Differential apoptotic response of human cancer cells to organoselenium compounds

PVM/MA-shelled selol nanocapsules promote cell cycle arrest in A549 lung adenocarcinoma cells

BACKGROUND

Selol is an oily mixture of selenitetriacylglycerides that was obtained as a semi-synthetic compound containing selenite. Selol is effective against cancerous cells and less toxic to normal cells compared with inorganic forms of selenite. However, Selol's hydrophobicity hinders its administration in vivo. Therefore, the present study aimed to produce a formulation of Selol nanocapsules (SPN) and to test its effectiveness against pulmonary adenocarcinoma cells (A549).

RESULTS

Nanocapsules were produced through an interfacial nanoprecipitation method. The polymer shell was composed of poly(methyl vinyl ether-co-maleic anhydride) (PVM/MA) copolymer. The obtained nanocapsules were monodisperse and stable. Both free Selol (S) and SPN reduced the viability of A549 cells, whereas S induced a greater reduction in non-tumor cell viability than SPN. The suppressor effect of SPN was primarily associated to the G2/M arrest of the cell cycle, as was corroborated by the down-regulations of the CCNB1 and CDC25C genes. Apoptosis and necrosis were induced by Selol in a discrete percentage of A549 cells. SPN also increased the production of reactive oxygen species, leading to oxidative cellular damage and to the overexpression of the GPX1, CYP1A1, BAX and BCL2 genes.

CONCLUSIONS

This study presents a stable formulation of PVM/MA-shelled Selol nanocapsules and provides the first demonstration that Selol promotes G2/M arrest in cancerous cells.

Selenium cytotoxicity in cancer

Selenium is an essential trace element with growth-modulating properties. Decades of research clearly demonstrate that selenium compounds inhibit the growth of malignant cells in diverse experimental model systems. However, the growth-modulating and cytotoxic mechanisms are diverse and far from clear. Lately, a remarkable tumour selective cytotoxicity of selenium compounds has been shown, indicating the potential of selenium in the treatment of cancer. Of particular interest are the redox-active selenium compounds exhibiting cytotoxic potential to tumour cells. These selenium compounds elicit complex patterns of pharmacodynamics and pharmacokinetics, leading to cell death pathways that differ among compounds. Modern oncology often focuses on targeted ligand-based therapeutic strategies that are specific to their molecular targets. These drugs are initially efficient, but the tumour cells often rapidly develop resistance against these drugs. In contrast, certain redox-active selenium compounds induce complex cascades of pro-death signalling at pharmacological concentrations with superior tumour specificity. The target molecules are often the ones that are important for the survival of cancer cells and often implicated in drug resistance. Therefore, the chemotherapeutic applications of selenium offer great possibilities of multi-target attacks on tumour cells. This MiniReview focuses on the tumour-specific cytotoxic effects of selenium, with special emphasis on cascades of cellular events induced by the major groups of pharmacologically active selenium compounds. Furthermore, the great pharmacological potential of selenium in the treatment of resistant cancers is discussed.

A bioassay-driven discovery of an unexpected selenophene and its cytotoxicity

During the reaction of methyl 3β-acetoxy-glycyrrhetinate (1) with SeO2 significant amounts of a cytotoxic hitherto unprecedented triterpenoic selenophene 3 are formed. This compound stops cell proliferation and acts by apoptosis.

Selenium compounds, apoptosis and other types of cell death: an overview for cancer therapy

Selenium (Se) is an essential trace element involved in different physiological functions of the human body and plays a role in cancer prevention and treatment. Induction of apoptosis is considered an important cellular event that can account for the cancer preventive effects of Se. The mechanisms of Se-induced apoptosis are associated with the chemical forms of Se and their metabolism as well as the type of cancer studied. So, some selenocompounds, such as SeO₂ involve the activation of caspase-3 while sodium selenite induces apoptosis in the absence of the activation of caspases. Modulation of mitochondrial functions has been reported to play a key role in the regulation of apoptosis and also to be one of the targets of Se compounds. Other mechanisms for apoptosis induction are the modulation of glutathione and reactive oxygen species levels, which may function as intracellular messengers to regulate signaling pathways, or the regulation of kinase, among others. Emerging evidence indicates the overlaps between the apoptosis and other types of cell death such as autophagy. In this review we report different processes of cell death induced by Se compounds in cancer treatment and prevention.

Substituted diaryl diselenides: cytotoxic and apoptotic effect in human colon adenocarcinoma cells

AIMS

To investigate the effects and study the underlying cell death mechanisms of diaryl diselenides, including: diphenyl diselenide (C(6)H(5)Se)(2); 4-chlorodiphenyl diselenide (4-ClC(6)H(4)Se)(2); 3-(trifluoromethyl)-diphenyl diselenide (3-CF(3)C(6)H(4)Se)(2) and 4-methoxydiphenyl diselenide (4-MeOC(6)H(4)Se)(2), on the human colon adenocarcinoma cell line HT-29.

MAIN METHODS

The viability of HT-29 cells after exposure to the diaryl diselenides and its substituted structures was based on the MTT assay. To verify if cell death was mediated throughout apoptosis mechanisms, flow cytometry and real-time PCR (qPCR) analyses were conducted.

KEY FINDINGS

The MTT assay and flow cytometry analyses showed that (3-CF(3)C(6)H(4)Se)(2) and (4-MeOC(6)H(4)Se)(2) induced cytotoxicity through apoptosis mechanisms in HT-29 cells. qPCR revealed there was an up-regulation of pro-apoptotic (Bax, casapase-9, caspase-8, apoptosis-inducing factor (AIF) and Endonuclease G (EndoG)) and cell-cycle arrest genes (p53 and p21) and down-regulation of anti-apoptotic (Bcl-2 and survivin) and Myc genes.

SIGNIFICANCE

These results demonstrate that (3-CF(3)C(6)H(4)Se)₂ and (4-MeOC(6)H(4)Se)(2) have the potential to induce apoptosis in HT-29 cells through the activation of caspase-dependent and independent pathways and through cell-cycle arrest.

Selenite targets eIF4E-binding protein-1 to inhibit translation initiation and induce the assembly of non-canonical stress granules

Stress granules (SGs) are large cytoplasmic ribonucleoprotein complexes that are assembled when cells are exposed to stress. SGs promote the survival of stressed cells by contributing to the reprogramming of protein expression as well as by blocking pro-apoptotic signaling cascades. These cytoprotective effects implicated SGs in the resistance of cancer cells to radiation and chemotherapy. We have found that sodium selenite, a selenium compound with chemotherapeutic potential, is a potent inducer of SG assembly. Selenite-induced SGs differ from canonical mammalian SGs in their morphology, composition and mechanism of assembly. Their assembly is induced primarily by eIF4E-binding protein1 (4EBP1)-mediated inhibition of translation initiation, which is reinforced by concurrent phosphorylation of eIF2α. Selenite-induced SGs lack several classical SG components, including proteins that contribute to pro-survival functions of canonical SGs. Our results reveal a new mechanism of mammalian SG assembly and provide insights into how selenite cytotoxicity may be exploited as an anti-neoplastic therapy.

Selenium: a double-edged sword for defense and offence in cancer

Selenium (Se) is an essential dietary component for animals including humans and is regarded as a protective agent against cancer. Although the mode of anticancer action of Se is not fully understood yet, several mechanisms, such as antioxidant protection by selenoenzymes, specific inhibition of tumor cell growth by Se metabolites, modulation of cell cycle and apoptosis, and effect on DNA repair have all been proposed. Despite the unsupported results of the last SELECT trial, the cancer-preventing activity of Se was demonstrated in majority of the epidemiological studies. Moreover, recent studies suggest that Se has a potential to be used not only in cancer prevention but also in cancer treatment where in combination with other anticancer drugs or radiation, it can increase efficacy of cancer therapy. In combating cancer cells, Se acts as pro-oxidant rather than antioxidant, inducing apoptosis through the generation of oxidative stress. Thus, the inorganic Se compound, sodium selenite (SeL), due to its prooxidant character, represents a promising alternative for cancer therapy. However, this Se compound is highly toxic compared to organic Se forms. Thus, the unregulated intake of dietary or pharmacological Se supplements mainly in the form of SeL has a potential to expose the body tissues to the toxic levels of Se with subsequent negative consequences on DNA integrity. Hence, due to a broad interest to exploit the positive effects of Se on human health and cancer therapy, studies investigating the negative effects such as toxicity and DNA damage induction resulting from high Se intake are also highly required. Here, we review a role of Se in cancer prevention and cancer therapy, as well as mechanisms underlying Se-induced toxicity and DNA injury. Since Saccharomyces cerevisiae has proven a powerful tool for addressing some important questions regarding Se biology, a part of this review is devoted to this model system.

Rapamycin suppresses ROS-dependent apoptosis caused by selenomethionine in A549 lung carcinoma cells

PURPOSE

Although selenium compounds possess chemotherapeutic features by inducing apoptosis in cancer cells with trivial side effects on normal cells, the mechanisms underlying its anti-cancer activity are insufficiently understood at the present. In this study, we investigated the effects of rapamycin on apoptosis induced by seleno-L-methionine (SeMet) or selenite in A549 cells.

METHODS

The effects of Se compounds, SeMet and selenite, on cell proliferation, apoptosis and its signaling pathway were investigated in established human adenocarcinoma cell line (A549). Cancer cells were treated with each Se during different periods. Cell apoptosis and signaling molecules were analyzed by flow cytometry (TUNEL method) or immunoblotting, respectively.

RESULTS

SeMet induces reactive oxygen species generation associated with the induction of apoptosis, because pretreatment of cells with N-acetyl-L-cysteine completely blocked SeMet-induced apoptosis. We also found that rapamycin completely suppressed the apoptosis of cells treated by SeMet, but not selenite. SeMet-induced apoptosis is significantly downregulated in combination with PI3 K family inhibitors (LY294002, wortmannin, PI-103, and 3-methyladenine). In addition, ROS generation was included in downstream signaling events associated with the phosphorylation of mTOR, because pretreatment of cells with rapamycin inhibited ROS generation.

CONCLUSION

These results suggest that SeMet-induced apoptosis is affected by the Akt/mTOR/ROS pathway in A549 cells. Akt serves an anti-survival function in the system of SeMet-treated lung cancer cells, but autophagic signaling remained unsolved.