Our previous studies on active constituents of Nigella sativa have indicated that cell death induced by
thymoquinone and
alpha-hederin was dose- and time-dependent, in a range of four
cancer cell lines. Both compounds elicited
necrosis and apoptosis with a higher incidence of the latter induced by
thymoquinone. As HEp-2 human laryngeal
carcinoma cells were the most susceptible, we sought to better understand the mechanisms involved by using
buthionine sulfoximine (BSO), a selective inhibitor of
glutathione (GSH) synthesis, to determine the importance of GSH in the apoptosis elicited, using
cisplatin as internal standard. BSO significantly enhanced
alpha-hederin- and
cisplatin- mediated toxicity as assessed by the MIT assay, without changes in apoptosis or
necrosis levels. Although the MTI assay did not indicate BSO potentiation of
thymoquinone, apoptosis levels were significantly enhanced following this combination, without changes in
necrosis.
Thymoquinone and
cisplatin significantly decreased GSH levels in a dose-dependent manner, with BSO pre-treatment synergistically depleting GSH levels in only
thymoquinone- treated cells. As the
caspase 3 inhibitor,
Z-DEVD-fmk significantly decreased
thymoquinone- and
cisplatin-induced apoptosis, GSH depletion and
caspase 3-activation mediate
thymoquinone-induced apoptosis, in this cell line.