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. Author manuscript; available in PMC: 2021 Feb 22.
Published in final edited form as: Vet Pathol. 2015 Mar 2;53(1):170–181. doi: 10.1177/0300985815571680

F344/NTac Rats Chronically Exposed to Bromodichloroacetic Acid Develop Mammary Adenocarcinomas With Mixed Luminal/Basal Phenotype and Tgfβ Dysregulation

Janice B Harvey 1,8, Hue-Hua L Hong 1, Sachin Bhusari 1, Thai-Vu Ton 1, Yu Wang 1,2, Julie F Foley 1,2, Shyamal D Peddada 3, Michelle Hooth 4, Michael DeVito 5, Abraham Nyska 6, Arun R Pandiri 1,7, Mark J Hoenerhoff 1,*
PMCID: PMC7899196  NIHMSID: NIHMS1666889  PMID: 25732176

Abstract

Breast cancer is the most common cancer, and the second-leading cause of cancer mortality in women in the United States. A recent 2-year National Toxicology Program (NTP) carcinogenicity study showed an increased incidence of proliferative mammary lesions (hyperplasia, fibroadenoma, adenocarcinoma) in F344/NTac rats exposed to bromodichloroacetic acid (BDCA), a disinfection byproduct in finished drinking water with widespread human exposure. We hypothesized that the increase in mammary tumors observed in BDCA-exposed F344/NTac rats may be due to underlying molecular changes relevant for human breast cancer. The objective of the study was to compare gene and protein expression, and mutation spectra of relevant human breast cancer genes between normal untreated mammary gland and mammary tumors from control and BDCA-exposed animals to identify molecular changes relevant for human cancer. Histologically, adenocarcinomas from control and BDCA-exposed animals were morphologically very similar, were estrogen/progesterone receptor positive, and displayed a mixed luminal/basal phenotype. Gene expression analysis showed a positive trend in the number of genes associated with human breast cancer, with proportionally more genes represented in the BDCA-treated tumor group. Additionally, a five-gene signature representing possible Tgf-β pathway activation in BDCA-treated adenocarcinomas was observed, suggesting that this pathway may be involved in the increased incidence of mammary tumors in BDCA-exposed animals.

Keywords: breast cancer, basal, F344/NTac rat, gene expression, luminal, transforming growth factor-beta, environmental pollutants

Introduction

Breast cancer is the most common cancer and the second most common cause of cancer-related death in women in the United States with an estimated 232,340 new cases and 39,620 deaths in 2013.3,57 Up to 10% of breast cancers are characterized as hereditary,37 while the remaining majority of tumors (90–95%) are sporadic, involving complex and multifactorial etiologies, including dietary, hormonal, environmental, and lifestyle factors.38 The complexity of this very heterogeneous disease is exemplified by the marked complexity of the molecular changes accompanying its pathogenesis.30,64 Prognosis is typically based on a variety of factors, including hormone receptor status and luminal/basal immunophenotype of the tumor cells. Specifically, tumors that lack expression of hormone receptors (estrogen receptor-alpha (ERα), progesterone receptor (PR) and those that express predominantly basal markers (cytokeratin (CK) 5, CK14, alpha-smooth muscle actin (α-SMA)) are associated with a particularly poor prognosis, whereas those that retain hormone receptor function and have a luminal phenotype (CK8, CK18, CK19) have a more favorable prognosis.11,25,26

Since the pathogenesis of the majority of breast cancers is multifactorial, the impact of environmental exposures that pose a potential human health hazard is of critical importance. Environmental factors are an overwhelming contributor to the causation of cancer in humans.36 A number of environmental contaminants and occupational exposures have been linked to an increased risk of mammary cancer in humans and/or rodent models.24,66 To date, the National Toxicology Program (NTP) has identified 51 chemicals associated with mammary tumors in rodents, with 37 showing clear evidence of carcinogenicity in either the rat or mouse (http://ntp.niehs.nih.gov/go/SA-39).47 Five chemicals were positive in both species (acrylamide, chloroprene, 1,2-dibromomethane, 1,2-dichloroethane, glycidol, and sulfallate). Chemically induced mammary gland tumors generally occurred more commonly in the rat, which is particularly relevant because many aspects of mammary carcinogenesis in the rat are similar to those in humans.15,25,56 A recent two-year NTP carcinogenicity study showed a dose dependent increase in the incidence of proliferative mammary lesions (hyperplasia, fibroadenoma, adenocarcinoma) in F344/NTac rats exposed to bromodichloroacetic acid (BDCA) in drinking water (NTP TR 583).50 The F344/NTac rat is a substrain of the F344/N rat, and was used by the NTP in the BDCA chronic bioassay. We consider both the F344/N and F344/NTac to be closely related, and the findings in this study apply to both rat strains. These strains have very similar genetic backgrounds since they are derived from the same founder strain. Following a workshop with input from several external experts, the NTP chose to continue the use of the F344/NTac rat in chronic bioassays of some chemicals, including the current BDCA study, because this rat strain is derived from the same parent stock and is genetically closest to the F344/N rat strain.32 So, while there is a possibility that the spontaneous tumor incidence and underlying molecular mechanisms may differ between these two closely related rat strains, their very similar genetic backgrounds minimize this possibility.

BDCA is a disinfection by-product that is a contaminant of finished drinking water, formed by the reaction of oxidizing agents containing chlorine with naturally occurring organic material and bromide ions in source water,54 and is one of the most common disinfection by-products in surface drinking water supplies in the United States.34 BDCA is therefore widespread in drinking water sources, and individuals are exposed to this compound in tap water through oral and dermal routes from oral consumption, cooking, bathing, and swimming.50 A similar compound, bromochloroacetic acid (BCAA), which is also a byproduct in the disinfection of drinking water, was previously shown to induce mammary tumors in both male and female F344/N rats and B6C3F1 mice.49 BDCA is not one of the haloacetic acids currently regulated by the Environmental Protection Agency (EPA),50 and due to potential widespread human exposure, determination of whether they pose a possible human health risk is critical.

Sporadic breast cancer in humans follows a paradigm of progressive acquisition of various molecular alterations including loss of tumor suppressor genes and overexpression of oncogenes and growth pathways in the progression from premalignant lesions to invasive cancer.4,31 Fibroadenomas are the most common tumor in the mammary gland of humans and rats, and it is suspected that some mammary adenocarcinomas may arise from fibroadenomas in both species.40 In National Toxicology Program studies, the F344/N rat develops a background rate of mammary fibroadenomas and adenocarcinomas of 51.4% and 3.9%, respectively.48 According to the Registry of Industrial Toxicology Animal (RITA) database, an industry sponsored database of peer-reviewed historical control tumor incidences, background mammary tumors (adenocarcinoma, adenocarcinoma arising in fibroadenoma, adenoma, fibroadenoma) are also fairly common in the Sprague-Dawley (58%) and Wistar rats (24%).22 Separating spontaneous lesions from treatment-related lesions can be difficult based upon these background incidences, particularly in studies in which a treatment-related tumor response is not obvious, since the level of spontaneous lesions present in treated groups is typically dictated by the number of spontaneous lesions in control animals.39 Thus, identification of molecular markers that separate spontaneous from chemically induced tumors is of great importance.

The identification of genetic alterations in chemically induced rodent tumors that are similar to those observed in human cancer lends further support that they are of potential human health risk.24 For example, overexpression and/or mutation of the epidermal growth factor receptor (EGFR) is associated with cell proliferation, angiogenesis and tumor progression in human breast cancer.16 Similarly, loss of function mutations in the phosphatase and tensin homologue (PTEN) tumor suppressor gene occur in a variety of human neoplasms including breast cancer, and are associated with hormone resistance.43 TP53 mutations are also common in triple negative breast cancer and are associated with epithelial-mesenchymal transition.9 The constitutive activation of the RAS signaling pathway is one of the most common alterations in human breast cancer, and while mutation of the HRAS gene is uncommon, overexpression of the protein is found in 60–70% of cases.44 Finally, CTNNB1 plays an important role in normal development as well as neoplastic transformation in the rodent and human mammary gland, and is often associated with hormone receptor negative breast cancers in people.27

Given the increased incidence of mammary tumors in F344/NTac rats exposed to BDCA, and the widespread human exposure in drinking and bathing water, the objective of this study was to 1) immunophenotype mammary tumors from control and BDCA-exposed animals based on human criteria, and 2) identify molecular changes in BDCA-exposed rats that are of relevance for human cancer risk. We hypothesized that the increased incidence of mammary tumors in BDCA-exposed animals may be associated with alterations in molecular pathways that are relevant for human cancer, including breast cancer. Identification of such relevant alterations would further strongly suggest that chronic exposure to this chemical poses a human cancer risk.

Materials and Methods

Animals, histology, and immunohistochemistry:

In the 2-year NTP bioassay, dose groups of 50 male and 50 female F344/NTac rats were exposed to 0, 250, 500, and 1000mg/L BDCA per dose group in drinking water. Routine tissues and tumors, including adenocarcinomas and fibroadenomas from BDCA exposed female F344/NTac rats, were collected at the end of the bioassay. Mammary gland lesions were diagnosed based on NTP standardized nomenclature and criteria.5 Tumor incidence and statistics were performed as previously described.50 Collection of paired frozen tissue from mammary tumors was performed based on incidence, size (>0.5cm in diameter) and viability (minimal hemorrhage and necrosis) criteria; at necropsy, tumors were sectioned in half, one half was flash frozen in liquid nitrogen for gene expression analysis, and the other half was fixed in 10% neutral buffered formalin (NBF) for histology and immunohistochemistry (IHC). While there was adequate spontaneous fibroadenoma frozen tissue available, there was a lack of frozen spontaneous adenocarcinoma tissue available from this study due to the lower tumor incidence. Therefore, spontaneous adenocarcinomas were collected from five age-matched vehicle control female rats from other chronic (2-year) NTP studies in F344/N rats (nickel sulfate hexahydrate; D&C Yellow #11; 2,2-bis(bromomethyl)-1,3,propanedial; 3,3`,4,4`-tetrachloroazobenzene). In addition, frozen vehicle-control mammary gland was not available from the BDCA study, so mammary glandular units (epithelium, myoepithelium, stroma) were collected by laser microdissection from five age-matched female F344/N rats obtained from the National Institute of Aging (NIA). Since normal mammary gland is predominantly adipose tissue, this sampling was considered to more appropriately represent the predominant cellular populations in normal mammary tissue that would be comparable to those in mammary tumor tissue. FFPE tumor samples were selected for immunohistochemistry based on the availability of paired collected frozen tissue, such that each formalin-fixed, paraffin-embedded (FFPE) tumor could be matched to its respective frozen sample used for gene expression analysis. Furthermore, it was necessary to use paired FFPE samples for immunohistochemistry as these tissues are processed immediately; FFPE samples not paired with frozen samples were subject to prolonged fixation (>4wks) and were thus not acceptable for immunohistochemistry. Seven FFPE normal mammary gland samples, eight FFPE mammary adenocarcinomas (four BDCA-treated, four spontaneous), and seven FFPE fibroadenomas (four BDCA-treated, three spontaneous) paired with frozen samples were used for immunohistochemistry based on these criteria. Samples collected for gene expression analysis, mutation analysis, and immunohistochemistry are listed in Supplemental Table 1. Paraffin-embedded samples were sectioned at 5μm and stained with hematoxylin and eosin or immunostained with the following antibodies using routine methods: mouse monoclonal anti-ERα (1:50, Beckman Coulter Inc, Brea CA), mouse monoclonal anti-PR (1:150, Beckman Coulter Inc, Brea CA), rabbit polyclonal anti-SMA (1:150, Abcam, Cambridge MA), mouse monoclonal anti-CK14 (1:50, Abcam, Cambridge MA) and mouse monoclonal anti-CK18 (1:800, Santa Cruz Biotechnology, Santa Cruz CA). Normal rat endometrium was used as a positive control for ERα, PR, SMA, and CK18, and normal rat skin was used as a positive control for CK14. Substitution of the primary antibody with normal serum from the species in which the primary antibody was raised was used for negative controls. Slides were washed, then labeled using the streptavidin ABC technique for detection of the primary antibody-specific protein. 3,3-diaminobenzidine (DAB) was used to visualize all immune reactions, and slides were counterstained with Mayer’s hematoxylin. The sections were dehydrated through graded alcohols, immersed in xylene, and mounted with coverslips. Positive immunoreactivity to ERα and PR was defined as nuclear staining, and cytoplasmic and membrane immunoreactivity was considered positive for CK14, CK18, and SMA.

Real Time Quantitative PCR Arrays and Data Analysis:

RNA was extracted from frozen samples from female rats (5 laser-captured samples of mammary epithelium from vehicle control, age-matched mammary gland, three spontaneous fibroadenomas from vehicle control rats, four fibroadenomas from BDCA-exposed rats, five spontaneous adenocarcinomas from vehicle control rats, and four adenocarcinomas from BDCA-exposed rats) using the Invitrogen TRIzol Kit (Supplemental Table 1). Fold increases and decreases in gene expression were determined by quantification of cDNA from spontaneous and BDCA-treated tumors relative to normal mammary epithelium from age-matched controls. The 18s RNA gene was used as the endogenous control for normalization of initial RNA levels. A rat PCR array (PARN-131Z, SA Biosciences, Frederick MD) representing 84 genes important in breast cancer pathogenesis was used to identify differential gene expression between spontaneous and BDCA-treated tumors. Quantitative differential gene expression levels were detected using arrays containing corresponding PCR primers and SABiosciences SYBR Green qPCR master mix, and the reactions were run on an ABI PRISM 7900HT Sequence Detection System (Foster City, CA) using the manufacturer’s protocols. Gene expression was normalized to Actb, and fold changes were calculated using the ΔΔCt method.53 To examine the respective gene changes in the progression from normal to benign (fibroadenoma) to malignant (adenocarcinoma) tumor, a trend analysis was performed to evaluate for an increasing or decreasing trend in gene expression. In addition, pairwise comparisons were performed for spontaneous and BDCA-treated adenocarcinomas (controlling for directional errors), using a residual bootstrap-based methodology17 to compute p values using 10,000 bootstrap samples. The statistical significance was obtained by controlling the false discovery rate (FDR) at a nominal 5% level (FDR < 0.05). Trend tests and pairwise comparisons were performed using ORIOGEN, version 4.01.19,51,52 Principal component analysis (PCA) and unsupervised hierarchical cluster analysis (HCA) were performed using Partek Genomics Suite, version 6.3 (St. Louis, MO).

Gene Mutation Analysis:

Using a DNeasy Tissue Kit (Qiagen, Valencia CA), DNA was isolated from FFPE sections of 10 mammary fibroadenomas (five spontaneous, five BDCA-treated) and 13 adenocarcinomas (five spontaneous, eight BDCA-treated) from BDCA-exposed female rats (Supplemental Table 1). Amplification reactions were carried out by semi-nested PCR using primer sets for rat Egfr (exons 18–21), Pten (exons 1–9), Tp53 (exons 5–8), Ctnnb1 (exon 2), and Hras (exon 2) (Supplemental Table 2). Controls lacking DNA were run with all sets of reactions. PCR products were sequenced with an automatic sequencer.

Results

Exposure of F344/NTac rats to BDCA in drinking water for two-years is associated with induction of mammary hyperplasia, fibroadenoma, and adenocarcinoma.

In the original 2-year NTP bioassay, exposure of 50 female F344/NTac rats to BDCA at doses of 250, 500, and 1000mg/L per dose group resulted in a treatment-related increase in the incidence of mammary gland hyperplasia, fibroadenoma, and carcinoma50 (Table 1). Approximately 56% of vehicle control female rats developed spontaneous fibroadenomas. In contrast to the normal vehicle control mammary gland (Figure 1), mammary hyperplasia was characterized by proliferation of glandular acini resulting in irregular nodules separated by fine fibrovascular stroma (lobular) and/or proliferation of ductal epithelial cells into multiple layers or papillary projections (ductal) (Figure 2). Mammary fibroadenomas were expansile, well demarcated proliferations of tubules and acini of well-differentiated glandular epithelial cells separated by abundant fibrous stroma (Figure 3). Mammary adenocarcinomas were characterized by infiltrative, generally poorly demarcated proliferations of solid lobules and irregular tubules and acini composed of poorly-differentiated glandular epithelial cells within scant fibrovascular stroma (Figure 4), often associated with variable necrosis and inflammatory cells.

Table 1.

Proliferative Mammary Gland Lesions in Control and BDCA-Treated Female Rats in the 2-year NTP Bioassaya

Dose (mg/L drinking water) 0 250 500 1000
Number examined 50 50 50 50
Hyperplasia 0 4 [1.3]b 2 [1.5] 10* [1.2]
Adenoma 1 (2%) 2 (4%) 3 (6%) 1 (2%)
Fibroadenoma, multiple 6 (12%) 34* (68%) 37* (74%) 27* (54%)
Fibroadenoma (including multiple) 28 (56%) 47 (94%)* 47 (94%)* 39 (78%)*
Adenocarcinoma 0 1 (2%) 3 (6%) 8 (16%)*
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a

Female F344/NTac rats were exposed to 0, 250, 500, or 1000mg/L bromodichloroacetic acid (BDCA) in drinking water for 2 years.40

b

Mean severity grade (1=minimal, 2=mild, 3=moderate, 4=severe).

*

Significantly different from controls (p < 0.001) using the Poly-3 test.40

Figure 1. Mammary gland, bromodichloroacetic acid-treated rat.

Figure 1.

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Normal mammary gland composed of glandular lumens lined by a single layer of well-differentiated glandular epithelium, often containing amorphous eosinophilic proteinaceous material. HE.

Figure 2. Mammary gland, bromodichloroacetic acid-treated rat.

Figure 2.

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Mammary hyperplasia, characterized by increased acini lined by multiple layers of plump epithelial cells. HE.

Figure 3. Mammary gland, bromodichloroacetic acid-treated rat.

Figure 3.

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Mammary fibroadenoma, composed of well-differentiated glandular structures separated by abundant fibrous stroma. HE.

Figure 4. Mammary gland, bromodichloroacetic acid-treated rat.

Figure 4.

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Mammary adenocarcinoma, characterized by an infiltrative proliferation of lobules, acini, and tubules of poorly differentiated glandular epithelial cells. Inset: higher magnification shows the cellular atypia and increased mitotic rate of epithelium in mammary adenocarcinoma. HE.

Mammary adenocarcinomas in F344/NTac rats harbor a mixed luminal/basal and hormone receptor immunophenotype and mutations observed in human breast cancer.

Spontaneous and BDCA-treated mammary adenocarcinomas were hormone receptor (ERα/PR) positive and immunoreactive to both luminal (CK18) and basal (CK14, α-SMA) markers (Table 2). Normal mammary gland (Figure 5a) and BDCA-treated fibroadenomas (Figure 6a) were negative for ERα, while 4/4 (100%) of BDCA-treated adenocarcinomas were positive (Figure 7a). Spontaneous fibroadenomas were negative for ERα, while 3/5 (40%) spontaneous adenocarcinomas were positive for ERα. Normal mammary gland shows a few positive cells for PR (Figure 5b), and 3/4 BDCA-treated fibroadenomas (Figure 6b) were negative for PR. In contrast, 3/4 BDCA-treated adenocarcinomas were positive for PR (Figure 7b). Two of 3 spontaneous fibroadenomas and 5/5 spontaneous adenocarcinomas were PR positive. In general, the number of cells immunoreactive for PR in each tumor was fewer than that for ERα in both spontaneous and BDCA-treated fibroadenomas and adenocarcinomas.

Table 2.

Results of Immunohistochemical Analysis of Mammary Tumors From Control and BDCA-Exposed Rats.

ER PR SMA CK14 CK18
Control Fibroadenomas 0/3 2/3 (67%) 5/5 (100%) 1/5 (20%) 5/5 (100%)
Adenocarcinomas 2/5 (40%) 5/5 (100%) 3/5 (60%) 5/5 (100%) 1/5 (20%)
BDCA-treated Fibroadenomas 0/4 1/4 (25%) 5/5 (100%) 0/5 5/5 (100%)
Adenocarcinomas 4/4 (100%) 3/4 (75%) 10/13 (77%) 11/13 (85%) 12/13 (92%)
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Figure 5. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 5.

Figure 5.

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Normal mammary gland is negative for estrogen receptor α (5a) and a few nuclei are positive for progesterone receptor (5b).

Figure 6. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 6.

Figure 6.

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Fibroadenomas are negative for estrogen receptor α (6a) and progesterone receptor (6b).

Figure 7. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 7.

Figure 7.

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Mammary adenocarcinomas show multifocal positive nuclear immunoreactivity to estrogen receptor α (7a) and progesterone receptor (7b)..

In control and BDCA-exposed animals, myoepithelium of normal mammary gland (Figure 8) and fibroadenomas (Figure 9) were positive for SMA. Glandular epithelial cells showed very faint immunoreactivity to SMA. In contrast, the basal epithelial layers in a majority of spontaneous (3/5) and BDCA adenocarcinomas (10/13) was strongly positive (Figure 10) for SMA. Normal mammary gland (Figure 11) and spontaneous fibroadenomas (Figure 12) were negative for CK14, and 1/5 fibroadenomas from BDCA-exposed animals showed focal expression for CK14. In contrast, 5/5 spontaneous adenocarcinomas and 11/13 adenocarcinomas from BDCA-exposed animals were diffusely positive (Figure 13). Luminal epithelial cells of normal mammary gland (Figure 14), all spontaneous and BDCA fibroadenomas (Figure 15), 1/5 spontaneous adenocarcinomas and 12/13 adenocarcinomas from BDCA-exposed animals (Figure 16) were positive for CK18.

Figure 8. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 8.

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Smooth muscle actin (SMA) immunoreactivity is restricted to myoepithelium of glandular acini in normal mammary gland. Inset: there is positive labeling in myoepithelium (arrowheads) of normal glandular units.

Figure 9. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 9.

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Positive immunoreactivity to SMA in myoepithelial cells surrounding glandular acini of a fibroadenoma, and faint cytoplasmic immunoreactivity in glandular epithelial cells.

Figure 10. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 10.

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Mammary adenocarcinomas show diffuse membrane and cytoplasmic labeling for SMA in basal layers of neoplastic epithelium.

Figure 11. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 11.

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Normal mammary epithelium is negative for CK14.

Figure 12. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 12.

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Fibroadenomas are negative for CK14.

Figure 13. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 13.

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Mammary adenocarcinomas show diffuse positive membrane and cytoplasmic labeling in basal cell layers for CK14.

Figure 14. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 14.

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Normal mammary gland shows positive CK18 immunoreactivity in luminal epithelial cells.

Figure 15. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 15.

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There is diffuse immunoreactivity for CK18 in luminal epithelial cells of fibroadenomas.

Figure 16. Mammary gland, bromodichloroacetic acid-treated rat. IHC.

Figure 16.

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Luminal epithelial layers of adenocarcinomas are diffusely positive for CK18.

Mammary adenocarcinomas in F344/NTac rats harbor low incidence of mutations observed in human breast cancer.

There were no mutations observed in Tp53, Pten, or Egfr in any of the spontaneous fibroadenomas evaluated. Interestingly, there were Tp53 mutations in 4/5 BDCA-treated fibroadenomas evaluated; however, these were synonymous mutations, in which a single base pair change did not result in a change in the resulting amino acid sequence (data not shown). Of the five spontaneous adenocarcinomas evaluated by mutation analysis, two harbored Tp53 mutations in exon 5 (Table 3), and of the eight BDCA-treated adenocarcinomas, one mid-dose animal harbored a mutation in exon 8 of Tp53 and also in exon 19 of Egfr, and two high-dose animals each had mutations in exon 7 of Pten and exon 20 of Egfr.

Table 3.

Summary of Gene Mutations in Mammary Adenocarcinomas from Control and BDCA Exposed Rats.a

Sample No. Dose (mg/L) Tp53 mutation Pten mutation Egfr mutation
Exon 5 Exon 8 Exons 1–9 Exons 18–21
I033 0 NMb NM NM NDc
LF325 0 Cdn155: GGT→TGT (Arg→Cys) NM NM NM
DF427 0 Cdn178: CGT→TGT (Arg→Cys) NM NM NM
DF401 0 NM NM NM NM
250 0 NM NM NM NM
559 250 ND ND NM NM
649 500 NM NM NM NM
647 500 NM NM NM NM
629 500 NM Cdn317: AAA→AAG (Lys→Lys) NM Exon19 Cdn739: GTG→ATG (Val→Met)
718 1000 NM NM Exon7 Cdn230: CCA→AGA (Gly→Arg) NM
749 1000 NM NM NM NM
741 1000 NM NM NM Exon20 Cdn805: GAA→AAA (Glu→Lys)
708 1000 ND ND NM NM
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a

F344/N Tac rats were treated with bromodichloroacetic acid (BDCA) at 0, 250, 500, or 1000mg/L in drinking water for 2 years.

b

NM=No Mutation

c

ND=Not Determined

No mutation at Tp53 exons 6 & 7, Hras exon 2 codon 61, and Ctnnb1 exon 2 for all the samples examined.

Spontaneous and BDCA-treated adenocarcinomas show gene expression alterations related to pathways of tumorigenesis in human breast cancer.

When comparing normal mammary gland, spontaneous fibroadenoma, and spontaneous adenocarcinoma, trend analysis reported a statistically significant upward (positive) trend in 37/84 genes, and a statistically significant downward (negative) trend in 15/84 genes (FDR < 0.05) (Table 4). In BDCA-exposed animals, trend analysis reported 49/84 genes with an upward trend, and 10/84 genes with a downward trend, when making the same comparisons. Spontaneous and BDCA-treated mammary tumors shared 34 of the upward trending genes, and nine of the downward trending genes. Three upward trending genes on the array were exclusive to spontaneous tumors, while 15 upward trending genes associated with breast cancer were exclusive to BDCA-treated tumors. Conversely, there were 6 genes exclusively trending downward in spontaneous tumors, while only one gene (Brca2) was exclusively trending downward in BDCA-treated tumors. While spontaneous and BDCA-treated tumors shared a majority of genes in terms of directionality of trend, BDCA-treated tumors had proportionally more genes that were exclusively upward trending, and conversely spontaneous tumors had proportionally more genes exclusively trending downward.

Table 4.

Significantly Altered* Genes by Trend Analysis in Fibroadenomas and Adenocarcinomas from Control and BDCA-Exposed Rats.

Positive Trend
 Shared Akt1, Apc, Atm, Ccnd1, Ccnd2, Cdh1, Cdk2, Cdkn1a, Csf1, Ctnnb1, Ctsd, Erbb2, Igfbp3, Jun, Krt18, Krt19, Krt8, Mapk8, Mki67, Myc, Nme1, Notch1, Nr3c1, Plau, Prdm2, Pten, Rassf1, Serpine1, Sfrp1, Slc39a6, Src, Tp53, Vegfa, Xbp1
 Spontaneous Birc5, Pgr, Pycard
 BDCA-induced Cdh13, Cdkn2a, Cst6, Gata3, Grb7, Id1, Mgmt, Mmp2, Mmp9, Sfn, Slit2, Snai2, Tgfb1, Thbs1, Twist1
Negative Trend
 Shared Abcb1a, Adam23, Ccna1, Egf, Esr2, Gli1, Rb1, Tff3, Tp73
 Spontaneous Gstp1, Hic1, Il6, Mmp2, Mmp9, Ptgs2
 BDCA-induced Brca2
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*

Significantly altered at FDR < 0.05

Genes that showed a statistically significant upward trend that were shared by both spontaneous and BDCA-treated mammary lesions (Table 4) included those involved in signal transduction including the PI3K/AKT, WNT, Notch, and MAPK pathways (Akt, Apc, Ctnnb1, Sfrp1, Notch1, Mapk8), glucocorticoid signaling (Igfbp3, Nme1, Nr3c1), cell cycle (Ccnd1, Ccnd2, Cdk2, Cdkn1a, Ki67, Myc, Pten, Rassf1, Tp53), cell adhesion (Cdh1, Csf1), DNA damage (Atm), matrix remodeling (Ctsd, Plau), epithelial-mesenchymal transition (Src, Notch1), angiogenesis (Vegfa), and luminal tumor classification markers (Krt8, Krt18, Krt19, Erbb2, Slc39a6, Xbp1). Genes that were found to trend upward exclusively in spontaneous tumors included Birc5 (anti-apoptosis, basal marker), Pgr (hormone receptor), and Asc (matrix remodeling). Conversely, there were several more genes exclusively trending upwards in BDCA-treated tumors, including genes associated with cell cycle (Cdkn2a, Sfn), matrix remodeling (Mmp2, Mmp9, Cst6), adhesion, angiogenesis, and epithelial-mesenchymal transition (Cdh13, Id1, Slit2, Tgfb1, Thbs2, Twist), hedgehog signaling (Snai2), and both luminal (Gata3) and basal (Grb7) tumor markers.

Those genes shared between spontaneous and BDCA-treated tumors with a statistically significant downward trend (Table 4) included xenobiotic transporters (Abcb1a), matrix remodeling (Adam23), cell cycle (Ccna1, Rb1), angiogenesis (Egf), Hedgehog signaling (Gli1), DNA damage (Tp73) and luminal tumor markers (Tff3).

BDCA-treated mammary adenocarcinomas are associated with upregulation of genes associated with the Tgfβ pathway, tumor progression and invasion

Pairwise comparison of spontaneous and BDCA-treated mammary adenocarcinomas was performed to identify differentially expressed genes relevant for human breast cancer. This analysis showed that of the 84 genes included on the PCR array, eight genes were significantly (p < 0.05) upregulated in BDCA-treated mammary adenocarcinomas compared to spontaneous tumors, five of which are associated with increased matrix remodeling, tumor progression, invasion, metastasis and Tgfβ pathway signaling (Mmp2, Mmp9, Id1, Vegfa, Thbs1) (Table 5).

Table 5. –

Significantly Upregulated* Genes Associated With Tgfβ Signaling, Breast Cancer Growth, Invasion, and Metastasis in BDCA-Treated Mammary Adenocarcinomas.a

Gene Fold change p-value Function Role in breast cancer References
Mmp9 11.232 0.0164 Extracellular matrix remodeling Cancer growth, migration and invasion Cupic et al., 2011
Mmp2 4.69 0.011 Extracellular matrix remodeling Cancer growth, migration and invasion Cupic et al., 2011
Id1 3.055 0.0154 Inhibition of differentiation Tumor invasion, metastasis, EMT Tobin et al., 2011
Vegfa 2.4 0.0381 Proangiogenic factor Cancer growth and metastasis Breier et al., 2002
Thbs1 2.04 0.0307 Extracellular matrix remodeling Tumor growth and metastasis Roberts 1996
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*

Significantly altered at FDR < 0.05

a

Compared to spontaneous adenocarcinomas based on pairwise comparison.

Discussion

Studies have shown that a majority of human cancer is due environmental factors.36 Whether environmental contaminants, occupational exposures, or nutritional/lifestyle factors, an overwhelming number of environmental exposures have been linked to an increased risk of mammary cancer in humans and/or rodent models.24,66 These environmental exposures pose a significant risk to human populations, not only because of their detrimental effects on human health, but also because numerous compounds are unregulated. The current 2-year chronic rat bioassay confirmed the carcinogenic effects of the environmental contaminant BDCA, with increased incidence of mammary hyperplasias, fibroadenomas, and adenocarcinomas. These findings resulted in a conclusion of clear evidence of carcinogenic activity of BDCA in female F344/NTac rats.50 Additional findings supporting clear evidence of carcinogenicity in F344/NTac rats included increased incidences of malignant mesothelioma and cutaneous epithelial tumors.50 There are no studies in the current literature which report carcinogenic activity of BDCA; however, a variety of reports describe carcinogenic activity of other haloacetic acids.12–14,42 One related haloacetic acid, bromochloroacetic acid, caused increased incidence of fibroadenoma of the mammary gland of rats.49 No studies report carcinogenic effects of BDCA in humans; however several have shown a link between bladder, rectal, and brain cancer risk and exposure to water disinfection by-products.7,41,46 This is the first report to assess the molecular phenotype of BDCA-induced mammary tumors in rats in the context of human breast cancer, and suggests that BDCA may be an environmental cause of human breast cancer. This is of particular importance because BDCA is not currently regulated by the Environmental Protection Agency (EPA),50 and due to potential widespread human exposure, determination of the possible human health risk is critical.

By comparing the gene and protein expression of spontaneous adenocarcinomas from other NTP studies to that of BDCA-treated lesions, we were able to perform a focused analysis to 1) characterize differences in luminal/basal phenotype 2) examine hormone status, and 3) evaluate differential gene expression, of these lesions in BDCA-treated F344/NTac rats. Histologically, mammary tumors from control animals and BDCA-exposed animals were morphologically very similar. These tumors had similar rates of metastasis, and there were no clear differences in local invasion or cellular atypia. Immunohistochemical analysis showed that adenocarcinomas from control and BDCA-treated animals in general possessed the same immunophenotype. Both showed an ERα/PR positive phenotype and a mixed luminal/basal phenotype. Human breast cancers are generally classified as luminal or basal based on immunohistochemistry or gene expression data, and this classification generally informs on biologic behavior and prognosis, with luminal tumors harboring a more favorable prognosis and less aggressive clinical course than basal tumors.58 However, not all human breast cancers necessarily fall into mutually exclusive groups based on luminal and basal markers; in fact, the classification of breast cancers based on basal (CK5, 14, 17) and luminal (CK8, 18, 19) markers remains controversial.25 For example, some breast cancers in humans may express both basal and luminal markers, and it has been shown that even in the normal breast, basal markers may be expressed in some of the duct or acinar luminal epithelial cells, in addition to basal epithelial or myoepithelial cells.20,21 In a study examining basal and luminal cytokeratin expression in 1944 human breast carcinomas, 27.4% of tumors expressed both luminal and basal cytokeratins,1 making the determination of biologic behavior difficult based upon the relative expression of these markers.

Mutation analysis of spontaneous and BDCA-treated mammary tumors illustrated possible molecular alterations associated with phenotype and exposure. For example, untreated fibroadenomas did not harbor mutations in any of the genes analyzed, whereas a majority of BDCA-treated fibroadenomas harbored synonymous Tp53 mutations. Only treatment related tumors exhibited Pten mutations (one fibroadenoma and one adenocarcinoma) and Egfr mutations (one fibroadenoma and two adenocarcinomas), whereas spontaneous tumors did not harbor mutations in these genes. The mutation spectra observed in treated fibroadenomas and adenocarcinomas and the increased variety of mutations in BDCA-treated adenocarcinomas may suggest increased DNA insult and damage leading to progression of tumorigenesis, or an increased incidence of neoplastic transformation in the mammary gland overall. However, a conclusion based on a small incidence of mutations in experimental groups of relatively small size such as this would be difficult. Ideally a larger sample number should be evaluated for additional mutations in these three genes as well as a broader number of genes to gain a better understanding of how these alterations relate to the increased incidence of tumors in this study.

Gene expression analysis showed that both spontaneous and BDCA-treated tumors expressed mostly luminal markers (CK8, 18, 19, Gata3, Slc39a6, Xbp1), but also Notch1, which is upregulated in basal type breast cancer and regulates a basal phenotype.23 The relevance of the dual expression of basal and luminal markers observed in mammary adenocarcinomas in BDCA-exposed F344/N rats is uncertain. It may point to a more aggressive phenotype, but it has also been shown in human breast cancer that more than half of ERα+/PR+ breast cancers contain a small population of neoplastic basal cells negative for hormone receptors, which can expand based on hormone therapy and Notch signaling,23 which could also account for a mixed luminal/basal phenotype. Alternatively, given that these tumors arise late in life and are not widely invasive or metastatic, their biologic behavior may reflect a less aggressive phenotype unrelated to an immunophenotype as seen in humans.

BDCA-treated and spontaneous mammary tumors shared a number of molecular alterations in gene expression related to cancer progression and growth. A number of genes that play a role in cell proliferation, growth and survival (Ccnd1, Ccnd2, Cdk2, Erbb2, Jun, Mki67, Myc, Akt1, Ctnnb1, Vegfa, Slc39a6) showed a positive increasing trend between normal mammary gland to fibroadenoma to adenocarcinoma in both groups (spontaneous and BDCA-treated), suggesting that these changes are common to the process of mammary carcinogenesis in rats and humans. However, importantly, there were a large proportion of genes related to cell cycle progression, matrix remodeling, cellular adhesion, and angiogenesis that were trending upwards exclusively in BDCA-treated lesions. Specifically, additional cell cycle mediators (Cdkn2a), matrix remodeling enzymes (Mmp2, Mmp9), and genes associated with cell adhesion, angiogenesis, invasion, and epithelial-to-mesenchymal transition (Id1, Tgfb1, Thbs2, Twist), important features of more aggressive human breast cancer, were overrepresented compared to spontaneous lesions. In addition, BDCA-treated tumors showed a decreasing trend in the expression of Brca2, an important DNA repair gene that when mutated in humans confers a 45–85% increased lifetime risk for breast cancer.45 Trend analysis comparing patterns of gene expression between spontaneous and BDCA-treated neoplastic mammary lesions suggest that BDCA-treated lesions represent a more aggressive phenotype, including increased cell cycle signaling, matrix remodeling, invasion, and epithelial-to-mesenchymal transition.

Lastly, results of a pairwise comparison between spontaneous and BDCA-treated adenocarcinomas identified a five gene signature of particular relevance to more aggressive breast cancer (Id1, Vegfa, Mmp2, Mmp9, Thbs1), which was significantly upregulated in BDCA-treated compared to spontaneous adenocarcinomas. Of particular interest is the fact that these genes are regulated in part by Tgfβ1, which showed a significantly increasing trend in BDCA-treated mammary carcinomas. TGFβ has been shown to act as a tumor suppressor in early stages of breast cancer development, but stimulate invasion and metastasis in later stages.2,61 It is commonly overexpressed in human breast cancer, associated with angiogenesis, epithelial-mesenchymal transition, tumor cell invasion, proliferation, and anti-apoptosis,45 and correlates with cancer progression and a poor prognosis.28 There is a paucity of data in the literature reporting involvement of this pathway in the development of spontaneous mammary tumors in the F344/NTac rat; however, some studies have shown that stimulation of rat mammary adenocarcinoma cells by TGFβ in experimental syngeneic models enhanced their tumorigenicity33 and metastatic behavior.63 While it is possible that BDCA is acting on pre-existing molecular pathways to promote a background spontaneous mammary tumor rate in the F344/NTac rat, these data suggest that BDCA could alter function of the TGF-β pathway separately from background alterations, through chemical-specific mechanisms.

TGFβ regulates the expression of MMP2 and 9,65 matrix metalloproteinases which function to degrade extracellular matrix, including type IV collagen,60 the main constituent of the basal lamina. Conversely, MMP2 and 9 also cleave adhesion molecules and activate growth factors, including TGFβ.18 In line with this, MMP2 and MMP9 have been shown to be associated with breast cancer cell growth, migration, and invasion.10 Inhibitor of differentiation-1 (Id1), a member of the helix-loop-helix family of genes found to be significantly upregulated in BDCA-treated adenocarcinomas, is also regulated by Tgfβ. Overexpression of ID1 in human breast cancer is associated with a more invasive phenotype, as well as epithelial-to-mesenchymal transition,62 a phenomenon associated with the acquisition of a more invasive and metastatic phenotype. While there are conflicting reports on the regulation of ID1 by TGFβ in various epithelial and fibroblast cell lines,8,29 ID1 has been shown to be strongly, albeit transiently, upregulated by TGFβ1 in mammary epithelial cells.35 Although the upregulation of Id1 in relationship to Tgfβ signaling in these tumors is uncertain, it may provide additional evidence of a Tgfβ-mediated mechanism in BDCA-treated adenocarcinomas. Thbs1 is overexpressed in many different types of human cancer, and in some forms of breast cancer (lobular carcinoma).55 The protein product of this gene is secreted in response to TGFβ stimulation, and plays a role in activation of a variety of proteases as well as latent TGFβ. Members of the vascular endothelial growth factor (Vegf) family play a central role in the regulation of angiogenesis, which plays an important role in the process of neoplastic growth. Angiogenesis is also regulated by a number of secreted growth factors, including members of the Tgfβ family.59 In fact, the induction of Vegf during tumor angiogenesis is mediated in part through Tgfβ stimulation.6

While adenocarcinomas in BDCA-exposed animals did not differ substantially in terms of morphologic features correlating with increased invasion, and there were no differences in rates of metastasis, changes in gene expression suggest that adenocarcinomas from BDCA-exposed animals are molecularly different from spontaneous tumors. The overrepresentation of Tgfβ mediators in adenocarcinomas from BDCA-exposed animals may suggest a correlation between activation of this pathway and the increased incidence of mammary tumors observed in BDCA-exposed animals. This pathway does not appear to be an important pathway in spontaneous mammary adenocarcinoma development in this strain, therefore these data show that in part, the process of mammary tumorigenesis in BDCA-exposed F344/NTac rats involves Tgfβ-dependent mechanisms, which are relevant for human breast cancer. Since this pathway plays such an important role in many human breast cancers, its involvement in mammary carcinogenesis in BDCA-exposed F344/NTac rats supports the hypothesis that BDCA exposure in humans may pose a significant health risk. The findings of this study are particularly important in terms of hazard identification for the general public, since not only is there an observation of an increased tumor incidence in rats exposed to this chemical, which has a wide human exposure, but also the presence of molecular alterations which are very relevant for development of human cancer. These data show that chronic exposure to this chemical should be considered a potential environmental human health hazard; the molecular findings strengthen the results of the carcinogenicity bioassay, help further define this hazard, and provide additional data for the regulatory community to consider. Additional studies on molecular mechanisms of carcinogenesis in animals exposed to BDCA and other water disinfection byproducts are warranted based on our identification of molecular alterations relevant for human cancer.

Supplementary Material

1

Acknowledgements:

We would like to thank the Cellular and Molecular Pathology Branch (CMPB) Necropsy Core Laboratory personnel, CMPB Histology, Immunohistochemistry, and Special Techniques Core Laboratories, and the NIEHS DNA Sequencing Core for their excellent technical expertise. This work was supported by the Division of the National Toxicology Program (DNTP), National Institutes of Environmental Health Sciences (NIEHS), National Institutes of Health (NIH).

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