LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 4
DISCUSSION
Numerous studies have implicated extracellular secreted signaling proteins in mediating the
bystander effect [289], but the strongest contenders have been reactive oxygen and nitrogen
species [299]. To investigate what factors are involved in radiation induced bystander effect, the
cells were treated with either L-NAME (NOS inhibitor) or cPTIO (NO scavenger). The effect of
cPTIO and L-NAME in bystander response between similar cell (EL-4 Vs EL-4) was different in
the fact that treatment with L-NAME reduces the bystander induction of NF-κB as well as iNOS
gene expression in EL-4 cells receiving medium from irradiated EL-4 cells (Fig. 3.4.5a, Lane 4)
but the treatment with cPTIO reduces the bystander induction of only NF-κB gene expression
and not iNOS gene expression (Fig. 3.4.5a, Lane 5). The above results demonstrate that the
activated NOS in the irradiated cell is essential for gene expression in the bystanding cell.
The addition of either L-NAME or cPTIO reduces the bystander induction of NF-κB as well as
iNOS gene expression in EL-4 cells receiving medium from LPS stimulated and irradiated RAW
264.7 cells (Fig. 3.4.5a, Lane 6 and 7), indicating that both activation of iNOS in the irradiated
cell and NO production play a role in cross bystander effect.
The treatment with L-NAME or cPTIO also reduces the bystander induction of DNA damage in
EL-4 cells receiving medium from irradiated EL-4 cells or LPS stimulated and irradiated RAW
264.7 cells (Fig. 3.4.5b, Lane 4, 5, 6 and 7), demonstrating that NO contributes to the DNA
damage in bystander EL-4 cells. NO is generated endogeneously from L-arginine by inducible
NO synthases [300, 301] that can be stimulated by radiation in mammalian cells [302]. It is
believed that NO itself does not induce DNA strand breaks, although one of its oxidation
product, peroxynitrite is toxic and can cause DNA damage by both attacking deoxyribose and
direct oxidation of purine and pyrimidine [303]. Radiation induced NO has the possibility of
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