Relationship between stages of mammary development and sensitivity to gamma-ray irradiation in mammary tumorigenesis in rats

Adverse outcome pathways for ionizing radiation and breast cancer involve direct and indirect DNA damage, oxidative stress, inflammation, genomic instability, and interaction with hormonal regulation of the breast

Knowledge about established breast carcinogens can support improved and modernized toxicological testing methods by identifying key mechanistic events. Ionizing radiation (IR) increases the risk of breast cancer, especially for women and for exposure at younger ages, and evidence overall supports a linear dose-response relationship. We used the Adverse Outcome Pathway (AOP) framework to outline and evaluate the evidence linking ionizing radiation with breast cancer from molecular initiating events to the adverse outcome through intermediate key events, creating a qualitative AOP. We identified key events based on review articles, searched PubMed for recent literature on key events and IR, and identified additional papers using references. We manually curated publications and evaluated data quality. Ionizing radiation directly and indirectly causes DNA damage and increases production of reactive oxygen and nitrogen species (RONS). RONS lead to DNA damage and epigenetic changes leading to mutations and genomic instability (GI). Proliferation amplifies the effects of DNA damage and mutations leading to the AO of breast cancer. Separately, RONS and DNA damage also increase inflammation. Inflammation contributes to direct and indirect effects (effects in cells not directly reached by IR) via positive feedback to RONS and DNA damage, and separately increases proliferation and breast cancer through pro-carcinogenic effects on cells and tissue. For example, gene expression changes alter inflammatory mediators, resulting in improved survival and growth of cancer cells and a more hospitable tissue environment. All of these events overlap at multiple points with events characteristic of "background" induction of breast carcinogenesis, including hormone-responsive proliferation, oxidative activity, and DNA damage. These overlaps make the breast particularly susceptible to ionizing radiation and reinforce that these biological activities are important characteristics of carcinogens. Agents that increase these biological processes should be considered potential breast carcinogens, and predictive methods are needed to identify chemicals that increase these processes. Techniques are available to measure RONS, DNA damage and mutation, cell proliferation, and some inflammatory proteins or processes. Improved assays are needed to measure GI and chronic inflammation, as well as the interaction with hormonally driven development and proliferation. Several methods measure diverse epigenetic changes, but it is not clear which changes are relevant to breast cancer. In addition, most toxicological assays are not conducted in mammary tissue, and so it is a priority to evaluate if results from other tissues are generalizable to breast, or to conduct assays in breast tissue. Developing and applying these assays to identify exposures of concern will facilitate efforts to reduce subsequent breast cancer risk.

Pre- and postpubertal irradiation induces mammary cancers with distinct expression of hormone receptors, ErbB ligands, and developmental genes in rats

Childhood exposure to carcinogens renders a higher risk of breast cancer. The molecular mechanisms underlying cancer development after such exposure are not, however, well understood. Here we examined how the mechanism of cancer development relates to the age at exposure to ionizing radiation (IR) or the carcinogen 1-methyl-1-nitrosourea (MNU). Pre- and postpubertal (3- and 7-wk-old, respectively) female Sprague-Dawley rats were whole-body γ-irradiated (2 Gy), injected intraperitoneally with MNU (20 mg/kg) or left untreated and were autopsied at 50 wk of age. Mammary carcinomas were examined for estrogen receptor (ER) α, progesterone receptor (PR) and ErbB ligand expression and for expression microarrays. Early histological changes of the ovaries were also evaluated. The incidence of mammary cancer was higher after postpubertal, rather than prepubertal, IR exposure; the inverse was true for MNU. Most cancers were positive for both ERα and PR except for the prepubertal IR group. Cancers of the prepubertal IR group expressed a different set of ErbB ligands from those of the other groups and did not overexpress Areg, which encodes an estrogen-regulated ErbB ligand, or other developmentally related genes including those for hormonally regulated mammary gland development. Prepubertal IR exposure resulted in ovarian dysfunction as revealed by a reduced follicular pool. Evidence thus suggests that mammary carcinogenesis induced by prepubertal IR exposure is independent of ovarian hormones but requires certain ErbB ligands; induction by postpubertal exposure depends on ovarian hormones and different ErbB ligands. In contrast, the mechanism of MNU-induced carcinogenesis was less influenced by the age at exposure.

Radiation-induced mammary carcinogenesis in rodent models: what's different from chemical carcinogenesis?

Ionizing radiation is one of a few well-characterized etiologic factors of human breast cancer. Laboratory rodents serve as useful experimental models for investigating dose responses and mechanisms of cancer development. Using these models, a lot of information has been accumulated about mammary gland cancer, which can be induced by both chemical carcinogens and radiation. In this review, we first list some experimental rodent models of breast cancer induction. We then focus on several topics that are important in understanding the mechanisms and risk modification of breast cancer development, and compare radiation and chemical carcinogenesis models. We will focus on the pathology and natural history of cancer development in these models, genetic changes observed in induced cancers, indirect effects of carcinogens, and finally risk modification by reproductive factors and age at exposure to the carcinogens. In addition, we summarize the knowledge available on mammary stem/progenitor cells as a potential target of carcinogens. Comparison of chemical and radiation carcinogenesis models on these topics indicates certain similarities, but it also indicates clear differences in several important aspects, such as genetic alterations of induced cancers and modification of susceptibility by age and reproductive factors. Identification of the target cell type and relevant translational research for human risk management may be among the important issues that are addressed by radiation carcinogenesis models.JRRS Incentive Award in 2009.

Acrylamide: review of toxicity data and dose-response analyses for cancer and noncancer effects

Acrylamide (ACR) is used in the manufacture of polyacrylamides and has recently been shown to form when foods, typically containing certain nutrients, are cooked at normal cooking temperatures (e.g., frying, grilling or baking). The toxicity of ACR has been extensively investigated. The major findings of these studies indicate that ACR is neurotoxic in animals and humans, and it has been shown to be a reproductive toxicant in animal models and a rodent carcinogen. Several reviews of ACR toxicity have been conducted and ACR has been categorized as to its potential to be a human carcinogen in these reviews. Allowable levels based on the toxicity data concurrently available had been developed by the U.S. EPA. New data have been published since the U.S. EPA review in 1991. The purpose of this investigation was to review the toxicity data, identify any new relevant data, and select those data to be used in dose-response modeling. Proposed revised cancer and noncancer toxicity values were estimated using the newest U.S. EPA guidelines for cancer risk assessment and noncancer hazard assessment. Assessment of noncancer endpoints using benchmark models resulted in a reference dose (RfD) of 0.83 microg/kg/day based on reproductive effects, and 1.2 microg/kg/day based on neurotoxicity. Thyroid tumors in male and female rats were the only endpoint relevant to human health and were selected to estimate the point of departure (POD) using the multistage model. Because the mode of action of acrylamide in thyroid tumor formation is not known with certainty, both linear and nonlinear low-dose extrapolations were conducted under the assumption that glycidamide or ACR, respectively, were the active agent. Under the U.S. EPA guidelines (2005), when a chemical produces rodent tumors by a nonlinear or threshold mode of action, an RfD is calculated using the most relevant POD and application of uncertainty factors. The RfD was estimated to be 1.5 microg/kg/day based on the use of the area under the curve (AUC) for ACR hemoglobin adducts under the assumption that the parent, ACR, is the proximate carcinogen in rodents by a nonlinear mode of action. When the mode of action in assumed to be linear in the low-dose region, a risk-specific dose corresponding to a specified level of risk (e.g., 1 x 10-5) is estimated, and, in the case of ACR, was 9.5 x 10-2 microg ACR/kg/day based on the use of the AUC for glycidamide adduct data. However, it should be noted that although this review was intended to be comprehensive, it is not exhaustive, as new data are being published continuously.

Does radiotherapy around the time of pregnancy for Hodgkin's disease modify the risk of breast cancer?

PURPOSE

To determine whether the risk of secondary breast cancer after radiotherapy (RT) for Hodgkin's disease is greater among women who underwent RT around time of pregnancy.

METHODS AND MATERIALS

The records of 382 women treated with RT for Hodgkin's disease were reviewed and divided into those who received RT around the time of pregnancy and those who were not pregnant. Comparisons of the overall incidence, actuarial rates, and latency to breast cancer between the two groups were made. Multivariate Cox regression modeling was performed to determine possible contributing factors.

RESULTS

Of the 382 women, 14 developed breast cancer (3.7%). The increase in the overall incidence (16.0% vs. 2.3%, p = 0.0001) and the actuarial rate of breast cancer among the women in the pregnant group (p = 0.011) was statistically significant. The women treated around the time of pregnancy had a 10- and 15-year actuarial rate of breast cancer of 6.7% and 32.6%, respectively. The 10-year and 15-year actuarial rate for the nonpregnant women was 0.4% and 1.7%, respectively. The median latency from RT to the diagnosis of breast cancer was 13.1 and 18.9 years for women in the pregnant and nonpregnant groups, respectively. In the multivariate analysis, pregnancy around the time of RT was the only variable associated with an increased risk of breast cancer. The risk was dependent on the length of time from pregnancy to RT, with women receiving RT during pregnancy and within 1 month of pregnancy having an increased risk of breast cancer compared with nonpregnant women and women irradiated later than 1 month after pregnancy (hazard ratio, 22.49; 95% confidence interval, 5.56-90.88; p <0.001).

CONCLUSION

The results of this study indicate that the risk of breast cancer after RT is greater with irradiation around the time of pregnancy. This suggests that pregnancy is a time of increased sensitivity of breast tissue to the carcinogenic effects of radiation. Because of the small sample size and limited follow-up, additional studies are recommended to confirm these findings.

Breast cancer in women after treatment for Hodgkin's disease

Although substantial progress has been made in the management of Hodgkin's lymphoma during the past 30 years, the development of secondary malignant diseases has emerged as a serious consequence of treatment. In particular, extended follow-up of patients with Hodgkin's disease has revealed an increased risk of breast cancer. We have systematically reviewed all published literature on breast cancer after treatment for Hodgkin's disease and show that high risk is particularly associated with treatment at a young age, mantle radiotherapy, and chemotherapy. Breast cancers in this context differ from sporadic disease because they develop in younger women, are associated with a high incidence of bilateral disease, and are generally located near the midline of the body. The risk of breast cancer is lower in patients who receive newer, combined modality treatments for Hodgkin's disease. In this review we discuss a protocol for formal follow-up and screening of patients who have recovered from Hodgkin's disease to aid early diagnosis and ensure the possibility of effective management.

Hormonal and genetic risk factors for breast cancer

Breast cancer is a major cause of female morbidity and mortality worldwide. In this review, we discuss the hormonal and genetic risk factors associated with breast cancer development and describe the currently available models for predicting an individual woman's risk. We highlight three more sophisticated surrogate markers of life-time oestrogen exposure (plasma oestradiol, mammographic breast density, bone mineral density) and propose that these may be used to improve estimates of a woman's absolute risk.

Enhancing effects of beta-estradiol 3-benzoate but not methoxychlor on the promotion/progression stage of chemically-induced mammary carcinogenesis in ovariectomized rats

Modifying effects of beta-estradiol 3-benzoate (EB) and methoxychlor (MXC), a pesticide which possesses weak estrogenic activity, on 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinogenesis were investigated in ovariectomized or intact female Sprague-Dawley rats. Twenty-eight weeks after a single DMBA (100 mg / kg body weight) initiation, when the incidence of mammary tumor-bearing rats had reached 75%, a number of the animals were subjected to ovariectomy in order to obtain 3 groups: i) tumor-bearing, ovariectomized group; ii) tumor-bearing, intact group; iii) no-tumor, ovariectomized group. Subsequently animals of each group were subjected to subcutaneous implantation of 0.5 mg EB or given diet containing 1000 ppm MXC for 13 weeks. Although the incidences, multiplicities and volumes of the palpable tumors gradually decreased after ovariectomy, EB treatment stimulated tumor growth in the tumor-bearing, ovariectomized group thereafter. A similar effect of EB treatment was also observed in the no-tumor, ovariectomized group. However, MXC did not show any effect in the tumor-bearing, or no-tumor ovariectomized groups, except that the multiplicity of tumors was significantly decreased by MXC treatment in the tumor-bearing, intact group. The results of our study suggest that MXC has no promotion / progression effect, but rather possesses a weak inhibitory effect, whereas the strongly estrogenic substance EB clearly enhanced DMBA-induced mammary tumorigenesis.

Prevention of radiation-induced mammary tumors

The radiation-induced rat mammary tumor model is useful for studying tumor prevention by treatment in the initiation or promotion stage. In anti-initiation experiments, the administration of radical scavengers or spin-trapping agents before or immediately after irradiation reduced the incidence of mammary tumors, suggesting that free radicals produced by exposure are a potent initiator. To evaluate the role of nitric oxide (NO) in the initiation, NO-specific scavengers or NO synthase inhibitors were administered during the initiation. These agents partially prevented the tumorigenesis, suggesting that radiation-induced NO contributes to tumor initiation. The administration of curcumin during irradiation reduced the incidence of the tumors in the presence of tumor promotor. In anti-promotion experiments on preventing diethylstilbestrol (DES)-dependent tumor development from mammary primodial cells exposed to radiation, tamoxifen decreased the tumor incidence. From the results, estrogen itself or prolactin induced by estrogen may be a promoter for the tumorigenesis. Bezafibrate and simvastatin, agents inducing hypolipidemia and hypocholesterolemia respectively, cause a decrease in the DES-dependent promotion of radiation-induced tumorigenesis. The simultaneous administration of curcumin and DES significantly reduces the development of mammary tumors in irradiated rats. In this review, the endocrinologic and pharmacologic significance of the anti-initiation and anti-promotion is discussed.

Breast cancer risk following irradiation for Hodgkin's disease

Radiation is commonly used to treat early-stage Hodgkin's disease. As the risk of recurrent Hodgkin's disease decreases as time from treatment elapses, the risk of radiation-induced breast cancer rises. Women irradiated between the time of puberty and the age of 30 are at the highest risk. The median time to breast cancer following radiation in this age group is approximately 15 years, usually when women are aged between 30 and 40, and often before regular breast screening is implemented. Risk factors associated with breast cancer development include age at irradiation, time since treatment and the radiation dose received. Current screening for breast cancer after Hodgkin's disease is inconsistent. In this article we review breast development, mechanisms of radiation-induced carcinogenesis, and findings from retrospective studies on Hodgkin's disease and breast cancer. We also review future considerations of management, including assessment of risk awareness in these women, guidelines for follow-up and screening, and chemoprevention both during and after treatment of Hodgkin's disease. The literature reviewed was obtained from Medline using the key words: breast cancer, Hodgkin's disease and radiation-induced cancer. The search was limited to English language literature. Other sources include reference lists in books and published papers.

Radiation-induced tumorigenesis of mammary glands in pituitary transplanted rats ovariectomized before onset of estrous cycle

The role of prolactin in the initiation of mammary tumorigenesis by radiation was evaluated in ovarian hormone-free rats. Rats were bilaterally ovariectomized at 23 days of age, and then, at 2.5 months of age, two pituitaries obtained from mature rats of the same strain were transplanted underneath the kidney capsule as a means of increasing the serum prolactin level to provide stimulation of development of mammary glands. After 2 weeks, the ovariectomized rats with ectopic pituitary glands were exposed to whole body irradiation of 2.6 Gy of gamma-rays from a 60Co source and then treated with diethylstilbestrol as a tumor promoter. For the control, ovariectomized rats without ectopic pituitary glands were exposed and treated in the same way as the experimental group. A significant increase of serum prolactin level was observed at the time of irradiation by the pituitary transplanted rats, and intense immunohistochemical reaction with a specific anti-prolactin antiserum was detected in the ectopic pituitary glands. Also, mammary glands in the pituitary transplanted rats, ovariectomized before puberty, showed lactiferous ducts without alveolar buds at the time of tumor initiation. The pituitary transplanted rats showed a significantly increased incidence of adenocarcinoma and fibroadenoma compared with the control. Many of the mammary tumors induced in the pituitary transplanted rats given radiation were estrogen receptor (ER)(+) progesterone receptor (PgR)(+) and ER(+)PgR(-) tumors, whereas ER(-)PgR(-) tumors were mainly obtained in the control rats. In the experimental group, many of the fibroadenomas had low concentrations of ER and no PgR, while the adenocarcinomas had moderate concentrations of ER and high PgR. These results suggest that hypersecretion of prolactin from the pituitary transplants developed lactiferous ducts and accelerated the tumorigenesis of mammary glands initiated by radiation in the absence of synergism with ovarian hormones.

Comparative effect of chlormadinone acetate and diethylstilbestrol as promoters in mammary tumorigenesis of rats irradiated with gamma-rays during lactation

The purpose of this study was to determine the promotional role of estrogen and progestin in the development of radiation-induced mammary tumors. To eliminate the effects of endogenous ovarian hormones on tumor promotion, all rats were ovariectomized immediately after the initiation by irradiation with 2.6 Gy gamma-rays at day 21 of lactation, and were divided into 3 groups. For the control experiment, rats were implanted with a cholesterol pellet 1 month after the irradiation. Only one rat developed a fibroadenoma (4.3% mammary tumor incidence) during the 1 year period of the implantation. In the other two groups, chlormadinone acetate (CMA) to increase progestin level or diethylstilbestrol (DES) to increase estrogenic activity were administered, respectively, as tumor promoters for 1 year. Treatment with CMA did not significantly increase the incidence of mammary tumors as compared with the controls. However, administration of DES resulted in a significantly higher mammary tumor incidence (79.3%) than control treatment. Compared with cholesterol administration, DES treatment caused an increase in prolactin concentration in serum (5-fold), and reduction of estradiol-17beta concentration (22% of control). These results suggest that DES ia a potent effective promoter for tumorigenesis of radiation-initiated mammary cells, but CMA is not. DES may act directly on the irradiated mammary cells by binding to ER, and indirectly by stimulating prolactin secretion from the pituitary glands.

Effect of hormonal status on the expression of the cyclooxygenase 1 and 2 genes and prostaglandin synthesis in rat mammary glands

Hormonal effects on mammary carcinogenesis have been linked to prostaglandin (PG) synthesis. The purpose of the present study was to examine the expression of the cyclooxygenase (COX) 1 and 2 genes and levels of PG synthesis in the mammary glands of rats that have different levels of susceptibility to mammary gland carcinogenesis associated with pregnancy, lactation, post-lactation involution, and ovariectomy. The expression of COX-1 mRNA, measured by Northern blot analysis, was similar in virgin, lactating, pregnant, and post-lactational animals of the same age. Ovariectomized animals exhibited significantly lower levels of COX-1 mRNA (approximately 40%) compared to the sham-operated controls or the ovariectomized animals treated with estradiol and progesterone. COX-2 mRNA, measured by RT-PCR, was detectable only in the mammary glands of lactating animals and ovariectomized animals administered estradiol and progesterone. Induction on COX-2 expression occurred in both stromal and epithelial cells in lactating rat mammary glands. COX enzymatic activities, determined by measuring the conversion rate of [1-14 C]-arachadonic acid to prostanoids, showed that lactating animals had a significantly higher activity compared to virgin (approximately 40%), pregnant (approximately 30%), or postlactational animals (approximately 40%). Ovariectomized animals had significantly lower COX enzymatic activity compared to the sham operated animals. Significant induction of COX activity, however, was observed in ovariectomized animals administered estradiol and progesterone. These changes in COX enzymatic activity were paralleled by similar changes in the mammary gland PGE2 content, measured by enzyme immunoassay. Our results suggest that the effect of hormones on the genesis of mammary cancer in the rat may be mediated, at least in part, by their effects on COX-2 expression and PG synthesis.

Radiation-induced mammary tumorigenesis in pseudopregnant rats mated with vasectomized partners

Pseudopregnancy was induced in Wistar-MS female rats by mating with male rats 1 month after vasectomy. Pseudopregnant rats received whole-body irradiation with 2.6 Gy gamma-rays at day 7 of pseudopregnancy and were then implanted with a DES pellet as a tumor promoter. In the control groups, virgin 2.5 month-old rats, and normal pregnant rats at day 7 of pregnancy were also exposed to the same dose of radiation. The incidence of mammary tumors in rats irradiated in pseudopregnancy (52.6%) was significantly higher than that in the irradiated virgin rats (20.8%), but was not significantly different from that observed in rats irradiated in normal pregnancy (66.7%). The appearance of the first mammary tumor in the rats irradiated in pseudopregnancy occurred 4 months earlier than that in the normal pregnancy group. The proportion of adenocarcinomas in total tumors was 16.7, 0 and 38.5% in rats irradiated in pseudopregnancy, virgin and pregnancy groups, respectively. The incidence of adenocarcinomas was not significantly different among the three groups.