Regulation of corticosterone secretion is modified by PFOS exposure at different levels of the hypothalamic–pituitary–adrenal axis in adult male rats
Introduction
Perfluorooctane sulfonate (PFOS) is a fluorinated organic compound declared as a Persistent Organic Pollutant (Stockholm Convention, 2009). It has been extensively used in industry, being the 3 M company the principal productor (United States Environmental Protection Agency (US-EPA), 2000). This compound is widely used as surfactant, in lubricants, polymers, fire fighting foams, in the photographic industry, as component of adhesives and paper coatings, in cosmetics, pesticides and food packaging, as well as in semiconductor production or in hydraulic fluids in the aviation industry. PFOS is a surface-active substance and it is used as water and fat repellent in textiles, carpets, etc. (United States Environmental Protection Agency (US-EPA), 2000Giesy and Kannan, 2001, Beach et al., 2006). This chemical is extremely persistent in the environment, it is neither hydrolyzed, nor photolyzed or degraded by any of the environmental conditions tested (Organisation for Economic Co-operation and Development (OECD), 2002). This fact has been shown by previous studies that have established that PFOS is present in many environmental compartments (Gómez et al., 2011, Corsolini et al., 2012, Kwadijk et al., 2013). Furthermore, studies focused on people not occupationally exposed to PFOS in the United States, Colombia, Brazil, Belgium, Italy, Poland, India, Malaysia and Korea, showed that PFOS is the most predominant perfluoroalkyl compound of those tested in blood, with a maximum value of 30 ng/ml (in United States and Poland) and a minimum of 3 ng/ml (in India) (Kannan et al., 2004).
PFOS can cross the blood–brain barrier and it is accumulated in many tissues like the adrenal gland, liver, kidney or brain regions, being the hypothalamus the one which presents the major concentration of PFOS in the brain of exposed rats, and it was three times higher than in other organs and physiological systems studied (Austin et al., 2003). According to several studies, PFOS can disrupt the endocrine system activity, fact evidenced in mammals as well as in other animal species (Austin et al., 2003, Chang et al., 2008, Chang et al., 2009, Ribes et al., 2010, Zhao et al., 2011, Knox et al., 2011, Fang et al., 2012, Pereiro, 2012, Spachmo and Arukwe, 2012, Du et al., 2013): it can alter the estrous cycle (Austin et al., 2003) and the glucocorticoid and thyroid metabolisms (Knox et al., 2011, Zhao et al., 2011). This chemical also exerts estrogenic activity (Fang et al., 2012) by modifying the gen expression of hypothalamic estrogen receptors in adult male rat (Lafuente et al., 2013), acting as an estrogen receptor agonist (Du et al., 2013, Fang et al., 2012) and interfering with steroidogenesis in mice and zebrafish (Du et al., 2013, Wan et al., 2011). PFOS also affects the hypothalamic–pituitary–testicular and the hypothalamic–pituitary–adrenal (HPA) axes function (Austin et al., 2003, Ribes et al., 2010). Similar endocrine effects were observed with other xenobiotics (Harvey et al., 2007, Lafuente, 2013). Furthermore, a clear relationship between PFOS exposure and stress was reported in both adult and pregnant mice (Fuentes et al., 2007a, Fuentes et al., 2007b, Ribes et al., 2010), fact likely related to its toxic effects on the HPA axis.
Although PFOS effects on thyroid hormones have been reported (Yu et al., 2009, Lopez-Espinosa et al., 2012), its toxic effects on adrenal gland are poorly known, with the exception of some studies (Austin et al., 2003, Fuentes et al., 2007a, Fuentes et al., 2007b, Ribes et al., 2010, Zheng et al., 2009, Zheng et al., 2011). However, the functional role of the adrenal gland is very important in health, metabolism, development, endocrine and immune systems activity, and it has been documented to be the most common toxicological target of all endocrine organs (Colby and Longhurst, 1992, Harvey et al., 2007, Ribelin, 1984). In fact, according to in vivo toxicology studies, the order of endocrine organ toxicity by frequency of reported effects was: adrenal gland > testis > thyroid > ovary > pancreas > pituitary gland > parathyroid glands, with the adrenal cortex, rather than the medulla, being the most frequent site of toxicity within the adrenal gland (Colby and Longhurst, 1992, Harvey et al., 2007, Ribelin, 1984). The high content of cytochrome P-450 enzymes in the adrenal cortex together with its remarkable tendency to accumulate hydrophobic substances, probably contributes to the extraordinary vulnerability of this endocrine gland to a high number of xenobiotics (Harvey et al., 2007, Hinson and Raven, 2006). In the context of adrenal/endocrine toxicology, the major priority should be to identify compounds causing functional suppression of the adrenal gland, since there are already clear examples of chemically induced adrenal suppression in human clinical toxicology (Harvey and Everett, 2003, Harvey and Everett, 2006, Harvey et al., 2007).
The adrenal gland is a vital organ. Inhibitory effects on the activity of the adrenal cortex induced by single brief exposures to some compounds at remarkably low dose levels and its lethal consequences in humans have been reported (Harvey et al., 2007). Corticosterone is involved in the regulation of important physiological functions such as immune system activity (Elftman et al., 2007); metabolism, nutrition and obesity (Pratchayasakul et al., 2011); behaviour and motor function (Metz et al., 2005); response to stress situations (Sun et al., 2010), and neurotransmission at the central level (Tse et al., 2012). Therefore, any alteration on corticosterone synthesis and/or secretion induced by PFOS exposure could lead to several negative consequences and different diseases.
On the other hand, the regulation of the HPA axis activity has been well established (Herman et al., 2012, Everds et al., 2013). The activation of this physiological axis is due to an adequate stimulation by circadian rhythms and/or stressful conditions. The main hormones and neuropeptides of this neuroendocrine axis are shown in Fig. 1. Moreover, ACTH receptors in the adrenal gland and the hypothalamus are indicated in this scheme (Fig. 1) because of its role. Synthesis and secretion of glucocorticoids (e.g. corticosterone) from the adrenal cortex are regulated by the adrenocorticotropic hormone (ACTH) secreted from chromophobic corticotroph cells located in the adenohypophysis (Herman et al., 2012). This pituitary hormone is a key signalling component of the HPA and its release is stimulated by the corticotrophin releasing hormone (CRH) (Preil et al., 2001), which is synthesized by the hypophysiotropic parvocellular neurons in the hypothalamic paraventricular nucleus of the anterior hypothalamus (de Kloet et al., 1998, Herman et al., 2012, Everds et al., 2013). Furthermore, the CRH is a mediator of endocrine, autonomic, and immune responses to stress (Vale et al., 1981, Owens and Nemeroff, 1991, De Souza, 1995, Brunson et al., 2001). This neuropeptide is also involved in the modulation of a wide range of behaviours including anxiety, arousal, motor function (Dunn and Berridge, 1990), learning, memory (Blank et al., 2002, Fenoglio et al., 2006), as well as in pathological conditions including anxiety, depression (Nemeroff and Vale, 2005) dementia (Rehman, 2002) and addiction (Koob, 2006).
Taking into account the regulatory mechanisms of the HPA axis activity, certain chemicals can alter adrenocortical function by modifying the gene expression and secretion of CRH at the hypothalamic level and/or ACTH release on pituitary gland, as well as directly acting on the adrenal cortex (Harvey et al., 2007, Kasperlik-Zaluska et al., 2005, Sonino et al., 2005). Proopiomelanocortin (POMC) is a multifunctional precursor protein that undergoes cell-specific processing to produce a number of bioactive peptides, including ACTH in anterior pituitary corticotroph cells (Bicknell, 2008). In addition, the gene expression of ACTH receptors (ACTHr) in both pituitary gland and hypothalamus could be involved in these toxic mechanisms. Therefore, adrenal toxicity may directly occur on this gland and/or secondarily in other tissues of the HPA axis such as hypothalamus and/or pituitary gland (Harvey et al., 2007). The importance of this fact is that these xenobiotics might alter the adrenal gland activity by specific mechanisms only likely to be detected by in vivo studies, with intact HPA axis function (Harvey et al., 2007).
Reactive oxygen species (ROS) and oxidative stress induced by PFOS exposure has been reported in both in vivo and in vivo studies in different species and tissues (Lee et al., 2012, Liu et al., 2007, Mao et al., 2012, Qian et al., 2010, Wang et al., 2013, Xu et al., 2013). This chemical induces apoptosis (Lee et al., 2012, Liu et al., 2007, Mao et al., 2012) and modifies the permeability of the endothelial barrier (Qian et al., 2010) through ROS. As a result, oxidative stress could be involved in the PFOS toxicity in the adrenal gland, leading to structural and functional alterations in this tissue.
In view of the facts above mentioned, the present study was undertaken (1) to evaluate the possible effects of PFOS exposure on the regulation of corticosterone secretion in adrenal and pituitary glands and at hypothalamic level in adult male rat, by analyzing the gene expression and/or concentration of CRH, POMC, ACTH, ACTHr and corticosterone; (2) to study the possible cell adaptation in the adrenal gland to PFOS toxicity by analyzing several antioxidant defense markers: the relative gene expression of nitric oxide synthase 1 (NOS1) and nitric oxide synthase 2 (NOS2) and the activity of superoxide dismutase (SOD); and finally (3) to evaluate the possible morphological alterations induced by PFOS in this endocrine gland through light and electron microscopy.
Section snippets
Animals and experimental design
Adult male Sprague–Dawley rats were obtained from the animal facilities of the University of Santiago (Santiago de Compostela, Spain), which were 60 day-old and their weigh was 305 ± 16.4 g at the beginning of the experiment. All animals were remained under constant environmental conditions (temperature of 22 ± 2 °C and an automatic day–night cycle (light: 09:00–21:00 h). Animals had been fed with compound feed and water ad libitum. This study has been conducted according to the European and Spanish
Results
Stress or behavioural alterations were not observed in PFOS exposed rats. Furthermore, the relative weight of the hypothalamus and pituitary gland did not change after the treatment with this compound, whilst the relative weight of the adrenal gland decreased in the rats treated with PFOS (Fig. 2; P ≤ 0.001 for the doses of 0.5 and 1.0 of PFOS/kg/day and P ≤ 0.05 for the doses of 3.0 and 6.0 mg of PFOS/kg/day vs. control group).
Discussion
In general terms, the obtained results suggest an inhibition of the HPA axis activity induced by oral administration of PFOS at the doses of 0.5, 1.0, 3.0 and 6.0 mg/kg/day in adult male rats, reflected on CRH, ACTH and corticosterone secretion.
The doses of PFOS administered in the present study were chosen according to the no-observed-adverse-effect level (NOAEL) and the lowest-observed-adverse-effect-level (LOAEL) of PFOS in rats. The NOAEL of this chemical is 1.25 mg/kg/day in Sprague–Dawley
Transparency document
The authors declare that there are no conflicts of interest.
Acknowledgements
This work was supported by grants from the Ministry of Science and Innovation, Spain (AGL2009-09061) and from the Xunta de Galicia (INCITE09 383 314 PR). The authors wish to acknowledge Mrs. Ana Ramos's inestimable collaboration in revising the use of English.
References (87)
- et al.
Thyroid hormone status and pituitary function in adult rats given oral doses of perfluorooctanesulfonate (PFOS)
Toxicology
(2008) - et al.
Gestational and lactational exposure to potassium perfluorooctanesulfonate (K + PFOS) in rats: toxicokinetics, thyroid hormone status, and related gene expression
Reprod. Toxicol.
(2009) - et al.
Perfluorinated compounds in surficial sediments of the Ganges river and adjacent Sundarban mangrove wetland, India
Mar. Pollut. Bull.
(2012) - et al.
Quantification of MYCN, DDX1, and NAG gene copy number in neuroblastoma using a real-time quantitative PCR assay
Mod. Pathol.
(2002) Corticotropin-releasing factor receptors: physiology, pharmacology, biochemistry and role in central nervous system and immune disorders
Psychoneuroendocrinology
(1995)- et al.
Physiological and behavioral responses to corticotropin-releasing factor administration: is CRF a mediator of anxiety or stress responses?
Brain Res Brain Res. Rev.
(1990) - et al.
Evaluation of hepatic and thyroid responses in male Sprague Dawley rats for up to eighty-four days following seven days of dietary exposure to potassium perfluorooctanesulfonate
Toxicology
(2012) - et al.
PFOS elicits transcriptional responses of the ER, AHR and PPAR pathways in Oryzias melastigma in a stage-specific manner
Aquat. Toxicol.
(2012) - et al.
Hippocampal neuroplasticity induced by early-life stress: functional and molecular aspects
Front Neuroendocrinol.
(2006) - et al.
Associations between PFOA, PFOS and changes in the expression of genes involved in cholesterol metabolism in humans
Environ. Int.
(2013)