Alexander Bürkle

Stress, DNA damage and ageing — an integrative approach

Ageing is highly complex, involving multiple mechanisms at different levels. Nevertheless, recent evidence suggests that several of the most important mechanisms are linked via endogenous stress-induced DNA damage caused by reactive oxygen species (ROS). Understanding how such damage contributes to age-related changes requires that we explain how these different mechanisms relate to each other and potentially interact. In this article, we review the contributions of stress-induced damage to cellular DNA through (i) the role of damage to nuclear DNA and its repair mediated via the actions of poly(ADP-ribose) polymerase-1, (ii) the role of damage to telomeric DNA and its contribution to telomere-driven cell senescence, and (iii) the role of damage to and the accumulation of mutations in mitochondrial DNA. We describe how an integrative approach to studying these mechanisms, coupled with computational modelling, may be of considerable importance in resolving some of the complexity of cellular ageing.

Increased poly(ADP-ribose) polymerase activity in lymphoblastoid cell lines from centenarians.

Abstractu2002Poly(ADP-ribosyl)ation is a posttranslational modification of nuclear proteins which is catalyzed by poly(ADP-ribose) polymerase and represents an immediate response of eukaryotic cells to oxidative and other types of DNA damage. Previously a strong correlation had been detected between maximal poly(ADP-ribose) polymerase activity in permeabilized mononuclear leukocytes of various mammalian species and species-specific life span. To study a possible relation between longevity and poly(ADP-ribosyl)ation in humans we measured maximal oligonucleotide-stimulated poly(ADP-ribose) polymerase activity in permeabilized, Epstein-Barr virus transformed lymphoblastoid cell lines from a French population of 49 centenarians and 51 controls aged 20–70 years. Maximal enzyme activity was significantly higher in centenarians than in controls [median of controls: 9035xa0cpm/106 cells (lower quartile: 6156; upper quartile: 11,410); median of centenarians: 10,380xa0cpm/106 cells (lower quartile: 7994; upper quartile: 12,991); P=0.031 by Mann-Whitney U test]. In a subset of 16 controls and 24 centenarians, cellular poly(ADP-ribose) polymerase content was determined by quantitative western blotting, thus allowing the calculation of specific enzyme activity. The latter was significantly higher in centenarians (P=0.006), the median value for centenarians being about 1.6-fold that of controls. Specific poly(ADP-ribose) polymerase activity was a more powerful parameter for differentiating between centenarians and controls than enzyme activity relative to cell number. In addition, in a genetic association study we analyzed 437 DNA samples (239 centenarians and 198 controls) by PCR amplification of a polymorphic dinucleotide repeat located in the promoter region of the poly(ADP-ribose) polymerase gene in an attempt to detect an association between this polymorphic marker and variability of enzyme activity or human longevity. However, this genetic analysis revealed no significant enrichment of any of the alleles or genotypes identified among centenarians or controls, but its power was limited by the relatively weak hetero-zygosity of this polymorphic marker in our population (51%). Viewed together with previous results on poly(ADP-ribose) polymerase activity in various mammalian species, the present data provide further evidence for the notion that longevity is associated with a high poly(ADP-ribosyl)ation capacity.

Poly(APD-ribosyl)ation, a DNA damage-driven protein modification and regulator of genomic instability

Activation of poly(ADP-ribose) polymerase-1 (PARP-1) is an immediate cellular reaction to DNA strand breakage as induced by alkylating agents, ionizing radiation or oxidants. The resulting formation of protein-coupled poly(ADP-ribose) facilitates survival of proliferating cells under conditions of DNA damage, probably via its contribution to DNA base-excision repair. Furthermore, based on recent results there is a role emerging for PARP-1 as a negative regulator of genomic instability in cells under genotoxic stress. Regarding possible applications for clinical cancer therapy with DNA-damaging agents, it appears that both inhibition and up-regulation of the poly(ADP-ribosyl)ation response in the malignant cells to be eradicated are promising strategies to improve the outcome of such therapy, albeit for different reasons.

PARP-1: a regulator of genomic stability linked with mammalian longevity.

Ageing of organisms is commonly defined as the time-dependent general decline of physiological functions accompanied by a progressive increase in the risk of morbidity and mortality. It appears that the major driving force of the ageing process is damage inflicted on cellular macromolecules, which interferes with their function. Such damage is mostly derived from low molecular weight reactive compounds that arise within the body during normal metabolism and are linked with important cellular functions such as oxygen transport, respiration, phagocyte activity or detoxification reactions. Most prominent among such endogenous damaging agents are reactive oxygen species (ROS), which can lead to a state termed oxidative stress, if produced in excessive amounts. It is the damage to DNA that may be particularly hazardous, since unlike other macromolecules DNA is subject to little if any turnover, which could dilute the damage. DNA damage and some of its irreversible consequences that are collectively referred to as agenomic instabilityo (e.g. chromosomal aberrations, sister chromatid exchange [SCE] as well as DNA translocations, deletions, amplifications and other kinds of mutations) would rapidly accumulate and disturb DNA replication, gene expression and ultimately cellular and tissue homeostasis if there were no effective cellular defence and repair systems in place. A central prediction of the adisposable soma theoryo is that longevity has evolved by allocation of increasing proportions of available bioenergy to somatic maintenance and repair pathways, thereby making cellular components, cells and organisms more resistant to stress (including genotoxic stress) and allowing for better functional preservation (including integrity and stability of the genome) over time.

New polymorphisms in the human poly(ADP-ribose) polymerase-1 coding sequence: lack of association with longevity or with increased cellular poly(ADP-ribosyl)ation capacity

Abstract. Poly(ADP-ribose) polymerase-1 (PARP-1) encoded by the PARP-1 gene, is a ubiquitous and abundant DNA-binding protein involved in the cellular response to various genotoxic agents. In a previous study we showed that maximal oligonucleotide-stimulated poly(ADP-ribosyl)ation was significantly higher in permeabilised lymphoblastoid cell lines from a French population of centenarians compared with controls aged 20–70xa0years, supporting the notion that longevity is associated with a genetically determined, high poly(ADP-ribosyl)ation capacity. Here, we describe four new genetic polymorphisms, three of which represent silent nucleotide variants (C402T, T1011C, G1215A), and one of which leads to a valine762-to-alanine exchange (T2444C). We undertook an association study between two of these polymorphisms and human longevity or poly(ADP-ribosyl)ation capacity in permeabilised lymphoblastoid cells. By analysing 648 DNA samples from a French population (324 centenarians and 324 controls) by fluorescent-allele-specific PCR, we showed the absence of any significant enrichment of any of the genotypes in the study of centenarians versus controls. Furthermore, we studied genotype distributions from individuals who had previously been tested for poly(ADP-ribosyl)ation capacity. None of the genotype combinations at any polymorphic site studied could be related to a high or low level of poly(ADP-ribosyl)ation capacity. Together, these results strongly suggest that the longevity-related differences in the poly(ADP-ribosyl)ation capacity of human lymphoblastoid cell lines cannot be explained by genetic polymorphisms in the PARP-1 coding sequence and that other mechanisms have to be considered as potential regulators of specific poly(ADP-ribosyl)ation capacity.

Negative regulation of alkylation‐induced sister‐chromatid exchange by poly(ADP‐ribose) polymerase‐1 activity

One of the earliest responses to DNA damage in eukaryotic cells is activation of poly(ADP‐ribose) polymerase‐1 (PARP‐1), a DNA strand break–dependent nuclear enzyme which covalently modifies proteins with poly(ADP‐ribose). Here, we show that conditional over‐expression of PARP‐1 in stably transfected hamster cells, which causes cellular over‐accumulation of poly(ADP‐ribose) by several‐fold, strongly suppresses alkylation‐induced sister‐chromatid exchange (SCE), while cytotoxicity of alkylation treatment is slightly enhanced. Viewed together with the known potentiation of SCE by abrogation of PARP‐1 activity, our results provide evidence that PARP‐1 activity is an important regulator of alkylation‐induced SCE formation, imposing a control that is strictly negative and commensurate with the level of enzyme activity. Int. J. Cancer 88:351–355, 2000.

Comparative characterisation of poly(ADP-ribose) polymerase-1 from two mammalian species with different life span

DNA damage induced in higher eukaryotes by alkylating agents, oxidants or ionising radiation triggers the synthesis of protein-conjugated poly(ADP-ribose) catalysed by poly(ADP-ribose) polymerase-1 (PARP-1). Previously, cellular poly(ADP-ribosyl)ation capacity has been shown to correlate positively with the life span of mammalian species [Proc. Natl. Acad. Sci. USA 89 (1992) 11,759-11,763]. Here, we have tested whether this correlation results from differences in kinetic parameters of the enzymatic activity of PARP-1. We therefore compared recombinant enzymes, expressed in a baculovirus system, from rat and man as two mammalian species with extremely divergent life span. In standard activity assays performed in the presence of histones as poly(ADP-ribose) acceptors both enzymes showed saturation kinetics with [NAD(+)]. The kinetic parameters (k(cat), k(m) and k(cat)/k(m)) of the two enzymes were not significantly different. However, in assays assessing the auto-poly(ADP-ribosyl)ation reaction, both enzymes displayed second-order kinetics with respect to [PARP-1], and up to two-fold higher specific activity was observed for human versus rat PARP-1. We conclude that the correlation of poly(ADP-ribosyl)ation capacity with life span is not reflected in the kinetic parameters, but that subtle differences in primary structure of PARP-1 from mammalian species of different longevity may control the extent of the automodification reaction.

Reduced poly(ADP‐ribosyl)ation in lymphocytes of laryngeal cancer patients: Results of a case‐control study

Poly(ADP‐ribose) polymerase (PARP), a nuclear enzyme that is catalytically activated by DNA strand breaks, plays a complex role in DNA repair. Using NAD+ as a precursor, it catalyzes the formation of ADP‐ribose polymers, which are attached to various proteins. Defects in DNA repair pathways have been associated with increased risks for cancer in humans. We investigated whether differences in the activity of PARP are associated with the risk for laryngeal cancer. In a case‐control study on genetic, lifestyle and occupational risk factors for laryngeal cancer, PARP activity was assessed as DNA damage–induced poly(ADP‐ribose) formation in human peripheral blood lymphocytes by quantitative immunofluorescence analysis. Polymer formation was determined as the cellular response to bleomycin, a well‐known inducer of DNA strand breaks, in lymphocytes from 69 laryngeal cancer patients and 125 healthy controls. The frequency of bleomycin‐induced polymer formation, measured as mean pixel intensity, was significantly lower in cases (74.6, SE = 3.7) than in controls (94.5, SE = 3.5) and not influenced by smoking, age or sex. There was no significant difference between cases (59.1, SE = 5.2) and controls (50.5, SE = 3.7) in basal polymer formation (in cells not treated with bleomycin). When the highest tertile of polymer formation was used as the reference, the odds ratio for the lowest tertile of bleomycin‐induced polymer formation was 3.79 (95% confidence interval 1.37–10.47, p = 0.01). Peripheral blood lymphocytes from laryngeal cancer patients thus showed significantly less bleomycin‐induced poly(ADP‐ribose) formation. Our results suggest that a reduced capacity of somatic cells to synthesize poly(ADP‐ribose) might be associated with an increased risk for laryngeal cancer. The underlying mechanism remains to be investigated.

Poly(ADP-Ribosyl)ation and Aging

Poly(ADP-ribosyl)ation is a DNA strand break-driven post-translational modification of proteins catalyzed by poly(ADP-ribose) polymerase-1 (PARP-1), with NAD+ serving as substrate. Poly(ADP-ribosyl)ation is triggered by DNA strand breaks, is functionally associated with DNA repair pathways and is a survival factor for cells under low to moderate levels of genotoxic stress. We have previously described a positive correlation between poly(ADP-ribosyl)ation capacity of mononuclear blood cells with longevity of mammalian species. Our comparison of purified recombinant human and rat PARP-1 revealed that this correlation might be explained in part by evolutionary sequence divergence. We have also developed molecular genetic approaches to modulate the poly(ADP-ribosyl)ation status in living cells. Our results revealed that PARP-1 acts as a negative regulator of DNA damage-induced genomic instability, the latter being known as an important driving force for carcinogenesis. Our recent data obtained in transgenic mice with selective expression of a dominant negative version of PARP-1 in basal skin keratinocytes indicate that PARP-1 activity suppresses skin papilloma formation in a two-stage skin carcinogenesis protocol. It is tempting to speculate that increased poly(ADP-ribosyl)ation capacity in long-lived species might help retard the accumulation of DNA damage and of mutations and thus slow down the rate of aging and of carcinogenesis more efficiently as compared with short-lived animals.

Role of poly(ADP-ribosyl)ation in DNA-PKcs- independent V(D)J recombination

V(D)J recombination is critical to the generation of a functional immune system. Intrinsic to the assembly of antigen receptor genes is the formation of endogenous DNA double-strand breaks, which normally are excluded from the cellular surveillance machinery because of their sequestration in a synaptic complex and/or rapid resolution. In cells deficient in double-strand break repair, such recombination-induced breaks fail to be joined promptly and therefore are at risk of being recognized as DNA damage. Poly(ADP-ribose) polymerase-1 is an important factor in the maintenance of genomic integrity and is believed to play a central role in DNA repair. Here we provide visual evidence that in a recombination inducible severe combined immunodeficient cell line poly(ADP-ribose) formation occurs during the resolution stage of V(D)J recombination where nascent opened coding ends are generated. Poly(ADP-ribose) formation appears to facilitate coding end resolution. Furthermore, formation of Mre11 foci coincide with these areas of poly(ADP-ribosyl)ation. In contrast, such a response is not observed in wild-type cells possessing a functional catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). Thus, V(D)J recombination invokes a DNA damage response in cells lacking DNA-PKcs activity, which in turn promotes DNA-PKcs-independent resolution of recombination intermediates.