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The multidrug resistant-associated protein 1 (MRP1) is a membrane-bound transport protein that is involved in the efflux of organic anions and has been implicated in multidrug resistance in cancer. MRP1 has also been reported to be ubiquitously expressed in normal tissues, including the brain. The presence of functional organic anion transporters in the blood-brain and blood-CSF barriers that influence the distribution of various compounds to the brain has long been known. The purpose of this study was to examine the role of MRP1 in the brain distribution of a model organic anion, fluorescein. The substrate specificity of MRP1 for fluorescein was initially determined by examining the accumulation of fluorescein in MDCKII MRP1-transfected cells. The distribution of fluorescein in the brain was then examined in wild-type and mrp1 gene knockout mice. The results show that in MDCKII MRP1-transfected cells, the accumulation of fluorescein was significantly lower (about 40% lower) than that in wild-type MDCKII cells. MRP1 inhibitors such as probenecid, MK-571, and LY402913 enhanced fluorescein accumulation in MDCKII MRP1-transfected cells to a greater extent than in wild-type MDCKII cells. In an in vivo study, after intravenous injection of fluorescein, the fluorescein brain-to-plasma concentration ratio in mrp1 knockout mice was not significantly different than that in wild-type mice. However, when probenecid was co-administered with fluorescein in wild-type mice, the fluorescein brain-to-plasma ratio was significantly increased (1.5-fold). These findings suggest that fluorescein is a substrate for MRP1. Furthermore, the in vivo study also suggests that MRP1 has a limited role in the transport and distribution of fluorescein in the brain. Therefore, other organic anion transport proteins, including the various isoforms of the MRP family, may be responsible for the accumulation and transport of organic anions in the brain., (Copyright 2001 Academic Press.)

Two major classes of plasma membrane proteins that actively extrude a wide range of structurally diverse hydrophobic amphipathic antineoplastic agents from cells, with different mechanisms of action, lead to multidrug resistance. To study the importance of these ATP-binding cassette transporters to the toxicity of cancer chemotherapy agents, we have used mice genetically deficient in both the mdr1a and mdr1b genes [mdr1a/1b(-/-) mice], the mrp1 gene [mrp1(-/-) mice], and the combined genes mdr1a/1b and mrp1 [mdr1a/1b(-/-), mrp1(-/-) mice] and embryonic fibroblasts derived from wild-type mice and from the three gene knockout animals. The consequences of export pump deficiencies were evaluated primarily using vincristine and etoposide. Mice deficient in the three genes, mdr1a/1b and mrp1, exhibited a 128-fold increase in toxicity to vincristine and a 3-5-fold increase in toxicity to etoposide; increased toxicity to embryonic fibroblast cells from triple knockout mice also occurred with vincristine and etoposide. Vincristine, which normally does not express toxicity to the bone marrow and to the gastrointestinal mucosa when used at therapeutic doses, caused extensive damage to these tissues in mdr1a/1b(-/-), mrp1(-/-) mice. The findings indicate that the P-glycoprotein and mrpl are compensatory transporters for vincristine and etoposide in the bone marrow and the gastrointestinal mucosa and emphasize the potential for increased toxicities by the combined inhibition of these efflux pumps.

Adaptive immune responses begin after antigen-bearing dendritic cells (DCs) traffic from peripheral tissues to lymph nodes. Here, we show that DC migration from skin to lymph nodes utilizes the leukotriene C(4) (LTC(4)) transporter multidrug resistance-associated protein 1 (MRP1). DC mobilization from the epidermis and trafficking into lymphatic vessels was greatly reduced in MRP1(-/-) mice, but migration was restored by exogenous cysteinyl leukotrienes LTC(4) or LTD(4). In vitro, these cysteinyl leukotrienes promoted optimal chemotaxis to the chemokine CCL19, but not to other related chemokines. Antagonism of CCL19 in vivo prevented DC migration out of the epidermis. Thus, MRP-1 regulates DC migration to lymph nodes, apparently by transporting LTC(4), which in turn promotes chemotaxis to CCL19 and mobilization of DCs from the epidermis.

The mrp (multidrug resistance protein) gene has been associated with the multidrug resistance of cancer cells in vitro and in vivo. To gain information on its physiological role, embryonic stem cells were used to generate mice homozygous for a disruption of the mrp gene, resulting in complete abrogation of mrp expression. No physiological abnormalities were observed, at least up to 4 months of age. Viability, fertility, and a range of histological, hematological, and serum-chemical parameters were similar in mrp(+/+) and mrp(-/-) mice. mrp(-/-) mice displayed an increased sensitivity to etoposide phosphate (2-fold) accompanied by greater bone marrow toxicity, whereas the acute toxicity of sodium arsenite was equivalent in mrp(+/+) and mrp(-/-) mice. Tissue levels of glutathione (GSH) were elevated in breast, lung, heart, kidney, muscle, colon, testes, bone marrow cells, blood mononuclear leukocytes, and blood erythrocytes of mrp(-/-) mice and were unchanged in organs known to express little if any mrp, such as the liver and small intestine. The increase in GSH was not due to an increase in the activity of gamma-glutamylcysteine synthetase, the rate-limiting enzyme for GSH synthesis. The findings demonstrate that mrp is dispensable for development and growth but exerts a role in drug detoxification and GSH metabolism.