**2. Telomeres and cellular senescence**

#### **2.1. Telomeres**

length, cell cycle arrest and cellular senescence occur in the cellular level. In the body, telo‐ meres gradually shorten with aging [2]. A number of observations suggest a close connection between telomere length, generally assessed in leukocytes, and mortality or age-related disease, suggesting telomere length as a "mitotic clock," a marker for individual biological aging [3]. In this regard, to date, telomere length in various diseases has been investigated, including cancers, immune insufficiency, and cardiovascular disease. In addition, metabolic diseases, such as obesity and diabetes mellitus (DM), have shown a strong association with telomere shorten‐ ing. Several endocrine disorders, such as polycystic ovary syndrome (PCOS), Cushing's syndrome, and acromegaly, are also reportedly associated with telomere shortening (**Figure**

Furthermore, several recent studies have focused on the pathophysiological role of telomeres for metabolic or endocrine diseases. Telomere shortening is one of the important causes of cellular senescence, and recently, it has emerged that cellular senescence plays a pivotal role in the aging and pathogenesis of age-related disease [4]. Actually, several clinical studies have shown that shortened telomeres at baseline are associated with an increased risk for develop‐

Here, we review the association and pathophysiological role of telomere length in metabolic and endocrine diseases. Furthermore, we discuss the mechanistic insight and significance of shortened telomeres and associated cellular senescence. Finally, we discuss a possible thera‐

**Figure 1.** Telomere shortening and human diseases. Shortened telomere length is associated with various disorders, including psychiatric disease, impaired immune function, and atherosclerotic disease as well as metabolic and endo‐

peutic approach for these diseases in the aspect of telomere shortening.

**1**).

crine diseases.

ment of age-related diseases.

144 Telomere - A Complex End of a Chromosome

Telomere is a dynamic complex at chromosome ends, which consists of repetitive DNA sequences [1,5]. In human cells, telomeres consist of thousands of "TTAGGG" tandem repeats. This base sequence is universal and consistent among most species. Human telomere complex consists of chromosomal-terminal tract of telomeric repeats bound by protective shelterin component proteins, with additional protective proteins. This complex, which binds specifi‐ cally to telomeres, forms a cap-like structure and prevents end-to-end fusion or damage of the chromosome ends.

The general chromosomal DNA replication cannot completely copy the DNA sequence in the ends of the linear chromosomes, which is called "end replication problem." During the course of cell divisions, this leads to attrition of chromosome ends. Therefore, normal telomere maintenance requires the ribonucleoprotein enzyme named telomerase, which can add telomeric repeat sequences to the end of the chromosomes [6]. However, in most of the human somatic cells, the levels of telomerase are limited and telomeres shorten throughout the life span. Other genetic or environmental factors can also contribute to telomere shortening; defects of telomere maintenance system, DNA replication stress, increased oxidative stress, chemical damage, and inflammatory status are involved in telomere shortening [1].

#### **2.2. Cellular senescence and senescence-associated secretary phenotype (SASP)**

Cellular senescence refers to the irreversible growth arrest that occurs when cells experience potentially oncogenic insults [7]. Telomere shortening is one of the most important causes of cellular senescence. Telomere shortening causes DNA damage response (DDR), a signaling pathway, in which cell cycle progression is blocked through an increased production of p53 and cyclin-dependent kinase (Cdk) inhibitor p21 protein. DDR subsequently induces cellular senescence. Recently, accumulating evidences suggest that senescent cells are also important for age-related pathologies, including metabolic diseases such as obesity and DM [8,9]. Elimination of senescent cells can delay age-related dysfunctions in mouse model [10], indicating that a presence of senescent cells itself plays a causal role in the process of aging. Aging tissues, in which senescent cells are increased, show a low-level chronic inflammation, termed "sterile inflammation" [11]. Sterile inflammation is, at least in part, derived from senescent cells, which secrete proinflammatory cytokines, chemokines, and proteases, which is called "senescence-associated secretary phenotype (SASP)" [12]. Proteins that are associated with SASP, such as tumor necrosis factor (TNF)-α, interleukin (IL)-6, matrix metalloprotei‐ nases (MMPs), and monocyte chemoattractant protein (MCP)-1, increase in multiple tissues with chronological aging in conjunction with sterile inflammation [13]. These SASP-related cytokines, such as TNF-α and IL-6, are reportedly associated with insulin resistance and the development of DM [14,15]. Although whether the SASP actually causes age-related diseases including metabolic diseases *in vivo* is still unclear, at least, telomere shortening and subse‐ quent cellular senescence have revealed a strong association with age-related diseases including metabolic diseases that are discussed in the following sections.

## **3. Telomeres in metabolic diseases**

#### **3.1. Obesity**

Obesity is a leading preventable cause of death and growing health problem worldwide with increasing rate in both adults and children. A number of studies have reported the association of telomere length with obesity. Although the results were inconsistent and the relationship of telomere shortening and obesity is still inconclusive, several large population-based studies showed telomere shortening in obesity. In cross-sectional epidemiological studies, shortened telomeres were associated with body mass index (BMI), waist-to-hip ratio, visceral fat, and weight gain [3,16,17]. Consistently, calorie-restricted diets and subsequent weight loss were associated with the increased telomere length in obese men [18]. These results indicated the relationship between obesity and shortened telomeres. As an underlying mechanism of telomere shortening in obesity, leptin might be involved. Leptin plays an essential role in the regulation of body fat mass, and it has some proinflammatory properties with increasing oxidative stress [19]. Valdes et al. reported that age, smoking, and serum leptin concentration were independently associated with telomere length, but BMI did not, suggesting that leptin may directly contribute the telomere shortening.

#### **3.2. Diabetes mellitus**

The number of patients with type 2 DM has drastically been increasing worldwide in associ‐ ation with the changes in lifestyle and increased prevalence of obesity. DM is categorized into several clinical types: type 1 DM (T1DM), type 2 DM (T2DM), gestational DM, and others. In particular, T2DM accounts for the majority of DM patients and the pathophysiology of T2DM has an age-related aspect. DM increases the risk of cardiovascular and cerebrovascular events, and cognitive dysfunction, which are known as age-related diseases. Telomere length in patients with DM has been examined in many studies [20]. In 1998, Jeanclos et al. [21] showed that patients with insulin-dependent DM (IDDM) had shorter telomeres in peripheral leuko‐ cytes than non-DM individuals. Patients with T2DM also showed shortened telomeres [22,23]. Furthermore, it has been reported that telomere shortening rate increased with the duration of T2DM [24]. As an underlying mechanism of the telomere shortening in DM, increased production of reactive oxygen species (ROS) caused by hyperglycemia and hyperinsulinemia is supposed. Polyol pathway activation, protein kinase C pathway activation, and increased production of advanced glycation end products (AGEs) also play a pathological role in increased levels of oxidative stress [25]. In fact, Sampson et al. [23] showed an inverse corre‐ lation between the level of oxidative stress marker, 8-hydroxy-deoxyguanosine, and the telomere length. These results suggest that the increased oxidative stress in DM may accelerate the telomere shortening.

To date, several studies have focused on the relationship between telomere length and the mortality and progression of DM complications. In a prospective follow-up study, telomere length in T1DM was associated with all cause of mortality [26]. However, the association of telomere length with DM complication has been controversial. Several studies exhibited that DM patients with shorter telomeres has tended to show severer complications of DM [27,28]. On the other hand, Astrup et al. [26] reported that telomere length did not differ between patients with and without nephropathy. Although further studies are needed, these results suggest that telomere length could be used as a surrogate marker for mortality and some of the morbidity in patients with DM.

#### **3.3. Hypertension**

**3. Telomeres in metabolic diseases**

146 Telomere - A Complex End of a Chromosome

may directly contribute the telomere shortening.

**3.2. Diabetes mellitus**

the telomere shortening.

Obesity is a leading preventable cause of death and growing health problem worldwide with increasing rate in both adults and children. A number of studies have reported the association of telomere length with obesity. Although the results were inconsistent and the relationship of telomere shortening and obesity is still inconclusive, several large population-based studies showed telomere shortening in obesity. In cross-sectional epidemiological studies, shortened telomeres were associated with body mass index (BMI), waist-to-hip ratio, visceral fat, and weight gain [3,16,17]. Consistently, calorie-restricted diets and subsequent weight loss were associated with the increased telomere length in obese men [18]. These results indicated the relationship between obesity and shortened telomeres. As an underlying mechanism of telomere shortening in obesity, leptin might be involved. Leptin plays an essential role in the regulation of body fat mass, and it has some proinflammatory properties with increasing oxidative stress [19]. Valdes et al. reported that age, smoking, and serum leptin concentration were independently associated with telomere length, but BMI did not, suggesting that leptin

The number of patients with type 2 DM has drastically been increasing worldwide in associ‐ ation with the changes in lifestyle and increased prevalence of obesity. DM is categorized into several clinical types: type 1 DM (T1DM), type 2 DM (T2DM), gestational DM, and others. In particular, T2DM accounts for the majority of DM patients and the pathophysiology of T2DM has an age-related aspect. DM increases the risk of cardiovascular and cerebrovascular events, and cognitive dysfunction, which are known as age-related diseases. Telomere length in patients with DM has been examined in many studies [20]. In 1998, Jeanclos et al. [21] showed that patients with insulin-dependent DM (IDDM) had shorter telomeres in peripheral leuko‐ cytes than non-DM individuals. Patients with T2DM also showed shortened telomeres [22,23]. Furthermore, it has been reported that telomere shortening rate increased with the duration of T2DM [24]. As an underlying mechanism of the telomere shortening in DM, increased production of reactive oxygen species (ROS) caused by hyperglycemia and hyperinsulinemia is supposed. Polyol pathway activation, protein kinase C pathway activation, and increased production of advanced glycation end products (AGEs) also play a pathological role in increased levels of oxidative stress [25]. In fact, Sampson et al. [23] showed an inverse corre‐ lation between the level of oxidative stress marker, 8-hydroxy-deoxyguanosine, and the telomere length. These results suggest that the increased oxidative stress in DM may accelerate

To date, several studies have focused on the relationship between telomere length and the mortality and progression of DM complications. In a prospective follow-up study, telomere length in T1DM was associated with all cause of mortality [26]. However, the association of telomere length with DM complication has been controversial. Several studies exhibited that DM patients with shorter telomeres has tended to show severer complications of DM [27,28].

**3.1. Obesity**

Hypertension can develop with various genetic and environmental factors, which is consid‐ ered to be an essential risk factor for cardiovascular or cerebrovascular diseases. However, most of the pathogenesis remains unclear. Recent evidence suggests that telomere length may be, at least in part, involved in the pathogenesis of hypertension [29]. Jeanclos et al. [30] reported the results of a twin study, in which 49 twin pairs were assessed their relation of blood pressure parameters with telomere length in leukocytes. They showed that telomere length were highly familial and negatively correlated with pulse pressure, implying that telomere shortening might be genetically regulated and associated with vascular aging. The Framing‐ ham Heart Study also demonstrated that hypertensive individuals exhibited shorter telomere length in leukocytes compared with normotensive individuals [31]. Furthermore, telomere shortening is associated with an increased atherosclerosis and cardiovascular risk [32–34]. These results suggest a close connection of telomeres and hypertension and its complications.

The underlying mechanism of telomere shortening in hypertension is still unclear. Mice lacking telomerase activity showed hypertension as a result of increased plasma endothelin-1 levels [35]. Telomerase activity was decreased in endothelial progenitor cells from both hypertensive rats and patients with essential hypertension [36]. These data suggest that endothelial cells play a key role in the association of telomere length and premature vascular aging.

#### **3.4. Nonalcoholic fatty liver disease/nonalcoholic steatohepatitis**

The role of telomeres in chronic liver diseases, such as viral hepatitis, nonalcoholic fatty liver disease (NAFLD)/nonalcoholic steatohepatitis (NASH), and liver cirrhosis, has been investi‐ gated [37]. It is well known that in these conditions, fibrosis generally determines the severity and prognosis of the disease. Older age and duration of chronic liver disease are the major risk factors for fibrosis [38].

Kitada et al. [39] first reported that telomere shortening was accelerated in hepatocyte of chronic liver disease, including chronic viral hepatitis or liver cirrhosis. Aikata et al. [40] also confirmed that telomere length was significantly shorter in the liver with chronic viral hepatitis or cirrhosis. Telomere length was significantly shorter in cirrhotic liver induced by broad etiologies compared with noncirrhotic liver [41]. Furthermore, telomerase-deficient mice showed impaired hepatic regenerative potential and developed liver cirrhosis. The regenera‐ tive potential of organ depends on the amount of the cells with sufficient telomere length, which reserves the potential for cell proliferation. Telomere shortening restricts the replicative capacity of these cells. Interestingly, adenoviral telomerase gene delivery inhibited the progression of liver cirrhosis [42]. These data indicate that telomere shortening in hepatocytes might impair the regenerative capacity in response to liver injury, which might result in liver fibrosis.
