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HEPATIC ENZYME INDUCERS INCREASE THYROXINE (T4) CATABOLISM IN HUMAN AND RAT HEPATOCYTES
Citation:
RICHARDSON, V. AND M. J. DEVITO. HEPATIC ENZYME INDUCERS INCREASE THYROXINE (T4) CATABOLISM IN HUMAN AND RAT HEPATOCYTES. Presented at Endocrine Society Meeting, San Diego, CA, June 19 - 22, 2010.
Impact/Purpose:
These results highlight the utility of hepatocytes to evaluate the potential species differences in the effects of thyroid hormone disruptors.
Description:
Nuclear receptor agonists such as 3-methylcholantrene (3-MC), phenobarbital (PB), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), and, pregnenolone-16a-carbonitrile (PCN) decrease serum thyroxine (T4) concentrations in rats. It appears that this decrease occurs through the induction of hepatic metabolizing enzymes resulting in an enhanced catabolism of T4. There is some evidence that this decrease in serum T4 occurs in humans, but the mechanism by which this happens is unclear. 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 (l000uM); PCB 153 (30uM) or AhR agonist, 3-MC (5uM). For PXR agonist, 5uM rifampicin (Rif) and 10uM PCN were used 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 [I125] _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 gamma counter. The predominant metabolite found in the media of control and treated hepatocytes was Ta-glucuronide (T4G). Initial studies show a 660-fold higher activity in T4G formation in unexposed rat hepatocytes as compared to unexposed human hepatocytes (0.002- vs. 0.000003 pmol T4G/min/mg protein). In rat hepatocytes, there was no significant increase in T4G formation with PB or PCN treatment compared to control; 3-MC and PCB 153 treatment increase T4G formation by 4 to 5-fold. In contrast, initial studies with human hepatocytes show significant increases in T4G formation following PB, Rif, 3-MC, and' PCB 153 treatment (100-, 17-, 10-, and 10-fold, respectively). These results highlight the utility of hepatocytes to evaluate the potential species differences in the effects of thyroid hormone disruptors. (This abstract of a proposed presentation, does not necessarily reflect USEP A or NIH policy