Slowing down Aging: Are we ready? (3)

Interventions to Slow Aging in Humans: Are We Ready? Longo et al., Aging Cell, 2015

translated by Evgenia Ryabtseva

(Continuation, the beginning of the article is here.)

Inhibitors of the TOR-mediated signaling mechanism

The relationship between the mTOR-mediated signaling mechanism and life expectancy, as well as healthy life expectancy, has been demonstrated in experiments on several important model organisms. For example, suppression of the activity of the mTOR-mediated signaling mechanism by genetic or pharmacological interventions leads to an increase in the lifespan of yeast, roundworms, fruit flies and mice (Johnson et al., 2013). This relationship is currently being studied in primate and human studies. Thus, the mTOR-mediated signaling mechanism is the main candidate for use in targeted interventions. The mTOR kinase exists in the form of two complexes: mTORC1 and mTORC2 (Laplante & Sabatini, 2012). The results of most studies indicate that the suppression of the activity of the mTORC1-mediated signaling mechanism has a positive effect on longevity (Johnson et al., 2013). However, suppression of the activity of mTORC1 and mTORC2 mediated signaling mechanisms separately also increases the lifespan of roundworms (Vellai et al., 2003; Soukas et al., 2009). In addition to sensitivity to insulin signals and insulin-like growth factor-1, mTORC1 is activated by the action of amino acids through RAG guanosine triphosphatase (GTPase) and reduces its activity under stress or lack of energy (Kim et al., 2013). In short, activation of mTORC1 triggers the process of protein translation and cell growth, whereas its inhibition blocks cell growth and induces stress response mechanisms, including autophagy (Laplante & Sabatini, 2012).

Many interventions that increase the lifespan of model organisms suppress the activity of the signaling mechanism mediated by mTORC1 (Johnson et al., 2013; Kennedy & Pennypacker, 2014). This group includes a low-calorie diet and a decrease in the protein content in the diet, suppression of the activity of the signaling mechanism mediated by insulin/insulin-like growth factor, as well as activation of AMP-dependent kinase and, possibly, sirtuins. This raises the question of whether the positive effects of these interventions depend, at least in part, on their effect on mTORC1. Another fundamental question concerns the mechanisms launched by mTORC1 that provide a positive impact on longevity. Both the main substrates of mTORC1 – S6 kinase and 4E-BP1 – are involved in the mechanisms of longevity. The loss of S6 kinase increases the lifespan of yeast, fruit flies, roundworms and mice (Johnson et al., 2013), and the increased activity of the repressor of mRNA translation 4E-BP1 ensures the longevity of at least fruit flies. The DAF-16/FOXO transcription factor (Seo et al., 2013), as well as the autophagy mechanism (Hansen et al., 2008) are needed to increase the lifespan of C.elegans roundworm mutants without S6 kinase. An interesting fact is that DAF-16/FOXO and autophagy are mandatory factors for increasing the life expectancy of roundworms, achieved by suppressing the signaling mechanism mediated by insulin/insulin-like growth factor (Kenyon et al., 1993; Melendez et al., 2003), which once again indicates the relationship between these two rearrangements.

An attractive aspect of considering mTOR as a target for aging–retarding interventions is the availability of rapamycin, a pharmacological agent that is a specific mTOR inhibitor and increases the lifespan of mice, to a greater extent in females (by 30% when used in high doses) (Harrison et al., 2009; Miller et al., 2014). Rapamycin is not an active inhibitor, it forms a three-molecular complex with mTOR kinase and FKBP protein (Brown et al., 1994; Sabatini et al., 1994; Marz et al., 2013). This leads to the destruction of the mTORC1 complex, which entails acute inhibition, and also prevents the entry of newly synthesized mTOR molecules into the mTORC1 and mTORC2 complexes, which ensures chronic inhibition of both complexes (Sarbassov et al., 2006).

Rapamycin has been studied for several decades. It has been tested in hundreds of clinical trials and approved for use in the treatment of a number of diseases, including kidney cancer (Lamming et al., 2013; Kennedy & Pennypacker, 2014). However, this drug has serious side effects that limit its recognition as an acceptable means to slow down aging. Side effects of rapamycin include metabolic disorders (hyperglycemia, hyperinsulinemia, insulin resistance, etc.), impaired proliferation of hematopoietic cells, etc. (Soefje et al., 2011). The undesirable effect of rapamycin on metabolism is attributed to the inhibition of mTORC2 (Lamming et al., 2012), which indicates the potential possibility of shifting the effectiveness/side effects ratio in the right direction through selective inhibition of mTORC1. Clinical safety studies of rapamycin have not been conducted on healthy volunteers, so it is unclear how dangerous this drug is for healthy adults. Currently, several clinical studies of rapamycin are being conducted with the participation of elderly people, which will help to assess the importance of the role of mTOR in human aging and determine whether this protein can be considered as a target for life-prolonging interventions.

In short, the research currently underway to study the role of mTOR in aging and to find out the possibility of its use as a target for life-prolonging interventions, with a high degree of probability, will help to understand these important issues in the near future.

Glycolysis inhibitors

Another actively studied strategy is to reproduce the effects of a low-calorie diet by inhibiting glycolysis enzymes (Minor et al., 2009). It is believed that this approach may be more effective than the effect on individual signaling mechanisms, but it can also cause a wider range of side effects (Minor et al., 2009). The first potential drug in this category is 2-deoxy-D-glucose, which inhibits phosphoglucose isomerase, providing a phenotype in the rat model similar to the effects of a low-calorie diet (Minor et al., 2009). However, other long-term studies have shown that concentrations necessary to trigger physiological reactions similar to reactions to a low-calorie diet have cardiotoxic effects (Ganapathy-Kanniappan & Geschwind, 2013).

Preliminary studies of a nutraceutical product made from avocado and containing a hexokinase inhibiting enzyme, the seven-carbon carbohydrate mannoheptulose, have brought very encouraging results (Minor et al., 2009). Studies have shown that avocado extract improves the sensitivity of the tissues of mice and dogs to insulin, and also increases the average life expectancy of roundworms and mice. In addition, there is currently great interest in the use of glycolysis inhibitors in the treatment of cancer, since many malignant cells use glucose as the main source of energy (Ingram & Roth, 2010).

Mice whose cells in culture have dysfunctional telomeres and enter the phase of physiological aging demonstrate increased energy demand and defects in mitochondrial biogenesis (Passos et al., 2010). In this context, the additional introduction of glucose into the diet improves the maintenance of body weight and tissues, as well as increases the life expectancy of animals (Missios et al., 2014). These results may seem illogical, since on their basis it can be assumed that an increase in glucose intake can contribute to maintaining energy and tissue homeostasis in old age. It should be noted that malnutrition is characteristic of more than 30% of geriatric patients. Moreover, the increased need for glucose in old age is similar to the protein requirement described above, which underscores the need for a more thorough approach to selecting the optimal age-appropriate diet.

Growth hormone/insulin-like growth factor-1 axis inhibitors

Despite the fact that complete suppression of the activity of insulin-like growth factor-1 (IGF-1) is lethal, studies in animal models have shown that a decrease in the level or activity of IGF-1 can increase life expectancy. In addition, polymorphisms of the gene encoding the human IGF-1 receptor are associated with exceptional longevity (Suh et al., 2008), and more recently Barzilai and his colleagues demonstrated that low concentrations of IGF-1 in blood plasma are a prognostic survival factor for long-lived people (Milman et al., 2014), especially for women with a history of cancer. In animals, dwarf long-lived mice without a growth hormone receptor (GHR-/-) are characterized by reduced IGF-1 levels and, despite obesity, high tissue sensitivity to insulin, as well as a low risk of cancer and diabetes mellitus (Zhou et al., 1997; Shevah & Laron, 2007; Ikeno et al., 2009). It is important that similar patterns are characteristic of patients with Laron syndrome who do not have a growth hormone receptor. Clinical studies involving such patients have not been conducted, but it is known that they do not suffer from diabetes mellitus and fatal tumors (Guevara-Aguirre et al., 2011; Steuerman et al., 2011). Thus, pharmaceutical interventions that directly reduce the level of IGF-1 in the adult body can improve health and increase life expectancy.

Pharmacological targets of IGF-1 activity suppression include molecules that directly or indirectly affect cells/tissues that produce growth hormone and/or IGF-1 or are sensitive to these factors. Human or humanized monoclonal antibodies and drugs directed against the IGF-1 receptor have been tested in clinical studies involving patients with severe forms of cancer (Warshamana-Greene et al., 2005; Carboni et al., 2009), but none of these drugs has been approved for clinical use. The authors are not aware of the development of antibodies against growth hormone or insulin-like growth factor-1, however, several classes of compounds inhibiting the growth hormone/insulin-like growth factor-1 axis have been approved for the treatment of patients with acromegaly. An agreed expert opinion on the use of these drugs has recently been developed (Giustina et al., 2014). Preparations of one of these classes – analogues of somatostatin – reduce the level of growth hormone in the blood serum by suppressing its secretion by somatotrophic cells of the pituitary gland, which ultimately leads to a decrease in serum IGF-1. Unfortunately, these drugs suppress the secretion of other endocrine hormones, including insulin. Moreover, only 20-50% of patients with acromegaly respond to these drugs, while their use is associated with serious side effects, including the formation of gallstones, diarrhea and anorexia. Therefore, the use of somatostatin analogues to increase life expectancy or the duration of a healthy life is currently unjustified.

The second drug approved for the treatment of acromegaly is the growth hormone receptor antagonist pegvisomant (Trainer et al., 2000; van der Lely et al., 2001; Kopchick et al., 2002; van der Lely & Kopchick, 2006). The uniqueness of this drug lies in the fact that it does not inhibit the secretion of growth hormone, but its activity by binding to the corresponding receptor and blocking it (Kopchick et al., 2002). It should be noted that the number of patients receiving this drug who have a dose-dependent decrease in the level of insulin-like growth factor-1 can reach 90% (Trainer et al., 2000; van der Lely et al., 2001; Kopchick et al., 2002; van der Lely & Kopchick, 2006). In addition, pegvisomant is an insulin receptor sensitivity enhancer that blocks the diabetogenic effect of growth hormone and thus has a positive effect on glucose metabolism. Based on this, pegvisomant can promote longevity and healthy aging by reducing the content of insulin-like growth factor-1 in blood serum and increasing tissue sensitivity to insulin. At the same time, very few serious side effects of prolonged use of pegvisomant have been reported to date, however, experts recommend monitoring liver function and tumor size (van der Lely et al., 2012). Thus, pegvisomant is an approved drug whose effect on longevity and healthy aging needs to be evaluated in clinical studies. Other classes of drugs that inhibit the growth hormone/insulin-like growth factor-1 axis include monoclonal antibodies against growth hormone or insulin-like growth factor-1, as well as small inhibitory RNAs (miRNAs) and new tyrosine kinase inhibitors specific to growth hormone or insulin-like growth factor-1 receptors.

Another approach to reducing the overall activity of insulin-like growth factor-1 is to inhibit its availability. For example, the elimination of PAPP-A, a protease that cleaves and inactivates IGFBP–4, a protein binding insulin-like growth factor–1, suppresses the signaling activity of this factor without affecting its content in blood serum. This not only increases the lifespan of mice, but also has other positive effects on their health, including in old age (Conover, 2012).

In general, a decrease in the activity of the growth hormone/insulin-like growth factor-1 somatotropic axis is perhaps the most studied and reproducible intervention that increases both life expectancy and healthy life expectancy in mice. In addition, deficiency of growth hormone and insulin-like growth factor-1 is one of the few well-described human phenotypes (Laron syndrome) that have only minor side effects on the adult body. 

Activators of signaling mechanisms mediated by sirtuins

Enzymes of the deacetylase family, known as sirtuins (SIRT1-7), provide longevity to representatives of various species and can mediate many positive effects of a low-calorie diet (Satoh et al., 2013). This makes them a very attractive therapeutic target. The first of the identified powerful sirtuin-activating compounds include several classes of plant metabolites, such as flavones, chalcones, stilbens and anthocyanidins. These phytochemical compounds directly activate SIRT1 in vitro by triggering an allosteric mechanism that reduces the level of the Km substrate (Howitz et al., 2003). Resveratrol (3,5,4'-trihydroxystilbene) is the most powerful of the currently known natural activators, the discovery of which launched work on the creation of synthetic analogues that are much more powerful, soluble and bioavailable (Hubbard & Sinclair, 2014). Many studies have shown the ability of resveratrol and synthetic sirtuin activators to induce physiological changes, as well as changes in gene expression similar to the effects of a low-calorie diet, manifested by improved health and increased life expectancy of various organisms, including yeast (S.cerevisiae), roundworms (C.elegans), fruit flies (D.melanogaster), toothed carp (N.furzeri) and honey bees (A.mellifera) (Hubbard & Sinclair, 2014).

An alternative approach to the activation of sirtuins, which increases the activity of the entire family of enzymes, uses their universal need for NAD+ (the reduced form of nicotinamide adenine dinucleotide). The level of this compound can be increased by increasing the levels of NAD precursors (nicotinamide mononucleotide or nicotinamide riboside), enzyme activation, NAD biosynthesis (Wang et al., 2014a), or inhibition of CD38 NAD hydrolase (Yoshino et al., 2011; Canto et al., 2012; Escande et al., 2012; Gomes et al. al., 2014).

Resveratrol increases the lifespan of mice kept on a high-calorie diet (Pearson et al., 2008), as well as in combination with standard food when fed every other day (Baur et al., 2006; Pearson et al., 2008), but not against the background of daily feeding with standard food (Pearson et al., 2008; Strong et al., 2013). Synthetic activators SRT1720 and SRT2104 also increase the lifespan of mice kept on both high-calorie and low-calorie diets and protect them from age-related changes in many tissues, including muscle tissue atrophy (Minor et al., 2011). In preclinical models, sirtuin-activating compounds have also demonstrated their promise as agents for the treatment of diseases and complications associated with aging, including cancer, type 2 diabetes, inflammation, cardiovascular diseases, stroke and fatty liver degeneration (Hubbard & Sinclair, 2014).

Significant progress has been made in the use of sirtuin-activating compounds in the treatment of inflammatory and autoimmune diseases (Hubbard & Sinclair, 2014). For example, SRT1720 had a positive effect on mouse models of chronic obstructive pulmonary disease and bronchial asthma. In rhesus macaques kept on a feed rich in fats and carbohydrates, resveratrol had an anti-inflammatory effect in visceral deposits of white adipose tissue (Jimenez-Gomez et al., 2013). Sirtuin-activating compounds may also be useful in neurodegeneration (Zhao et al., 2013), as demonstrated in mouse models of Alzheimer's and Parkinson's diseases, as well as multiple sclerosis (Graff et al., 2013; Hubbard & Sinclair, 2014). Sirtuin-activating compounds prevent the effects of obesity and age-related metabolic decline and eliminate their manifestations. Resveratrol and sirtuin-activating compounds protected against the development of obesity, improved tissue sensitivity to insulin and mitochondrial function, and also prevented the development of fatty liver degeneration in mice kept on a high-calorie diet (Baur & Sinclair, 2006; Baur et al., 2006). These effects were also observed in primates on a diet rich in fats and carbohydrates (Fiori et al., 2013; Jimenez-Gomez et al., 2013). The results of studies conducted on mouse models of obesity and aging have demonstrated that increasing the level of NAD+ protects mice from metabolic decay and aging (Yoshino et al., 2011; Canto et al., 2012; Escande et al., 2012), and also eliminates mitochondrial function disorders, inflammation and muscle tissue atrophy (Gomes et al., 2014).

Researchers have accumulated a significant amount of data on the use of sirtuin-activating compounds in the treatment of people (Hubbard & Sinclair, 2014). Resveratrol had mixed effects, so its effect on tissue sensitivity to insulin (Ghanim et al., 2011; Smoliga et al., 2011) and the formation of phenotypes similar to those associated with a low-calorie diet were observed in elderly and obese patients (Timmers et al., 2011), but not in people with normal body weight and glucose tolerance (Yoshino et al., 2012). This indicates the ability of sirtuin-activating compounds to restore homeostasis mainly in people with metabolic disorders. Meta-analysis of six unique databases, including 196 patients with type 2 diabetes mellitus (104 in the resveratrol group, 92 in the placebo control group), revealed a statistically significant (P < 0.05) positive effect of resveratrol-containing supplements on systolic blood pressure, as well as hemoglobin A1c and creatinine levels compared to placebo. At the same time, there was no effect on fasting blood glucose, the results of assessing insulin resistance using a homeostatic model, diastolic blood pressure, as well as the content of insulin, triglycerides, low and high density lipoproteins in the blood (Hausenblas et al., 2014). Synthetic sirtuin-activating compounds have already passed clinical trials and are currently under clinical development (Venkatasubramanian et al., 2013; Hubbard & Sinclair, 2014). Clinical trials of NAD+ are planned for the next few years.

In general, the role of sirtuins and their activators in increasing life expectancy, as well as in the prevention and treatment of a wide range of diseases in mouse models is a proven fact. However, due to long-standing contradictions in this area (Ledford, 2010; Couzin-Frankel, 2011), the seminar participants did not reach a full consensus on the ability of sirtuin activators to slow down aging.

End: pharmacological and some other methods aimed at slowing down the aging process.

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09.09.2015