ISSN 1990-519X, Cell and Tissue Biology, 2011, Vol. 5, No. 4, pp. 383–387. © Pleiades Publishing, Ltd., 2011. Original Russian Text © D.V. Firsanov, A.V. Kropotov, V.M. Mikhailov, 2011, published in Tsitologiya, Vol. 53, No. 5, 2011, pp. 355–358. 383 1 DNA double-strand breaks (DSBs) are the most dangerous lesions in eukaryotic cells. If unrepairable they can cause cell death or carcinogenesis. One of the earliest responses to DSBs caused by ionizing radia- tion (IR) or by chemicals is the phosphorylation of core variant histone H2AX at 139-serine in megabase chromatin domains around DSB sites with formation of discrete nuclear γ-H2AX foci (Rogakou et al., 1998, 1999; Tomilin et al., 2001). Members of the phos- phatidyl-inositol-3-kinase (PI3-K) protein kinase (PIKK) family, including ataxia telangiectasia mutated (ATM), DNA-PK and ATR (ATM and Rad3-related) are all thought to phosphorylate histone H2AX in vivo (Motoyama, Naka, 2004; Stiff et al., 2004). In human cells the maximum induction of γ-H2AX is observed about 1 h after IR. Then it is slowly eliminated, and kinetics of elimination corre- lates with the kinetics of DSBs rejoining (Nazarov et al., 2003; Svetlova et al., 2007). It was shown that the time of half elimination of γ-H2AX correlated with radio-sensitivity in human cell lines and mammalian tissues (Olive, Banath, 2004), and that variations existed between mammalian tissues in dynamics of γ- 1 The article was translated by the authors. H2AX formation and elimination after IR (Gavrilov et al., 2006; Koike et al., 2008). Forskolin is a natural compound found in the coleus herb that activates the enzyme adenylate cyclase, increases the concentration of intracellular cyclic AMP and decreases the level of γ-H2AX in cul- tured cells after IR (Solovjeva et al., 2009). It is well known that poly-(ADP-ribose) polymerase (PARP) is activated at DNA damage sites and it ribosilates pro- teins around DNA damage using nicotinamide ade- nine dinucleotide (NAD + ) as a substrate (Surjyana et al., 2010). Overactivation of PARP can cause NAD + pool depletion and cell energy crisis thus leading cells to necrotic death (Carson et al., 1986). It was shown that NAD + content of cells influences responses to DNA damaging agents (Surjyana et al., 2010). It was also reported there that NAD + could be a free radical scavenger (Surjyana et al., 2010). Thus energy pro- cesses in cells could play a crucial role in DNA damage response and DSBs repair efficiency. Here we examined the influence of exogenous NADP on histone γ-H2AX formation in mouse heart cells after IR. We have found that exogenous injection of NADP in different doses increases the level of γ-H2AX in mouse heart cells of C57Bl/6 mice after IR Exogenous NADP Increases the Level of Histone H2AX Phosphorylation in Mouse Heart Cells after Ionizing Radiation 1 D. V. Firsanov, A. V. Kropotov, and V. M. Mikhailov Institute of Cytology, Russian Academy of Sciences e-mail: dfirsanov@gmail.com Abstract—Phosphorylation of replacement histone H2AX occurs in megabase chromatin domains around DNA double-strand breaks (DSBs), and this modification called γ-H2AX can be used as an effective marker for DSBs repair and DNA damage response. Using Western blotting and immunohistochemistry techniques we have studied here the influence of exogenous nicotinamide adenine dinucleotide phosphate (NADP), which can potentially increase the level of intracellular NAD + , on the level of γ-H2AX formation in mouse heart cells after ionizing radiation (IR). We have found that injection of NADP in different doses immediately after IR causes an increased level of γ-H2AX in mouse heart cells 20 min after IR at the dose of 3 Gy com- pared to control mice after IR exposure. It indicates that there could be a relationship between intracellular NAD + content and DNA damage response in vivo. Keywords: Ionizing radiation; Histone H2AX phosphorylation; NADP; DNA damage response. Abbreviations: γ-H2AX, histone H2AX with phosphorylated C-terminal serine; DSB, double-strand DNA break; IR, ionizing radiation; NADP, Nicotinamide adenine dinucleotide phosphate DOI: 10.1134/S1990519X11040055