333.2.3 Discussion
It has been reported that approximately 5–18 % of ART can bind to hemoproteins
such as hemoglobin, but does not react with heme-free globins (Meshnick et al. 1991 ).
Among versatile hemoproteins, ART’s targets for exerting antitumor activity are cur-
rently unidentified. Because NOS was previously proved a cytochrome P450-like
hemoprotein (Chinje and Stratford 1991 ), we planed to examine whether the formation
of ART-heme adducts might be correlated with the elevation of NO levels as well as
whether they would accompany with the elevation or decline of survival rates in HepG 2.
If this is the case, ART conjugation to NOS should lead to its inhibition and
induction. Indeed, we detected a dramatic increase of A 415 following the exposure
of HepG 2 to ART upon incubation for 24 h, suggesting the de novo biosynthesis of
hemoproteins including NOS. As indicated earlier, NO is able to inhibit the pro-
gressions of cysteine reduction and Fenton reaction (Gusarov and Nudler 2005 ).
Therefore, NO is likely exploited by bacteria to protect from oxidative damage,
and NO confers bacteria resistance to antibiotics (Gusarov et al. 2009 ).
With equal importance, it has been filed that NO can regulate cancer growth,
migration, invasion, survival, angiogenesis, and metastasis in a concentration-dependent
manner (Ridnour et al. 2006 ; Jang and Kim 2002 ). Evidence supporting a beneficial
role of NO to tumor survival comes from a finding that NO levels are higher in
cancerous tissues compared to their normal counterparts (Wink et al. 1993 ). This find-
ing was later deciphered as that NO is harnessed to protect against cellular damage and
cytotoxicity from ROS and organic peroxides (Wink et al. 1995 ).
Our results indicated that a lower concentration (50 μM) of ART allows tumor
cells survival with a higher rate, whereas higher concentrations (100–200 μM)
of ART cause tumor cell death. Interestingly, 50 μM ART triggers NO burst, but
100–200 μM ART does not. Therefore, it can be deduced that ART-induced pro-
tective NO should benefit to tumor cell survival. This conclusion is supported by
a previous observation that the cytotoxicity of ART in the mouse macrophage cell
line RAW 264.7 is associated with the inhibition of NOS (Konkimalla et al. 2008 ).
Additionally, we did not detect any significant changes of H 2 O 2 levels after ART
treatment although CAT is also a hemoprotein. This is likely because ART only
inhibits the heme-harboring CAT, but does not inhibit the heme-independent GSH-
POX, which can detoxify H 2 O 2 and may maintain a homeostatic redox state.
In summary, a low dose of ART does not exert an antitumor role because it
induces the production of a protective NO level. In contrast, a high dose of ART
exhibits a potent antitumor effect by its direct cytotoxicity.
3.2.4 Conclusions
ART kills cancer cells with uncertain mechanisms. Here, we report that ART can
exert its antitumor activity by alkylating hemoproteins such as NOS in tumor
cells. The conjugation of ART with the heme moiety of hemoproteins was
3.2 ART Diminishes NO-Conferred Anticytotoxicity ...