Differential expression of selenoproteins by human skin cells and protection by selenium from UVB-radiation-induced cell death

Effect of selenium on N-nitrosodiethylamine-induced multistage hepatocarcinogenesis with reference to lipid peroxidation and enzymic antioxidants

We have studied the relationship between antioxidant and anticancer properties of selenium (Se) in multistage hepatocarcinogenesis induced by N-nitrosodiethylamine (DEN). In this study we have observed an increased level of lipid peroxide (LPO) products and decreased antioxidant enzyme activities (superoxide dismutase and catalase) in hepatoma and surrounding liver tissues of cancer-bearing animals. Selenium (Se) was supplemented either before initiation or during initiation and selection/promotion phases of hepatocarcinogenesis and was found to be effective in altering hepatic lipid peroxidation and antioxidant enzyme activities to a statistically significant level measured either in the hepatoma or in the surrounding liver tissues. These alterations inclined towards normal in a time-dependent manner on selenium supplementation. Furthermore, increased levels of lipid peroxidation and decreased levels of antioxidants (superoxide dismutase and catalase) were also observed in distant organs of cancer-bearing rats other than the tumour-bearing site. These alterations are brought back to normal levels upon Se treatment. Our results confirm the fact that Se is particularly protective in limiting the action of DEN by its antioxidant property.

Selenium, ultraviolet radiation and the skin

Selenium (Se) is a dietary trace mineral in which there has recently been a surge of interest, in both the popular and the scientific press, because of its demonstrated anti-carcinogenic and anti-inflammatory properties in humans. In this short review, I will explain why Se is an important component of cellular anti-oxidant defences and review its protective effects against UV radiation-induced damage to skin cells. Although little is known about whether selenium can protect human skin from UV-induced damage, clinical studies are underway and the anti-oxidant may offer considerable benefits.

Thioredoxin reductase as a pathophysiological factor and drug target

Human cytosolic thioredoxin reductase (TrxR), a homodimeric protein containing 1 selenocysteine and 1 FAD per subunit of 55 kDa, catalyses the NADPH-dependent reduction of thioredoxin disulfide and of numerous other oxidized cell constituents. As a general reducing enzyme with little substrate specificity, it also contributes to redox homeostasis and is involved in prevention, intervention and repair of damage caused by H2O2-based oxidative stress. Being a selenite-reducing enzyme as well as a selenol-containing enzyme, human TrxR plays a central role in selenium (patho)physiology. Both dietary selenium deficiency and selenium oversupplementation, a lifestyle phenomenon of our time, appear to interfere with the activity of TrxR. Selenocysteine 496 of human TrxR is a major target of the anti-rheumatic gold-containing drug auranofin, the formal Ki for the stoichiometric inhibition being 4 nM. The hypothesis that TrxR and extracellular thioredoxin play a pathophysiologic role in chronic diseases such as rheumatoid arthritis, Sjögren's syndrom, AIDS, and certain malignancies, is substantiated by biochemical, virological, and clinical evidence. Reduced thioredoxin acts as an autocrine growth factor in various tumour diseases, as a chemoattractant, and it synergises with interleukins 1 and 2. The effects of anti-tumour drugs such as carmustine and cisplatin can be explained in part by the inhibition of TrxR. Consistently, high levels of the enzyme can support drug resistance. TrxRs from different organisms such as Escherichia coli, Mycobacterium leprae, Plasmodium falciparum, Drosophila melanogaster, and man show a surprising diversity in their chemical mechanism of thioredoxin reduction. This is the basis for attempts to develop specific TrxR inhibitors as drugs against bacterial infections like leprosy and parasitic diseases like amebiasis and malaria.

Reproductive outcomes in a population exposed long-term to inorganic selenium via drinking water

Despite being an essential element in mammals and lower animals at very low doses, selenium is recognized as a teratogen and a growth-inhibiting substance in several animal species. Some inorganic selenium compounds have also been shown to be mutagenic and pro-oxidant: however, very little is known about their effects on human reproduction. Between 1972 and 1988, a few thousand residents in the municipality of Reggio Emilia, northern Italy, were accidentally exposed to drinking water with unusually high levels of inorganic selenium (selenate), range 7-9 microg/l, through a local public water supply system. We found no deleterious effect on overall body weight and length of newborns and stillborns delivered by 18 women previously exposed to high-selenium tapwater. Rates of spontaneous abortions, however, were increased slightly (RR = 1.73; 95% CI = 0.62-4.80), compared with rates among unexposed women from the same municipality. From 1980 through 1988 the prevalence of congenital malformations at birth among the exposed population was similar to that expected, but estimates were imprecise owing to small numbers. Results do not suggest marked effects on human reproduction from chronic exposure to selenate in drinking water at < 10 microg/l.

Selenite negatively regulates caspase-3 through a redox mechanism

Selenium, an essential biological trace element, exerts its modulatory effects in a variety of cellular events including cell survival and death. In our study we observed that selenite protects HEK293 cells from cell death induced by ultraviolet B radiation (UVB). Exposure of HEK293 cells to UVB radiation resulted in the activation of caspase-3-like protease activity, and pretreatment of the cells with z-DEVD-fmk (N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone), a caspase-3 inhibitor, prevented UVB-induced cell death. Interestingly, enzymatic activity of caspase-3-like protease in cell lysates of UVB-exposed cells was repressed in vitro by the presence of selenite. Selenite also inhibited the in vitro activity of purified recombinant caspase-3 in cleaving Ac-DEVD-pNA (N-acetyl-Asp-Glu-Asp-p-nitroanilide) or ICAD(L) (inhibitor of a caspase-activated deoxyribonuclease) and in the induction of DNA fragmentation. The inhibitory action of selenite on a recombinant active caspase-3 could be reversed by sulfhydryl reducing agents, such as dithiothreitol and beta-mercaptoethanol. Furthermore, pretreatment of cells with selenite suppressed the stimulation of the caspase-3-like protease activity in UVB-exposed cells, whereas dithiothreitol and beta-mercaptoethanol reversed this suppression of the enzymatic activity. Taken together, our data suggest that selenite inhibits caspase-3-like protease activity through a redox mechanism and that inhibition of caspase-3-like protease activity may be the mechanism by which selenite exerts its protective effect against UVB-induced cell death.

Dehydroascorbic acid uptake in a human keratinocyte cell line (HaCaT) is glutathione-independent

Vitamin C plays an important role in neutralizing toxic free radicals formed during oxidative metabolism or UV exposure of human skin. This study was performed to investigate the mechanisms that regulate the homoeostasis of vitamin C in HaCaT cells by identifying the events involved in the transport and in the reduction of dehydroascorbic acid. Dehydroascorbic acid accumulated to a greater extent and faster compared with ascorbic acid; its transport appeared to be mediated by hexose transporters and was entirely distinct from ascorbic acid transport. Dehydroascorbate reductase activity was unaffected by glutathione depletion, although it was sensitive to thiol protein reagents. These observations, as well as the subcellular distribution of this enzymic activity and the cofactor specificity, indicate that thioredoxin reductase and lipoamide dehydrogenase play an important role in this reduction process. HaCaT cells were able to enhance their dehydroascorbic acid reductase activity in response to oxidative stress.

Responsiveness of selenoproteins to dietary selenium

Selenocysteine-containing enzymes that have been identified in mammals include the glutathione peroxidase family (GPX1, GPX2, GPX3, and GPX4), one or more iodothyronine deiodinases and two thioredixin reductases. Selenoprotein P, a glycoprotein that contains 10 selenocysteine residues per 43 kDa polypeptide and selenoprotein W, a 10 kDa muscle protein, are unidentified as to function. Levels of all of these selenocysteine-containing proteins in various tissues are affected to different extents by selenium availability. Increased amounts of selenoproteins observed in response to selenium supplementation were shown in several studies to correlate with increases in the corresponding mRNA levels. In general, selenoprotein levels in brain are less sensitive to dietary selenium fluctuation than the corresponding selenoprotein levels in other tissues.

The effects of ultraviolet radiation on the human immune system

The adverse outcome of increased ultraviolet (UV) irradiation on human health is currently of concern. While many experiments have been carried out in rodent models, fewer have been designed to test the effects of UV exposure in human subjects. This review concentrates on the modulations induced in the human immune system by UV, and outlines changes in antigen presentation by Langerhans cells and macrophages, in the activities of natural killer cells and T cells, and in cytokine regulation. Precautionary measures which might be taken to help protect people against the immunosuppressive action of UV irradiation are considered.