Does ABCG2 Need a Heterodimer Partner? Expression and Functional Evaluation of ABCG2 (Arg 482)*

Inhibitory effects of daiokanzoto (da-huang-gan-cao-tang) on p-glycoprotein

We have studied the effects of various Kampo medicines on P-glycoprotein (P-gp), a drug transporter, in vitro. The present study focused on Daiokanzoto (Da-Huang-Gan-Cao-Tang), which shows the most potent inhibitory effects on P-gp among the 50 Kampo medicines studied, and investigated the P-gp inhibitory effects of Daiokanzoto herbal ingredients (rhubarb and licorice root) and their components by an ATPase assay using human P-gp membrane. Both rhubarb and licorice root significantly inhibited ATPase activity, and the effects of rhubarb were more potent than those of licorice root. The content of rhubarb in Daiokanzoto is double that in licorice root, and the inhibition patterns of Daiokanzoto and rhubarb involve both competitive and noncompetitive inhibition, suggesting that the inhibitory effects of Daiokanzoto are mainly due to rhubarb. Concerning the components of rhubarb, concentration-dependent inhibitory effects were observed for (−)-catechin gallate, (−)-epicatechin gallate, and (−)-epigallocatechin gallate. In conclusion, rhubarb may cause changes in the drug dispositions of P-gp substrates through the inhibition of P-gp. It appears that attention should be given to the interactions between these drugs and Kampo medicines containing rhubarb as an herbal ingredient.

Effects of Kampo medicines on P-glycoprotein

The Kampo medicines are more and more often used in recent years, usually together with the western drugs. The need for the investigation of drug interactions between Kampo medicines and western drugs are, therefore, widely recognized. Among the various possible causes for the drug-drug interactions, those related to pharmacokinetics such as drug metabolism and transport are regarded as most frequent and clinically important. In the present study, the effects of Kampo medicines on the P-glycoprotein (P-gp), one of the major drug transporters, were investigated in in vitro studies using human P-gp membranes. The P-gp activity in the presence and absence of commonly used 50 Kampo medicines was evaluated by the ATPase assay detecting the inorganic phosphate produced by the ATP hydrolysis. The ATPase activity was inhibited by most of the Kampo medicines studied, indicating the possibility of their inhibiting the P-gp. The degree of inhibition in the presence of verapamil, a P-gp substrate, showed a significant correlation with that in the absence of verapamil. Furthermore, the inhibitory effect of the Kampo medicines on the ATPase activity correlated with their licorice root (kanzo) content, suggesting the contribution of licorice root in the P-gp inhibition. Because licorice root is one of the most common ingredients in the Kampo medicines and is also often used in the food as a sweetener, it might be necessary to pay attention on the interaction between the licorice root-containing drug/food and the number of drugs transported by P-gp.

Effect of hydrocotarnine on cytochrome P450 and P-glycoprotein

The injectable form of oxycodone contains hydrocotarnine that is supposed to potentiate the analgesic effect of oxycodone with unknown mechanism(s). In this study, the effects of hydrocotarnine on the cytochrome P450 (CYP) and P-glycoprotein (P-gp) were investigated. Hydrocotarnine did not induce a significant change in the metabolic activities of CYP2C9, 2C19, and 2E1 in an in vitro study using human CYP recombinants. Although weak inhibitory effects were observed on CYP3A4 and 2D6, these interactions did not seem to be clinically relevant. Hydrocotarnine also did not cause a significant change in the ATPase activity of human P-gp membranes, suggesting that it is not an inhibitor of P-gp. Furthermore, mice were intraperitoneally injected with hydrocotarnine for 14 days and the mRNA levels of major CYP isozymes and P-gp in the liver and small intestine were determined by real-time RT-PCR. As a result, none of the mRNAs investigated showed a significant change in their levels by hydrocotarnine treatment. In conclusion, it is unlikely that the potentiation of oxycodone effect by hydrocotarnine involves its effect on CYP and P-gp. The findings also demonstrate that hydrocotarnine is unlikely to cause drug interactions via CYP or P-gp.

Breast cancer resistance protein (BCRP/ABCG2) is expressed by progenitor cells/reactive ductules and hepatocytes and its expression pattern is influenced by disease etiology and species type: possible functional consequences

Breast cancer resistance protein (BCRP/ABCG2) is an ATP-binding cassette transport protein that is expressed in several organs including the liver. Previous studies have shown that ABC transport proteins play an important pathophysiological role in several liver diseases. However, to date, expression pattern and possible role of BCRP in human liver diseases and animal models have not been studied in detail. Here we investigated the expression pattern of BCRP in normal liver, chronic parenchymal and biliary human liver diseases, and parallel in different rat models of liver diseases. Expression was studied by immunohistochemistry and additionally by RT-PCR analysis in Thy-1-positive rat oval cells. Bile ducts, hepatic progenitor cells, reactive bile ductules, and blood vessel endothelium were immunoreactive for BCRP in normal liver and all types of human liver diseases and in rat models. BCRP was expressed by the canalicular membrane of hepatocytes in normal and diseased human liver, but never in rat liver. Remarkably, there was also expression of BCRP at the basolateral pole of human hepatocytes, and this was most pronounced in chronic biliary diseases. In conclusion, BCRP positivity in the progenitor cells/reactive ductules could contribute to the resistance of these cells to cytotoxic agents and xenotoxins. Basolateral hepatocytic expression in chronic biliary diseases may be an adaptive mechanism to pump bile constituents back into the sinusoidal blood. Strong differences between human and rat liver must be taken into account in future studies with animal models.

A functional study on polymorphism of the ATP-binding cassette transporter ABCG2: critical role of arginine-482 in methotrexate transport

Overexpression of the ATP-binding cassette transporter ABCG2 reportedly causes multidrug resistance, whereas altered drug-resistance profiles and substrate specificity are implicated for certain variant forms of ABCG2. At least three variant forms of ABCG2 have been hitherto documented on the basis of their amino acid moieties (i.e., arginine, glycine and threonine) at position 482. In the present study we have generated those ABCG2 variants by site-directed mutagenesis and expressed them in HEK-293 cells. Exogenous expression of the Arg(482), Gly(482), and Thr(482) variant forms of ABCG2 conferred HEK-293 cell resistance toward mitoxantrone 15-, 47- and 54-fold, respectively, as compared with mock-transfected HEK-293 cells. The transport activity of those variants was examined by using plasma-membrane vesicles prepared from ABCG2-overexpressing HEK-293 cells. [Arg(482)]ABCG2 transports [(3)H]methotrexate in an ATP-dependent manner; however, no transport activity was observed with the other variants (Gly(482) and Thr(482)). Transport of methotrexate by [Arg(482)]ABCG2 was significantly inhibited by mitoxantrone, doxorubicin and rhodamine 123, but not by S -octylglutathione. Furthermore, ABCG2 was found to exist in the plasma membrane as a homodimer bound via cysteinyl disulphide bond(s). Treatment with mercaptoethanol decreased its apparent molecular mass from 140 to 70 kDa. Nevertheless, ATP-dependent transport of methotrexate by [Arg(482)]ABCG2 was little affected by such mercaptoethanol treatment. It is concluded that Arg(482) is a critical amino acid moiety in the substrate specificity and transport of ABCG2 for certain drugs, such as methotrexate.

Expression and Functional Characterization of Human ABC Transporter ABCG2 Variants in Insect Cells

Hitherto three variant forms of ABCG2 have been documented on the basis of their amino acid moieties (i.e., Arg, Gly, and Thr) at the position 482. In the present study, we have generated those variants of ABCG2 by site-directed mutagenesis and expressed them in Sf9 insect cells. The apparent molecular weight of the expressed ABCG2 variants was 130,000 under non-reductive conditions, whereas it was reduced to 65, 000 by treatment with mercaptoethanol. It is suggested that ABCG2 exists in the plasma membrane of Sf9 cells as a homodimer bound through cysteinyl disulfide bond(s). Both ATPase activity and drug transport of ABCG2 variants were examined by using plasma membrane fractions prepared from ABCG2-overexpressing Sf9 cells. The ATPase activity of the plasma membrane expressing ABCG2 (Gly-482) was significantly enhanced by prazosin. In contrast, ABCG2 (Arg-482) transports [(3)H]methotrexate in an ATP-dependent manner; however, no transport activity was observed with the other variants (Gly-482 and Thr-482). It is strongly suggested that the amino acid moiety at the position of 482 is critical for the substrate specificity of ABCG2.