The nitric oxide–iron interplay in mammalian cells: Transport and storage of dinitrosyl iron complexes

Nitrogen monoxide (NO) is a vital effector and messenger molecule that plays roles in a variety of biological processes. Many of the functions of NO are mediated by its high affinity for iron (Fe) in the active centres of proteins. Indeed, NO possesses a rich coordination chemistry with this metal and the formation of dinitrosyl–dithiolato–Fe complexes (DNICs) is well known to occur intracellularly. In mammals, NO produced by activated macrophages acts as a cytotoxic effector against tumour cells by binding and releasing cancer cell Fe that is vital for proliferation. Glucose metabolism and the subsequent generation of glutathione (GSH) are critical for NO-mediated Fe efflux and this process occurs by active transport. Our previous studies showed that GSH is required for Fe mobilisation from tumour cells and we hypothesized it was effluxed with Fe as a dinitrosyl–diglutathionyl–Fe complex (DNDGIC). It is well known that Fe and GSH release from cells induces apoptosis, a crucial property for a cytotoxic effector like NO. Furthermore, NO-mediated Fe release is mediated from cells expressing the GSH transporter, multi-drug resistance protein 1 (MRP1). Interestingly, the glutathione-S-transferase (GST) enzymes act to bind DNDGICs with high affinity and some members of the GST family act as storage intermediates for these complexes. Since the GST enzymes and MRP1 form a coordinated system for removing toxic substances from cells, it is possible to hypothesize these molecules regulate NO levels by binding and transporting DNDGICs.     

Nuclear transport of histone 2b in mammalian cells is signal- and energy-dependent and different from the importin α/β-mediated process

Histone 2b nuclear transport was investigated using the digitonin-permeabilized cell system and the rat liver resealed nuclear envelope system. In permeabilized cells, maximal uptake of histone 2b is dependent on cytosolic components and an appropriate energy source. Addition of the recombinant proteins importin alpha/beta, and Ran, as well as ATP and GTP, to cytosol-depleted permeabilized cells does not enhance the uptake of histone 2b in contrast to that of nucleoplasmin serving as a control. Nuclear import of histone 2b cannot be blocked by addition of an excess of a nuclear localization signal-bearing peptide or nucleoplasmin. Similar results were obtained with resealed nuclear envelopes. As shown previously, resealed vesicles respond to the importin signal for the uptake of nuclear localization signal-bearing proteins which allows investigation of the import mechanism independent of intranuclear binding to chromatin. Uptake of histone 2b therefore seems to be an energy-requiring transport mechanism different from the import of proteins bearing a typical nuclear localization signal.     

The state of DNA methylation of human ϵ-globin gene in erythroid and non-erythroid cells

The extent of DNA methylation within the embryonic human ϵ-globin gene domain was studied in erythroid and non-erythroid cell lines. The results obtained show that the human ϵ-globin gene is totally methylated at all sites tested in tissues where it is not expressed, i.e. blood leucocytes. In the erythroid cell lines, K562 and PUTKO, both forced to embryonic differentiation by induction with haemin, the level of methylation is reduced compared with that observed in blood leucocytes. In the nonerythroid cell lines HeLa and Raji, where the human ϵ-globin gene is not expressed, the overall level of methylation in all sites tested is lower compared with that in erythroid cell lines.     

Multiple aggregates and aggresomes of C-terminal truncated human αA-crystallins in mammalian cells and protection by αB-crystallin

Background

Cleavage of 11 (αA162), 5 (αA168) and 1 (αA172) residues from the C-terminus of αA-crystallin creates structurally and functionally different proteins. The formation of these post-translationally modified αA-crystallins is enhanced in diabetes. In the present study, the fate of the truncated αA-crystallins expressed in living mammalian cells in the presence and absence of native αA- or αB-crystallin has been studied by laser scanning confocal microscopy (LSM).

Methodology/Principal Findings

YFP tagged αAwt, αA162, αA168 and αA172, were individually transfected or co-transfected with CFP tagged αAwt or αBwt, expressed in HeLa cells and studied by LSM. Difference in protein aggregation was not caused by different level of α-crystallin expression because Western blotting results showed nearly same level of expression of the various α-crystallins. The FRET-acceptor photo-bleaching protocol was followed to study in situ protein-protein interaction. αA172 interacted with αAwt and αBwt better than αA168 and αA162, interaction of αBwt being two-fold stronger than that of αAwt. Furthermore, aggresomes were detected in cells individually expressing αA162 and αA168 constructs and co-expression with αBwt significantly sequestered the aggresomes. There was no sequestration of aggresomes with αAwt co-expression with the truncated constructs, αA162 and αA168. Double immunocytochemistry technique was used for co-localization of γ-tubulin with αA-crystallin to demonstrate the perinuclear aggregates were aggresomes.

Conclusions/Significance

αA172 showed the strongest interaction with both αAwt and αBwt. Native αB-crystallin provided protection to partially unfolded truncated αA-crystallins whereas native αA-crystallin did not. Aggresomes were detected in cells expressing αA162 and αA168 and αBwt co-expression with these constructs diminished the aggresome formation. Co-localization of γ-tubulin in perinuclear aggregates validates for aggresomes.     

The involvement of p62–Keap1–Nrf2 antioxidative signaling pathway and JNK in the protection of natural flavonoid quercetin against hepatotoxicity

Quercetin, one of the most abundant dietary flavonoids, is reported to have protective function against various hepatotoxicant-induced hepatotoxicity. The present study aims to investigate the critical role of the nuclear factor erythroid 2-related factor 2 (Nrf2) antioxidative signaling pathway in the protection of quercetin against hepatotoxicity. Quercetin prevented the cytotoxicity induced by a variety of hepatotoxicants including clivorine (Cliv), acetaminophen (APAP), ethanol, carbon tetrachloride (CCl₄), and toosendanin (TSN) in human normal liver L-02 cells. Quercetin induced the nuclear translocation of Nrf2, along with the increased expression of the antioxidant responsive element (ARE)-dependent genes like catalytic or modify subunit of glutamate-cysteine ligase (GCLC/GCLM), and heme oxygenase-1 (HO-1). In addition, the HO-1 inhibitor zinc protoporphyrin (ZnPP) and the GCL inhibitor L-buthionine-(S,R)-sulfoximine (BSO) both reduced the hepatoprotection induced by quercetin. Quercetin had no effect on kelch-like ECH-associated protein-1(Keap1) expression, but molecular docking results indicated the potential interaction of quercetin with the Nrf2-binding site in Keap1 protein. Quercetin increased the expression of p62, and p62 siRNA decreased quercetin-induced hepatoprotection. Quercetin induced the activation of c-Jun N-terminal kinase (JNK) in hepatocytes. JNK inhibitor SP600125 and JNK siRNA both reduced quercetin-induced hepatoprotection. SP600125 and JNK siRNA decreased the increased p62 expression induced by quercetin. In addition, SP600125 also decreased the increased mRNA and protein expression of GCLC, GCLM, and HO-1 induced by quercetin. Taken together, our present study demonstrates that quercetin prevents hepatotoxicity by inducing p62 expression, inhibiting the binding of Keap1 to Nrf2, and thus leading to the increased expression of antioxidative genes dependent on Nrf2. Meanwhile, our study indicates that JNK plays some regulation in this process.     

Mechanisms and consequences of impaired lipid trafficking in Niemann–Pick type C1-deficient mammalian cells

Niemann–Pick C disease is a fatal progressive neurodegenerative disorder caused in 95% of cases by mutations in the NPC1 gene; the remaining 5% of cases result from mutations in the NPC2 gene. The major biochemical manifestation of NPC1 deficiency is an abnormal sequestration of lipids, including cholesterol and glycosphingolipids, in late endosomes/lysosomes (LE/L) of all cells. In this review, we summarize the current knowledge of the NPC1 protein in mammalian cells with particular focus on how defects in NPC1 alter lipid trafficking and neuronal functions. NPC1 is a protein of LE/L and is predicted to contain thirteen transmembrane domains, five of which constitute a sterol-sensing domain. The precise function of NPC1, and the mechanism by which NPC1 and NPC2 (both cholesterol binding proteins) act together to promote the movement of cholesterol and other lipids out of the LE/L, have not yet been established. Recent evidence suggests that the sequestration of cholesterol in LE/L of cells of the brain (neurons and glial cells) contributes to the widespread death and dysfunction of neurons in the brain. Potential therapies include treatments that promote the removal of cholesterol and glycosphingolipids from LE/L. Currently, the most promising approach for extending life-span and improving the quality of life for NPC patients is a combination of several treatments each of which individually modestly slows disease progression.     

The occurrence and properties of histone H1° in quiescent rabbit tissue

• 1.1. The issue of the existence and the properties of histone H1° in quiescent rabbit tissues was approached by electrophoretic, Chromatographic and immunochemical techniques.
• 2.2. The results demonstrated that the rabbit tissues did contain a typical histone H1°, its properties being comparable to those of H1° from all other sources studied thus far.

     

Recruitment of the cohesin loading factor NIPBL to DNA double-strand breaks depends on MDC1, RNF168 and HP1γ in human cells

The cohesin loading factor NIPBL is required for cohesin to associate with chromosomes and plays a role in DNA double-strand break (DSB) repair. Although the NIPBL homolog Scc2 is recruited to an enzymatically generated DSB and promotes cohesin-dependent DSB repair in yeast, the mechanism of the recruitment remains poorly understood. Here we show that the human NIPBL is recruited to the sites of DNA damage generated by micro-irradiation as well as to the sites of DSBs induced by homing endonuclease, I-PpoI. The recruitment of NIPBL was impaired by RNAi-mediated knockdown of MDC1 or RNF168, both of which also accumulate at DSBs. We also show that the recruitment of NIPBL to the sites of DNA damage is mediated by its C-terminal region containing HEAT repeats and Heterochromatin protein 1 (HP1) interacting motif. Furthermore, NIPBL accumulation at damaged sites was also compromised by HP1γ depletion. Taken together, our study reveals that human NIPBL is a novel protein recruited to DSB sites, and the recruitment is controlled by MDC1, RNF168 and HP1γ.     

An improved lentiviral system for efficient expression and purification of β-defensins in mammalian cells

β-Defensins are a family of conserved small cationic antimicrobial peptides with different significant biological functions. The majority of mammalian β-defensins are expressed in epididymis, and many of them are predicted to have post-translational modifications. However, only a few of its members have been well studied due to the limitations of expressing and purifying bioactive proteins with correct post-translational modifications efficiently. Here we developed a novel Fc tagged lentiviral system and Fc tagged prokaryotic expression systems provided new options for β-defensins expression and purification. The novel lentiviral system contains a secretive signal peptide, an N-terminal IgG Fc tag, a green fluorescent protein (GFP), and a puromycin selection marker to facilitate efficient expression and fast purification of β-defensins by protein A magnetic or agarose beads. It also enables stable and large-scale expression of β-defensins with regular biological activities and post-translational modification. Purified β-defensins such as Bin1b and a novel human β-defensin hBD129 showed antimicrobial activity, immuno-regulatory activity, and expected post-translational phosphorylation, which were not found in Escherichia coli (E. coli) in expressed form. Furthermore, we successfully applied the novel system to identify mBin1b interacting proteins, explaining Bin1b in a better way. These results suggest that the novel lentiviral system is a powerful approach to produce correct post-translational processed β-defensins with bioactivities and is useful to identify their interacting proteins. This study has laid the foundation for future studies to characterize function and mechanism of novel β-defensins.     

Methylation of deoxyribonucleic acid in cultured mammalian cells by N-methyl-N′-nitro-N-nitrosoguanidine. The influence of cellular thiol concentrations on the extent of methylation and the 6-oxygen atom of guanine as a site of methylation

1. In neutral aqueous solution N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) yields salts of nitrocyanamide as u.v.-absorbing products. With cysteine, as found independently by Schulz & McCalla (1969), the principal product is 2-nitràminothiazoline-4-carboxylic acid. Both these reactions liberate the methylating species; thiols enhance the rate markedly at neutral pH values. An alternative reaction with thiols gives cystine, presumably via the unstable S-nitrosocysteine. 2. Thiols (glutathione or N-acetylcysteine) in vitro at about the concentration found in mammalian cells enhance the rate of methylation of DNA markedly over that in neutral solution. 3. Treatment of cultured mammalian cells with MNNG results in rapid methylation of nucleic acids, the extent being greater the higher the thiol content of the cells. Rodent embryo cells are more extensively methylated than mouse L-cells of the same thiol content. Cellular thiol concentrations are decreased by MNNG. Proteins are less methylated by MNNG than are nucleic acids. 4. Methylation of cells by dimethyl sulphate does not depend on cellular thiol content and protein is not less methylated than nucleic acids. Methylation by MNNG may therefore be thiol-stimulated in cells. 5. Both in vitro and in cells about 7% of the methylation of DNA by MNNG occurs at the 6-oxygen atom of guanine. The major products 7-methylguanine and 3-methyladenine are given by both MNNG and dimethyl sulphate, but dimethyl sulphate does not yield O(6)-methylguanine. Possible reaction mechanisms to account for this difference between these methylating agents and its possible significance as a determinant of their biological effects are discussed.     

Global DNA methylation profile at LINE-1 repeats and promoter methylation of genes involved in DNA damage response and repair pathways in human peripheral blood mononuclear cells in response to γ-radiation

Molecular and Cellular Biochemistry - DNA methylation is an epigenetic mechanism, which plays an important role in gene regulation. The present study evaluated DNA methylation profile of LINE1...     

The IL-7Rα pathway is quantitatively and functionally altered in CD8 T cells in multiple sclerosis

The IL-7Rα single nucleotide polymorphism rs6897932 is associated with an increased risk for multiple sclerosis (MS). IL-7Rα is a promising candidate to be involved in autoimmunity, because it regulates T cell homeostasis, proliferation, and antiapoptotic signaling. However, the exact underlying mechanisms in the pathogenesis of MS are poorly understood. We investigated whether CD4 and CD8 lymphocyte subsets differed in IL-7Rα expression and functionality in 78 MS patients compared with 59 healthy controls (HC). A significantly higher frequency of IL-7Rα(+) CD8 effector memory (CD8EM) was found in MS. Moreover, IL-7Rα membrane expression was significantly increased in MS in naive and memory CD8 (all p < 0.05) with a similar trend in CD8EM (p = 0.055). No correlation was found between the expression level or frequency of IL-7Rα(+)CD8(+) and rs6897932 risk allele carriership. Upon IL-7 stimulation, MS patients had stronger STAT5 activation in CD8EM compared with HC. IL-7 stimulation had a differential effect on both mRNA and protein expression of granzyme A and granzyme B between MS and HC. Stainings of different lesions in postmortem MS brain material showed expression of IL-7 and CD8(+)IL-7Rα(+) in preactive, but not in active, demyelinating MS lesions, indicating involvement of IL-7Rα(+) lymphocytes in lesion development. The intralesional production of IL-7 in combination with the lower threshold for IL-7-induced cytotoxicity in MS may enhance the pathogenicity of these CD8 T cells. This is of special interest in light of the established demyelinating and cytotoxic actions of granzyme A.     

The C-terminal extension of Lsm4 interacts directly with the 3′ end of the histone mRNP and is required for efficient histone mRNA degradation

Metazoan replication-dependent histone mRNAs are the only known eukaryotic mRNAs that lack a poly(A) tail, ending instead in a conserved stem–loop sequence, which is bound to the stem–loop binding protein (SLBP) on the histone mRNP. Histone mRNAs are rapidly degraded when DNA synthesis is inhibited in S phase in mammalian cells. Rapid degradation of histone mRNAs is initiated by oligouridylation of the 3′ end of histone mRNAs and requires the cytoplasmic Lsm1-7 complex, which can bind to the oligo(U) tail. An exonuclease, 3′hExo, forms a ternary complex with SLBP and the stem–loop and is required for the initiation of histone mRNA degradation. The Lsm1-7 complex is also involved in degradation of polyadenylated mRNAs. It binds to the oligo(A) tail remaining after deadenylation, inhibiting translation and recruiting the enzymes required for decapping. Whether the Lsm1-7 complex interacts directly with other components of the mRNP is not known. We report here that the C-terminal extension of Lsm4 interacts directly with the histone mRNP, contacting both SLBP and 3′hExo. Mutants in the C-terminal tail of Lsm4 that prevent SLBP and 3′hExo binding reduce the rate of histone mRNA degradation when DNA synthesis is inhibited.     

The α isoform of topoisomerase II is required for hypercompaction of mitotic chromosomes in human cells

As proliferating cells transit from interphase into M-phase, chromatin undergoes extensive reorganization, and topoisomerase (topo) IIα, the major isoform of this enzyme present in cycling vertebrate cells, plays a key role in this process. In this study, a human cell line conditional null mutant for topo IIα and a derivative expressing an auxin-inducible degron (AID)-tagged version of the protein have been used to distinguish real mitotic chromosome functions of topo IIα from its more general role in DNA metabolism and to investigate whether topo IIβ makes any contribution to mitotic chromosome formation. We show that topo IIβ does contribute, with endogenous levels being sufficient for the initial stages of axial shortening. However, a significant effect of topo IIα depletion, seen with or without the co-depletion of topo IIβ, is the failure of chromosomes to hypercompact when delayed in M-phase. This requires much higher levels of topo II protein and is impaired by drugs or mutations that affect enzyme activity. A prolonged delay at the G2/M border results in hyperefficient axial shortening, a process that is topo IIα-dependent. Rapid depletion of topo IIα has allowed us to show that its function during late G2 and M-phase is truly required for shaping mitotic chromosomes.