MP2 Amphibian Research and Monitoring
Ballroom F, Level 4
2:10 PM - 5:30 PM, Monday, 10 November 2003
Chair: Rostad, Colleen ,
Co-chair: Battaglin, William ,
(105) Multiple molecular mechanisms underlie dioxin insensitivity in the frog, Xenopus laevis.
Powell, W1, Klimova, T1, Rowatt, A1, Susman, T1, 1 Kenyon College, Gambier, OH, USA
ABSTRACT- Halogenated aromatic hydrocarbons, including 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD), are potent developmental toxicants in most vertebrates. However, several ranid frog species are resistant to TCDD toxicity, and although assessments of TCDD toxicity in the pipid Xenopus laevis vary, some reports suggest this species is also relatively insensitive, especially during early life stages. We have used X. laevis as a model for investigating the molecular mechanisms of TCDD insensitivity in frogs. X. laevis tadpoles express two distinct transcripts encoding aryl hydrocarbon receptors (AHRs), AHR1
and AHR1
. Sharing 86% amino acid identity, the encoded proteins likely represent the products of recently duplicated genes. Biochemical analysis of AHR1 indicates that while TCDD enables sequence-specific DNA interaction, it binds the receptor with an affinity approximately twenty-fold lower than that for mouse AHR. Furthermore, the expression of functional AHRs may be limited in early life stages. While TCDD induces the expression of CYP1A6 and CYP1A7 mRNAs in stage 54 tadpoles (∼1 month), it does not alter their expression at stage 47 (∼5 days) or earlier. Finally, like other frogs, X. laevis embryos eliminate TCDD very rapidly. Embryos (stage 42) exposed to [3H]TCDD eliminated their TCDD burden with a half life less than one day. Taken together, these studies suggest that reduced affinity of AHR for TCDD, reduced AHR activity at early life stages, and rapid TCDD elimination each contribute to the relative insensitivity of X. laevis to TCDD. An understanding of these molecular mechanisms should aid amphibian ecotoxicology and refine the use of frog embryos as a model (e.g. in FETAX) for determining developmental toxicity of samples containing dioxin-like compounds in vertebrates. [NIH R15-ES11130]
Key words: Xenopus laevis, amphibians, TCDD, AH receptor