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Telomere Length and Oxidative Stress and Its Relation with Metabolic Syndrome Components in the Aging

Simple Summary

A link between telomere length and some age-related diseases has been identified, including metabolic syndrome. So far, there is no mechanism to explain the origin or cause of telomere shortening in this syndrome; however, oxidative stress is a constant factor. Therefore, we reviewed scientific evidence that supported the association between oxidative stress and telomere length dynamics, also examining how each of the metabolic syndrome components individually affects the length. In this regard, there is strong scientific evidence that an increase in the number of metabolic syndrome components is associated with a shorter telomere length, oxidative damage at the lipid and DNA level, and inflammation, as well as its other components, such as obesity, hyperglycemia, and hypertension, while for dyslipidemia, there is a little more discrepancy. The difficulty for the correct treatment of metabolic syndrome lies in its multifactorial nature. Hence, there is a need to carry out more studies on healthy lifestyles during aging to prevent and reduce oxidative damage and telomere wear during aging, and consequently the progression of chronic degenerative diseases, thus improving the living conditions of older people.

Abstract

A great amount of scientific evidence supports that Oxidative Stress (OxS) can contribute to telomeric attrition and also plays an important role in the development of certain age-related diseases, among them the metabolic syndrome (MetS), which is characterised by clinical and biochemical alterations such as obesity, dyslipidaemia, arterial hypertension, hyperglycaemia, and insulin resistance, all of which are considered as risk factors for type 2 diabetes mellitus (T2DM) and cardiovascular diseases, which are associated in turn with an increase of OxS. In this sense, we review scientific evidence that supports the association between OxS with telomere length (TL) dynamics and the relationship with MetS components in aging. It was analysed whether each MetS component affects the telomere length separately or if they all affect it together. Likewise, this review provides a summary of the structure and function of telomeres and telomerase, the mechanisms of telomeric DNA repair, how telomere length may influence the fate of cells or be linked to inflammation and the development of age-related diseases, and finally, how the lifestyles can affect telomere length.

At the Beginning of the End and in the Middle of the Beginning: Structure and Maintenance of Telomeric DNA Repeats and Interstitial Telomeric Sequences

Tandem DNA repeats derived from the ancestral (TTAGGG)n run were first detected at chromosome ends of the majority of living organisms, hence the name telomeric DNA repeats. Subsequently, it has become clear that telomeric motifs are also present within chromosomes, and they were suitably called interstitial telomeric sequences (ITSs). It is well known that telomeric DNA repeats play a key role in chromosome stability, preventing end-to-end fusions and precluding the recurrent DNA loss during replication. Recent data suggest that ITSs are also important genomic elements as they confer its karyotype plasticity. In fact, ITSs appeared to be among the most unstable microsatellite sequences as they are highly length polymorphic and can trigger chromosomal fragility and gross chromosomal rearrangements. Importantly, mechanisms responsible for their instability appear to be similar to the mechanisms that maintain the length of genuine telomeres. This review compares the mechanisms of maintenance and dynamic properties of telomeric repeats and ITSs and discusses the implications of these dynamics on genome stability.

Subtelomeric proteins negatively regulate telomere elongation in budding yeast

The Tbf1 and Reb1 proteins are present in yeast subtelomeric regions. We establish in this work that they inhibit telomerase-dependent lengthening of telomere. For example, tethering the N-terminal domain of Tbf1 and Reb1 in a subtelomeric region shortens that telomere proportionally to the number of domains bound. We further identified a 90 amino-acid long sequence within the N-terminal domain of Tbf1 that is necessary but not sufficient for its length regulation properties. The role of the subtelomeric factors in telomere length regulation is antagonized by TEL1 and does not correlate with a global telomere derepression. We show that the absence of TEL1 induces an alteration in the structure of telomeric chromatin, as defined biochemically by an increased susceptibility to nucleases and a greater heterogeneity of products. We propose that the absence of TEL1 modifies the organization of the telomeres, which allows Tbf1 and Reb1 to cis-inhibit telomerase. The involvement of subtelomeric factors in telomere length regulation provides a possible mechanism for the chromosome-specific length setting observed at yeast and human telomeres.

Telomere maintenance, function and evolution: the yeast paradigm

Telomeres are multifunctional genetic elements that cap chromosome ends, playing essential roles in genome stability, chromosome higher-order organization and proliferation control. The telomere field has largely benefited from the study of unicellular eukaryotic organisms such as yeasts. Easy cultivation in laboratory conditions and powerful genetics have placed mainly Saccharomyces cerevisiae, Kluveromyces lactis and Schizosaccharomyces pombe as crucial model organisms for telomere biology research. Studies in these species have made it possible to elucidate the basic mechanisms of telomere maintenance, function and evolution. Moreover, comparative genomic analyses show that telomeres have evolved rapidly among yeast species and functional plasticity emerges as one of the driving forces of this evolution. This provides a precious opportunity to further our understanding of telomere biology.

The N-terminal and C-terminal domains of RAP1 are dispensable for chromatin opening and GCN4-mediated HIS4 activation in budding yeast

Repressor activator protein 1 (RAP1) assists GCN4-mediated HIS4 activation by overcoming some repressive aspect of chromatin structure to facilitate GCN4 binding. RAP1 also participates in other nuclear processes, and discrete domains of RAP1 have been shown to have specific properties including DNA binding, DNA bending, transcriptional activation, and silencing and telomere functions. To investigate whether specific domains of RAP1 are required to "open" chromatin and help GCN4 to activate the HIS4 gene, we examined the abilities of different truncated RAP1 proteins to perturb positioned nucleosomes via a nucleosomal RAP1 site in a yeast episome in vivo, and we tested HIS4 activation in yeast strains harboring truncated RAP1 mutants. We found that neither the DNA bending domain nor the putative activation domain of RAP1 is required for its ability to perturb the chromatin structure of a plasmid containing a RAP1 site. Similarly, neither the putative activation domain nor the N-terminal DNA-bending domain was required for GCN4-mediated activation of HIS4. We also used a rap1(ts) mutant to show that continuous occupancy of the HIS4 promoter by RAP1 is required for GCN4-mediated gene activation.

Alternative Mechanisms of Transcriptional Activation by Rap1p

Single Rap1p DNA-binding sites are poor activators of transcription of yeast minimal promoters, even when fully occupied in vivo. This low efficiency is due to two independent repression mechanisms as follows: one that requires the presence of histones, and one that requires Hrs1p, a component of the RNA polymerase II mediator complex. Both repression mechanisms were greatly reduced for constructs with tandemly arranged sites. In these constructs, UASrpg sequences (ACACCCATACATTT) activated better than telomere-like sequences (ACACCCACACACCC) in an orientation-dependent manner. Both mutations in the SWI/SNF complex and a deletion of amino acids 597--629 of Rap1p (Tox domain) decreased synergistic effects of contiguous telomeric sites. Conversely, deletion of amino acids 700--798 of Rap1p (Sil domain) made UASrpg and telomeric sites functionally indistinguishable. We propose that the Sil domain masks the main transactivation domain of Rap1p in Rap1p-telomere complexes, where the Tox domain behaves as a secondary activation domain, probably by interacting with chromatin-remodeling complexes. Rap1p DNA-binding sites in ribosomal protein gene promoters are mainly UASrpg-like; their replacement by telomeric sequences in one of these promoters (RPS17B) decreased transcription by two-thirds. The functional differences between UASrpgs and telomeric sequences may thus contribute to the differential expression of Rap1p-regulated promoters in vivo.

RAP, RAP, open up! New wrinkles for RAP1 in yeast

RAP1 (repressor/activator protein 1) from budding yeast is well known for its involvement in gene activation and repression, telomere structure and function, and replication. Recent studies have examined additional roles for RAP1 in heterochromatin boundary-element formation, creation of hotspots for meiotic recombination, and chromatin opening. These studies provide new insight into the ability of this abundant DNA-binding protein to participate in a diverse array of functions taking place in a chromatin environment.

Chromatin opening and transactivator potentiation by RAP1 in Saccharomyces cerevisiae

Transcriptional activators function in vivo via binding sites that may be packaged into chromatin. Here we show that whereas the transcriptional activator GAL4 is strongly able to perturb chromatin structure via a nucleosomal binding site in yeast, GCN4 does so poorly. Correspondingly, GCN4 requires assistance from an accessory protein, RAP1, for activation of the HIS4 promoter, whereas GAL4 does not. The requirement for RAP1 for GCN4-mediated HIS4 activation is dictated by the DNA-binding domain of GCN4 and not the activation domain, suggesting that RAP1 assists GCN4 in gaining access to its binding site. Consistent with this, overexpression of GCN4 partially alleviates the requirement for RAP1, whereas HIS4 activation via a weak GAL4 binding site requires RAP1. RAP1 is extremely effective at interfering with positioning of a nucleosome containing its binding site, consistent with a role in opening chromatin at the HIS4 promoter. Furthermore, increasing the spacing between binding sites for RAP1 and GCN4 by 5 or 10 bp does not impair HIS4 activation, indicating that cooperative protein-protein interactions are not involved in transcriptional facilitation by RAP1. We conclude that an important role of RAP1 is to assist activator binding by opening chromatin.

Proteins that bind to double-stranded regions of telomeric DNA

In budding yeast, the DNA-binding protein Rap1p orchestrates a negative feedback on regulation of telomere length and the organization of a heterochromatin-like telomeric compartment. Recent studies have led to the identification of functionally related telomeric proteins from fission yeast and mammals. These advances underline the key role played by the proteins that bind to the duplex part of telomeric DNA and reveal an important structural diversity among telomeric proteins.

Stn1, a new Saccharomyces cerevisiae protein, is implicated in telomere size regulation in association with Cdc13

We have isolated STN1, an essential Saccharomyces cerevisiae gene, as a suppressor of the cdc13-1 mutation. A synthetic lethal interaction between a temperature-sensitive mutant allele of STN1, stn1-13, and cdc13-1 was observed. Stn1 and Cdc13 proteins displayed a physical interaction by two-hybrid analysis. As shown previously for cdc13-1, stn1-13 cells at the restrictive temperature accumulate single-stranded DNA in subtelomeric regions of the chromosomes, but to a lesser extent than cdc13-1 cells. In addition, both Cdc13 and Stn1 were found to be involved in the regulation of telomere length, mutations in STN1 or CDC13 conferring an increase in telomere size. Loss of Stn1 function activated the RAD9 and MEC3 G2/M checkpoints, therefore confirming that DNA damage is generated. We propose that Stn1 functions in telomere metabolism during late S phase in cooperation with Cdc13.

Nuclear organization and transcriptional silencing in yeast

Transcriptional repression at the yeast silent mating type loci requires the formation of a nucleoprotein complex at specific cis-acting elements called silencers, which in turn promotes the binding of a histone-associated Sir-protein complex to adjacent chromatin. A similar mechanism of long-range transcriptional repression appears to function near telomeric repeat sequences, where it has been demonstrated that Sir3p is a limiting factor for the propagation of silencing. A combined immunofluorescence/in situ hybridization method for budding yeast was developed that maintains the three-dimensional structure of the nucleus. In wild-type cells the immunostaining of Sir3p, Sir4p and Rap1 colocalizes with Y' subtelomeric sequences detected by in situ hybridization. All three antigens and the subtelomeric in situ hybridization signals are clustered in foci, which are often adjacent to, but not coincident with, nuclear pores. This colocalization of Rap1, Sir3p and Sir4p with telomeres is lost in sir mutants, and also when Sir4p is overexpressed. To test whether the natural positioning of the two HM loci, located roughly 10 and 25 kb from the ends of chromosome III, is important for silencer function, a reporter gene flanked by wild-type silencer elements was integrated at various internal sites on other yeast chromosomes. We find that integration at internal loci situated far from telomeres abrogates the ability of silencers to repress the reporter gene. Silencing can be restored by creation of a telomere at 13 kb from the reporter construct, or by insertion of 340 bp of yeast telomeric repeat sequence at this site without chromosomal truncation. Elevation of the internal nuclear pools of Sir1p, Sir3p and Sir4p can relieve the lack of repression at the LYS2 locus in an additive manner, suggesting that in wild-type cells silencer function is facilitated by its juxtaposition to a pool of highly concentrated Sir proteins, such as those created by telomere clustering.

Evidence for silencing compartments within the yeast nucleus: a role for telomere proximity and Sir protein concentration in silencer-mediated repression

Transcriptional repression at the silent mating-type loci in yeast requires the targeting of silent information regulator (Sir) proteins through specific interactions formed at cis-acting silencer elements. We show here that a reporter gene flanked by two functional silencers is not repressed when integrated at >200 kb from a telomere. Repression is restored by creation of a new telomere 13 kb from the integrated reporter or by elevated expression of SIR1, SIR3, and/or SIR4. Coupled expression represses in an additive manner, suggesting that all three factors are in limiting concentrations. When overexpressed, Sir3 and Sir4 are dispersed throughout the nucleoplasm, in contrast to wild-type cells where they are clustered in a limited number of foci together with telomeres. Efficient silencer function thus seems to require either proximity to a pool of concentrated Sir proteins, that is, proximity to telomeres, or delocalization of the silencing factors.

A novel mechanism for telomere size control in Saccharomyces cerevisiae

One of the central requirements for eukaryotic chromosome stability is the maintenance of the simple sequence tracts at telomeres. In this study, we use genetic and physical assays to reveal the nature of a novel mechanism by which telomere length is controlled. This mechanism, telomeric rapid deletion (TRD), is capable of reducing elongated telomeres to wild-type tract length in an apparently single-division process. The deletion of telomeres to wild-type lengths is stimulated by the hpr1 mutation, suggesting that TRD in these cells is the consequence of an intrachromatid pathway. Paradoxically, TRD is also dependent on the lengths of the majority of nonhomologous telomeres in the cell. Defects in the chromatin-organizing protein Sir3p increase the rate of hpr1-induced rapid deletion and specifically change the spectrum of rapid deletion events. We propose a model in which interactions among telosomes of nonhomologous chromosomes form higher order complexes that restrict the access of the intrachromatid recombination machinery to telomeres. This mechanism of size control is distinct from that mediated through telomerase and is likely to maintain telomere length within a narrow distribution.

The crystal structure of the DNA-binding domain of yeast RAP1 in complex with telomeric DNA

Telomeres, the nucleoprotein complexes at the ends of eukaryotic chromosomes, are essential for chromosome stability. In the yeast S. cerevisiae, telomeric DNA is bound in a sequence-specific manner by RAP1, a multifunctional protein also involved in transcriptional regulation. Here we report the crystal structure of the DNA-binding domain of RAP1 in complex with telomeric DNA site at 2.25 A resolution. The protein contains two similar domains that bind DNA in a tandem orientation, recognizing a tandemly repeated DNA sequence. The domains are structurally related to the homeodomain and the proto-oncogene Myb, but show novel features in their DNA-binding mode. A structured linker between the domains and a long C-terminal tail contribute to the binding specificity. This structure provides insight into the recognition of the conserved telomeric DNA sequences by a protein.

Heterochromatin and gene regulation in Drosophila

We have recently learned more about the biochemistry of heterochromatin and about how heterochromatic environments affect gene function. New findings have emphasized the distinctions between telomeric and pericentric heterochromatin in Drosophila and have suggested a mosaic structure within pericentric heterochromatin. Theories concerning the mechanism of inactivation of euchromatic genes in heterochromatic environments have been tested using transgenes inserted into heterochromatin. The current data support a competition/chromatin structure model, in which multiprotein repressor complexes compete with transcriptional activators to assemble an active or inactive chromatin structure.

The telobox, a Myb-related telomeric DNA binding motif found in proteins from yeast, plants and human

The yeast TTAGGG binding factor 1 (Tbf1) was identified and cloned through its ability to interact with vertebrate telomeric repeats in vitro. We show here that a sequence of 60 amino acids located in its C-terminus is critical for DNA binding. This sequence exhibits homologies with Myb repeats and is conserved among five proteins from plants, two of which are known to bind telomeric-related sequences, and two proteins from human, including the telomeric repeat binding factor (TRF) and the predicted C-terminal polypeptide, called orf2, from a yet unknown protein. We demonstrate that the 111 C-terminal residues of TRF and the 64 orf2 residues are able to bind the human telomeric repeats specifically. We propose to call the particular Myb-related motif found in these proteins the 'telobox'. Antibodies directed against the Tbf1 telobox detect two proteins in nuclear and mitotic chromosome extracts from human cell lines. Moreover, both proteins bind specifically to telomeric repeats in vitro. TRF is likely to correspond to one of them. Based on their high affinity for the telomeric repeat, we predict that TRF and orf2 play an important role at human telomeres.