Pathophysiologic role of oxidative stress-induced poly(ADP-ribose) polymerase-1 activation: focus on cell death and transcriptional regulation.

PARP-1 is a nuclear enzyme activated by DNA breaks. Activated PARP-1 cleaves NAD into nicotinamide and ADP-ribose and polymerizes the latter covalently coupled to nuclear acceptor proteins. Poly(ADP-ribosyl)ation has been implicated in the regulation of a diverse array of cellular processes ranging from DNA repair, chromatin organization, transcription, replication to protein degradation. On the 'dark side' of poly(ADP-ribosyl)ation, PARP-1 activation has been shown to contribute to tissue injury in shock, diabetes, myocardial or cerebral ischemia reperfusion and various forms of inflammation, as proven by pharmacological studies as well as experiments utilizing PARP-1 knockout animals. To our current knowledge, two mechanisms are responsible for the beneficial effects of PARP inhibitors in inflammatory, neurodegenerative and ischemia-reperfusion-based diseases: (i) inhibition of cell death caused by over-activation of PARP-1; (ii) inhibition of inflammatory signal transduction and production of inflammatory mediators. Here we review the possible regulatory mechanisms (e.g. calcium signaling, metabolism, density-dependent signaling, kinase cascades) of the PARP-1-mediated cell death pathway and discuss recent developments shedding new light on the complex role of PARP-1 in the regulation of the expression of inflammatory mediators.

Partial protection by poly(ADP-ribose) polymerase inhibitors from nitroxyl-induced cytotoxity in thymocytes.

Nitroxyl (NO(-)/HNO), has been proposed to be one of the NO(*)-derived cytotoxic species. Although the biological effect of nitroxyl is largely unknown, it has been reported to cause DNA breakage and cytotoxicity. We have therefore investigated whether NO(-)/HNO-induced DNA single-strand breakage activates the nuclear nick sensor enzyme poly(ADP-ribose) polymerase (PARP) and whether PARP activation affects the mode of NO(-)/HNO- induced cell death. NO(-)/HNO generated from Angeli's salt (AS, sodium trioxodinitrate) (0-300 microM) induced DNA single-strand breakage, PARP activation, and a concentration-dependent cytotoxicity in murine thymocytes. AS-induced cell death was also accompanied by decreased mitochondrial membrane potential and increased secondary superoxide production. The cytotoxicity of AS, as measured by propidium iodide uptake, was abolished by electron acceptors potassium ferricyanide, TEMPOL, the intracellular calcium chelator BAPTA-AM, and by PARP inhibitors 3-aminobenzamide (3-AB) and PJ-34. The cytoprotective effect of 3-AB was paralleled by increased output of AS-induced apoptotic parameters such as phosphatidylserine exposure, caspase activation, and DNA fragmentation. No significant increase in tyrosine nitration could be observed in AS-treated thymocytes as opposed to peroxynitrite-treated cells, indicating that tyrosine nitration is not likely to contribute to NO(-)/HNO-induced cytotoxicity. Our results demonstrate that NO(-)/HNO-induced PARP activation shifts the default apoptotic cell death toward necrosis in thymocytes. However, as total PARP inhibition resulted only in 30% cytoprotection, PARP-independent mechanisms dominate NO(-)/HNO-induced cytotoxicity in thymocytes.

Peroxynitrite production, DNA breakage, and poly(ADP-ribose) polymerase activation in a mouse model of oxazolone-induced contact hypersensitivity.

Peroxynitrite-induced poly(ADP-ribose) polymerase activation has been implicated in the pathogenesis of various inflammatory conditions. Here we have investigated whether peroxynitrite and poly(ADP-ribose) polymerase may play a role in the pathophysiology of the elicitation phase of contact hypersensitivity. We have detected nitrotyrosine, DNA breakage, and poly(ADP-ribose) polymerase activation in the epidermis of mice in an oxazolone-induced contact hypersensitivity model. As tyrosine nitration is mostly mediated by peroxynitrite, a nitric-oxide-derived cytotoxic oxidant capable of causing DNA breakage, we have applied peroxynitrite directly on mouse skin and showed poly(ADP-ribose) polymerase activation in keratinocytes and in some scattered dermal cells. We have also investigated the cellular effects of peroxynitrite in HaCaT cells, a human keratinocyte cell line. We found that peroxynitrite inhibited cell proliferation and at higher concentrations also caused cytotoxicity. Peroxynitrite activates poly(ADP-ribose) polymerase in HaCaT cells and poly(ADP-ribose) polymerase activation contributes to peroxynitrite-induced cytotoxicity, as indicated by the cytoprotective effect of the poly(ADP-ribose) polymerase inhibitor 3-aminobenzamide. The cytoprotective effect of 3-aminobenzamide cannot be attributed to inhibition of apoptosis, as apoptotic parameters (caspase activation and DNA fragmentation) were not reduced in the presence of 3-aminobenzamide in peroxynitrite-treated cells. Moreover, poly(ADP-ribose) polymerase inhibition by 3-aminobenzamide dose-dependently reduced interferon-induced intercellular adhesion molecule 1 expression as well as interleukin-1beta-induced interleukin-8 expression. Our results indicate that peroxynitrite and poly(ADP-ribose) polymerase regulate keratinocyte function and death in contact hypersensitivity.