Metabolism of Selenite in Human Lung Cancer Cells: X-Ray Absorption and Fluorescence Studies

Abstract

Selenite is an inorganic form of selenium that has a cytotoxic effect against several human cancer cell lines: one or more selenite metabolites are considered to be responsible for its toxicity. X-ray absorption spectroscopy was used to monitor Se speciation in A549 human lung cancer cells incubated with selenite over 72 h. As anticipated, selenodiglutathione and elemental Se both comprised a large proportion of Se in the cells between 4 and 72 h after treatment, which is in accordance with the reductive metabolism of selenite in the presence of glutathione and glutathione reductase/NADPH system. Selenocystine was also present in the cells, but was only detected as a significant component between 24 h and 48 h concomitant with a decrease in the proportion of selenocysteine and the viability of the cells. The change in speciation from the selenol, selenocysteine, to the diselenide, selenocystine, is indicative of a change in the redox status of the cells to a more oxidizing environment, likely brought about by metabolites of selenite. X-ray fluorescence microscopy of single cells treated with selenite for 24 h revealed a punctate distribution of Se in the cytoplasm. The accumulation of Se was associated with a greater than two-fold increase in Cu, which was colocalized with Se. Selenium K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed Se-Se and Se-S bonding, but not Se-Cu bonding, despite the spatial association of Se and Cu. Microprobe X-ray absorption near-edge structure spectroscopy (μ-XANES) showed that the highly localized Se species was mostly elemental Se.

XAS studies of Se speciation in selenite-fed rats

“…Also within this region, is the spectrum of a pellet of 1 μM selenite-treated human lung cancer cells that, like the rat tissue spectra, was poorly fit by a linear combination of model compound spectra. 24 The position of the rat tissue spectra in relation to model compounds suggests a mixture of species including diselenides, selenotrisulfides and deprotonated selenoamino acids (likely protein-bound), which is in accord with the fits of Se–Se and Se–S to the EXAFS of liver and kidney tissues (Table 1). …”

“…The fits for most spectra, regardless of noise level, were poor and thus no useful information was obtained regarding the speciation of Se in the tissues. The poor fits were surprising given the successful fitting of Se-treated cells we have previously reported, 23,24 but comparison of some rat tissue spectra to the spectra of selected Se model compounds and Se-treated cell spectra revealed the distinctly different appearance of the rat tissue spectra (Figure 2d). As opposed to the cell and model compound spectra, which generally consist of an intense peak at low energy followed by a less intense peak at higher energies, the two peaks in the tissue spectra tend to be of a similar intensity (the ratio of the high-energy to low-energy peaks ranging from about 0.80 to 1.00).…”

“…24 Cancer cell lines are commonly used as models of selenium metabolism due to the interest anti-cancer activities of selenium compounds. Direct comparison of the XANES spectra collected from the frozen, hydrated rat tissues with those collected from selenite-treated A549 lung cancer cells underscored the differences between these two models of Se metabolism.…”

Dynamic equilibrium of endogenous selenium nanoparticles in selenite-exposed cancer cells: a deep insight into the interaction between endogenous SeNPs and proteins

“…15,25,26 The percentage of elemental Se detected by EXAFS increased from 37% at 4 h to 55% at 48 h, before decreasing to 49% at 72 h in lung cancer H549 cells. 15 This result supports our finding that endogenous SeNP assembly is a dynamic flux process in cancer cells. Hyperdose selenium was usually recommended during cancer therapy according to experience.…”

Influência dos polimorfismos Pro198Leu, -602A/G e Arg5Pro na atividade da enzima glutationa peroxidase e no estado nutricional de indivíduos adultos com relação ao selênio

“…Em experimentos com animais, na dieta ou água para beber, sua faixa de adequação está na ordem de 0,1 a 1,0 µg/g (2). (2,27,28,29,30 (30,31,32).…”