Studies of tamoxifen as a promoter of hepatocarcinogenesis in female Fischer F344 rats

Effect of tamoxifen treatment on catalase (CAT) and superoxide dismutase (SOD) expression and localization in the hen oviduct

The imbalance between free reactive oxygen species (ROS) generation and removal (e.g., by antioxidative enzymes) leads to the damage of important biomolecules and cells. Earlier studies in hens showed that treatment with tamoxifen (TMX; estrogen receptor inhibitor) modulates oxidative stress and causes the reproductive system regression realized by cell apoptosis. The aim of the present study was, therefore, to examine the expression and immunolocalization of the key enzymatic antioxidants, i.e. catalase (CAT) and superoxide dismutase (SOD), in the chicken oviduct following TMX treatment. Laying hens were treated daily with TMX until a pause in egg-laying occurred and then euthanized on day 8 of the experiment. Quantitative real-time PCR and western blot analyses showed the presence of CAT and SOD transcripts and proteins, respectively, in all oviductal segments, i.e., the infundibulum, magnum, isthmus, shell gland and vagina. In control hens (laying), the mRNA expression of CAT was the highest in the shell gland, lower in the isthmus and the lowest in other oviductal parts, whereas protein expression was the highest in the magnum, lower in the isthmus and the lowest in other segments. The SOD transcript and protein abundances only were lower in the magnum than in other segments. Immunoreactive CAT and SOD products were localized in all layers of the oviductal wall, but the intensity of staining depended on the cell type. TMX treatment affected CAT and SOD expression and the effect of TMX depended on gene, protein, cell type and oviductal part. Generally, CAT expression was elevated, while SOD expression was decreased under TMX treatment. These results point to the importance of CAT and SOD in the maintenance of proper oviduct health and function. Changes in ROS scavenging enzymes after estrogen receptor blockage indicate the significance of estrogen in the regulation of oxidative status in the avian oviduct.

The SERM Saga, Something from Nothing: American Cancer Society/SSO Basic Science Lecture

BACKGROUND

The discovery of nonsteroidal antiestrogens created a new group of medicines looking for an application; however, at the time, cytotoxic chemotherapy was the modality of choice to treat all cancers. Antiestrogens were orphan drugs until 1971, with the passing of the National Cancer Act. This enabled laboratory innovations to aid patient care.

METHODS

This article traces the strategic application of tamoxifen to treat breast cancer by targeting the estrogen receptor (ER), deploying long-term adjuvant tamoxifen therapy, and becoming the first chemopreventive for any cancer. Laboratory discoveries from the University of Wisconsin Comprehensive Cancer Center (UWCCC) are described that address a broad range of biological issues with tamoxifen. These translated to improvements in clinical care.

RESULTS

Tamoxifen was studied extensively at UWCCC in the 1980s for the development of acquired resistance to long-term therapy. Additionally, the long-term metabolism of tamoxifen and regulation of growth factors were also studied. A concern with tamoxifen use for chemoprevention was that an antiestrogen would increase bone loss and atherosclerosis. Laboratory studies with tamoxifen and keoxifene (subsequently named raloxifene) demonstrated that 'nonsteroidal antiestrogens' maintained bone density, and this translated into successful clinical trials with tamoxifen at UWCCC. However, tamoxifen also increased endometrial cancer growth; this discovery in the laboratory translated into changes in clinical care. Selective estrogen receptor modulators (SERMs) were born at UWCCC.

CONCLUSIONS

There are now five US FDA-approved SERMs, all with discovery origins at UWCCC. Women's health was revolutionized as SERMs have the ability to treat multiple diseases by switching target sites around a woman's body on or off.

Rodent Oncology: Diseases, Diagnostics, and Therapeutics

Cancer incidence in rodent species varies dramatically from a common occurrence in mice and rats to just a limited number of documented cases in chinchillas and degus. This article summarizes common tumors, both benign and malignant, that have been reported to occur in rodents. Outlined are clinical signs, diagnostics, and treatments that have been described for rodents presenting with specific neoplasms.

Adventures in hepatocarcinogenesis

Neoplasia is a heritably altered, relatively autonomous growth of tissue. Hepatocarcinogenesis, the pathogenesis of neoplasia in liver, as modeled in the rat exhibits three distinct, quantifiable stages: initiation, promotion, and progression. Simple mutations and/or epigenetic alterations may result in the irreversible stage of initiation. The stage of promotion results from selective enhancement of cell replication and selective inhibition of cellular apoptosis of initiated cells dependent on the genetic and/or epigenetic alterations of the latter. The irreversible stage of progression results from initial karyotypic alterations that evolve into greater degrees of genomic instability. The initial genomic alteration in the transition from promotion to progression may involve primarily epigenetic mechanisms driven by epigenetic and genetic alterations fixed during the stage of promotion.

Spontaneous tumors of small mammals

The most common tumor of guinea pigs is bronchogenic papillary adenoma; of hedgehogs is mammary gland adenocarcinoma; of hamsters is adrenal cortical adenoma; of gerbils is ovarian granulosa cell and theca cell tumors; of mice is pulmonary carcinoma; and of rats is mammary fibroadenoma. A relatively low incidence of tumors is described for chinchillas and hamsters, whereas the incidence of tumors is high for gerbils, hedgehogs, mice, and rats. Limited literature regarding neoplasia exists for prairie dogs, sugar gliders, and chinchillas.

Genotoxic Mechanism of Tamoxifen in Developing Endometrial Cancer

Increased risk of developing endometrial cancers has been observed in women treated with tamoxifen (TAM), a widely used drug for breast cancer therapy and chemoprevention. The carcinogenic effect may be due to genotoxic DNA damage induced by TAM. In fact, TAM-DNA adducts were detected in the endometrium of women treated with this drug. TAM is alpha-hydroxylated by cytochrome P450 3A4 followed by O-sulfonation by hydroxysteroid sulfotransferase, and reacts with guanine residues in DNA, resulting in the formation of alpha-(N2-deoxyguanosinyl)tamoxifen adducts. During this metabolic process, short-lived carbocations are produced at the ethyl moiety of TAM as reactive intermediates. TAM-DNA adducts promote primarily G -->T transversions in mammalian cells. The same mutations have been frequently detected at codon 12 of the K-ras gene in the endometrial tissue of women treated with this drug. TAM-DNA adducts, if not readily repaired, may act as initiators, leading to development of endometrial cancers. The reactivity of TAM metabolites with DNA is inhibited in toremifene, where the hydrogen atom has been replaced by a chlorine atom at the ethyl moiety. Therefore, toremifene may be a safer alternative to TAM. This article describes an overview of the mechanism of TAM-DNA adduct formation, mutagenic events of this adduct, and detection of TAM-DNA adducts in the endometrium of women treated with TAM.

On the promoting action of tamoxifen in a model of hepatocarcinogenesis induced by p-dimethylaminoazobenzene in CF1 mice

BACKGROUND AND AIMS

Tamoxifen (TMX) has proven to be an effective palliative treatment for advanced breast cancer with low reported incidence of side effects. TMX has been demonstrated to be an initiator and/or a promoter in the rat model of hepatocarcinogenesis. To document the long-term effect of TMX in mice treated with p-dimethylaminoazobenzene (DAB), we have investigated the time response action of these drugs on different biochemical parameters.

METHODS

A group of animals was placed on dietary DAB (0.5%, w/w) during a period of 28 weeks. Control animals received a standard laboratory diet. Two other groups of non-treated and DAB-treated animals received TMX citrate (0.025%, w/w) in the diet since day 20.

RESULTS

The activities of the enzymes involved in heme synthesis and degradation as evaluated in the DAB group was not further affected by TMX. DAB and/or TMX treatment significantly increased the content of total cytochrome P450 and also the activity of glutathione S-transferase indicating liver damage. In all treated groups oxidative stress and an adaptive response of the natural defense system (catalase and superoxide dismutase) were demonstrated. Histological and morphological studies revealed liver cell hyperplasia in DAB treated group; however, only in the DAB+TMX group solid, trabecular and acinar hepatocellular carcinoma was confirmed at the end of the experimental trial.

CONCLUSION

We have demonstrated that TMX produced changes in hepatic enzyme activities which may be relevant for the metabolism and disposition of this and/or other drugs. Because liver tumors could be initiated and promoted by several agents which need to be activated, the possible hazard of TMX should be considered. This study reports that long-term treatment with TMX enhances hepatocarcinogenesis induced by DAB. The widespread use of TMX as an anticancer agent adds to the significance of this study.

Characterization of the major DNA adduct formed by alpha-hydroxy-N-desmethyltamoxifen in vitro and in vivo

Tamoxifen is hepatocarcinogenic in rats and has been associated with an increased risk of endometrial cancer in women. Recent reports suggest that it may be genotoxic in humans. N-Desmethyltamoxifen is a major tamoxifen metabolite that has been proposed to be responsible for one of the major adducts detected in liver DNA of rats treated with tamoxifen. The metabolic activation of N-desmethyltamoxifen to DNA binding products may involve oxidation to alpha-hydroxy-N-desmethyltamoxifen followed by esterification. In the study presented here, we report the synthesis of alpha-hydroxy-N-desmethyltamoxifen and the characterization of the major adduct obtained from alpha-sulfoxy-N-desmethyltamoxifen in vitro as (E)-alpha-(deoxyguanosin-N(2)-yl)-N-desmethyltamoxifen. In addition, we use (32)P-postlabeling in combination with HPLC to compare the adducts formed in the livers of female Sprague-Dawley rats treated by gavage with tamoxifen or equimolar doses of alpha-hydroxy-N-desmethyltamoxifen. We conclude that one of the major adducts formed in vivo and previously suggested to derive from N-desmethyltamoxifen is chromatographically identical to alpha-(deoxyguanosin-N(2)-yl)-N-desmethyltamoxifen.

Comparison of the DNA adducts formed by tamoxifen and 4-hydroxytamoxifen in vivo

Tamoxifen is a liver carcinogen in rats and has been associated with an increased risk of endometrial cancer in women. Recent reports of DNA adducts in leukocyte and endometrial samples from women treated with tamoxifen suggest that it may be genotoxic to humans. One of the proposed pathways for the metabolic activation of tamoxifen involves oxidation to 4-hydroxytamoxifen, which may be further oxidized to an electrophilic quinone methide. In the present study, we compared the extent of DNA adduct formation in female Sprague-Dawley rats treated by gavage with seven daily doses of 54 micromol/kg tamoxifen or 4-hydroxytamoxifen and killed 24 h after the last dose. Liver weights and microsomal rates of ethoxyresorufin O-deethylation, 4-dimethylaminopyrine N-demethylation and p-nitrophenol oxidation were not altered by tamoxifen or 4-hydroxytamoxifen treatment. Uterine weights were decreased significantly and uterine peroxidase activity was decreased marginally in treated as compared with control rats. DNA adducts were assayed by 32P-post-labeling in combination with HPLC. Two major DNA adducts were detected in liver DNA from rats administered tamoxifen. These adducts had retention times comparable with those obtained from in vitro reactions of alpha-acetoxytamoxifen and 4-hydroxytamoxifen quinone methide with DNA. Hepatic DNA adduct levels in rats administered 4-hydroxytamoxifen did not differ from those observed in control rats. Likewise, adduct levels in uterus DNA from rats treated with tamoxifen or 4-hydroxytamoxifen were not different from those detected in control rats. These data suggest that a metabolic pathway involving 4-hydroxytamoxifen is not a major pathway in the activation of tamoxifen to a DNA-binding derivative in Sprague-Dawley rats.

Results of three life-span experimental carcinogenicity and anticarcinogenicity studies on tamoxifen in rats

Tamoxifen was submitted to carcinogenicity bioassays on Sprague-Dawley rats (of the colony used at the Cancer Research Center in the Castle of Bentivoglio of the European Ramazzini Foundation of Oncology and Environmental Sciences) at the dose of 3.3 mg/kg b.w., by stomach tube, in three experiments. In the first experiment the drug was administered once daily, 6 days a week to male and female rats, 8 weeks old at start for their life span. In the second experiment, the drug was administered to female rats, 12 weeks old at start, once daily for 8 consecutive days every 8 weeks for their life span. In the third experiment the drug was administered to female rats, 56 weeks old at start, 6 times weekly for 40 weeks; and then the animals were kept alive for their life span. In the first experiment, a mild increase in hepatocarcinomas with low grading was detected. In the first and second experiments, a borderline increase in uterine malignancies was found. No carcinogenic effect was observed in the third experiment. In the three experiments, tamoxifen showed a strong, long-lasting chemopreventive effect on mammary benign tumors and cancers. The presented data also indicate that tamoxifen treatment reduces the incidence of other tumors: pituitary adenomas, adrenal pheochromocytomas, islet cell pancreatic tumors, Leydig cell testicular tumors, and polyps of the uterus.

Chemicals with carcinogenic activity in the rodent liver; mechanistic evaluation of human risk

A wide variety of chemicals, both naturally occurring and synthetic, have exhibited carcinogenic activity in rodent liver. Some are clearly DNA reactive whereas others produce only epigenetic effects. Hepatocarcinogens are categorized according to these properties and the characteristics of examples of both types are reviewed. DNA-reactive rodent hepatocarcinogens represent human cancer risk even at non-toxic exposures, whereas epigenetic agents pose either no risk because their effects are specific to rodents, or a risk only at high exposures at which they produce the same cellular effects in humans that are the basis for their carcinogenic activity in rodents.

Tamoxifen and secondary tumours. An update

The nonsteroidal antiestrogen tamoxifen is the most widely used anticancer drug. In women with breast cancer, adjuvant therapy with tamoxifen reduces relapse and improves overall survival. In advanced breast cancer, the response rate is more than 50% in hormonal dependent disease. In women treated with adjuvant tamoxifen the incidence of new primary breast cancers is decreased. This latter observation has led to the initiation of prevention trials. In 1989 the first report from a large prospective randomised trial showed a significant increase of endometrial carcinoma among women treated with adjuvant tamoxifen. This effect may be linked to the somewhat paradoxical estrogenic properties of tamoxifen. The endometrial effects should be considered in the long term use of tamoxifen, and should also be taken into account in the evaluation of the prevention trials. Animal data indicate that tamoxifen can induce tumours in other organ systems, for example the liver, but no increase in primary liver cancer has been reported from the randomised trials. In some of these trials an increase in other gastrointestinal cancers (e.g. colon and gastric carcinoma) has been observed. The mechanism behind this may be different from that of the endometrium. In animal systems, tamoxifen has shown to induce DNA damage, with formation of DNA adducts. The risk of secondary gastrointestinal cancer needs to be further evaluated. The adverse effects of tamoxifen have led to the development of new anti-estrogenic drugs and other estrogen reducing agents (e.g. aromatase inhibitors).

Safety assessment of pharmaceuticals: examples of inadequate assessments and a mechanistic approach to assuring adequate assessment

For a conventional organic new chemical entity (NCE) being developed as a pharmaceutical, standard regulatory safety assessment studies are required. Early in development, an NCE should undergo a safety/benefit analysis to justify further development. This analysis is made easier and more effective when comprehensive nonclinical data are available. One of the most important aspects of nonclinical toxicologic studies is to provide information on absence of potential carcinogenicity in humans. To avoid human exposures to potentially carcinogenic agents, even in early development of an NCE, the Decision Point Approach to carcinogen testing provides a useful guide to acquisition of mechanistically relevant data for risk assessment.

A realistic clinical perspective of tamoxifen and endometrial carcinogenesis

Tamoxifen has been the endocrine treatment of choice for all stages of breast cancer for nearly a decade. Millions of women are currently receiving tamoxifen worldwide, while large-scale randomised trials have been launched aiming to investigate the drug's merit as a preventive agent. However, there are now concerns about tamoxifen's potential carcinogenicity. The goal of this review is to address these concerns, re-evaluate the available data from laboratory biological models and those from clinical reports and put the whole issue into perspective. Our focus is the association between tamoxifen and the increased frequency of endometrial tumours, while key issues, such as the role of duration of tamoxifen therapy, are also addressed. Finally, we discuss the various monitoring strategies for early detection of endometrial lesions and pertinent problems most likely to be encountered by clinicians taking care of patients who are receiving tamoxifen.

Promotion potential of tamoxifen on hepatocarcinogenesis in female SD or F344 rats initiated with diethylnitrosamine

The liver promotion potential of tamoxifen (TAM), which has been widely used in the treatment of hormone-dependent breast cancers, was investigated using female SD or F344 rat initiated with diethylnitrosamine (DEN). In Experiment 1, 45 newborn female SD rats were administered DEN (100 mg/kg, i.p.) (Groups 1 and 2) or saline (Group 3) 24 h after birth. After weaning at week 3, Groups 1 and 3 were subcutaneously injected with TAM citrate (1 mg/rat per day), suspended in corn oil, in the subscapular region, while Group 2 was given the vehicle alone (s.c.) daily for 9 weeks, and killed at week 12. In Experiment 2, 70 female F344 rats at 7 weeks of age were divided into five groups. All animals were initially given DEN (200 mg/kg i.p.) for initiation. Two weeks later Groups 1-4 were given diets containing 100, 250, 500 ppm TAM, or 500 ppm PB for 6 weeks, respectively, while Group 5 was administered basal diet as a control for the same period. The rats were subjected to two-thirds partial hepatectomy (PH) at week 3 and were killed at week 8. The enhanced development of glutathione S-transferase-placental form (GST-P)-positive liver cell foci after DEN exposure in both newborn SD and adult F344 rat medium-term liver bioassay models (Experiments 1 and 2). This suggests that TAM exerts promotion potential for hepatocarcinogenesis in female rats.

Molecular mechanisms of antiestrogen action in breast cancer

The success of antiestrogen therapy to treat all stages of breast cancer, and the evaluation of tamoxifen as a preventive for breast cancer in normal women, have focused attention on the molecular mechanisms of antiestrogen action and mechanisms of drug resistance. The overall goal of research is to enhance current therapies and to develop new approaches for breast cancer treatment and prevention. Recent studies show that tamoxifen and the new pure antiestrogens appear to have different mechanisms of action: tamoxifen and related compounds cause a change in the folding of the steroid binding domain that prevents gene activation whereas the pure antiestrogens cause a reduced interaction at response elements and cause a rapid loss of receptor complexes. Tamoxifen treatment produces changes in the cellular and circulating levels of growth factors that could influence both receptor negative or receptor positive tumor growth and the metastatic potential of a tumor. These events may explain the survival advantage observed with tamoxifen therapy. However, the current therapeutic challenge is to avoid drug resistance during long-term tamoxifen therapy. Numerous explanations for drug resistance to tamoxifen have been suggested, including elevated estrogen levels, increased tumor antiestrogen binding sites, receptor mutations, and impaired signal transduction. However, it is probable that multiple mechanisms evolve to facilitate tumor survival. Most importantly, current research is examining mechanisms responsible for the beneficial actions of tamoxifen on bones and lipids as well as the potentially deleterious effects of tamoxifen on liver and endometrial carcinogenesis and retinopathy. The urgent need to understand antiestrogenic drug mechanisms and toxicity is being facilitated by the application of the technology developed for basic molecular biology.