Genetics of aging: implications for drug discovery and development

¹ From Elixir Pharmaceuticals, Cambridge, MA

² Presented at the conference "Living Well to 100: Nutrition, Genetics, Inflammation," held in Boston, MA, November 15–16, 2004.

³ Reprints not available. Address correspondence to BJ Geesaman, Medical Development, Elixir Pharmaceuticals, 1 Kendall Square Building 1000, Cambridge, MA 02139. E-mail: geesaman{at}elixirpharm.com.

ABSTRACT

Aging is not a passive activity, but an actively regulated metabolic process. Specific genes have been identified that regulate aging, although aging, and consequently longevity, is only partially under genetic influence. It is also possible to increase life span by environmental modification; for example, caloric restriction can increase life span. Because human life span is long, directly studying aging in humans is impractical. Fortunately, significant insights into aging can be achieved by studying short-lived organisms, such as yeast, worms, and fruit flies. Many of the molecular pathways regulating aging in these lower organisms are conserved in mammals and overlap with pathways regulating metabolism. For example, an insulin–growth hormone signaling system has been implicated in regulating aging and longevity in both worms and mammals. Furthermore, the dysregulation of glucose homeostasis is a hallmark of aging in humans. In fact, type 2 diabetes, a disease of glucose homeostasis, can be conceptualized as a form of accelerated aging. Consistent with this, aging and diabetes are both common risk factors for a wide range of diseases. Because aging and diabetes are intimately related at a molecular level, diabetes may be able to provide the link between disease treatment (eg, diabetes) and the prevention of age-related diseases. If specific molecular pathways controlling the rate of aging can be modulated genetically, then perhaps they can be modulated pharmacologically. These insights may ultimately have an important impact on the discovery and development of drugs to both treat and prevent a wide range of diseases.

Key Words: Aging • life span • longevity • age-related disease • diabetes • Caenorhabditis elegans, Sir2 • daf-2 • daf-16 • insulin • insulin-like growth factor • caloric restriction • metformin • acarbose

INTRODUCTION

Life span is an outcome that partially depends on the rate of aging. The rate at which we age (ie, the gradual decay of homeostatic mechanisms) affects our susceptibility to disease and our ability to recover from illness and other stressors. The rate of aging is not fixed but is an actively regulated metabolic process. Research has shown that single gene changes in yeast, worms, and rodents can significantly alter life span (1-4). The observation that the first-degree relatives of human centenarians are both disease resistant and have longer than average life spans implies a genetic component to human longevity (5, 6). Current estimates based on twin studies are that the human life span is 25% heritable (6-9).

Environmental manipulation, such as caloric restriction (CR), can also influence the rate of aging and extend life span. Caloric restriction is defined as a low-calorie diet that does not cause malnutrition. As early as the 1930s, it was observed that mice subjected to CR lived 30% longer than controls fed ad libitum (10). Since then, the effects of CR on life span and aging have been shown for a wide variety of short-lived species (11-13). Also, these species subjected to CR appear to be less susceptible to a wide range of diseases, including diabetes, neurodegeneration, cancer, and vascular ischemia (14, 15).

Investigations with longer-lived species, such as rhesus monkeys, found that CR produces results similar to those observed in rodents (16). For many decades, CR was the only regimen known to promote longevity in mammals (17). In addition to increased longevity, CR can delay the onset of several diseases, including obesity (18-20), kidney disease (18, 21), autoimmune disease (22), diabetes (19), and possibly Parkinson disease (23) and Alzheimer disease (24).

Studies of CR in humans suggest similar beneficial effects. Several observational studies reported decreased morbidity among those who consumed less food (25, 26). Studies involving CR-like regimens reported improved disease risk profiles among those whose energy intake was reduced (27, 28). A recent study observed, not surprisingly, that subjects in a CR group had lower body mass indexes and percentages of body fat than did controls (29). Also, the CR subjects had lower serum total cholesterol, LDL cholesterol, ratio of total to HDL cholesterol, triacylglycerol, fasting glucose, fasting insulin, and systolic and diastolic blood pressure, whereas HDL cholesterol was higher in the CR group than in the control group. Carotid artery intima media thickness was also lower. Furthermore, serum concentrations of the inflammatory markers C-reactive protein and platelet-derived growth factor AB were lower, which indicates that CR may reduce inflammation and hence delay the onset of age-associated diseases, specifically vascular disease.

Although the beneficial effects of CR have been observed for many years, the mechanisms of action remain unclear at the molecular level. Extension of life span is a multidimensional phenomenon, and CR can influence it in many ways, including through metabolic, neuroendocrine, and apoptotic pathways (30). Antiaging trials in humans raise ethical and economic concerns and would take decades to complete. Therefore, lower organisms such as yeast, worms (Caenorhabditis elegans), and fruit flies (Drosophila) are used for antiaging experiments. Because these organisms have a short life span and are easy to manipulate, we can quickly determine how specific genetic polymorphisms interact with CR with respect to longevity. Fortunately, the molecular pathways that regulate aging in these lower organisms are highly conserved in mammals as well. Therefore, we can draw parallels between lower organisms and mammals, including humans, to determine whether these genetic manipulations have a similar effect on aging and longevity. Once the pathways and genes mediating the effects of CR on longevity are identified, we can select the most tractable molecular targets and develop interventions to beneficially modulate the activities of their gene products.

ANTIAGING MOLECULAR PATHWAYS

Several potential pathways linking CR with longevity have been identified. For example, in yeast, the Sir2 (silencing information regulator 2) protein has been implicated in controlling aging via transcriptional silencing (31, 32). Mutations of the SIR2 allele reduce mean life span by 50%, whereas overexpression of SIR2 increases life span beyond that of the wild-type strain (1). Transcriptional silencing of chromosomal material results in a closed, inaccessible chromatin structure, making the genes coded on that section of DNA essentially inactive.

SIR2 is highly conserved throughout evolution; SIR2 in yeast is homologous to SIR-2.1 in C. elegans, dSir2 in Drosophila, and SIRT1 mammals. Research into the role of SIR2 on longevity in C. elegans, Drosophila, and mice has yielded similar results as with yeast. In C. elegans, increased dosage of SIR-2.1 extended life span by up to 50% (33), whereas in Drosophila, dSir2 knockout resulted in the shortening of life span (34). Research in mice has shown that SIRT1 is important for embryonic development (3, 4). Although SIRT1 knockout mice were viable, they were smaller and did not survive as long as their wild-type littermates and had several developmental defects (3, 4). Mammalian cells and tissues overexpressing SIRT1 have been shown to be resistant to apoptosis and neural and retinal degeneration and to be prolipolytic. A transgenic rodent ubiquitously overexpressing SIRT1 has yet to be developed, and it will be interesting to see whether this animal is predictably long-lived.

The potential link between Sir2 and CR is cellular energy charge, as reflected by NAD⁺ concentration. NAD⁺ is a coenzyme that participates in many redox reactions in cells, including glycolysis and cellular respiration. Under conditions of CR, NAD⁺ concentrations are high, allosterically enhancing Sir2 activity. In yeast subjected to CR (usually by limiting the glucose content of the growth medium from 2% to 0.5%), glucose metabolism shifts from glycolysis to oxidative respiration. Because oxidative respiration produces more ATP per molecule of glucose than does glycolysis, this shift in metabolism is a way of conserving energy when food is scarce. An increased reliance on respiration for energy production changes the redox balance (NAD⁺/NADH) to favor increased intracellular NAD⁺ concentration. A move toward respiration, as occurs in a CR environment, therefore has the effect of increasing Sir2 activity by increasing NAD⁺ availability.

Aging research in C. elegans has identified additional genes associated with life span regulation, specifically, the dauer formation genes DAF-2 and DAF-16. Interestingly, SIR-2.1 has been shown to influence DAF-2 and DAF-16 (33, 35), and all these genes are likely part of a single conserved molecular pathway. To understand the role DAF-2 and DAF-16 play in aging, we must consider the developmental cycle of C. elegans. Under optimal growth conditions, this species of worm develops through 4 potential larval stages culminating in adulthood. If, on the other hand, it is subjected to environmental stress, for example, food scarcity or overcrowding, the larvae go into an alternative state known as dauer. This is an arrested, nonreproductive state that is resistant to adverse environmental conditions and is adapted for long-term survival (36). Aging during this stage is halted; the postdauer life span is not affected by a prolonged dauer stage of up to 2 mo (37).

Kenyon et al (2) characterized the genetic pathway of aging in C. elegans by showing that mutations in DAF-2 and DAF-16 had major effects on longevity. Completely knocking out DAF-2 activity induces the dauer state, whereas partially down-regulating DAF-2 activity avoids the dauer state but results in increased longevity and disease resistance (38). The activity of both genes is influenced by caloric restriction; DAF-2 is inhibited by low energy availability, which in turn de-represses DAF-16. Under normal conditions, DAF-16 is under negative regulation by DAF-2. Therefore, when food is available, DAF-2 is "on" and DAF-16 is "off," thereby promoting normal adult development.

In lower organisms, genes have been identified that influence longevity, and changes to the nutritional environment can influence the expression of these genes. But are these genes conserved, and what roles do they play in mammals?

INSULIN–INSULIN-LIKE GROWTH FACTOR SIGNALING AND AGING

Molecular pathways regulating aging in lower organisms are, in fact, highly conserved in mammals, including the DAF-2 pathway. In C. elegans, the DAF-2 gene encodes a member of the insulin receptor family. Receptors for insulin and insulin-like growth factors (IGFs) are central regulators of energy metabolism and organismal growth. In mammals, the homologues of daf-2 are the insulin and IGF-1 receptors (38). Whereas C. elegans and Drosophila have a single cell-surface receptor transducing insulin-like signals, mammals have developed distinct hormonal pathways for insulin and IGF-1, each of which is characterized by its own dedicated receptor. The homologues of daf-16 in mammals are the forkhead transcription factors FOXO1-FOXO3 (30). The FOXO transcription factors are targets of the insulin receptor signal pathway and modulate cell growth and proliferation. Decreased signaling by insulin–IGF-1 activates forkhead transcription factors, which has been shown to increase resistance to stress in mammalian cells (39), a relation similar to that previously described for DAF-2 and DAF-16 in C. elegans.

In mammals, the IGF-1 receptor regulates embryonic and postembryonic growth (40) and longevity (41). Mice with one inactive IGF-1 receptor gene (leaving the second copy intact) live on average 26% longer than do their wild-type littermates, do not develop dwarfism, and maintain normal energy metabolism, and their nutrient uptake, physical activity, fertility, and reproduction are unaffected (41). Several strains of dwarf rodents with impaired growth hormone–IGF-1 sensitivity are both long-lived and disease resistant. Finally, knocking out either the IGF-1 or the insulin receptor in the retinal tissue of mice provides resistance to retinal disease, and transgenic mice overexpressing IGF-1 in the retina exhibit diabetes-like eye disease (42, 43).

The insulin receptor regulates glucose homeostasis and embryonic growth (44). A mutation in this receptor was implicated in a case of leprechaunism (44), and a complete knockout of the insulin receptor gene in mice proved lethal. Conversely, mice in which the insulin receptor was selectively knocked out in adipose tissue experienced an 18% increase in life span compared with controls and a 50–70% reduction in fat mass, despite the fact that they ate normally or even more than did the controls. Moreover, they were protected against obesity and its related metabolic disorders, including type 2 diabetes (45).

TYPE 2 DIABETES AS A MODEL FOR AGING

These data suggest that life span regulation appears to be under insulin-like metabolic control. One disease directly related to insulin resistance is type 2 diabetes, and type 2 diabetes provides a disease model of accelerated aging that is amenable to lifestyle and pharmacologic intervention. Both aging and type 2 diabetes are associated with micro- and macrovascular diseases, including cardiovascular disease, retinopathy, nephropathy, and neuropathy, with many similarities.

Because the mechanisms of aging and metabolic control are related, certain approaches to treating diabetes may also have a positive effect on aging. The beneficial effects of CR on longevity have been described previously. Interestingly, in obese patients with type 2 diabetes, CR has an important regulatory effect that is independent of weight loss. Patients subjected to a 7-d period of CR experienced substantial reductions in fasting plasma glucose and triacylglycerol concentrations and increases in insulin sensitivity and secretion. These improvements in metabolic control were approximately one-half those achieved after a weight loss of 12.7 ± 2 kg.

Furthermore, metformin hydrochloride, a therapy that improves insulin sensitivity by increasing peripheral glucose uptake and utilization and decreasing gluconeogenesis by the liver, has been clinically proven to prevent diabetes (46). In patients with type 2 diabetes, it has also been proven to reduce the risk of any diabetes-related endpoint, diabetes-related death, and all-cause mortality (47).

Although the cellular mechanisms of action of metformin are not fully understood, it is thought that its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization (48). Metformin activates AMP-activated protein kinase (AMPK), an enzyme activated by low energy availability (adverse environment, food stress) in hepatocytes (49). As a result, acetyl-CoA carboxylase activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK is required for metformin's inhibitory effect on glucose production by hepatocytes. Activation of AMPK provides a unified explanation for the beneficial effects of metformin: activating AMPK increases life span in C. elegans, and metformin has been shown to increase life span in rodents.

CONCLUSIONS

The rate of aging involves well-regulated metabolic processes. Studies in both humans and nonhuman models indicate that alteration of metabolism, as can be achieved by CR, reduces the rate of aging, the level of inflammation, and the susceptibility to certain chronic diseases of aging. Specific genetic variations that alter these metabolic processes appear to determine 25% of the variance observed in life span. Because both aging and diabetes result in similar metabolic disturbances that increase disease susceptibility, the clinical effects of the compounds to treat diabetes may provide insight into aging and antiaging therapy. Further study is needed to validate their relative actions within the molecular pathways identified as being common to aging and age-related diseases. Once the pathways and genes involved with longevity are identified, we can select the most appropriate molecular targets for drug discovery and other interventions. If modulating antiaging genes can both treat and prevent age-related disease, then studying the molecular biology of aging and identifying common metabolic pathways with age-associated disease will have important effects on drug discovery and development and preventive approaches to extend healthy life span.

ACKNOWLEDGMENTS

The author is the Vice President of Medical Development at Elixer Pharmaceuticals.

REFERENCES