J Thorac Cardiovasc Surg 2004;128:324-325
© 2004 The American Association for Thoracic Surgery
Reply to the Editor
Gábor Szabó, MD, PhDa,
Csaba Szabó, MD, PhD, DScb
a Department of Cardiac Surgery, University of Heidelberg, Im Neuenheimer Feld 110, 69120 Heidelberg, Germany
b Inotek Pharmaceuticals Corporation, Beverly, MA 01915, USA
In response to the comments of Bloch and Mehlhorn on the effects of poly–adenosine diphosphate–ribose polymerase (PARP) inhibition after cardioplegic arrest and reperfusion, we agree that the PARP enzyme family has a complex regulatory role under several physiologic and pathologic conditions. After a decade of intensive research, we reported on the role of PARP in general1 and focusing on ischemia-reperfusion injury.2,3 To date, the role of PARP in physiologic DNA repair, apoptosis, and necrosis can be summarized as follows: depending on the severity of DNA damage, genotoxic stimuli can trigger three different pathways. In the case of mild DNA damage, PARP facilitates DNA repair and thus survival. However, the exact physiologic role of PARP still remains to be clarified; many authors have suggested that PARP is an abundant enzyme with limited role under physiologic conditions. More severe DNA damage induces apoptotic cell death, during which caspases, the main executor enzymes of apoptotic process, inactivate PARP, cleaving it into two fragments (p89 and p24) and thus PARP cleavage by caspases is a marker of apoptotic cell death. This pathway allows cells with irreparable DNA damage to become eliminated in a safe way. Much of the cell death related literature focuses on PARP cleavage (as opposed to PARP activation). The most severe DNA damage may cause excessive PARP activation, depleting oxidized nicotinamide adenine dinucleotide (NAD+) and adenosine triphosphate stores. NAD+/ATP depletion blocks apoptosis and results in necrosis. This pathway, which we proposed in the article under discussion, has no relationship to the PARP cleavage pathway. As mentioned previously, the cleaved form of PARP is catalytically inactive; PARP cleavage has been considered as an endogenous mechanism that serves to prevent PARP-dependent metabolic suppression and necrosis.
Considering the previously mentioned mechanisms, the cited study of Fischer and colleagues4 is not contrary to but supportive of our results. They found caspase activation but no apoptotic cell death and no PARP cleavage in a similar model of cardiopulmonary bypass and cardiac arrest. On the basis of these findings, Bloch and Mehlhorn criticized our study, suggesting that pharmacologic PARP inhibition may be detrimental by inhibiting physiologic DNA repair and promoting apoptosis.
We do believe that the fact they did not find apoptotic cell death and PARP cleavage (inactivated PARP) after cardioplegic arrest indicates that the primary form of cell damage is the necrotic (or prenecrotic) pathway, with concomitant PARP activation instead of PARP cleavage. It is also evident that cardioplegic arrest leads to adenosine triphosphate depletion, and therefore it is not surprising that no or negligible apoptosis occurs during reperfusion, because apoptosis is an adenosine triphosphate–dependent process. The loss of the ability of the cells to undergo apoptosis results in missing PARP cleavage and turns the cells toward PARP activation and metabolic suppression. Indeed, there is immunohistochemical evidence of PARP activation in different models of cardioplegic arrest and reperfusion.2,5 It is also important to note that cellular PARP activation does not always need to culminate in full-fledged cellular necrosis (complete breakdown of cell membrane integrity and release of cellular content into the extracellular space); a partial, and reversible suppression of cellular energetic pools can be associated with reversible cell dysfunction. These phenomena are sometimes termed prenecrosis or cytopathic hypoxia and are reversible by PARP inhibition.6,7
The questions raised about additional functions of PARP, its potential toxicity, and its role in aging were far beyond the scope of our study; however, we refer to a previous review article in which they were discussed and considered in detail.1 We note, nevertheless, that in addition to participating in the NAD depletion/prenecrosis/necrosis pathway PARP activity plays an active role in the transcription of various proinflammatory genes. Suppression of this process thus may provide additional cardioprotective benefits during and after cardioplegia.
The reference made by Bloch and Mehlhorn to stroke and NAD depletion does not have any direct relationship to our current work, but for the record we note that the vast majority of studies (more than 20 published reports, reviewed in Virág and Szabó1) demonstrate the marked protective effect of PARP inhibitors and genetic PARP deficiency in various models of stroke. In most experimental models of stroke (similar to myocardial ischemia) a prenecrotic/necrotic pathway of cell death (as opposed to apoptosis or PARP cleavage) appears to play the dominant role and can be beneficially affected by PARP inhibition.
Whether inhibition or scavenging of upstream effectors of apoptosis or necrosis, such as reactive oxygen species, would be more effective by reduction of endogenous PARP activation in addition to the inhibition of DNA damage and subsequent cell death remains to be clarified. We have preclinical experience with the novel porphyrinic peroxynitrite decomposition catalyst FP15,8,9 which shows beneficial effects during regional ischemia/reperfusion8,9 and after cardioplegic arrest (unpublished observations).
In conclusion, we maintain that PARP inhibition is a promising concept for reducing cell damage during ischemia and reperfusion. Even if alternative cardiac surgical techniques such as beating-heart surgery minimize ischemia-reperfusion injury, PARP inhibition may improve postoperative outcome in those cases in which cardioplegia is still necessary.
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References
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- Virág L, Szabó C. The therapeutic potential of poly(ADP-ribose) polymerase inhibitors. Pharmacol Rev. 2002;54:375–429[Abstract/Free Full Text]
- Szabó G, Liaudet L, Hagl S, Szabó C. Poly(ADP-ribose) polymerase activation in the reperfused myocardium. Cardiovasc Res. 2004;61:471-80
- Liaudet L, Szabó G, Szabó C. Oxidative stress and regional ischemia-reperfusion injury: the peroxynitrite-poly (ADP-ribose) polymerase connection. Coron Artery Dis. 2003;14:115–122[Medline]
- Fischer UM, Klass O, Stock U, Easo J, Geissler HJ, Fischer JH, et al. Cardioplegic arrest induces apoptosis signal-pathway in myocardial endothelial cells and cardiac myocytes. Eur J Cardiothorac Surg. 2003;23:984–990[Abstract/Free Full Text]
- Szabó G, Bährle S, Stumpf N, Sonnenberg K, Szabó EE, Pacher P, et al. Poly (ADP-Ribose) polymerase inhibition reduces reperfusion injury after heart transplantation. Circ Res. 2002;90:100–106[Abstract/Free Full Text]
- Khan AU, Delude RL, Han YY, Sappington PL, Han X, Carcillo JA, et al. Liposomal NAD(+) prevents diminished O(2) consumption by immunostimulated Caco-2 cells. Am J Physiol Lung Cell Mol Physiol. 2002;282:L1082–1091[Abstract/Free Full Text]
- Szabó C. Pathophysiological aspects of cellular pyridine nucleotide metabolism: focus on the vascular endothelium. Review. Acta Physiol Hung. 2003;90:175–193[Medline]
- Bianchi C, Wakiyama H, Faro R, Khan T, McCully JD, Levitsky S, et al. A novel peroxynitrite decomposer catalyst (FP-15) reduces myocardial infarct size in an in vivo peroxynitrite decomposer and acute ischemia-reperfusion in pigs. Ann Thorac Surg. 2002;74:1201–1207[Abstract/Free Full Text]
- Pacher P, Liaudet L, Bai P, Mabley JG, Kaminski PM, Virág L, et al. Potent metalloporphyrin peroxynitrite decomposition catalyst protects against the development of doxorubicin-induced cardiac dysfunction. Circulation. 2003;107:896–904[Abstract/Free Full Text]
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