ATP Binding Cassette (ABC) transporters form a special family of membrane proteins, characterized by homologous ATP-binding, and large, multispanning transmembrane domains. Several members of this family are primary active transporters, which significantly modulate the absorption, metabolism, cellular effectivity and toxicity of pharmacological agents. This review provides a general overview of the human ABC transporters, their expression, localization and basic mechanism of action. Then we shortly deal with the human ABC transporters as targets of therapeutic interventions in medicine, including cancer drug resistance, lipid and other metabolic disorders, and even gene therapy applications. We place a special emphasis on the three major groups of ABC transporters involved in cancer multidrug resistance (MDR). These are the classical P-glycoprotein (MDR1, ABCB1), the multidrug resistance associated proteins (MRPs, in the ABCC subfamily), and the ABCG2 protein, an ABC half-transporter. All these proteins catalyze an ATP-dependent active transport of chemically unrelated compounds, including anticancer drugs. MDR1 (P-glycoprotein) and ABCG2 preferentially extrude large hydrophobic, positively charged molecules, while the members of the MRP family can extrude both hydrophobic uncharged molecules and water-soluble anionic compounds. Based on the physiological expression and role of these transporters, we provide examples for their role in Absorption-Distribution-Metabolism-Excretion (ADME) and toxicology, and describe several basic assays which can be applied for screening drug interactions with ABC transporters in the course of drug research and development.

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ABSTRACT:The exporter ABCC2 (cMOAT, MRP2) is a membrane-bound protein on the apical side of enterocytes and hepatic biliary vessels that transports leukotriene C 4 , glutathione, some conjugated bile salts, drugs, xenobiotics, and phytonutrients. The latter class includes quercetin, a bioactive flavonoid found in foods such as onions, apples, tea, and wine. There is no available three-dimensional (3D) structure of ABCC2. We have ABCC2 (cMOAT, MRP2) is a member of the family of ATP binding cassette (ABC) transporters. Lack of ABCC2 expression in humans leads to the Dubin-Johnson syndrome, an autosomal dominant hereditary disease (König et al., 1999). This disease is manifested by chronic hyperbilirubinemia due to reduced biliary secretion of bilirubin conjugates (Payen et al., 2002). ABCC2 is a transmembrane protein that uses the energy of ATP hydrolysis to translocate its substrates across biological membranes and transports a wide variety of compounds, including various endobiotics and xenobiotics. Recent studies suggest that ABCC2 influences oral bioavailability (Dietrich et al., 2003), and its inhibition decreases the elimination of xenobiotics. It is structurally closely related to ABCC1 (MRP1) and the substrate selectivities of ABCC1 and ABCC2 overlap (Gerk and Vore, 2002) to a large extent.The 1545-amino acid human ABCC2 contains two nucleotidebinding domains and up to 17 transmembrane helices distributed within three transmembrane domains (TMD), 1, 2, and 3. Classified in the same MRP family, human ABCC1 and human ABCC2 share 48% sequence identity as well as a similar membrane topology, implying structural and functional similarity. It has been shown that the aminoterminal TMD-1 of ABCC1 is not essential for substrate transport. Experimental efforts to characterize the substrate binding/transport have therefore been focused on transmembrane segments TM6 to TM17 of TMD-2 (TM6 to TM11) and TMD-3 (TM12 to TM17). To date, high-resolution 3D structures for ABCC1 and ABCC2 are still not available. The 3D structures for TMD-2 and -3 of ABCC1 have been obtained by homology modeling (Campbell et al., 2004). As revealed in the predicted 3D model, TMD-2 and -3 form a channel, which allows for the transportation of ABCC1 substrates. Together with biochemical studies, the 3D structural model for ABCC1 has provided further insight on the transport mechanisms (Campbell et al., 2004).Quercetin is an anticarcinogenic flavonoid that affects phase II

ABSTRACT:The ATPase assay using membrane preparations from recombinant baculovirus-infected Spodoptera frugiperda ovarian (Sf9) cells is widely used to detect the interaction of compounds with different ATP-binding cassette transporters. However, Sf9 membrane preparations containing the wild-type ABCG2 transporter show an elevated baseline vanadate-sensitive ATPase activity, which cannot be further stimulated by substrates of ABCG2. Therefore, this assay system cannot be used for the detection of ABCG2 substrates. To overcome this difficulty we 1) purified membranes from a selected human cell line expressing wild-type ABCG2, and 2) inhibited the baseline ATPase activity with different inhibitors. In our modified assay, ABCG2 substrates were able to stimulate the baseline ATPase activity of ABCG2 expressed in membranes of human cells. Furthermore, using the specific ABCG2 inhibitors Ko143 or Ko134 allowed us to suppress the baseline vanadate-sensitive ATPase activity. Substrates of ABCG2 could stimulate this suppressed baseline ATPase, resulting in a better signal-to-background ratio and a robust assay to detect substrates of the ABCG2 transporter. The ATPase assay and the direct vesicular transport measurements for estrone-3-sulfate were in good accordance.