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Effects of hepatic enzyme inducers on thyroxine (T4) catabolism in primary rat hepatocytes
Citation:
RICHARDSON, V. AND M. J. DEVITO. Effects of hepatic enzyme inducers on thyroxine (T4) catabolism in primary rat hepatocytes. Presented at Society of Toxicology 49th Annual meeting, Salt Lake City, UT, March 07 - 11, 2010.
Impact/Purpose:
Using primary rat hepatocytes, we compared the effects of constitutive androstane receptor (CAR) and aryl hydrocarbon receptor (AhR) agonists on T4catabolism.
Description:
Nuclear receptor agonists such as phenobarbital (PB), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), and 3-methylcholantrene (3-MC) decrease circulating thyroxine (T4) concentrations in rats. It is suspected that this decrease occurs through the induction of hepatic metabolizing enzymes resulting in an enhanced catabolism of T4. Using primary rat hepatocytes, we compared the effects of constitutive androstane receptor (CAR) and aryl hydrocarbon receptor (AhR) agonists on T4catabolism. 48 hours after plating, fresh sandwich-cultured hepatocytes from male Sprague-Dawley rats were treated with one of the following: CAR agonists, PB (100 or 1000uM) or PCB 153 (3 or 30uM) or AhR agonist, 3-MC (1 or SuM). Control and treated cells were exposed to a final concentration of 0.1% dimethylsulfoxide (DMSO). After 72 hours, media were removed and replaced with 0.05uM [1125]_T4 (rat median serum concentration). After 24 hours, media and cell lysates were collected for analysis. T4 and its metabolites were separated using an Ultra Performance Liquid Chromatography (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 Ts-glucuronide (T4G). Following a 24 hour incubationof0.05to 100.0uM[1125 ]_T4with controlhepatocytes,Vmaxand Km for T4G formation was 8.0 pmollmg protein/min and 37.6uM, respectively. PB did not significantly increase the formation of T4G in comparison to control hepatocytes. However, initial studies with 3-MC (1 and SuM) and PCB 153 (3 and 30uM) show significant increases in T4G in media (2.7-, 3.4-, 1.8-and 3.7-fold, respectively). These results suggest that unlike PB, pretreatment with 3-MC or PCB 153 increases the catabolism of T4 in rat hepatocytes. These observations also demonstrate the utility of primary rat hepatocytes for screening thyroid hormone disruptors. (This is an abstract of a proposed presentation and does not necessarily reflect USEPA or NIH policy