Genetic variability and evolutionary diversification of membrane ABC transporters in plants

BACKGROUND

ATP-binding cassette proteins have been recognized as playing a crucial role in the regulation of growth and resistance processes in all kingdoms of life. They have been deeply studied in vertebrates because of their role in drug resistance, but much less is known about ABC superfamily functions in plants.

RESULTS

Recently released plant genome sequences allowed us to identify 803 ABC transporters in four vascular plants (Oryza. sativa, Solanum lycopersicum, Solanum tuberosum and Vitis vinifera) and 76 transporters in the green alga Volvox carteri, by comparing them with those reannotated in Arabidopsis thaliana and the yeast Saccharomyces cerevisiae. Retrieved proteins have been phylogenetically analysed to infer orthologous relationships. Most orthologous relationships in the A, D, E and F subfamilies were found, and interesting expansions within the ABCG subfamily were observed and discussed. A high level of purifying selection is acting in the five ABC subfamilies A, B, C, D and E. However, evolutionary rates of recent duplicate genes could influence vascular plant genome diversification. The transcription profiles of ABC genes within tomato organs revealed a broad functional role for some transporters and a more specific activity for others, suggesting the presence of key ABC regulators in tomato.

CONCLUSIONS

The findings achieved in this work could contribute to address several biological questions concerning the evolution of the relationship between genomes of different species. Plant ABC protein inventories obtained could be a valuable tool both for basic and applied studies. Indeed, interpolation of the putative role of gene functions can accelerate the discovering of new ABC superfamily members.

Simvastatin-induced compartmentalisation of doxorubicin sharpens up nuclear topoisomerase II inhibition in human rhabdomyosarcoma cells

Tumours, which are initially sensitive to cytotoxic agents, often develop resistance to a broad spectrum of structurally unrelated drugs. The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors have been shown to inhibit ATP-binding cassette (ABC) transporters but have also impact on glycosylation of such proteins. Doxorubicin is a substrate for ABC transporters like P-glycoprotein (ABCB1) which is present in human RD rhabdomyosarcoma cells. It was therefore the aim of this study to identify the compartmentalisation and action of doxorubicin in simvastatin-treated RD cells. Due to autofluorescence of doxorubicin, intracellular distribution was monitored by confocal microscopy. The biological effects were traced on the level of colony formation, caspase activation and DNA injury. Here we show that simvastatin treatment leads to ABCB1 inhibition and down-regulation of the transporter. Consequently, these cells accumulate significant amounts of doxorubicin, predominantly in the nucleus and lysosomes. While clearance of the anthracycline into lysosomes is not altered by simvastatin treatment, it significantly enhanced nuclear accumulation in a HMG-CoA reductase-independent manner. Thus, in such treated cells, topoisomerase II activity is significantly inhibited, which is further corroborated by augmented double-strand DNA breaks. Moreover, colony formation was synergistically inhibited by the combination of simvastatin and doxorubicin. Given the fact that ABCB1 expression correlates with an adverse prognosis in many tumours, adjuvant chemotherapy including statins might represent a novel therapeutic concept to overcome ABCB1-mediated multidrug resistance by direct inhibition and down-regulation.

Pharmacogenetics of drug metabolizing enzymes and transporters: effects on pharmacokinetics and pharmacodynamics of anticancer agents

There is wide interpatient variability in drug response and toxicity to standard doses of most anticancer medications. Genetic polymorphisms in genes encoding metabolic enzymes, receptors and drug transporters targeted by anticancer medications are often found, in part, to be responsible for the observed variability. Approximately 80% of all sequence variations residing in genes is in the form of single nucleotide polymorphisms or SNPs. The location of SNPs can be in the protein coding sequence, regulatory regions or at exon-intron boundaries of genes. Adverse drug reactions resulting from these sequence variations are due to changes in the activity of the encoded protein (in many instances the protein is non-functional) or perturbations in the level of gene expression. The goal of pharmacogenetic testing is to identify genetic polymorphisms that predispose patients to an adverse drug reaction, thereby allowing the health care provider to make informed decisions pertaining to the type of drug, dosage and dosage scheduling to be administered.

Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8(-/-)). The G5G8(-/-) mice had a 2- to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an approximately 30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8(-/-) mice when compared with wild-type animals (mean = 0.4 vs. 5.5 micromol ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8(-/-) mice and increased 2.4- and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.

Dual effects of ADP and adenylylimidodiphosphate on CFTR channel kinetics show binding to two different nucleotide binding sites

The CFTR chloride channel is regulated by phosphorylation by protein kinases, especially PKA, and by nucleotides interacting with the two nucleotide binding domains, NBD-A and NBD-B. Giant excised inside-out membrane patches from Xenopus oocytes expressing human epithelial cystic fibrosis transmembrane conductance regulator (CFTR) were tested for their chloride conductance in response to the application of PKA and nucleotides. Rapid changes in the concentration of ATP, its nonhydrolyzable analogue adenylylimidodiphosphate (AMP-PNP), its photolabile derivative ATP-P3-[1-(2-nitrophenyl)ethyl]ester, or ADP led to changes in chloride conductance with characteristic time constants, which reflected interaction of CFTR with these nucleotides. The conductance changes of strongly phosphorylated channels were slower than those of partially phosphorylated CFTR. AMP-PNP decelerated relaxations of conductance increase and decay, whereas ATP-P3-[1-(2-nitrophenyl)ethyl]ester only decelerated the conductance increase upon ATP addition. ADP decelerated the conductance increase upon ATP addition and accelerated the conductance decay upon ATP withdrawal. The results present the first direct evidence that AMP-PNP binds to two sites on the CFTR. The effects of ADP also suggest two different binding sites because of the two different modes of inhibition observed: it competes with ATP for binding (to NBD-A) on the closed channel, but it also binds to channels opened by ATP, which might either reflect binding to NBD-A (i.e., product inhibition in the hydrolysis cycle) or allosteric binding to NBD-B, which accelerates the hydrolysis cycle at NBD-A.