Studies on the Interactions between Breast Cancer Resistance Protein (BCRP/ABCG2) and Dietary Phytochemicals: Implications for Phytochemical Bioavailability

Abstract

Breast cancer resistance protein (BCRP/ABCG2) is a member of the ATP-binding cassette (ABC) transporter superfamily. Ubiquitously expressed in the apical membrane of human tissues, ABCG2 is recognised as an efflux transporter that regulates the absorption and disposition of various therapeutic agents, and confers multidrug resistance (MDR) in cancer chemotherapy. In addition to therapeutic agents, ABCG2 is known to interact with phytochemicals, which are plant secondary metabolites that encompass a broad range of small molecules widely viewed as the active components that confer, or in part contribute to, the health-promoting and disease-preventive effects associated with the consumption of fruits and vegetables. The goals of this thesis were to determine if selected phytochemicals are inhibitors and substrates of ABCG2, and if a co-administered phytochemical inhibitor could improve the uptake of a phytochemical substrate by modulating ABCG2-mediated efflux. The ABCG2-inhibition screening program employing the mitoxantrone accumulation and membrane vesicular transport assays led to the identification of 28 novel ABCG2 inhibitors. An ABCG2- inhibitory effect was also detected in seven plant extracts and fruit juices. Activity-guided fractionation of a hop extract also indicated hop-derived α- and β-acids as potential ABCG2 inhibitors. From the identified inhibitors, selected phytochemicals were tested in the ATPase assay and a substrate-type relationship was indicated for the prenylflavonoids and ellagic acid, but not the polyacetylenes. Subsequently, the prenylflavonoids were investigated in the bidirectional transport assay to determine if they are subjected to direct efflux by ABCG2. Strong evidence of a direct transport is shown for isoxanthohumol. For 8-prenylnaringenin and 8-prenylnaringenin sulphate, there is considerable evidence that at least one of them is a substrate of ABCG2. In addition, 8-prenylnaringenin and a hop extract were able to improve the absorptive transport of resveratrol in the assay, providing a proof of principle for the phytochemical co-administration strategy in targeting ABCG2. These results may have a significant impact in several research areas. Firstly, the discovery of novel ABCG2 inhibitors broadens the pool of candidates that may be investigated as ‘next-generation’ MDR reversal agents for use in cancer chemotherapy. Secondly, evidence is provided for the potential of dietary phytochemicals to interfere with the transport and consequently pharmacokinetics of concurrent food- or drug-derived ABCG2 substrates, which may result in undesirable food/herb-drug interactions. Thirdly, the demonstration that the cellular absorption and secretion of a phytochemical ABCG2 substrate can be modulated by manipulating ABCG2 transport provides a framework for further investigations into enhancing phytochemical in vivo bioavailability by targeting ABCG2-mediated efflux.