Expression and subcellular localization of efflux transporter ABCG2/BCRP in important tissue barriers of lactating dairy cows, sheep and goats

Expression of efflux transporter ABCG2/BCRP in tissues barriers has shown to be associated with altered pharmaco- and toxicokinetics of xenobiotics. Until now, little is known about the functional expression of this transporter in dairy animals. We therefore systematically examined the expression and subcellular localization of ABCG2/BCRP in small intestine, colon, lung, liver, kidney and mammary gland in lactating cows, sheep and goats. Carrier expression was investigated by RT-PCR and Western blot analysis showing highest expression of ABCG2/BCRP in small intestine and mammary gland, high levels in liver and moderate amounts of protein in lung, colon and kidney. Regarding subcellular localization, BCRP was predominantly found at the apical plasma membrane of small intestine, colon, bronchial epithelium, bile ducts and overall in endothelial structures in all tested species. In the mammary gland, there was strong apical staining of the alveolar epithelial cells and most of the ducts in all dairy ruminants. We also detected significantly elevated protein expression in lactating mammary gland compared with nonlactating cows, sheep and goats. Our results contribute to the role of BCRP in cytoprotection and disposition in important tissue barriers and may have important implications for veterinary pharmacotherapy of dairy animals using drugs identified as BCRP substrates.