Environmental Health Sciences– a Primer for Green Chemists
Environmental health science (EHS) is the study of the effects of biological agents, chemicals, radiation, as well as social, psychological and physical factors on human health for the prevention of disease and the promotion of health and well-being. The specific issue addressed in this forum, the effects of chemical exposure on human health, concerns the sources and routes of exposure, pharmacokinetics—what the body does to the chemical (i.e. absorption, distribution, metabolism and excretion of chemicals) and pharmacodynamics—what the chemical does to the body at the level of the cell, the organ, organ systems, and the whole body.
This study of the effects of xenobiotic (i.e. synthetic, environmental pollutants) exposures integrates the complex interaction among chemicals, developmental mechanisms and genetic expression that informs the physiology and function of an organism. Xenobiotics can exert adverse effects on human health by disrupting or interacting with multiple cellular communication pathways that direct growth, development and normal physiological function. These interactions or disturbances can result in heritable and non-heritable changes in gene expression and function without causing a change in DNA sequence. Studying the complex interaction between xenobiotics and the whole organism requires consideration of multiple factors of time (e.g. when exposure occurs and effects are observed, transgenerational exposure and effects) and space (e.g. population effects, complex mixtures of chemicals, indirect effects, inter-, intra-species and individual variability).
Key principles and concepts
1. Timing of exposure
2. Mechanisms of action
4. Assessing exposures and effects
5. Dose-response relationships
6. Individual and population complexity
1. Timing of exposure is critical
The timing of exposure and the dose determines the effect(s). Exposure during critical windows of development (periods of rapid growth and differentiation) can result in increased susceptibility to disease. There are multiple critical windows of development, but early life exposures (i.e. prenatal and neonatal) represent the most vulnerable period because they can result in permanent or at least persistent changes over the life span of the organism. (see Linda Birnbaum’s work)
The theory of the fetal origins of disease contends that disruption of fetal programming of communications systems that direct growth, development and functioning can result in long-term and often permanent (or persistent) changes that manifest as increased risk of disease and/or a syndrome of related diseases and disorders, such as the testicular dysgenesis syndrome and metabolic disorders (see Louis Guillette and Bruce Blumberg’s work).
Timing of exposure is also an important factor to consider in in vitro (cell-based studies). The stage of development of the cell during exposure can have an effect on the results of assays.
2. Mechanistic complexities
Nuclear receptor binding:
Most early receptor-based research on xenobiotics focused on chemicals that interact with estrogen receptors, androgen receptors and thyroid receptors—the EAT receptors. The mechanistic model was fairly simple. Chemicals are transported into the cell and bound to receptors, forming DNA binding units. These binding units then enter into the nucleus and bind to specific sequence, which subsequently induces specific genes that direct cell function.
Over the past decade, receptor-based research on xenobiotics has become increasingly complex. There are 49 members of the nuclear receptor superfamily. Nuclear receptors are evolutionary ancient and diverse—found in all animals examined. This suggests that there may be hundreds of chemicals capable of interacting with nuclear receptors across animal species. Researchers now explore xenobiotic interaction with glucocorticoid receptors, progesterone receptors, thyroid hormone receptors, retinoid X receptors, and peroxisome proliferator activated receptors (PPAR), and the list continues to grow. (see Joe Thornton’s work)
Xenobiotics can act as agonists (binding that elicits a response) or antagonists (blocking the action of an agonist, or anti-agonist) to the nuclear receptor superfamily. Compounds can also act as agonists and antagonists for the same receptor in the same tissue. They can have tissue-specific and species-specific effects, and can be active at different dose ranges depending on the system—what are called selective nuclear receptor modulators. This action is well defined for drugs. For example, tamoxifen, an anti-cancer drug, is a selective estrogen receptor modulator—it is an estrogen receptor antagonist in the breast but an estrogen receptor agonist in the endometrium (Keri, Ho et al. 2007).
Membrane receptor activity:
In 2005, Cheryl Watson’s laboratory at the University of Texas published a paper that demonstrated bisphenol A and other assumed “weak” xenoestrogens can bind to cell membrane estrogen receptors (mER) and elicit cellular level effects (e.g. increases in calcium influx) at much lower levels than those observed with nuclear receptor binding (Wozniak, Bulayeva et al. 2005). Just as substances may be selective nuclear receptor modulators, there are also selective membrane receptor modulators. Xenobiotics may also act as agonists or antagonists to cell membrane receptors.
Indirect receptor-ligand binding effects:
Xenobiotics may also influence receptor-ligand binding (e.g. interaction with transcription co-regulators), receptor expression (e.g. down- or up-regulating expression), or steroidogenesis (the making of different steroid hormones). For example, some pesticides can inhibit aromatase, the key enzyme involved in converting androgens to estrogens. Compounds may also inhibit critical proteins involved in blocking toxins from entering the cell or essential to removing toxins from the cell (e.g. efflux transporter proteins, CYP450 enzymes). (See David Epel’s research)
3. Considering epigenetics
Chemicals can alter gene expression and change gene function without causing mutation, resulting in altered phenotypic expression and complex human diseases such as cancer, obesity and diabetes, infertility and reproductive disorders, and neurological impairment. These epigenetic changes of gene expression can be heritable (germline dependent effects) and non-heritable (changes in the somatic cells which have no genetic information). “EDCs [endocrine disrupting chemicals] do not act on genes alone but on developmental mechanisms that integrate genetic and epigenetic interactions, resulting in the phenotype”. Understanding the “ecology of gene expression” means considering the broader context in which a gene is expressed—its spatial and temporal relation to other genes, proteins, cells and tissues. (Crews and McLachlan, 2006).
Mechanisms of inducing epigenetic change include DNA methylation (usually associated with gene silencing) and histone modification (associated with gene expression).
*image from Rex Hess (modified)
4. Complexities in measuring exposure and effects
The ‘real world’ exposure experience involves a complex mixture of chemicals. Research on mixtures of xenobiotics with similar mechanisms of action demonstrates additive and greater-than-additive effects.
The duration of the dose and route of exposure must be considered. For example, was the dose given continuously (e.g. mini pump), in a short burst (e.g. injection) or via feed or gavage (i.e. forced feeding), when and for how long. These factors together with the pharmacokinetics of a chemical are integrated to determine the internal dose.
The endpoints examined extend beyond the classic, adverse effects measured in toxicology, such as organ weight (e.g. measuring estrogenic effects by weight of the uterus). Such endpoints include physiological changes such as alteration in estrous cycling, organizational changes in reproductive tissues, behavioral changes, and other endpoints that indicate functional changes and/or increased susceptibility to disease and pathology. There may be a spectrum of related effects that result from developmental exposures, such as such as testicular dysgenesis syndrome (See Louis Guillette’s research) and obesity and metabolic disorders (See Bruce Blumberg’s research).
5. Dose-response relationships
Traditional toxicological testing uses high doses to predict low dose responses based on the assumption that the ‘dose makes the poison.’ In other words, with an increase in dose, there is an increase in effect (i.e. monotonic dose-response relationship). However, the dose-response of hormones, synthetic hormones and endocrine disrupting chemicals at low doses, physiologically active levels, can be non-monotonic (U or inverted U shaped) or J-shaped. For these chemicals, low dose effects could not be predicted using monotonic dose-response models.
Consider the drug tamoxifen. When administered at high doses, it acts like an antiestrogen and suppresses the proliferation of breast cancer cells; however, as Wade Welshons lab at the University of Missouri has shown, at levels 10,000 times lower, tamoxifen acts like an estrogen and can promote proliferation of breast cancer cells. (See Fred vom Saal’s research)
6. Individual and population complexities
Humans and wildlife develop and live in complex environments. The development of each individual is a dynamic process influenced by its genetic make-up, its own endogenous hormones and biochemistry, and complex stressors in the environment. In light of such complexities, in vitro models, while extremely useful and necessary for understanding mechanisms of action at the cellular level, should be used to inform rather than predict whole body responses to exposure.
There is also considerable complexity in the response across species and strains. Exposure effects observed in one species or strain may not be observed in another; rather than undermining the reliability of the research, this can inform our understanding of human impacts as well as the broader ecological effects at the population level. Moreover, because humans do not represent a homogenous species, wildlife research and diverse animal models are necessary (See Louis Guillette’s research).
There may be indirect population level effects of a given compound or groups of compounds. For example, a population crash due to disrupted reproductive success in one species (prey) may result in the decline of another (predator) due to the loss of a food supply.
Toxicology then… environmental health science today
|Regulatory toxicology||Environmental Health Science in the 21st Century|
|The dose makes the poison for all stages of life.
The dose-response relationship is monotonic.
High dose testing is used to predict safety at low doses.
|Timing of exposure is as critical as the dose level.
Low doses may have effects not predicted from high dose testing (i.e. non-monotonic dose-response relationship).
Environmentally relevant levels of chemicals are tested.
|Adverse effects are defined by a limited set of absolute endpoints. Classic toxicological endpoints include gross morphological changes, death, body weight loss, change in organ weight, spontaneous abortion, etc.||Adverse effects can include indicators of increased susceptibility to disease, functional changes that can lead to pathology or abnormal function, or a syndrome of related effects.
Effects studied include important human chronic diseases and abnormalities.
Adverse effects may be transgenerational; therefore, effects may be seen in the offspring rather than the exposed adult, and can be passed down through the germline.
|Chemicals are examined individually.||Mixtures of chemicals are examined. Effects may be additive and greater-than-additive.|
Keri, R. A., S. M. Ho, et al. (2007). “An evaluation of evidence for the carcinogenic activity of bisphenol A.” Reprod Toxicol 24(2): 240-52.
Newbold, R. R., E. Padilla-Banks, et al. (2008). “Effects of endocrine disruptors on obesity.” Int J Androl 31(2): 201-8.
Newbold, R. R., E. Padilla-Banks, et al. (2007). “Perinatal exposure to environmental estrogens and the development of obesity.” Mol Nutr Food Res 51(7): 912-7.
Newbold, R. R., E. Padilla-Banks, et al. (2007). “Developmental exposure to endocrine disruptors and the obesity epidemic.” Reprod Toxicol 23(3): 290-6.
Wozniak, A. L., N. N. Bulayeva, et al. (2005). “Xenoestrogens at picomolar to nanomolar concentrations trigger membrane estrogen receptor-alpha-mediated Ca2+ fluxes and prolactin release in GH3/B6 pituitary tumor cells.” Environ Health Perspect 113(4): 431-9.