To evaluate whether alterations in the multidrug-resistance (MDR)-1 gene correlate with intestinal MDR-1 expression and uptake of orally administered P-glycoprotein (PGP) substrates, we analyzed the MDR-1 sequence in 21 volunteers whose PGP expression and function in the duodenum had been determined by Western blots and quantitative immunohistology ( n = 21) or by plasma concentrations after orally administered digoxin ( n = 8 + 14). We observed a significant correlation of a polymorphism in exon 26 (C3435T) of MDR-1 with expression levels and function of MDR-1. Individuals homozygous for this polymorphism had significantly lower duodenal MDR-1 expression and the highest digoxin plasma levels. Homozygosity for this variant was observed in 24% of our sample population ( n = 188). This polymorphism is expected to affect the absorption and tissue concentrations of numerous other substrates of MDR-1.

Intestinal P-glycoprotein, which is encoded by the MDR1 gene, plays an important role in the absorption and presystemic elimination of many xenobiotics. Hence, an understanding of the factors regulating its expression and function is of substantial interest. In addition to genetic factors, exposure to drugs such as rifampin can profoundly affect its expression. So far, the mechanisms by which rifampin induces MDR1 expression are poorly understood. Recent studies demonstrate that the nuclear receptor PXR (pregnane X receptor) is involved in xenobiotic induction of CYP3A4. Because CYP3A4 and MDR1 are often co-induced, we investigated whether a similar mechanism is also involved in MDR1 induction. The human colon carcinoma cell line LS174T was used as an intestinal model to study induction because in these cells the endogenous MDR1 gene is highly inducible by rifampin. The 5-upstream region of human MDR1 was examined for the presence of potential PXR response elements. Several binding sites were identified that form a complex regulatory cluster at about ؊8 kilobase pairs. Only one DR4 motif within this cluster is necessary for induction by rifampin. We conclude that induction of MDR1 is mediated by a DR4 motif in the upstream enhancer at about ؊8 kilobase pairs, to which PXR binds. The MDR11 gene product P-glycoprotein (P-gp) plays an important role in the transport of hydrophobic xenobiotics and peptides from the inside to the outside of cells. Initially discovered in cancer cells as a mechanism responsible for resistance against certain cytostatic drugs (reviewed in Ref. 1), it was later shown that P-gp is also expressed in different nonmalignant cells of various organs. In agreement with its assumed physiologic role as a defense mechanism against potential toxic substances present in the diet and from environmental exposure, it is expressed in the brush border membrane of the mature enterocytes, the canalicular membrane of hepatocytes, the brush border of proximal renal tubular cells, and the luminal side of endothelial cells of brain capillaries (2, 3).In the gut P-gp functions as an efflux pump, actively transporting substances back into the intestinal lumen (4) and, hence, has an important role for the absorption and presystemic elimination of many chemicals including drugs. The level of intestinal P-gp expression shows wide interindividual differences (5) controlled by both genetic and environmental factors. Recently a genetic polymorphism of the MDR1 gene has been reported that affects the P-gp expression in the epithelial cell lining of the small intestine (6). In addition to this genetic polymorphism, environmental factors can affect the expression of P-gp. A number of drugs and steroid hormones have been shown to induce P-gp expression (7-11). For instance, the antibiotic rifampin not only induces intestinal cytochrome P 450 3A4 enzyme but also elicits a significant increase of intestinal P-gp (11). As a consequence, the plasma concentrations of orally administered digoxin are dramatically reduced. So fa...

CYP3A proteins comprise a significant portion of the hepatic cytochrome P450 (CYP) protein and they metabolize around 50% of drugs currently in use. The dissection of the individual contributions of the four CYP3A genes identified in humans to overall hepatic CYP3A activity has been hampered by sequence and functional similarities. We have investigated the expression of CYP3A5 and its genetic determinants in a panel of 183 Caucasian liver samples. CYP3A5 expression is increased in 10% of livers in this ethnic group. Using a high density map of CYP3A5 variants, we searched for genetic markers of the increased CYP3A5 expression. In agreement with an independent, recent study, we report that a SNP within intron 3 (g.6986G>A) is the primary cause of the CYP3A5 protein polymorphism. The frequencies of the g.6986A variant which allow for normal splicing of CYP3A5 transcripts are 5% in Caucasians, 29% in Japanese, 27% in Chinese, 30% in Koreans and 73% in African-Americans. In the last ethnic group, the expression of CYP3A5 in some individuals who carry the g.6986A variant is affected adversely by a frame shift mutation (CYP3A5*7, D348., q = 0.10). In summary, these results should add to efforts to identify clinically relevant, CYP3A5-specific reactions and to further elucidate traits responsible for variable expression of the entire CYP3A family.

An important function of hepatocytes is the biotransformation and elimination of various drugs, many of which are organic cations and are taken up by organic cation transporters (OCTs) of the solute carrier family 22 (SLC22). Because interindividual variability of OCT expression may affect response to cationic drugs such as metformin, we systematically investigated genetic and nongenetic factors of OCT1/SLC22A1 and OCT3/SLC22A3 expression in human liver. OCT1 and OCT3 expression (messenger RNA [mRNA], protein) was analyzed in liver tissue samples from 150 Caucasian subjects. Hepatic OCTs were localized by way of immunofluorescence microscopy. Matrixassisted laser desorption/ionization time-of-flight mass spectrometry and genome-wide singlenucleotide polymorphism microarray technology served to genotype 92 variants in the SLC22A1-A3/ OCT1-3 gene cluster. Transport of metformin by recombinant human OCT1 and OCT3 was compared using transfected cells. OCT1 mRNA and protein expression varied 113-and 83-fold, respectively; OCT3 mRNA expression varied 27-fold. OCT1 transcript levels were on average 15-fold higher compared with OCT3. We localized the OCT3 protein to the basolateral hepatocyte membrane and identified metformin as an OCT3 substrate. OCT1 and OCT3 expression are independent of age and sex but were significantly reduced in liver donors diagnosed as cholestatic (P < 0.01). Several haplotypes for OCT1 and OCT3 were identified. Multivariate analysis adjusted for multiple testing showed that only the OCT1-Arg61Cys variant (rs12208357) strongly correlated with decreased OCT1 protein expression (P < 0.0001), and four variants in OCT3 (rs2292334, rs2048327, rs1810126, rs3088442) were associated with reduced OCT3 mRNA levels (P ‫؍‬ 0.03). Conclusion: We identified cholestasis and genetic variants as critical determinants for considerable interindividual variability of hepatic OCT1 and OCT3 expression. This indicates consequences for hepatic elimination of and response to OCT substrates such as metformin.