Contribution of Abcc10 (Mrp7) to in vivo paclitaxel resistance as assessed in Abcc10(-/-) mice

Modulation of the ATPase and transport activities of broad-acting multidrug resistance factor ABCC10 (MRP7)

The cell surface molecule ABCC10 is a broad-acting transporter of xenobiotics, including cancer drugs, such as taxanes, epothilone B, and modulators of the estrogen pathway. Abcc10(-/-) mice exhibit increased tissue sensitivity and lethality resulting from paclitaxel exposure compared with wild-type counterparts, arguing ABCC10 functions as a major determinant of taxane sensitivity in mice. To better understand the mechanistic basis of ABCC10 action, we characterized the biochemical and vectorial transport properties of this protein. Using crude membranes in an ABCC10 overexpression system, we found that the ABCC10 transport substrates estrogen estradiol-glucuronide (E(2)17βG) and leukotriene C4 (LTC(4)) significantly stimulated ABCC10 beryllium fluoride (BeFx)-sensitive ATPase activity. We also defined the E(2)17βG antagonist, tamoxifen, as a novel substrate and stimulator of ABCC10. In addition, a number of cytotoxic substrates, including docetaxel, paclitaxel, and Ara-C, increased the ABCC10 basal ATPase activity. We determined that ABCC10 localizes to the basolateral cell surface, using transepithelial well assays to establish that ABCC10-overexpressing LLC-PK1 cells exported [(3)H]-docetaxel from the apical to the basolateral side. Importantly, we found that the clinically valuable multikinase inhibitor sorafenib, and a natural alkaloid, cepharanthine, inhibited ABCC10 docetaxel transport activity. Thus, concomitant use of these agents might restore the intracellular accumulation and potency of ABCC10-exported cytotoxic drugs, such as paclitaxel. Overall, our work could seed future efforts to identify inhibitors and other physiologic substrates of ABCC10.

PDE5 inhibitors, sildenafil and vardenafil, reverse multidrug resistance by inhibiting the efflux function of multidrug resistance protein 7 (ATP-binding Cassette C10) transporter

Phosphodiesterase type 5 (PDE5) inhibitors are widely used in the treatment of male erectile dysfunction and pulmonary hypertension. Recently, several groups have evaluated the ability of PDE5 inhibitors for their anticancer activities. Previously, we had shown that sildenafil, vardenafil and tadalafil could reverse P-glycoprotein (ATP-binding cassette B1)-mediated MDR. In the present study, we determined whether these PDE5 inhibitors have the potential to reverse multidrug resistance protein 7 (MRP7; ATP-binding cassette C10)-mediated MDR. We found that sildenafil and vardenafil dose-dependently enhanced the sensitivity of MRP7-transfected HEK293 cells to paclitaxel, docetaxel and vinblastine, while tadalafil had only a minimal effect. Accumulation and efflux experiments demonstrated that sildenafil and vardenafil increased the intracellular accumulation of [(3)H]-paclitaxel by inhibiting the efflux of [(3 H]-paclitaxel in HEK/MRP7 cells. In addition, immunoblot and immunofluorescence analyses indicated that no significant alterations of MRP7 protein expression and localization in plasma membranes were found after treatment with sildenafil, vardenafil or tadalafil. These results demonstrate that sildenafil and vardenafil reverse MRP7-mediated a MDR through inhibition of the drug efflux function of MRP7. Our findings indicate a potentially novel use of PDE5 inhibitors as an adjuvant chemotherapeutic agent in clinical practice.

The role of transporters in the toxicity of nucleoside and nucleotide analogs

INTRODUCTION

Two families of nucleoside analogs have been developed to treat viral infections and cancer, but these compounds can cause tissue- and cell-specific toxicity related to their uptake and subcellular activity, which are dictated by host enzymes and transporters. Cellular uptake of these compounds requires nucleoside transporters that share functional similarities but differ in substrate specificity. Tissue-specific cellular expression of these transporters enables nucleoside analogs to produce their tissue-specific toxic effects, a limiting factor in the treatment of retroviruses and cancer.

AREAS COVERED

This review discusses the families of nucleoside transporters and how they mediate cellular uptake of nucleoside analogs. Specific focus is placed on examples of known cases of transporter-mediated cellular toxicity and classification of the toxicities resulting. Efflux transporters are also explored as a contributor to analog toxicity and cell-specific effects.

EXPERT OPINION

Efforts to modulate transporter uptake/clearance remain long-term goals of oncologists and virologists. Accordingly, subcellular approaches that either increase or decrease intracellular nucleoside analog concentrations are eagerly sought and include transporter inhibitors and targeting transporter expression. However, additional understanding of nucleoside transporter kinetics, tissue expression and genetic polymorphisms is required to design better molecules and better therapies.

Arsenic-glutathione conjugate transport by the human multidrug resistance proteins (MRPs/ABCCs)

Millions of people world-wide are chronically exposed to inorganic forms of the environmental toxicant arsenic in drinking water. This has led to a public health crisis because arsenic is a human carcinogen, and causes a myriad of other adverse health effects. In order to prevent and treat arsenic-induced toxicity it is critical to understand the cellular handling of this metalloid. A large body of literature describes the importance of the cellular tripeptide glutathione (γ-Glu-Cys-Gly,GSH/GS) in the excretion of arsenic. The triglutathione conjugate of arsenite [As(III)(GS)(3)] and the diglutathione conjugate of monomethylarsonous acid [MMA(III)(GS)(2)] have been isolated from rat bile and mouse urine, and account for the majority of excreted arsenic, suggesting these are important transportable forms. The ATP-binding cassette (ABC) transporter proteins, multidrug resistance protein 1 (MRP1/ABCC1) and the related protein MRP2 (ABCC2), are thought to play an important role in arsenic detoxification through the cellular efflux of arsenic-GSH conjugates. Current knowledge on the cellular handling of arsenic with a special emphasis on the transport pathways of the arsenic-GSH conjugates As(III)(GS)(3), MMA(III)(GS)(2), and dimethylarsenic glutathione DMA(III)(GS), as well as, the seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-) are reviewed.

Multidrug resistance associated proteins in multidrug resistance

Multidrug resistance proteins (MRPs) are members of the C family of a group of proteins named ATP-binding cassette (ABC) transporters. These ABC transporters together form the largest branch of proteins within the human body. The MRP family comprises of 13 members, of which MRP1 to MRP9 are the major transporters indicated to cause multidrug resistance in tumor cells by extruding anticancer drugs out of the cell. They are mainly lipophilic anionic transporters and are reported to transport free or conjugates of glutathione (GSH), glucuronate, or sulphate. In addition, MRP1 to MRP3 can transport neutral organic drugs in free form in the presence of free GSH. Collectively, MRPs can transport drugs that differ structurally and mechanistically, including natural anticancer drugs, nucleoside analogs, antimetabolites, and tyrosine kinase inhibitors. Many of these MRPs transport physiologically important anions such as leukotriene C4, bilirubin glucuronide, and cyclic nucleotides. This review focuses mainly on the physiological functions, cellular resistance characteristics, and probable in vivo role of MRP1 to MRP9.