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

Redox-active selenium compounds--from toxicity and cell death to cancer treatment

Selenium is generally known as an antioxidant due to its presence in selenoproteins as selenocysteine, but it is also toxic. The toxic effects of selenium are, however, strictly concentration and chemical species dependent. One class of selenium compounds is a potent inhibitor of cell growth with remarkable tumor specificity. These redox active compounds are pro-oxidative and highly cytotoxic to tumor cells and are promising candidates to be used in chemotherapy against cancer. Herein we elaborate upon the major forms of dietary selenium compounds, their metabolic pathways, and their antioxidant and pro-oxidant potentials with emphasis on cytotoxic mechanisms. Relative cytotoxicity of inorganic selenite and organic selenocystine compounds to different cancer cells are presented as evidence to our perspective. Furthermore, new novel classes of selenium compounds specifically designed to target tumor cells are presented and the potential of selenium in modern oncology is extensively discussed.

Histological lesion of spleen and inhibition of splenocyte proliferation in broilers fed on diets excess in selenium

The purpose of this 42-day study was to investigate the effects of excess dietary selenium on immune function by determining morphological changes of spleen and cell cycle of splenocyte. Three hundred 1-day-old avian broilers were fed on a basic diet (0.2 mg/kg selenium) or the same diet amended to contain 1, 5, 10, and 15 mg/kg selenium (Se) supplied as sodium selenite (n = 60/group). Anatomically, the spleens were shrunken in volume with pallecent color. Histopathologically, lymphopenia in splenic nodules and periarterial lymphatic sheaths and congestion of the red pulp were observed in 5, 10, and 15 mg/kg Se group. By flow cytometry method, the percentage of G(0)/G(1) phase splenocytes was significantly increased, whereas the percentages of S phase and G(2)+M phase splenocytes and the proliferation index were markedly decreased in 5, 10, and 15 mg/kg Se groups when compared with those of 0.2 mg/kg group. The results confirmed that excess dietary Se as sodium selenite in the range of 5~15 mg/kg caused growth retardation of spleen by cell cycle blockage in young chickens.

Detection of cytotoxic activity of lectin on human colon adenocarcinoma (Sw480) and epithelial cervical carcinoma (C33-A)

Lectins comprise a heterogeneous class of proteins that recognize the carbohydrate moieties of glycoconjugates with high specificity. Numerous studies have shown that lectins are capable of recognizing specific carbohydrate moieties displayed by malignant cells or tissues. The present work was performed to investigate the effects of tepary bean (Phaseolus acutifolius) lectins on proliferation, colony formation, and alteration of DNA synthesis of human malignant cells. Tepary bean lectin showed dose dependent  effects on the inhibition of viability as well as on colony formation in two human malignant cells lines (C33-A, Sw480); By contrast, tepary bean lectin only showed significant effects on DNA synthesis on Sw480 cells. Our results provide evidence of the anti- proliferative and cytotoxic effects of the tepary bean lectins on C33-A and Sw480 cells lines.

Modulation of redox status in human lung cell lines by organoselenocompounds: selenazolidines, selenomethionine, and methylseleninic acid

Cancer prevention strategies utilizing selenium-containing compounds have demonstrated reduced cancer mortality and efficacy for some cancer types but considerable differences in cellular effects exist among the selenocompounds employed. The variability of the effects on cell viability, redox modulation, and disruption of subcellular compartments by the conventional selenium-containing amino acid, selenomethionine, the oxidized selenosugar metabolite, methylseleninic acid, and selenazolidines was investigated in A549 and BEAS-2B human lung cell lines. Selenomethionine had little effect whereas methylseleninic acid increased cellular thiols and stress in the endoplasmic reticulum. The cyclohexylselenazolidine increased mild oxidative stress in the adenocarcinoma cell line, A549, but the effects were attenuated in the normal, but virally transformed cell line, BEAS-2B. These data demonstrate that all selenocompounds are not equal and that the form of the organic selenocompound is a major determinant in the expected cellular response.

Identification of new proteins in follicular fluid from mature human follicles by direct sample rehydration method of two-dimensional polyacrylamide gel electrophoresis

Human follicular fluid (HFF) includes various biologically active proteins which can affect follicle growth and oocyte fertilization. Thus far, these proteins from mature follicles in human follicular fluid have been poorly characterized. Here, two-dimensional polyacrylamide gel electrophoresis (2-DE) with matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) was used to identify new proteins in HFF. Mature follicular fluids were obtained from five females after oocyte collection during in vitro fertilization (IVF). We directly rehydrated HFF samples, obtained high-resolution 2-DE maps, and processed them for 2-DE and MALDI-MS. One hundred eighty spots were detected and 10 of these spots were identified. By the 2-DE database, six of them had been reported, as proteins already existing in HFF. Hormone sensitive lipase (HSL), Unnamed protein product 1 (UPP1), Unnamed protein product 2 (UPP2), and apolipoprotein A-IV precursor were newly detected. HSL and apolipoprotein A-IV participate in lipid metabolism. UPP1 has a homology with selenocysteine lyase. We found by RT-PCR that these genes are expressed from human primary granulosa cells. The proteins identified here may emerge as potential candidates for specific functions during folliculogenesis, hormone secretion regulation, or oocyte maturation. Further functional analysis of these proteins is necessitated to determine their biological implications.

Inhibitory effect of selenite on invasion of HT1080 tumor cells

Selenium, an essential biological trace element, has been shown to reduce and prevent the incidence of cancer. Our previous studies have shown that selenite is involved in the chemoprevention of cancer and induction of apoptosis of cancer cells. In this study, we demonstrate that selenite also inhibits the invasion of tumor cells. Cancer cell invasion requires coordinated processes, such as changes in cell-cell and cell-matrix adhesion, degradation of the extracellular matrix, and cell migration. We found that selenite inhibited invasion of HT1080 human fibrosarcoma cells. Adhesion of HT1080 cells to the collagen matrix was also inhibited by treatment with selenite, but cell-cell interaction and cell motility were not affected by selenite. Moreover, selenite reduced expression of matrix metalloproteinase-2 and -9 and urokinase-type plasminogen activator, which are involved in matrix degradation, but increased a tissue inhibitor of metalloproteinase-1. This inhibitory effect of selenite on the protease expressions was mediated by the suppression of transcription factors, NF-kappaB and AP-1. However, selenate showed no remarkable effect on all the steps of cancer cell invasion.

Se-methylselenocysteine induces apoptosis through caspase activation in HL-60 cells

Apoptosis, a programmed process of cell suicide, has been proposed as the most plausible mechanism for the chemopreventive activities of selenocompounds. In our study, we found that Se-methylselenocysteine (MSC) induced apoptosis through caspase activation in human promyelocytic leukemia (HL-60) cells. Measurements of cytotoxicity, DNA fragmentation and apoptotic morphology revealed that MSC was more efficient at inducing apoptosis than selenite, but was less toxic. Moreover, MSC increased both the apoptotic cleavage of poly(ADP-ribose) polymerase (PARP) and caspase-3 activity, whereas selenite did not. We next examined whether caspases and serine proteases are required for the apoptotic induction by MSC. A general caspase inhibitor, z-VAD-fmk, dramatically decreased cytotoxicity in MSC-treated HL-60 cells and several other apoptotic features, such as, caspase-3 activation, the apoptotic DNA ladder, TUNEL-positive staining and the DNA double-strand break. Interestingly, a general serine protease inhibitor, AAPV-cmk, also effectively inhibited MSC-mediated cytotoxicity and apoptosis. These results demonstrate that MSC is a selenocompound that efficiently induces apoptosis in leukemia cells and that proteolytic machinery, in particular caspase-3, is necessary for MSC-induced apoptosis. On the other hand, selenite-induced cell death could be derived from necrosis rather than apoptosis, since selenite did not significantly induce several apoptotic phenomena, including the activation of caspase-3.

Selenite supplementation decreases expression of MAZ in HT29 human colon adenocarcinoma cells

Low dietary intake of the essential trace element selenium can increase the risk of colon cancer. Utilizing RNA arbitrarily primed polymerase chain reaction (RAP-PCR), we sought to identify genes differentially expressed in HT29 human colon adenocarcinoma cells cultured with or without supplemental sodium selenite. One cDNA fragment, present at lower levels in samples from cells supplemented with selenite, had 97% nucleotide sequence identity with a sequence from the 3'-untranslated region of myc-associated zinc-finger protein (MAZ) cDNA. Northern blot analysis showed that steady-state levels of mRNA detected using this fragment as a probe were three times greater in unsupplemented (Se-) than in supplemented (Se+) samples. When a duplicate Northern blot was probed with a 300-bp fragment from the open reading frame of an MAZ cDNA clone, signal intensity was 2.2 times greater in Se- than in Se+ lanes. The MAZ protein has been shown to be a transcription regulator of the c-myc protooncogene. Signal intensity on a Northern blot probed with a segment of c-myc Exon 1 cDNA was 94% greater in Se- than in Se+ lanes. These findings are consistent with the established role for MAZ in regulating c-myc gene expression. They also suggest a molecular mechanism by which selenium intake may affect risk of colon cancer.

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.

On the nature of selenium toxicity and carcinostatic activity

Selenium toxicity was first confirmed in 1933 to occur in livestock that consumed plants of the genus Astragalus, Xylorrhiza, Oonopsis, and Stanleya in the western regions of the United States. In 1957 selenium was identified as an essential nutrient for laboratory rats and soon thereafter for chickens and sheep. Essentiality for mammalian species was established in 1973 with the discovery that the enzyme glutathione peroxidase contained selenium. During this same period of time, human epidemiological evidence suggested that selenium possessed anticarcinogenic effects. Since the 1970s, many animal studies have confirmed the human epidemiologic evidence that selenium compounds possess carcinostatic activity. Less progress has been made in explaining why many of these compounds of selenium are toxic and why these same compounds are carcinostatic. In 1988 the observation was made that oxidation of glutathione by selenite produced superoxide, opening a new area for selenium research. This present paper, drawing information from the literature on selenium metabolism in plants and animals, selenium toxicology, selenium cytotoxicity, and selenium carcinostatic activity in animals over the last sixty years, sets forth a probable biochemical catalytic mechanism that encompasses both selenium toxicity and selenium carcinostatic activity. The thesis presented here for scrutiny is that compounds of selenium are toxic owing to their prooxidant catalytic activity to produce superoxide (O2.-), hydrogen peroxide, and very likely other cascading oxyradicals. The toxicity of selenium compounds is countered by plant and animal methylation reactions and antioxidant defenses. As carcinostasis is mostly known to occur at supranutritional levels of selenium in animals, carcinostasis appears to be directly correlated to selenium toxicity. The catalytic toxic selenium specie appears to be the metabolic selenide (RSe-) anion.

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.

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.