The A-B-C of small-molecule ABC transport protein modulators: From inhibition to activation-a case study of multidrug resistance-associated protein 1 (ABCC1).

Several mechanisms of pharmacokinetic, metabolic, and regulatory nature have been elucidated to take part or act in concert in the phenomenon of multidrug resistance (MDR). MDR is characterized by cross-resistance of cells against chemotherapeutic agents, which are used for treatment of e.g., cancer, bacterial infections, or human immunodeficiency virus (HIV) infections. One group of proteins that combines all three stated aspects-the metabolism and distribution of drugs as well as their own regulation-is adenosine triphosphate-binding cassette (ABC) transporters. These efflux pumps use the energy of adenosine triphosphate hydrolysis for drug translocation from the membrane and the cytosol to the extracellular space, often with cotransport of a cosubstrate. Multidrug resistance-associated protein 1 (MRP1, ABCC1) had been discovered as one major key player in cancer-related MDR. The xenobiotic substrates include anthracyclines, vinca alkaloids, podophyllotoxins, as well as glutathione (GSH)-adducts of certain cytostatics. Contrary to other transport proteins involved in cancer-related MDR the activity of MRP1 is related to the GSH content of cells. A modern strategy to overcome MRP1-associated MDR is besides its inhibition the activation of GSH efflux, enforcing cell death due to cellular stress. In addition, it has recently been found that MRP1 contributes to the β-amyloid protein clearance in Alzheimer's disease (AD). Collectively, transport activation of MRP1 is of therapeutic value, and furthermore helps to elucidate the transport protein function and the mechanisms behind it. This review is meant to summarize the known concepts of MRP1 activation, which might contribute to a further understanding of MRP1 in particular and ABC transporters in general.