DEVELOPMENT OF YEAST BASED STEROID BIOASSAY FOR SCREENING SOME NATURAL ENDOCRINE DISRUPTORS

Abstract

Evidence has been accumulating which ind icates that humans and domestic and wildlife species have suffered adverse health consequences from exposure to environmental chemicals that interact with the endocrine system. To date, these health problems have been identified primarily in domestic or wildlife species with relatively high exposures to organochlorine compounds, including l,l,l-trichloro-2,2-bis (p-chlorophenyl) ethane (DDT) and its metabolites, polychlorinated biphenyls (PCBs) and dioxins, or to naturally occurring plant estrogens. It is not known if similar effects are occurring in the general human population, but again there is evidence of adverse effects in populations with relatively high exposures. Several reports (Medical Research Council, 1995) of declines in the quality and decreases in the quantity of sperm production in humans over the last four decades and reported increases in incidences of certain cancers (breast, prostate, testicular) that may have an endocrine related basis have led to speculation about environmental etiologies. However, considerable scientific uncertainty remains regarding the causes of these reported effects. Nevertheless, it is known that the normal functions of all organ systems are regulated by endocrine factors, and small disturbances in endocrine function, especially during certain stages of the life cycle such as development, pregnancy, and lactation, can lead to profound and lasting effects. The critical issue is whether sufficiently high levels of endocrine-disrupting chemicals exist in the ambient environment to exert adverse health effects on the general population. Current methodologies for assessing human and wildlife health effects (e.g., the generation of data in accordance with testing guidelines developed by the U.S. Environmental Protection Agency [U.S. EPA]) are generally targeted at detecting effects rather than mechanisms, and may not adequately evaluate effects on the endocrine system. This is particularly true for exposures that occur during critical developmental periods when the endocrine system plays a key role in regulating essential physiological and morphological processes. Given the potential scope of the problem, the possibility of serious adverse effects on the health of human and wildlife populations, and the broad occurrence and persistence of some endocrine-disrupting agents in the environment, it is important to focus on the available resources for research on the most critical gaps in our knowledge base so that more informed regulatory and public health decisions can be made in the future. The broad nature of the problem necessitates a coordinated effort on both the national and the international levels. In response to the growing public health concerns related to chemicals in the environment that have the potential to act as endocrine disruptors, the Office of Research and Development of the U.S. EPA held a workshop on April 10-13, 1995, in Raleigh, North Carolina, to begin developing a research strategy related to endocrinedisrupting chemicals. An environmental endocrine disrupter was broadly defined as "an exogenous agent that interferes with the production, release, transport, metabolism, binding, action or elimination of natural hormones in the body responsible for the maintenance of homeostasis and the regulation of developmental processes" (adapted from EDSTAC, 1998). This definition reflects a growing awareness that the issue of endocrine disruptors in the environment extends considerably beyond that of exogenous (anti)estrogens and includes (anti)androgens and agents that act on other components of the endocrine system such as the thyroid and pituitary glands. In many cases, these endocrine disruptors share no apparent structural similarities to traditional steroids. Endocrine disruptors include natural products (phytoestrogens, e.g., genistein) (Martin et al., 1978; Miksicek et al., 1993; Verdeal et al., 1979), pharmaceuticals (i.e., diethylstilbestrol, ethynyl estradiol) (McLachlan et al, 1984), environmental pollutants (i.e., DDT, polychlorinated biphenyls, dioxins, polyaromatic hydrocarbons) (Clemons et al., 1996; Fieldenet al., 1997; Jansen et al., 1993; Kelce et al., 1995; Safe, 1995), and industrially relevant chemicals (i.e., alkylphenols, bisphenol A) (Korach, 1993; Krishnan et al., 1993; Nimrod et al., 1996; Wilcox et al., 1995; Fisch et al., 1996; Fisch et al., 1996). However, recent scientific evidences reveal the existence of newly defined class of environmental contaminants which may have variable chemical structures and can alter the normal endocrine physiology (Cargouet et al., 2004). Endocrine disrupting compounds can be categorized as (anti)androgenic, (anti)progestagenic and (anti)estrogenic based on with which steroid it's alike or mimicking. Mostly they enter the animal and/or human system through oral ingestion of their diet as well as contaminated water and possibly generate an agonistic and/or antagonistic effect. Endocrine disrupting compounds generally exert their effects either by indirectly targeting the arylhydrocarbon receptor (AhR) (Indarto and Izawa, 2001) or by direct binding of these compounds to steroid receptors (Sanderson and Vanden Berg, 2003; Rice et al., 2006). Thereby, they upregulate and/or downregulate the promoter of the target gene (Lin et al., 2006; Lyssimachouet al., 2006). Evidences also suggests that various environmental as well as therapeutic compounds acts as endocrine disruptors yielding sex hormone disease or disorders (Satoh et al., 2001; Roy et al., 2005; Sone et al., 2005; Guillete, 2006; Buck et al., 2006; Darbre, 2006; Massart et al., 2006; Maffini et al., 2006; Chen et al., 2007). Chronic exposure in a very low dose of these compounds may disturb the delicate hormone balance and results in serious reproductive anomaly (Ralph et al., 2003). Different types of chemicals and their byproducts are being discharged daily to surface water streams and it retains after the rigorous effluent treatment process through effluent treatment plant (Chatterjee et al., 2007; Chatterjee et al., 2008, Kumar et al., 2008). Hence there is an increasing need of potential and robust in vitro bioassay system for mass screening of these steroidogenic compounds. This would help in their identification and potential remediation.