Review ArticleTelomere length assessment: Biomarker of chronic oxidative stress?
Introduction
Telomeres are nucleoprotein structures at the end of chromosomes. They prevent chromosomal ends from being recognized as double-strand breaks and protect them from end to end fusion and degradation [1], [2], [3]. Telomeres consist of stretches of repetitive DNA with a high G-C content [4], [5]. Fig. 1 illustrates the structure of the telomeres. In humans, the telomere terminus consists of 4 to 15 kbp of the hexanucleotides 5′-TTAGGG-3′ [6], [7]. At each chromosomal end, the G-rich telomeric DNA strand runs 5′ to 3′ toward the terminus and protrudes 100–150 nucleotides beyond the complementary C-rich strand [3]. It is protected from degradation by intercalation into the double-stranded telomere DNA, forming a telomeric loop (t-loop) [8].
In most proliferating cells telomere length is dynamic, and with each cell division the lengths of telomeres in human somatic cells decrease gradually by 20 to 200 bp. Partially, this loss of base pairs is a consequence of the so-called end-replication problem. DNA polymerases are not able to replicate chromosomes completely, since one RNA primer remains on each daughter DNA strand. The last primers are removed by a 5′→3′ exonuclease, but DNA polymerases cannot fill the gaps because there is no 3′-OH available to which a nucleotide can be added. As a consequence, the replication machinery must leave a small region at the end (a piece of the telomere) uncopied. The end-replication problem leads to chromosome shortening with each round of cell division. Eventually, this will lead to elimination of the telomeres and, as a consequence, apoptosis or an irreversible growth arrest of the cells [9]. These senescent cells are irreversibly arrested in the G1 phase of the cell cycle [10].
Besides the end-replication problem, other factors may contribute to telomere shortening.
Since human telomeres contain G-rich overhangs, the action of a 5′-to-3′-specific exonuclease was suggested. The action of this exonuclease would shorten each telomere by half the overhang length per round of replication and it was suggested to be involved in the final degradation of RNA primers from the lagging strand of the DNA [11], [12], [13].
Both the end-replication problem and the existence of an exonuclease only partially account for the loss of telomeres seen in cells, and therefore it was suggested that other mechanisms may be involved in accelerated telomere shortening. One of these mechanisms, DNA damage induced by oxidative stress, has been extensively studied and described by von Zglinicki and co-workers [6], [8], [11], [14], [15], [16], [17], [18]. This led to the hypothesis that measurement of telomere length can be used as a biomarker of chronic oxidative stress. In this review, therefore, the measurement of telomere length as a potential biomarker of oxidative stress and chronic inflammation, and various models to investigate this relationship, will be discussed.
Section snippets
Factors influencing telomere regulation
Before discussing the role of oxidative stress in telomere shortening, other factors that are involved in telomere regulation will be discussed. In contrast to stem cells and germ-line cells, telomeres in somatic cells shorten with each cell division. Cell proliferation is therefore considered to be one of the most important causes of telomere shortening. For instance, human diploid fibroblasts have a limited replicative life span in vitro due to telomere attrition, which leads to cellular
Telomere length in different cell types and tissues
Telomere length has been determined in a variety of tissues and it was observed that telomere length varies considerably from tissue to tissue. Cells in different tissues differ enormously in their turnover rate. Many of the differentiated cells that need continual replacement are unable to divide themselves. Replacement cells need to be generated from a stock of precursor cells, the stem cells or progenitor cells [21]. Most mature cells of the hematopoietic system are relatively short-lived. A
Proteins involved in telomere regulation
Various proteins play an important role in telomere length regulation. A complex formed by six telomere-specific proteins, namely telomeric repeat binding factors 1 and 2 (TRF1 and TRF2), TRF1-interacting protein 2 (TIN2), repressor activator protein 1 (Rap1), tripeptidyl peptidase 1 (TPP1), and protection of telomeres 1 (POT1), associates with the telomere sequence and protects the chromosome ends. These and some other important proteins at human telomeres, namely tankyrase 1 and 2,
How does oxidative stress cause telomere shortening?
The contribution to telomere loss by oxidative DNA damage was, in many cases, reported to overrate the contribution by the end-replication problem [6]. Due to their high content of guanines, telomeres were demonstrated highly sensitive to damage by oxidative stress [51], [52]. In addition, oxidative damage to bases has been reported to accumulate over the life span of a cell or an organism and may also significantly contribute to senescence. Senescent cells were found to contain 30% more
Antioxidants and telomere shortening
Since oxidative modification and shortening of telomeres are induced by ROS, it is expected that antioxidants may be preventive. This appears to be supported by several studies. For example, Furumoto et al. [66] found an age-dependent telomere shortening in human vascular endothelial cells (HUVECs) in vitro, as determined by Southern blot analysis. This could be slowed down by ascorbate-2-O-phosphate (Asc2P), an oxidation-resistant derivative of vitamin C [66]. In this study, addition of 130 μM
Chronic inflammatory diseases and oxidative stress
Additional evidence for the role of oxidative stress in telomere shortening was found in mechanistic studies and population studies that investigated the association between telomere length and diseases in which chronic oxidative stress and inflammation play a major role.
In an in vitro study performed by Xu et al. [72], a relationship between homocysteine and aging was demonstrated. Homocysteine has been reported to exert atherogenic effects and this has been ascribed to increased hydrogen
Possible models to investigate the relationship between telomere length and oxidative stress
Various models have been developed and applied to investigate the relationship between telomere length and oxidative stress. The most common models used are in vitro studies, in which different cell lines are exposed to an oxidative stressor, such as hydrogen peroxide or tert-butyl hydroperoxide, for several passage numbers. Several studies have shown that chronic oxidative stress leads to enhanced telomere shortening in cultured cells [11], [17], [51], [52], [63], [99], [100], [101]. In a
Other important features of telomere length
A reliable assessment of telomere length is also important in investigating the relationship between oxidative stress and telomere length regulation, and there are different methods that are applied to analyze telomere length [113], [114]. Describing these different methods would be beyond the scope of this review. The choice for a specific method to determine telomere length will depend on the type and aim of the study. Each method has its advantages and disadvantages that make it more or less
Concluding remarks
According to the studies reviewed and discussed in the previous sections, telomere length appears a promising genetic marker for chronic oxidative stress. Since antioxidants positively and ROS negatively influence telomere length, a relationship between chronic oxidative stress and telomere shortening appears to be supported by in vitro as well as in vivo experimental data. However, it must be taken into account that telomere length is determined not only by environmental factors, but also by
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