Selenium metabolism in cancer cells: the combined application of XAS and XFM techniques to the problem of selenium speciation in biological systems

Abstract

Determining the speciation of selenium in vivo is crucial to understanding the biological activity of this essential element, which is a popular dietary supplement due to its anti-cancer properties. Hyphenated techniques that combine separation and detection methods are traditionally and effectively used in selenium speciation analysis, but require extensive sample preparation that may affect speciation. Synchrotron-based X-ray absorption and fluorescence techniques offer an alternative approach to selenium speciation analysis that requires minimal sample preparation. We present a brief summary of some key HPLC-ICP-MS and ESI-MS/MS studies of the speciation of selenium in cells and rat tissues. We review the results of a top-down approach to selenium speciation in human lung cancer cells that aims to link the speciation and distribution of selenium to its biological activity using a combination of X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy (XFM). The results of this approach highlight the distinct fates of selenomethionine, methylselenocysteine and selenite in terms of their speciation and distribution within cells: organic selenium metabolites were widely distributed throughout the cells, whereas inorganic selenium metabolites were compartmentalized and associated with copper. New data from the XFM mapping of electrophoretically-separated cell lysates show the distribution of selenium in the proteins of selenomethionine-treated cells. Future applications of this top-down approach are discussed.

Figure 1

The structures, names and abbreviations of Se compounds referred to in this review.

Figure 2

An overview of the metabolism of the dietary selenium compounds selenite, SeMet and MeSeCys. See text for references.

Figure 3

Optical micrographs (top left), scattered X-ray (XS) and XFM elemental distribution maps of A549 cells treated with (a) 50 µM SeMet (the white arrow indicates the perinuclear accumulation of Se), (b) 50 µM MeSeCys or (c) 5 µM selenite. The range of elemental area densities (quantified from standards and expressed in micrograms per square centimeter) is given in the bottom corner of each map.

Figure 4

Library of Se K-edge X-ray absorption spectra of model selenium compounds used in the linear combination fitting of experimental spectra. XANES spectra of cells treated with 100 µM SeMet (blue), 100 µM MeSeCys (red) or 5 µM selenite (orange) are also shown for comparison.

Figure 5

Photograph (top right) and XFM elemental distribution maps of S, Se and an overlay of S and Se in an SDS-PAGE blot of the lysates of cells treated with selenium compounds for 24 h. The wells contain lysates of cells treated with (a) 5 µM selenite, (b) PBS alone (as vehicle control), (c) 50 µM MeSeCys, (d) 50 µM SeMet, (e) 1 µM selenite or (f) 100 µM SeMet. The color scheme indicates the relative concentration within each elemental map.