Interactions between doxorubicin and the human iron regulatory system

Anthracyclines are included in clinical treatments against various malignancies, but severe cardiotoxic side-effects and the development of resistance mechanisms limit their usefulness. Many aspects of the cellular response to anthracyclines remain debated. The status of the main regulator of iron homeostasis, namely the RNA-binding activity of iron regulatory proteins (IRPs), has been assessed herein for two types of human tumor cells and their derived doxorubicin-resistant sublines. IRPs were always fully activated in the latter, whereas only partial activation occurred in the former. Doxorubicin exposure reversibly inactivated IRP1 in small cell lung carcinoma (GLC(4)) and myelogenous leukemia (K562) cell lines, but was without effect in their derived doxorubicin-resistant sublines. In contrast, adding doxorubicin to cytosolic fractions of untreated cells or to purified IRPs led to the irreversible alteration of the RNA-binding activity of IRP1. In these different conditions, interaction between doxorubicin and the iron regulatory system disturbs iron metabolism, and cells having developed a resistance mechanism are tuned to maximize the iron supply. The results reported herein may lead the path toward a better therapeutic management of cancer patients receiving doxorubicin by discriminating between the antiproliferative and cardiotoxic properties of this anthracycline.     

Cloning of novel human SEC14p-like proteins: ligand binding and functional properties

We describe the cloning and expression of two novel genes highly similar to the tocopherol-associated protein (hTAP/SEC14L2/SPF). Immunoprecipitation of the three recombinant hTAPs and extraction of their associated lipid-soluble molecules indicates that they bind not just tocopherols, but also phosphatidylinositol, phosphatidylcholine, and phosphatidylglycerol. Ligand competition analysis by isoelectric point mobility shift assay indicates that phosphatidylcholine, tocopherols, and tocopheryl-succinate compete with phosphatidylinositol binding to hTAPs. To investigate a possible function of hTAPs on enzymes involved in phospholipids metabolism, the activity of recombinant phosphatidylinositol 3-kinase (PI3Kgamma/p110gamma) was tested. Recombinant hTAPs reduce in vitro the activity of the recombinant catalytic subunit of PI3Kgamma and stimulate it in the presence of alpha-tocopherol up to 5-fold. Immunoprecipitation of hTAP1 from cells results in co-precipitation of PI3-kinase activity, indicating a physical contact between the two proteins at a cellular level. In summary, hTAPs may modulate, in a tocopherol-sensitive manner, phosphatidylinositol-3-kinase, a central enzyme in signal transduction, cell proliferation, and apoptosis. It is possible that other phosphatidylinositol- and phosphatidylcholine-dependent signaling pathways are modulated by hTAPs and tocopherols, possibly by transporting and presenting these ligands to the corresponding enzymes.     

Vacuolar system distribution in Arabidopsis tissues,visualized using GFP fusion proteins

Green fluorescent protein (GFP) allows the direct visualization of gene expression and the subcellular localization of fusion proteins in living cells. The localization of different GFP fusion proteins in the secretory system was studied in stably transformed Arabidopsis plants cv. Wassilewskaja. Secreted GFP (SGFP) and GFP retained in the ER (GFP-KDEL) confirmed patterns already known, but two vacuolar GFPs (GFP-Chi and Aleu-GFP) labelled the Arabidopsis vacuolar system for the first time, the organization of which appears to depend on cell differentiation. GFP stability in the vacuoles may depend on pH or degradation, but these vacuolar markers can, nevertheless, be used as a tool for physiological studies making these plants suitable for mutagenesis and gene-tagging experiments.     

Critical amino acid residues determine the binding affinity and the Ca2+ release efficacy of maurocalcine in skeletal muscle cells

Maurocalcine (MCa) is a 33 amino acid residue peptide toxin isolated from the scorpion Scorpio maurus palmatus. MCa and mutated analogues were chemically synthesized, and their interaction with the skeletal muscle ryanodine receptor (RyR1) was studied on purified RyR1, sarcoplasmic reticulum (SR) vesicles, and cultured myotubes. MCa strongly potentiates [3H]ryanodine binding on SR vesicles (7-fold at pCa 5) with an apparent EC50 of 12 nm. MCa decreases the sensitivity of [3H]ryanodine binding to inhibitory high Ca2+ concentrations and increases it to the stimulatory low Ca2+ concentrations. In the presence of MCa, purified RyR1 channels show long-lasting openings characterized by a conductance equivalent to 60% of the full conductance. This effect correlates with a global increase in Ca2+ efflux as demonstrated by MCa effects on Ca2+ release from SR vesicles. In addition, we show for the first time that external application of MCa to cultured myotubes produces a cytosolic Ca2+ increase due to Ca2+ release from 4-chloro-m-cresol-sensitive intracellular stores. Using various MCa mutants, we identified a critical role of Arg24 for MCa binding onto RyR1. All of the other MCa mutants are still able to modify [3H]ryanodine binding although with a decreased EC50 and a lower stimulation efficacy. All of the active mutants produce both the appearance of a subconductance state and Ca2+ release from SR vesicles. Overall, these data identify some amino acid residues of MCa that support the effect of this toxin on ryanodine binding, RyR1 biophysical properties, and Ca2+ release from SR.     

Binding discrimination of MutS to a set of lesions and compound lesions (base damage and mismatch) reveals its potential role as a cisplatin-damaged DNA sensing protein

The DNA mismatch repair (MMR) system plays a critical role in sensitizing both prokaryotic and eukaryotic cells to the clinically potent anticancer drug cisplatin. It is thought to mediate cytotoxicity through recognition of cisplatin DNA lesions. This drug generates a range of lesions that may also give rise to compound lesions resulting from the misincorporation of a base during translesion synthesis. Using gel mobility shift competition assays and surface plasmon resonance, we have analyzed the interaction of Escherichia coli MutS protein with site-specifically modified DNA oligonucleotides containing each of the four cisplatin cross-links or a set of compound lesions. The major 1,2-d(GpG) cisplatin intrastrand cross-link was recognized with only a 1.5-fold specificity, whereas a 47-fold specificity was found with a natural G/T containing DNA substrate. The rate of association, kon, for binding to the 1,2-d(GpG) adduct was 3.1 x 104 m-1 s-1 and the specificity of binding was essentially dependent on koff. DNA duplexes containing a single 1,2-d(ApG), 1,3-d(GpCpG) adduct, and an interstrand cross-link of cisplatin were not preferentially recognized. Among 12 DNA substrates, each containing a different cisplatin compound lesion derived from replicative misincorporation of one base opposite either of the 1,2-intrastrand adducts, 10 were specifically recognized including those that are more likely formed in vivo based on cisplatin mutation spectra. Moreover, among these lesions, two compound lesions formed when an adenine was misincorporated opposite a 1,2-d(GpG) adduct were not substrates for the MutY-dependent mismatch repair pathway. The ability of MutS to sense differentially various platinated DNA substrates suggests that cisplatin compound lesions formed during misincorporation of a base opposite either adducted base of both 1,2-intrastrand cross-links are more plausible critical lesions for MMR-mediated cisplatin cytotoxicity.     

Reversion of the lethal phenotype of an HIV-1 integrase mutant virus by overexpression of the same integrase mutant protein

The human immunodeficiency virus type 1 (HIV-1) integrase (IN) is essential for integration of viral DNA into host cell chromatin. We have reported previously (Priet, S., Navarro, J. M., Gros, N., Querat, G., and Sire, J. (2003) J. Biol. Chem. 278, 4566-4571) that IN also plays a role in the packaging of the host uracil DNA glycosylase UNG2 into viral particles and that the region of IN encompassing residues 170-180 was responsible for the interaction with UNG2 and for its packaging into virions. In this work, we aimed to investigate the replication of HIV-1 viruses rendered deficient in virion-associated UNG2 by single or double point mutations in the region 170-180 of IN. We show that the L172A/K173A IN mutant virus was deficient for UNG2 packaging and was defective for replication because of a blockage at the stage of proviral DNA integration in host cell DNA. In vitro assays using long term repeat mimics, however, demonstrate that the L172A/K173A IN mutant was catalytically active. Moreover, trans-complementation experiments show that the viral propagation of L172A/K173A viruses could be rescued by the overexpression of Vpr.L172A/K173A IN fusion protein in a dose-dependent manner and that this rescue is independent of UNG2 packaging. Altogether, our data indicate that L172A/K173A mutations of IN induce a subtle defect in the function of IN, which nevertheless dramatically impairs viral replication. Unexpectedly, this blockage of replication could be overcome by forcing the packaging of higher amounts of this same mutated integrase. This is the first study reporting that blockage of the integration process of HIV-1 provirus carrying a mutation of IN could be alleviated by increasing amounts of IN even carrying the same mutations.