Carcinogenesis Advance Access originally published online on August 3, 2009
Carcinogenesis 2009 30(10):1686-1695; doi:10.1093/carcin/bgp192
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Published by Oxford University Press 2009.
Intercellular communication of cellular stress monitored by 
-H2AX induction
Laboratory of Molecular Pharmacology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20952, USA
* To whom correspondence should be addressed. Tel: 301 435 8668; Fax: 301 402 0752; Email: dickeyj{at}mail.nih.gov
Correspondence may also be addressed to William M. Bonner. Tel: 301 496 5942; Fax: 301 402 0752; Email: bonnerw{at}mail.nih.gov
When cells are exposed to ionizing radiation (IR), unexposed cells that share media with damaged cells exhibit similar effects to irradiated cells including increased levels of DNA double-strand breaks (DSBs). Hypothesizing that this effect, known as the radiation-induced bystander effect, may be a specific instance of communication between damaged and undamaged cells regardless of damage source, we demonstrated that exposure of target cells to non-IR induces bystander damage in non-targeted cells as measured by 
-H2AX and 53BP1 focal formation. Initially, bystander damage was found primarily in S-phase cells, but at later times, non-S-phase cells were also affected. In addition, media from undamaged malignant and senescent cells also was found to induce DSBs in primary cultures. Media conditioned on cells targeted with either ionizing or non-IR as well as on undamaged malignant and senescent cells contained elevated levels of several cytokines. One of these, transforming growth factor beta (TGF-β), and nitric oxide (NO) were found to elevate numbers of -H2AX/53BP1 foci in normal cell cultures similar to levels found in bystander cells, and this elevation was abrogated by NO synthase inhibitors, TGF-β blocking antibody and antioxidants. These findings support the hypothesis that damage in bystander cells results from their exposure to cytokines or reactive compounds released from stressed cells, regardless of damage source. These results have implications for oncogenesis in that they indicate that damaged normal cells or undamaged tumor cells may induce genomic instability, leading to an increased risk of oncogenic transformation in other cells with which they share media or contact directly.
Abbreviations: BrdU, bromodeoxyuridine; DSB, double-strand break; fpc, foci per cell; IR, ionizing radiation; NBF, normal breast fibroblast; NHF, normal human fibroblast; NO, nitric oxide; PAI-1, plasminogen activator inhibitor 1; PCNA, proliferating cell nuclear antigen; PrEC, prostate epithelial cell; RIBE, radiation-induced bystander effect; SDS, sodium dodecyl sulfate; UVA, ultraviolet A; UVC, ultraviolet C
Received June 12, 2009; revised July 17, 2009; accepted July 25, 2009.