**5. Oxidative stress**

and younger controls observed that cases were less likely to be deficient in complement C4

The cellular communication has a fundamental role in regulating the reaction of the immune system to a possible danger [36]. In this scenario, a key role may be played by Toll-like receptor 4 (TLR4) that initiated both innate and clonotypic immunity to Gram-negative bacteria and to other agents. A SNP in TLR4 gene, ASP299GLY, was known to regulate the receptor signaling that the presence of 896G allele seems to be attenuated with a minor risk to develop carotid atherosclerosis and less intima-media thickness in the common carotid artery. In addition, 896G TLR4 allele shows a significantly lower frequency in patients affected by acute myocardial infarction with respect to controls, whereas centenarians show higher frequency [37]. This is in agreement with the hypothesis that genetic basis of inflammation might play an opposite role in CVD and in longevity because people genetically predisposed to a weak inflammatory activity less likely develop CVD and, at the same time, without any serious

TLR4 activates the inflammatory cell via the NF-kB pathway by inducing the expression of a variety of cytokines; some of these have been shown to be involved in atherosclerosis and reciprocally in longevity. In a previous study, conducted by Candore et al. [39], it was demonstrated that the 896 G allele carriers produce low levels of the pro-inflammatory cytokines IL-6 and tumor necrosis factor (TNF)-α and a higher level of the anti-inflammatory cytokine IL-10. Cytokines are the expression of a network involving genes, polymorphisms and environment, and are involved both in inflammation and anti-inflammation. Pro-inflammatory cytokines seem to play a pathogenic role in age-related diseases, and in previous study, it was demonstrated that genetic variations located within their promoter regions may influence the susceptibility to agerelated diseases, by increasing gene transcription and therefore cytokine production [40, 41]. Conversely, successful ageing seems to be associated to genetic variations determining increased production of anti-inflammatory cytokines or decreased production of pro-inflammatory cytokines, suggesting a role for the control of the inflammatory state in the attainment of longevity. IL-1, IL-2, IL-6, IL-12, IL-15, IL-18, IL-22, IL-23, TNF-α and interferon (IFN)-γ were described as pro-inflammatory cytokines, while IL-1Ra, IL-4, IL-10 and TGF-β1 as anti-inflammatory cytokines. High levels of IL-1, together with IL-6, tumor necrosis factor (TNF) and interferon (IFN)-γ, are associated with increased risk of morbidity and mortality in the older subject. Two studies have investigated the role of genetic variability of IL-1 gene cluster and a possible association with longevity [42, 43]. Neither study showed statistically significant differences comparing the allele frequencies, genotype frequencies and haplotype frequencies between

Also, IL-6 cytokine polymorphisms have been linked to longevity. Several data suggest that IL-6 −174C/G locus variability seems to modulate individual susceptibility to common causes of morbidity and mortality among the oldest subjects (e.g. type 2 diabetes, CVD and dementia) and therefore interferes with an individual's ability to reach the extreme limits of human lifespan [44–47]. A meta-analysis, conducted in Europeans, analyzed that data regarding long-lived subjects and controls from eight case-control studies showed no association

long genes, suggesting a potential role of immunity in lifespan [35].

10 Senescence - Physiology or Pathology

infectious disease complication, more likely live longer [38].

long-lived patients and youth nor between males or females.

The role of the oxidative stress response in healthy ageing and longevity is a hot topic in the field of human ageing studies.

The free radical theory of ageing, proposed in 1956, suggests that free radical-induced accumulation of damage to cellular macromolecules is a primary driving force of ageing and a major determinant of lifespan [57]. Under normal conditions ROS (including NADPH oxidases (NOX), mitochondria, xanthine oxidase, monoamine oxidase and nitric oxide synthase) were maintained at the physiological levels by several endogenous antioxidant systems, such as superoxide dismutatase (SOD), catalase, glutathione peroxidases and glutathione reductase (GR). Other antioxidant systems involving thiol-disulfide oxidoreductase systems include the cytosolic proteins thioredoxin (TRX) and glutaredoxin (GRX). ROS at physiological levels can interact with redox state and play a role in mediating cell signaling, while at pathological levels can result in oxidative damage to cellular components that activate several cell death pathways.

The close interrelationship of redox balance to oxidative stress has in recent years become a more prominent aspect of the free radical theory of ageing that was extended to implicate the mitochondrial production of ROS [58]. In fact, studies in long-lived species showed the presence of reduced oxidative damage [59], reduced mitochondrial free radical production [60], increased antioxidant defences [60] and increased resistance to oxidative stress both in vivo and in vitro [61]. However, a lack of correlation of oxidation with lifespan [62], or even an increase in oxidative damage/stress associated with long lifespan, has also been reported [63].

In addition, in support of the importance of genetic factors in the ageing organism's ability to counteract the negative effects of oxidative stress, genetic modifications of the stress response with age have also been reported. These modifications may minimize health risks, and they may increase the individual's possibilities of achieving a longer life [64].

Several studies suggest that both genetic factors and modifiable lifestyle habits have major impact on the oxidative stress response, but the relative contribution of genes and lifestyle in promoting an efficient stress response in cells is difficult to estimate. Data collected in the literature shows that in experimental organisms and in exceptionally long-lived individuals, among the antioxidant enzymes, a major role in longevity seems to be attributed to genes SOD2 and GPX [65].

Knowing that oxidative stress accelerates telomere loss, the genes encoding the telomere maintenance pathway, mainly telomerase reverse transcriptase (TERT) and telomerase RNA component (TERC), are also important to happen [66, 67]. In addition, in support of gender difference in longevity, some genes involved in the stress response pathway, like heat shock protein A1A (HSPA1A) or paraoxonase 1 (PON1), show sex-specific effects [68].

A study in Chinese centenarians and nonagenarians and younger controls identified significant genotype differences in the GNB3 and eNOS genes, whose variants have been implicated in hypertension and vascular function via nitric oxide (NO) generation, respectively [69]. Another study found that variants of two NO synthase genes, NOS1 and NOS2, decrease the probability of attaining longevity, suggesting that NO production and signaling may be involved in ageing [70].

Mitochondrial genetic variability, both germ line and somatic, influences the stress response and is associated with human ageing/longevity. In both physiological and pathological conditions, a strict coordination between nuclear and mitochondrial genomes is necessary to ensure the biosynthesis and functional activity of mitochondria [71]. In normal conditions, signals from the nucleus to mitochondrion are essential for maintaining an adequate mitochondrial structure and function. The mitochondrial replication and transcription were modulated by several nuclear-encoded transcription factors and coactivators, such as transcription factors which bind to the promoter regions of mtDNA (transcription factors A (Tfam) and B (mtTFB) that enhancing the rate of transcription initiation of mtDNA genes and mitochondrial biogenesis) and nuclear respiratory factors NRF-1 and NRF-2 and the peroxisome proliferatoractivated receptor-γ coactivator-1 (PGC-1) family coactivators (PGC-1α, PGC-1α,and related coactivator PRC) [71–73].

Although many association studies explored the effect of genetic variability at candidate genes belonging to the oxidative stress pathway in relation to age-related clinical conditions [74], as well as with human longevity, few papers investigated their role on the quality of human ageing and in particular on the functional decline characterizing human senescence. Most of the studies investigate the association of genes related to oxidative stress with cognitive ability and cognitive ageing in healthy older people [75–79] and found an involvement of apolipoprotein E (APOE), catechol-O-methyltransferase (COMT), brain-derived neurotrophic factor (BDNF) and dystrobrevin-binding protein 1 (DTNBP1) genes in cognitive ability in older people.

A recent work conducted by Dato et al. [80] investigated the association between 311 SNPs at 38 genes belonging to the oxidative stress pathway with functional status at very older age. They found associations for TXNRD1 variability with activities of daily living and walking speed, NDUFS1 and UCP3 with handgrip strength and walking speed and GCLC and UCP2 with walking speed. They also found that the association between genetic variability in the pro-oxidant-antioxidant pathway and functional status at old age is influenced by sex and in particular in nonagenarian females. From these data it is possible to speculate that prooxidant-antioxidant pathway is able to modulate physical and cognitive performance after the ninth decade of life, finally influencing extreme survival.

Thus, the balance between pro-oxidants and enzymatic antioxidant systems may be of particular importance in the elderly, whose nutritional deficiencies and sedentary lifestyle concur with a depletion of dietary antioxidants and increased susceptibility to oxidative stress.
