Furthermore stress hormones can indirectly induce DNA damage

Furthermore, stress hormones can indirectly induce DNA damage, for example by altering DNA repair and transcriptional regulation of the cell cycle. It has been found that stress hormones induce the up-regulation of checkpoint kinase 1 (Chk1) and checkpoint kinase 2 (Chk2) and proto-oncogene cell division cycle 25A (CDC25A), which is involved in cell cycle delay following DNA damage (Flint et al., 2007). DNA damage can block genome replication and transcription, and if it is not repaired it leads to genomic instability and an increased cancer risk. On the other hand, cells have evolved mechanisms, collectively termed the DNA-damage response (DDR), to detect DNA lesions, signal their presence and promote their repair (Jackson and Bartek, 2009). The nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1) acts as DNA damage sensor and is crucial for initiating DNA repair. Activation of PARP-1 is an immediate cellular response to DNA strand breakage and plays an important role in regulating genomic stability (Bürkle, 2001). Activated PARP-1 sequentially adds ADP-ribosyl moieties from nicotinamide JP 1302 dihydrochloride dinucleotide (NAD+) onto target proteins, thus creating long, linear and branched chains of poly(ADP-ribose) (PAR). It should be noted that NAD+ pools are replenished under consumption of adenosine triphosphate (ATP), and therefore massive PAR formation may lead to a strain on ATP pools (Ha and Snyder, 1999). PARylation of target proteins, especially auto-PARylation of PARP-1, is triggered when PARP-1 binds to sites of DNA damage, and this leads to recruitment of DNA repair proteins (Wei and Yu, 2016). At the same time, PARP-1 is involved in the regulation of cell death and inflammation in response to genotoxic stress (Laudisi et al., 2011) and might therefore be a potential mediator at the interface of DNA damage and cellular dysfunction. Excessive cell damage, however, triggers caspase dependent apoptosis (Elmore, 2007). PARP-1 itself is proteolytically cleaved into 89 kDa and 24 kDa fragments by caspases during the process, rendering PARP-1 fragmentation a well-accepted early marker of apoptosis (Germain et al., 1999; Kaufmann et al., 1993). Interestingly, under certain conditions PARP-1 activation can also lead to translocation of apoptosis-inducing factor (AIF) from the mitochondria to the nucleus (Yu et al., 2002), thus leading to caspase-independent cell death. In rats used as a model of posttraumatic stress disorder (PTSD), gene expression of caspase-9 and caspase-3 was significantly increased (Xiao et al., 2011), indicating a potential role of apoptosis in mediating stress-induced effects. Further, epinephrine is known to induce apoptosis in human endothelial cells, and this effect is associated with activation of Fas cell surface death receptor by the Fas ligand (Fas-FasL) and caspase-3 pathways (Romeo et al., 2000). Further studies show a positive correlation between daily stress and apoptosis in human lymphocytes cultured ex vivo (Sakami et al., 2002), and elevated levels of epinephrine drives porcine retinal pigment epithelial cells into apoptosis in vitro (Sibayan et al., 2000). However, although there is evidence that psychological stress induces DNA damage, impairs DNA repair and induces apoptosis, to our knowledge no data has been reported about its effect on PARP activity. In our previous in vivo study, we have shown that in patients suffering from posttraumatic stress disorder (PTSD) there is accumulation of DNA strand breaks in peripheral blood mononuclear cells and that narrative exposure therapy can reverse this effect (Morath et al., 2014). PTSD patients experience, for prolonged time periods, acute stress as a consequence of frequent flashbacks, during which they repeatedly re-experience their trauma. PTSD has been associated with higher baseline levels of norepinephrine (Liberzon et al., 1999) and epinephrine (Kosten et al., 1987). Early studies have shown that peripheral epinephrine release at the time of exposure to trauma can predict subsequent development of PTSD (Yehuda et al., 1998). Furthermore, administration of the β-adrenergic receptor antagonist propranolol, shortly after exposure to psychological trauma, reduces PTSD symptom severity (Pitman et al., 2002). We hypothesized that chronic release of epinephrine also affects the DNA damage response and we were interested in investigating the effect of epinephrine on PARP-1 as a sensor of DNA damage and DNA repair initiator. For this purpose we used, isoproterenol, a synthetic sympathomimetic amine that is structurally related to epinephrine. Although isoproterenol has similar intrinsic efficacy across all three receptor subtypes (Baker, 2010), adrenergic-binding sites expressed on immune cells are almost exclusively of the β2 subtype (Kin and Sanders, 2006) and therefore, in this case, isoproterenol acts almost exclusively on β2-AR.