Regulation of mdrlb gene expression in Fischer, Wistar and Sprague-Dawley rats in vivo and in vitro

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Abstract
Administration of 2-acetylaminofluorene (2-AAF) to rats increased mdrlb expression in Fischer, Wistar and Sprague--Dawley rat livers; however, the response was much smaller in Sprague-Dawley livers. To investigate the basis of this difference we further examined the regulation of the mdrlb gene in hepatocytes isolated from Fischer, Sprague-Dawley and Wistar rats. A time-dependent increase in basal expression of mdrlb but not mdr2 was observed in hepatocytes isolated from all three strains of rats. After 4 days in culture, a larger increase in mdrlb mRNA levels was observed in Fischer and Wistar rat hepatocytes (3.5- and 4.6-fold respectively) than Sprague-Dawley hepatocytes (2-fold). Treatment of primary hepatocytes with 2-AAF caused an induction of mdrlb expression that varied among the three strains. Notably, Sprague-Dawley hepatocytes were not responsive to 2-AAF. In contrast to the parent compound, the electrophilic metabolites N-hydroxy-2-acetylaminofluorene and N-acetoxy-2-acetylaminofluorene caused a dose-dependent induction of mdrlb expression in both Fischer and Sprague-Dawley hepatocytes, indicating that differences in the metabolic activation of 2-AAF between the strains may account for the differences in mdr1b induction by 2-AAF. Hepatocytes isolated from all three strains of rats showed an equivalent induction of mdr1b after treatment with cycloheximide. Nuclear run-on assays demonstrated that the increases in mdrlb expression with time in culture and after xenobiotic treatment were due to increased transcription.

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