Lack of modifying effects of an estrogenic compound atrazine on 7,12-dimethylbenz(a)anthracene-induced ovarian carcinogenesis in rats

Preclinical models of epithelial ovarian cancer: practical considerations and challenges for a meaningful application

Despite many improvements in ovarian cancer diagnosis and treatment, until now, conventional chemotherapy and new biological drugs have not been shown to cure the disease, and the overall prognosis remains poor. Over 90% of ovarian malignancies are categorized as epithelial ovarian cancers (EOC), a collection of different types of neoplasms with distinctive disease biology, response to chemotherapy, and outcome. Advances in our understanding of the histopathology and molecular features of EOC subtypes, as well as the cellular origins of these cancers, have given a boost to the development of clinically relevant experimental models. The overall goal of this review is to provide a comprehensive description of the available preclinical investigational approaches aimed at better characterizing disease development and progression and at identifying new therapeutic strategies. Systems discussed comprise monolayer (2D) and three-dimensional (3D) cultures of established and primary cancer cell lines, organoids and patient-derived explants, animal models, including carcinogen-induced, syngeneic, genetically engineered mouse, xenografts, patient-derived xenografts (PDX), humanized PDX, and the zebrafish and the laying hen models. Recent advances in tumour-on-a-chip platforms are also detailed. The critical analysis of strengths and weaknesses of each experimental model will aid in identifying opportunities to optimize their translational value.

Optimization of 7,12-dimethylbenz(a)anthracene-induced rat epithelial ovarian tumors

Ovarian carcinoma is the second most common malignant tumor of the female reproductive system and an notable cause of cancer death. The detection and diagnosis of early ovarian carcinomas are still clinical challenges, which calls for imaging studies using early ovarian carcinoma animal models. The present study aimed to optimize the 7,12-dimethylbenz(a)anthracene (DMBA)-induced model of rat ovarian tumors by investigating the delivery methods, induction dose and time of DMBA exposure, and explored the morphological features of tumors using MRI. Three schemes were performed. In scheme one the ovary was covered with absorbable hemostatic gauze loaded with a high concentration of liquid DMBA. For this scheme, 150 Sprague-Dawley rats were divided into three groups depending on the DMBA dose (1.0, 2.0 and 3.0 mg). In scheme two DMBA solution was injected under the ovarian capsule. For this scheme, 159 rats were divided into 0.5, 1.0 and 1.5 mg DMBA groups. In scheme three the ovary was covered with absorbable gauze loaded with a high concentration of solid DMBA. For this scheme 161 rats were divided into 1.0, 2.0 and 3.0 mg DMBA groups. Each group of the three schemes was further subdivided into 60-, 90-, 120-, 150- and 180-day groups. In scheme two, the tumor formation rate was 75.6% (99/131), which was the highest in the 1.5 mg group (86.4%, 38/44) and reached 100% (10/10) on day 120. The induced tumors were serous in 93.9% (93/99) of tumors. Borderline ovarian tumors accounted for 19.2% (19/99) of all tumors, and ovarian cancer accounted for 46.5% (46/99). The mean maximum diameter (MMD) of borderline ovarian tumors was 10.29±3.41 mm, and that of ovarian cancer was 15.19±7.10 mm. MMD of the solid components increased with increasing malignancy. Cystic, cystic-solid and solid tumors were observed. The ovarian subcapsular injection of 1.5 mg DMBA was the best scheme for the rat ovarian tumor model. The present model is ideal for investigating the occurrence, development and imaging of ovarian tumors.

Diverse animal models to examine potential role(s) and mechanism of endocrine disrupting chemicals on the tumor progression and prevention: Do they have tumorigenic or anti-tumorigenic property?

Acting as hormone mimics or antagonists in the interaction with hormone receptors, endocrine disrupting chemicals (EDCs) have the potentials of disturbing the endocrine system in sex steroid hormone-controlled organs and tissues. These effects may lead to the disruption of major regulatory mechanisms, the onset of developmental disorders, and carcinogenesis. Especially, among diverse EDCs, xenoestrogens such as bisphenol A, dioxins, and di(2-ethylhexyl)phthalate, have been shown to activate estrogen receptors (ERs) and to modulate cellular functions induced by ERs. Furthermore, they appear to be closely related with carcinogenicity in estrogen-dependant cancers, including breast, ovary, and prostate cancers. In in vivo animal models, prenatal exposure to xenoestrogens changed the development of the mouse reproductive organs and increased the susceptibility to further carcinogenic exposure and tumor occurence in adults. Unlike EDCs, which are chemically synthesized, several phytoestrogens such as genistein and resveratrol showed chemopreventive effects on specific cancers by contending with ER binding and regulating normal ER action in target tissues of mice. These results support the notion that a diet containing high levels of phytoestrogens can have protective effects on estrogen-related diseases. In spite of the diverse evidences of EDCs and phytoestrogens on causation and prevention of estrogen-dependant cancers provided in this article, there are still disputable questions about the dose-response effect of EDCs or chemopreventive potentials of phytoestrogens. As a wide range of EDCs including phytoestrogens have been remarkably increasing in the environment with the rapid growth in our industrial society and more closely affecting human and wildlife, the potential risks of EDCs in endocrine disruption and carcinogenesis are important issues and needed to be verified in detail.

Simultaneous optical coherence tomography and laser induced fluorescence imaging in rat model of ovarian carcinogenesis

Determining if an ovarian mass is benign or malignant is an ongoing clinical challenge. The development of reliable animal models provides means to evaluate new diagnostic tools to more accurately determine if an ovary has benign or malignant features. Although sex cord-stromal tumors (SCST) account for 0.1–0.5% of ovarian malignancies, they have similar appearances to more aggressive epithelial cancers and can serve as a prototype for developing better diagnostic methods for ovarian cancer. Optical coherence tomography (OCT) and laser-induced fluorescence (LIF) spectroscopy are non-destructive optical imaging modalities. OCT provides architectural cross-sectional images at near histological resolutions and LIF provides biochemical information. We utilize combined OCT-LIF to image ovaries in post-menopausal ovarian carcinogenesis rat models, evaluating normal cyclic, acyclic and neoplastic ovaries. Eighty-three female Fisher rats were exposed to combinations of control sesame oil, 4-vinyl cyclohexene diepoxide (VCD) to induce ovarian failure,and/or 7,12-dimethylbenz[a]anthracene (DMBA) to induce carcinogenesis. Three or five months post-treatment, 162 ovaries were harvested and imaged with OCT-LIF: 40 cyclic, 105 acyclic and 17 SCST. OCT identified various follicle stages,corpora lutea (CL), CL remnants, epithelial invaginations/inclusions and allowed for characterization of both cystic and solid SCST. Signal attenuation comparisons between CL and solid SCST revealed statistically significant increases in attenuation among CL. LIF characterized spectral differences in cyclic, acyclic and neoplastic ovaries attributed to collagen, NADH/FAD and hemoglobin absorption. We present combined OCT-LIF imaging in a rat ovarian carcinogenesis model, providing preliminary criteria for normal cyclic, acyclic and SCST ovaries which support the potential of OCT-LIF for ovarian imaging.

Expression of R-ras oncogenes in the hermaphroditic fish Kryptolebias marmoratus, exposed to endocrine disrupting chemicals

The hermaphroditic fish Krytolebias marmoratus is a potential fish model for study of tumour development. Recently, sequences and expression of some oncogenes and tumor suppressor gene have been studied in K. marmoratus. To get a better understanding of oncogene expression at different development stage, and in different tissues three R-ras genes were cloned and fully sequenced. Expression of these R-ras genes (R-ras1, R-ras2, R-ras3) was also studied in fish exposed to endocrine-disrupting chemicals (EDCs). Liver showed the highest level of expression compared to other tissues, even though each R-ras gene showed different expression patterns in tissues. Interestingly, in secondary male (ovary atresia stage), expression levels of three R-ras genes was lower compared to hermaphrodites. At different developmental stages, R-ras2 gene showed most pronounced expression at early embryogenesis but at stage 5 (hatchling stage) and juvenile stage, R-ras3 gene showed the highest expression. After the juvenile stage, R-ras1 gene was upregulated compared to other R-ras genes, which showed the highest expression at the hermaphroditic stage. When fish were exposed to 17-beta-estradiol (E2), a natural estrogen and tamoxifen, a nonsteroidal estrogen antagonist and three EDCs viz., 4-n-nonylphenol (NP), bisphenol A (BPA), and 4-tert-octylphenol (OP), all the three R-ras genes were induced, except in the fish exposed to tamoxifen. These results suggest that EDCs modulate the expression of R-ras genes and thus affect subsequent signal transduction and tumor development.

Ovarian neoplasm development by 7,12-dimethylbenz[a]anthracene (DMBA) in a chemically-induced rat model of ovarian failure

OBJECTIVES

The objectives were to determine the time course for ovarian failure in rats caused by 4-vinylcyclohexene diepoxide (VCD) and develop a model for ovarian cancer in which ovarian neoplasms were chemically induced in an animal that was follicle depleted, but retained residual ovarian tissue.

METHODS

Initially, female Fisher 344 rats were treated with VCD (to induce ovarian failure) or vehicle control (sesame oil). Three or 6 months after treatment, ovaries were collected and processed for histological evaluation for confirmation of ovarian failure. A further set of female rats was assigned to four groups exposed to combinations of vehicle control, VCD and/or DMBA (directly applied to the ovary) in a novel model for examining early stages of ovarian neoplasia.

RESULTS

Three and 6 months following VCD dosing there was a significant reduction of ovarian weight and follicle number. Treatment with DMBA subsequent to VCD resulted in tumors in 42% of animals at 3 months and 57% at 5 months. All neoplasms were classified Sertoli-Leydig cell tumors (SLCT). No tumor occurred in animals treated with vehicle or DMBA alone.

CONCLUSIONS

These studies demonstrate that the VCD-treated rat can be used as a model for peri- and post-menopause. DMBA induction of ovarian neoplasms was greater in those rats treated with VCD. Whether this increase was due to tumor initiation by VCD or was the result of ovarian failure cannot be distinguished from these results. This represents the only animal model to date for sex cord stromal tumors.

Atrazine-induced changes in aromatase activity in estrogen sensitive target tissues

Atrazine (ATR) is a pesticide used widely throughout North America. Although not directly estrogenic, ATR treatment has been shown to increase aromatase activity in tumor cell lines. Thus, it is suggested that ATR can increase local tissue estrogen levels in estrogen sensitive target tissues through increased aromatase activity. Therefore the effect of ATR on aromatase activity was measured in human granulosa-lutein cell cultures, cells that abundantly express aromatase, and endometrial stromal cell (ESC) cultures, cells that do not express aromatase. Aromatase activity was quantified by the tritiated water method and the specificity of the assay was confirmed by co-incubation with 4-hydroxyandrostenedione, an irreversible inhibitor of the catalytic activity of aromatase. Aromatase activity in ATR treated (1-10 microm) granulosa-lutein cells was increased more than 2-fold compared with control cultures. There were no treatment related changes in cellular protein and thus it is suggested that the ATR-induced change in aromatase activity was not due to an increase in cell number. ATR-treatment had no effect on ESC aromatase activity at any concentration tested. Similarly, there was no effect of ATR treatment on human recombinant aromatase activity in our cell-free test system. Therefore it is concluded that microm concentrations of ATR can increase aromatase activity of human granulosa cells but not ESC and this effect is not elicited at the enzyme level.

Modulatory effect of environmental endocrine disruptors on N-ras oncogene expression in the hermaphroditic fish, Kryptolebias marmoratus

Kryptolebias marmoratus is the only known internally self-fertilizing vertebrate. It shows high susceptibility to many chemical carcinogens and has been proposed as a potential cancer model species alternative to mammals. Since use of this fish species is expected to rise in cancer research, regulation of oncogenes from K. marmoratus needs proper understanding. We cloned and deduced full-length sequence of cDNA of N-ras oncogene from K. marmoratus. Study of expression profile of N-ras by using quantitative real-time RT-PCR revealed that brain had the highest level of expression compared to other tissues. Some embryonic stages showed more N-ras expression than juveniles and adults. Exposure to two environmental endocrine disrupting chemicals (EDCs), bisphenol A (BPA) and 4-nonylphenyl (NP) caused up-regulation of N-ras in gonad, intestine and liver of hermaphrodite K. marmoratus. It is suggested that K. marmoratus may be a suitable model species for oncogene expression studies. The observed EDC-induced expression of N-ras supports the assumption that EDC exposure may predispose the host to the risk of environmental carcinogenesis.

Atrazine interaction with estrogen expression systems

More than 40 publications have described results of atrazine responses in 17 estrogen-dependent systems and in more than a dozen different reporter and estrogen receptor-binding studies in vitro. Results from these studies have consistently failed to demonstrate that atrazine acts as an estrogen agonist. Moreover, a variety of indices of estrogen-dependent activity, in models that encompass cell incubations to whole animals, have failed to respond to atrazine. Researchers in more than a dozen laboratories have examined rats, rat tissues, human and prokaryotic cells, in addition to tissues from reptile, fish, amphibian, avian, molluscan, and insect sources, without eliciting estrogenic-like responses from atrazine. In contrast, studies of atrazine ability to antagonize estrogen-mediated responses have yielded equivocal results. Results of several studies show inhibition of estrogen-like activities by atrazine, yet many other tests have yielded negative results. Generally, in vivo models have more consistently shown that atrazine inhibits estrogen-mediated responses, whereas in more specific in vitro systems, inhibition is seldom observed. The implication is that in vivo effects of atrazine may result from inhibition of factors that are indirectly connected to the genomic interaction of estrogen (e.g., at the receptor). Potential targets of atrazine may be downstream of the ligand-receptor binding event. Atrazine may also interact with other, less specific, factors that are necessary for the completion of the estrogen-mediated response. Moreover, the apparent inhibition of cytosolic-ER binding by atrazine may, similarly, be relatively nonspecific. Observed inhibitory responses occur only at extreme doses or concentrations, i.e., several orders of magnitude greater than the level of estradiol presence in each test system. It is probable that the inhibitory effects result from very low affinity and/or low specificity interactions, which are unlikely to occur in nature. We conclude that atrazine is not an estrogen receptor agonist, but it may be a weak antagonist, when present at a high concentration under conditions of disequilibrium with estrogen. These conditions are not expected to occur as a result of normal environmental exposure.

Influence of atrazine administration and reduction of calorie intake on prostate carcinogenesis in probasin/SV40 T antigen transgenic rats

Atrazine, which has been used worldwide as a pesticide, is now known to exert endocrine disrupting (antiandrogenic) effects in mammals. In this study, modifying effects of dietary feeding of 500 and 1000 p.p.m. atrazine on the development of androgen-dependent prostate cancer were investigated using male probasin/SV40 T antigen transgenic (TG) rats. As administration of atrazine has now been identified as causing a decrease in bodyweight, a dietary-restricted TG rat group was also included in order to elucidate the influence of reduction of calorie intake per se on the development of prostate cancer. At week 13, almost the entire lobes of the prostate were occupied with tumor lesions, with no clear intergroup differences in the incidences and multiplicities. Therefore, morphometrical assessment ratios of the prostate epithelial area to the whole prostate tissue area were evaluated. The ratio in the lateral lobe of the 1000 p.p.m. atrazine-treated group was significantly decreased, and there was a tendency to decrease in the ratios in the dorsal lobe of the atrazine-treated groups. However, dietary restriction itself without atrazine treatment caused the same reduction to a similar or greater extent. Testosterone levels were not affected by atrazine administration or dietary restriction. Our results indicate that the observed atrazine-related suppression of prostate carcinogenesis was probably caused by the decrease in calorie intake, rather than by atrazine-related endocrine disruption.