Apolipoprotein A-I directly interacts with extracellular domain 1 of human ABCA1.

ATP-binding cassette transporter A1 (ABCA1) is critical for the generation of nascent high-density lipoprotein (HDL) and plays important roles in cholesterol homeostasis. ABCA1 has two large extracellular domains (ECDs), which may interact directly with apolipoprotein A-I (apoA-I). However, the molecular mechanisms underlying HDL formation and the importance of ABCA1-apoA-I interactions in HDL formation remain unclear. We investigated the ABCA1-apoA-I interaction in photo-activated crosslinking experiments using sulfo-SBED-labeled apoA-I. ApoA-I bound to cells expressing ABCA1, but not to untransfected cells or cells expressing non-functional ABCA1. Binding was inhibited by sulfo-SBED-labeled apoA-I, and crosslinking of sulfo-SBED-labeled apoA-I with ABCA1 was inhibited by non-labeled apoA-I, suggesting that sulfo-SBED-labeled apoA-I specifically binds and crosslinks with functional ABCA1. Proteolytic digestion of crosslinked ABCA1 revealed that apoA-I bound the N-terminal half of ABCA1, and that the first ECD of ABCA1 is an apoA-I binding site. Abbreviations: ABC: ATP-binding cassette; apoA-I: apolipoprotein A-I; ATP: adenosine triphosphate; CHAPS: 3-(3-cholamidepropyl)dimethylammonio-1- propanesulphonate; DTT: dithiothreitol; ECD: extra cellular domain; EDTA: ethylenediaminetetraacetic acid; GFP: green fluorescent protein; HA: hemagglutinin; HDL: high density lipoprotein; HEK: human embryonic kidney; HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; sulfo-SBED: (sulfosuccinimidyl-2-[6-(biotinamido)-2-(p-azidobenzamido)hexanoamido] ethyl-1,3'-dithiopropionate; NHS-ester, N-hydroxysuccinimide-ester.

Upregulation of cellular glutathione levels in human ABCB5- and murine Abcb5-transfected cells.

BACKGROUND: Previously, we have demonstrated that human ABCB5 is a full-sized ATP-binding cassette transporter that shares strong homology with ABCB1/P-glycoprotein. ABCB5-transfected cells showed resistance to taxanes and anthracyclines. Herein, we further screened ABCB5 substrates, and explored the mechanism of resistance. METHODS: Sensitivity of the cells to test compounds was evaluated using cell growth inhibition assay. Cellular levels of buthionine sulfoximine (BSO), glutathione and amino acids were measured using HPLC and an enzyme-based assay. Cellular and vesicular transport of glutathione was evaluated by a radiolabeled substrate. Expression levels of glutathione-metabolizing enzymes were assessed by RT-PCR. RESULTS: Human ABCB5-transfected 293/B5-11 cells and murine Abcb5-transfected 293/mb5-8 cells showed 6.5- and 14-fold higher resistance to BSO than the mock-transfected 293/mock cells, respectively. BSO is an inhibitor of gamma-glutamylcysteine ligase (GCL), which is a key enzyme of glutathione synthesis. 293/B5-11 and 293/mb5-8 cells also showed resistance to methionine sulfoximine, another GCL inhibitor. A cellular uptake experiment revealed that BSO accumulation in 293/B5-11 and 293/mb5-8 cells was similar to that in 293/mock cells, suggesting that BSO is not an ABCB5 substrate. The cellular glutathione content in 293/B5-11 and 293/mb5-8 cells was significantly higher than that in 293/mock cells. Evaluation of the BSO effect on the cellular glutathione content showed that compared with 293/mock cells the BSO concentration required for a 50 % reduction in glutathione content in 293/B5-11 and 293/mb5-8 cells was approximately 2- to 3-fold higher. This result suggests that the BSO resistance of the ABCB5- and Abcb5-transfected cells can be attributed to the reduced effect of BSO on the transfectants. Cellular and vesicular transport assays showed that the transport of radiolabeled glutathione in 293/B5-11 cells was similar to that in 293/mock cells. The mRNA expression of genes encoding glutathione-metabolizing enzymes in 293/B5-11 cells was similar to that in 293/mock cells. The cellular content of Glu, a precursor of glutathione, in 293/B5-11 and 293/mb5-8 cells was higher than that in 293/mock cells. CONCLUSIONS: ABCB5/Abcb5-transfected cells showed resistance to BSO, which is not a substrate of ABCB5. Our results suggest that ABCB5/Abcb5 upregulates cellular glutathione levels to protect cells from various poisons.

Expression of human ABCB5 confers resistance to taxanes and anthracyclines.

Human ABCB5, an ATP-binding cassette (ABC) transporter gene, has two major mRNA species. One transcript encodes an 812 amino acid polypeptide, ABCB5ß, with a transmembrane domain and a nucleotide-binding domain. We isolated the cDNA of another ABCB5 mRNA that encodes a 1257 amino acid polypeptide. The translated ABCB5 protein is a full-sized ABC transporter that has an internally duplicated structure with two transmembrane domains and two nucleotide-binding domains. The 5' and 3' parts of the ABCB5 mRNA were expressed in the prostate and testis. HEK293 cells transfected with the ABCB5 cDNA expressed a 150-160kDa protein. The ABCB5 transfectants showed approximately 1.5-fold higher resistance to doxorubicin, and 2- to 3-fold higher resistance to paclitaxel and docetaxel. Cellular uptake of radiolabeled paclitaxel and docetaxel in the transfectants was lower than that in the parental HEK293 cells. Treatment of the transfectants with ABCB5-targeted siRNA lowered their resistance to docetaxel. Revertant cells that express a reduced amount of ABCB5 also showed a lowered level of docetaxel resistance. These results indicated that the expression of ABCB5 conferred resistance to taxanes and anthracyclines. Membrane vesicles prepared from ABCB5 baculovirus-infected Sf21 cells showed higher vanadate-sensitive ATPase activity than the Sf21 control vesicles. The k(m) and V(max) values of ATPase activity in the ABCB5 vesicles were 1.8mM and 65nmol/min/mg protein, respectively. ABCB5 ATPase activity was 1.25-fold higher in the presence of 100µM docetaxel than it was in the absence of docetaxel. These results indicates that the full-length ABCB5 protein has ATPase activity that is sensitive to docetaxel.