Abstract ATP-binding Cassette Transporters Research Articles

P-Glycoprotein4 Displays Auxin Efflux Transporter–Like Action in Arabidopsis Root Hair Cells and Tobacco Cells

ATP binding cassette (ABC) transporters transport diverse substrates across membranes in various organisms. However, plant ABC transporters have only been scantily characterized. By taking advantage of the auxin-sensitive Arabidopsis thaliana root hair cell and tobacco (Nicotiana tabacum) suspension cell systems, we show here that Arabidopsis P-glycoprotein4 (PGP4) displays auxin efflux activity in plant cells. Root hair cell-specific overexpression of PGP4 (PGP4ox) and known auxin efflux transporters, such as PGP1, PGP19, and PIN-FORMEDs, decreased root hair elongation, whereas overexpression of the influx transporter AUXIN-RESISTANT1 enhanced root hair length. PGP4ox-mediated root hair shortening was rescued by the application of auxin or an auxin efflux inhibitor. These results indicate that the increased auxin efflux activity conferred by PGP4 reduces auxin levels in the root hair cell and consequently inhibits root hair elongation. PGP4ox in tobacco suspension cells also increased auxin efflux. PGP4 proteins were targeted to the plasma membrane of Arabidopsis root hair cells and tobacco cells without any clear subcellular polarity. Brefeldin A partially interfered with the trafficking of PGP4 reversibly, and this was rescued by pretreatment with auxin. These results suggest that PGP4 is an auxin efflux transporter in plants and that its trafficking to the plasma membrane involves both BFA-sensitive and -insensitive pathways.

Identification of the tree shrew ATP-binding cassette transporter A1 (ABCA1) and its expression in tissues

ATP-binding cassette transporter A1 (ABCA1) modulates plasma levels of high density lipoprotein (HDL), a cardiovascular protecting factor. Tree shrew was considered to be an animal protected from atherosclerosis characterized by high proportion of HDL in plasma. The cDNA clones and expression of tree shrew ABCA1 was identified using SMART-RACE and Real-Time PCR techniques respectively. The nucleotide sequence of tree shrew ABCA1 covered 7,762 bp, including a 6,786 bp coding region which encoded a 2,261 amino acids protein with the high identity to human ABCA1 (95%). Tree shrew ABCA1 was expressed in various tissues, the highest in lung, followed by liver, kidney, spleen and cardiac muscle in turn from high to medium expression levels. This pattern was partially different from that of human ABCA1 which was low in kidney and cardiac muscle. This work could shed new light on its role of ABCA1 in the distinctive HDL metabolism in tree shrew.

Targeted degradation of ABC transporters in health and disease

ATP binding cassette (ABC) transporters comprise an extended protein family involved in the transport of a broad spectrum of solutes across membranes. They consist of a common architecture including two ATP-binding domains converting chemical energy into conformational changes and two transmembrane domains facilitating transport via alternating access. This review focuses on the biogenesis, and more precisely, on the degradation of mammalian ABC transporters in the endoplasmic reticulum (ER). We enlighten the ER-associated degradation pathway in the context of misfolded, misassembled or tightly regulated ABC transporters with a closer view on the cystic fibrosis transmembrane conductance regulator (CFTR) and the transporter associated with antigen processing (TAP), which plays an essential role in the adaptive immunity. Three rather different scenarios affecting the stability and degradation of ABC transporters are discussed: (1) misfolded domains caused by a lack of proper intra- and intermolecular contacts within the ABC transporters, (2) deficient assembly with auxiliary factors, and (3) arrest and accumulation of an intermediate or 'dead-end' state in the transport cycle, which is prone to be recognized by the ER-associated degradation machinery.

Abstract 827: Total Body ABCG1-Deficiency Increases Atherosclerotic Lesion Development In LDL-Receptor Knockout Mice

ATP-binding cassette transporter G1 (ABCG1) facilitates the efflux of cholesterol from macrophages to HDL. As ABCG1 plays a role in preventing cellular lipid accumulation, macrophage ABCG1 expression has been suggested to protect against atherosclerosis. In bone marrow transplantation studies, however, we and others obtained different effects of specific macrophage ABCG1-deficiency on atherosclerotic lesion development under different experimental settings. The objective of this study was to further define the effect of ABCG1-deficiency on lipid metabolism and atherosclerotic lesion development. ABCG1 knockout (KO) mice upon atherogenic diet feeding develop only modest atherosclerotic lesions, therefore, we recently cross-bred ABCG1 KO mice with LDL-receptor KO (LDLr KO) mice. LDLr KO and LDLr/ABCG1 double KO (dKO) littermates were fed a high-fat, high-cholesterol Western-type diet (WTD) containing 0.25% cholesterol and 15% fat for 10 weeks to induce atherosclerotic lesion formation. Both on chow diet and on WTD, serum lipid levels and lipoprotein profiles showed no significant differences between the LDLr KO and LDLr/ABCG1 dKO mice. However, after 10 weeks of diet feeding, a significant 1.5-fold increase in atherosclerotic lesion size was observed in the aortic root of LDLr/ABCG1 dKO mice (130 ± 12x10 3 μm 2 [n = 8] compared to 88 ± 11x10 3 μm 2 [n=7] for LDLr KO mice; p =0.026). In addition, macrophage-rich areas of the lungs and spleens of LDLr/ABCG1 dKO mice exhibited excessive lipid accumulation compared with LDLr KO mice. Furthermore, in vitro studies using bone marrow derived macrophages of LDLr KO and LDLr/ABCG1 dKO mice showed that disruption of ABCG1 results in a 15% decrease in cholesterol efflux to HDL ( p< 0.001), whereas the cholesterol efflux to ApoAI was unaffected. These studies reveal that total-body ABCG1 deficiency leads to increased atherosclerotic lesion development and massive lipid accumulation in macrophages of the lung and spleen, possibly due to impaired efflux of cholesterol from macrophages deficient in ABCG1. In conclusion, the expression of ABCG1 protects against atherosclerotic lesion development.

R219K Polymorphism of ATP Binding Cassette Transporter A1 Related with Low HDL in Overweight/Obese Thai Males

ATP binding cassette transporter A1 (ABCA1) plays a role in the initial stage of removing cholesterol from the body via cholesterol efflux. Mutations of this gene cause wide-ranging HDL deficiency, as evident in Tangier disease and familial hypoalphalipoproteinemia. The aim of this study was to elucidate whether the presence of ABCA1 gene polymorphism could be a risk factor for overweight/obesity. The presence of R219K and I883M genetic variant was determined by PCR-RFLP analysis in 112 overweight/obese and 117 control subjects of both sexes. Statistical analysis was performed to find an association between polymorphism and lipid data. Overweight/obese men carrying the mutant allele of R219K had lower level of HDL than the control (p = 0.006). However, no positive association was observed using bivariate logistic regression analysis. On the contrary, there was no difference in HDL level among genotypes in I883M polymorphism. Both polymorphisms appeared to be common in Thai ethnic groups. No difference was detected in genotype frequency between the two populations for both polymorphisms. Although the lower level of HDL in overweight/obese men carrying R219K in comparison to the control suggests the possible involvement of this gene with obesity, further investigations are needed to prove the influence of ABCA1 gene polymorphism on HDL level and to determine whether it could be a genetic determinant of obesity.

Osmotic Upshift Transiently Inhibits Uptake via ABC Transporters in Gram-Negative Bacteria

ATP-binding cassette transporters from several rhizobia and Salmonella enterica serovar Typhimurium, but not secondarily coupled systems, were inhibited by high concentrations (100 to 500 mM) of various osmolytes, an effect reversed by the removal of the osmolyte. ABC systems were also inactivated in isolated pea bacteroids, probably due to the obligatory use of high-osmolarity isolation media. Measurement of nutrient cycling in isolated pea bacteroids is impeded by this effect.

Molecular diagnosis of ATP-binding cassette transporter-related diseases

ATP-binding cassette (ABC) transporters are involved in a variety of physiologic processes such as xenobiotic defense, lipid metabolism, ion homeostasis and immune functions. A large number of ABC proteins have been causatively linked to rare and common human genetic diseases including familial high-density lipoprotein deficiency, retinopathies, cystic fibrosis, diabetes and cardiomyopathies. Furthermore, genetic variations in ABC transporter genes and dysregulated expression patterns of these molecules significantly contribute to drug resistance in human cancer cells and alter the pharmacokinetic properties of a variety of drugs. In order to analyze DNA sequence alterations or define disease-associated mRNA expression patterns of the complete ABC transporter superfamily, novel high-throughput molecular methods such as quantitative real-time PCR and DNA microarray analysis are emerging. The aim of this review is to provide an overview and to present some examples of human ABC transporters involved in monogenic diseases, cancer and pharmacogenetics. Methodologic aspects of molecular diagnostics applied to analyze genetic variations, mRNA and protein expression levels and functional characteristics of ABC transporters are discussed.

The ABC Transporters MDR1 and MRP2: Multiple Functions in Disposition of Xenobiotics and Drug Resistance

ATP‐binding cassette (ABC) transporters comprise one of the largest membrane bound protein families. They are involved in transport of numerous compounds. These proteins transport substrates against a concentration gradient with ATP hydrolysis as a driving force across the membrane. Mammalian ABC proteins have important physiological, pharmacological and toxicological functions including the transport of lipids, bile salts, drugs, toxic and environmental agents. The efflux pumps serve both as natural defense mechanisms and influence the bioavailability and disposition of drugs. In general terms, the transporters remove xenobiotics from the cellular environment. For example, in cancer cells, over expression of these molecules may confer to multidrug resistance against cytostatic drugs. In addition, based on diverse structural characteristics and a broad substrate specifity, ABC transport proteins alter the intracellular concentration of a variety of therapeutically used compounds and toxicologically relevant agents. We review the function of the human multidrug resistance protein MDR1, (P‐glycoprotein, ABCB1) and the multidrug resistance protein MRP2 (ABCC2). We focus on four topics namely 1) structure and physiological functions of these transporters, 2) substrates e.g., drugs, xenotoxins, and environmental toxicants including their conjugates, 3) drug–drug interactions, and the role of chemosensitizers which may be able to reverse drug resistance, and 4) pharmacologically and toxicologically relevant genetic polymorphisms in transport proteins and their clinical implications.

ATP binding cassette transporter A1--key roles in cellular lipid transport and atherosclerosis.

ATP-binding cassette transporter A1 (ABCA1) was recently recognized as the mutant molecule responsible for Tangier disease with low HDL levels, accumulation of cholesteryl esters in tissues, and increased risk of cardiovascular disease. Extensive studies for the past 2 years have recognized the critical role of ABCA1 in cholesterol and phospholipid trafficking. Since the removal of cholesterol from tissues is a key step in the prevention of atherosclerosis, significant attention has been focused on this molecule. Natural ABCA1 mutations in Tangier disease (TD) patients and WHAM chickens together with induced mutation in ABCA1 knock-out mice unequivocally established the important role of ABCA1 in maintaining circulating HDL levels and promoting cholesterol efflux from the arterial wall. Mice lacking ABCA1 showed similar phenotypes observed in Tangier disease patients with low levels of HDL. Further understanding of the roles of ABCA1 in lipid transport and atherosclerosis became clear from studies with ABCA1 transgenic mice. These mice showed enhanced cholesterol efflux from macrophages and reduced atherosclerotic lesion formation. The promoter of the ABCA1 gene has been mapped to a large extent, with the exception of cAMP response element. The present review summarizes recent developments on the role of ABCA1 in cholesterol efflux and prevention of atherosclerosis. Given the antiatherogenic properties of ABCA1, this molecule can serve as an appropriate target for developing drugs to treat individuals with low levels of HDL.

ABCG subfamily of human ATP-binding cassette proteins

Abstract ATP-binding cassette (ABC) proteins form one of the largest known protein families and have been found in all known organisms. Most members of the human ABC protein family are membrane-spanning transporters that use energy derived from the hydrolysis of ATP to transport specific substrates across cell membranes. Mutations in certain human ABC transporters of the subfamilies A, B, C, and D have been shown to cause a wide variety of inherited diseases such as the lung condition cystic fibrosis, the nervous degenerative condition adrenoleukodystrophy (of Lorenzo’s Oil fame), hereditary macular degeneration of the eye (Stargardt’s disease), and inherited deficiency of circulating high-density lipoproteins (Tangier disease or familial hypoalphalipoproteinemia). Very recent studies showed that mutations in two members of the subfamily G of human ABC transporters (ABCG5 and ABCG8) cause a condition called sitosterolemia in which plant sterols accumulate in the body and may be responsible for influencing total body sterol homeostasis. In addition, other members of the subfamily G, namely ABCG1 and ABCG4, have also been shown to be involved in cellular lipid trafficking and are thought to play important roles during foam cell formation of human macrophages. By contrast, ABCG2 is a multidrug resistance transporter. In this review, we focus on the current knowledge and physiological background of the members of the subfamily G. We also present new insights on the evolutionary relationship of human and nonhuman ABCG proteins.

Hormonal control of a gene encoding a putative PDR5-like ABC transporter in periwinkle

ATP binding cassette (ABC) transporters retain increasing attention since their implication in multidrug resistance has been demonstrated in several prokaryotic and eukaryotic cells. Many ABC proteins are known in various organisms but only few in higher plants. We report here on the characterization of a partial 955 bp clone (Cr-ABC1) isolated from a Catharanthus roseus cDNA library, which shows high homology (65% identity) with the TUR2 cDNA previously isolated from the water lens Spirodela polyrrhiza. Accumulation of Cr-ABC1 transcripts in cultured C. roseus cells was enhanced by cytokinin or methyl jasmonate addition to, or auxin suppression from the culture medium. In whole plants, the gene was mainly expressed in flowers.