Investigation of effects of myricetin on thyroid-gonadal axis of male rats at prepubertal period
Introduction
In recent years, there is an increasing concern about the relationship between human male reproductive disorders and environmental factors. It was reported that some compounds have estrogenic and androgenic properties caused male reproductive tract disorders such as increased testicular tumor, reduced sperm count, cryptorchidism and hypospadias. Human are exposed to these environmental chemicals like bisphenol A and phthalate with food packaging, synthetic hormones in pharmaceutics, pesticides in nourishments, solvents in cleaning products, detergents. Also, environmental estrogens include phytoestrogens in fruits and vegetables all day long (Pflieger-Bruss, 2004, Castro et al., 2014). Phytoestrogens’ chemical structure is similar to estradiol, an endogenous estrogen. As a result, these biologically active plant compounds can bind to estrogen receptors in various cells (Aguilla et al., 2013). Therefore, they may alter concentrations of endogenous sex hormones and they may alter sex differentiation and increase the risk of reproductive tract tumors or developmental disorders especially during fetal development (Kasum, 2002).
Phytoestrogens are found in several plants and they are daily consumed by people and animals. They are comprised four major classes: isoflavones, flavonoids, coumestans and lignans, are polyphenolic antioxidants (Ganry, 2005). Polyphenolic compounds like flavonoids have come to researchers’ attention because of their potent antioxidant features and their ability of preventing from various diseases related to oxidative stress such as cancer, coronary heart disease and aging (Park and Kim, 2011).
Flavonoids also act like a regulator role on hormones such as androgens, estrogens and thyroid hormones (Agrawal, 2011). Studies show that, in mammals, thyroid system plays a critical role in the development of many organs and systems, including the reproductive tract (Mendis-Handagama, 2005). When flavanoids are consumed at large quantities, it is expected that they might be major cause of histological changes in the thyroid such as thyroid gland growth and goiter by increasing TSH levels (Santos et al., 2011).
Opposite of this, Yin (1999) demonstrated that flavonoids exhibit antiproliferative activity in vitro against human thyroid carcinoma cells and they suggested that flavonoids may contribute to new drug developments for the treatment of thyroid cancer. They have antiviral, antiplatelet, anti-allergic, anti-inflammatuar, antitumor and antioxidant activity. Especially, flavonols have strong antioxidant activity (Ferreyra, 2012).
Myricetin belongs to flavonol group present in vegetables such as broccoli (62.5 mg/kg), red pepper (171.5 mg/kg), green bean (47.0 mg/kg), Chinese cabbage (31.0 mg/kg), garlic (693.0 mg/kg) (Miean, 2001) and in fruits such as currant (71 mg/kg), bilberry (14–142 mg/kg), blackberry (23–26 mg/kg) (Hakkinen, 1999) and in black tea (303 mg/kg) (Miean, 2001).
When myricetin is consumed, some part of it is absorbed in gastrointestinal system and the rest of it is metabolized by gastrointestinal microflora. Liver is the main responsible organ for metabolism of myricetin (Ong, 1997). It was showed that among flavonoids (quercetin, kaempferol, catechin and rutin), myricetin had the highest antioxidant capacity. Comparing to others, myricetin has more phenolic hydroxyl groups. It was proved that their antioxidant activity increase depending on a number of phenolic hydroxyl group (Pekkarinen, 1999). These phenolic groups provide binding to estrogen. Thus, myricetin may act like estrogen agonists or antagonists (Makela, 1995). At another study, Tiwari et al. (2009) showed that myricetin has an ability of preventing cardiotoxic effects induced by isoproterenol. Kang (2011) proved that myricetin was a substantial chemo preventive agent in skin carcinogenesis. Knekt (2002) found that men with higher myricetin intakes had a lower prostate cancer risk. Myricetin has also bactericidal properties. It has been reported that myricetin exhibited inhibitory activity against development of Burkholderia cepacia multidrug resistant bacteria and vancomysin resistant enterococci (Xu, 2001).
This product is purchased in tablets as dietary supplement due to its benefits on human health. However, there are not enough comprehensive and long term in vivo animal and human experiments about adverse effects of these supplements.
In the present study, we hypothesized that exposure to myricetin in the EDSTAC male pubertal protocol would alter the development of the male rat reproductive system and thyroid function.
Section snippets
Chemicals
17α-ethinyl estradiol (%98) and myricetin (%96) were purchased from Sigma-Aldrich. Chemicals were dissolved in corn oil (vehicle). ALT, AST, urea, total protein, albumin, creatine, glucose and LDH kits were bought from Audit Diagnostics (Ankara, Turkey). Thyroxin (T4), triiodothyronine (T3) and thyroid-stimulating hormone (TSH) were purchased from Calbiotech.
Animals
Twenty one day male Wistar rats were purchased from the Experimental Animals Production Center, Hacettepe University in Ankara, Turkey.
Body and organ weights
The body and organ weights of control and treatment groups are given in Table 1. The body weights were decreased statistically in 0.7 and 7 μg/kg/day EE treatment groups, compared with the oil control group. Otherwise, the body weight in 25 mg/kg/day myricetin dose group was increased compared to the EE dose groups and in 50 mg/kg/day of myricetin dose group. The absolute and relative weight of right and left testis was decreased in 7 μg/kg/day EE treatment group compared to the oil and 0.7
Discussion
In the last ten years, physiological and pharmacological roles of bioactive compounds found in plants have been the focus of attention by scientists. Some epidemiological and experimental studies show that consumption of foods which are rich from phytoestrogens provides the beneficial health effects in human. Previous studies have shown that myricetin has antioxidant, anti-inflammatory and potent anticancer effects (Weng and Yen, 2012, Sun et al., 2012). A number of factors like age at
Conflict of interest
The authors declare that there are no conflicts of interest.
Acknowledgment
This research was supported by Health Sciences Research Group of TUBITAK (Project number is 111S013).
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