Selenium compounds as a novel class of experimental cancer chemotherapeutics

Abstract

Selenium is an essential micronutrient for humans, it has a narrow margin between antioxidant and pro-oxidant effects. Redox-active selenium compounds have the potency to increase ROS levels in cancer cells, providing a plausible window for therapeutic intervention. Redox-active selenium compounds such as sodium selenite (Se), selenocystine (SeC), and Se-methylselenocysteine (MSC) have been shown to inhibit growth, angiogenesis, and induce apoptosis by altering the redox potential (oxidative stress) in various tumor cells in vitro. Different selenium compounds produce different metabolites that act on tumor cells through multiple pathways. Sodium selenite is readily reduced to hydrogen selenide (HSe-) by extracellular cysteine, whereas selenocysteine is reduced to HSe- by enzymatic conversion by selenocysteine lyase. Another important selenium compound, MSC, is a prodrug metabolized to methylselenol by kynurenine aminotransferase 1 (KYAT1 or CCBL1). Hydrogen selenide (HSe-) and methylselenol (MS) are two important intermediate metabolites that are highly redox-active by inducing the production of ROS and initiating cell death via redox-regulated signaling pathways. Hydrogen selenide is more readily taken up by the cell compared to selenite. These intermediate molecules can effectively redox cycle with oxygen in the presence of NADPH and thiols, thus enhancing oxidative stress in malignant cells. Nevertheless, the anti-cancer properties of selenium compounds have not been fully characterized. In this work, our objective was to describe the anti-cancer properties of various selenium compounds using different methods and experimental models that are easily translatable from in vitro to in vivo.

Selenite at physiological concentrations in combination with ATRA completely abolished the expression of the PML/RARα oncoprotein and increased the expression of the transcription factors RAR, PU.1 and FOXO3A, providing a plausible basis for the increased differentiation in cells of acute promyelocytic leukemia (APL). The extracellular milieu is important for selenite cytotoxicity, i.e. selenite is readily reduced to hydrogen selenide (HSe-) by extracellular cysteine, the xCT (cystine/glutamate transporter) antiporter is very important for HSe- turnover. Diphenyl diselenide, a small- molecule compound, increases the expression of xCT and its key regulatory genes such as NRF2 and ATF4 in vitro. When diphenyl diselenide was co-incubated with selenite or selenocysteine, we observed multiple sensitizing effects in almost all cancer cell lines tested. This provides a strong correlation between extracellular thiols and the cytotoxicity of selenite and selenocysteine. Kynurenine aminotransferase 1 (KYAT1 or CCBL1) is a PLP-dependent enzyme and plays an important role in MSC metabolism. KYAT1 has dual enzyme activity, transamination and β-elimination towards the single substrate. MSC is considered a prodrug that is not toxic as long as it is not metabolized by KYAT1. MSC is reduced by transamination to β-methylselenopyruvate (MSP) and by β-elimination to monomethylselenol. Several assays exist to determine the transamination activity of KYAT1, but very few simple assays exist to determine the β-elimination activity of KYAT1, which is not reliable because it is not a direct measure of MS. We introduced a simple novel coupled assay to determine the β-elimination activity of KYAT1. This assay method combines two enzyme systems, i.e. thioredoxin reductase1 (TrxR1) and KYAT1. MS is an excellent substrate for thioredoxin reductase1. MSC is metabolized to MS by β-elimination activity, and this can be used as a substrate for TrxR1, which is monitored spectrophotometrically by the oxidation of NADPH.

Overexpression of KYAT1 may be an advantage in exploring the anti-tumor property of MSC, as it plays an important role in MSC metabolism. Both metabolites of MSC (MSP and MS) play critical roles in anti-tumor activity. MSP is known to inhibit HDAC activity, while MS has been shown to increase the formation of ROS and induce redox imbalance in the tumor. We used therapeutic mRNA techniques to induce KYAT1 expression using a lipid nanoparticle (LNPs)-based delivery system in hepatocellular carcinoma (HCC) cells. the addition of antisense microRNA122 (HCC-specific) with KYAT1mRNA showed precise targeting of HCC cells. Our results demonstrate successful targeted therapy in HCC cells with MSC. The choice of the model system is very important in drug screening. Cell culture, 2D and 3D models are widely used, but the reproducibility is very low when transferred to in vivo. Our group has established an ex vivo slice culture model for pancreatic ductal adenocarcinoma (PDAC). We used this ex vivo model to test the anti-cancer properties of sodium selenite and MSC. Our results, both by histology and transcriptomics data, show that sodium selenite at a concentration of 15 μM (concentration below MTD in humans) exhibited pronounced anti-tumor activity by targeting multiple hallmark genes that support cancer growth and progression.

In this work, we have shown that redox-active selenium compounds as potential anti-cancer agents by (1) mechanisms to facilitate uptake by altering the expression of SLC7A11 (xCT) through small-molecule pharmacological compounds, (2) increasing the metabolizing enzymes (KYAT1) using different methods and targeted therapy (3) used different model (ex vivo) to mimic the in vivo settings.