37In the ASC group, CAT activity from 50 U/mg of total proteins in the control drops
to 5 U/mg of total proteins as tested after 24 h, and 10 U/mg of total proteins as tested
after 48 h. In the ART group, GSH content (100 mg/g of total proteins) and GSH-POX
activity (150–230 U/g of total proteins) are slightly lower than those of the control
(90 mg/g of total proteins and 200–230 units/g of total proteins, respectively), whereas
CAT activity (20 U/mg of total proteins) is significantly lower than that of the control
(50 U/mg of total proteins). These results confirmed that pro-oxidants can potentate
ART’s antitumor activity by suppressing the antioxidative capacity of tumor cells.
3.3.3 Discussion
The sensitizer of radiotherapy, buthionine sulfoximine (BSO), is the most com-
monly used GSH inhibitor, leading to the decline of GSH levels by inhibiting
γ-GSH synthase responsible for GSH biosynthesis (Clark et al. 1984 ). Depletion
of GSH is an important step toward the apoptosis of tumor cells via a ROS-
dependent signaling pathway. The rapid proliferation of tumor cells generally
leads to the massive production of ROS, but tumor cells can accordingly induce
a high level of antioxidant enzymes to scavenge ROS. If GSH is depleted, tumor
cells should be unable to scavenge O 2 − via GSH-POX.
In previous work conducted by other authors, ART was combined with trans-
ferrins to increase the antitumor capacity of ART because transferrins enable the
accumulation of more bioavailable iron and allow a potent burst of ROS (Lai and
Singh 2001 ). It is also known that H 2 O 2 can also induce the apoptosis of tumor
cells (Simizu et al. 1998 ). Therefore, the tumor-killing potential of ART should be
greatly enhanced when H 2 O 2 is increased due to the inhibition of CAT and POX.
It is anticipated that ART as an alkylator of hemoproteins including CAT should
Fig. 3.2 Comparison of antioxidant activities in HepG 2 cells after treatment by ART or ASC for
24 and 48 h. ART artemisinin; ASC artemisinin-sensitizing compound; CAT catalase; CK control;
GSH glutathione (reduced); GSH-POX glutathione(reduced)-peroxidase. A single asterisk (*)
represents significant difference from the control (P < 0.05)
3.3 Pro-oxidant Agents Synergize ART in Killing Tumor Cells