Presenter: Cynthia V. Rider
Advisor(s): Gerald A. LeBlanc
Author(s): Cynthia V. Rider and Gerald A. LeBlanc
Graduate Program: Environmental and Molecular Toxicology

Title: Atrazine Stimulates Stress-Responsive Hemoglobin Induction in Daphnia magna: Is it Hormonal or Hypoxic?

Abstract: Hemoglobin accumulation in daphnids is an important adaptive response that is stimulated by two distinct molecular pathways: 1) a hypoxia pathway mediated by hypoxia inducible factor (HIF) binding to cis-acting hypoxia response elements and 2) an endocrine pathway stimulated by juvenoid hormones interacting with a juvenoid response element (JRE). We found that the ubiquitous herbicide atrazine induced expression of hemoglobin in Daphnia magna despite having no clear relationship to either hypoxia or juvenoid hormones. We assessed the joint action of atrazine and the potent juvenoid pyriproxyfen to determine whether these compounds induced hemoglobin gene expression via the same signaling pathway. The concentration addition model describes joint effects of chemicals with the same mechanism of action and response addition describes the joint effects of chemicals with different mechanisms of action. Therefore, conformation of joint effects to concentration addition would indicate that atrazine functions as a juvenoid hormone to induce hemoglobin. Whereas, conformation to a model for response addition would imply that atrazine induces hemoglobin through either the HIF-mediated pathway or some other non-juvenoid pathway. Atrazine-mediated changes in hemoglobin expression were evaluated using real-time RT PCR with primers specific to each of the three sequenced D. magna hemoglobin genes (dhb1, dhb2, and dhb3). Atrazine significantly induced dhb2 gene expression (7-fold), while having little effect on the expression of dhb1 and dhb3 (<2-fold). The combined action of atrazine and pyriproxyfen mixtures on dhb2 expression was measured and compared to effects predicted using concentration addition and response addition models. Induction of dhb2 by the combination did not significantly deviate from response addition predictions but did significantly vary from concentration addition predictions. Results from this study demonstrate that molecular interactions among chemicals conform to standard mixture modeling approaches and that atrazine induces hb2 gene expression through a pathway that does not involve the JRE.