Polychlorinated biphenyl exposure delays metamorphosis and alters thyroid hormone system gene expression in developing Xenopus laevis

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that disrupt thyroid hormone (TH) system function in numerous species. Previous studies have shown delayed metamorphosis in developing Xenopus laevis frogs exposed to PCBs, but the underlying molecular mechanisms have not been thoroughly investigated. In this research, developing X. laevis tadpoles were exposed to environmentally relevant concentrations (5, 50 ppb) of Aroclor 1254 (A1254), a PCB mixture, dissolved in water and 0.25% dimethyl sulfoxide. Quantitative real-time reverse transcriptase polymerase chain reaction was used to measure expression of several TH system genes, other genes that regulate growth and development, and a xenobiotic response gene. Exposure to 50 ppb A1254 significantly delayed metamorphosis and significantly altered gene expression of three thyroid system genes: transthyretin and types II and III deiodinase. Since all three genes regulate the amount of available, biologically active TH, PCB-induced changes in the expression of these genes may underlie alterations in metamorphic timing.