Significance of selenium-labeled proteins for selenium's chemopreventive functions

Human selenium binding protein-1 (hSP56) is a negative regulator of HIF-1α and suppresses the malignant characteristics of prostate cancer cells

In the present study, we demonstrate that ectopic expression of 56-kDa human selenium binding protein-1 (hSP56) in PC-3 cells that do not normally express hSP56 results in a marked inhibition of cell growth in vitro and in vivo. Down-regulation of hSP56 in LNCaP cells that normally express hSP56 results in enhanced anchorage-independent growth. PC-3 cells expressing hSP56 exhibit a significant reduction of hypoxia inducible protein (HIF)-1α protein levels under hypoxic conditions without altering HIF-1α mRNA (HIF1A) levels. Taken together, our findings strongly suggest that hSP56 plays a critical role in prostate cells by mechanisms including negative regulation of HIF-1α, thus identifying hSP56 as a candidate anti-oncogene product. [BMB Reports 2014; 47(7): 411-416]

Selenite reduction by the thioredoxin system: kinetics and identification of protein-bound selenide

Selenite (SeO(3)(2-)) assimilation into a bacterial selenoprotein depends on thioredoxin (trx) reductase in Esherichia coli, but the molecular mechanism has not been elucidated. The mineral-oil overlay method made it possible to carry out anaerobic enzyme assay, which demonstrated an initial lag-phase followed by time-dependent steady NADPH consumption with a positive cooperativity toward selenite and trx. SDS-PAGE/autoradiography using (75)Se-labeled selenite as substrate revealed the formation of trx-bound selenium in the reaction mixture. The protein-bound selenium has metabolic significance in being stabilized in the divalent state, and it also produced the selenopersulfide (-S-SeH) form by the catalysis of E. coli trx reductase (TrxB).

The antitumor effects of selenium compound Na5SeV5O18·3H2O in K562 cell

With an approach to study the anti-tumor effects and mechanism of selenium compound, we investigated the anti-tumor activity and mechanism of Na5SeV5O18.H20 (NaSeVO) in K562 cells. The results showed that 0.625-20 mg/L NaSeVO could significantly inhibit the proliferation of K562 cells in vitro in a time- and concentration-dependent manner as determined by microculture tetrazolium (MTT) assay, the IC50 values were 14.41 (4.45-46.60) and 3.45 (2.29-5.22) mg/L after 48 h and 72 h treatment with NaSeVO respectively. In vivo experiments demonstrated that i.p. administration of 5, 10 mg/kg NaSeVO exhibited an significant inhibitory effect on the growth of transplantation tumor sarcoma 180 (S180) and hepatoma 22 (H22) in mice, with inhibition rate 26.8% and 58.4% on S180 and 31.3% and 47.4% on H22, respectively. Cell cycle studies indicated that the proportion of G0/G1 phase was increased at 2.5 mg/ L while decreased at 10 mg/L after treatment for 24, 48 h. Whereas S phase was decreased at 2.5-5 mg/L and markedly increased at 10 mg/L after treatment for 48 h. After treatment for 24 h, 10 mg/L NaSeVO also markedly increased S and G2/M phases. Take together, the result clearly showed that NaSeVO markedly increased S and G2/M phases at 10 mg/L. The study of immunocytochemistry showed that the expression bcl-2 is significantly inhibited by 10 mg/L NaSeVO, and bax increased. Morphology observation also revealed typical apoptotic features. NaSeVO also significantly caused the accumulation of Ca2+ and Mg2+, reactive oxygen species (ROS) and the reduction of pH value and mitochondrial membrane potential in K562 cells as compared with control by confocal laser scanning microscope. These results suggest that NaSeVO has anti-tumor effects and its mechanism is attributed partially to apoptosis induced by the elevation of intracellular Ca2+, Mg2+ and ROS concentration, and a reduction of pH value and mitochondria membrane potential (MMP).

The major selenium-containing protein in human peripheral granulocytes

Previously, a selenium-containing protein with subunit molecular weight of 15 kDa was found in peripheral human granulocytes. In continuation of this work, the present communication accounts for purification, identification, and characterization of this major selenium-containing protein. The protein was purified on a heparin-Sepharose column followed by Sephacryl S-200 column chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis visualized two bands with subunit molecular weights around 15 kDa. o-Phthaldialdehyde precolumn derivatization and reverse-phase high-performance liquid chromatography showed that the protein contains selenocysteine or selenocystine residues. High-performance gel filtration and isoelectric focusing revealed that the protein had an apparent molecular weight of 32 kDa and a pI value of 7.9. The addition of the protein synthesis inhibitor puromycin to the cell culture medium decreased the 15-kDa protein synthesis. These data suggest that the major selenium-containing protein in peripheral human granulocytes might be a protein with two subunits around 15 kDa. Enzyme studies showed that the protein had peroxidase activity assayed with H2O2 as a substrate and O-dianisidine as a hydrogen donor. This enzymatic activity competed with glutathione peroxidase on the consumption of H2O2, leading to an "inhibiton" of glutathione peroxidase (GSH-Px) activity. Sodium azide could eliminate the inhibition of the protein to GSH-Px. All of the above results implicated that the protein might be a H2O2-dependent selenium containing peroxidase different from GSH-Px. Therefore, the biological function of the protein could be related to eliminating H2O2 generated in the respiratory burst reaction of granulocytes, thus protecting these cells from oxidative damage during phagocytosis.

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.

Isolation, characterization, and chromosomal mapping of a novel cDNA clone encoding human selenium binding protein

We have isolated the full-length human 56 kDa selenium binding protein (hSP56) cDNA clone, which is the human homolog of mouse 56 kDa selenium binding protein. The cDNA is 1,668 bp long and has an open reading frame encoding 472 amino acids. The calculated molecular weight is 52.25 kDa and the estimated isoelectric point is 6.13. Using Northern blot hybridization, we found that this 56 kDa selenium binding protein is expressed in mouse heart with an intermediate level between those found in liver/lung/kidney and intestine. We have also successfully expressed hSP56 in Escherichia coli using the expression vector-pAED4. The hSP56 gene is located at human chromosome 1q21-22).

A new selenoprotein from human lung adenocarcinoma cells: purification, properties, and thioredoxin reductase activity

We report the isolation and characterization of a new selenoprotein from a human lung adenocarcinoma cell line, NCI-H441. Cells were grown in RPMI-1640 medium containing 10% (vol/vol) fetal bovine serum and 0.1 microM [75Se]selenite. A 75Se-labeled protein was isolated from sonic extracts of the cells by chromatography on DE-23, phenyl-Sepharose, heparin-agarose, and butyl-Sepharose. The protein, a homodimer of 57-kDa subunits, was shown to contain selenium in the form of selenocysteine; hydrolysis of the protein alkylated with either iodoacetate or 3-bromopropionate yielded Se-carboxymethyl-selenocysteine or Se-carboxyethyl-selenocysteine, respectively. The selenoprotein showed two isoelectric points at pH 5.2 and pH 5.3. It was distinguished from selenoprotein P by N-glycosidase assay and by the periodate-dansylhydrazine test, which indicated no detectable amounts of glycosyl groups on the protein. The selenoprotein contains FAD as a prosthetic group and catalyzes NADPH-dependent reduction of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), and reduction of insulin in the presence of thioredoxin (Trx). The specific activity was determined to be 31 units/mg by DTNB assay. Apparent Km values for DTNB, Escherichia coli Trx, and rat Trx were 116, 34, and 3.7 microM, respectively. DTNB reduction was inhibited by 0.2 mM arsenite. Although the subunit composition and catalytic properties are similar to those of mammalian thioredoxin reductase (TR), the human lung selenoprotein failed to react with anti-rat liver TR polyclonal antibody in immunoblot assays. The selenocysteine-containing TR from the adenocarcinoma cells may be a variant form distinct from rat liver TR.

Decreased glutathione peroxidase activity in mice in response to nafenopin is caused by changes in selenium metabolism

The activity of selenium-dependent glutathione peroxidase is known to be reduced in the liver of both rats and mice after exposure to nafenopin, as well as other peroxisome proliferators. The mechanism for this down-regulation is not known, but might involve changes in incorporation of selenium into selenoproteins. In this paper we show that both incorporation of selenium into selenoproteins and the level of selenium in liver is reduced in mice treated with nafenopin. The activity of selenium dependent glutathione peroxidase (GPx), as well as incorporation of selenium into its 23 kD subunit were found to be decreased. Contrary to what might have been expected, the decreased GPx activity was detected concomitantly with a slight increase in mRNA levels after 10 days of treatment, while a small decrease in mRNA levels was detected in treated animals after 26 weeks, together with the decrease in GPx-activity. Incorporation of selenium into liver fatty acid binding protein (L-FABP) was also decreased, even though large increases in protein and mRNA levels were detected. Taken together these data suggest that the decrease in GPx-activity in response to nafenopin is due to post-transcriptional mechanisms, involving changes in selenium metabolism.

Effects of organic and inorganic selenium compounds on rat mammary tumor cells

To explore cellular effects of potent organoselenium chemopreventive agents we have used a rat mammary tumor cell line. We demonstrate that 1,4-phenylenebis(methylene) selenocyanate (p-XSC) at a dose of 5 microM is a more potent inhibitor of DNA, RNA and protein synthesis as well as of mitochondrial transmembrane potential than its chemopreventive counterparts benzyl selenocyanate (BSC) and sodium selenite. These differences were also reflected in reduced growth rate by 24 and 48 hr. Cell-cycle and cell-morphology analysis revealed that higher doses of p-XSC (10 microM) caused DNA fragmentation which was accompanied with partial loss of nuclear stainability, whereas BSC caused a noticeable change in cell-cycle distribution and extensive micronucleation. Overall, our results point to cellular targets of selenium compounds which may mediate their chemopreventive activities in mammary tissues.

Chemoprevention of cancer by organoselenium compounds

A major research goal of our laboratories is the development of new organoselenium cancer chemopreventive agents with less toxicity compared to some of the historical selenium compounds, such as sodium selenite. Ideally, such agents would be employed to inhibit tumor development in different organs caused by a variety of chemical carcinogens, particularly those present in the human environment. A series of organoselenium compounds has been synthesized and evaluated for their chemopreventive efficacy in vivo. Parallel to these studies, short-term in vitro and in vivo assays were employed to understand the mechanism of action and to rapidly evaluate their efficacy in eventual long-term preclinical investigations. We demonstrated that one of the most effective of these organoselenium compounds, 1,4-phenylenebis(methylene)selenocyanate (p-XSC, Fig. 1), is capable of inhibiting tumors in the mammary glands, colon, and lung of laboratory animals. Dietary p-XSC inhibited mammary tumor development induced by 7,12-dimethylbenz(a)anthracene (DMBA) during both the initiation and post-initiation phases of carcinogenesis in female CD rats. p-XSC inhibited DMBA-DNA adduct formation in the mammary glands. In collaboration with other laboratories, we demonstrated that p-XSC inhibited thymidine kinase in mammary tumor cell lines derived from both humans and rats. Employing mammary carcinoma cell lines, p-XSC was also shown to inhibit cell growth and induce a dose-dependent increase in cell death by apoptosis. In these assays p-XSC appears superior to selenite and to its sulfur analog, 1,4-phenylenebis(methylene)thiocyanate. Dietary p-XSC decreased colon tumor induction by azoxymethane in F344 rats during both phases of carcinogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)