Nuclear receptor targets for environmental pollutants. Modulation of transcriptional activity of pregnane x receptor and peroxisome proliferator-activated receptor gamma in polar bears and zebrafish

Liganded nuclear receptors can be viewed as transcriptional dimmers whose activity is adjusted by molecular signals. Since the chemical footprint of humans continue to increase, the numbers of chemicals in the environment that can tune the activity of nuclear receptors inside organisms also keep growing. As several nuclear receptor are central regulators in biological and toxicological pathways knowledge about how xenobiotics can modulate the transcriptional activity of nuclear receptors is important. Studies of nuclear receptor increase insight into toxic mechanisms and also contribute to more precise risk assessment of chemicals. The focus of this study is on how xenobiotics can modulate the activity two xenobiotics activated receptors, the pregnane X receptor (a. k. a. steroid and xenobiotic receptor, PXR/SXR/NR1I2) and the peroxisome proliferator-activated receptor gamma (a. k. a. PPARG/NR1C3). Paper I is a comparative study of the ability of the PXR orthologs from humans and polar bears to be activated by a 51 compound test panel consisting of pharmaceuticals, pesticides, polychlorinated biphenyls (PCBs), brominated flame retardants (BFRs) and industrial compounds. It was shown that polar bear PXR is activated by xenobiotics and that PXR is a promiscuous receptor that likely functions as a xenosensor in polar bears. Four environmental pollutants, HBCDD, toxaphene, 4-nonylphenol and TBBPA, and the cholesterol lowering drug SR12813, activated polar bear PXR more strongly than human PXR. Our findings show that polar bear PXR is activated by structurally different xenobiotics. While polar bear PXR is promiscuous, it is somewhat less promiscuous than human PXR. Paper II focuses on how environmental pollutants, present in liver and adipose tissue of polar bears, can modulate the transcriptional activity of PPARG and adipogenesis. Extracts of persistent organic pollutants from polar bear liver and adipose tissue induced lipid accumulation in 3T3-L1 cells. Synthetic mixtures composed to reflect the POP composition of the extract from polar bear adipose tissue did not induce adipogenesis in murine preadipocytes (3T3-L1) nor in adipocyte-derived stem cells from polar bears (ASCs). In contrast, the synthetic mixtures and some single compounds, such as PCB153, bisphenol A, HBCDD, DDE, oxychlordane and endosulfan, inhibited lipid accumulation. These results suggest that the total burden of persistent organic pollutants in polar bear can modulate adipogenesis in murine preadipocytes. Paper III focuses on genomic variation in the Pxr gene from four strains of zebrafish, the AB Tübingen (AB/Tü), Singapore wild type (SWT), Tupfel long fin (TL) and a strain of unknown origin (UNK). Due to several missense mutations and indels in the Pxr genes from these strains, functionally different Pxr variants are encoded. PxrAB/Tü was activated more strongly by the antifungal drug clotrimazole, and also formed a stronger interaction with this compound. PxrUNK formed the weakest interaction to butyl 4-aminobenzoate, and was activated the least by this compound. Zebrafish is commonly used as a model species in toxicology. The occurrence of functionally different variants of zebrafish Pxrs could have implications for risk assessment. Based on our results, the choice of strain for use in toxicity testing may therefore be of high importance.