Xenoestrogen effects on fish: molecular to physiological approaches

Estrogen hormones, or compounds that mimic them (xenoestrogens), are found in surface waters worldwide mainly due to discharges to the environment from anthropogenic sources, such as sewage treatment plant (STP) effluents. Fish are adversely affected by these xenoestrogens in the areas of reproduction, growth, immune function and offspring fitness. To identify and explore the effects of xenoestrogens on salmonids, a series of toxicological studies were performed using comprehensive panels of assays evaluating immune, osmoregulatory, hematological, metabolic, reproductive and endocrine-related parameters in response to xenoestrogen- and STP effluent-exposures. Most studies included a post-treatment depuration phase to determine recovery timeframes of altered parameters. In male or juvenile fish, xenoestrogens are known to induce vitellogenesis, the mature female-related process of making eggs under estrogen receptor (ER) control that is initiated by the hepatic production of egg proteins, vitellogenin (VTG) and vitelline envelope proteins (VEPs). In three studies, induction of liver somatic index (LSI), plasma VTG protein or hepatic gene transcripts (ERs, VTG and VEPs) were found due to exposure to a tertiary-treated STP effluent, a synthetic wastewater, and estrogen hormones. In Rainbow trout exposed to xenoestrogens, LSI and transcripts recovered to baseline values after treatments ceased, while VTG protein concentrations remained elevated through recovery phases. Thus, gene expression alterations were only useful as indicators of existing exposure, while VTG protein levels were indicative of both present and prior exposure. Xenoestrogens affected several immune system functions, including reduced leukocyte counts, potentially increasing susceptibility to pathogens. Isolated leukocytes from head kidney and peripheral blood contained ERα and ERβ transcripts, although only ERα1 and ERα2 were altered by xenoestrogen exposure, providing a direct mechanistic pathway for xenoestrogen modulation of the immune system. Reductions in burst swimming performance were found in Rainbow trout exposed to xenoestrogens, with potential mechanisms suggested involving altered osmoregulation (evidenced by reduced chloride ions) and decreased blood oxygen carrying capacity via a reduction in red blood cell counts. Overall, most but not all altered parameters recovered from exposure to xenoestrogens, and the adverse effects of environmentally-relevant concentrations of xenoestrogens on many biological functions showed that xenoestrogens pose a hazard to wild juvenile fish.