Wednesday, January 13, 2016
Thyroidal and estrogenic activities in manure (and associated groundwater run-off)
When poultry manure is applied to agricultural land, the hormones present in it can be transferred to soil. These substances can make their way into surface and groundwater by leaching or by run-off, thereby contaminating aquatic ecosystems. Several studies have identified animal breeding activities as a major source of hormone input into the environment (Chen et al., 2010 and Hanselman et al., 2006). To date, most research addressing the presence of EDs in manure has focused almost exclusively on reproductive hormones, particularly on estrogenic compounds (Hotchkiss et al., 2008). These substances can emulate the action of endogenous estrogens by activating the estrogen receptor (Er). This receptor acts as a ligand-activated transcription factor that induces the expression of estrogen-dependent genes (Beato and Klug, 2000), which are related mainly to reproduction, differentiation and growth.
Estrogen excretion associated with farm chicken amounts to 2.8 tons per year in the European Union (EU) and 2.7 tons in the USA (Lange et al., 2002). Steroid hormones show high chemical stability, and they are excreted in the free form or as conjugates, which are almost immediately biotransformed to the free form (Panter et al., 1999 and Shore and Shemesh, 2003). 17β-Estradiol (E2) and estrone are the estrogens of greatest concern because they show a higher potency than other estrogens and can be found in the environment in concentrations above their lowest observable effect level (LOEL) for fish and plants (10 ng/l) (Miles-Richardson et al., 1999 and Shore et al., 1993).
More recently, the thyroid axis has also been recognized as a target of EDs (Jugan et al., 2010, Schmutzler et al., 2007 and Sciarrillo et al., 2008). The thyroid hormones (THs) triiodothyronine (T3) and thyroxine (T4) play a crucial role in the maintenance of a normal physiological state in vertebrates, notably in the control of development, growth, energy provision, reproduction and behavior (Yen, 2001). In amphibians and teleost fish, they also have essential functions in the regulation of metamorphosis, in the transition from larval to juvenile stages, and in smoltification in salmonids (Brown et al., 2004, Kloas, 2002 and Power et al., 2001). In recent years, many chemicals, including some PCBs, tetrabromobisphenol A (TBBPA), and polybrominated diphenyl ethers (PBDEs) (Boas et al., 2006), have been suspected of acting as thyroid disruptors. These substances may compete with endogenous hormones for binding to transport proteins (i.e. transthyretin) and/or to TH receptors (Tr), thus acting as either agonists or antagonists and disrupting TH homeostasis (Boas et al., 2006 and Kashiwagi et al., 2009). Trs, together with steroid receptors, belong to the nuclear receptor family and act as ligand-dependent transcription factors.
In contrast to estrogenic disruptors, studies devoted to thyroidal disruptors in environmental samples are scarce and have been limited to the detection of these compounds in rivers (Jugan et al., 2009), effluents from wastewater treatment plants (WWTPs) (Li et al., 2011 and Murata and Yamauchi, 2008) and drinking water (Shi et al., 2012). To our knowledge, the thyroid-disrupting effects of manure have not been addressed, in spite of the detection of THs or products of their degradation in livestock residues. TH degradation is initiated by deiodination in peripheral tissues. Thereafter THs are decarboxylated and conjugated to hydrophilic products by means of glucuronidation or sulfatation and excreted via bile through feces or urine. Part of the conjugated THs can be hydrolyzed in the colon and excreted as free compounds like T3 and T4 (Brouwer et al., 1998 and Schuur et al., 1999).
In addition to natural hormones, various chemicals with endocrine activity have been detected in manure, including PAH, dioxins, phthalates, bisphenol A and nonylphenol (Combalbert et al., 2012). All these substances affect the total hormonal activity of a manure sample.