Aliases: ABC16, Lith1, PFIC2, PGY4, SPGP
Gene name: TP-binding cassette, sub-family B (MDR/TAP), member 11 (ABCB11)
Bile salt export pump (BSEP/Bsep) is one of the major transporters of interest in drug development due to its relevance to hepatotoxicity. Rodents, dogs and monkeys are species often used in metabolic, pharmacokinetic and toxicity studies. In order to interpret data generated in these species, it is necessary to understand the functional characteristics of the animal orthologs of human BSEP.
BSEP/Bsep is highly conserved in vertebrates. Rat, mouse and dog orthologs of human BSEP are also close relatives, sharing 82, 80, and 89.4% similarity, respectively, with the human protein [1-3].
Preclinical animals are often less sensitive than humans to bile acid-mediated drug-induced liver injury. Only about 50% of incidences of clinical hepatotoxicity can be successfully predicted using rodent models; thus, results obtained in these models are not readily translatable to the human clinical setting. Potential reasons include species differences in bile acid composition, in the metabolism and disposition of drugs, as well as in the substrate and/or inhibitor specificities of bile salt transporters. It was shown previously that although the affinities for the monovalent bile acid conjugates are very similar across species, the preferred substrate of human BSEP is taurocholate, whereas it is taurochenodeoxycholate for the rodent ortholog . Differences among species in Bsep activity and expression levels have also been demonstrated in liver tissue as well as fresh and cryopreserved hepatocytes [5-7].
An important difference between these species is their sensitivity to impairment of Bsep transport function . Loss-of-function mutations of BSEP in humans lead to progressive intrahepatic cholestasis that is fatal unless liver transplantation is performed. In contrast, Bsep-null mice develop only mild cholestasis, probably because overexpression of both P-glycoprotein genes compensates for lost Bsep function . Even if cholestasis is induced by bile duct ligation or chemical treatment, Bsep-null mice successfully evade hepatic injury by upregulating bile acid detoxification / hydroxylation pathways . A recent study on Bsep knockdown rats gave insight into the diverse effects of different Bsep inhibitors on bile salt homeostasis .
In vitro assays using hepatocytes, sandwich cultured hepatocytes, or membrane vesicles expressing the Bsep protein showed marked inter-species differences in the inhibitory potencies of selected drugs. The conclusion of these studies was that drugs are less able to inhibit mouse Bsep than the corresponding orthologs from human or other species [12, 13]. In some cases, inter-species differences in toxicity may be explained by the species-dependent effects of drugs on transporter and/or metabolic enzyme expression .
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