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 β (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.
Keywords: RNA polymerase II; epigenetics; histone; ioxynil; tetrabromobisphenol A; thyroid hormone receptor.