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In Vitro Metabolism of Thyroxine by Rat and Human Hepatocytes
Citation:
Richardson, V., S. Ferguson, Y. Sey, AND M. DeVito. In Vitro Metabolism of Thyroxine by Rat and Human Hepatocytes. XENOBIOTICA. Taylor & Francis, Inc., Philadelphia, PA, 44(5):391-403, (2014).
Impact/Purpose:
The objective of this study was to compare the metabolism of T4 in fresh primary rat and human sandwich-cultured hepatocytes. The hepatocytes were fed daily with William’s E media containing 0.1% DMSO. Up to five days after plating, cell culture media was replaced with media containing [125I]-T4 at median serum concentrations observed in rats and humans of 0.05µM and 0.1µM, respectively. Up to 24 hours after [125I]-T4 administration, media was collected and T4 and its metabolites were separated by UPLC. UPLC fractions were collected and quantified by gamma spectroscopy. The metabolites analyzed in media of hepatocytes include T3, rT3, T4G, and T4S. Rat hepatocytes had as much as 13-times higher concentrations of metabolites in the media compared to human hepatocytes. Metabolite distribution in the media of rat versus human hepatocytes was as follows: T4G, (91.6 vs. 5.3%); T4S, (3.6 vs. 4.4%); T3+rT3, (4.9 vs. 90.3%). The results indicate that basal hepatic activity of T4 glucuronidation is greater in cultured rat hepatocytes compared to cultured human hepatocytes. These data also suggest that glucuronidation may be a more important pathway for T4 metabolism in rats and deiodination may be a favored pathway in humans. However, with PCB 153 exposure these data support glucuronidation as a primary route of T4 metabolism in both rat and humans.
Description:
The liver metabolizes thyroxine (T4) through two major pathways: deiodination and conjugation. Rodents utilize both pathways, but it is uncertain to what degree different species employ deiodination and conjugation in the metabolism of T4. The objective of this study was to compare the metabolism of T4 in fresh primary rat and human sandwich-cultured hepatocytes. The hepatocytes were fed daily with William’s E media containing 0.1% DMSO. Up to five days after plating, cell culture media was replaced with media containing [125I]-T4 at median serum concentrations observed in rats and humans of 0.05µM and 0.1µM, respectively. Up to 24 hours after [125I]-T4 administration, media was collected and T4 and its metabolites were separated by UPLC. UPLC fractions were collected and quantified by gamma spectroscopy. The metabolites analyzed in media of hepatocytes include T3, rT3, T4G, and T4S. Rat hepatocytes had as much as 13-times higher concentrations of metabolites in the media compared to human hepatocytes. Metabolite distribution in the media of rat versus human hepatocytes was as follows: T4G, (91.6 vs. 5.3%); T4S, (3.6 vs. 4.4%); T3+rT3, (4.9 vs. 90.3%). The results indicate that basal hepatic activity of T4 glucuronidation is greater in cultured rat hepatocytes compared to cultured human hepatocytes. These data also suggest that glucuronidation may be a more important pathway for T4 metabolism in rats and deiodination may be a favored pathway in humans. However, with PCB 153 exposure these data support glucuronidation as a primary route of T4 metabolism in both rat and humans.