title: Inhibition of endogenous thyroid hormone receptor-beta and peroxisome proliferator-activated receptor-alpha activities by humic acid in a human-derived liver cell line.
abstract: Humic acid (HA), know to be ubiquitous in the natural environment, is present in almost all soil, surface water, and plants. Earlier studies indicate that HA can affect thyroid economy via binding with iodide, inhibiting both thyroid peroxidase and hepatic 5'-deiodinase in rodents. However, the effect of HA, a peroxisome proliferator in rodents, on thyroid hormone receptor (TR) and peroxisome proliferator-activated receptor (PPAR) in human cells has not yet been examined. In this study, we demonstrate that the malic enzyme activity and the transcriptional activities of endogenous TR and PPAR were inhibited after treatment with HA in human hepatocyte Chang liver cell line. Although the protein expression levels of TR-beta, PPAR-alpha and retinoid X receptor-alpha (RXRalpha) were not changed significantly by HA treatment, both the binding abilities of endogenous TR-beta on thyroid hormone response element (TRE) and PPAR-alpha on the PPAR response element (PPRE) were inhibited by HA treatment. The study of the subcellular distribution of HA, relying on the inherent HA fluorescence, showed that HA distributed in the intracellular compartments including cytoplasm and nucleus. The 50% binding inhibition values (CI(50)) of HA on ME-TRE (malic enzyme gene-TRE) and ACOX-PPRE (acylCoA oxidase gene-PPRE) were 19.31 and 19.94 microg/mL, respectively. These results suggest that HA-induced endemic goiter may link in part to the disruption of TRbeta and PPARalpha function in human Chang liver cells. This model may be useful in the investigation of environmental goitrogens.
authors.affiliation: Graduate Institute of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei, Taiwan, Republic of China.
authors.forename: Mei-Ling
authors.initials: ML
authors.lastname: Yang
authors.forename: Tien-Shang
authors.initials: TS
authors.lastname: Huang
authors.forename: Yashang
authors.initials: Y
authors.lastname: Lee
authors.forename: Tsung-Hwa
authors.initials: TH
authors.lastname: Chen
authors.forename: Shu-Yi
authors.initials: SY
authors.lastname: Chen
authors.forename: Fung-Jou
authors.initials: FJ
authors.lastname: Lu
mesh_terms: D015153:Blotting, Western; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D002614:Chelating Agents / Q000494:pharmacology*; D004247:DNA / Q000032:analysis / Q000378:metabolism; D004586:Electrophoresis; D006801:Humans; D006812:Humic Substances / Q000378:metabolism / Q000494:pharmacology*; D008099:Liver / Q000378:metabolism*; D008291:Malate Dehydrogenase / Q000037:antagonists & inhibitors / Q000378:metabolism; D008856:Microscopy, Fluorescence; D010957:Plasmids / Q000235:genetics; D018160:Receptors, Cytoplasmic and Nuclear / Q000037:antagonists & inhibitors* / Q000235:genetics; D018168:Receptors, Retinoic Acid / Q000037:antagonists & inhibitors / Q000235:genetics; D011988:Receptors, Thyroid Hormone / Q000037:antagonists & inhibitors* / Q000235:genetics; D047488:Retinoid X Receptors; D013347:Subcellular Fractions / Q000378:metabolism; D037042:Thyroid Hormone Receptors beta; D014157:Transcription Factors / Q000037:antagonists & inhibitors* / Q000235:genetics; D014162:Transfection
chemical_list: D002614:Chelating Agents; D006812:Humic Substances; D018160:Receptors, Cytoplasmic and Nuclear; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone; D047488:Retinoid X Receptors; D037042:Thyroid Hormone Receptors beta; D014157:Transcription Factors; D004247:DNA; D008291:Malate Dehydrogenase