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LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

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Cancer Investigation, 28:615–622, 2010<br />

ISSN: 0735-7907 print / 1532-4192 online<br />

Copyright c○ Informa Healthcare USA, Inc.<br />

DOI: 10.3109/07357901003630991<br />

ORIGINAL ARTICLE<br />

Cellular and Molecular Biology<br />

Low Energy Proton Beam Induces Efficient Cell<br />

Killing in A549 Lung Adenocarcinoma Cells<br />

<strong>Somnath</strong> <strong>Ghosh</strong>, 1 Nagesh N. Bhat, 2 S. Santra, 3 R. G. Thomas, 3 S.K. Gupta, 3 R.K. Choudhury, 3 and Malini Krishna 1<br />

Radiation Biology and Health Sciences Division, 1 Radiological Physics and Advisory Division, 2 Nuclear Physics Division, <strong>Bhabha</strong> Atomic<br />

Research Centre, Trombay, Mumbai, India 3<br />

Cancer Invest Downloaded from informahealthcare.com by University of Chicago Library on 06/10/11<br />

For personal use only.<br />

ABSTRACT<br />

The aim of the current study was to determine the signaling differences between γ- and<br />

proton beam-irradiations. A549 lung adenocarcinoma cells were irradiated with 2 Gy proton<br />

beam or γ-radiation. Proton beam was found to be more cytotoxic than γ-radiation. Proton<br />

beam-irradiated cells showed phosphorylation of H2AX, ATM, Chk2, and p53. The mechanism<br />

of excessive cell killing in proton beam-irradiated cells was found to be upregulation of Bax<br />

and downregulation of Bcl-2. The noteworthy finding of this study is the biphasic activation of<br />

the sensor proteins, ATM, and DNA-PK and no activation of ATR by proton irradiation.<br />

INTRODUCTION<br />

Exposureof mammalian cells to ionizing radiation leads to<br />

the activation of several signaling pathways that result in apoptosis.<br />

Although, the end point, apoptosis is effectively activated<br />

by conventional X-ray treatment, the development of radioresistance<br />

following therapy remains a major cause for concern.<br />

Clinicians are now favoring proton beam therapy to avoid the<br />

issue (1–6). However, the mechanism of cell killing by proton<br />

beam is not yet well delineated.<br />

Lung cancer is a frequently occurring, mostly lethal disease<br />

in all countries worldwide (7). It is one of the most commonly<br />

diagnosed types of cancer and has the highest death rate (8, 9).<br />

Overall, fewer than 10% of people with primary lung cancer<br />

are alive 5 years after diagnosis. Depending on tumor size, location,<br />

and histology, several treatment options are available,<br />

including surgery and radiotherapy (RT), both often combined<br />

with chemotherapy (10). A major problem remains local tumor<br />

control (11, 12). Therefore, new ways to deliver radiotherapy<br />

beyond photons have been sought for, including protons (13, 14)<br />

Keywords Proton beam, ATM, p53, Bax, Bcl-2<br />

Correspondence to:<br />

Malini Krishna<br />

Radiation Biology and Health Sciences Division<br />

<strong>Bhabha</strong> Atomic Research Centre<br />

Trombay, Mumbai<br />

India, 400085<br />

email: malini@barc.gov.in; malinik00@gmail.com<br />

and carbon ions (15). Furthermore, treatment with charged particle<br />

radiation has several potential advantages over treatment<br />

with γ or X-ray radiation such as the Bragg peak enables precise<br />

localization of the radiation dose, an inverted depth-dose<br />

distribution, a higher relative biological effectiveness (RBE),<br />

and a lower cellular capability for repair of radiation injury.<br />

Because of their superior dose distribution, a therapeutic gain<br />

can be expected with charged particles (1–4). Although proton<br />

beams are currently being used for the treatment of many cancers<br />

(1–6), the mechanism of cell killing and its variance from<br />

γ -irradiation is not well understood. Understanding the specific<br />

biological effects of charged particle radiation on cancer cells<br />

could also provide valuable insights for the design of novel<br />

therapeutic applications for the treatment of cancers, which are<br />

resistant to many types of therapies. Moreover, therapies can be<br />

designed only if the signaling pathway is understood.<br />

A cytological manifestation of nuclear activity in response<br />

to ionizing radiation (IR) is the formation of the socalled<br />

IR-induced foci (IRIF) (16). IRIFs are dynamic, microscopically<br />

discernible structures containing thousands of<br />

copies of proteins, including γ H2AX, ataxia telangiectasia<br />

mutated (ATM), BRCA1, 53BP1, MDC1, RAD51, and the<br />

MRE11/RAD50/NBS1 (MRN) complex, which accumulate in<br />

the vicinity of a DNA double-strand breaks (DSB) (17, 18). It is<br />

being appreciated lately that DNA repair pathways activated by<br />

charged particle radiation might be different from those studied<br />

after γ -irradiation.<br />

When exposed to ionizing radiation, eukaryotic cells also<br />

activate checkpoint pathways to delay the progression of the<br />

615

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