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THYROXINE (T4) CATABOLISM IN HUMAN AND RAT HEPATOCYTES INCREASES FOLLOWING EXPOSURE TO HEPATIC ENZYME INDUCERS
Citation:
RICHARDSON, V. AND M. J. DeVito. THYROXINE (T4) CATABOLISM IN HUMAN AND RAT HEPATOCYTES INCREASES FOLLOWING EXPOSURE TO HEPATIC ENZYME INDUCERS. Presented at Society of Toxicology (SOT) Annual Meeting, San Francisco, CA, March 11 - 15, 2012.
Impact/Purpose:
As a potential in vitro screening tool, the results highlight the utility of primary SCH in evaluating species differences and TH disruption.
Description:
Nuclear receptor agonists phenobarbital (PB), 3-methylcholanthrene (3MC), pregnenolone-16a-carbonitrile (PCN), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), and 2,2' ,4,4'-tetrabromodiphenyl ether (BDE 47) decrease serum thyroxine (T4) in rats. This decrease is thought to occur through the induction of hepatic metabolizing enzymes resulting in enhanced catabolism and elimination of T4. It is evident that serum T4 decreases occur in humans, but the mechanism is uncertain. Using rat and human hepatocytes, we compared effects of aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR) agonists on T4catabolism. Fresh sandwich-cultured hepatocytes (SCH) from human or male Sprague-Dawley rats were treated (72 hrs) with CAR agonists, PB (1000uM), PCB 153 (30uM), and BDE 47 (30uM), AhR agonist, 3MC (5uM) or PXR agonists 5uM rifampicin (Rif) and 10uM PCN. Media were removed and replaced with 0.05 or 0.1 uM 1251_T4 (rat or human median serum concentration). After 24hrs, media were collected for analysis. T4 and its metabolites were separated on an UPLC from which fractions were collected and quantified on a gamma counter. T4G is the most abundant conjugated metabolite. T4G formation was 50-fold higher in unexposed rat SCH compared to unexposed human SCH (5.2 vs. 0.1 fmol T4G/min/mg protein). In rat SCH, T4G was unchanged by PB treatment compared to control. PCN, 3MC, PCB 153 and BDE 47 treatment increased T4G by 1.4-, 5.0-, 2.6-, and 1.4-fold respectively. In contrast, T4G in human SCH increased following PB, Rif, 3MC, PCB 153 and BDE 47 treatment by as much as 8-, 10-, 12:.,6.7-and, 5.9-fold respectively. The data suggest glucuronidation is a primary route of T4 metabolism in rat and humans. Variability between human donors suggests possible polymorphisms in human T4 metabolizing enzymes. As a potential in vitro screening tool, the results highlight the utility of primary SCH in evaluating species differences and TH disruption. (This abstract does not reflect USEPA policy)