Involvement of cellular sulfhydryl compounds in the inhibition of RNA synthesis by selenite

Age-dependent assessment of selenium nanoparticles: biodistribution and toxicity study in young and adult rats

Aim: To study the biodistribution and toxicology of selenium nanoparticles (SeNPs) versus their bulk counterpart in young and adult male rats in a 28-day study. Methods: SeNPs were synthesized and conjugated with indocyanine green to assess comparative biodistribution by in vivo imaging and further characterized by transmission electron microscopy, Fourier transform infrared, scanning electron microscopy/energy dispersive x-ray spectroscopy, UV and ζ-analysis. The toxicity of bulk selenium was evaluated relative to its nano form by hematology indices, redox, inflammatory markers and histopathology. Results: Indocyanine green-conjugated nanoparticles showed preferential accumulation in the liver, followed by testis and kidney. The protective effect of SeNPs was more significantly observed in young livers than in adults compared with the bulk counterpart. Conclusion: Age-dependent monitoring and diagnosis of toxicity may need different biomarkers of selenium and may also provide better understanding of SeNPs as therapeutics.

Conserved structural elements in glutathione transferase homologues encoded in the genome ofEscherichia coli

Multiple sequence alignments of the eight glutathione (GSH) transferase homologues encoded in the genome of Escherichia coli were used to define a consensus sequence for the proteins. The consensus sequence was analyzed in the context of the three-dimensional structure of the gst gene product (EGST) obtained from two different crystal forms of the enzyme. The enzyme consists of two domains. The N-terminal region (domain I) has a thioredoxin-like alpha/beta-fold, while the C-terminal domain (domain II) is all alpha-helical. The majority of the consensus residues (12/17) reside in the N-terminal domain. Fifteen of the 17 residues are involved in hydrophobic core interactions, turns, or electrostatic interactions between the two domains. The results suggest that all of the homologues retain a well-defined group of structural elements both in and between the N-terminal alpha/beta domain and the C-terminal domain. The conservation of two key residues for the recognition motif for the gamma-glutamyl-portion of GSH indicates that the homologues may interact with GSH or GSH analogues such as glutathionylspermidine or alpha-amino acids. The genome context of two of the homologues forms the basis for a hypothesis that the b2989 and yibF gene products are involved in glutathionylspermidine and selenium biochemistry, respectively.

Dietary Selenium Intake Modulates Thyroid Hormone and Energy Metabolism in Men

Most studies of selenium and thyroid hormone have used sodium selenite in rats. However, rats regulate thyroid hormone differently, and selenite, which has unique pharmacologic activities, does not occur in foods. We hypothesized that selenium in food would have different effects in humans. Healthy men were fed foods naturally high or low in selenium for 120 d while confined to a metabolic research unit. Selenium intake for all subjects was 47 microg/d (595 nmol/d) for the first 21 d, and then changed to either 14 (n = 6) or 297 (n = 5) microg/d (177 nmol/d or 3.8 micromol/d) for the remaining 99 d, causing significant changes in blood selenium and glutathione peroxidase. Serum 3,3',5-triiodothyronine (T3) decreased in the high selenium group, increased in the low selenium group, and was significantly different between groups from d 45 onward. A compensatory increase of thyrotropin occurred in the high selenium group as T3 decreased. The changes in T3 were opposite in direction to those reported in rats, but were consistent with other metabolic changes. By d 64, the high selenium group started to gain weight, whereas the low selenium group began to lose weight, and the weight changes were significantly different between groups from d 92 onward. Decreases of serum T3 and compensatory increases in thyrotropin suggest that a subclinical hypothyroid response was induced in the high selenium group, leading to body weight increases. Increases of serum T3 and serum triacylglycerol accompanied by losses of body fat suggest that a subclinical hyperthyroid response was induced in the low selenium group, leading to body weight decreases.

Sensitivity of melphalan-resistant tumors to selenite in vivo

Previous studies have demonstrated that melphalan-resistant human ovarian tumor cells exhibit a higher degree of sensitivity to the cytotoxic effects of selenite in vitro than comparable drug-sensitive cells (P.B. Caffrey, G.D. Frenkel, Selenite cytotoxicity in drug resistant and non-resistant human ovarian tumor cells, Cancer Res. 52 (1992) 4812-4816; P.B. Caffrey, G.D. Frenkel, The development of drug resistance by tumor cells in vitro is accompanied by the development of sensitivity to selenite, Cancer Lett. 81 (1994) 59-65). We have now examined the sensitivity of drug-resistant tumors to selenite in vivo. A2780 human ovarian tumor cells, or their melphalan-resistant derivative (A2780ME) cells were injected subcutaneously into nude mice and the resulting tumors were found to be melphalan-sensitive and -resistant, respectively, in vivo. Treatment with selenite (2 mg/kg Se s.c.), which had no overt toxic effect on the animals, resulted in a significant decrease in the rate of growth of the melphalan-resistant tumors, but not on the rate of growth of the drug-sensitive tumors. Thus, melphalan-resistant ovarian tumors are also more sensitive to selenite treatment in vivo.

Selenocompounds induce a redox modulation of protein kinase C in the cell, compartmentally independent from cytosolic glutathione: its role in inhibition of tumor promotion

Since selenite and other redox-active selenocompounds can modify protein kinase C (PKC) in the test tube, we have determined whether or not this redox regulation occurs inside the cell despite having high concentrations of GSH and the role of this regulation in the inhibition of tumor promotion. By using phorbol ester-promoted JB6 epidermal cell transformation assay, the concentrations of selenite, selenocystine, and selenodiglutathione which are optimal for chemopreventive activity were determined. At such concentrations (0.5 to 2 microM) in the cells treated with these agents, only a slight but transient decrease in PKC activity was observed when measured with a low (5 microM), but not with a high (100 microM) concentration of ATP. However, when the cells were serum starved or pretreated with 2-deoxyglucose, there was a pronounced but transient inactivation of PKC when assayed with both low and high concentrations of ATP. The inactivation was reversed in the cell by an endogenous mechanism or by treatment with thiol agents in the test tube. In spite of a substantial (90%) depletion of GSH in the cells by pretreatment with buthionine sulfoximine, there was no further increase in the redox modification of PKC by selenite as well as no change in the inhibitory effect of selenite on the phorbol ester-stimulated induction of ornithine decarboxylase, which is an intermediate marker related to cell transformation. While GSH is known to influence certain actions of selenium, it may not be required to mediate the effects of selenite tested in this study. The water-soluble cytosolic GSH did not interfere with the redox modification of PKC probably due to the shielding of the cysteine-rich region of the enzyme by a weak hydrophobic association with the membrane. Due to the presence of cofactors in the crude cell extracts, PKC was more sensitive to selenite than in the purified form and was inactivated by low concentrations of selenite (IC50 = 0.05 microM). This modification was reversed by thiol agents as well as by NADPH. A protein disulfide reductase, which can regenerate PKC, was present in the homogenate. Conceivably, selenite and other selenocompounds induce a redox modification of cellular PKC, compartmentally independent from the cytosolic GSH, but intimately connected to a NADPH-dependent reductase system, to mediate, at least in part, some of the cancer-preventive actions.

Cancer-preventive selenocompounds induce a specific redox modification of cysteine-rich regions in Ca(2+)-dependent isoenzymes of protein kinase C

Since protein kinase C (PKC) serves as a receptor for phorbol ester type tumor promoters and oxidants and has unique redox-active cysteine-rich regions, we have determined whether various chemopreventive selenocompounds could affect this enzyme. At lower concentrations, selenite decreased the kinase activity (IC50 = 0.5 microM), while at higher concentrations it decreased phorbol ester binding. However, when the catalytic and regulatory domains of PKC were separated by proteolysis, the catalytic domain retained its sensitivity to selenite, while the regulatory domain lost its sensitivity. Cysteine residues were quantitated in PKC modified with selenite by using 5,5'-dithiobis(2-nitrobenzoic acid) and also by using 2-nitro-5-thiosulfobenzoic acid after sulfitolysis. At lower concentrations, selenite induced a modification of four cysteine residues resulting in the formation of two disulfides, while at higher concentrations it induced a modification of seven to eight cysteine residues resulting in the formation of three to four disulfides. Contrary to selenite, selenocystine and selenodiglutathione (GSSeSG) readily inactivated the kinase activity, but not the phorbol ester binding. These two agents induced a two-stage modification of PKC; a limited modification at low concentrations leads to a loss of affinity for ATP, while an excessive modification at high concentrations leads to a loss of Vmax. Selenocystine and GSSeSG were 100,000-fold more potent than GSSG in inactivating PKC. The isoenzymes alpha, beta, and gamma exhibited an identical susceptibility to these selenocompounds. These results suggested that the cysteine residues present within the catalytic domain of these isoenzymes, although apart in the sequence, may be clustered in the tertiary structure to react with selenite, as well as may be in close proximity to some of the cysteines in the regulatory domain. Selenite did not affect protein kinase A, whereas GSSeSG and selenocystine inactivated the catalytic subunit after dissociation from the regulatory subunit at concentrations 100- and 800-fold, respectively, higher than that required for PKC inactivation. All three selenocompounds did not affect the activities of phosphorylase kinase and protein phosphatase 2A. Taken together, these results suggest that the accessible redox-active cysteine residues present in the PKC catalytic domain can react with certain specificity with redox-active selenocompounds such as selenite, selenocystine, and GSSeSG relative to other protein kinases tested.

Time-course of inhibition of cellular nucleic acid synthesis by selenite

The relationship between intracellular sulfhydryl(SH) compounds and the kinetics of the inhibitory effect of selenite on cellular nucleic acid synthesis has been examined. In A549 cells, with a relatively high SH level, exposure to low concentrations of selenite caused inhibition even after short exposure times. In contrast, in VA cells, with a relatively low level of SH compounds, selenite had no significant effect at short exposure times, but inhibited significantly with longer exposures. Selenodicysteine, the product of the reaction of selenite with cysteine (an important intracellular SH compound), inhibited synthesis in both cell types at short exposure times. Exposure of cells to diethylmaleate, which decreased the level of intracellular SH compounds, reduced the inhibitory effect of a short exposure to selenite but did not affect a long exposure. These results indicate that the reaction of selenite with intracellular SH compounds may be a determining factor in the kinetics of its inhibitory effect on cellular DNA and RNA synthesis.

The development of drug resistance by tumor cells in vitro is accompanied by the development of sensitivity to selenite

The effects of selenite on cell viability and proliferation in a line of drug-sensitive human ovarian tumor (A2780) cells were compared with its effects on a melphalan-resistant derivative of these cells (A2780-ME) which had been developed in vitro (Hamilton et al. (1985) Biochemical Pharmacol., 34, 2583-2586). With the A2780-ME cells there was a 50% decrease in the number of viable cells (i.e. which exclude Trypan Blue dye) after exposure to less than 100 microM selenite for 6 h. In contrast, exposure to more than 300 microM selenite was required to achieve the same effect in the parent line. Similarly, exposure to 10 microM selenite resulted in a 50% decrease in A2780-ME cell proliferation, whereas this treatment had only a small inhibitory effect on proliferation of the parent cells. Thus, the development of melphalan resistance in vitro was accompanied by the development of selenite sensitivity. Pre-exposure of the two cell types to buthionine sulfoximine eliminated the difference in their intracellular glutathione levels, as well as most of their differential sensitivity to selenite. Furthermore, the two cell types did not exhibit a difference in sensitivity to selenodiglutathione, the product of the reaction of selenite with glutathione. Thus, the increase in intracellular glutathione, which has been shown to be responsible for the development of drug resistance in these cells is also responsible for the development of selenite sensitivity.

Protein synthesis is not required for the inhibitory effect of selenite on cell colony formation and RNA synthesis

Selenite has been shown to undergo intracellular metabolism that results in its conversion to other low molecular weight Se-containing species and also to its incorporation into a selenocysteine residue in selenoprotein. In order to investigate whether the incorporation into protein is required for the cytotoxic effects of selenite, we have examined whether inhibition of protein synthesis prevents the inhibitory effect of selenite on the ability of cells to form colonies or to synthesize RNA. We have found that treatment of HeLa cells with cycloheximide inhibited protein synthesis by > 90% but had no effect on the inhibitory effect of selenite on cell colony formation or RNA synthesis. Since protein synthesis is not necessary for these cytotoxic effects of selenite they are unlikely to result from an increase in the synthesis of selenoproteins.

Glutathione mercaptides as transport forms of metals

Among the many cellular functions of GSH, the roles of this tripeptide in metal transport, storage, and metabolism have recently received considerable attention. Although these roles had often been overlooked, they are critical for normal cellular metabolism and for protection from xenobiotics. Indeed, a number of the protective and regulatory functions of GSH are related to its ability to chelate reactive metals. GSH functions in the mobilization and delivery of metals between ligands, in the transport of metals across cell membranes, as a source of cysteine for metal binding, and as a reductant or cofactor in redox reactions involving metals. However, the interaction between GSH and metals can also produce or exacerbate cell injury. For example, GSH appears to be involved in the renal accumulation and toxicity of a number of metals, and in the carcinogenicity of chromium. Additional work is clearly needed to identify the mechanisms involved, and to better define the roles of GSH in metal homeostasis.

Effect of cell density on the inhibition of tumor cell attachment and nucleic acid synthesis by selenite

The effect of cell density on the sensitivity of tumor cells to selenite has been examined. The inhibitory effect of selenite on cellular DNA and RNA synthesis was significantly greater in higher density cultures of HeLa cells and A2780 ovarian tumor cells. High-density cells were also more sensitive to the inhibitory effect of selenite on cell attachment. This difference could not be accounted for by a higher intracellular level of glutathione, since there was no significant difference between the cells at high or low density. The high-density cells were found to take up more selenium per cell during the exposure period; the resulting higher level of intracellular Se could explain their greater sensitivity to selenite. This hypothesis is supported by the observation that DNA synthesis in nuclei isolated from high-density cells did not exhibit higher sensitivity to inhibition by selenite than synthesis in nuclei isolated from low-density cells.

Effects of diheptyldiselenide (DDS) on human tumor cell lines and on peripheral blood mononuclear cells

In vitro effects of graded concentrations of diheptyldiselenide (DDS) on human tumor cell proliferation, and on the proliferative responses and immunological functions of peripheral blood mononuclear cells (MNC) were investigated. The agent significantly decreased tumor cell proliferation in a dose and time dependent manner. Proliferative responses of MNC to phytohemagglutinin (PHA) and interleukin-2 (IL-2) were also significantly depressed when MNCs were exposed to DDS (250 microM for 18 h) led to a significant increase in NK activity only in MNC samples showing very limited baseline NK function. On the other hand, generation of LAK cells was significantly inhibited by DDS. However, when the agent was added to the effector and target cell mixture during the 4 h 51Cr release cytotoxicity assay, no influence was found on NK and LAK-mediated target cell lysis. These studies show that high concentrations of DDS inhibit tumor cell proliferation and could also impair certain proliferative-dependent immune functions, without directly affecting cell-mediated cytolytic activity of effector cells.

Selenite-induced inhibition of colony formation by buthionine sulfoximine-sensitive and resistant cell lines

We previously demonstrated that treatment of HeLa cells with buthionine sulfoximine (BSO), which decreases the level of cellular glutathione, resulted in a decrease in the potency of selenite in inhibiting cell colony formation. We have now examined the effect of selenite on normal human lung fibroblast (CCL-210) cells, which resemble HeLa cells in their sensitivity to BSO, and on human lung adenocarcinoma (A549) cells, which are relatively insensitive to BSO. We have found that BSO treatment caused an approximately fourfold decrease in selenite potency in the CCL-210 cells, but had no significant effect on its potency in A549 cells. These results support the hypothesis that for selenite to exert its cytotoxic effect, it must undergo the reaction with an SH compound to form the selenotrisulfide. As a result of the lower sensitivity of the tumor cells to BSO, it was possible to achieve a large differential sensitivity to the cytotoxic effect of selenite.

Inhibition by selenium of DNA and RNA synthesis in normal and malignant human cells in vitro

Several studies have demonstrated differences between normal and malignant cells in their sensitivity to various effects of selenite. We have compared the effect of selenite on DNA and RNA synthesis in two pairs of normal and malignant human cell lines. One pair of cells, CCL-210 (normal lung fibroblasts) and A549 (lung adenocarcinoma cells), exhibited a large difference in their sensitivity to selenite but no significant difference in their sensitivity to selenodiglutathione. They also had a large difference in the level of intracellular sulfhydryl (SH) compounds. In contrast the other pair of cells, WI-38 (normal fetal lung fibroblasts) and WI-38VA (SV-40 transformed WI-38 cells) both had low levels of intracellular SH compounds and exhibited similar (low) sensitivity to selenite. Our results indicate that differences between normal and malignant cells in their sensitivity to selenite could be due to a difference in the reaction of selenite with intracellular sulfhydryl compounds to form selenotrisulfides.

Temporal changes in tissue glutathione in response to chemical form, dose, and duration of selenium treatment. Relevance to cancer chemoprevention by selenium

Selenium has been reported to affect glutathione (GSH) concentrations in short-term animal-feeding experiments. Given the central role that this tripeptide plays in maintaining cellular homeostasis, it was hypothesized that perturbations in glutathione metabolism induced by selenium might account for its cancer chemopreventive activity. In the present study, four experiments were conducted in which the effect of acute, short-, or long-term exposure to selenium was assessed. Selenium was provided as either sodium selenite or D,L-selenomethionine. Selenite was observed to induce a biphasic response in total liver GSH. Injected selenium caused an acute reduction in GSH, whereas short-term feeding (up to 8 wk) increased both total GSH and oxidized glutathione (GSSH), an effect that gradually diminished in magnitude with prolonged feeding. Our data suggest that such changes are unlikely to account for the chemopreventive activity of selenium for the following reasons: Perturbations in glutathione metabolism occurred only at doses of selenite that approached toxicity. These doses are higher than what would be required for producing cancer chemoprevention. The transient nature of these changes also contrasts with the need for a continuous supplementation of selenite in suppression of tumorigenesis. Furthermore, selenomethionine was found to have little activity in altering glutathione metabolism, even though it compares favorably with selenite as a cancer chemopreventive agent. Nonetheless, these findings do not discount the possibility that sulfhydryl compounds, such as glutathione, might be used to modify the toxicity and/or enhance the cancer prophylactic activity of selenium compounds.

Effect of selenium compounds and thiols on human mammary tumor cells

The effect on cell viability and growth rate of sodium selenite, selenocystine, sodium selenate, and selenomethionine at selenium concentrations of 6.25 and 12.5 uM was studied in vitro on cells of the human mammary tumor cell line HTB123/DU4475. Selenite and selenocystine affected both cell viability and growth rate of the tumor cells at these selenium concentrations. Selenite and selenocystine decreased intracellular glutathione concentrations, but did not affect tumor cell glutathione peroxidase activity. After six days of exposure to either selenate or selenomethionine, the viability of tumor cells remained stable, but cell growth, as measured by numbers of cells, was retarded. Neither selenate nor selenomethionine produced changes in concentrations of intracellular glutathione. The toxic effect of selenite on tumor cells was enhanced by addition of 0.25 mM glutathione to the growth medium. Preincubation of the tumor cells with 62.5 uM buthionine sulfoximine decreased cellular glutathione to 15% of controls at 24 h and enhanced the toxicity of selenite toward the tumor cells. Glutathione, 2-mercaptoethanol, and L-cysteine were all toxic to the tumor cells in a dose-dependent manner.

Products of the reaction of selenite with intracellular sulfhydryl compounds

The usual first step in the intracellular metabolism of exogenous selenite is its chemical reaction with glutathione to form selenodiglutathione (1). We have investigated whether selenite also reacts intracellularly with other SH compounds. HeLa cells were exposed to [75Se]selenite and lysed with SDS. Cellular proteins and nucleic acids were precipitated with trichloroacetic acid, and the acid-soluble fraction was analyzed by ion-exchange thin-layer chromatography (ion-exchange TLC) and autoradiography. In control cells, the major [75Se]-containing species detected can be identified by its mobility as selenodiglutathione. Two other species were detected, which can be identified as selenodimercaptoethylamine and the mixed selenotrisulfide of mercaptoethylamine and glutathione. In contrast, in cells that were depleted of glutathione (by treatment with buthionine sulfoximine), very little, if any, selenodiglutathione was detected. However, new [75Se]-containing species were detected, which can be identified as selenodicysteine and the mixed selenotrisulfide of cysteine and glutathione. The same species were detected when [75Se]selenite was added to the acid-soluble fraction of a cell extract (as opposed to living cells), confirming that these compounds can be formed by nonenzymatic reactions.