Aliases: ABC15, ABCP, AI428558, Bcrp1, MXR, MXR1
Gene name: ATP-binding cassette, sub-family G, member 2 (Abcg2)
Like their human counterpart, rodent orthologues of BCRP are ATP-binding cassette half-transporters with a broad range of endogenous and xenobiotic substrates. The rodent Bcrp gene is located on chromosome 6, at a distance of 2829cM from the centromere. The mouse Bcrp cDNA encodes a 657-amino-acid protein with 81% identity and 86% similarity to human BCRP. Homology is particularly high in the ATP-binding region, and the hydrophobic regions of mouse Bcrp and human BCRP, comprising all six transmembrane domains, are almost identical .
The tissue localization of Bcrp in mice and rats is similar to that in humans: rodent Bcrp is widely expressed on the apical plasma membrane of several tissues such as the liver, kidney, and intestinal tract, as well as at barriers such as the placenta, testes, and brain capillary endothelial cells . Mouse Bcrp expression was highest in the kidney, followed by the liver, ileum, and testes. Inter-species differences in the expression levels of BCRP between rodents and humans have been observed in a number of studies. In the rodent gastrointestinal tract, Bcrp mRNA levels are generally higher in rats than in mice . Li et. al. performed absolute quantification of BCRP/Bcrp protein in the liver of different species by LC-MS/MS, and established the rank order as dog > rat > monkey ≈ human . Slightly discordant results were obtained in a more recent study, where four efflux drug transporters were quantified in the liver and kidney across the same species using targeted quantitative proteomics by isotope dilution nanoLC-MS/MS. In membrane fractions from the liver, the rank order of BCRP/Bcrp levels was dog > monkey > human ≈ rat. In kidney samples, rat Bcrp levels were substantially higher than in human, monkey and dog . BCRP levels at the blood-brain barrier, measured by quantitative targeted absolute proteomics, were reported to be essentially similar between murine and human, albeit slightly (1.85-fold) higher in human . Strong expression of BCRP/Bcrp protein was shown in the brain microvessels of human, mouse, and rat, with immunofluorescence staining of frozen sections revealing slightly higher levels in mouse and rat than in human .
Gender-specific differences in BCRP/Bcrp expression have been established in both man and rodents, although these findings have not been uniformly confirmed by all studies . Rat Bcrp mRNA levels were higher in the male kidney, and BCRP/Bcrp protein levels were significantly higher in the male versus female liver in both mice and humans [3, 9].
Bcrp-null mice  and rats  have been developed and widely used to investigate the impact of Bcrp on the pharmacokinetics and tissue distribution of different substrates. Animal models using Bcrp-null mice have demonstrated the significance of Bcrp in limiting intestinal absorption of many Bcrp substrates. Topotecan, a chemotherapeutic agent, induced BCRP/Bcrp overexpression in selected cell lines, thereby reducing drug bioavailability. Higher topotecan exposure in Bcrp-/- mice after oral administration was reported . Jonker et. al. observed diet-dependent photosensitization (lesions on the skin) in Bcrp1 -/- mice. These mice were extremely sensitive to the phototoxin pheophorbide a, which is a porphyrin catabolite of chlorophyll . Based on this result, humans with mutant BCRP may be at risk of diet-dependent phototoxicity and protoporphyria. Sulfasalazine, an anti-inflammatory drug and BCRP/Bcrp substrate, had 111 times higher plasma concentration in Bcrp1 knockout mice after oral administration, as compared with wild-type mice . Sulfasalazine oral bioavailability was markedly increased (1,4% versus 30%) in Bcrp knockout rats, which was in good agreement with the 4% to 37% increase reported in mice . Human and rodent BCRP/Bcrp also localize to the canalicular membrane of hepatocytes, where they are involved in biliary excretion. In triple knockout Mrp2; Mrp3; Bcrp1-/- mice, 67% of intravenously dosed methotrexate was present in the liver after one hour of administration. In wild-type mice only 7% of the dose remained . This suggests concerted roles of Mrp2, Mrp3, and Bcrp in the biliary excretion of metabolites.
BCRP/Bcrp in the placenta can limit the penetration of drugs from the maternal plasma into the fetus. In the human placenta, multidrug resistance protein 1 (MDR1)/ABCB1 and BCRP are highly expressed in the apical microvillous membrane of the syncytiotrophoblastic monolayer. In rodents the syncytiotrophoblasts, as opposed to humans, form a bilayer, and the cognate transporters are expressed on the apical membrane of placental syncytiotrophoblastic layer II . Fetal topotecan concentration to maternal plasma concentration was 2-fold higher in Bcrp-/- compared with wild type mouse fetuses, indicating that Bcrp in the placenta can limit the penetration of substrate drugs .
Bakhsheshian et. al. compared mouse and human BCRP function, using BCRP-expressing, mitoxantrone-selected mouse and human cell sublines. They found that the fluorescent substrates examined were transported similarly by human and mouse BCRP , which would substantiate the use of mouse models in evaluating the clinical roles of BCRP. A few studies, however, reported inter-species differences in substrate specificity or interaction with inhibitors. For example, bisphenol A (BPA), used in the manufacture of polycarbonate plastics and epoxy resins, interacts with human BCRP as a potential substrate, but not with rat Bcrp. Its glucuronide conjugate, BPA-glucuronide (BPA-G), appeared to be a non-substrate for either rat or human BCRP . Another study revealed that human BCRP is more sensitive to inhibition by the mycotoxin fumitremorgin C than mouse Bcrp, as the human transporter has significantly lower IC50 values than mouse Bcrp .
Species-specific differences associated with some BCRP substrates may limit the use of knockout models in predicting the role of BCRP in humans. In a recent study, however, the generation and characterization of a humanized BCRP mouse model has been announced. In the new strain, the human BCRP transporter is expressed under the control of the murine Bcrp promoter . This hBCRP mouse model may provide a novel tool to study the in vivo role of human BCRP.
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