Ioxynil and Tetrabromobisphenol A Suppress Thyroid-Hormone-Induced Activation of Transcriptional Elongation Mediated by Histone Modifications and RNA Polymerase II Phosphorylation

To elucidate molecular mechanisms by which the phenolic herbicide ioxynil (IOX) and the brominated flame retardant tetrabromobisphenol A (TBBPA) exert thyroid hormone (TH) disrupting activity, we investigated the effects of the chemicals on the histone and RNA polymerase II (RNAPII) modifications in Xenopus laevis XL58-TRE-Luc cells in direct TH-response genes encoding TH receptor beta (Thrb) and TH-induced basic leucine zipper protein (Thibz) using chromatin immunoprecipitation assays. For both the thrb and thibz genes, 3,3',5-triiodothyronine (T3) enhanced the amounts of gene transcripts and increased the amounts of acetylated histone H4 (H4Ac), trimethylated histone H3 lysine 4 (H3K4me3) and phosphorylated RNAPII serine 5 (RNAPIIS5P), epigenetic markers of gene activation at 5' regulatory regions, and the amounts of trimethylated histone H3 lysine 36 (H3K36me3) and phosphorylated RNAPII serine 2 (RNAPIIS2P), epigenetic markers of activation of transcriptional elongation at protein coding regions. Treatment with IOX and TBBPA reduced the amounts of the thrb transcript and suppressed the T3-induced modifications of H3K4me3, RNAPIIS5P, H3K36me3, and RNAPIIS2P. In the thibz gene, IOX and TBBPA did not suppress the T3-induced histone and RNAPII modifications except for H3K36me3 in the TBBPA treatment, despite both chemicals decreasing the T3-induced transcription. Our results demonstrate that IOX and TBBPA affect TH-induced histone and RNAPII modifications, which are involved in early and progressive stages of RNAPII transcriptional elongation, in direct TH-response genes, in somewhat target gene-dependent and chemical-specific manners. Both IOX and TBBPA are likely to influence epigenetically a cascade of TH receptor-mediated gene regulation.