Demonstration of nuclear compartmentalization of glutathione in hepatocytes

Cocaine-induced liver injury in mice elicits specific changes in DNA ploidy and induces programmed death of hepatocytes

Liver injury was induced by a single dose (60 mg/kg) of cocaine in male albino Swiss mice untreated or pretreated with phenobarbital (in drinking water 1 gm/L), for 5 days before cocaine administration. One parameter of liver injury, serum isocitrate dehydrogenase activity, showed sharp increases at 24 hr of cocaine treatment; we also noted decrease hepatic levels of ATP, GSH, cytochrome P-450 and NADPH/NADP+ ratio and increases in malondialdehyde concentration. Histopathological study of liver slices showed perivenous and periportal necrosis induced by cocaine in untreated mice and mice pretreated with phenobarbital, respectively. A regenerative postnecrotic response, which peaked at 48 hr, was demonstrated by the appearance of mitotic cells. Mitotic index analysis showed that proliferative cells appear to be unevenly distributed in the hepatic acinus and were mainly located in the vicinity of the damaged acinar region. Genomic DNA ploidy and the distribution of DNA in the phases of the cell cycle were studied in nuclei of isolated hepatocytes. At 12 hr of cocaine administration, both in untreated and phenobarbital-pretreated mice, the following changes were observed: a sharp decrease in tetraploid (4N) cells (40% to 17% and 25% to 6%, respectively) and octoploid (8N) cells (5% to 2% and 2% to 1%, respectively), together with the appearance of a hypodiploid population (13% and 31%, respectively). Hypodiploid population was characterized as apoptotic cells by detection of DNA fragmentation in agarose gel. These results suggest that a significant percentage of cell death induced by cocaine occurs by means of the apoptosis death program. Comparison of the initial values of DNA ploidy with those obtained at 7 days of cocaine administration showed remarkable increases in polyploid populations (4N and 8N) and a decrease in diploid cells (2N), indicating that the process of differentiation occurs when liver restores its functionality.

Oxidative metal release from metallothionein via zinc-thiol/disulfide interchange

Mammalian metallothionein has been postulated to play a pivotal role in cellular zinc distribution. All seven of its metal atoms are bound with high thermodynamic stability in two clusters buried deeply in the molecule. If the protein is to function in metal delivery, there must be a biological mechanism to facilitate metal release. One means to achieve this would be a labilization of the clusters by interaction of metallothionein with an appropriate cellular ligand. To search for such a mediator, we have designed a rapid radiochromatographic method that can detect changes in the zinc content of 65Zn-labeled metallothionein in response to other biomolecules. Using this methodology, we have established that rabbit liver metallothionein 2 interacts with glutathione disulfide with concomitant release of zinc. Under conditions of pseudo-first-order kinetics, the monophasic reaction depends linearly on the concentration of glutathione disulfide in the range from 5 to 30 mM with a second-order rate constant k = 4.9 x 10(-3)s-1.M-1 (pH 8.6; 25 degrees C). Apparently, zinc release does not involve direct access of glutathione disulfide to the inner coordination sphere of the metals. Rather it appears that the solvent-accessible zinc-bound thiolates in two clefts of each domain of metallothionein [Robbins, A. H., McRee, D. E., Williamson, M., Collett, S. A., Xuong, N. H., Furey, W. F., Wang, B. C. & Stout, C. D. (1991) J. Mol. Biol. 221, 1269-1293] participate in a thiol/disulfide interchange with glutathione disulfide. This rate-limiting initial S-thiolation, which occurs with indistinguishable rates in both clusters, then causes the clusters to collapse and release their zinc. Such a mechanism of metal release would link the control of the metal content of metallothionein to the cellular glutathione redox status and raises important questions about the physiological implications of this observation with regard to a role of glutathione in zinc metabolism and in making zinc available for other biomolecules.

Distribution of the monochlorobimane-glutathione conjugate between nucleus and cytosol in isolated hepatocytes

The nuclear/cytoplasmic distributions of the GSH and N-acetylcysteine conjugates of monochlorobimane (MCB) have been studied in rat hepatocytes in primary culture after microinjection into the cytoplasm. The immediately occurring fluorescence (within 1-2 s) in the nucleus was much greater than that in the cytoplasm. Other water-soluble fluorescent markers such as Cascade Blue, 6-carboxyfluorescein and Lucifer Yellow also exhibited a similar nuclear localization. MCB, which is poorly soluble in water, gave similar results whether exposed to the exterior of the cell or microinjected as a solution in ethanol. It is concluded that the fluorophores studied here diffuse into the nucleus and accumulate there, including MCB-GSH formed intracellularly from MCB and GSH. Therefore MCB may be of limited value in estimating concentrations of GSH in the nucleus and the cytoplasm, as recently suggested by Bellomo, Vairetti, Stivala, Mirabelli, Richelmi and Orrenius [(1992) Proc. Natl. Acad. Sci. U.S.A. 89, 4412-4416].

Biochemical manipulation of intracellular glutathione levels influences cytotoxicity to isolated human lymphocytes by sulfur mustard

Glutathione (GSH) is the major nonprotein thiol that can protect cells from damage due to electrophilic alkylating agents by forming conjugates with the agent. Sulfur mustard (HD) is an electrophilic alkylating agent that has potent mutagenic, carcinogenic, cytotoxic, and vesicant properties. Compounds that elevate or reduce intracellular levels of GSH may produce changes in cytotoxicity induced by sulfur mustard. Pretreatment of human peripheral blood lymphocytes (PBL) for 72 hr with 1 mM buthionine sulfoximine (BSO), which reduces intracellular GSH content to approximately 26% of control, appears to sensitize these in vitro cells to the cytotoxic effects of 10 microM HD but not to higher HD concentrations. Pretreatment of PBL for 48 hr with 10 mM N-acetyl cysteine (NAC), which elevates intracellular glutathione levels to 122% of control, appears to partially protect these in vitro cells from the cytotoxic effects of 10 microM HD but not to higher HD concentrations. Augmentation of intracellular levels of glutathione may provide partial protection against cytotoxicity of sulfur mustard.

Antioxidants inhibit stimulation of HIV transcription

In studies presented here, we demonstrate that antioxidants regulate NF-kappa B activation and signal transduction pathways leading to HIV expression. We show (1) that N-acetyl-L-cysteine (NAC), an antioxidant and an efficient glutathione (GSH) precursor, inhibits NF-kappa B activation and HIV expression under conditions in which GSH is depleted and NAC cannot be converted to GSH, (2) that the D-stereoisomer of NAC and a wide variety of chemically unrelated antioxidants also inhibit NF-kappa B activation and/or transcription directed by the HIV LTR, and (3) that depletion of GSH, the principal intracellular antioxidant, augments HIV production in an acute infection model. Taken together, these findings suggest direct antioxidant action as the mechanism for inhibition of HIV transcription by NAC. They also confirm that GSH, acting in its capacity as an antioxidant, regulates HIV expression and that exogenous antioxidants can potentiate this regulation.

Metallothionein protects DNA from oxidative damage

Metallothionein (MT) is a potent hydroxyl radical scavenger but its antioxidant properties in vivo have not been defined. Most of the recent results indicate that it does not afford protection to cells against the lethal action of oxidative stress. However, the possibility that MT confers protection against oxidative damage to a specific cellular target, such as DNA, had not been considered. We compared V79 Chinese hamster cells enriched in and depleted of MT in terms of DNA-strand scission. Zinc induces an increase in MT content of V79 Chinese hamster cells, without concomitant increase in the GSH level. These induced cells are more resistant to the production of DNA-strand scission by H2O2 than the parental cells. Conversely, cells rendered partially deprived of MT, by transfection with a plasmid vector in which the MT-I cDNA is antisense oriented in relation to a simian virus 40 promoter, became more susceptible to the DNA-damaging action of H2O2. The transfected cells did not exhibit alterations of GSH, superoxide dismutase- and H2O2-destroying enzymes. Indirect immunofluorescence indicated that most of the MT was concentrated in the cell nucleus. Neither overexpression nor lower expression of MT resulted in differential resistance to the killing action of H2O2. However, the combined high nuclear concentration of MT and its excellent hydroxyl scavenger properties confer protection to DNA from hydroxyl radical attack.