Nitric oxide-dependent pro-oxidant and pro-apoptotic effect of metallothioneins in HL-60 cells challenged with cupric nitrilotriacetate

All Roads Lead to Auxin: Post-translational Regulation of Auxin Transport by Multiple Hormonal Pathways

Auxin is a key hormonal regulator, that governs plant growth and development in concert with other hormonal pathways. The unique feature of auxin is its polar, cell-to-cell transport that leads to the formation of local auxin maxima and gradients, which coordinate initiation and patterning of plant organs. The molecular machinery mediating polar auxin transport is one of the important points of interaction with other hormones. Multiple hormonal pathways converge at the regulation of auxin transport and form a regulatory network that integrates various developmental and environmental inputs to steer plant development. In this review, we discuss recent advances in understanding the mechanisms that underlie regulation of polar auxin transport by multiple hormonal pathways. Specifically, we focus on the post-translational mechanisms that contribute to fine-tuning of the abundance and polarity of auxin transporters at the plasma membrane and thereby enable rapid modification of the auxin flow to coordinate plant growth and development.

Non-enzymatic modifications in metallothioneins connected to lipid membrane damages: structural and biomimetic studies under reductive radical stress

Metallothioneins (MTs) are small cysteine-rich proteins with the ability to coordinate heavy metal atoms through metal-thiolate bonds, which are widely distributed among the animal and plant kingdoms. Multifunctional roles for MTs have been proposed, including their ability to scavenger various radicals and reactive oxygen species. In the present article we summarize available information of four MT polypeptides from different organisms, forming metal complexes with Zn(II), Cd(II) or Cu (I) ions. Non-enzymatic modifications of MTs under ionizing radiations and their consequences on the lipidic membrane compartment were studied by Raman spectroscopy and a biomimetic model, respectively. The latter is based on liposome technology and allows to measure the trans unsaturated fatty acid content as a result of reductive radical stress on MTs.

BIOLOGICAL SIGNIFICANCE

The effect of radical stress on the cell metabolism and functions is a very active field of research connecting various disciplines in life sciences. In this contest tandem radical damage has been the subject of recent investigations that pointed out its harmfulness in the general scenario of establishing the consequences of radical stress. By using biomimetic models of tandem damage we have for the first time tested the capability of metallothioneins (MTs), small metalloproteins rich of Cys residues, to damage another cell compartment like lipid membranes when they are undergone to reductive radical stress. The connection of MT reactivity with membrane lipid transformation can give a contribution to the puzzling context of radical stress occurring to biomolecules and the role as biological signaling. To this purpose, MT polypeptides from different organisms, exhibiting different sequence peculiarities, have been analyzed here. The spectroscopic analysis of these systems has allowed to identify modifications affecting metal-thiolate clusters, cystines, and Met residues, acting as efficient interceptors of reducing radical species. The chemical mechanism involving sulfur-containing moieties under reductive conditions discloses new scenarios that bring to the loss of sulfur-centered radicals by desulfurization reactions that change the natural sequences of MTs. Ala is a genetically coded amino acid, therefore the mutation of Cys to Ala occurring to a sequence by the radical process so far discussed, corresponds to a post-translational modification. Research on such mutation connected also to a free radical stress will be important to contribute for a complete picture of the degeneration associated to diseases and aging. Analogously, the Met to Aba mutation occurring after reductive stress transforms a natural amino acid into a natural, non-genetically-coded congener. Aba corresponds to a homologation of the alkyl chains normally present in genetically codified amino acids, such as methyl (in Ala) and isopropyl (in Leu), with an ethyl unit. Based on alkyl substitution, this modification can therefore be studied in order to understand its general consequences on the structure-activity relationships in proteins and, in particular, on molecular interactions. This article is part of a Special issue entitled: Posttranslational Protein modifications in biology and Medicine.

A critical role for increased labile zinc in reducing sensitivity of cultured sheep pulmonary artery endothelial cells to LPS-induced apoptosis

We previously noted an important signaling role for decreased labile intracellular zinc ([ Zn ] (i)) in LPS-induced apoptosis in cultured sheep pulmonary artery endothelial cells (SPAEC) (Tang ZL, Wasserloos KJ, Liu X, Stitt MS, Reynolds IJ, Pitt BR, St Croix CM. Mol Cell Biochem 234-235: 211-217, 2002; Thambiayya K, Wasserloos KJ, Huang Z, Kagan VE, St Croix CM, Pitt BR. Am J Physiol Lung Cell Mol Physiol 300: L624-632, 2011). In the present study, we used small interfering RNA (siRNA) to important contributors of zinc homeostasis [ SLC39A14 or Zrt/Irt-like protein 14 (ZIP14), a zinc importer; metallothionein (MT), a zinc binding protein ] to define molecular pathways by which extracellular zinc or nitric oxide (NO) increase labile [ Zn ] (i) [ e.g., zinc-sensitive fluorophore (FluoZin-3) detectable and/or chelatable by N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine ] and reduce the sensitivity of SPAEC to LPS. Addition of 10 μM zinc to serum-free medium of SPAEC increased [ Zn ] (i) and abolished LPS-induced apoptosis (e.g., increased annexin V binding). The increase in [ Zn ] (i) and the protective effect of extracellular zinc were sensitive to reduction in ZIP14 expression (by siRNA), but not affected by collectively knocking down major isoforms of sheep MT (sMT-Ia, -Ib, -Ic, and -II). Pretreatment of wild-type SPAEC with 250 μM of the NO donor S-nitroso-N-acetylpenicillamine (SNAP) increased labile zinc in a relatively similar fashion to addition of extracellular zinc and reduced sensitivity of SPAEC to LPS-induced apoptosis (e.g., caspase-3/7 activation) in a N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine-sensitive fashion. The antiapoptotic effects of SNAP were insensitive to siRNA knockdown of ZIP14, but were abolished (along with SNAP-induced increase in [ Zn ] (i)) when SPAEC were pretreated with siRNA to sheep MT. Zinc was able to directly inhibit recombinant caspase-3 activity in an in vitro assay. Collectively, these data show that increases in labile [ Zn ] (i) are an important component of ZIP14- or NO-mediated resistance to LPS-induced apoptosis. Cytoprotection via ZIP14 appeared to be secondary to transcellular movement of extracellular zinc, whereas NO-mediated protection was secondary to S-nitrosation of MT and redistribution of [ Zn ] (i).

Unraveling the response of plant cells to cytotoxic saponins: role of metallothionein and nitric oxide

A wide range of pharmacological properties are ascribed to natural saponins, in addition to their biological activities against herbivores, plant soil-borne pathogens and pests. As for animal cells, the cytotoxicity and the chemopreventive role of saponins are mediated by a complex network of signal transduction pathways which include reactive oxygen species (ROS) and nitric oxide (NO). The involvement of other relevant components of the saponin-related signaling routes, such as the Tumor Necrosis Factor(TNF)-α, the interleukin(IL)-6 and the Nuclear Transcription Factor-kB (NF-κB), has been highlighted in animal cells. By contrast, information concerning the response of plant cells to saponins and the related signal transduction pathways is almost missing. To date, there are only a few common features which link plant and animal cells in their response to saponins, such as the early burst in ROS and NO production and the induction of metallothioneins (MTs), small cysteine-rich, metal-binding proteins. This aspect is discussed in the present paper in view of the recent hypothesis that MTs and NO are part of a novel signal transduction pathway participating in the cell response to oxidative stress.

Maternal and fetal oxidative stress and intrapartum term fever

OBJECTIVE

The association between maternal chorioamnionitis and fetal oxidative stress has not been well established.

STUDY DESIGN

A nested case control study was performed within a prospective cohort of term nulliparous women: 20 cases (intrapartum fever of >100.4 degrees F) and 20 afebrile controls. Oxidative stress was assessed using ThioGlo-1 (TG-1; Calbiochem, San Diego, CA) fluorescent sulfhydryl detection. Median levels (+/- interquartile range) of protein-thiol sulfhydryls were compared.

RESULTS

In early labor, maternal oxidative stress (lower protein sulfhydryls) was significantly higher in those women who subsequently had intrapartum fever develop (79.87 +/- 22.88 vs 127.73 +/- 43.79 counts/second per microg protein; P < .001). In contrast, cord serum sulfhydryls were not different between groups (75.77 +/- 14.00 vs 75.04 +/- 17.83 counts/second per microg protein; P = .99)

CONCLUSION

Our data suggest that the term human fetus is protected from maternal oxidative stress associated with intrapartum fever. However, maternal oxidative status in early labor is associated with subsequent intrapartum fever. Optimal fetal neuroprotection will require a more precise knowledge of pathogenic mechanisms.

Protein-passivated Fe(3)O(4) nanoparticles: low toxicity and rapid heating for thermal therapy

Thermotherapy is a promising technique for the minimally invasive elimination of solid tumors. Here we report the fabrication of protein-coated iron oxide NPs (12 nm core) for use as thermal therapeutic agents. These albumin-passivated NPs are stable under physiological conditions, with rapid heating and cell killing capacity upon alternating magnetic field (AMF) exposure. The mode of action is specific: no measurable cytotoxicity was observed for the particle without AMF or for AMF exposure without the particle.

Oxidative stress and protein oxidation in the brain of water drinking and alcohol drinking rats administered the HIV envelope protein, gp120

Possible roles of oxidative stress and protein oxidation on alcohol-induced augmentation of cerebral neuropathy in gp120 administered alcohol preferring rats drinking either pure water (W rats) or a free-choice ethanol and water (E rats) for 90 days. This study showed that peripherally administered gp120 accumulated into the brain, liver, and RBCs samples from water drinking - gp120 administered rats (Wg rats) and ethanol drinking - gp120 administered rats (Eg rats), although gp120 levels in samples from Eg rats were significantly greater than the levels in samples from Wg rats. The brain samples from ethanol drinking-saline administered (EC) and Wg rats exhibited comparable levels of free radicals that were significantly lower than the levels in Eg rats. Peroxiredoxin-I (PrxI) activity in the brain samples exhibited the following pattern: Wg >> >> WC >> EC > Eg. Total protein-carbonyl and carbonylated hippocampal cholinergic neurostimulating peptide precursor protein levels, but not N-acetylaspartate or N-acetyl aspartylglutamate or total protein-thiol levels, paralleled the free radical levels in the brain of all four groups. This suggests PrxI inhibition may be more sensitive indicator of oxidative stress than measuring free radicals or metabolites. As PrxI oxidation in WC, Wg, and EC rats was reversible, while PrxI oxidation in Eg rats was not, we suggest that alcohol drinking and gp120 together hyperoxidized and inactivated PrxI that suppressed free radical neutralization in the brain of Eg rats. In conclusion, chronic alcohol drinking, by carbonylating and hyperoxidizing free radical neutralization proteins, augmented the gp120-induced oxidative stress that may be associated with an increase in severity of the brain neuropathy.

Nitric oxide-induced nuclear translocation of the metal responsive transcription factor, MTF-1 is mediated by zinc release from metallothionein

We previously showed that the major Zn-binding protein, metallothionein (MT) is a critical target for nitric oxide (NO) with resultant increases in labile Zn. We now show that NO donors also affected the activity of the metal responsive transcription factor MTF-1 that translocates from the cytosol to the nucleus in response to physiologically relevant increases in intracellular Zn and transactivates MT gene expression. Exposing mouse lung endothelial cells (MLEC) to ZnCl(2) or the NO donor, S-Nitroso-N-acetylpenicillamine (SNAP, 200 microM), caused nuclear translocation of a reporter molecule consisting of enhanced green fluorescent protein (EGFP) fused to MTF-1 (pEGFP-MTF-1). In separate experiments, NO donors induced increases in MT protein levels (Western blot). In contrast, NO did not cause nuclear translocation of EGFP-MTF-1 in MLEC from MT knockouts, demonstrating a central role for MT in mediating this response. These data suggest that S-nitrosation of Zn-thiolate clusters in MT and subsequent alterations in Zn homeostasis are participants in intracellular NO signaling pathways affecting gene expression.

Nitric oxide and zinc homeostasis in acute lung injury

Among putative small molecules that affect sensitivity to acute lung injury, zinc and nitric oxide are potentially unique by virtue of their interdependence and dual capacities to be cytoprotective or injurious. Nitric oxide and zinc appear to be linked via an intracellular signaling pathway involving S-nitrosation of metallothoinein--itself a small protein known to be an important inducible gene product that may modify lung injury. In the present article, we summarize recent efforts using genetic and fluorescence optical imaging techniques to: (1) demonstrate that S-nitrosation of metallothionein affects intracellular zinc homeostasis in intact pulmonary endothelial cells; and (2) reveal a protective role for this pathway in hyperoxic and LPS-induced injury.

Catfish egg lectin causes rapid activation of multidrug resistance 1 P-glycoprotein as a lipid translocase

Rhamnose-binding lectin from catfish (Silurus asotus) eggs (SAL) has the ability to induce externalization of phosphatidylserine (PS), followed by cell shrinkage in globotriaosylceramide (Gb3)-expressing Burkitt's lymphoma Raji cells. Because phospholipid scramblase and aminophospholipid translocase did not participate in SAL-induced PS externalization, we examined the relationship of ATP-binding cassette (ABC) transporters, such as multidrug resistance (MDR) 1 P-glycoprotein (MDR1 P-gp) and MDR-associated protein 1 (MRP1), for translocation of PS. Since cyclosporin A (MDR1 P-gp inhibitor) but not MK571 (MRP1 inhibitor) inhibited SAL-induced PS externalization, it was suggested that MDR1 P-gp is involved in this phenomenon. On the other hand, SAL activated both of the ABC transporters for efflux of rhodamine123 (MDR1 P-gp substrate, Rho123) and 5-carboxyfluorescein diacetate (MRP1 substrate, 5-CFDA) in Raji cells. In contrast, SAL did not activate these two transporters in Gb3-negative cell lines, such as K562 and doxorubicin-resistant K562 cells, involving not only PS externalization but also efflux of Rho123 or 5-CFDA. Since Gb3 and both transporters in Raji cells are located in the glycosphingolipid-enriched microdomain (GEM), it is suggested that the binding of SAL to Gb3 localized in the GEM specifically induces MDR1 P-gp activation in Raji cells.

Long-term cadmium exposure accelerates age-related mitochondrial changes in renal epithelial cells

Long-term cadmium exposure leads to mitochondrial dysfunction in the proximal tubular epithelial cells. Mitochondrial DNA deletion may contribute to the pathogenesis of cadmium-induced nephropathy. The aim of our study is to clarify the accumulation of mitochondrial DNA deletion and mitochondrial dysfunction in the renal cortex of rats injected three times/week with 1 ml of 1 mM CdCl2 or saline for 80 weeks. After 40-week cadmium injection, mitochondrial number diminished, and cadmium in the renal cortex reached a saturation level. At this time interval, nearly 30% of cadmium in the whole cell fraction was found in the mitochondria. Cytochrome c oxidase (COX) activity in the proximal tubular epithelial cells decreased after 40-week exposure of cadmium. Oxidized phosphatidylcholine (oxPC) started to accumulate in the cytochrome c-positive mitochondria in some tubular epithelial cells after 80-week exposure. After 40 weeks, accumulation of the 4834-bp deletion in mitochondrial DNA was evident in both control and cadmium-treated groups. However, the amount of accumulated mitochondrial DNA deletion tended to increase after 40-week exposure, and was significantly greater after 80 weeks of exposure, compared to the control. Our results indicate that long-term cadmium exposure in rats accelerates accumulation of 4834-bp mitochondrial DNA deletions and impairment of mitochondrial function associated with accumulation of oxidized product.

Nitric oxide-induced modification of protein thiolate clusters as determined by spectral fluorescence resonance energy transfer in live endothelial cells

Low-molecular-weight S-nitrosothiols are found in many tissues and affect a diverse array of signaling pathways via decomposition to *NO or exchange of their -NO function with thiol-containing proteins (transnitrosation). We used spectral laser scanning confocal imaging to visualize the effects of D- and L-stereoisomers of S-nitrosocysteine ethyl ester (SNCEE) on fluorescence resonance energy transfer (FRET)-based reporters that are targets for the following NO-related modifications: (a) S-nitrosation, via the cysteine-rich protein metallothionein (FRET-MT), and (b) nitrosyl-heme-Fe, via guanosine 3',5'-cyclic monophosphate (cygnet-2). Conformational changes consistent with S-nitrosation of FRET-MT were specific to l-SNCEE. In addition, they were reversed by dithiothreitol (DTT) but unaffected by exogenous oxyhemoglobin. In contrast, d- and l-SNCEE had comparable effects on cygnet-2, likely via activation of soluble guanylyl cyclase (sGC) by *NO as they were sensitive to the sGC inhibitor 1H-[1,2,4]-oxadiazolo[4,3-alpha] quinoxalin-1-one and exogenous oxyhemoglobin. These data demonstrate the utility of spectral laser scanning confocal imaging in revealing subtle aspects of NO signal transduction in live cells. Stereoselective transnitrosation of MT emphasizes the specificity of posttranslational modification as a component of NO signaling.

Regulation of zinc homeostasis by inducible NO synthase-derived NO: nuclear metallothionein translocation and intranuclear Zn2+ release

Zn2+ is critical for the functional and structural integrity of cells and contributes to a number of important processes including gene expression. It has been shown that NO exogenously applied via NO donors resulting in nitrosative stress leads to cytoplasmic Zn2+ release from the zinc storing protein metallothionein (MT) and probably other proteins that complex Zn2+ via cysteine thiols. We show here that, in cytokine-activated murine aortic endothelial cells, NO derived from the inducible NO synthase (iNOS) induces a transient nuclear release of Zn2+. This nuclear Zn2+ release depends on the presence of MT as shown by the lack of this effect in activated endothelial cells from MT-deficient mice and temporally correlates with nuclear MT translocation. Data also show that NO is an essential but not sufficient signal for MT-mediated Zn2+ trafficking from the cytoplasm into the nucleus. In addition, we found that, endogenously via iNOS, synthesized NO increases the constitutive mRNA expression of both MT-1 and MT-2 genes and that nitrosative stress exogenously applied via an NO donor increases constitutive MT mRNA expression via intracellular Zn2+ release. In conclusion, we here provide evidence for a signaling mechanism based on iNOS-derived NO through the regulation of intracellular Zn2+ trafficking and homeostasis.

Quantification of oxidative/nitrosative modification of CYS(34) in human serum albumin using a fluorescence-based SDS-PAGE assay

The SH group represented by cysteine in proteins is fundamental to the redox regulation of protein structure and function. Albumin is the most abundant serum protein whose redox modification modulates its physiologic function, as well as serves as a biomarker of oxidative stress. Measurement of selective Cys modification (S-oxidation/nitrosation, electrophilic substitution) on specific proteins, however, is problematic within complex biological mixtures such as plasma. We have utilized a maleimide fluorogenic SH reagent, ThioGlo-1, to develop a fluorescence-based quantitative assay of SH modification of human serum albumin (hSA) using SDS-PAGE. Fully reduced native albumin containing one free SH (Cys(34)) per molecule was utilized as a model protein to characterize the kinetics of ThioGlo-1 reaction using a solution-based spectrofluorometric assay. Optimum labeling of hSA Cys(34) was achieved within 10 min at 60 degrees C using a threefold molar excess of ThioGlo-1 relative to hSA and required SDS. Comparison of the solution spectrofluorometric assay to fluorescent image analysis of hSA bands localized by SDS-PAGE revealed that SH groups in hSA could be quantified after gel electrophoresis. The solution- and gel-based methods were in excellent concordance in their ability to quantify SH modification of hSA following exposure to phenoxyl radicals and nitric oxide. The application of ThioGlo-1 staining and SDS-PAGE quantified the degree of hSA modification in complex human plasma exposed to oxidative or nitrosative stress and revealed that hSA is more sensitive to S modification than other SH-containing plasma proteins.

Redox sensor function of metallothioneins

In summary, the redox conversions of MT cysteines are likely to be the principal mechanisms for regulation of metal binding and release by this protein. Oxidative and/or nitrosative challenges can serve to promote metal ion release from MT to render their delivery to specific target proteins. It is tempting to consider the potential roles of MTs as redox sensors because of their high sensitivity to cysteine modification, as well as their potential to amplify signals by releasing multiple metal ions. In other words, MTs may act early in a biological signaling cascade that triggers metal-dependent biochemical and cellular responses. Alternatively, uncontrolled release of metals by excessive oxidative stress may contribute to metal toxicity. Because oxidative and nitrosative signaling is ubiquitous within cells, the physiological function of MT demands that efficient recycling of modified cysteines be operative. Little is known regarding the potential mechanisms for the regeneration of MT after oxidative/nitrosative modification, but they may involve endogenous dithiols, such as thioredoxin, and pharmacologically relevant dithiols, such as dihydrolipoate.

Chronic exposure of HepG2 cells to excess copper results in depletion of glutathione and induction of metallothionein

Metallothionein (MT) and reduced glutathione (GSH) play an important role in the intracellular handling of copper by preventing the generation and favouring the removal of copper-derived free radicals. The present study addressed the changes in MT and GSH that follow chronic (2 or 5 weeks) exposure of human hepatoblastoma cells (HepG2) to excess copper. Copper treatment (100 microM, 2 weeks) led to a 28-fold elevation in intracellular copper. Concomitantly, cells exhibited a seven-fold increase in total MT and an increment in its saturation with copper from 45 to 86%. Around 38% of copper in the cytosolic fraction could be accounted for by MT. GSH equivalents were substantially lowered (to 37% of basal levels) in treated cells, with only part of it being accounted for by an increase in GSSG. Copper-treatment induced no changes in catalase or GSH-peroxidase activities but it was associated with a small reduction in SOD (20%) and GSH-reductase (26%) activities. Copper-loaded cells did not differ from controls in their basal oxidative tone; however, when exposed to tert-butylhydroperoxide they exhibited a markedly greater susceptibility to undergo both oxidative stress and cell lysis. It is proposed that chronic exposure of HepG2 cells to excess copper is accompanied by "adaptive changes" in GSH and MT metabolism that would render cells substantially more susceptibility to undergo oxidative stress-related cytotoxicity.