Superfund Research Program

Molecular Mechanisms of Endocrine Disruption in Largemouth Bass

Project Leader: Nancy D. Denslow
Grant Number: R01ES015449
Funding Period: 2006-2009

Program Links

Final Progress Reports

Year:   2010 

Organochlorine pesticide (OCP) contamination of the north shore of Lake Apopka originated with agricultural practices in the muck farms which received multiple yearly applications of high concentrations of DDT, methoxychlor, dieldrin, toxaphene, and chlordane, among others. While this practice ended in the 1970’s, sediments are still highly contaminated and have resulted in bird mortality and poor fish reproduction. To study the role of sediment contamination on reproductive success, largemouth bass were introduced into a mesocosm built in the contaminated area for 3 months and developed tissue burdens up to 20 parts per million (by wet weight) and altered gene expression patterns by microarray analysis.

The research group evaluated a large number of physiological and molecular endpoints to study individual effects of p,p-DDE, methoxychlor (MXC), dieldrin (DIEL) and toxaphene (TOX) in largemouth bass administered the OCPs in their diets to achieve similar body burdens. Each of these OCPs activates distinct biochemical pathways that lead to dysfunction of reproduction at different points in the HPG axis. P,p’-DDE and MXC act both as antiandrogens and estrogens in gene expression experiments, with p,p’-DDE functioning as a stronger antiandrogen of the two. DIEL appeared to have minimal impact on sex hormone regulated pathways, but instead was more active in the brain, affecting genes and proteins normally associated with Parkinson’s disease, Alzheimer’s, DNA damage, inflammation, among others. TOX exposure also produced a unique expression pattern. The model compound ethinyl estradiol (EE2) appeared to be bioconcentrated from the diet and gene expression patterns varied widely with dose applied. Changes in gonadal histology after exposure in the feeding study suggest major alterations to the reproductive system. The researchers also examined ex vivo gonadal steroidogenesis for all the test compounds, and found that exposure to EE2 lowered E2 synthesis in females, but not in males. In vivo TOX, p,p’-DDE, MXC or EE2 exposure in males prevented increased testosterone synthesis in response to gonadotropin stimulation. Ex vivo MXC and TOX exposure caused concentration-dependent decreases in basal and stimulated estradiol synthesis, respectively. These studies begin to explain the biochemical basis for the complex changes seen in the field and are a first step at understanding the effects of mixtures of OCPs on fish reproduction.