Exposure to tetrabromobisphenol-A alters TH-associated gene expression and tadpole metamorphosis in the Pacific tree frog Pseudacris regilla

Presently in the environment, there exist a number of chemical contaminants which share structural similarity with key naturally occurring regulatory hormones. These hormones play pivotal roles in the normal growth and development of wildlife species and humans. In particular, biphenolic chemical compounds may have the potential to act as agonists or antagonists of thyroid hormone (TH) action. We investigated whether there was any biological effect of exposure to low concentrations of the brominated fire retardant, tetrabromobisphenol-A (TBBPA), on the TH-mediated process of metamorphosis of the Pacific tree frog, Pseudacris regilla. Tadpoles exposed to 10 nM (5.4 μg/L) TBBPA showed an increase in TH-mediated expression of gelatinase B mRNA within 48 h in the tadpole tail which was associated with increased tail resorption by 96 h. Treatment with 100 nM (54.4 μg/L) TBBPA resulted in increased TH-mediated thyroid hormone receptor alpha mRNA expression in the tadpole brain and reduced levels of PCNA transcript in the tail. TBBPA alone was also found to alter the mRNA abundance of thyroid hormone receptor alpha in tail, gelatinase B in brain, and PCNA in both tissues of premetamorphic tadpoles. Interestingly, expression of thyroid hormone receptor beta mRNA was not affected by exposure to TBBPA either alone or in the presence of TH. The results suggest that exposure to low levels of TBBPA may act as an agonist of TH action and potentiate TH-mediated gene expression leading to accelerated anuran metamorphosis.