**3. Aging**

Aging can be understood as a natural result between destructive processes that act on cells and organs over a lifetime and the responses that promote homeostasis, vitality, and longevity [18]. Aging is associated with the decline of a number of physiological systems, such as when they reach critical levels of functionality. The time of this decline depends on variables such as genetics, development, metabolism, and lifestyle [19].

The sarcopenia in the elderly is a significant public health problem because it leads to gradual slowing of movement, increasing the risk of injury, and loss of independence. The mass and muscle strength are lost due to a decline in neuromuscular transmission, the structure, function, metabolism, and muscle performance. Muscle, bone, and their metabolic needs must be considered as a single entity. Muscle strength, energy balance, and bone health influence the ability to perform physical tasks of daily life and the ease with which these tasks can be performed determines the degree of independence that an individual can keep up with advancing age [19].

Cross-sectional studies indicate that muscle mass decreases with aging in the early third decade [20] and that approximately 10%–15% of the mass is lost between the ages of 20 and 50. In women with menopause, muscle mass decreases at an accelerated rate of approximately 1% per year, affecting muscle strength and bone integrity, with a negative impact on activities of daily living [21]. In addition, older, postmenopausal women are more likely to develop abdominal adiposity and highest lipase activity, which is associated with increased systemic levels of inflammatory cytokines and free fatty acids [22].

Similar to what happens in obese individuals, elderly individuals also exhibit elevated levels of inflammatory cytokines such as interleukin-6 (IL-6), interleukin-1 (IL-1), TNF-α and Creactive protein (CRP) in the extracellular space (e.g., serum and plasma). This pro-inflamma‐ tory profile has no acute characteristic such as after acute traumas (sepsis, stroke, e.g.), but is a low-grade chronic inflammation present in elderly individuals that has been defined as "inflammaging." Epidemiological studies have shown that "inflammaging" is a risk factor in the accelerated decline of muscle mass and strength, and these changes in muscle performance can be a critical step in mediating the causal link between "inflammaging" and disability [23].

Another fact is that increased extracellular HSPs levels (mainly 70kDa isoforms as eHSP72) are correlated with oxidative damage and stress in diabetes and in obesity. Moreover, the content of the plasma eHSP72 is higher in T2DM obesity compared to DM or only obese subjects, suggesting eHSP70 levels as biomarker of glucose homeostasis unbalance [15]. Together, eHSP70 and pro-inflammatory cytokines represent a link between metabolic and immune related events. During severe stress response that can cause insulin resistance, we can observe the action of inflammatory cytokines such as IL-1 and TNF-α [16]. Under hypoglyce‐ mic conditions, as a part of the homeostatic stress response, HSP70 is secreted to the blood‐ stream and may be purely a danger signal to all the tissues of the body for the enhancement of immune and metabolic surveillance state or actively participate in glycemic control under stressful situations [17]. Additionally, eHSP70 can bind receptors in immune cells that induce

Aging can be understood as a natural result between destructive processes that act on cells and organs over a lifetime and the responses that promote homeostasis, vitality, and longevity [18]. Aging is associated with the decline of a number of physiological systems, such as when they reach critical levels of functionality. The time of this decline depends on variables such

The sarcopenia in the elderly is a significant public health problem because it leads to gradual slowing of movement, increasing the risk of injury, and loss of independence. The mass and muscle strength are lost due to a decline in neuromuscular transmission, the structure, function, metabolism, and muscle performance. Muscle, bone, and their metabolic needs must be considered as a single entity. Muscle strength, energy balance, and bone health influence the ability to perform physical tasks of daily life and the ease with which these tasks can be performed determines the degree of independence that an individual can keep up with

Cross-sectional studies indicate that muscle mass decreases with aging in the early third decade [20] and that approximately 10%–15% of the mass is lost between the ages of 20 and 50. In women with menopause, muscle mass decreases at an accelerated rate of approximately 1% per year, affecting muscle strength and bone integrity, with a negative impact on activities of daily living [21]. In addition, older, postmenopausal women are more likely to develop abdominal adiposity and highest lipase activity, which is associated with increased systemic

Similar to what happens in obese individuals, elderly individuals also exhibit elevated levels of inflammatory cytokines such as interleukin-6 (IL-6), interleukin-1 (IL-1), TNF-α and Creactive protein (CRP) in the extracellular space (e.g., serum and plasma). This pro-inflamma‐ tory profile has no acute characteristic such as after acute traumas (sepsis, stroke, e.g.), but is a low-grade chronic inflammation present in elderly individuals that has been defined as "inflammaging." Epidemiological studies have shown that "inflammaging" is a risk factor in

pro-inflammatory cytokine release.

as genetics, development, metabolism, and lifestyle [19].

levels of inflammatory cytokines and free fatty acids [22].

**3. Aging**

90 Muscle Cell and Tissue

advancing age [19].

The pathogenesis of T2DM is established by a vicious cycle of metabolically induced inflam‐ mation, impaired insulin responsiveness, and loss of homeostatic signaling. Approach in metabolic, pro-inflammatory cytokines may influence the state of the insulin response and can lead to insulin resistance in acute or chronic form. Adipocytes and macrophages secrete inflammatory cytokines (e.g., TNF-α) that activate the serine-threonine kinases JNK and inhibitor of κB kinase (IKK-β) in insulin sensitive organs, such as liver, skeletal muscle, and adipose tissue. JNK and IKK-β both impair the function of the insulin receptor and interfere with downstream signaling [24].

Cytokines released into the bloodstream can bind to receptors and activate intracellular signaling pathways, thus facilitating the activation of phosphorylation of JNK and inhibiting the signal transduction of insulin. JNK is also activated by stress and fatty acids [25]. Situations of inflammatory stress can also affect the process of regulating the availability of non-insulin dependent glucose as the activation of AMP-activated protein kinase (AMPK) enzyme sensitive to intracellular energy status [25, 26]. This pathway is frequently studied in the animal model of insulin resistance (or in some cases T2DM) by high fat diet protocols (Goettems-Fiorin et al., manuscript in preparation). These studies of insulin resistance show that several structural and metabolic changes in the muscle are correlated with increased levels of fasting glucose and/or response to glucose intolerance during glucose tolerance test (GTT).

Aging and tissue degeneration also involve the accumulation of damage to cellular macro‐ molecules. Chemical damage due to oxidative stress, glycation, and the addition of sugar residues has the capacity to modify both DNA and proteins. Situations of inflammatory stress, as described above, are able to activate the expression of genes that perform the cytoprotection in many tissues, especially when we talk about the muscle tissue. The muscle, which is rich in protein chains, uses the expression of HSPs to repair the damage that can be induced by both the consumption of high-fat diets and aging.

It has been assumed that HSPs, particularly 70kDa (HSP70) levels, generally decrease during normal aging processes. As HSP70 functions to chaperone cytosolic proteins to allow appro‐ priate refolding or degradation by ubiquination pathways, these reductions in HSP70 have been implicated in the aging process, as cells accumulate oxidation products without adequate cellular protection [27]. Age-dependent decline in the heat shock response is also observed in muscle tissues. In these tissues, vigorous contraction leads to the induction of HSP that is cytoprotective in nature. These effects are severely blunted in the muscles of older animals and aging humans, suggesting that decline in muscle mass and force generation may be related to loss of HSP expression [18].

The leak of HSP70 expression in muscles of older subjects represents a failure to exercise adaptation, in terms of both structural and metabolic characteristics. Loss of heat shock factor 1 (HSF1) activity and expression, the transcription factor of HSP70, which is at the same time the result and consequence of obesity and aging, induce an impairment of muscle mass maintenance and insulin signaling (Figure 3). The relevance of HSP70 expression in the muscle can be identified since the pro-inflammatory mediators JNK and NF-κB can be inhibited by the HSP70 expression, which is low in age, T2DM, obesity, and finally in sedentary subjects, suggesting that active muscle can be a non-pharmacological option to prevention or repair this "meta-inflammation" process that harm the human health.

Older adults with diabetes are vulnerable to accelerated loss of lean body mass. Declines in lower extremity muscle mass in particular, can be associated with decreased muscle strength and lead to muscle weakness, poor lower-extremity performance, and mobility loss, all of which have been reported in persons with diabetes [28]. Aging also produces muscle changes such as decrease in type II fibers, decline in oxidative capacity of type IIa and I fibers, and forming a favorable development of metabolic alterations. During aging, changes in the morphological structure of the skeletal muscle and also in the number and function of mitochondria are observed in the decline in this tissue. Studies have consistently shown that PGC-1α, a transcription factor that promotes mitochondrial biogenesis, decline with aging and in many age-related chronic diseases suggesting that such decline may explain the progressive mitochondrial dysfunction with aging [23].

**Figure 3.** Aging alterations in the muscle related to insulin resistance.

As previously described, both the lifestyle and aging promote body changes, which may be directly related to skeletal muscle dysfunctions. Changes in composition and functional capacity of type 1 and type 2 fibers, which reduce the mass and muscle strength, occur due to molecular disorders affecting the muscle, whether caused by the high consumption of HFD and physical inactivity or by aging. Myosteatosis, reduction of PGC-1α development of a lowgrade inflammatory with increased expression of pro-inflammatory cytokines such as TNF- α and IL-6, and reduction of HSPs, represent the body's response to aging is quite similar to lifestyle (hipercaloric food intake and sedentarism). The term "inflammaging", described in several studies highlights the low-grade inflammatory condition in the aging process, with an increase of pro-inflammatory cytokines (IL-1, IL-6 and TNF-α) that promotes the activation of JNK and IKK-β in the muscle interfering with the sensitivity to insulin (Figure 3). Thus, many subclinical process of insulin resistance development occurs in the muscle and activating this organ can be an important strategy to obtain a healthy aging process.
