The human multidrug resistance-associated protein (MRP) family currently has seven members. The ability of several of these membrane proteins to transport a wide range of anticancer drugs out of cells and their presence in many tumors make them prime suspects in unexplained cases of drug resistance, although proof that they contribute to clinical drug resistance is still lacking. Recent studies have begun to clarify the function of the MRP family members. MRPs are organic anion transporters; i.e., they transport anionic drugs, exemplified by methotrexate, and neutral drugs conjugated to acidic ligands, such as glutathione (GSH), glucuronate, or sulfate. However, MRP1, MRP2, and MRP3 can also cause resistance to neutral organic drugs that are not known to be conjugated to acidic ligands by transporting these drugs together with free GSH. MRP1 can even confer resistance to arsenite and MRP2 to cisplatin, again probably by transporting these compounds in complexes with GSH. MRP4 overexpression is associated with high-level resistance to the nucleoside analogues 9-(2-phosphonylmethoxyethyl) adenine and azidothymidine, both of which are used as anti-human immunodeficiency virus drugs. MRPs may, therefore, also have a role in resistance against nucleoside analogues used in cancer chemotherapy. Mice without Mrp1, a high-affinity leukotriene C(4) transporter, have an altered response to inflammatory stimuli but are otherwise healthy and fertile. MRP2 is the major transporter responsible for the secretion of bilirubin glucuronides into bile, and humans without MRP2 develop a mild liver disease known as the Dubin-Johnson syndrome. The physiologic functions of the other MRPs are not known. Whether long-term inhibition of MRPs in humans can be tolerated (assuming that suitable inhibitors will be found) remains to be determined.

The human multidrug resistance protein (MRP) family contains at least six members: MRP1, the godfather of the family and well known as the multidrug resistance protein, and five homologs, called MRP2-6. In this review, we summarize what is known about the protein structure, the expression in tissues, the routing in cells, the physiological functions, the substrate specificity, and the role in multidrug resistance of the individual members of the MRP family.

Prostaglandins are involved in a wide variety of physiological and pathophysiological processes, but the mechanism of prostaglandin release from cells is not completely understood. Although poorly membrane permeable, prostaglandins are believed to exit cells by passive diffusion. We have investigated the interaction between prostaglandins and members of the ATP-binding cassette (ABC) transporter ABCC [multidrug resistance protein (MRP)] family of membrane export pumps. In inside-out membrane vesicles derived from insect cells or HEK293 cells, MRP4 catalyzed the time-and ATP-dependent uptake of prostaglandin E 1 (PGE1) and PGE2. In contrast, MRP1, MRP2, MRP3, and MRP5 did not transport PGE 1 or PGE 2. The MRP4-mediated transport of PGE1 and PGE2 displayed saturation kinetics, with K m values of 2.1 and 3.4 M, respectively. Further studies showed that PGF 1␣, PGF2␣, PGA1, and thromboxane B 2 were high-affinity inhibitors (and therefore presumably substrates) of MRP4. Furthermore, several nonsteroidal antiinflammatory drugs were potent inhibitors of MRP4 at concentrations that did not inhibit MRP1. In cells expressing the prostaglandin transporter PGT, the steady-state accumulation of PGE 1 and PGE2 was reduced proportional to MRP4 expression. Inhibition of MRP4 by an MRP4-specific RNA interference construct or by indomethacin reversed this accumulation deficit. Together, these data suggest that MRP4 can release prostaglandins from cells, and that, in addition to inhibiting prostaglandin synthesis, some nonsteroidal antiinflammatory drugs might also act by inhibiting this release. P rostaglandins are key mediators in the regulation of many physiological processes. They are involved in inflammatory responses and tumorigenesis, and their synthesis and metabolism are tightly regulated (1). The first step in prostaglandin synthesis is the production of arachidonic acid, which is released from membrane lipid primarily by cytosolic phospholipase A 2 (1). Arachidonic acid is then oxidized to the intermediate prostaglandin H 2 (PGH 2 ) by PGH synthases, also known as cyclooxygenase (COX)-1 and -2, and the recently identified COX-3 (2). These enzymes are known clinically as the targets of aspirin and other nonsteroidal antiinf lammatory drugs (NSAIDs) (3). Moreover, several recent studies have shown a link between COX-2 expression and carcinogenesis. Prostaglandins are overproduced by a variety of tumors, leading to the suggested prophylactic use of COX-2 inhibitors to decrease the incidence of colon cancer (4, 5). After COX-mediated synthesis, PGH 2 is further converted by tissue-specific prostaglandin synthases into PGE 2 , PGF 2␣ , PGD 2 , prostacyclin, or thromboxane B 2 , the biologically active molecules (1).Prostaglandins are formed and secreted by most cells, and act as autocrine-or paracrine-signaling molecules. In many cases they exert their effects extracellularly via interaction with a family of G protein-coupled membrane receptors (reviewed in ref. 6), although some prostaglandins interact with the nuclear horm...

Loss of Crumbs homologue 1 (CRB1) function causes either the eye disease Leber congenital amaurosis or progressive retinitis pigmentosa, depending on the amount of residual CRB1 activity and the genetic background. Crb1 localizes specifically to the sub-apical region adjacent to the adherens junction complex at the outer limiting membrane in the retina. We show that it is associated here with multiple PDZ protein 1 (Mupp1), protein associated with Lin-7 (Pals1 or Mpp5) and Mpp4. We have produced Crb1-/- mice completely lacking any functional Crb1. Although the retinas are initially normal, by 3-9 months the Crb1-/- retinas develop localized lesions where the integrity of the outer limiting membrane is lost and giant half rosettes are formed. After delamination of the photoreceptor layer, neuronal cell death occurs in the inner and outer nuclear layers of the retina. On moderate exposure to light for 3 days at 3 months of age, the number of severe focal retinal lesions significantly increases in the Crb1-/- retina. Crb2, Crb3 and Crb1 interacting proteins remain localized to the sub-apical region and therefore are not sufficient to maintain cell adhesion during light exposure in Crb1-/- retinas. Thus we propose that during light exposure Crb1 is essential to maintain, but not assemble, adherens junctions between photoreceptors and Müller glia cells and prevents retinal disorganization and dystrophy. Hence, light may be an influential factor in the development of the corresponding human diseases.

Two prominent members of the ATP-binding cassette superfamily of transmembrane proteins, multidrug resistance 1 (MDR1) Pglycoprotein and multidrug resistance protein 1 (MRP1), can mediate the cellular extrusion of xenobiotics and (anticancer) drugs from normal and tumor cells. The MRP subfamily consists of at least six members, and here we report the functional characterization of human MRP5. We found resistance against the thiopurine anticancer drugs, 6-mercaptopurine (6-MP) and thioguanine, and the anti-HIV drug 9-(2-phosphonylmethoxyethyl)adenine (PMEA) in MRP5-transfected cells. This resistance is due to an increased extrusion of PMEA and 6-thioinosine monophosphate from the cells that overproduce MRP5. In polarized Madin-Darby canine kidney II (MDCKII) cells transfected with an MRP5 cDNA construct, MRP5 is routed to the basolateral membrane and these cells transport S-(2,4-dinitrophenyl)glutathione and glutathione preferentially toward the basal compartment. Inhibitors of organic anion transport inhibit transport mediated by MRP5. We speculate that MRP5 might play a role in some cases of unexplained resistance to thiopurines in acute lymphoblastic leukemia and/or to antiretroviral nucleoside analogs in HIV-infected patients.