Date of Award

8-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biological Sciences

Committee Member

William S Baldwin, Committee Chair

Committee Member

Lisa J Bain

Committee Member

Charles D Rice

Committee Member

Christopher A Saski

Abstract

Atrazine is an herbicide with several known toxic effects, including interactions with other chemicals. Atrazine increases the toxicity of several organophosphates and has been shown to reduce the toxicity of triclosan to D. magna in a concentration dependent manner. Atrazine is a potent activator in vitro of the xenobiotic-sensing nuclear receptor, HR96, related to vertebrate constitutive androstane receptor (CAR) and pregnane X-receptor (PXR). HR96 is a promiscuous nuclear receptor that is activated and inhibited by a variety of xenobiotics, and regulates transcription of Phase I-III enzymes, as well as genes involved in cholesterol homeostasis and triglyceride uptake. RNA sequencing (RNAseq) was performed to determine if atrazine is inducing phase I-III detoxification enzymes in vivo, and estimate its potential for mixture interactions. RNAseq analysis demonstrates induction of glutathione S-transferases (GSTs), cytochrome P450s (CYPs), glucosyltransferases, and xenobiotic transporters, of which several are verified by qPCR. Pathway analysis demonstrates changes in drug, glutathione, and sphingolipid metabolism, indicative of HR96 activation. Based on our RNAseq data, we hypothesized as to which environmentally relevant chemicals may show altered toxicity with co-exposure to atrazine. Acute toxicity tests were performed to determine individual LC50 and Hillslope values as were toxicity tests with binary mixtures containing atrazine. The observed mixture toxicity was compared with modeled mixture toxicity using the Computational Approach to the Toxicity Assessment of Mixtures (CATAM) to assess whether atrazine is exerting antagonism, additivity, or synergistic toxicity in accordance with our hypothesis. Atrazine mixtures with triclosan decreased toxicity as expected; atrazine mixtures with parathion, endosulfan, and to a lesser extent p-nonylphenol increased toxicity. In summary, exposure to atrazine activates HR96, and induces phase I-III detoxification genes that are likely responsible for mixture interactions.

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