Introduction

Skeletal muscle contractions power body movements and are crucial for maintaining homeostasis. Skeletal muscle tissue accounts for almost half of the human body mass. Given its central role in human mobility and metabolic function, any deterioration in the contractile, material, and metabolic properties of skeletal muscle has an extremely important effect on human health. Sarcopenia is widely accepted as causing an age-related decline of muscle mass, quality, and strength. In addition, it is often used to describe both the cellular processes (denervation, mitochondrial dysfunction, and inflammatory and hormonal changes) and the outcomes such as decreased muscle strength, mobility, and function; a greater risk of falls; and reduced energy needs. Sarcopenia can be considered “primary” (or age-related) when no other cause is evident but aging itself [27]. Primary sarcopenia is especially associated with physical inactivity, both derived by a reduction of physical exercise during leisure time or work-related. Secondary sarcopenia usually occurs when one or more identifiable causes coexist. This condition is a proxy of chronic or acute diseases that are highly prevalent in older persons, such as Parkinson’s disease, diabetes, chronic heart failure, chronic obstructive pulmonary disease, stroke, and hip fracture. Von Haeling et al. [109] estimated its prevalence at 5–13% for elderly people aged 60–70 years and 11–50% for those aged 80 years or older. The lean muscle mass generally contributes up to ~ 50% of the total body weight in young adults, but declines with aging to 25% at 75–80 years old. The loss of muscle mass is the most notable in the lower limb muscle groups, with the cross-sectional area of the vastus lateralis being reduced by as much as 40% between the age of 20 and 80 years old. At the muscle fiber level, sarcopenia is characterized by specific type II muscle fiber atrophy, fiber necrosis, and fiber-type grouping.

Several possible mechanisms of age-related muscle atrophy have been described. Age-related muscle loss is a result of reductions in the size and number of muscle fibers, possibly due to a multi-factorial process that involves physical activity, nutritional intake, metabolic homeostasis, oxidative stress, hormonal changes, and life span. The specific contribution of each of these factors is unknown, but there is emerging evidence that the disruption of several positive regulators (Akt and serum response factor) of muscle hypertrophy with age is an important feature in the progression of sarcopenia [88]. Intriguingly, more recent studies indicated an apparent functional defect in autophagy- and myostatin-dependent signaling in sarcopenic muscle [67, 117, 119]. In contrast, many investigators failed to demonstrate age-related enhancement in the levels of common negative regulators [atrophy gene-1 (atrogin-1), NF-κB (nuclear factor-kappaB), and calpain] in senescent mammalian muscles [88, 91, 93].

The progression of age-related muscle wasting and weakness is effectively prevented by the combination of resistance training and amino acid-containing supplements [39, 88]. In contrast, sarcopenia has been most attenuated by mild caloric restriction (CR) in all mammals except humans. Many researchers have considered the strategy of inhibiting myostatin to treat various muscle disorders, such as muscular dystrophy, cachexia, and sarcopenia. In addition, more recent studies indicated the possible application of new supplements (e.g., ursolic acid) to prevent muscle atrophy. Furthermore, pharmacological treatment with the fibroblast growth factor (FGF) 19 markedly ameliorates sarcopenia and muscle atrophy induced by glucocorticoid probably via an obligate co-receptor for FGF15/19, β-klotho [12]. This review outlines the molecular mechanism of muscle atrophy in sarcopenia and considers several recent strategies for inhibiting this phenomenon.

Molecular mechanism regulating sarcopenia

The adaptive changes of ubiquitin-proteasome system in sarcopenic muscle

ATP-dependent ubiquitin-proteasome system (UPS) is essential for regulating protein degradation. The degradation of a protein via UPS involves two steps: (1) tagging of the substrate by covalent attachment of multiple ubiquitin molecules and (2) degradation of the tagged protein by the 26S proteasome complex with the release of free and reusable ubiquitin. The ubiquitination of proteins is regulated by at least three enzymes: ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin ligase (E3). Consistent increases in gene expressions of two important E3 ubiquitin ligases (atrogin-1 and MuRF1) have been observed in a wide range of in vivo models of skeletal muscle atrophy including diabetes, cancer, renal failure, denervation, unweighting, and glucocorticoid or cytokine treatment [14, 61]. The importance of these atrophy-regulated genes in muscle wasting was confirmed through knockout studies in mice where an absence of atrogin-1 or MuRF1 attenuated denervation-, fasting-, and dexamethasone-induced muscle atrophies [5, 26, 39].

Atrogin-1 and/or MuRF1 messenger RNA (mRNA) levels in aged muscle increase [24, 87, 116] or remain unchanged in humans and rats or decrease in rats [32, 40, 114]. Even when the mRNA expression of these atrogenes increased in sarcopenic muscles, this was a very limited response (1.5- to 2.5-fold). Although many previous studies investigated the mRNA levels of these ubiquitin ligases in aged mammalian muscle, the protein levels in sarcopenic muscles did not correspond to age-related changes in the mRNA of several ubiquitin ligases. For example, Edström et al. [40] indicated the marked upregulation of phosphorylated Akt and forkhead box O (FOXO)4 in the gastrocnemus muscle of aged female rats, probably contributing to the downregulation of atrogin-1 and MuRF1 mRNA. In addition, Léger et al. [63], using human subjects aged 70 years old, demonstrated decreases in nuclear FOXO1 and FOXO3a in spite of there being no apparent age-related changes in the atrogin-1 and MuRF1 mRNA. Interestingly, recent findings indicated that atrogin-1-knockout mice are short-lived and experience a greater loss of muscle mass during aging than control mice [97], indicating that the activity of this E3 ubiquitin ligase is required to preserve muscle mass during aging in mice. Moreover, MuRF1-null mice show a more marked decay of muscle strength during aging than controls, although the muscle mass is at least partly preserved in these mice [55]. As indicated by Sandri et al. [97], the chronic inhibition of these atrogenes should not be considered a therapeutic target to counteract sarcopenia because this does not prevent muscle loss but instead exacerbates weakness.

Adaptation of autophagy-linked signaling in muscle with age

Macroautophagy (herein autophagy) is a ubiquitous catabolic process that involves the bulk degradation of cytoplasmic components through a lysosomal pathway [81, 95, 96]. This process is characterized by the engulfment of part of the cytoplasm inside double-membrane vesicles called autophagosomes. Autophagosomes subsequently fuse with lysosomes to form autophagolysosomes in which the cytoplasmic cargo is degraded and the degradation products are recycled. The turnover of most long-lived proteins, biological membranes, and whole organelles such as the mitochondria and ribosomes is mediated by autophagy [28].

At first glance, autophagy was considered a coarse, non-selective, degradative system, but a closer investigation revealed a different truth. Autophagy represents an extremely refined collector of altered organelles, abnormal protein aggregates, and pathogens, similar to a selective recycling center rather than a general landfill [85]. The selectivity of the autophagy process is conferred by a growing number of specific cargo receptors, including p62/SQSTM1, Nbr1, Nix (Bnip3L), and optineurin [100]. These adaptor proteins are equipped with both a cargo-binding domain, with the capability to recognize and attach directly to molecular tags on organelles, and at the same time a microtubule-associated protein 1 light chain 3 (LC3)-interacting domain, able to recruit and bind essential autophagosome membrane proteins. The de novo formation of autophagosomes is regulated by at least three molecular complexes: the LC3 conjugation system and the regulatory complexes governed by unc51-like kinase-1 and Beclin-1. The conjugation complex is composed of different proteins encoded by autophagy-related genes (Atg) [71]. The Atg12-Atg5-Atg16L1 complex, along with Atg7, plays an essential role in the conjugation of LC3 to phosphatidylethanolamine, which is required for the elongation and closure of the isolation membrane [71].

A decline in autophagy during normal aging has been described in invertebrates and higher organisms [29]. Inefficient autophagy has been attributed to play a major role in the apparent age-related accumulation of damaged cellular components, such as undegradable lysosome-bound lipofuscin, protein aggregates, and damaged mitochondria [29]. Demontis and Perrimon [31] showed that the function of the autophagy/lysosome system of protein degradation declined during aging in the skeletal muscle of Drosophila. This results in the progressive accumulation of poly-ubiquitin protein aggregates in senescent Drosophila muscle. Intriguingly, the overexpression of FOXO increases the expression of many autophagy genes, preserves the function of the autophagy pathway, and prevents the accumulation of poly-ubiquitin protein aggregates in sarcopenic Drosophila muscle [31]. Several investigators reported the autophagic changes in aged mammalian skeletal muscle [46, 70, 113, 117]. Compared with those in young male Fischer 344 rats, amounts of Beclin-1 were significantly increased in the plantaris muscles of senescent rats [117]. Using Western blot of fractionated homogenates and immunofluorescence microscopy, we recently demonstrated the selective induction of p62/SQSTM1 and beclin-1 but not LC3 in the cytosol of sarcopenic muscle fibers of mice [92]. In addition, we also observed significantly smaller p62/SQSTM1-positive muscle fibers in aged muscle compared with the surrounding p62/SQSTM1-negative fibers [92]. In contrast, aging did not influence the amounts of Atg7 and Atg9 proteins in rat plantaris muscle [117]. More recently, Western blot analysis by Wohlgemuth et al. [117] clearly showed a marked increase in the amount of LC3 in muscle during aging. However, they could not demonstrate an aging-related increase of the ratio of LC3-II to LC3-I, a better biochemical marker to assess ongoing autophagy. In contrast, Wenz et al. [113] noted a significant increase in the ratio of LC3-II to LC3-I during aging (3 vs. 22 months) in the biceps femoris muscle of wild-type mice. None of the studies determining the transcript level of autophagy-linked molecules found a significant increase with age [46, 70, 117]. Not all contributors to autophagy signaling change similarly at both mRNA and protein levels in senescent skeletal muscle. Intriguingly, a more recent study [19] using biopsy samples of young and aged human volunteers clearly showed an age-dependent autophagic defect such as decrease in the amount of Atg7 protein and in the ratio of LC3-II/LC3-I protein. The activity of lysosomal enzymes (cathepsin B, cathepsin L, etc.) also modulates the autophagy-dependent sarcopenia [10]. Moon et al. [72] demonstrated that exercise (running) increased cathepsin B levels in mouse gastrocnemius muscle and plasma. However, there is no systematic research dealing with age-related changes in the activity of lysosomal enzymes. Therefore, sarcopenia would include a partial defect of autophagy signaling, although more exhaustive investigation is needed in this field.

Pharmacological approach

Myostatin inhibition

Myostatin was first discovered during screening for novel members of the transforming growth factor-β superfamily and was shown to be a potent negative regulator of muscle growth [62]. Mutations of myostatin can lead to marked hypertrophy and/or hyperplasia in developing animals, as evidenced by knockout experiments in mice. Moreover, mouse skeletal muscles engineered to overexpress the myostatin propeptide, the naturally occurring myostatin inhibitor follistatin, or a dominant negative form of activin receptor IIB (ActRIIB: the main myostatin receptor [62]) all display similar, if not greater, increases in size.

Myostatin levels increase with muscle atrophy due to unloading in mice and humans [91] and with severe muscle wasting in HIV patients. The increased levels of myostatin are widely accepted to lead to muscle wasting [90]. Although many researchers consider myostatin levels to increase with age, studies using sarcopenic muscles have yielded conflicting results [18, 63, 91]. Intriguingly, Carlson et al. [18] showed enhanced levels of Smad3 (possible myostatin downstream regulator) but not myostatin in sarcopenic muscles of mice. More recently, McKay et al. [67] observed more abundant myostatin-positive type II-associated stem cells in older than in younger males after muscle loading in spite of there being no difference in stem cell-specific myostatin levels at the baseline. Therefore, it is possible that myostatin-dependent signaling is activated in sarcopenic mammalian muscles.

Many researchers have conducted experiments to inhibit myostatin in models of muscle disorders such as Duchenne muscular dystrophy, amyotrophic lateral sclerosis, and cancer cachexia [53, 74, 76]. In addition, several investigators examined the effect of inhibiting myostatin to counteract sarcopenia using animals only. Lebrasseur et al. [60] reported several positive effects of 4 weeks of treatment with a myostatin inhibitor (PF-354, 24 mg/kg) in aged mice. They showed that PF-354-treated mice exhibited a significantly greater muscle mass and increased performance such as treadmill time, distance to exhaustion, and habitual activity. In addition, the PF-354-treated mice exhibited decreased levels of negative regulators (phosphorylated Smad3 and MuRF1) in aged muscle. More recently, Murphy et al. [75] showed, with once weekly injections to 21-month-old senescent mice, that a lower dose of PF-354 (10 mg/kg) significantly increased the fiber size and in situ force of hind limb muscle. Blocking myostatin enhances muscle protein synthesis by potentially relieving the inhibition normally imposed on the Akt/mammalian target of rapamycin (mTOR) signaling pathway by myostatin. More recently, a proof-of-concept, randomized, phase two trial of a myostatin antibody (LY2495655: LY) was conducted in older weak fallers. Becker et al. [11] tested whether the subcutaneous injection of LY (315 mg) increases the appendicular lean body mass (aLBM) and improves physical performance in individuals aged 75 years or older (Argentina, Australia, France, Germany, Sweden, and the USA). At 24 weeks, the least square mean change in aLBM significantly increased in the LY group (0.303 kg) compared with the placebo group (− 0.123 kg). Becker et al. [11] demonstrated that several functional measures of muscle power (stair climbing time, chair rise with arms, and fast gait speed) improved significantly from the baseline to week 24 in frail elderly subjects after treatment with LY. These lines of evidence clearly highlight the therapeutic potential of the antibody-directed inhibition of myostatin for treating sarcopenia.

Angiotensin-converting enzyme inhibitors

ACE inhibitors have long been used as a treatment for not only secondary stroke prevention but also cardiovascular disease. It has been suggested that ACE inhibitors may have a beneficial effect on skeletal muscle. ACE inhibitors may improve muscle function through improvements in endothelial function, metabolic function, anti-inflammatory effects, and angiogenesis, thereby improving skeletal muscle blood flow. ACE inhibitors can increase mitochondrial numbers and insulin-like growth factor-I (IGF-I) levels, thereby helping to counter sarcopenia [64].

The long-term use of ACE inhibitors may attenuate the decline in muscle strength and walking speed in older hypertensive people and induce a greater lower limb lean muscle mass when compared with users of other antihypertensive agents [84]. ACE inhibitors have been shown to improve the exercise capacity in both younger and older people with heart failure [37, 84], but they do not improve the grip strength [99]. Few interventional studies using ACE inhibitors for physical function have been undertaken. Using functionally impaired older people without hearing loss, Sumukadas et al. [103] demonstrated that treatment with ACE inhibitors increases the 6-min walking distance to a similar extent to that achieved after 6 months of exercise training. However, a study comparing the effects of nifedipine with ACE inhibitors in older people found no difference between treatments in muscle strength, walking distance, or functional performance [17]. Frail subjects exhibit slower walking speeds and a tendency to have more cardiovascular problems. Further evidence is required before recommending ACE inhibitors to counter the effects of sarcopenia. However, ACE inhibitors are associated with cardiovascular benefits and as older people frequently have underlying cardiovascular problems, these agents are already commonly prescribed [3, 20].

Hormonal approach

Testosterone

In males, levels of testosterone decrease by 1% per year and those of bioavailable testosterone by 2% per year from age 30 [43]. In women, testosterone levels drop rapidly between the ages of 20 to 45. Testosterone increases muscle protein synthesis, and its effects on the muscle are modulated by several factors including the genetic background, nutrition, and exercise. Systemic reviews [13] have indicated that supplementation with testosterone attenuates some sarcopenic characteristics such as decreases in the muscle mass [44] and grip strength [8]. A study of long-term (6 months) supraphysiological treatment with testosterone in a placebo-controlled study showed increased leg lean body mass and leg and arm strengths [101]. Although there are significant increases in strength among elderly males given high doses of testosterone, the potential risks may outweigh the benefits. Risks associated with testosterone therapy in older men include sleep apnea, thrombotic complications, and an increased risk of prostate cancer.

These side effects have driven the necessity for drugs that demonstrate improved therapeutic profiles. Novel, non-steroidal compounds, called selective androgen receptor modulators, have shown tissue-selective activity and improved pharmacokinetic properties. Whether these drugs are effective in treating sarcopenia has yet to be clarified. Dehydroepiandrosterone (DHEA) is marketed as a nutritional supplement in the USA and is available over the counter. Unlike testosterone and estrogen, DHEA is a hormone precursor that is converted into sex hormones in specific target tissues. However, supplementation with DHEA for the elderly increases the bone density and levels of testosterone and estradiol, but does not significantly modulate the muscle size, strength, or function [30].

Ghrelin

Ghrelin is a 28-amino-acid peptide, which is mainly produced by cells in the stomach, intestines, and hypothalamus [57]. For ghrelin to express its characteristic hormonal activity, a post-translational modification involving the conjugation of octanoic acid to Ser-3 of proghrelin is required. Ghrelin is a natural ligand for the growth hormone (GH)-secretagogue receptor that possesses a unique fatty acid modification. Ghrelin plays an important role in several physiological situations, including the stimulation of GH secretion and regulation of energy homeostasis by enhancing food intake and promoting adiposity. In contrast, ghrelin acts on monocytes and T lymphocytes to suppress their production of anorectic proinflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α [36]. Ghrelin and low-molecular-weight agonists of the ghrelin receptor are considered attractive candidates for the treatment of cachexia [1] because of their combined anabolic effects on skeletal muscle and appetite. Nagaya et al. [78] intravenously gave human ghrelin (2 μg/kg, twice daily, 3 weeks) to patients with chronic obstructive pulmonary disease. This treatment led to significant increases in the lean body mass, handgrip strength, and Karnofsky performance score [77]. In addition, Nagaya et al. [77] demonstrated that intravenous treatment with human ghrelin (2 μg/kg, twice daily, 3 weeks) significantly improved several parameters (e.g., lean body mass measured by dual-energy X-ray absorption and left ventricular ejection fraction) in patients with chronic heart failure [4]. In a long-term (1 year) placebo-controlled study involving healthy older adults (over 60 years old) given an oral ghrelin mimetic (MK-677), an increase in appetite was observed [77]. The study did not show a significant increase in strength or function in the ghrelin-mimetic treatment group, when compared with the placebo group [4]. Anamorelin HCl (ONO-7643) is a potent and selective novel ghrelin receptor agonist that mimics the N-terminal-active core of ghrelin [80]. More recently, Pietra et al. [86] demonstrated that oral treatment with anamolelin HCl (ANAM) to rats significantly and dose-dependently increased food intake and body weight at all dose levels (3, 10, and 30 mg/kg) compared with the control and significantly increased GH levels at 10 or 30 mg/kg doses. In addition, phase 3 trials of two types of anamorelin (ROMANA1 and ROMANA2) were conducted in patients with non-small-cell lung cancer and cachexia [104]. Twelve weeks of both anamorelin treatments induced significant increases in the lean body mass but not handgrip strength in these cachectic patients with very low levels of adverse effects (hyperglycemia < 1%). However, a more recent review pointed out that heterogeneity existed in the clinical effects of anamorelin [6]. Therefore, further validation of this trial is necessary by increasing the sample size, varying the range of doses during treatment, and observing other outcomes.

Vitamin D

Vitamin D has been traditionally considered a key regulator of bone metabolism and calcium and phosphorus homeostases through negative feedback with the parathyroid hormone. Today, approximately 1 billion people worldwide, mostly elderly, have vitamin D deficiency. The prevalence of low vitamin D concentrations in subjects older than 65 years of age has been estimated at approximately 50% [115], but this figure is highly variable because it is influenced by sociodemographic, clinical, therapeutic, and environmental factors. Similarly, there is an age-dependent reduction found in vitamin D receptor expression in skeletal muscle [98]. Prolonged vitamin D deficiency has been associated with severe muscle weakness, which improves with vitamin D supplementation.

A large body of evidence demonstrates that low vitamin D concentrations represent an independent risk factor for falls in the elderly [42, 102]. Using community-living elderly and nursing home residents with low vitamin D levels, Annweiler et al. [2] showed that supplementation with vitamin D in double-blind trials increased muscle strength and performance and reduced the risk of falling. In contrast, a positive effect of vitamin D supplementation on fall events has been refused by another group [94]. Cesari et al. [22] attributed these contradictory findings to the selection criteria adopted to recruit study populations, adherence to the intervention, or the extreme heterogeneity of cutoff points defining the status of deficiency. Although a more comprehensive knowledge of vitamin D-related mechanisms may provide a very useful tool to prevent muscle atrophy in older persons (sarcopenia), treatment with vitamin D may exhibit no significant effect on patients without vitamin D deficiency.

Supplemental approach

Amino acids

Many Americans consume more than the recommended dietary allowance (RDA) of protein; however, research shows that a significant number of elderly people do not meet the estimated average requirement, let alone the RDA [45]; 32 to 41% of women and 22 to 38% of men aged 50 or older consume less than the RDA of protein [56]. Epidemiological studies show that protein intake is positively associated with the preservation of the muscle mass. Many reviews indicate that certain nutritional interventions such as a high protein intake or an increased intake of essential amino acids and the branched chain amino acid leucine along with resistance training may help to attenuate fiber atrophy in sarcopenic muscle by the modulation of both anabolic and catabolic pathways [33, 52, 106]. In particular, leucine can be considered a regulatory amino acid with unique characteristics. It plays several roles in muscle metabolism regulation, which include the translational control of protein synthesis [83], glucose homeostasis [79], and nitrogen donation for the synthesis of muscle alanine and glutamine [83]. Considering these findings, the use of leucine as an antiatrophic agent is biologically justified.

Oral post-exercise amino acid supplementation had a synergistic effect on the contraction-induced escalation of muscle protein synthesis following acute resistance exercise [38, 111]. Human studies have shown that amino acids play a role in the phosphorylation of translational factors called eularyotic initiation factors, especially eukaryotic initiation factor 4F and p70S6K, through an mTOR-mediated mechanism [38, 39]. Treatment with amino acids has been shown to induce additive hypertrophy in response to continuous resistance training [41]. Furthermore, the administration of many essential amino acids tends to have a positive effect on the muscle mass and protein synthesis both under normal conditions [34, 82] and with resistance training [39]. In contrast, Godard et al. [48] failed to gain an additive effect of the long-term supplementation of several amino acids and carbohydrates with resistance training. Unfortunately, they conducted the examination of total muscle cross-sectional areas by only magnetic resonance imaging and did not perform a detailed morphological analysis using biopsy samples. Although the combination of both resistance training and amino acid supplementation is recommended to prevent sarcopenia [88, 89, 110], recent systematic reviews [9, 105] deny such an additional effect of supplementation above exercise. Beudart et al. [9] indicated that an additional effect of nutritional intervention on muscle mass was only found in eight randomized controlled trials (RCTs) (23.5%), which were all of high quality. Although exercise intervention increased muscle strength in almost all of the studies involving elderly people (29/25 RCTs), dietary supplementation showed additional benefits in only a small number of studies (8/35 RCTs, 22.8%). Therefore, such an additional effect of supplementation with amino acids may be recognized only in sarcopenic elderly with undernutrition.

Ursolic acid

A pentacylic triterpenoid, ursolic acid is the major waxy component in apple peel, and it is also found in many other edible plants. Ursolic acid is thought to be the active component in a variety of folkloric antidiabetic herbal medicines, since it exerts beneficial effects in animal models of diabetes and hyperlipidemia [112]. As predicted by connectivity mapping, Kunkel et al. [58] showed that ursolic acid reduced skeletal two distinct atrophy-inducing stresses (fasting and muscle denervation). Ursolic acid might increase muscle mass by inhibiting atrophy-associated skeletal muscle gene expression. Intriguingly, Kunkel et al. [58] found that acute treatment of fasted mice with ursolic acid reduced atrogin-1 and MuRF1 mRNA levels. In addition, chronic treatment of unstressed normal mice with ursolic acid reduced atrogin-1 and MuRF1 mRNA and induced muscle hypertrophy. Ursolic acid may increase skeletal muscle Akt phosphorylation in vivo, but it is unknown whether it directly influences skeletal muscle or whether it requires IGF-I or insulin, which is always present in healthy animals. To address these issues, Kunkel et al. [58] studied serum-starved skeletal myotubes and found that ursolic acid rapidly stimulated IGF-I receptor and insulin receptor activity. Importantly, ursolic acid alone was not sufficient to increase activation of the IGF-I receptor or insulin receptor. Rather, its effects also required IGF-I and insulin, respectively. More recently, Yu et al. [118] conducted long-term treatment with ursolic acid (100 mg/kg by oral gavage, 21 days) of a mouse model of chronic kidney disease (CKD). They demonstrated that supplementation with ursolic acid markedly attenuated muscle atrophy (tibialis anterior) induced by CKD by decreasing the expression of myostatin (both serum and muscle) and inflammatory cytokines (IL-6, TNF-α, etc.), which are initiators of muscle-specific ubiquitin-E3 ligases (e.g., atrogin-1, MuRF-1, and MUSA1). Intriguingly, they found that levels of phosphorylation of NF-κB (p65) and STAT3 and p38 were significantly suppressed in CKD-induced atrophic muscle by ursolic acid. These results clearly indicate that ursolic acid has an anti-inflammatory property. Further research is needed to elucidate the effect of supplementation with ursolic acid on skeletal muscle and the attenuation of muscle wasting (e.g., sarcopenia).

Antioxidant supplementation

Free radicals are highly reactive chemical species with a single unpaired electron in their outer orbit seeking to pair with another free electron. In particular, reactive oxygen species (ROS), derived from oxidative metabolism, show higher reactivity than O2. ROS are constantly generated in cells of aerobic organisms, particularly skeletal muscle, by the addition of a single electron to an oxygen molecule, with the subsequent damage of biological macromolecules (e.g., lipids). The interaction of ROS with normal cellular structures leads to potentially non-reversible modifications, with the consequent cellular loss of function and death. ROS production has been shown to increase in skeletal muscle during aging [108]. During the aging process, it is probable that increased levels of ROS lead to the modification of mitochondrial DNA and result in increases in myonuclear apoptosis [65].

In the case of investigations of diabetes, antioxidant supplementation may effectively prevent muscle atrophy [15]. The effect on cancer cachexia is partial but significant. In contrast, the data on antioxidant supplementation for mammalian sarcopenia are extremely limited and controversial, despite the clinical relevance and marked interest (both from research and commercial points of view). Several studies have investigated the possibility of delaying the aging process by enhancing the antioxidative capacity [16, 51]. For example, resveratrol, a natural poly-phenol found in grapes, peanuts, and berries, has shown a protective effect against oxidative stress in skeletal muscle. Although most human studies have analyzed the relationship between dietary antioxidant supplementation and physical performance or muscle strength measures, the effect is still largely controversial. As pointed out by a more recent review [21], there are currently no trials verifying the effects of antioxidant supplementation on sarcopenia (as identified by one of the several consensus definitions provided by international groups of experts). As proposed by Bonetto et al. [15], oxidative stress may behave as an additional factor to amplify the wasting stimuli but probably would not play a leading role in many other cases and for which the effectiveness of antioxidant therapy was not demonstrated. Very intriguingly, a recent statement from the Society on Sarcopenia, Cachexia, and Wasting Disease does not mention antioxidant supplementation as a possible method to manage sarcopenia in older persons [73].

Another candidate

Caloric restriction

CR, which typically involves consuming 20–40% fewer calories than normal, preserves mitochondrial health and attenuates sarcopenia. CR is recognized as the most robust intervention to retard both primary aging (natural age-related deterioration) and secondary aging (accelerated aging due to disease and negative lifestyle behaviors), thereby increasing the life span of many species. While CR studies in primates and humans are largely ongoing, studies in rodents have consistently shown that CR extends the maximum life span by up to 50% and reduces the incidence of many age-associated diseases. These protective effects are likely attributable to the ability of CR to reduce the incidence of mitochondrial abnormalities (mitochondrial proton leakage) and attenuate oxidative stress. In rodents, CR may modulate the mitochondrial efficiency, content, and function via decreased proton leakage, which is, in turn, facilitated by a shift to a less oxidative milieu. In terms of mitochondrial content and function, CR does not affect the gene expression, protein level, or activity of citrate synthase [50]. More recently, Lanza et al. [59] demonstrated that CR decreases whole-muscle protein synthesis and fractional synthesis rates of individual proteins in rodents. In addition, their analysis using representative transmission electron micrographs showed no attenuation of the reduction in mitochondrial density with aging. Taking these findings together, Lanza et al. [59] concluded that CR preserves the mitochondrial function by protecting the integrity and function of existing cellular components rather than by increasing mitochondrial biogenesis. Furthermore, CR may counteract the age-related increases in proapoptotic signaling in skeletal muscle [35]. Noticeably, CR has been shown to modulate the majority of the apoptotic pathways involved in age-associated skeletal muscle loss, such as mitochondrion-, cytokine/receptor-, and Ca2+/ER-stress-mediated signaling [35]. For example, CR markedly inhibits increases in several mediators of the TNF-mediated pathway of apoptosis (TNF-α, TNF-receptor 1, cleaved caspase-3 and cleaved caspase-8) possibly by enhancing the production of a muscle-derived anabolic cytokine, IL-15, which competes with TNF-mediated signaling. In addition, the combination of CR with exercise training has been suggested to counteract the apoptosis associated with sarcopenia more effectively.

How does CR modulate sarcopenia irrespective of the mitochondrial function and apoptosis? It is intriguing that many different studies have shown that peroxisome proliferator-activated receptor (PPAR)γ coactivator-1 (PGC-1)α is increased with CR in various organs such as the brain, liver, heart, and brown and visceral adipose tissues [49]. Baker et al. [7] showed a significant increase in PGC-1α in the gastrocnemius muscle of rats after a 40% CR diet beginning at 16 weeks of age. PGC-1α binds to and coactivates many transcription factors in addition to PPARγ, including most nuclear factors. Therefore, PGC-1α has various roles, such as in fatty acid oxidation, myokine secretion, the activation of autophagy, and neuromuscular junction gene induction, as well as the upregulation of mitochondrial biogenesis [23]. Indeed, Valdez et al. [107] demonstrated that lifelong CR significantly decreased the incidence of pre- and post-synaptic abnormalities (e.g., axonal swelling and synaptic detachment) in 24-month-old mice and the age-related loss of motor neurons probably due to PGC-1α induction. Since the level of basal autophagy in the skeletal muscle has been shown to decrease with age [29, 117], the normal function of autophagy by CR may attenuate the atrophy of muscle fibers with age. However, CR in mice did not modulate the level of several autophagy-linked molecules (Beclin-1, Atg9, and LC3) at the protein level, except for Atg7 in sarcopenic muscles of rats [117]. It remains to be further elucidated whether CR activates autophagy signaling to inhibit muscle atrophy.

Recently, investigators conducting a large study of CR in non-human primates presented positive results for both all-cause and age-related mortalities [25]. This was in contrast to another ongoing study showing no significant differences in either all-cause or age-related mortality [66]. In addition, another more recent editorial opinion was published [54]. The effect of mortality by CR should be thoroughly studied in non-human primates. On the other hand, the effect of CR to attenuate sarcopenia in the mouse, rat, and rhesus monkey [68, 69] is well-known. We hope to elucidate whether CR also attenuates sarcopenia in human subjects and to what extent dietary restriction can be applied to human populations. Figure 1 represents an overview of pharmacological, hormonal, and nutritional interventions for sarcopenia.

Fig. 1
figure 1

Pharmacological, hormonal, and nutritional interventions affect different mediators in sarcopenic muscle. Recent findings suggest that the myostatin-Smad pathway inhibits protein synthesis probably by blocking the functional role of Akt. Treatment with an ACE inhibitor, ursolic acid, and testosterone upregulates the amount of IGF-I and then stimulates protein synthesis by activating the Akt/mTOR/p70S6K pathway. Supplementation with testosterone enhances protein synthesis by stimulating mTOR. Abundant serum GH, which is induced by ghrelin, activates JAK2-STAT5 signaling to promote the muscle-specific gene transcription necessary for hypertrophy. A recent finding [47] indicated that vitamin D enhances follistatin expression and, in turn, blocks the functional role of myostatin in vitro. A direct role for vitamin D in muscle fibers remains to be elucidated. Sarcopenic muscle exhibits a marked defect of autophagy-dependent signaling, which is effectively ameliorated by caloric restriction. ALK, activin receptor-like kinase; ActRIIB, activin receptor IIB; IGF-I, insulin-like growth factor I; TSC, tuberous sclerosis complex; TORC1, component of TOR signaling complex 1; Rheb, Ras homolog enriched in brain; mTORC1, mammalian target of rapamycin complex 1; eIF4E, eukaryotic initiation factor 4E; FOXO, forkhead box O; LC3, microtuble-associated protein light chain 3; BNIP BCL2, adenovirus E1B 19 kd-interacting protein; Atg, autophagy-related genes; JAK2, Janus kinase 2; STAT5, signal transducer and activator of transcription 5

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Conclusion

The recent advances in our understanding of muscle biology have led to new hopes for hormonal, pharmacological, and nutritional treatments of muscle wasting.

Although resistance training combined with amino acid-containing supplementation is usually recommended to prevent age-related muscle wasting and weakness [88, 89, 110], supplementation with proteins (amino acids) only did not influence sarcopenic symptoms. A myostatin-inhibiting strategy is the most important candidate to prevent sarcopenia in humans. Milder CR (15–25%) would also be effective for age-related muscle atrophy in humans, although some evidence-based modification is necessary. Supplementation with ursolic acid and ghrelin is an intriguing candidate to combat sarcopenia, although systematic and fundamental research on this treatment has not been conducted, even in rodent models of sarcopenia.