Age-related changes in the regulation of autophagic proteolysis in rat isolated hepatocytes

Autophagy as a therapeutic target in diabetic nephropathy

Diabetic nephropathy is a serious complication of diabetes mellitus, and its prevalence has been increasing worldwide. Therefore, there is an urgent need to identify a new therapeutic target to prevent diabetic nephropathy. Autophagy is a major catabolic pathway involved in degrading and recycling macromolecules and damaged organelles to maintain intracellular homeostasis. The study of autophagy in mammalian systems is advancing rapidly and has revealed that it is involved in the pathogenesis of various metabolic or age-related diseases. The functional role of autophagy in the kidneys is also currently under intense investigation although, until recently, evidence showing the involvement of autophagy in the pathogenesis of diabetic nephropathy has been limited. We provide a systematic review of autophagy and discuss the therapeutic potential of autophagy in diabetic nephropathy to help future investigations in this field.

Age-related increases of macroautophagy and chaperone-mediated autophagy in rat nucleus pulposus

OBJECTIVE

Excessive apoptosis plays an important role in the progression of intervertebral disc degeneration. However, the effect of autophagy, another type of programmed cell death, on the pathogenesis of disc degeneration is still unclear. Macroautophagy and chaperone-mediated autophagy (CMA) change and intervertebral disc degeneration aggravates with age. This study aims at examining the expression changes of light chain 3 (LC3), lysosome-associated membrane protein 2A (LAMP-2A), and Hsc70, the indicator substrates of macroautophagy and CMA, in rat nucleus pulposus (NP) to prove that macroautophagy and CMA are both related with age.

METHODS

Female Sprague-Dawley rats of 3, 12, and 24 months (n = 8 per age) were used in this study. Autophagic vacuoles in NP cells were detected by transmission electron microscopy. In NP, the expressions of LC3-II and LAMP-2A protein and mRNA were examined by immunohistochemistry and reverse transcription polymerase chain reaction, respectively. LC3-II, LC3-I, and LAMP-2A protein were also measured by western blot. The mRNA and protein level of myocyte enhancer factor-2D regulated by LAMP-2A and Hsc70 were detected by reverse transcriptase polymerase chain reaction and western blot, respectively.

RESULTS

Transmission electron microscopy showed more autophagic vacuoles in 12- and 24-month groups than in 3-month group. Expression of LC3-II and LC3-II/LC3-I in 24-month group was significantly higher than in 3-month group (p < 0.05). Meanwhile, LAMP-2A expression was significantly higher in 24-month group than in 3-month group (p < 0.05). However, lower expression of Hsc70 and myocyte enhancer factor-2D was found in the 24-month rats than in 3-month group (p < 0.05, p < 0.05, respectively).

CONCLUSION

Macroautophagy and CMA were present and increased with age in rat NP.

Cellular stress response pathways and ageing: intricate molecular relationships

Ageing is driven by the inexorable and stochastic accumulation of damage in biomolecules vital for proper cellular function. Although this process is fundamentally haphazard and uncontrollable, senescent decline and ageing is broadly influenced by genetic and extrinsic factors. Numerous gene mutations and treatments have been shown to extend the lifespan of diverse organisms ranging from the unicellular Saccharomyces cerevisiae to primates. It is becoming increasingly apparent that most such interventions ultimately interface with cellular stress response mechanisms, suggesting that longevity is intimately related to the ability of the organism to effectively cope with both intrinsic and extrinsic stress. Here, we survey the molecular mechanisms that link ageing to main stress response pathways, and mediate age-related changes in the effectiveness of the response to stress. We also discuss how each pathway contributes to modulate the ageing process. A better understanding of the dynamics and reciprocal interplay between stress responses and ageing is critical for the development of novel therapeutic strategies that exploit endogenous stress combat pathways against age-associated pathologies.

Aging as an event of proteostasis collapse

Aging cells accumulate damaged and misfolded proteins through a functional decline in their protein homeostasis (proteostasis) machinery, leading to reduced cellular viability and the development of protein misfolding diseases such as Alzheimer's and Huntington's. Metabolic signaling pathways that regulate the aging process, mediated by insulin/IGF-1 signaling, dietary restriction, and reduced mitochondrial function, can modulate the proteostasis machinery in many ways to maintain a youthful proteome for longer and prevent the onset of age-associated diseases. These mechanisms therefore represent potential therapeutic targets in the prevention and treatment of such pathologies.

Selective autophagy in the maintenance of cellular homeostasis in aging organisms

Altered cellular homeostasis, accumulation of damaged non-functional organelles and presence of protein inclusions are characteristics shared by almost all types of differentiated cells in aged organisms. Cells rely on quality control mechanisms to prevent the occurrence of these events and the subsequent cellular compromise associated with them. What goes wrong in aging cells? Growing evidence supports gradual malfunctioning with age of the cellular quality control systems. In this review, we focus on autophagy, a catabolic process that contributes to the maintenance of cellular homeostasis through the degradation of unwanted and damaged components in lysosomes. We describe recent advances on the molecular characterization of this process, its different variants and the multiplicity of functions attributed to them. Autophagic dysfunction has been identified in severe human disorders, many of which worsen with age. We comment on the contribution of an adequate autophagic function to longevity, and the negative impact on health-span of the age-dependent decline in autophagic function.

New insights into the role of mitochondria in aging: mitochondrial dynamics and more

A decline in mitochondrial function plays a key role in the aging process and increases the incidence of age-related disorders. A deeper understanding of the intricate nature of mitochondrial dynamics, which is described as the balance between mitochondrial fusion and fission, has revealed that functional and structural alterations in mitochondrial morphology are important factors in several key pathologies associated with aging. Indeed, a recent wave of studies has demonstrated the pleiotropic role of fusion and fission proteins in numerous cellular processes, including mitochondrial metabolism, redox signaling, the maintenance of mitochondrial DNA and cell death. Additionally, mitochondrial fusion and fission, together with autophagy, have been proposed to form a quality-maintenance mechanism that facilitates the removal of damaged mitochondria from the cell, a process that is particularly important to forestall aging. Thus, dysfunctional regulation of mitochondrial dynamics might be one of the intrinsic causes of mitochondrial dysfunction, which contributes to oxidative stress and cell death during the aging process. In this Commentary, we discuss recent studies that have converged at a consensus regarding the involvement of mitochondrial dynamics in key cellular processes, and introduce a possible link between abnormal mitochondrial dynamics and aging.

Regulation of mammalian autophagy in physiology and pathophysiology

(Macro)autophagy is a bulk degradation process that mediates the clearance of long-lived proteins and organelles. Autophagy is initiated by double-membraned structures, which engulf portions of cytoplasm. The resulting autophagosomes ultimately fuse with lysosomes, where their contents are degraded. Although the term autophagy was first used in 1963, the field has witnessed dramatic growth in the last 5 years, partly as a consequence of the discovery of key components of its cellular machinery. In this review we focus on mammalian autophagy, and we give an overview of the understanding of its machinery and the signaling cascades that regulate it. As recent studies have also shown that autophagy is critical in a range of normal human physiological processes, and defective autophagy is associated with diverse diseases, including neurodegeneration, lysosomal storage diseases, cancers, and Crohn's disease, we discuss the roles of autophagy in health and disease, while trying to critically evaluate if the coincidence between autophagy and these conditions is causal or an epiphenomenon. Finally, we consider the possibility of autophagy upregulation as a therapeutic approach for various conditions.

Eaten alive: a history of macroautophagy

Macroautophagy (hereafter autophagy), or 'self-eating', is a conserved cellular pathway that controls protein and organelle degradation, and has essential roles in survival, development and homeostasis. Autophagy is also integral to human health and is involved in physiology, development, lifespan and a wide range of diseases, including cancer, neurodegeneration and microbial infection. Although research on this topic began in the late 1950s, substantial progress in the molecular study of autophagy has taken place during only the past 15 years. This review traces the key findings that led to our current molecular understanding of this complex process.

Models of accelerated sarcopenia: critical pieces for solving the puzzle of age-related muscle atrophy

Sarcopenia, the age-related loss of skeletal muscle mass, is a significant public health concern that continues to grow in relevance as the population ages. Certain conditions have the strong potential to coincide with sarcopenia to accelerate the progression of muscle atrophy in older adults. Among these conditions are co-morbid diseases common to older individuals such as cancer, kidney disease, diabetes, and peripheral artery disease. Furthermore, behaviors such as poor nutrition and physical inactivity are well-known to contribute to sarcopenia development. However, we argue that these behaviors are not inherent to the development of sarcopenia but rather accelerate its progression. In the present review, we discuss how these factors affect systemic and cellular mechanisms that contribute to skeletal muscle atrophy. In addition, we describe gaps in the literature concerning the role of these factors in accelerating sarcopenia progression. Elucidating biochemical pathways related to accelerated muscle atrophy may allow for improved discovery of therapeutic treatments related to sarcopenia.

Autophagy and aging: lessons from progeria models

Autophagy is an evolutionarily conserved process essential for cellular homeostasis and organismal viability. In fact, this pathway is one of the major protein degradation mechanisms in eukaryotic cells. It has been repeatedly reported that the autophagic activity of living cells decreases with age, probably contributing to the accumulation of damaged macromolecules and organelles during aging. Moreover, autophagy modulation in different model organisms has yielded very promising results suggesting that the maintenance of a proper autophagic activity contributes to extend longevity. On the other hand, recent findings have shown that distinct premature-aging murine models exhibit an extensive basal activation of autophagy instead of the characteristic decline in this process occurring during normal aging. This unexpected autophagic increase in progeroid models is usually associated with a series of metabolic alterations resembling those occurring under calorie restriction or in other situations reported to prolong life-span. In this chapter, we will discuss the current knowledge on the relationship between the autophagy pathway and aging with a special emphasis on the unexpected and novel link between premature aging and autophagy up-regulation.

Skeletal muscle autophagy and apoptosis during aging: effects of calorie restriction and life-long exercise

Sarcopenia, loss of muscle mass and function, is a common feature of aging. Oxidative damage and apoptosis are likely underlying factors. Autophagy, a process for the degradation of cellular constituents, may be a mechanism to combat cell damage and death. We investigated the effect of age on autophagy and apoptosis in plantaris muscle of male Fischer 344 rats that were either fed ad libitum, or mild, life-long calorie restricted (CR) alone or combined with life-long voluntary exercise. Upstream autophagy-regulatory proteins were either upregulated with age (Beclin-1) or unchanged (Atg7 and 9). LC3 gene and protein expression pattern as well as LAMP-2 gene expression, both downstream regulators of autophagy, however, suggested an age-related decline in autophagic degradation. Atg protein expression and LC3 and LAMP-2 gene expression were improved in CR rats with or without exercise. The age-related increase in oxidative damage and apoptosis were attenuated by the treatments. Both, oxidative damage and apoptosis correlated negatively with autophagy. We conclude that mild CR attenuates the age-related impairment of autophagy in rodent skeletal muscle, which might be one of the mechanisms by which CR attenuates age-related cellular damage and cell death in skeletal muscle in vivo.

mTOR's role in ageing: protein synthesis or autophagy?

The molecular and cellular mechanisms that regulate ageing are currently under scrutiny because ageing is linked to many human diseases. The nutrient sensing TOR pathway is emerging as a key regulator of ageing. TOR signaling is complex affecting several crucial cellular functions and two such functions, which show clear effects on ageing, are protein synthesis and autophagy. In this article we discuss the relative importance of both these processes in ageing, identify how TOR regulates translation and autophagy and speculate on links between the TOR signaling network and ageing pathways.

Oxidative modifications, mitochondrial dysfunction, and impaired protein degradation in Parkinson's disease: how neurons are lost in the Bermuda triangle

While numerous hypotheses have been proposed to explain the molecular mechanisms underlying the pathogenesis of neurodegenerative diseases, the theory of oxidative stress has received considerable support. Although many correlations have been established and encouraging evidence has been obtained, conclusive proof of causation for the oxidative stress hypothesis is lacking and potential cures have not emerged. Therefore it is likely that other factors, possibly in coordination with oxidative stress, contribute to neuron death. Using Parkinson's disease (PD) as the paradigm, this review explores the hypothesis that oxidative modifications, mitochondrial functional disruption, and impairment of protein degradation constitute three interrelated molecular pathways that execute neuron death. These intertwined events are the consequence of environmental exposure, genetic factors, and endogenous risks and constitute a "Bermuda triangle" that may be considered the underlying cause of neurodegenerative pathogenesis.

Physiological functions of autophagy

The field of autophagy research has advanced rapidly in recent years, with important discoveries made in relation to both molecular mechanisms and physiological functions. Initially, autophagy was thought to be primarily a response to starvation. Although this might be true in lower eukaryotes, this catabolic process exerts various physiological functions in higher eukaryotes. This review summarizes the physiological roles of autophagy in amino acid pool maintenance, intracellular quality control, development, cell death, tumor suppression and anti-aging.

Sarcopenia of aging: underlying cellular mechanisms and protection by calorie restriction

Sarcopenia, the loss of muscle mass and function, is a common feature of aging and impacts on individual health and quality of life. Several cellular mechanisms have been involved in the pathogenesis of this syndrome, including mitochondrial dysfunction, altered apoptotic and autophagic signaling, and, more recently, trace metal dyshomeostasis. Calorie restriction (CR) without malnutrition has been shown to ameliorate the age-related loss of muscle mass in a variety a species. Mechanisms of protection span from preservation of mitochondrial functional and structural integrity to mitochondrial biogenesis, reduction of oxidative stress, and favorable modulation of apoptotic and autophagic signaling pathways. Importantly, preliminary evidence indicates that moderate CR may promote muscle mitochondrial biogenesis in middle-aged human subjects. Further research is warranted to investigate whether CR may represent a safe and efficient strategy to delay the onset and mitigate the progression of sarcopenia in older adults.

Autophagy and aging: keeping that old broom working

Autophagy, a highly conserved mechanism of quality control inside cells, is essential for the maintenance of cellular homeostasis and for the orchestration of an efficient cellular response to stress. The decrease in autophagic activity observed in almost all cells and tissues as organisms age was proposed to contribute to different aspects of the aging phenotype and to the aggravation of detrimental age-related diseases. The recent advances in our understanding of the molecular mechanisms underlying autophagy and the identification of the subset of genes involved in this process has enabled the use of genetic manipulations to start testing this hypothesis. Here, I review the recent genetic evidence in support of tight connections between autophagy, health span and aging.

A role for autophagy in the extension of lifespan by dietary restriction in C. elegans

In many organisms, dietary restriction appears to extend lifespan, at least in part, by down-regulating the nutrient-sensor TOR (Target Of Rapamycin). TOR inhibition elicits autophagy, the large-scale recycling of cytoplasmic macromolecules and organelles. In this study, we asked whether autophagy might contribute to the lifespan extension induced by dietary restriction in C. elegans. We find that dietary restriction and TOR inhibition produce an autophagic phenotype and that inhibiting genes required for autophagy prevents dietary restriction and TOR inhibition from extending lifespan. The longevity response to dietary restriction in C. elegans requires the PHA-4 transcription factor. We find that the autophagic response to dietary restriction also requires PHA-4 activity, indicating that autophagy is a transcriptionally regulated response to food limitation. In spite of the rejuvenating effect that autophagy is predicted to have on cells, our findings suggest that autophagy is not sufficient to extend lifespan. Long-lived daf-2 insulin/IGF-1 receptor mutants require both autophagy and the transcription factor DAF-16/FOXO for their longevity, but we find that autophagy takes place in the absence of DAF-16. Perhaps autophagy is not sufficient for lifespan extension because although it provides raw material for new macromolecular synthesis, DAF-16/FOXO must program the cells to recycle this raw material into cell-protective longevity proteins.

The role of autophagy in age-related neurodegeneration

Most age-related neurodegenerative diseases are characterized by accumulation of aberrant protein aggregates in affected brain regions. In many cases, these proteinaceous deposits are composed of ubiquitin conjugates, suggesting a failure in the clearance of proteins targeted for degradation. The 2 principal routes of intracellular protein catabolism are the ubiquitin proteasome system and the autophagy-lysosome system (autophagy). Both of these degradation pathways have been implicated as playing important roles in the pathogenesis of neurodegenerative disease. Here we describe autophagy and review the evidence suggesting that impairment of autophagy contributes to the initiation or progression of age-related neurodegeneration. We also review recent evidence indicating that autophagy may be exploited to remove toxic protein species, suggesting novel strategies for therapeutic intervention for a class of diseases for which no effective treatments presently exist.

Autophagy in the heart and liver during normal aging and calorie restriction

Autophagy is a highly regulated intracellular process for the degradation of cellular constituents and essential for the maintenance of a healthy cell. We evaluated the effects of age and life-long calorie restriction on autophagy in heart and liver of young (6 months) and old (26 months) Fisher 344 rats. We observed that the occurrence of autophagic vacuoles was higher in heart than liver. The occurrence of autophagic vacuoles was not affected by age in either tissue, but was increased with calorie restriction in heart but not in liver. Next, we examined the expression of proteins involved in the formation and maturation of autophagosomes (beclin-1, LC3, Atg7, Atg9) or associated with autolysosomes and lysosomes (LAMP-1; cathepsin D). In hearts of both ad libitum-fed and calorie-restricted rats, we observed an increase in expression of beclin-1 and procathepsin D, but not mature cathepsin D, and a decrease in expression of LAMP-1 because of aging. In hearts, calorie restriction stimulated the expression of Atg7 and Atg9 and the lipidation of Atg8 (elevated LC3-II/I ratios) in aged rats. In hearts of ad libitum-fed rats, expression of Atg7 and lipidation of Atg8 were unaffected by age, while the cellular levels of Atg9 were lower in aged animals. Furthermore, we observed that the age- and diet-dependent expression levels of those proteins differed between heart and liver. In conclusion, autophagy in heart and liver did not decrease with age in ad libitum-fed rats, but was enhanced by calorie restriction in the heart. Thus, calorie restriction may mediate some of its beneficial effects by stimulating autophagy in the heart, indicating the potential for cardioprotective therapies.

Autophagy and neurodegeneration

All cellular components are subjected to continuous surveillance by intracellular quality control systems. The major players involved in this quality control are molecular chaperones, which detect the abnormal components, and proteases, which eliminate them from the cell. Malfunctioning of the cellular surveillance systems inexorably leads to cell toxicity, and often cell death, due to the accumulation of unwanted nonfunctional components inside cells. In this work, we review the contribution of the autophagic system to cellular quality control and the consequences that autophagy malfunction has on cellular function. Special emphasis is made on the recently identified role of this system in maintenance of neuronal homeostasis and in the links currently established between alterations in the autophagic system and major neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and Huntington's disease.

Altered dynamics of the lysosomal receptor for chaperone-mediated autophagy with age

Rates of autophagy, the mechanism responsible for lysosomal clearance of cellular components, decrease with age. We have previously described an age-related decline in chaperone-mediated autophagy (CMA), a selective form of autophagy, by which particular cytosolic proteins are delivered to lysosomes after binding to the lysosome-associated membrane protein type 2A (LAMP-2A), a receptor for this pathway. Rates of CMA decrease with age because of a decrease in the levels of LAMP-2A. In this work we have investigated the reasons for the reduced levels of LAMP-2A with age. While transcriptional rates of LAMP-2A remain unchanged with age, the dynamics and stability of the receptor in the lysosomal compartment are altered. The mobilization of the lysosomal lumenal LAMP-2A to the membrane when CMA is activated is altered in lysosomes from old animals, leading to the presence of an unstable pool of lumenal LAMP-2A. By contrast, the regulated cleavage of LAMP-2A at the lysosomal membrane is reduced owing to altered association of the receptor and the protease responsible for its cleavage to particular membrane microdomain regions. We conclude that age-related changes at the lysosomal membrane are responsible for the altered turnover of the CMA receptor in old organisms and the consequent decline in this pathway.

The involvement of macroautophagy in aging and anti-aging interventions

Macroautophagy is a process that sequesters and degrades organelles and macromolecular constituents of cytoplasm for cellular restructuring and repair, and as a source of nutrients for metabolic use in early starvation. Extensive evidence has been reported that macroautophagy process declines with increasing age. This impairment, probably due to ad libitum feeding, may cause accumulation of altered structures leading to the age-related decline in cell functions. It has been suggested that caloric restriction (CR) and disruption of insulin-like signals contrast the process of aging by prolonged stimulation of macroautophagy. According to this hypothesis, it is shown that life-long weekly administration of an anti-lipolytic drug decreases glucose and insulin levels, stimulates autophagy and intensifies anti-aging effects of submaximal CR.

Stimulation of Macroautophagy Can Rescue Older Cells from 8-OHdG mtDNA Accumulation: A Safe and Easy Way to Meet Goals in the SENS Agenda

Reduction of oxidative stress within mitochondria is a major focus and important part in the SENS agenda. The age-related accumulation of mitochondria rich in oxidatively altered DNA may be a biomarker of malfunctioning and increased oxidative stress. Macroautophagy is the cell repair mechanism responsible for the disposal of excess or altered mitochondria under the inhibitory control of nutrition and insulin, and may mediate the antiaging effects of caloric restriction. The authors investigated the effects of stimulation of macroautophagy by the injection of an antilipolytic agent on the age-related accumulation of oxidatively altered mitochondrial DNA (mtDNA) in rat liver cells. Results showed that treatment rescued older cells from the accumulation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the mtDNA in less than 6 hours. It is concluded that the age-related changes in mtDNA and function are likely to be the consequence of a failure of macroautophagy in the recognition and disposal of a small number of severely injured mitochondria, and that easy and safe ways are available to counteract this change.

You say lipofuscin, we say ceroid: defining autofluorescent storage material

Accumulation of intracellular autofluorescent material or "aging pigment" has been characterized as a normal aging event. Certain diseases also exhibit a similar accumulation of intracellular autofluorescent material. However, autofluorescent storage material associated with aging and disease has distinct characteristics. Lipofuscin is a common term for aging pigments, whereas ceroid is used to describe pathologically derived storage material, for example, in the neuronal ceroid lipofuscinoses (NCLs). NCLs are a family of neurodegenerative diseases that are characterized by an accumulation of autofluorescent storage material (ceroid) in the lysosome, which has been termed "lipofuscin-like". There have been many studies that describe this autofluorescent storage material, but what is it? Is this accumulation lipofuscin or ceroid? In this review we will try to answer the following questions: (1) What is lipofuscin and ceroid? (2) What contributes to the accumulation of this storage material in one or the other? (3) Does this material have an effect on cellular function? Studying parallels between the accumulation of lipofuscin and ceroid may provide insight into the biological relevance of these phenomena.

Aging, lipofuscin formation, and free radical-mediated inhibition of cellular proteolytic systems

Alterations in a wide array of physiological functions are a normal consequence of aging. Importantly, aged individuals exhibit an enhanced susceptibility to various degenerative diseases and appear less able than their young and adult counterparts to withstand (patho)physiological stress. Elucidation of mechanisms at play in the aging process would benefit the development of effective strategies for enhancing the quality of life for the elderly. It is likely that decrements in cellular and physiological function that occur during aging are the net result of numerous interacting factors. The current review focuses on the potential contribution(s) of free radical-mediated modifications to protein structure/function and alterations in the activities of two major proteolytic systems within cells, lysosomes and the proteasome, to the age-dependent accumulation of fluorescent intracellular granules, termed lipofuscin. Specifically, aging appears to influence the interplay between the occurrences of free radical-derived modifications to protein and the ability of cells to carry out critical proteolytic functions. We present immunochemical and ultrastructural evidence demonstrating the occurrence of a fluorescent protein cross-link derived from free radical-mediated reaction(s) within lipofuscin granules of rat cerebral cortex neurons. In addition, we provide evidence that a fluorophore-modified protein present in lipofuscin granules is the alpha subunit of F1F0-ATP synthase, a mitochondrial protein. It has previously been shown that protein(s) bearing this particular fluorescent cross-link are resistant to proteolysis and can inhibit the proteasome in a non-competitive fashion (J. Biol. Chem. 269 (1994a) 21639; FEBS Lett. 405 (1997) 21). Therefore, the current findings demonstrate that free radical-mediated modifications to protein(s) that lead to the production of inhibitor(s) of cellular proteolytic systems are present on specific protein components of lipofuscin. In addition, the mitochondrial origin of one of these proteins indicates specific intracellular pathways likely to be influenced by free radical events and participate in the formation of lipofuscin. The results of these studies are related to previous in vitro and in vivo observations in the field, thus shedding light on potential consequences to cellular function. In addition, future research directions suggested by the available evidence are discussed.

Autophagy and aging--when "all you can eat" is yourself

A recent paper provides evidence that macroautophagy is an essential downstream pathway for one of the mutations known to extend life span. Autophagy, or the degradation of intracellular components by the lysosomal system, was thought for a long time to be a catabolic process responsible for cellular cleanup. However, in recent years, we have learned that autophagy comes in different sizes and shapes, macroautophagy being one of them, and that this cellular maid plays many more roles than previously anticipated. Activation of autophagy is essential in physiological processes as diverse as morphogenesis, cellular differentiation, tissue remodeling, and cellular defense, among others. Furthermore, its participation in different pathological conditions, including cancer and neurodegeneration, is presently a subject of intense investigation. A role in aging has now been added to this growing list of autophagy functions. The activity of different forms of autophagy decreases with age, and this reduced function has been blamed for the accumulation of damaged proteins in old organisms. Research such as that covered in this Perspective shows that there is much more than trash to worry about when autophagy is not functioning properly.

The anti-ageing effects of caloric restriction may involve stimulation of macroautophagy and lysosomal degradation, and can be intensified pharmacologically

Caloric restriction (CR) and a reduced growth hormone (GH)-insulin-like growth factor (IGF-1) axis are associated with an extension of lifespan across taxa. Evidence is reviewed showing that CR and reduced insulin of GH-IGF-1 axis may exhibit their effects at least partly by their common stimulatory action on autophagy, the cell repair mechanism responsible for the housekeeping of cell membranes and organelles including the free radical generators peroxisomes and mitochondria. It is shown that the life-long weekly administration of an anti-lipolytic drug may decrease glucose and insulin levels and stimulate autophagy and intensify anti-ageing effects of submaximal CR.

Proteolysis, free radicals, and aging

Aging is accompanied by declines in cellular proteolytic capacity. Proteolytic processing is an important step in numerous cellular processes required for normal metabolic function. These include regulation of protein turnover, degradation of altered forms of protein, signal transduction, protein sorting/trafficking, receptor-mediated endo- and exocytosis, stress/immune responses, and activation of gene transcription. Thus, loss of cellular proteolytic function is likely to contribute to the enhanced fragility of cells from senescent relative to young and adult organisms. Free radicals have been implicated as contributing factors to observed age-dependent declines in proteolytic capacity. The current review offers an overview of the evidence linking free radical events to functional alterations in the lysosomal system and the proteasome, two major pathways by which proteins are degraded within cells. Implications for future investigations in the field are discussed in light of these findings.

Age-related changes in the autophagic proteolysis of rat isolated liver cells: effects of antiaging dietary restrictions

Autophagy is a process that sequesters and degrades organelles and macromolecular constituents of cytoplasm for cellular restructuring and repair and as a source of nutrients for metabolic use in early starvation. The effects of two antiaging dietary regimens (initiated in rats at the age of 2 months), namely, 40% dietary restriction (DR) and every-other-day ad-libitum feeding, that exhibited different effects on metabolism and similar effects on longevity on the age-related changes in the regulation of autophagic proteolysis were studied by monitoring the rate of valine release in the incubation medium from isolated liver cells of male albino Sprague-Dawley rats aged 2, 6, 12, 18, 24, and 27 months. (The liver cells were incubated in vitro with added amino acids and 10(-7) M insulin or glucagon.) Age-matched male albino Sprague-Dawley rats fed ad libitum served as a control. Results show that in ad-libitum-fed rats, after a transient increase by age 6 months, autophagic proteolysis and regulation by amino acid exhibit a dramatic age-related decline, and that the age-related changes are prevented by dietary antiaging intervention. A comparison shows that the protective effects of DR and every-other-day ad-libitum feeding are partially different in 24-month-old rats (but the beneficial effects of the two diets on regulation of autophagic proteolysis are always similar). With regard to endocrine regulation, results confirm that the liver cell response to glucagon (but not to insulin) declines with increasing age, and they show that antiaging DRs significantly improve the effects of glucagon (and have no effect on the response to insulin). The interactions of age by diet, glucagon (and in older rats, insulin), and amino acids are significant. It is concluded that DR significantly improves the susceptibility of liver cells to lysosomal degradation, and it prevents decline with increasing age. It is suggested that improved liver autophagy and lysosomal degradation might be part of the antiaging mechanisms of DR.