Changes in chromatin accessibility in mouse liver following TCPOBOP exposure

This work is part of a larger study where we investigated activation of the nuclear receptor and transcription factor CAR (Nr1i3) by its specific agonist ligand TCPOBOP (1,4-bis[2-(3,5-dichloropyridyloxy)]benzene), which dysregulates hundreds of genes in mouse liver and is linked to male-biased hepatocarcinogenesis. We used DNase-seq and DNase hypersensitivity site (DHS) analysis to identify several thousand genomic regions (∆DHS) where short-term exposure to TCPOBOP induces localized changes (increases or decreases) in mouse liver chromatin accessibility, many of which cluster together with TCPOBOP-responsive genes. Sites of chromatin opening were highly enriched nearby genes induced by TCPOBOP and chromatin closing was highly enriched nearby genes repressed by TCPOBOP, consistent with TCPOBOP-responsive ∆DHS serving as enhancers and promoters that positively regulate CAR-responsive genes. Gene expression changes lagged behind chromatin opening or closing for a subset of TCPOBOP-responsive ∆DHS. DHS that were specifically responsive to TCPOBOP in male liver were significantly enriched for genomic regions with a basal male bias in chromatin accessibility; however, the male-biased response of hepatocellular carcinoma-related genes to TCPOBOP was not associated with a correspondingly male-biased ∆DHS response. These studies elucidate the genome-wide organization of CAR-responsive genes and of the thousands of associated genomic sites where TCPOBOP exposure induces both rapid and persistent changes in chromatin accessibility. Nuclei were isolated from male and female CD1 mouse liver following exposure to TCPOBOP for either 3 h or 27 h, followed by limited DNase-I digestion. Two biological replicate pools, comprised of DNase fragments released from n=3-5 individual mouse liver nuclei, were sequenced per control orTCPOBOP treatment group.