**4. Occupational and environmental exposures**

Different approaches are used to evaluate effects and risks of exposure to chemical, physical, and biological agents during routine work or where an individual lives. Biomarker is a general term for analysis of the interaction of a biological system and an environmental agent. There are three classes of biomarkers: (a) of exposure, that involve exogenous substances or their metabolites, or a product of interaction between a xenobiotic agent and some target cells or molecules; (b) of effect, that are biochemical, physiological or behavioral parameters, or other changes within the body and, depending upon the extent, they can be recognized in association with a disease or as a potential risk for the development of a disease; and (c) of susceptibility, that refers to the inherent or acquired ability of an organism to respond to changes in exposure to xenobiotic specific substances [76–78].

There is growing evidence that telomeric stability may be affected by environmental and/or occupational exposure, as some of those factors have been related to inflammation and chronic diseases. Occupational exposures related to shorter telomeres include polycyclic aromatic hydrocarbons (PAHs), benzene and toluene, particulate matter and long‐term exposure to lead (reviewed by Zhang et al. [41]). PAHs are known for generating DNA adducts and, therefore, genomic instability. A recent study has shown the relationship between telomere shortening and pesticide use in workers associated with the agricultural industry [79]. Lead, in turn, induces double‐strand breaks in DNA, particularly in lagging strands on telomeres (for a review, see [41]). Working as a hairdresser has been associated with increasing risk of cancer, due to cancer‐related DNA alterations. Telomere length was shortened in a group of Sweden hairdressers [80]. The authors suggest that, as hairdressers are exposed to strong oxidative agents, it is likely that oxidative stress is the main reason for telomere shortening in these workers [80].

In another meta‐analysis with regard to telomere length and cancer risk population studies, authors reviewed more than 50 publications [50]. Their results revealed heterogeneous association between different cancer types. In opposition to other reviews, they did not observe a significant association of short telomeres with the overall risk of cancer. Still, shorter telomeres were found associated with increased risk of gastrointestinal and head and neck cancers, similar to prior review [46]. Both are mainly cases of epithelial malignancies, which mostly appear to develop from morphologically defined precursor lesions termed intraepi‐ thelial neoplasia. The meta‐analyses also revealed a significant dose‐response association of gastrointestinal tumor and head and neck cancer with telomere length. The authors also highlight that telomere length is critically shortened in more than 90% of intraepithelial neoplasias. It is accepted that telomeres have different roles in different types of cancer, but again, this review indicates that short telomeres may be risk factors for tumors, especially of the digestive system [50]. A shorter TL in individuals with cancer when compared to healthy controls is biologically plausible. The accumulated mutations from critically shortened telomeres, genetic lesions, and inactivated tumor suppressor checkpoints may ultimately

For many years, oncogenesis has been linked to telomerase activity in somatic cells [6, 7]. In fact, overexpression of telomerase is enough to neutralize the natural telomere shortening and to indefinitely extend the replicative lifespan of cultured cells when genomic instability is lacking, turning them into cancerous cells. The active telomerase complex may be more necessary to cancerous cells than to normal somatic cells due to its chromosomal aneuploidy and rapid cell division cycle [47, 75]. Thus, it seems conflicting that shortened telomeres are linked to several types of cancer. However, the mechanism of the shortened telomere rela‐ tionship with cancer is through genomic instability. In the oncogenesis process, the inactivation of senescence pathways by some viral oncogenes, mutations on key‐genes or chemical substances allows cells to bypass replicative checkpoints. This enables the propagation of cells with damaged telomere leading to end‐to‐end fusions and genome instability, and then to age‐

Different approaches are used to evaluate effects and risks of exposure to chemical, physical, and biological agents during routine work or where an individual lives. Biomarker is a general term for analysis of the interaction of a biological system and an environmental agent. There are three classes of biomarkers: (a) of exposure, that involve exogenous substances or their metabolites, or a product of interaction between a xenobiotic agent and some target cells or molecules; (b) of effect, that are biochemical, physiological or behavioral parameters, or other changes within the body and, depending upon the extent, they can be recognized in association with a disease or as a potential risk for the development of a disease; and (c) of susceptibility, that refers to the inherent or acquired ability of an organism to respond to changes in exposure

result in cancer [6, 7, 47, 52].

170 Telomere - A Complex End of a Chromosome

associated diseases, like cancer [43].

to xenobiotic specific substances [76–78].

**4. Occupational and environmental exposures**

A study from our group observed shortening telomeres in individuals occupationally chron‐ ically exposed to low doses of pesticides at tobacco farms [33], in addition to various kinds of DNA damages already found in those individuals [81–84], corroborated by several other studies [85, 86]. Pesticides are known for inducing oxidative stress [33, 82, 87, 88], and although not all action mechanism of these chemicals are clear, induction of oxidative stress and of ROS seems to be involved. Senescent cells present 30% more modified guanines within their DNA and fourfold more free 8‐oxo‐dG basis, contributing to telomeric loss through oxidative stress [89]. Recently, our group was able to show two different pathways involving the ubiquitin proteasome system (UPS) by which pesticides and nicotine influence telomere length. Using System Biology approach tools, we evaluated proteins involved in telomere maintenance and their relation to pesticides used in tobacco crops at Brazil, including the natural pesticide of tobacco leaves, nicotine. In this interaction network of proteins related to telomere length and tobacco pesticides, it is important to highlight the ubiquitination bioprocess of proteins involved in some clusters of interaction [90]. The UPS is a highly conserved cell pathway that plays an important role in the selective degradation of proteins that are essential for several cell functions [91]. Some works have shown the role of ubiquitination in maintaining telomeric length [30, 31].

Longer telomeres may also reflect a health problem. Persistent organic pollutant (POPs) was associated with telomere elongation, but the mechanism remains unknown. Increasing telomerase activity and, therefore, longer telomeres were also observed in occupational exposure to arsenic [41]. The Agricultural Health Study (AHS) analyzed farmers with regard to telomere length and both cumulative and recent use of several pesticides [92]. Shorter telomeres were found associated with the lifetime use of two pesticides, and one with recent use, while only alachlor was significantly associated with longer telomeres for both cumulative and recent use [92]. The Chernobyl nuclear power plant accident forced workers to clean up the region. Some studies revealed high occurrence of age‐associated degenerative diseases, cardiovascular disorders, and cancer among them (reviewed by Reste et al. [93]). However, when telomere length was analyzed, longer telomeres were found for the workers undertaking excavation and deactivation, and in workers with cancer. The authors suggest that the exposure to ionizing radiation led to longer telomeres through telomerase activation, which could potentiate carcinogenesis [93]. Even so, it is possible to highlight that most occupational exposures induce telomere shortening.

Environmental factors can also trigger epigenetic changes [94], which can also be related to telomere maintenance [67, 95]. Previous studies suggested that DNA methylation plays an important role in maintaining genomic stability, and is highly sensitive to environmental exposure [96, 97]. The impact of adverse exposure on telomere shortening starts at a very early developmental stage. Environmental exposure to lead in children appears to be associated with shorter telomeres [98], as prenatal exposure to toxic agents also seems to be a predictor of telomere imbalance. Neonatal umbilical cord blood showed a positive association between shortened fetal telomere length and smoking during pregnancy [99]. Even with the concept that high variation in telomere length between individuals is already present before birth and could increase due to environmental exposure, 128 Indian newborns from high‐level natural background radiation areas showed no evidence of telomere length attrition [100]. On the other hand, placental tissues of over 200 twins were evaluated with regard to telomere length [101]. The aim was to verify if maternal residential traffic exposure was associated with telomere length. Maternal residential proximity to a major road was linked to shortened placental telomere length, while maternal residence closer to more wooded sites increased placental telomere length by 3.6%. As traffic exposure is an important source of free radicals that are known for accelerating aging, the air pollution‐related adverse outcomes started early in life [101].

A group of Italian pregnant women living close to waste landfill sites was analyzed with regard to telomere length to investigate if pollution, as an environmental stressor, could affect their health. The authors observed that pollution from illegal waste sites was significantly associated with shorter telomere length, higher oxidative stress levels, and lower telomerase activity, which are known factors of cellular senescence and aging‐related meiotic dysfunction in women [102]. Even low levels of cadmium shortened buccal cell telomere length in adolescents environmentally exposed to this metal [103]. Arsenic exposure to drinking water increased telomere length in individuals from West Bengal, India. This effect was telomerase‐dependent but did not exhibit an overexpression of alternative lengthening of telomere‐associated proteins TRF1 and TRF2 [104]. Some environmental toxic metals can produce epigenetic changes, such as DNA methylation, loss of expression of tumor suppressor gene *p16*, among others [105], eventually leading to telomere dynamics alterations.
