Se-methylselenocysteine Research Articles

Se-(methyl)selenocysteine (MSC) potentiates the antitumor activity of irinotecan against human tumor xenografts and protects against drug induced toxicity

Se-(methyl)selenocysteine (MSC) potentiates the antitumor activity of irinotecan against human tumor xenografts and protects against drug induced toxicity

Enhanced 7-ethyl-10-hydroxycamptothecin (SN-38) lethality by methylselenocysteine is associated with Chk2 phosphorylation at threonine-68 and down-regulation of Cdc6 expression.

Methylselenocysteine (MSC) is an organic selenium compound in preventative clinical trials involving prostate, lung, and colon carcinoma. We found that methioninase-activated MSC potentiates 7-ethyl-10-hydroxycamptothecin (SN-38)-induced cell lethality in vitro in the p53-defective human head and neck carcinoma A253 cells. Activated MSC increases chk2 phosphorylation at threonine-68 induced by SN-38, with no significant effect on chk1 phosphorylation. Cell cycle arrest induced by SN-38, however, was not abrogated or potentiated by MSC. These results suggest that the enhanced cellular lethality of SN-38 by MSC was not associated with cell cycle regulation pathways. Because chk2, in addition to its role in cell cycle arrest, can induce apoptosis by phosphorylation/activation, we examined whether increased chk2 phosphorylation could induce preapoptotic DNA fragmentation. DNA damage analysis showed that megabase DNA fragmentation is decreased, accompanied by the increased 30 to 300 kilobase pairs of DNA fragmentation after exposure to SN-38 with MSC, compared with SN-38 alone. No significant changes in the amount of DNA fragments were observed in cells treated with SN-38 or MSC alone. Moreover, proteolytic destruction of DNA replication-associated proteins cdc6, MCM2, and cdc25A may induce a DNA damage checkpoint response. The observed down-regulation of DNA replication proteins cdc6, MCM2, and cdc25A after exposure to SN-38 with MSC further indicates a relationship between drug response and DNA damage. Exposure to SN-38 with MSC resulted in a significant increase of poly(ADP-ribose) polymerasecleavage and caspase 3 activation. All together, the data support the hypothesis that enhanced lethality of this combination is associated with increased chk2 phosphorylation at Thr68 and down-regulation of specific DNA replication-associated proteins, which result in poly(ADP-ribose) polymerase cleavage, caspase 3 activation, and the induction of 30 to 300 kilobase pairs of DNA fragmentation.

Selenium and its relationship to cancer: an update

Selenomethionine (Semet) is the major seleno-compound in cereal grains and enriched yeast whereas Se-methylselenocysteine (SeMCYS) is the major seleno-compound in Se-accumulator plants and some plants of economic importance such as garlic and broccoli exposed to excess Se. Animals can metabolize both Semet and SeMCYS. Epidemiological studies have indicated an inverse relationship between Se intake and the incidence of certain cancers. Blood or plasma levels of Se are usually lower in patients with cancer than those without this disorder, but inconsistent results have been found with toenail-Se values and the incidence of cancer. There have been eight trials with human subjects conducted on the influence of Se on cancer incidence or biomarkers, and except for one, all have shown a positive benefit of Se on cancer reduction or biomarkers of this disorder. This is consistent with about 100 small-animal studies where Se has been shown to reduce the incidence of tumours in most of these trials. Se-enriched yeast is the major form of Se used in trials with human subjects. In the mammary-tumour model, SeMCYS has been shown to be the most effective seleno-compound identified so far in reduction of tumours. Several mechanisms have been proposed on the mechanism whereby Se reduces tumours. Even though SeMCYS was shown to be the most effective seleno-compound in the reduction of mammary tumours, it may not be the most effective seleno-compound for reduction of colon tumours.

Selenium speciation analysis of selenium-enriched supplements by HPLC with ultrasonic nebulisation ICP-MS and electrospray MS/MS detection

Size-exclusion, anion-exchange and reversed-phase ion-pair (RP-IP) HPLC were used in combination with ICP-(Q)MS for selenium (Se)-specific detection and quantitation of Se-compounds in extracts of Se-yeast and Se-methylselenocysteine (SeMC) based supplements. On-line electrospray ionisation (ESI)-MS/MS combined with RP-IP HPLC allowed characterisation of such materials in terms of species identification. Accelerated solvent extraction (ASE) was evaluated for extraction of the Se-compounds from the complex matrices in water. Alternatively, digestion with proteolytic enzymes was used for yeast protein hydrolysis. The use of ultrasonic nebulisation in combination with HPLC-ICP-(Q)MS for Se speciation led to detection limits up to six-fold lower (as low as ng l−1 levels) than those obtained with pneumatic nebulisation. Such enhancement of the ICP-MS capabilities for Se detection, combined with an improved separation using the newly developed RP-IP-HPLC method, allowed quantification of minor Se species such as SeMC in yeast extracts. Identification of the main compounds in yeast and SeleniumMC™ tablet hydrolysates as SeMet and SeMC, respectively, was accomplished by on-line RP-IP HPLC with ESI-MS/MS. Identification of SeMC in yeast digests, on the basis of retention time, molecular mass determination for the [M + H]+ 80Se ions (m/z 184) and detection of its product ions, is reported here for the first time. The mass spectral confirmation for SeMC in yeast is of interest, as this species is believed to be metabolised in animals and humans to methylselenol (CH3SeH), an anti-carcinogenic Se-metabolite.

The antioxidant role of selenium and seleno-compounds

Selenium (Se) is an essential trace element for animals and humans that is obtained from dietary sources including cereals, grains and vegetables. The Se content of plants varies considerably according to its concentration in soil. Plants convert Se mainly into Se-methionine (Se-Met) and incorporate it into protein in place of methionine (Met). Selenocystine (Se-Cys), methyl-Se-Cys and γ-glutamyl-Se-methyl-Cys are not significantly incorporated into plant protein and are at relatively low levels irrespective of soil Se content. Higher animals are unable to synthesize Se-Met and only Se-Cys was detected in rats supplemented with Se as selenite. Renal regulation is the mode by which whole body Se is controlled. Se is concentrated in hair and nail and it occurs almost exclusively in organic compounds. The potentiating effect of Se deficiency on lipid peroxidation is enhanced in some tissues by concurrent deficiency of copper or manganese. In the in vitro system, the chemical form of Se is an important factor in eliciting cellular responses. Although the cytotoxic mechanisms of selenite and other redoxing Se compounds are still unclear, it has been suggested that they derive from their ability to catalyze the oxidation of thiols and to produce superoxide simultaneously. Selenite-induced cytotoxicity and apoptosis in human carcinoma cells can be inhibited with copper (CuSO 4) as an antioxidant. High doses of selenite result in induction of 8-hydroxydeoxyguanosine (8-OHdG) in mouse skin cell DNA and in primary human keratinocytes. It may cause DNA fragmentation and decreased DNA synthesis, cell growth inhibition, DNA synthesis, blockade of the cell cycle at the S/G 2-M phase and cell death by necrosis. In contrast, in cells treated with methylselenocyanate or Se methylselenocysteine, the cell cycle progression was blocked at the G 1 phase and cell death was predominantly induced by apoptosis.

Selenium compound analysis by particle beam/hollow cathode optical emission spectroscopy (PB/HC-OES): monitoring of carbon and hydrogen emission from organoselenium compounds

By virtue of the underlying fundamental process, speciation via atomic spectroscopy methods is only accomplished by inference from empirical formulae. The coupling of particle beam sample introduction and hollow cathode optical emission spectroscopy (PB/HC-OES) is applied here for empirical formula determinations of a group of organoselenium compounds. In this application, the carbon and hydrogen optical emission from the organoselenium compounds was monitored and the analyte element response (C (I) and H (I)) to glow discharge operation conditions was studied. Analytical response curves for integrated C and H emission intensities, as a function of both single analyte (selenomethionine) concentration and the empirical formula of the model compounds, were obtained. In all cases, a variability of less than 10% RSD for triplicate injections of 200 µL samples is observed. The detection limits of 0.16 ppm of selenomethionine (0.05 ppm of elemental C) and 0.034 ppm of selenomethionine (0.0019 ppm of elemental H) were obtained over the concentration range of 0.01–1000 ppm. Within the 100 µL injections, these values equate to absolute masses of 16 ng of selenomethionine (i.e. 5 ng of elemental carbon) and 3.4 ng of selenomethionine (i.e. 0.2 ng of elemental hydrogen). The corresponding detection limits for the selenium content in Se-(methyl)selenocysteine were determined to be 11 ng of elemental Se. The correlation of H (I)/C (I) emission intensity values to the atomic H/C values in different organoselenium compounds demonstrates that the PB/HC-OES system is capable of selenium speciation by means of the determination of empirical formulae.

New concepts in selenium chemoprevention.

This article highlights some recent advances in selenium cancer chemoprevention research. It has been well documented that the chemical transformation of selenium to a monomethylated metabolite is an important step in achieving cancer prevention. Studies with the rat mammary carcinogenesis model suggested that methylselenocysteine (MSC), a good precursor for generating methylselenol endogenously, is able to block clonal expansion of premalignant lesions in the mammary gland. This finding supports the notion that selenium intervenes at an early stage of carcinogenesis. In addition to decreasing cell proliferation of the transformed colonies in vivo, MSC also enhances apoptosis. These same cellular responses are replicated with human premalignant breast cells grown in culture. cDNA microarray analysis indicated that selenium affects a multitude of molecular targets. Based on this information, a number of signaling pathways are proposed that could potentially provide insight into how selenium might block cell cycle progression and induce cell death.

Effects of methylselenocysteine on PKC activity, cdk2 phosphorylation and gadd gene expression in synchronized mouse mammary epithelial tumor cells

Methylselenocysteine (MSC), an organic selenium compound is an effective chemopreventive agent against mammary cell growth both in vivo and in vitro but its mechanism of action is still not understood. We have previously demonstrated that MSC is able to inhibit growth in a synchronized TM6 mouse mammary epithelial tumor cell line at 16 h time point followed by apoptosis at 48 h. The decrease in cdk2 kinase activity was coincident with prolonged arrest of cells in S-phase. The present set of experiments showed that cdk2 phosphorylation was reduced by 72% in the MSC-treated cells at 16 h time point. Expression for gadd34, 45 and 153 was elevated 2.5 to 7 fold following MSC treatment only after 16 h time point. In order to investigate a possible upstream target for MSC, we analyzed protein kinase C (PKC) in this model. Total PKC activity was reduced in TM6 cells by MSC (50 μM) within 30 min of treatment, both in cytosolic (55.4 and 77.6%) and membrane (35.2 and 34.1%) fractions for calcium-dependent and independent PKCs, respectively. PMA significantly elevated the PKC activity in membrane fraction (P<0.01) and MSC inhibited this activation by more than 57%. The effect of MSC was selenium specific as selenomethionine and sulfurmethyl-l-cysteine (SMC) did not alter PKC activity either in cytosolic or membrane fraction. Immunoblot analysis showed that PKC-α was translocated to the membrane by PMA and MSC did not alter this translocation. PKC-δ was faintly detectable in membrane fractions of control and MSC-treated cells. MSC treatment slightly reduced levels of PKC-ε (in cytosolic and membrane fractions) and PKC-ζ (cytosolic fractions). The data presented herein suggest that PKC is a potential upstream target for MSC that may trigger one or all of the downstream effects; i.e. the decrease of cdk2 kinase activity, decreased DNA synthesis, elevation of gadd gene expression and finally apoptosis.

Inhibition of cdk2 kinase activity by methylselenocysteine in synchronized mouse mammary epithelial tumor cells.

Methylselenocysteine (MSC), an organic selenium compound has significant anticarcinogenic activity against mammary tumorigenesis. Previous experiments have demonstrated that MSC and inorganic selenite inhibit mammary cell (TM6 cell line) growth through different pathways. The present investigation demonstrated that MSC arrested cells in S phase during the TM6 cell cycle, which was followed by cells entering apoptosis at 48 h. Methylselenocysteine specifically affected the cdk2 kinase activity of the TM6 cells (54% reduction) at 16 h after release from growth arrest. The cdk4 kinase activity did not change during the cell cycle, confirming that cells had passed the G1 checkpoint and had entered S phase. The amount of cyclin E associated with cdk2 was increased by MSC by the 12 h time point, thereby facilitating entry of cells into S phase. Afterwards, cyclin E and cyclin A associated with cdk2 did not change for the remainder of the cell cycle. The data demonstrate that inhibition of mammary cell growth by MSC is mediated by alterations in progression of cells through S phase. The decrease in cdk2 kinase activity is coincident with prolonged arrest in S phase. One consequence of prolonged arrest may be apoptosis.

Identification of selenium species in selenium-enriched garlic, onion and broccoli using high-performance ion chromatography with inductively coupled plasma mass spectrometry detection

Six standard selenium species including selenocystine, methyl selenocysteine, selenite, selenomethionine, allyl selenocysteine and selenate have been separated by high-performance ion chromatography on a Hamilton PRPX-100 column and detected by ICP-MS. Selenium enriched vegetables were analysed. Five selenium species and several unknown peaks were detected.

The mechanism of material exchange between a highly rarefied gas and its solution in a molten metal, with special reference to the degassing of metal melt in a vacuum: T Kraus, trans 6 vac metall conf American Vacuum Society, 1964, 50–87.

The purpose of this study was to identify conditions that will increase the sensitivity of drug-resistant cancer cells. Selenium derivatives have been shown to present anti-cancer properties in the clinic. Currently, selenate, selenite, selenomethionine (SeMet), methyl-selenocysteine (MSC), and methaneselenic acid (MSA) are the most common selenium derivatives used as drugs in humans. Herein, we tested whether these selenium derivatives can sensitize KBV20C cancer cells, which are highly resistant to anti-cancer drugs such as vincristine. All five drugs could sensitize KBV20C cells to the same extent as they sensitized the sensitive parent KB cells, suggesting that selenium-derived drugs can be used for drug-resistant cancer cells. We also observed that these drugs did not inhibit the P-glycoprotein (P-gp) pumping-out ability, suggesting that the sensitization by selenium-derived drugs does not depend on P-gp activity in resistant KBV20C cells. Interestingly, using a cell viability assay, microscopic observation, and Hoechst staining, we found that selenate highly sensitized drug-resistant KBV20C cells by activating the apoptotic pathway, when compared to sensitive KB cells.Furthermore, we investigated why selenate sensitizes resistant KBV20C cells. Selenate-induced toxicity was associated with an increase in G2-phase cell cycle arrest in KBV20C cells, suggesting that the selenate-induced increase in apoptosis resulted from cell cycle arrest in resistant KBV20C cells. Our findings may contribute to the development of selenate-based therapies for patients resistant to cancer drugs.