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THYROXINE (T4) CATABOLISM IN HUMAN AND RAT HEPATOCYTES INCREASES FOLLOWING EXPOSURE TO PROTOTYPICAL HEPATIC ENZYME INDUCERS
Citation:
RICHARDSON, V., Y. M. SEY, AND M. DeVito. THYROXINE (T4) CATABOLISM IN HUMAN AND RAT HEPATOCYTES INCREASES FOLLOWING EXPOSURE TO PROTOTYPICAL HEPATIC ENZYME INDUCERS. Presented at International Society for the Study of Xenobiotics (ISSX), Atlanta, GA, October 16 - 20, 2011.
Impact/Purpose:
As a possible in vitro screening tool for environmental toxicants, the results highlight the utility of primary hepatocytes in evaluating potential species differences and human health risk
Description:
Nuclear receptor agonists such as phenobarbital (PB), 3-methylcholantrene (3MC), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), and, pregnenolone-16a-carbonitrile (PCN) decrease serum thyroxine (T4) concentrations in rats. This decrease is thought to occur through the induction of hepatic metabolizing enzymes resulting in the enhanced catabolism and elimination of T4. There is some evidence that this decrease in serum T4 occurs in humans, but the mechanism by which this transpires is uncertain. Using primary rat and human hepatocytes, we compared the effects of aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR) agonists on T4 catabolism. 48 hours after plating, fresh sandwich-cultured hepatocytes from male human or Sprague-Dawley rats were treated with the following: CAR agonists, PB (1000uM); PCB 153 (30uM) or AhR agonist, 3-MC (5uM). 5uM rifampicin (Rif) and 10uM PCN were used as PXR agonists for human and rat hepatocytes, respectively. Control and treated cells were exposed to a final concentration of 0.1% DMSO. After 72 hours, media were removed and replaced with 0.05uM (rat median serum concentration) or 0.1 uM [1 125]_T4 (human median serum concentration). After 24 hours, media were collected for analysis. T4 and its metabolites were separated on an UPLC from which fractions were collected and quantified on a garrlma counter. Of the conjugated metabolites, T4G is the most abundant as compared to T4S. Studies show a 50-fold higher activity in T4G formation in unexposed rat hepatocytes as compared to unexposed human hepatocytes (5.2 vs. 0.1 fmol T4G/min/mg protein). In rat hepatocytes, there was no significant increase in T4G appearance with PB treatment compared to control; PCN, 3-MC and PCB 153 treatment increase T4G formation by 1.4-, 5.0-, and 3.8-fold, respectively. In contrast, experiments with human hepatocytes showed significant increases in T4G appearance following PB, Rif, 3-MC, and PCB 153 treatment by as much as 8-, 10-, 12-, and 12-fold, respectively. Increases in T4G showed to be variable between human donors suggesting possible polymorphisms in human UGT genes. As a possible in vitro screening tool for environmental toxicants, the results highlight the utility of primary hepatocytes in evaluating potential species differences and human health risk.(This abstract of a proposed presentation, does not necessarily reflect USEPA or NIH