcripts. Nucleotide sequencing of RT-PCR products confirmed more than 99% homology with human parathyroid CaR complementary DNA. Although insufficient quantities of mRNA were isolated from normal and fibrocystic tissue for Northern analysis, a single 5.2-kb CaR transcript was expressed in malignant breast tissue similar to the major CaR transcript in human parathyroid. Localization of CaR protein by immunohistochemistry showed specific CaR staining of the ductal epithelial cells of the breast in all three tissue types. These findings indicate the presence of CaR mRNA and protein in the breast, providing indirect evidence that the CaR may have some role(s) in the control of Ca²⁺ transport, epithelial cell proliferation, and/or other processes in normal and abnormal breast tissue.

	Introduction

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A CELL SURFACE, extracellular calcium (Ca²⁺_o)-sensing receptor (CaR) has been cloned from bovine parathyroid (1) and rat kidney (2), which is a key mediator of the direct effects of Ca²⁺_o on several of the tissues that are intimately involved in calcium homeostasis. Subsequently, the same receptor has been cloned from human parathyroid gland (3) and kidney (4), a rat thyroidal C cell line (5), and rat brain (6).

Ca²⁺_o plays an important role in the breast, not only in its normal physiological function, but in pathologic states as well. Ca²⁺_o is obviously an important constituent of milk, and lactating mothers produce breast milk containing about 200 mg Ca²⁺_o daily (7). A diagnostically important characteristic of breast cancer is the tendency to form microcalcifications (8). In addition, in vitro studies have shown that increases in Ca²⁺_o within the physiological range induce senescence and terminal differentiation of normal human breast epithelial cells in culture (9, 10). It is not known how Ca²⁺_o is involved in these diverse physiological processes of the breast.

The present study was undertaken to evaluate the expression of CaR in human breast. Specifically, we assayed for CaR transcripts by RT-PCR and Northern analysis in normal breast, fibrocystic breast tissue, and/or ductal carcinoma of the breast. To assess the degree of homology of CaR transcripts expressed in breast tissues with the human parathyroid CaR (HuPCaR) complementary DNA (cDNA), nucleotide sequencing was performed on RT-PCR products amplified from each tissue. In addition, immunohistochemistry was performed using antibodies directed against the extracellular domain of the CaR and provided further localization and confirmation of expression of the CaR in sections of normal, fibrocystic, and malignant human breast tissues.

	Materials and Methods

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Patients

Normal and fibrocystic tissues were obtained from two patients for each tissue type during surgery for various breast diseases. Ductal carcinoma tissue was procured during lumpectomy in two patients. This study was approved by the investigational review board at Brigham and Women’s Hospital (Boston, MA).

Tissues and reagents

Tissue fragments were frozen after surgical removal and stored at -70 C. Histopathological examination by light microscopy was routinely performed on each tissue sample using standard techniques. For immunohistochemistry, tissues were snap-frozen, embedded in OCT compound (Miles, Elkhart, IN), and maintained in 2-methylbutane at -70 C until further use.

Isolation of total ribonucleic acid (RNA)

Total RNA was isolated as described previously (1, 2, 11). Briefly, 1 g tissue was homogenized in 8 mL 4 mol/L guanidine isothiocyanate, 25 mmol/L sodium citrate, and 1.12 mg/mL ß-mercaptoethanol. The homogenate was layered onto 4 mL of a 5.7 mol/L CsCl-25 mmol/L sodium acetate cushion and sedimented at 32,000 rpm at 18 C for 18 h. The RNA pellet was resuspended in water and concentrated by precipitation with sodium acetate and ethanol.

RT-PCR of CaR transcripts

A 425-bp product was amplified from RNA isolated from breast tissues by RT-PCR using primers specific for the human parathyroid cDNA based on previously described methodology (5). Total RNA (5 µg) for each of the tissues was reverse transcribed at 42 C (1 h) using random primers and the avian myeloblastoma virus reverse transcriptase from the cDNA Cycle Kit (Invitrogen, San Diego, CA). Sense and antisense primers from the 5'-end of the human parathyroid CaR cDNA were used to amplify a 425-bp DNA fragment. The PCR buffer contained 29.75 µL water, 5 µL 10 Thermo DNA poly 10x reaction buffer (Promega), 3.75 µL MgCl₂ (25 mmol/L), 2 µL deoxynucleotide triphosphates (2.5 mmol/L each), 3 µL single-stranded cDNA, 3 µL each of the sense and antisense primers based on the sequence of the HuPCaR cDNA [primer 2009, 5'-CGGGGTACCTTAAGCACCTACGGCATCTAA-3' (bp 1745–1774); primer 2017, 5'-GCTCTAGAGTTAACGCGATCCCAAAGGG-CTC-3' (bp 2200–2230), respectively] and 0.5 µL Taq polymerase (2.5 U; Pharmacia LKB Biotechnology, Piscataway, NJ). Optimal cycling conditions were as follows: 92 C for 1 min, 55 C for 1 min, and 72 C for 1.5 min; cycling under these conditions was repeated 29 times, followed by a final extension step at 72 C for 8 min.

Nucleotide sequencing of CaR RT-PCR products

Primers 2009 and 2017 contained restriction sites for AflII and HpaI, respectively. Before sequencing, PCR products were digested with AflII and HpaI and ligated into the plasmid, pBluescript II SK^- (Stratagene, La Jolla, CA). Bidirectional sequencing was performed by the dideoxy chain termination method (2) using an Applied Biosystems model 373A automated sequencer (Department of Genetics, Children’s Hospital, Boston, MA). Further nucleotide and amino acid analyses were carried out using GeneWorks software (version 2.3.1, IntelliGenetics, Mountain View, CA).

Northern analysis of messenger RNA (mRNA) encoding for the CaR

Polyadenylated [poly(A)⁺] RNA selection was carried out using oligo(deoxythymidine)-cellulose chromatography (1, 2). Aliquots of 5 µg denatured poly(A)⁺ RNA were electrophoresed on 2.2 mol/L formaldehyde-1% agarose gels and transferred overnight to nylon membranes (Duralon, Stratagene) (1, 2). A 486-bp KpnI-XbaI fragment corresponding to nucleotides 1745–2230 of the HuPCaR cDNA was subcloned into the pBluescript SK⁺ vector (Stratagene). The plasmid was then linearized with KpnI, and a [³²P]UTP-labeled riboprobe was synthesized using the T3 polymerase site in the vector with the MAXIscript T3 kit (Ambion, Austin, TX). Nylon membranes were prehybridized for 2 h at 65 C in a solution consisting of 50% formamide, 4 x Denhardt’s solution (50 x Denhardt’s solution = 5 g Ficoll, 5 g polyvinylpyrrolidone, and 5 g BSA), 5 x SSPE (20 x SSPE = 2.98 mol/L NaCl and 0.02 mol/L ethylenediamine tetraacetate in 0.2 mol/L phosphate buffer, pH 7.0), 0.5% SDS, 25 mmol/L Tris (pH 7.5), 10.7 µg/mL yeast transfer RNA, and 250 µg/mL salmon sperm DNA. Labeled probe (2 x 10⁶ cpm/mL) was then added, and the membranes were hybridized overnight at the same temperature. Washing was carried out twice at low stringency (2 x SSC [20 x SSC = 3 mol/L NaCl and 0.3 mol/L Na₃-citrate(2H₂O)] and 0.1% SDS at room temperature) for 20–25 min and once at high stringency (0.1 x SSC-0.1% SDS at 65 C) for 15 min. ³²P-Labeled bands were quantified using a PhosphorImager (Molecular Dynamics, Sunnyvale, CA) (12).

Immunohistochemistry

Immunohistochemistry was performed using techniques modified from those described previously (12, 13, 14). Sections (5 µm thick) were prepared on a cryostat and were postfixed in acetone for 10 min at -20 C. Endogenous peroxidases were inhibited by incubating the sections in Dako peroxidase blocking reagent (Dako Corp., Carpenteria, CA) for 10 min, followed by treatment with Dako protein block serum-free solution (Dako Corp.) for 1 h. The sections were then incubated overnight at 4 C with 10 µg/mL primary anti-CaR antisera in blocking solution (Dako Corp.; either affinity-purified rabbit polyclonal antiserum 4637 raised against a peptide corresponding to amino acids 345–359 of the bovine parathyroid CaR or protein A-purified rabbit polyclonal antiserum 4641 raised against a peptide corresponding to residues 215–237 of BoPCaR). Characterization of these antisera has been detailed previously (12, 13, 14). In both cases, the corresponding sequences of the immunogenic peptides were identical in the human and bovine CaRs. Control sections were prepared by incubation with protein A-purified preimmune serum or with anti-CaR antiserum preabsorbed with synthetic CaR peptide (10 µg Ab/20 µg peptide for antiserum 4637; 10 µg Ab/40 µg peptide for antiserum 4641) against which the antibody was raised. After washing the sections three times with 0.5% BSA in PBS for 10 min each time, peroxidase-coupled, goat antirabbit IgG (1:100; Sigma) was added and incubated for 1 h at room temperature. The slides were then washed in PBS three times for 10 min each time, and the color reaction was developed using the Dako AEC Substrate System (Dako Corp.) for about 5 min. The color reaction was stopped by washing three times in water.

	Results

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Amplification and sequencing of RT-PCR products derived from human breast CaR

In RNA prepared from normal and fibrocystic breast tissue as well as from ductal carcinoma of the breast, RT-PCR amplified 425-bp DNA fragments of the expected size for CaR transcripts, as shown in Fig. 1A. These PCR products were of the same size as those obtained from the HuPCaR cDNA as a positive control. The primers that were employed spanned at least one intron, precluding amplification of products of the same size from contaminating genomic DNA; furthermore, no PCR products were amplified when the reverse transcriptase was omitted from the reaction (not shown). Nucleotide sequencing of RT-PCR products obtained from each tissue revealed more than 99% nucleotide identity with the corresponding HuPCaR clone, indicating that this PCR product was derived from bona fide CaR transcripts (Fig. 1B).

View larger version (38K): [in this window] [in a new window]   Figure 1. A, RT-PCR amplification of CaR-related sequences from human breast, performed as described in Materials and Methods. The size of the CaR product was estimated to be 425 bp, as indicated. Lane 1, One hundred-base pair DNA ladder; lane 2, PCR product from normal breast; lane 3, PCR product from fibrocystic breast; lane 4, PCR product from ductal carcinoma of the breast; lane 5, PCR product from HuPCaR cDNA. B, The nucleotide sequence of a PCR product amplified from breast tissue (designated HuBrCaR) is aligned with the sequence of the 4.0-kb HuPCaR cDNA clone (3). Mismatches are indicated by underlined letters. Primer sequences are represented as solid bars with arrowheads under the corresponding sequence of the CaR cDNA, and nucleotide numbering corresponds to the coding sequence of HuPCaR.

Northern analysis (Fig. 2) performed at high stringency on the poly(A)⁺ RNA isolated from the ductal carcinoma tissue that was used for RT-PCR revealed a single 5.2-kilobase (kb) transcript, similar in size to one of the principal transcripts observed in human parathyroid gland and kidney. The latter organs, however, expressed an additional 4.0-kb transcript in prior studies (3, 4).

View larger version (72K): [in this window] [in a new window]   Figure 2. Northern blot analysis of CaR transcripts in human breast ductal carcinoma. Total RNA was extracted, poly(A)+ enriched, and analyzed by Northern analysis with a 32P-labeled riboprobe derived from the 5'-region of HuPCaR as described in Materials and Methods. Lane 1, Patient 1; lane 2, patient 2. The arrowhead indicates the major transcript at 5.2 kb.

Immunohistochemistry using two specific anti-CaR antisera directed at highly conserved epitopes within the extracellular domain identified strong CaR immunoreactivity along the epithelial cells lining the ducts of the breast as well as in neoplastic breast epithelial cells in two separate ductal carcinomas (Fig. 3). Staining was abolished by preincubation of the antibody with an excess of the peptide against which the respective antisera were raised, thereby confirming the specificity of the reaction.

View larger version (128K): [in this window] [in a new window]   Figure 3. Immunohistochemical staining of human breast tissue with a specific polyclonal antiserum directed against the extracellular domain of the CaR, performed as described in Materials and Methods. A, Normal breast tissue at x40 magnification. B, Normal breast tissue at x100 magnification. C, Fibrocystic breast tissue at x40 magnification. D, Ductal carcinoma at x40 magnification. E, Ductal carcinoma at x100 magnification. F, A representative negative control from fibrocystic breast tissue using antiserum preabsorbed with the peptide against which it was raised. Similar results were observed using a second anti-CaR antiserum directed against a different epitope within the receptor’s extracellular domain as described in Materials and Methods.

	Discussion

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Our Northern analysis clearly demonstrates that CaR mRNA is present in the breast. The 5.2-kb transcript is similar in size to one of the major transcripts seen in human parathyroid. The parathyroid gland, however, expresses an additional 4.0-kb band. It is interesting to note that the sequence of the PCR product that was amplified from reverse transcribed CaR transcripts in all three types of breast tissue, including the 5.2-kb transcript observed in ductal carcinoma tissue, was the same as that of the 4.0-kb HuPCaR cDNA. The latter exhibits an in-frame 30-nucleotide deletion compared to the corresponding region of the sequence of the 5.2-kb clone isolated from human parathyroid (3). As the functions of the CaRs encoded by the 4.0- and 5.2-kb human parathyroid cDNA clones exhibited no obvious differences when expressed in Xenopus laevis oocytes (3), the 30-nucleotide deletion present in the transcripts amplified from human breast tissue by RT-PCR is of uncertain significance.

By immunohistochemistry we demonstrated the presence of CaR protein in all three breast tissue types. Strong immunostaining was observed along the epithelial cells of the ducts in normal and fibrocystic breast tissue. Although there is a dearth of data about the regulation of the transport of Ca²⁺ into milk, the presence in the receptor in ductal cells raises the possibility of a role for the CaR in such transport processes. Furthermore, metastatic cancer of the breast is known to spread readily to bone (15). We have recently shown that numerous normal cells within the bone marrow express the CaR (16). Perhaps the presence of the CaR on normal and malignant mammary cells contributes to the tendency of these and other malignant cells to metastasize to bone, in which there are high local levels of Ca²⁺_o related to bone resorption (17).

In summary, our data demonstrate the presence of CaR mRNA and protein in normal and malignant human breast tissue. Given the importance of Ca²⁺_o in the breast, it is not difficult to imagine that the CaR may play diverse roles in the physiology and pathophysiology of breast tissue. Further studies are required to evaluate the role(s) of the CaR in the ductal epithelial cells shown to express it in this study.

	Footnotes

 
¹ This work was supported by grants from the Howard Hughes Medical Institute (Research Training Fellowship for Medical Students to I.C.), the NIH (DK-44571–02 to D.I.S.; DK-41415 and DK-48330 to E.M.B.), the St. Giles Foundation (to E.M.B.), The Stanley Foundation (to E.M.B.), and NPS Pharmaceuticals (to E.M.B.).

Received September 3, 1997.

Revised October 6, 1997.

Accepted October 28, 1997.

	References

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This Article Abstract Full Text (PDF) Submit a related Letter to the Editor Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Cheng, I. Articles by Brown, E. M. Search for Related Content PubMed PubMed Citation Articles by Cheng, I. Articles by Brown, E. M.