Dr. Hamilton’s principal research interests are in the areas of molecular toxicology, metals toxicology, developmental toxicology, gene regulation, pathophysiology associated with toxicant exposures, and the use of -omics technologies to understand the environmental etiology of human disease. The primary focus of his research over the past decade has been on the molecular toxicology of arsenic and other toxic metals. The current focus of the laboratory is on three principal research directions related to this interest.
The first area is focused on understanding the molecular and mechanistic basis for the effects of arsenic as an endocrine disruptor, which was first discovered and reported by Dr. Hamilton’s lab. They have demonstrated in a series of studies that arsenic is a very potent endocrine disruptor at extremely low concentrations at or below the current U.S. drinking water standard, i.e., 10 ppb. This was first demonstrated with the steroid hormone receptor for glucocorticoids, but has since been shown to also occur with the steroid receptors for estrogen, progesterone, androgen and mineralocorticoids, i.e., all five steroid receptor classes. Similar effects have also been seen with other non-steroid nuclear hormone receptors, i.e., those for thyroid hormone and retinoic acid. Interestingly, the mechanism for this appears to be unique since arsenic does not act as a ligand for these receptors, i.e., it is neither an agonist or competitive antagonist, nor does arsenic appear to interfere with normal hormone binding, activation of the receptor, translocation to nuclear chromatin, or binding to hormone-responsive DNA elements that regulate hormone-responsive genes. However, in the presence of arsenic these hormone-activated, chromatin-bound receptors function abnormally as transcription factors, with either greatly enhanced gene signaling at very low doses or greatly suppressed signaling at slightly higher doses. The shared effects of arsenic on all these different receptors that represent two entirely different classes of nuclear hormone receptors, despite their lack of absolute shared sequence or structure, suggests that there is a common regulatory component or other shared machinery which is the actual molecular target(s) for arsenic. Current research in this area is focused on precisely how arsenic is able to elicit these effects on receptor-mediated gene expression at the cell and molecular level.
The broad effects of arsenic on this suite of important hormone pathways also suggests an important role of arsenic-mediated endocrine disruption on arsenic’s ability to increase the risk of various cancers, type 2 diabetes, reproductive and developmental effects, vascular and cardiovascular disease, neurological and cognitive disorders, and the growing list of other known pathophysiological consequences on humans and on natural populations that are exposed chronically to arsenic environmentally in food or water. Thus, a second major focus of the lab is to investigate these pathophysiological consequences of such endocrine disruption using model whole animal systems, and also in collaboration with epidemiologists and ecologists studying human or natural populations, respectively. Recent work from the lab has shown that arsenic can profoundly disrupt certain developmental or physiological programs that are critically dependent on hormone receptors that have been shown to be disrupted by low dose arsenic. For example, arsenic at very low doses, equivalent to human drinking water levels of concern, blocks thyroid hormone-dependent tadpole metamorphosis in the frog, Xenopus. Likewise, arsenic at similar levels disrupts the ability of the euryhaline fish, Fundulus, to adapt to changes in water salinity equivalent to the changing salt marsh tides, a process which is regulated by the glucocorticoid hormone, cortisol, and its control of a key salt regulatory protein, CFTR (the same protein which, when mutated, causes the human disease, cystic fibrosis). Current research is extending these studies to other systems to determine what other effects, at what levels, and the extent to which such endocrine disruption can explain the myriad adverse effects of arsenic observed in exposed populations.
The third area focuses on using genomics and proteomics tools to investigate more broadly the effects of arsenic on gene and protein expression in model systems in order to understand its overall biological effects. These experiments are useful both to test hypotheses and to generate new avenues of research based on biological discovery. Previous work in the lab has shown, using whole genome microarrays, that arsenic broadly affects hormone regulation of gene expression at low doses. For example, the lab demonstrated that the synthetic glucocorticoid hormone, dexamethasone, significantly alters expression of over a thousand genes in mouse liver, and that low doses of arsenic affect the hormone regulation of virtually all of these genes. Conversely, in the lungs of the mice in these same experiments, it was observed that the dominant effect of arsenic at low doses is to profoundly alter immune response, and this is now a new avenue of research in the lab based on this discovery. The lab has also pioneered the use of microarrays in environmentally relevant species, particularly the aquatic freshwater zooplankton, Daphnia, and the marine fish, Fundulus. These two species are ideal because they can be used both in controlled laboratory experiments and also in the environment as sentinel species for natural populations. The lab is continuing to develop and apply genomics tools in these species in collaboration with other laboratories in order to establish them as model organisms for use in their own studies but also broadly shared within a larger research community. Related to this genomics research, the lab has been pioneering the development and application of new analytical tools and methods for obtaining richer and more accurate biological information from the large data sets that are generated in a typical whole genome microarray, which allows comparisons among different treatments and different experimental species.