Abstract 3789: Harnessing the electrophilic character of michael acceptor- and isothiocyanato-centric compounds as thioredoxin reductase inhibitors
Authors: Chew Eng-Hui , Kaminska Kamila K. , Gan Fei-Fei , Zeng Wanling , Neo Rui-Yi , Reddy Shridhivya A. , Thirumoorthi Navanita d/o T. , Chai Christina , Bertrand Helene , Wells Geoff 
Published: May 2015

Due to the high proliferation rate of cancer cells, reactive oxygen species, a metabolic by-product of aerobic respiration, poses a threat to cell survival. Cancer cells therefore rely heavily on intracellular redox systems to attain redox homeostasis. The thioredoxin and glutathione systems are the two major thiol redox systems that play a key role in maintaining intracellular redox homeostasis. The thioredoxin system comprises thioredoxin, thioredoxin reductase and NADPH. Thioredoxin reductase catalyzes the NADPH-dependent reduction of the active site disulfide of oxidized thioredoxin. Through a conserved redox active dithiol/disulfide motif (-Cys-xx-Cys-), thioredoxins participate in reversible thiol-disulfide exchange reactions so as to protect cellular proteins from oxidative damage. Thioredoxin is also involved in cell signaling pathways that regulate cell proliferation and apoptosis. However, in cancer, the biological effects of the thioredoxin system contribute to tumor growth and progression.

Accumulating evidence has indicated that the selenocysteine-dependent thioredoxin reductase enzyme is a valid molecular target for anticancer drug development. Numerous natural products and synthetic compounds, including several clinically used anti-cancer agents have been recognized to target thioredoxin reductase. In our laboratory, we had evaluated the possible inhibitory effect of a panel of structurally diversified naturally-occurring compounds and their synthetic derivatives on thioredoxin reductase activity. These compounds contain either an α,β-unsaturated carbonyl (also known as Michael acceptor) or isothiocyanato (N=C=S) moiety, which contribute towards their electrophilic propensities. Analogs bearing superior structural modifications were found to display anti-proliferative activities against HCT116 colorectal and MCF-7 breast carcinoma cells at GI50 values in the sub-micromolar range, as well as inhibit activity of recombinant rat thioredoxin reductase with half-maximal inhibitory concentrations of 10-20 micromolar. Moreover, treatment of the susceptible cancer cells with these compounds at apoptosis inducing doses led to a reduction in cellular thioredoxin reductase activity. At a molecular level, the tested compounds inhibited thioredoxin reductase, which caused thioredoxin oxidation and dissociation of the thioredoxin-apoptosis signal-regulating kinase 1 (ASK1) complexes. This led to ASK1-dependent activation of the JNK and p38 MAPK pathways to result in apoptosis induction.

In conclusion, from the results obtained, we propose that thioredoxin reductase inhibition is a critical cellular event that contributes to the proapoptotic effects of electrophilic bioactive compounds.

Citation Format: Eng-Hui Chew, Kamila K. Kaminska, Fei-Fei Gan, Wanling Zeng, Rui-Yi Neo, Shridhivya A. Reddy, Navanita d/o T. Thirumoorthi, Christina Chai, Helene Bertrand, Geoff Wells. Harnessing the electrophilic character of michael acceptor- and isothiocyanato-centric compounds as thioredoxin reductase inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3789. doi:10.1158/1538-7445.AM2014-3789