Long term inhibition by estradiol or progesterone of melatonin secretion after administration of a mammary carcinogen, the dimethyl benz(a)anthracene, in Sprague-Dawley female rat; inhibitory effect of Melatonin on mammary carcinogenesis

Chemically induced carcinogenesis in rodent models of aging: assessing organismal resilience to genotoxic stressors in geroscience research

There is significant overlap between the cellular and molecular mechanisms of aging and pathways contributing to carcinogenesis, including the role of genome maintenance pathways. In the field of geroscience analysis of novel genetic mouse models with either a shortened, or an extended, lifespan provides a unique opportunity to evaluate the synergistic roles of longevity assurance pathways in cancer resistance and regulation of lifespan and to develop novel targets for interventions that both delay aging and prevent carcinogenesis. There is a growing need for robust assays to assess the susceptibility of cancer in these models. The present review focuses on a well-characterized method frequently used in cancer research, which can be adapted to study resilience to genotoxic stress and susceptibility to genotoxic stress-induced carcinogenesis in geroscience research namely, chemical carcinogenesis induced by treatment with 7,12-dimethylbenz(a)anthracene (DMBA). Recent progress in understanding how longer-living mice may achieve resistance to chemical carcinogenesis and how these pathways are modulated by anti-aging interventions is reviewed. Strain-specific differences in sensitivity to DMBA-induced carcinogenesis are also explored and contrasted with mouse lifespan. The clinical relevance of inhibition of DMBA-induced carcinogenesis for the pathogenesis of mammary adenocarcinomas in older human subjects is discussed. Finally, the potential role of insulin-like growth factor-1 (IGF-1) in the regulation of pathways responsible for cellular resilience to DMBA-induced mutagenesis is discussed.

Metformin and melatonin improve histopathological outcome of NMU-induced mammary tumors in rats

OBJECTIVE

Numerous reports showed inhibition of carcinogenesis after metformin (MF) and melatonin (MEL) administration. However, most in vivo studies used standard diet type, with relatively low fat content. As increase in fat intake may have a considerable impact on malignant transformation, we evaluated the effects of these two substances in a model of mammary carcinogenesis in rats fed a high-fat diet (10%).

METHODS

Mammary tumors were induced by N-methyl-N-nitrosourea (NMU) in female rats of sensitive Sprague-Dawley strain. MF was administered in a diet (0.2%), MEL was administered in drinking water (20 mg/L). The chemoprevention was initiated 12 days prior to tumor initiation, both substances were administered through the termination of the experiment on 16th week after carcinogen application. Analysis of basic parameters of tumor growth, histopathological profile, and serum IGF-1 level were performed together with immunohistochemical detection of Ki67 (proliferation marker) and caspase-3 and BCL-2 (apoptosis markers) in mammary cancer cells.

RESULTS

Although neither tumor incidence nor frequency were changed after MF and/or MEL administration, MF and MEL decreased high-grade/low-grade (HG/LG) tumor ratio. MEL decreased proliferation in mammary cancer cells; positive correlations between histological grade and Ki67 expressions were found after single administration of both MF and MEL. Serum IGF-1 levels were reduced to the level of intact rats in all groups receiving chemoprevention.

CONCLUSIONS

MF and MEL administration did not inhibit growth of NMU-induced mammary tumors in rats in a significant manner but both substances ameliorated tumor histopathological profile. Surprisingly, combined treatment had no such effect.

Dimethyl-Benz(a)anthracene: A mammary carcinogen and a neuroendocrine disruptor

Polycyclic Aromatic Hydrocarbons (PAHs) are potent carcinogens. Among these, dimethylbenz(a)anthracene (DMBA) is well known for its capacity to induce mammary carcinomas in female Sprague-Dawley (SD) rats. Ovariectomy suppresses the susceptibility of this model to DMBA, thus suggesting that the inducible action of the carcinogen depends on ovarian hormones. The promotion of DMBA-induced adenocarcinoma is accompanied by a series of neuroendocrine disruptions of both Hypothalamo-Pituitary-Gonadal (HPG) and Hypothalamo-Pituitary-Adrenal (HPA) axes and of the secretion of melatonin during the latency period of 2 months that precedes the occurrence of the first mammary tumor. The present review analyses the various neuroendocrine disruptions that occur along the HPG and the HPA axes, and the marked inhibitory effect of the carcinogen on melatonin secretion. The possible relationships between the neuroendocrine disruptions, which essentially consist in an increased pre-ovulatory secretion of 17β-estradiol and prolactin, associated with a marked reduction of melatonin secretion, and the decrease in gene expression of the receptors for aryl-hydrocarbons receptor (AhR) and 17β-estradiol (ERα; ERβ) are also discussed.

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Polycyclic Aromatic Hydrocarbons influence promotion of breast tumorigenesis.

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Dimethylbenz(a)anthracene (DMBA) alters neuroendocrine axes and melatonin secretion.

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DMBA modulates the activity of aryl hydrocarbon and 17β-estradiol receptors.

Melatonin Suppresses the Growth of Ovarian Cancer Cell Lines (OVCAR-429 and PA-1) and Potentiates the Effect of G1 Arrest by Targeting CDKs

Melatonin is found in animals as well as plants. In animals, it is a hormone that anticipates the daily onset of darkness and regulates physiological functions, such as sleep timing, blood pressure, and reproduction. Melatonin has also been found to have anti-tumor properties. Malignant cancers are the most common cause of death, and the mortality rate of ovarian tumor is the highest among gynecological diseases. This study investigated the anti-tumor effects of melatonin on the ovarian cancer lines, OVCAR-429 and PA-1. We observed the accumulation of melatonin-treated cells in the G₁ phase due to the down-regulation of CDK 2 and 4. Our results suggest that in addition to the known effects on prevention, melatonin may also provide anti-tumor activity in established ovarian cancer.

Upregulated Myc expression in N-methyl nitrosourea (MNU)- induced rat mammary tumours

BACKGROUND

The most common incident cancer and cause of cancer-related deaths in women is breast cancer. The Myc gene is upregulated in many cancer types including breast cancer, and it is considered as a potential anti-cancer drug target. The present study was conducted to evaluate the Myc (gene and protein) expression pattern in an experimental mammary tumour model in rats.

MATERIALS AND METHODS

Thirty six Sprague Dawley rats were divided into: Experimental group (26 animals), which received the chemical carcinogen N-methyl nitrosourea (MNU) and a control group (10 animals), which received vehicle only. c-Myc oncoprotein and its mRNA expression pattern were evaluated using immunohistochemistry (IHC) and semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR), respectively, in normal rat mammary tissue and mammary tumours. The rat glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene was used as internal control for semi-quantitative RT-PCR.

RESULTS

Histopathological examination of mammary tissues and tumours from MNU treated animals revealed the presence of premalignant lesions, benign tumours, in situ carcinomas and invasive carcinomas. Immunohistochemical evaluation of tumour tissues showed upregulation and heterogeneous cellular localization of c-Myc oncoprotein. The expression levels of c-Myc oncoprotein were significantly elevated (75- 91%) in all the tumours. Semi-quantitative RT-PCR revealed increased expression of c-Myc mRNA in mammary tumours compared to normal mammary tissues.

CONCLUSIONS

Further large-scale investigation study is needed to adopt this experimental rat mammary tumour model as an in vivo model to study anti-cancer strategies directed against Myc or its downstream partners at the transcriptional or post-transcriptional level.

N-methylnitrosourea induced breast cancer in rat, the histopathology of the resulting tumours and its drawbacks as a model

Several animal models of breast cancer have been developed to study various aspects of breast cancer biology. Substantial evidence suggests that the N-methylnitrosourea (MNU) animal model mimics human breast cancer in many respects. It has therefore been used extensively to evaluate preventive and therapeutic agents for human breast cancer. Chemically induced rodent models are also suitable for studying malignant progression. Recently, Liska et al. [7] established two protocols of MNU administration depending on the animal's age and number of applications of carcinogen, with the aim of investigating the advanced stages of mammary gland tumours. We used the same protocol as Liska but have obtained substantially different results. These results are presented and discussed in the frame of suggested key drawbacks of the MNU induced breast cancer rat model, as a contribution to the debate about the suitability of that model for evaluating preventive and therapeutic agents.

The Anti-tumor Activity of Pineal Melatonin and Cancer Enhancing Life Styles in Industrialized Societies

This review discusses the potential role of the anti-tumor activity of pineal melatonin for the aetiology and prevention of cancers related to life-styles in industrialized societies, e.g. frequent long-distance flights as well as chronic night shift work leading to circadian disturbances of neuroendocrine parameters including melatonin. Experimental studies show that melatonin controls not only the growth of well-differentiated cancers, but also possesses anti-carcinogenic properties. Therefore, it is plausible that disturbances of circadian melatonin rhythmicity could be functionally involved in elevated cancer risks among aircrew members and nurses frequently working on night shifts. Due to the suppression of melatonin by light it can be assumed that too much artificial light at night could, at least in part, be responsible for generally increasing rates of e.g. breast cancer in industrialized countries. It is discussed under which conditions a transient substitutional therapy with melatonin could be justified or which forms of living could help to physiologically foster melatonin secretion to optimise control over cancerous growth and development.

Long-term dysregulation of circadian and 17-beta estradiol-induced LH, prolactin and corticosterone secretion after dimethylbenz (a) anthracene administration in the Sprague-Dawley female rat

A single intragastric administration of 7,12-dimethylbenz (a) anthracene (DMBA) has been shown, when given at 55-60 days of age, to induce mammary tumors in young cycling female Sprague-Dawley rats. The appearance of the tumors is preceded by a series of neuroendocrine disturbances of the hypothalamo-pituitary-gonadal (HPG) axis, including attenuation of the preovulatory Luteinizing Hormone (LH) and Gonadotropin-Releasing Hormone (GnRH) release and amplification of the preovulatory 17beta-Estradiol (E(2)) surge. In this study, we examined the hypothesis that a single administration of DMBA could also, in the long range, induce disturbances of others neuroendocrine axis, like the Hypothalamic-Pituitary-Adrenal (HPA) axis and/or the Lactotroph axis. Sprague-Dawley rats, 55-60 days of age, received, on the day of Estrous of the Estrous cycle, a single administration of 15 mg of DMBA delivered by intragastric intubation. Then, they were ovariectomized 5 days later. One month later, (1) Two groups of animal were sacrificed by decapitation at 09:00 a.m. and 05:00 p.m. to record the circadian rhythm of plasma LH, Prolactin (PRL) and corticosterone, (2) Three other groups of animal were sacrificed by decapitation at three different times after a morning subcutaneous administration of 50 microg/kg of Estradiol Benzoate (EB), to induce a negative and positive feed-back of the secretion of LH. Then, plasma LH, PRL and corticosterone concentrations were measured. After DMBA administration, (1) the negative--but not the positive--LH feed-back was seen, (2) the PRL circadian rhythm was blunted and the corticosterone circadian rhythm was almost absent, (3) the increase in PRL or Corticosterone plasma concentration was significantly reduced. In conclusion, a single administration of DMBA provokes a long-term dysregulation of not only the HPG axis but also of the lactotroph and HPA axis. These dysregulations, along with the already evidenced long-term inhibition of DMBA upon Melatonin secretion from the pineal gland, might accelerate the promotion of mammary tumors induced by the mammary carcinogen.

Melatonin increases the survival time of animals with untreated mammary tumours: Neuroendocrine stabilization

The aim of this study was to evaluate the therapeutic effect of melatonin, the main hormone of the pineal gland, on rats with advanced and untreated mammary tumours. Mammary tumours were chemically induced in Sprague-Dawley rats with the carcinogen 9,10-dimethyl-1,2-bezanthracene (DMBA). After the appearance of tumours the effect of melatonin (5 mg/ml per rat per day) was then evaluated on the survival time, tumour multiplicity, and tumour volume until the death of the animals. In addition, the variations in prolactin, noradrenaline and adrenaline concentrations, and in the percentage of NK cells were evaluated after one month of the treatment with melatonin. Daily administration of melatonin increased significantly the survival time of tumour-bearing animals (p<0.05 with respect to the control non-melatonin-receiving rats). The increased survival time did not correlate, however, with changes in either tumour multiplicity or tumour growth rate. Animals with mammary tumours exhibited an increase (p<0.05 with respect to healthy animals) in prolactin and catecholamine concentrations. The administration of melatonin stabilized the hormone levels, returning them to those in the basal-healthy animals. Rats with mammary tumours also presented lower percentages of NK cells, which were not increased by the administration of melatonin. The results strongly suggest that melatonin per se is beneficial during advanced breast cancer. It increases survival time, maybe by improving the homeostatic and neuroendocrine equilibrium which is imbalanced during advanced breast cancer.

Preventive and curative effect of melatonin on mammary carcinogenesis induced by dimethylbenz[a]anthracene in the female Sprague-Dawley rat

Introduction

It has been well documented that the pineal hormone, melatonin, which plays a major role in the control of reproduction in mammals, also plays a role in the incidence and growth of breast and mammary cancer. The curative effect of melatonin on the growth of dimethylbenz [a]anthracene-induced (DMBA-induced) mammary adenocarcinoma (ADK) has been previously well documented in the female Sprague–Dawley rat. However, the preventive effect of melatonin in limiting the frequency of cancer initiation has not been well documented.

Methods

The aim of this study was to compare the potency of melatonin to limit the frequency of mammary cancer initiation with its potency to inhibit tumor progression once initiation, at 55 days of age, was achieved. The present study compared the effect of preventive treatment with melatonin (10 mg/kg daily) administered for only 15 days before the administration of DMBA with the effect of long-term (6-month) curative treatment with the same dose of melatonin starting the day after DMBA administration. The rats were followed up for a year after the administration of the DMBA.

Results

The results clearly showed almost identical preventive and curative effects of melatonin on the growth of DMBA-induced mammary ADK. Many hypotheses have been proposed to explain the inhibitory effects of melatonin. However, the mechanisms responsible for its strong preventive effect are still a matter of debate. At least, it can be envisaged that the artificial amplification of the intensity of the circadian rhythm of melatonin could markedly reduce the DNA damage provoked by DMBA and therefore the frequency of cancer initiation.

Conclusion

In view of the present results, obtained in the female Sprague–Dawley rat, it can be envisaged that the long-term inhibition of mammary ADK promotion by a brief, preventive treatment with melatonin could also reduce the risk of breast cancer induced in women by unidentified environmental factors.