Abstract 3785: Isothiocyanates vs. isoselenocyanates: Reactivity toward glutathione and cysteinyl residues in cell and cell-free systems
Authors: Melissa Gimbor, Dhimant Desai, Julian Spallholz, Shantu Amin, Arun K. Sharma
Journal: Cancer Research (2010)

Abstract Isothiocyanates (ITCs) are naturally occurring compounds found in cruciferous vegetables. Both naturally occurring phenylalkyl ITCs (BITC, PEITC) and longer chain synthetic ITCs (PBITC, PHITC) have been studied extensively and are shown to inhibit carcinogenesis due to inhibition of CYP450 enzymes, as well as induction of Phase II detoxification enzymes. These actions are thought to be mediated by the reactivity of the ITC functional group (N=C=S) with sulfhydryl groups of cell thiols, mainly glutathione and proteins (cysteinyl residues). Our lab has modified both the naturally occurring and synthetic phenyl alkyl ITCs by isosterically replacing sulfur with selenium to make isoselenocyanates (ISCs): ISC1, ISC2, ISC4, and ISC6. We have found these compounds to be more potent in cancer cell viability assays and in xenograft mouse model at inhibiting tumor growth than corresponding ITCs. Our hypothesis is the better potency of ISC compounds may be due to their modified reactivity rates toward cell thiols. To test this, A549 lung cancer cells were treated with our panel of ITC compounds or corresponding ISC compounds from 500 nM to 15 µM for 1 hour. GSH depletion appeared to be dose dependent for all compounds except for hexyl chain compounds PHITC and ISC6. For the remaining compounds, ISC compounds depleted GSH in cells to a greater extent than corresponding ITCs. 15 µM of ISC1, ISC2, and ISC4 depleted GSH levels to 58%, 53%, and 63% of control, respectively, whereas BITC, PEITC, and PBITC only depleted GSH to 69%, 86%, and 86%, respectively. These results may be related to redox cycling capabilities which measures the reactivity of compounds with GSH and O2 to form superoxide. In a chemiluminesence assay measuring redox cycling, ISC compounds showed a much greater ability to redox cycle compared to corresponding ITCs. The redox cycling capabilities were as follows: ISC2≫PEITC&amp;gt;ISC1=ISC4=ISC6&amp;gt;PHITC&amp;gt;PBITC&amp;gt;BITC. In a cell free system, overall binding of ITC and ISC compounds were analyzed for GSH using the Ellman assay. In this cell free system both ITCs and ISCs showed a trend of increasing binding with decreasing chain length: BITC&amp;gt;PEITC&amp;gt;PBITC&amp;gt;PHITC and ISC1&amp;gt;ISC2&amp;gt;ISC4&amp;gt;ISC6. However, corresponding ITC and ISC compounds bound similarly to GSH. Similar experiment using tubulin protein exhibited the same trend of reactivity with ITCs, as with GSH. However, among ISC compounds, ISC2 showed the greatest binding followed by ISC1=ISC4&amp;gt;ISC6. These results suggest that cell environment is playing a role in the reactivity of these compounds with glutathione and proteins. In conclusion, ISC compounds depleted GSH in cells to a greater extent than corresponding ITC compounds and were also better able to redox cycle. Future studies will analyze the exact rate of reactivity of ISC compounds toward GSH, identifying protein targets of these compounds, and binding preferences of different proteins. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3785.