Using real-time quantitative TaqMan RT-PCR to evaluate the role of dexamethasone in gene regulation of rat P-glycoproteins mdr1a/1b and cytochrome P450 3A1/2.

Cytochromes P450 (CYPs) and p-glycoproteins (Pgps) are believed to play important roles in drug absorption, metabolism, and elimination. Numerous drugs and environmental chemicals can modulate expression of these two classes of genes in different species. The present study investigated the effect of dexamethasone (Dex) on gene expression on both message and protein levels of mdr1a, mdr1b, CYP3A1, and CYP3A2 in small intestine, colon, liver, kidney, and brain microvessels of the rats treated orally with Dex at 1 or 20 mg/kg/day for 3 days. The basal expression of mdr1a mRNA was highest in the brain microvessels followed by colon, small intestine, liver, and kidney, and mdr1b mRNA was highest in the brain microvessels followed by kidney, liver, colon, and small intestine. After Dex treatment, mdr1a mRNA was increased by 5.5- and 10.7-fold in the small intestine, decreased extensively by 85-90% in the liver, and showed little or no change in the colon, kidney, and brain microvessels compared to the control rats. A similar pattern was observed for mdr1b mRNA. CYP3A1 mRNA was increased in all tissues examined. CYP3A2 mRNA was not significantly changed with the exception that at 20 mg/kg CYP3A2 mRNA was increased 5- and 30-fold in the colon and kidney. In general, Western blot analyses were consistent with mRNA changes. CYP3A protein expression was increased in all tissues examined. The disparity of the impact of Dex on the CYP 3A and Pgp expression in these studies suggest that the regulation of Pgp expression is very complex and is difficult to predict solely based on the PXR response to xenobiotics.