Invited review
Potential roles of poly (ADP-ribose) polymerase in male reproduction

Introduction
Poly (ADP-ribose) polymerase (PARP) is a nuclear enzyme important for the detection of DNA strand breaks caused by genotoxic agents such as reactive oxygen species (ROS), ionizing radiation and alkylating agents or those caused by enzymatic incision of DNA-base lesions [1]. In testicular germ cells, PARP has a particularly well-researched role in base excision repair (BER), which is one of the primary repair mechanisms to resolve DNA lesions caused by endogenous and exogenous processes [2-6]. However, a similar role for PARP in human ejaculated spermatozoa is still being investigated and remains to be controversial [7-10].
Male germ cells are exposed to a wide variety of endogenous and exogenous genotoxic agents [11, 12]. Endogenous agents include reactive oxygen and nitrogen species generated during the metabolic activities of cells [13, 14]. Exogenous agents include various environmental factors that can inflict damage to genomic DNA. These genotoxic agents can introduce DNA lesions in the form of DNA single-strand breaks, double-strand breaks, base damage, inter- and intra-stand cross links and DNA-protein cross-links.
To counteract this wide spectrum of DNA lesions, eukaryotic cells possess efficient DNA repair pathways. In addition to BER, these include nucleotide excision repair (NER), mismatch repair (MMR), non-homologous end-joining (NHEJ), and homologous recombination (HR). These five major pathways operate alone or in combination to remove the wide array of DNA lesions and restore genomic stability. Some proteins like PARP participate in multiple DNA repair pathways [15-17].
Not only does PARP have a well-defined role in DNA repair, it can also serve as biochemical marker of caspase-dependent apoptosis [15, 21]. During apoptosis, numerous DNA strand breaks lead to PARP activation. This may be an attempt by the dying cell to repair the DNA damage caused by nuclease activation [18]. However, this attempt to repair damage proves futile as PARP is cleaved by caspase-3 into a catalytic fragment of 89 kDa and DNA binding unit of 24 kDa [19, 20].
Recently, there has been growing interest in the role of PARP in the management malignancy including the use of PARP inhibition as an adjuvant therapy with chemotherapeutic drugs [22]. As our focus lies primarily in male infertility, we highlight in this review the possibilities of PARP involvement in male infertility as well as the possible role of PARP modulation to control DNA damage in male germ cells. This may reveal a new therapeutic option for the correction and repair of sperm DNA damage.
Structural and functional classifications of poly (ADP-ribose) polymerase
The PARP family contains 18 homologues with a conserved catalytic domain made up of 50 amino acids that serve as the “PARP signature” [23]. This is the site where PARP chains are initiated and elongated and where branching of the chains can occur [24]. Besides this catalytic domain, other PARP family members may have other domains, including DNA binding domains, macrodomains, BRCT (BRCA1 C terminus) domain found in proteins responding to DNA damage, ankyrin repeats, and WWE domains found in proteins associated with ubiquitination. All of these special types of domains contribute to the unique functions of each family member [23, 25].
Poly (ADP-ribose) polymerase family members can be divided into several subcategories based on each protein’s established functional domains and precise functions. The first category consists of the DNA-dependent PARP (PARP-1 and PARP-2), which are activated by DNA strand breaks. The second group is the tankyrases (tankyrase-1 and tankyrase-2), which serve diverse functions such as telomere regulation and mitotic segregation. The third group is the CCCH-type PARP (PARP-12, PARP-13, and TCDD-inducible PARP), which contains special CCCH-type zinc fingers. Lastly, the fourth group (PARP-9, PARP-14, and PARP-15) consists of macro-PARP that have one to three macro-domains connected to a PARP domain. PARP-8, PARP-11, PARP-16, and PARP-6 do not have sufficient known domains functions to be assigned a role [26].
A recent classification system by Hassa and Hottiger compared the catalytic domain sequences of these enzymes. They divided the PARP family into three separate groups: group 1 consists of PARP-1, PARP-1b (short PARP-1, PARP-2, and PARP-3). Group 2 consists of only PARP-4, and group 3 consists of tankyrase-1, tankyrase-2a, and its isoform tankyrase-2b (also known as PARP-5 and PARP-6a/b) [24]. The various PARP enzymes also can have different subcellular localization patterns. PARP-1 and PARP-2 are considered nuclear enzymes and are found in the nucleus of cells. In contrast, the tankyrases and PARP-3 are found in both the nucleus and cytoplasm [27].
Perhaps the best studied member of the PARP family is PARP-1, a 113 kD enzyme encoded by the ADPRT gene in humans located on chromosome 1 [28, 29]. The protein structure of PARP-1 is very well characterized. It is made up of four functional domains, including a DNA binding domain consisting of structures known as zinc fingers that can bind to DNA breaks. A second domain contains the nuclear localization signal (NLS), which ensures PARP-1 is found in the nucleus, and it is also a site of cleavage by caspase-3. PARP-1 also includes an automodification domain and a domain that holds the enzyme’s catalytic activity [23].
Modulation of poly (ADP-ribose) polymerase activity
Modulation of PARP activity is important for exploring this enzyme’s therapeutic options. Several types of molecules have been identified as activators of PARP activity, including histones, the common target of PARP activity. Although histones are modified by PARP-1, histones H1 and H3 actually can activate PARP-1. An important enzyme involved in regulating histone structure, SIRT-1 (a histone deacetylase) enzyme also is involved in regulating PARP-1 activity. In the absence of SIRT-1, PARP is not regulated, and cell death regulated by apoptosis-inducing factor (AIF) occurs [49]. Magnesium ions, calcium ions, and polyamines are allosteric activators of the auto-(poly-ADP-ribosyl)ation activity of these enzymes. It should be noted that calcium ions also play an important role in oxidative stress pathophysiology [50].
Poly (ADP-ribose) polymerase has an extensive list of inhibitors that are used extensively to study PARP activity. Purines such as hypoxanthine and inosine are endogenous inhibitors [51]. Interestingly, caffeine derivatives can inhibit PARP-1 activity as well [52-55], in addition tetracycline derivative also have the ability to inhibit PARP-1 activity [56]. Phosphorylation of PARP is a more complicated type of modulation. It was found that phosphorylation of PARP-1 by ERK 1 and 2 (extracellular signal-regulated kinases) was important for regulating neuronal cell death [57]. Poly (ADP-ribose) polymerase also can be phosphorylated by DNA-PK (DNA-dependent protein kinase) a major protein that takes part in double-stranded break repair. DNA-PK phosphorylates PARP and suppresses PARP activity [58].
Poly (ADP-ribose) polymerase in the context of male reproduction
Poly (ADP-ribose) polymerase plays a crucial role in maintaining the genomic integrity in a variety of cell types. Perhaps nowhere is this genomic integrity more important than in germ cells. Cases of male infertility are associated with abnormal sperm chromatin and DNA structure. The problems that arise in genomic integrity of sperm come from a variety of sources, including spermatogenesis defects, abortive apoptosis, problems with spermatid maturation, and oxidative stress [76]. Problems in spermatogenesis could include double-stranded breaks that are not resolved after crossing over during meiosis I [76].
The role of PARP in male fertility is not as well defined as its role in cellular processes. However, there is enough evidence to suggest that such a role exists due to the documented presence of PARP in the testis, during spermatogenesis, and just recently in ejaculated spermatozoa [72]. Furthermore, PARP as an important DNA repair enzyme could maintain the sperm genomic integrity. Similarly, the role of PARP in cell death pathways may have important implications for the elimination of abnormal spermatozoa, especially during the processes of spermatogenesis.
Germ cell apoptosis
Apoptosis is a normal component of mammalian spermatogenesis. It is orchestrated spontaneously during the entirety of spermatogenesis to produce mature spermatozoa and to eliminate any abnormal spermatozoa. In fact, a very large number of spermatozoa die during spermatogenesis. This may be due to the ability of the Sertoli cells to maintain only a limited number of germ cells so excess cells must be eliminated. Apoptosis also may function to destroy cells that do not make it past certain cellular checkpoints [59]. In a recent study by Codelia et al., the cell death pathway involved in pubertal rat spermatogenesis was identified as the extrinsic pathway of cell death involving the Fas-FasL system. The study also showed significantly less cleaved PARP and a reduction in the number of apoptotic germ cells when using a caspase-8 inhibitor and a pan-caspase inhibitor [61]. Thus, PARP cleavage may play a key role in the cellular death pathways of spermatogenesis.
The significant presence of PARP in human spermatocytes during maturation arrest was suggested to represent the greater amount of DNA strand breaks occurring during spermatogenesis impairment [70]. Poly (ADP-ribose) polymerase-2 also has been implicated in abnormal spermatogenesis. In a recent study by Dantzer et al., an increased incidence of apoptosis was found in the testis of PARP-2 null mice, specifically in the spermatocyte and spermatid layers. However, the layers containing spermatogonia and preleptotene spermatocytes did not show any markers for apoptosis. Chromosome segregation was abnormal during metaphase I, and spindle assembly was also abnormal in these PARP-2-deficient mice. Thus, the decrease in fertility seen in these mice could be related to defective meiosis I and spermiogenesis [71].
Poly (ADP-ribose) polymerase in ejaculated spermatozoa
The quest to detect PARP in ejaculated spermatozoa has demonstrated success only recently. Initially, the presence of PARP-1 in human ejaculated sperm samples when analyzing semen for apoptotic markers was not detected (Taylor, Weng et al. 2004). However, in a recent study by Jha et al., several PARP isoforms were detected in ejaculated spermatozoa, including PARP-1, PARP-2, and PARP-9. Immunolocalization patterns showed that PARP was found near the acrosomal regions in sperm heads. Furthermore, a direct correlation was seen between sperm maturity and the presence of PARP. An increased presence of PARP-1, PARP-2, and PARP-9 was seen in mature sperm samples when compared with immature sperm samples of fertile and infertile men. In addition, a possible relationship between PARP and male infertility was also demonstrated. Poly (ADP-ribose) polymerase activity was then modulated to determine its role in responding to oxidative and chemical damage in sperm. In the presence of the PARP inhibitor 3-aminobenzamide, chemical- and oxidative stress- induced apoptosis increased by nearly twofold. This recent finding suggested that PARP could play an important protective role for spermatozoa responding to oxidative and chemical damage [72].
The research presented by our group was the first in showing the presence of cleaved PARP in ejaculated spermatozoa. The presence of cleaved PARP was similar in mature and immature sperm samples following exposure to oxidative stress and chemical damage. Modulating PARP activity was also shown to alter the incidence of cell death. When the same sperm samples were exposed to PARP inhibitors after chemical and oxidative stress, apoptosis decreased [75].
Poly (ADP-ribose) polymerase and oxidative stress
Oxidative stress can cause DNA damage, inflammation, and modification of proteins. Poly (ADP-ribose) polymerase responds to all three of these changes that can occur in the cell as a result of oxidative damage. However, there is a great deal of variability in PARP activation as a result of this type of stress; the cell’s metabolic stage or its microenvironment could affect the activity of PARP [38]. In response to DNA damage caused by ROS, PARP-1 recruits the DNA repair protein XRCC1 to damage sites [40]. DNA is not the only structure modified by ROS; histones could be damaged as well. Ullrich et al. showed that PARP can activate the 20S proteosome involved in breaking down oxidatively damaged histones. Also, in response to oxidative stress caused by exposure of histones to hydrogen peroxide, PARP, PAR, and the nuclear proteosome all bind together [41]. This could be important for protecting DNA against oxidative damage because it has been found that a condensed chromatin structure may prevent DNA strand breaks induced by hydroxyl radicals [42].
Poly (ADP-ribose) polymerase and aging
Two main aspects of PARP activity may influence aging: PARP’s regulation of immune responses through its interaction with NF-kB and PARP’s central role in maintaining genomic stability through DNA repair, telomere maintenance, spindle stability, and cell death [43]. Both PARP-1 and PARP-2 are involved in telomere functioning. PARP-1 and PARP-2 bind to TRFII (telomere repeat binding factor II) and modify it, affecting its ability to bind to telomere regions [44, 45]. Poly (ADP-ribose) polymerase-1 also is found at telomere regions of DNA damaged by genotoxic agents, and it may play a role in preventing damage to genomic instability [45]. Tankyrases also are involved in telomere regulation. Tankyrase can bind to TRF1, a telomeric factor that shortens telomere length, and TRF-1 is in turn modified by ADP-ribosylation, thus making it more difficult for TRF-1 to bind to telomeric regions of DNA in vitro [46]. It has even been shown that mice lacking PARP exhibited telomere shortening, a symptom of ageing [47].
Chromosomal instability increases with an organism’s age. Poly (ADP-ribose) polymerase is essential for chromosomal stability through its role in DNA repair. Poly (ADP-ribose) polymerase-1-deficient mice created by Simbulan-Rosenthal et al. showed a higher incidence of chromosomal aberrations and polyploidy. Reintroduction of PARP in the form of cDNA allowed for restoration of chromosomal integrity [48].
Potential therapeutic options for poly (ADP-ribose) polymerase
The key role of PARP in cell death has made it an attractive candidate for modulation in cancer therapies. Poly (ADP-ribose) polymerase inhibition is currently being explored as an adjunct to chemotherapy. It is based on the hypothesis that inactivating PARP will render malignant cells exposed to chemotherapy unable to repair DNA damage resulting from therapy, leading to their death. Non-malignant cells will not be susceptible to cell death at these low doses of chemotherapy. Thus PARP-inhibitors are way of sensitizing cancerous cells to chemotherapy [82]. Inactivating PARP through cleavage is also being explored. In a recent study by Zhang et al., PARP cleavage through activation of caspases induced cytotoxicity in human leukemia cells [83].
Whether PARP can function in therapy for male infertility remains to be seen. Our group has previously reported that PARP inhibition may protect against chemically induced injury of ejaculated spermatozoa in vitro, but it could not protect against damage induced by oxidative stress [75]. PARP inhibition may have a potential role in testicular cancer as well as cancer that may have spread to the testes. Inflammatory processes as result of infections could also be another area to explore in terms of PARP and male fertility.
Future directions
Poly (ADP-ribose) polymerase homologues seem to have diverse roles in spermatogenesis and even in ejaculated sperm. Their expression has showed a correlation with spermatozoa maturity and fertility potential. The presence of cleaved PARP in ejaculated spermatozoa confirms this hypothesis. The available preliminary data show that PARP inhibition may protect against chemically induced damage in sperm. This will pave the way for future studies to confirm these findings and to examine the role of PARP in other types of induced sperm damage.
Poly (ADP-ribose) polymerase modulation may also create new therapeutic options for infertile patients, especially those suffering from sperm DNA damage, oxidative stress-induced sperm DNA damage, or perhaps even idiopathic male infertility. Poly (ADP-ribose) polymerase inhibition may be used as an in vitro treatment for inducing death in spermatozoa that carry damaged DNA. Cleaved PARP could serve as an apoptotic marker that may prove useful for identifying healthy spermatozoa. Not only this, but the therapeutic potential of PARP as an anti-tumor agent could help point the way to new ways of preserving fertility in cancer patients even after the genotoxic stress of chemotherapy and radiation. The current widespread use of assisted reproductive technologies and the limited knowledge available about the consequences of sperm DNA damage both demand further exploration of DNA repair mechanisms in spermatozoa. Poly (ADP-ribose) polymerase may hold the key to a better understanding of these repair mechanisms inherent in spermatozoa and the importance of such mechanisms in producing healthy pregnancies.
References

1. Dantzer F, Schreiber V, Niedergang C, et al. Involvement of poly(ADP-ribose) polymerase in base excision repair. Biochimie 1999; 81: 69-75.
2. Chen J, Tomkinson AE, Ramos W, et al. Mammalian DNA ligase III: molecular cloning, chromosomal localization, and expression in spermatocytes undergoing meiotic recombination. Mol Cell Biol 1995; 15: 5412-22.
3. Di Meglio S, Tramontano F, Cimmino G, Jones R, Quesada P. Dual role for poly(ADP-ribose)polymerase-1 and -2 and poly(ADP-ribose)glycohydrolase as DNA-repair and pro-apoptotic factors in rat germinal cells exposed to nitric oxide donors. Biochim Biophys Acta 2004; 1692: 35-44.
4. Sinha Hikim AP, Lue Y, Diaz-Romero M, Yen PH, Wang C, Swerdloff RS. Deciphering the pathways of germ cell apoptosis in the testis. J Steroid Biochem Mol Biol 2003; 85: 175-82.
5. Meyer-Ficca ML, Scherthan H, Bürkle A, Meyer RG. Poly(ADP-ribosyl)ation during chromatin remodeling steps in rat spermiogenesis. Chromosoma 2005; 114: 67-74.
6. Atorino L, Di Meglio S, Farina B, Jones R, Quesada P. Rat germinal cells require PARP for repair of DNA damage induced by gamma-irradiation and H₂O2 treatment. Eur J Cell Biol 2001; 80: 222-9.
7. Blanc-Layrac G, Bringuier AF, Guillot R, Feldmann G. Morphological and biochemical analysis of cell death in human ejaculated spermatozoa. Cell Mol Biol (Noisy-le-grand) 2000; 46: 187-97.
8. Taylor SL, Weng SL, Fox P, et al. Somatic cell apoptosis markers and pathways in human ejaculated sperm: potential utility as indicators of sperm quality. Mol Hum Reprod 2004; 10: 825-34.
9. Sakkas D, Mariethoz E, Manicardi G, Bizzaro D, Bianchi PG, Bianchi U. Origin of DNA damage in ejaculated human spermatozoa. Rev Reprod 1999; 4: 31-7.
10. Barroso G, Morshedi M, Oehninger S. Analysis of DNA fragmentation, plasma membrane translocation of phosphatidylserine and oxidative stress in human spermatozoa. Hum Reprod 2000; 15: 1338-44.
11. Kappes F, Fahrer J, Khodadoust MS, et al. DEK is a poly (ADP-ribose) acceptor in apoptosis and mediates resistance to genotoxic stress. Mol Cell Biol 2008; 28: 3245-57.
12. Tripathi DN, Jena GB. Astaxanthin inhibits cytotoxic and genotoxic effects of cyclophosphamide in mice germ cells. Toxicology 2008; 248: 96-103.
13. Low GK, Fok ED, Ting AP, Hande MP. Oxidative damage induced genotoxic effects in human fibroblasts from Xeroderma Pigmentosum group A patients. Int J Biochem Cell Biol 2008; 40: 2583-95.
14. Fahmy MA, Hassan NH, Farghaly AA, Hassan EE. Studies on the genotoxic effect of beryllium chloride and the possible protective role of selenium/vitamins A, C and E. Mutat Res 2008; 652: 103-11.
15. Pacher P, Szabo C. Role of the peroxynitrite-poly(ADP-ribose) polymerase pathway in human disease. Am J Pathol 2008; 173: 2-13.
16. Sakamoto-Hojo ET, Balajee AS. Targeting poly (ADP) ribose polymerase I (PARP-1) and PARP-1 interacting proteins for cancer treatment. Anticancer Agents Med Chem 2008; 8: 402-16.
17. Adhikari S, Choudhury S, Mitra PS, Dubash JJ, Sajankila SP, Roy R. Targeting base excision repair for chemosensitization. Anticancer Agents Med Chem 2008; 8: 351-7.
18. Ohashi Y, Ueda K, Kawaichi M, Hayaishi O. Activation of DNA ligase by poly(ADP-ribose) in chromatin. Proc Natl Acad Sci U S A 1983; 80: 3604-7.
19. D'Amours D, Desnoyers S, D'Silva I, Poirier GG. Poly(ADP-ribosyl)ation reactions in the regulation of nuclear functions. Biochem J 1999; 342: 249-68.
20. Tewari M, Quan LT, O'Rourke K, et al. Yama/CPP32 beta, a mammalian homolog of CED-3, is a CrmA-inhibitable protease that cleaves the death substrate poly(ADP-ribose) polymerase. Cell 1995; 81: 801-9.
21. Duriez PJ, Shah GM. Cleavage of poly(ADP-ribose) polymerase: a sensitive parameter to study cell death. Biochem Cell Biol 1997; 75: 337-49.
22. Gero D, Szabo C. Poly(ADP-ribose) polymerase: a new therapeutic target? Curr Opin Anaesthesiol 2008; 21: 111-21.
23. Ame JC, Spenlehauer C, de Murcia G. The PARP superfamily. Bioessays 2004; 26: 882-93.
24. Hassa PO, Hottiger MO. The diverse biological roles of mammalian PARPS, a small but powerful family of poly-ADP-ribose polymerases. Front Biosci 2008; 13: 3046-82.
25. Wacker DA, Frizzell KM, Zhang T, Kraus WL. Regulation of chromatin structure and chromatin-dependent transcription by poly(ADP-ribose) polymerase-1: possible targets for drug-based therapies. Subcell Biochem 2007; 41: 45-69.
26. Schreiber V, Dantzer F, Ame JC, de Murcia G. Poly(ADP-ribose): novel functions for an old molecule. Nat Rev Mol Cell Biol 2006; 7: 517-28.
27. Rouleau M, Aubin RA, Poirier GG. Poly(ADP-ribosyl)ated chromatin domains: access granted. J Cell Sci 2004; 117: 815-25.
28. Baumgartner M, Schneider R, Auer B, Herzog H, Schweiger M, Hirsch-Kauffmann M. Fluorescence in situ mapping of the human nuclear NAD+ ADP-ribosyltransferase gene (ADPRT) and two secondary sites to human chromosomal bands 1q42, 13q34, and 14q24. Cytogenet Cell Genet 1992; 61: 172-4.
29. Cherney BW, McBride OW, Chen DF, et al. cDNA sequence, protein structure, and chromosomal location of the human gene for poly(ADP-ribose) polymerase. Proc Natl Acad Sci U S A 1987; 84: 8370-4.
30. Diefenbach J, Bürkle A. Introduction to poly(ADP-ribose) metabolism. Cell Mol Life Sci 2005; 62: 721-30.
31. Mendoza-Alvarez H, Alvarez-Gonzalez R. Poly(ADP-ribose) polymerase is a catalytic dimer and the automodification reaction is intermolecular. J Biol Chem 1993; 268: 22575-80.
32. Poirier GG, de Murcia G, Jongstra-Bilen J, Niedergang C, Mandel P. Poly(ADP-ribosyl)ation of polynucleosomes causes relaxation of chromatin structure. Proc Natl Acad Sci U S A 1982; 79: 3423-7.
33. Adamietz P, Rudolph A. ADP-ribosylation of nuclear proteins in vivo. Identification of histone H2B as a major acceptor for mono- and poly(ADP-ribose) in dimethyl sulfate-treated hepatoma AH 7974 cells. J Biol Chem 1984; 259: 6841-6.
34. Mendoza-Alvarez H, Alvarez-Gonzalez R. Regulation of p53 sequence-specific DNA-binding by covalent poly(ADP-ribosyl)ation. J Biol Chem 2001; 276: 36425-30.
35. Yoshihara K, Itaya A, Tanaka Y, et al. Inhibition of DNA polymerase alpha, DNA polymerase beta, terminal deoxynucleotidyl transferase, and DNA ligase II by poly(ADP-ribosyl)ation reaction in vitro. Biochem Biophys Res Commun 1985; 128: 61-7.
36. Scovassi AI, Mariani C, Negroni M, Negri C, Bertazzoni U. ADP-ribosylation of nonhistone proteins in HeLa cells: modification of DNA topoisomerase II. Exp Cell Res 1993; 206: 177-81.
37. Kameoka M, Ota K, Tetsuka T, et al. Evidence for regulation of NF-kappaB by poly(ADP-ribose) polymerase. Biochem J 2000; 346: 641-9.
38. Erdélyi K, Bakondi E, Gergely P, Szabó C, Virág L. Pathophysiologic role of oxidative stress-induced poly(ADP-ribose) polymerase-1 activation: focus on cell death and transcriptional regulation. Cell Mol Life Sci 2005; 62: 751-9.
39. Oliver FJ, Menissier-de Murcia J, Nacci C, et al. Resistance to endotoxic shock as a consequence of defective NF-kappaB activation in poly (ADP-ribose) polymerase-1 deficient mice. EMBO J 1999; 18: 4446-54.
40. El-Khamisy SF, Masutani M, Suzuki H, Caldecott KW. A requirement for PARP-1 for the assembly or stability of XRCC1 nuclear foci at sites of oxidative DNA damage. Nucleic Acids Res 2003; 31: 5526-33.
41. Ullrich O, Reinheckel T, Sitte N, Hass R, Grune T, Davies KJ. Poly-ADP ribose polymerase activates nuclear proteasome to degrade oxidatively damaged histones. Proc Natl Acad Sci U S A 1999; 96: 6223-8.
42. Ljungman M, Hanawalt PC. Efficient protection against oxidative DNA damage in chromatin. Mol Carcinog 1992; 5: 264-9.
43. Beneke S, Bürkle A. Poly(ADP-ribosyl)ation in mammalian ageing. Nucleic Acids Res 2007; 35: 7456-65.
44. Dantzer F, Giraud-Panis MJ, Jaco I, et al. Functional interaction between poly(ADP-Ribose) polymerase 2 (PARP-2) and TRF2: PARP activity negatively regulates TRF2. Mol Cell Biol 2004; 24: 1595-607.
45. Gomez M, Wu J, Schreiber V, et al. PARP1 Is a TRF2-associated poly(ADP-ribose)polymerase and protects eroded telomeres. Mol Biol Cell 2006; 17: 1686-96.
46. Smith S, Giriat I, Schmitt A, de Lange T. Tankyrase, a poly(ADP-ribose) polymerase at human telomeres. Science 1998; 282: 1484-7.
47. d'Adda di Fagagna F, Hande MP, Tong WM, Lansdorp PM, Wang ZQ, Jackson SP. Functions of poly(ADP-ribose) polymerase in controlling telomere length and chromosomal stability. Nat Genet 1999; 23: 76-80.
48. Simbulan-Rosenthal CM, Haddad BR, Rosenthal DS, et al. Chromosomal aberrations in PARP(-/-) mice: genome stabilization in immortalized cells by reintroduction of poly(ADP-ribose) polymerase cDNA. Proc Natl Acad Sci USA 1999; 96: 13191-6.
49. Kolthur-Seetharam U, Dantzer F, McBurney MW, de Murcia G, Sassone-Corsi P. Control of AIF-mediated cell death by the functional interplay of SIRT1 and PARP-1 in response to DNA damage. Cell Cycle 2006; 5: 873-7.
50. Ermak G, Davies KJ. Calcium and oxidative stress: from cell signaling to cell death. Mol Immunol 2002; 38: 713-21.
51. Bauer PI, Kenesi E, Mendeleyev J, Kun E. The influence of ATP on poly(ADP-ribose) metabolism. Int J Mol Med 2005; 16: 321-4.
52. Geraets L, Moonen HJ, Brauers K, et al. Flavone as PARP-1 inhibitor: its effect on lipopolysaccharide induced gene-expression. Eur J Pharmacol 2007; 573: 241-8.
53. Geraets L, Moonen HJ, Brauers K, Wouters EF, Bast A, Hageman GJ. Dietary flavones and flavonoles are inhibitors of poly(ADP-ribose)polymerase-1 in pulmonary epithelial cells. J Nutr 2007; 137: 2190-5.
54. Geraets L, Moonen HJ, Wouters EF, Bast A, Hageman GJ. Caffeine metabolites are inhibitors of the nuclear enzyme poly(ADP-ribose)polymerase-1 at physiological concentrations. Biochem Pharmacol 2006; 72: 902-10.
55. Moonen HJ, Geraets L, Vaarhorst A, Bast A, Wouters EF, Hageman GJ. Theophylline prevents NAD+ depletion via PARP-1 inhibition in human pulmonary epithelial cells. Biochem Biophys Res Commun 2005; 338: 1805-10.
56. Calandria C, Irurzun A, Barco A, Carrasco L. Individual expression of poliovirus 2Apro and 3Cpro induces activation of caspase-3 and PARP cleavage in HeLa cells. Virus Res 2004; 104: 39-49.
57. Kauppinen TM, Chan WY, Suh SW, Wiggins AK, Huang EJ, Swanson RA. Direct phosphorylation and regulation of poly(ADP-ribose) polymerase-1 by extracellular signal-regulated kinases 1/2. Proc Natl Acad Sci U S A 2006; 103: 7136-41.
58. Ariumi Y, Masutani M, Copeland TD, et al. Suppression of the poly(ADP-ribose) polymerase activity by DNA-dependent protein kinase in vitro. Oncogene 1999; 18: 4616-25.
59. Baum JS, St George JP, McCall K. Programmed cell death in the germline. Semin Cell Dev Biol 2005; 16: 245-59.
60. Print CG, Loveland KL. Germ cell suicide: new insights into apoptosis during spermatogenesis. Bioessays 2000; 22: 423-30.
61. Codelia VA, Cisternas P, Moreno RD. Relevance of caspase activity during apoptosis in pubertal rat spermatogenesis. Mol Reprod Dev 2008; 75: 881-9.
62. Hikim AP, Vera Y, Vernet D, et al. Involvement of nitric oxide-mediated intrinsic pathway signaling in age-related increase in germ cell apoptosis in male Brown-Norway rats. J Gerontol A Biol Sci Med Sci 2005; 60: 702-8.
63. Miura M, Sasagawa I, Suzuki Y, Nakada T, Fujii J. Apoptosis and expression of apoptosis-related genes in the mouse testis following heat exposure. Fertil Steril 2002; 77: 787-93.
64. Pentikäinen V, Suomalainen L, Erkkilä K, et al. Nuclear factor-kappa B activation in human testicular apoptosis. Am J Pathol 2002; 160: 205-18.
65. Chiarugi A. Poly(ADP-ribosyl)ation and stroke. Pharmacol Res 2005; 52: 15-24.
66. Hadziselimovic F, Geneto R, Emmons LR. Increased apoptosis in the contralateral testes of patients with testicular torsion as a factor for infertility. J Urol 1998; 160: 1158-60.
67. Lin WW, Lamb DJ, Wheeler TM, Lipshultz LI, Kim ED. In situ end-labeling of human testicular tissue demonstrates increased apoptosis in conditions of abnormal spermatogenesis. Fertil Steril 1997; 68: 1065-9.
68. Kim SK, Yoon YD, Park YS, Seo JT, Kim JH. Involvement of the Fas-Fas ligand system and active caspase-3 in abnormal apoptosis in human testes with maturation arrest and Sertoli cell-only syndrome. Fertil Steril 2007; 87: 547-53.
69. Tesarik J, Greco E, Cohen-Bacrie P, Mendoza C. Germ cell apoptosis in men with complete and incomplete spermiogenesis failure. Mol Hum Reprod 1998; 4: 757-62.
70. Maymon BB, Cohen-Armon M, Yavetz H, et al. Role of poly(ADP-ribosyl)ation during human spermatogenesis. Fertil Steril 2006; 86: 1402-7.
71. Dantzer F, Mark M, Quenet D, et al. Poly(ADP-ribose) polymerase-2 contributes to the fidelity of male meiosis I and spermiogenesis. Proc Natl Acad Sci U S A 2006; 103: 14854-9.
72. Jha R, Agarwal A, Mahfouz R, et al. Determination of Poly (ADP-ribose) polymerase (PARP) homologues in human ejaculated sperm and its correlation with sperm maturation. Fertil Steril 2008 [Epub ahead of print].
73. Yu SW, Wang H, Poitras MF, et al. Mediation of poly(ADP-ribose) polymerase-1-dependent cell death by apoptosis-inducing factor. Science 2002; 297: 259-63.
74. El-Domyati MM, Al-Din AB, Barakat MT, El-Fakahany HM, Xu J, Sakkas D. Deoxyribonucleic acid repair and apoptosis in testicular germ cells of aging fertile men: the role of the poly(adenosine diphosphate-ribosyl)ation pathway. Fertil Steril 2008 [Epub ahead of print].
75. Mahfouz RZ, Sharma RK, Poenicke K, et al. Evaluation of poly(ADP-ribose) polymerase cleavage (cPARP) in ejaculated human sperm fractions after induction of apoptosis. Fertil Steril 2008 [Epub ahead of print].
76. Erenpreiss J, Spano M, Erenpreisa J, Bungum M, Giwercman A. Sperm chromatin structure and male fertility: biological and clinical aspects. Asian J Androl 2006; 8: 11-29.
77. Meseguer M, Santiso R, Garrido N, Fernandez JL. The effect of cancer on sperm DNA fragmentation as measured by the sperm chromatin dispersion test. Fertil Steril 2008; 90: 225-7.
78. O'Flaherty C, Vaisheva F, Hales BF, Chan P, Robaire B. Characterization of sperm chromatin quality in testicular cancer and Hodgkin's lymphoma patients prior to chemotherapy. Hum Reprod 2008; 23: 1044-52.
79. Edelstein A, Yavetz H, Kleiman SE, et al. Deoxyribonucleic acid-damaged sperm in cryopreserved-thawed specimens from cancer patients and healthy men. Fertil Steril 2008; 90: 205-8.
80. Sta°hl O, Eberhard J, Jepson K, et al. Sperm DNA integrity in testicular cancer patients. Hum Reprod 2006; 21: 3199-205.
81. Spermon JR, Ramos L, Wetzels AM, et al. Sperm integrity pre- and post-chemotherapy in men with testicular germ cell cancer. Hum Reprod 2006; 21: 1781-6.
82. de la Lastra CA, Villegas I, Sánchez-Fidalgo S. Poly(ADP-ribose) polymerase inhibitors: new pharmacological functions and potential clinical implications. Curr Pharm Des 2007; 13: 933-62.
83. Zhang P, Li H, Chen D, Ni J, Kang Y, Wang S. Oleanolic acid induces apoptosis in human leukemia cells through caspase activation and poly(ADP-ribose) polymerase cleavage. Acta Biochim Biophys Sin (Shanghai) 2007; 39: 803-9.