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Phenethyl Isothiocyanate Triggers Apoptosis in Jurkat Cells Made Resistant by the Overexpression of Bcl-2

¹ Free Radical Research Group, Department of Pathology, Christchurch School of Medicine and Health Sciences, University of Otago and ² School of Biological Sciences, University of Canterbury, Christchurch, New Zealand

Requests for reprints: Mark B. Hampton, Free Radical Research Group, Department of Pathology, Christchurch School of Medicine and Health Sciences, P.O. Box 4345, Christchurch, New Zealand. Phone: 64-3-364-1524; Fax: 64-3-364-1083; E-mail: mark.hampton{at}chmeds.ac.nz.

Isothiocyanates are a class of naturally occuring chemopreventive agents known to be effective at triggering apoptosis. In this study, we show that whereas overexpression of the oncoprotein Bcl-2 renders Jurkat T-lymphoma cells resistant to a range of cytotoxic agents, phenethyl isothiocyanate is able to overcome the inhibitory action of Bcl-2 and trigger apoptosis. A 50-fold increase in Bcl-2 expression shifted the dose-response curve, with an increase in the phenethyl isothiocyanate LD₅₀ from 7 to 15 µmol/L, but there was still a complete loss in cell viability at doses in excess of 20 µmol/L. At these concentrations, cytotoxicity was strongly associated with caspase activation, phosphatidylserine exposure, and morphologic changes characteristic of apoptosis. Cytotoxicity was inhibited by treatment of the cells with a broad-spectrum caspase inhibitor. A structure-activity analysis showed that the phenethyl and benzyl isothiocyanates were most effective at triggering apoptosis in cells overexpressing Bcl-2 whereas phenyl isothiocyanate and benzyl thiocyanate had no proapoptotic activity. Allyl isothiocyanate also had limited efficacy despite its ability to trigger apoptosis in the parental Jurkat cell line. From this information, we propose that isothiocyanates modify a key cysteine residue in an apoptosis regulatory protein and that the aromatic side chain facilitates access to the target site. An in-depth investigation of the cellular targets of the aromatic isothiocyanates is warranted.(Cancer Res 2006; 66(13): 6772-7)

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