LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 4
DISCUSSION
4.2 Proton beam induced signaling in mammalian cells and its comparison with Gamma
irradiation.
After establishing that A549 cells were relatively more radioresistant if these cells were exposed
to fractionated irradiation, the variance in signaling pattern and effectiveness of cell killing with
the change in LET was studies in A549 cells.
More than 2 fold decrease in survival of the A549 cells as assessed by the clonogenic cell
survival assay was indicative of the fact that the proton beam is more efficient in killing the
tumor cells and may be a more preferred mode of treatment (Fig.3.2.1). The mechanisms of cell
death caused by proton treatment and the signaling pathways that ensue have not yet been
investigated in detail although proton beam therapy is in use for many cancers [259-264].
The massive activation of DNA-PK and ATM after proton irradiation when compared
with gamma (Fig.3.2.2 A &B) was more or less expected and may be due to the fact that DNA
damage activates Phosphatidylinositol 3-kinase-like kinases (DNA-PK, ATM), which then
amplify and channel the signal by activating downstream kinases (Chk1 and Chk2). These, in
turn, phosphorylate target proteins such as p53, Cdc25A and Cdc25C [11] to delay the cell cycle,
a process called the DNA damage checkpoint. In addition to checkpoint control, the downstream
kinases also modulate DNA repair and trigger apoptosis [13]. The lack of activation of ATR after
proton irradiation was however intriguing and was found to be reflected no activation of Chk1
(Fig.3.2.2).
Phosphorylation of H2AX even at 4 hours in A549 cells that had been exposed to proton
beam indicates the presence of large number of double strand breaks at that time indicating very
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