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Chronic Kidney Disease: A Clinical Model of Premature Aging
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American Journal of Kidney Diseases
Article in Press
published online 28 January 2013
Premature aging is a process associated with a progressive accumulation of deleterious changes over time, an impairment of physiologic functions, and an increase in the risk of disease and death. Regardless of genetic background, aging can be accelerated by the lifestyle choices and environmental conditions to which our genes are exposed. Chronic kidney disease is a common condition that promotes cellular senescence and premature aging through toxic alterations in the internal milieu. This occurs through several mechanisms, including DNA and mitochondria damage, increased reactive oxygen species generation, persistent inflammation, stem cell exhaustion, phosphate toxicity, decreased klotho expression, and telomere attrition. Because recent evidence suggests that both increased local signaling of growth factors (through the nutrient-sensing mammalian target of rapamycin) and decreased klotho expression are important modulators of aging, interventions that target these should be tested in this prematurely aged population.
The prevalence of chronic kidney disease (CKD) has reached epidemic proportions, and today ~10% of the population shows signs of decreased kidney function.1 Patients with CKD are at increased risk of premature death, mainly due to a high risk of cardiovascular disease and infections, which often occur in combination with protein-energy wasting.1 Cardiovascular risk increases early in the course of CKD progression,2 and the non-normalized cardiovascular mortality risk in European patients starting dialysis therapy is 15-fold higher than that in the general population,3 with the relative death risk being even higher in the United States.4 Because the uremic phenotype is characterized by many features of aging, such as osteoporosis, atherosclerosis, poor wound healing, sarcopenia, infections, inflammation, oxidative stress, insulin resistance, frailty, hypogonadism, infertility, skin athrophy, cognitive dysfunction, and disability, CKD could be seen as a premature aging (or progeroid) syndrome. Because kidneys are among the organs most sensitive to the aging process,5 the link between aging and decreased kidney function is birectional. Yang and Fogo6 have suggested that manipulation of cell senescence, which is an important mechanism for preventing the proliferation of potential cancer cells, may be a future way to manipulate the age-associated decrease in kidney function.
The Biological Process of Aging
As a consequence of improvements in life conditions and medical care, today humans can live longer than 100 years, a considerably longer time than that of our ancestors. The longest documented human life span is that of Jeanne Calment (1875-1997), a French woman who lived long enough to both meet Vincent van Gogh and experience the internet. Research in aging is a young field and aging has turned out to be a complex process controlled by many transcription factors and signaling pathways.7 Although aging seems to occur in most species, many animals living in the natural environment do not become senescent because they die of disease, starvation, and predation before they reach old age.8, 9 Even so, the phenomenon of negligible senescence, which is characterized by an attenuated age-related change in reproductive and physiologic functions, as well as no observable age-related gradual increase in mortality rate,9 has been documented. Among long-lived animal species such as turtles and rougheye rockfish, the naked mole rat has attracted much interest because recent data show that this eusocial mammal lives up to 8 times longer than mice. In addition, naked mole rats are extremely resistant to neoplasia, oxidants, toxins, and oxygen deprivation.10 Lewis et al11 recently demonstrated that enhanced cell signaling through the tumor suppressor protein p53 and the transcription factor Nrf2 protects cells in naked mole rats, suggesting that further studies of the role of these proteins in the aging process are warranted. Improved understanding of the processes that have evolved in these specific species to increase healthy life spans provides unique opportunities to develop novel treatment strategies against human aging.
Box 1. Examples of Main Theories of Aging
·Evolutionary theory: based on Darwin's theory of natural selection
·Free radical theory: oxidative stress is considered a major cause of premature aging
·Mitochondrial theory: extension of the free radical theory
·Gene regulation theory: cellular senescence is the result of changes in gene expression
·Inflammation hypothesis: inflammation is considered a major part of the aging process
·Telomere theory: a limitation in replicative capacity after a certain number of cell divisions
·Immune theory: the immune system is a powerful mechanism to face stressors
·Neuroendocrine theory: aging is due to changes in endocrine and neural function
·Neuroendocrine-immuno theory: a combination of the immune and neuroendocrine theories
·Phosphate retention theory: a novel theory based on the finding that dietary restriction of phosphate attenuates the aging characteristics in klotho null mice13
Source: Tosato et al.12
Box 1. Examples of Main Theories of Aging
Box 2. Examples of Interventions That May Affect Aging Processes
Nutritional and lifestyle interventions
·Caloric restriction137
·Red wine117
·Fish oil (omega-3 fatty acids)157
·Phosphate restriction118
·Physical exercise155
Pharmacologic interventions
·SIRT activation: resveratrol146
·Increased klotho expression: drugs that alter DNA hypermethylation,119 inhibition of NF-κB,90 PPAR-γ agonists,120 thyroid hormones,121 ACE inhibition,122 vitamin D123
·mTOR inhibition: rapamycin,164 metformin,165 resveratrol147
·Stabilization of telomeres: statins,124 estrogens,125 telomerase reactivation,126 vitamin D127
·Limitation of DNA damage: inhibition of NF-κB,128 antioxidants129
·Phosphate lowering: phosphate binders,130 blocking the intestinal phosphate transporter Npt2b131
Abbreviations: ACE, angiotensin-converting enzyme; mTOR, mammalian target of rapamycin; NF-κB, nuclear factor κB; PPAR, peroxisome proliferator-activated receptor; SIRT, sirtuins.
Aging commonly is defined as the progressive accumulation of deleterious changes in cells and tissues that are responsible for deterioration in physiologic functions coupled with increased vulnerability and risk of death. Tosato et al12 discussed several different hypotheses of aging (summarized in Box 1). The permanent and irreversible growth arrest of cell senescence is a central paradigm of aging. Although senescent cells remain viable, they are unable to divide and their morphologic characteristics change and undergo significant transcriptional changes accompanied by delayed repair, as well as alterations in nuclear structure, gene expression, protein processing, and levels of growth factors (Fig 1). Recent evidence suggests that the mammalian target of rapamycin (mTOR) is involved in the hypersecretory senescent phenotype.15 Among tumor suppressor proteins that are crucial in the induction of senescence, p53 (the "guardian of the genome") has an especially important role in protecting against DNA damage, oxidative stress, and telomere attrition. Cellular senescence may occur prematurely in response to a variety of stress factors, such as oxidative stress, DNA damage, and inflammation.16 Notably, decreased kidney function per se and the uremic mileu affect most of the factors known to accelerate aging, including DNA damage, inflammation, phosphate toxicity, klotho deficiency, oxidative stress, exhaustion of stem cells, and telomere shortening.17
The Antagonistic Pleiotrophy Hypothesis
Of the multiple theories to explain exceptional longevity, the most robust has centered on the decreased signaling of anabolic hormones: growth hormone (GH), insulin-like growth factor (IGF), and insulin. Despite ample evidence in the literature that deficiencies in GH and IGF-1 contribute to several aspects of the natural aging process, animal studies show that disrupting the signaling pathways for these hormones exerts antiaging effects.18 Potential mechanisms linking decreased signaling of these hormones with delayed aging include increased hepatic sensitivity to insulin actions, decreased plasma glucose levels, increased resistance to oxidative stress, and decreased mTOR signaling.18 Thus, familial longevity usually is associated with better insulin sensitivity.19 At least 7 genetic mouse models (including mice null for either GH receptor/binding protein or mice heterozygous for the IGF-1 receptor) have been reported to show increased life span and a delay in aging-related diseases.20 Sonntag et al,21 who recently summarized the literature on GH and IGF-1 and aging processes, suggested that the perceived contradictory roles of these anabolic hormones are explained by their differential effects on health during specific life span stages. In this context, the antagonistic pleiotrophy hypothesis22 should be mentioned; this hypothesis posits that gene products have opposite effects on biological fitness at different stages of life. Based on this hypothesis, Blagosklonny23 suggested that although mTOR activation by IGF-1, GH, insulin, and nutrients may provide a selective survival advantage to young males (because it stimulates muscle growth and increases competitive and reproductive ability), mTOR overactivation may accelerate age-related diseases (Fig 2). It recently was shown that increased mTOR signaling in hypothalamic neurons that express pro-opiomelanocortin contribute to age-dependent obesity.24 Moreover, interventions that inhibit mTOR, such as rapamycin and caloric restriction, lead to changes in gene expression and to increased life span in both animals25 and humans.26 Thus, because single-gene mutations in those genes involved in insulin/IGF and mTOR signaling pathways extend life span,27 dampening the mTOR pathway may protect from age-related diseases.
Pregnancy-associated plasma protein A (PAPP-A) was identified first in high concentrations in pregnant women in 1974 and its function was unknown for 25 years. Today we have learned that this protein is stimulated by proinflammatory cytokines and has a role in the progression of atherosclerotic plaque development through enhanced local IGF-1 bioavailability.28 Mice null for PAPP-A are resistant to the development of atherosclerosis29 and live ~30% longer than their wild-type littermates.30 Because elevated PAPP-A levels have been found in patients with CKD stage 4,31 the role of this protein in premature uremic vascular aging needs further study.
Persistent Inflammation and Cellular Senescence
Because most chronic diseases are associated with inflammation, it can be hypothesized that an increased inflammatory load increases the risk of age-related pathologic states and decreases survival. Elderly individuals often exhibit chronic inflammation, which is characterized by immune system dysregulation and increased inflammatory cytokine production.32 The Newcastle 85+ Study recently confirmed the importance of inflammatory markers in frailty of the very old.33 As discussed by Brod,34 unregulated inflammation shortens human functional longevity and has an important role in the cause, progression, and shortened life span of patients with autoimmine diseases, presenile dementia, osteoporosis, diabetes, and atherosclerosis.
The growing importance of inflammation as a cause of aging is exemplified by the neologism "inflamma-aging."35 The consequences of inflammation also provide additional support for the antagonistic pleiotropy hypothesis in that its beneficial effects, which early in life work to neutralize dangerous and harmful agents, may become harmful themselves at an older age.35 Of potential relevance is the observation that as human immunodeficiency virus (HIV)-infected patients live longer (due to successful treatment with antiretroviral therapy), this patient group develops premature susceptibility to the age-related morbidities and adaptive changes in the immune system ("immune-senescence") seen in older adults.36 It has been suggested that cytomegalovirus contributes to the development of premature aging and immunosenescence in HIV-infected patients.37
In terms of kidney disease, because increased levels of the proinflammatory cytokine interleukin 6 (IL-6) are a common feature of uremia,38 it is of interest that this cytokine regulates oncogene-induced senescence.39 Because senescent cells secrete multiple growth factors, proinflammatory cytokines, and chemokines,40 they promote further inflammation (Fig 1). Acosta et al41 showed that senescent cells trigger a feedback loop that reinforces growth arrest through a secretory network acting through CXCR2-binding chemokines. This links cellular senescence with endothelial dysfunction and the inflammatory process of atherosclerosis.42
The fact that cellular senescence impairs the sucessful reprogramming of pluripotent stem cells43 may explain why marrow-derived stromal cells44 and endothelial progenitor cells45 diminish in uremia. Endothelial progenitor cells and proinflammatory and proatherogenic CD14++CD16+ monocytes,46 which are highly expressed in uremia,47 differentiate from the same CD34+ progenitor cells.48 Girndt and Seibert49 suggested that in the inflamed uremic milieu, stem cells differentiate to aggressive monocytes at the expense of reduced production of endothelial repair cells. Depleted levels of CD34+50 and CD14+51 endothelial progenitor cells are associated with decreased kidney function in patients with coronary artery disease. Thus, because the expression of nonclassical CD14+CD16+ monocytes increases with age,52 endothelial injury,53 and telomere attrition,46 senescent monocytes are linked to cardiovascular disease, aging, and CKD. Another cause of accelerated senescence in human endothelial progenitor cells is carbamylated low-density lipoproteins,54 a group of modified proteins known to have increased levels in the inflamed and oxidated uremic milieu. Because nocturnal hemodialysis was reported to be associated with the restoration of abnormal endothelial progenitor cell biology,55 further studies are needed to determine the effect of dialysis treatment per se on the aging process. Finally, because vitamin K-dependent proteins are essential for progenitor cell proliferation,56 additional investigations should explore whether vitamin K depletion57 and/or warfarin treatment contribute to stem cell exhaustion in patients with CKD.
Telomere Length and Aging
Shortening (or attrition) of telomeres reflects not only cellular senescence, but also stem cell exhaustion, cellular hyperactivation, and a hypersecretory phenotype. Telomere length and integrity are important for optimal chromosome function and genomic integrity, and as telomeres within cells become shorter, they assume a more unstable phenotype that triggers senescence.58 Telomeres are synthesized by the telomerases that maintain chromosome length. In certain rare forms of progeria, such as Hutchinson-Gilford progeria syndrome, premature cellular senescence results from progerin-induced telomere dysfunction.59
In the general population, accelerated telomere attrition is associated with cardiovascular risk factors, such as age, male sex, obesity, smoking, dyslipidemia, sedentary lifestyle, and mental stress.60 Moreover, Shiels et al61 showed that shorter telomeres were associated with lower socioeconomic status, inflammation, and poor diet. Because cells of the immune system are under enormous proliferative demand and stress telomere function, telomere dynamics is critical for preventing immunosenescence.62 Infectious diseases such as cytomegalovirus63 cause telomere attrition,64 and telomere length correlates with both IL-6 level65 and the cumulative inflammatory load.66 Oxidative stress67 and visceral fat accumulation68 also are linked to accelerated aging and shorter telomeres. Because telomere shortening in the general population is associated with increased risk of premature myocardial infarction,69 coronary heart disease,69 and death,70 telomere length is a useful aging biomarker.
Given the possibility to prevent telomere attrition by various nutritional and pharmacologic interventions, nephrologists should pay more attention to this fascinating "biological clock."71 Ramirez et al72 demonstrated that mononuclear cells from hemodialysis patients have decreased telomere length compared with controls and that the percentage of cells with short telomeres correlates positively with C-reactive protein level. We also have shown a correlation between telomere attrition and inflammation (as indicated by IL-6 level).73 In our study of hemodialysis patients, decreased telomere length was associated with increased all-cause mortality independent of age, sex, and IL-6 levels.73 Finally, Westhoff et al74 showed that in telomerase-deficient mice, critical telomere attrition in the kidneys spurs greater apoptosis and senescence, which in turn diminishes postinjury regenerative capacity.
Oxidative Stress and DNA Damage as Promoters of Aging
Mutations and DNA damage that lead to dysfunctional proteins and modified DNA structure have been considered a main cause of aging since the late 1950s.75 Werner syndrome and Hutchinson-Gilford progeria syndrome are rare progeroid syndromes that clinically resemble accelerated aging. Because these diseases are characterized by defects in DNA repair and processing, this implies that increased DNA damage accelerates a decrease in physiologic processes and the development of the aged phenotype.76 A causal contribution between DNA damage and aging was demonstrated by Niedernhofer et al,77 who showed that when cytotoxic DNA damage is unrepaired, it triggers a highly conserved metabolic response involving the IGF-1/insulin pathway that redirects resources from growth to life extension.
The free radical and mitochondria theory of aging, which suggests that the aging process involves the initiation of free radical reactions, may explain why females live longer than males. Mitochondrial oxidative stress is higher in males and higher levels of estrogens protect females by upregulating the expression of antioxidant longevity-related genes through NF-κB (nuclear factor κB).78 Because decreased mitochondrial function is an important factor in aging and increases the incidence of age-related disorders,79 mitochondrial dysfunction may contribute to the growing burden of CKD in the aging population.80 Because increased oxidative stress of DNA is a feature of uremia81 and seems to be related to many features of the aged phenotype, including atherosclerosis,82 a defective response to cellular DNA damage may accumulate over time and contribute to premature aging. Mutations in genomic stability genes, such as FAN1 (a DNA repair nuclease), recently were reported to connect a DNA damage response to progressive loss of kidney function.83
Dysregulation of the Klotho-FGF-23 axis, Phosphate Toxicity, and Aging
The discovery of the progeric phenotype of mice that lack a functional variant of a protein that subsequently was named klotho84 has evoked much interest among nephrologists because this "elixir of youth"85 is highly expressed in the kidney.84 Klotho has a short membrane-spanning domain and two large extracellular domains86; it exists as a transmembrane protein but also has two circulating isoforms, one of which is a shedded variant of the membrane form. Diminished tissue expression is a cardinal feature of CKD and is observed at the very earliest stages of the disease.87 Thus, klotho has been proposed as a relevant biomarker for risk exposure associated with kidney failure. Currently, the cause of suppressed klotho levels in CKD is unknown. However, downregulation of klotho gene expression by uremic toxins and subsequent gene hypermethylation may be one plausible mechanism,88 which implies that epigenetic dysregulation may cause some of the physiologic changes associated with aging.89 Moreover, the fact that inflammation downregulates klotho expression through NF-κB90 confirms the observed relationship between inflammation and accelerated organ aging.
Although a lack of klotho is associated with an aging-like phenotype, the underlying mechanisms are largely elusive. Importantly, to date there is no bioassay available for adequate detection of circulating klotho level because the recently reported assay yields substantially higher serum and urine levels than anticipated compared with immunoprecipitation techniques.87, 91 Another question is whether a decreased klotho level in tissue (predominantly the kidneys) promotes aging through direct/local mechanisms or systemic effects of klotho deficiency. Several observations support the latter theory, namely that decreased systemic klotho level is critical for accelerated aging beyond the protein's direct effects in mineral metabolism.92, 93 In this regard, in vitro studies provide evidence that soluble klotho functions as a hormone per se and directly counteracts senescence through several alternate mechanisms. Systemic delivery of recombinant wild-type klotho can ameliorate the aging phenotype in klotho mutant mice.94 Possible mechanisms include protection against oxidative stress by activating the transcription factor FOXO and increasing the expression of superoxide dismutase downstream,95 endogenous anti-inflammatory effects,96 inhibition of endothelial cell senescence,97 antifibrotic properties,98 and prevention of vascular calcification.87 Because suppressing the insulin/IGF-1 signaling pathway seems to have an evolutionary role for extending life,23 the ability to suppress insulin/IGF-1 signaling also may account for klotho's antiaging properties.99
An alternate explanation for the link between klotho and antiaging may be its role in systemic regulation of mineral metabolism.100 Klotho promotes renal calcium absorption and renal phosphate excretion92, 93 in addition to functioning as a permissive coreceptor for the phosphate and vitamin D-regulating hormone fibroblast growth factor 23 (FGF-23).101 Importantly, the aging characteristics of klotho null mice are attenuated by dietary restrictions of phosphate and 1,25-dihydroxyvitamin D or by the elimination of vitamin D toxicity through ablation of the vitamin D receptor or the enzyme responsible for its activation (CYP27B1).102, 103, 104 Collectively, the aging phenotype associated with klotho deficiency therefore may be due to both the intrinsic antiaging properties of klotho and secondary systemic alterations in mineral metabolism.
The finding that klotho and FGF-23 function in a common signal transduction pathway was derived from the observation that FGF-23 and klotho-deficient mice developed similar aged phenotypes, including growth arrest, nephrocalcinosis, hyperphosphatemia, hypervitaminosis D, osteopenia, hypercalcemia, emphysema, gonad atrophy, and vascular calcification.84, 101 The discovery of the klotho-FGF-23 endocrine system is important not only because it has added new dimensions to the classic view of the endocrine regulation of phosphate, but also because it indirectly linked phosphate to vascular aging. Hyperphosphatemia has been identified as a significant cardiovascular risk and mortality predictor not only in dialysis patients105 and patients with myocardial infarction,106 but also in individuals without CKD.107 Moreover, hyperphosphatemia has been determined to be a cardiovascular risk factor for left ventricular hypertrophy in community-dwelling young adults.108 Ohnishi and Razzaque103 found that phosphate toxicity accelerates the mammalian aging process. Moreover, there is a strong inverse correlation between longevity and serum phosphate levels in different animals (Fig 3), which further supports this hypothesis.13
Experimental studies show that phosphate promotes both vascular calcification (by increasing the genetic transcription of proteins that are involved in osteoblast function-bone formation and stimulation of matrix mineralization109) and apoptosis of vascular smooth muscle cells.110 Because maintaining normal phosphate concentrations seems to be fundamental for a healthy long life, it seems logical that evolution has created many backup systems (ie, phosphatonins, such as FGF-23, FGF-7, and MEPE) aside from parathyroid hormone that protect the organism from phosphate toxicity.111
If the number of functional nephrons decreases to a level that fails to excrete ingested phosphate, vascular calcification and decreased life span follow. Because klotho deficiency induces FGF-23 resistance,100 which leads to exceptionally elevated FGF-23 levels in end-stage renal disease,112 it has been proposed that FGF-23 itself could modify the vascular phenotype. This is supported by epidemiologic data linking FGF-23 to high mortality, vascular dysfunction, and left ventricular hypertrophy across all strata of kidney function.113, 114 Also, experimental evidence suggests that FGF-23 may cause cardiac hypertrophy115 and modulate vascular calcification.116
Can Interventions Delay Aging?
Since the Spanish explorer Juan Ponce de Leon (1474-1521) searched for the fountain of youth in the mythical land of Bimini, humans have dreamed of preventing aging with medicines or diets. Although no such remedy works yet in humans, animal studies have shown that life expectancy can be modified.12 Box 2 lists examples of nutritional, lifestyle, and pharmacologic interventions that have been suggested as having the potential to affect aging processes through different mechanisms. It generally is believed that the best predictor of longevity are the genes we are born with. However, Hjelmborg et al132 uncovered scant evidence supporting the idea that genetics affects the age of death for individuals younger than 60 years, whereas in adults 60 years and older, the authors found genetics to have a moderate influence that increased over time.
Nevertheless, it has been demonstrated that genes for insulin/IGF-1 signaling and telomere maintenance pathways are relevant for human longevity.133 Centenarians have gene variants that optimally tune levels of pro- and anti-inflammatory molecules and reduce the effects of a lifetime's worth of environmental insults and stressors.134 Moreover, functional gene variants of klotho135 and apolipoprotein E136 are associated with longevity.
Among the interventions that may affect life expectancy, the effects of caloric restriction (typically by 20%-40% of ad libitum consumption) while maintaining essential nutrient requirements have been studied the most, given that this intervention consistently increases the life span of mice.137 Studies in primate species show conflicting results: Colman et al138 reported improved survival and delayed disease onset with caloric restriction, whereas Mattison et al139 recently reported that a caloric restriction regimen did not improve survival outcomes in young and older rhesus monkeys. Because of the ethical and logistical limitations of the research design and the length of a human life span, it is difficult to design studies that answer whether caloric restriction prolongs life in humans. Whereas caloric restriction reduces oxidative stress, improves cardiometabolic risk profile,140 and deactivates the mTOR pathway,141 it does not affect telomere dynamics in rhesus monkeys.142 Indirect support for a benefit of caloric restriction on life expectancy is provided by studies of the Okinawan population in Japan, in whom the prevalence of centenarians is highest in the world and caloric intake is 20% lower than in the rest of Japan and 40% lower than in the United States.12 Interestingly, higher glucose levels are associated with an older perceived age in nondiabetic patients and diabetic individuals.143 While religious fasts are mainly undertaken for spiritual purposes, they have the potential to downregulate mTOR and affect physical health. Because caloric restriction suppresses both apoptosis in aged rat kidneys144 and the accumulation of long-chain glycosphingolipids, which have a role in mammalian aging,145 there is a rationale for caloric restriction in obese patients in the earlier stages of CKD. However, in later stages of CKD, when the risk of protein-energy wasting increases, caloric restriction is not advocated.
Because the beneficial effects of caloric restriction during aging may be mediated through sirtuins (SIRT1-SIRT7), the SIRT1 activator resveratrol, which accounts for the beneficial cardiovascular effects of red wine, has attracted much recent interest because this polyphenol activates both PPAR (peroxisome proliferator-activated receptor) and endothelial nitric oxide synthase and inhibits cyclooxygenase.146 It also has been reported that resveratrol has anti-inflammatory and antioxidative effects mediated through both mTOR inhibition147 and stimulation of the antioxidant-activated transcription factor Nrf2.148 Accumulating in vivo evidence from disease and stress models suggests that resveratrol has a protective role and promotes human health.146
Red wine polyphenols also may preserve endothelial function during aging.149
Huang et al117 showed in a randomized trial that consuming red wine increased the number and functional capacity of circulating endothelial progenitor cells. Long-term moderate red wine consumption and equivalent oral pharmacologic doses of resveratrol increased telomere length, decreased p53 expression, and preserved vascular function indexes in normal rats; however, life span was not extended.150 Because decreased mitochondrial oxidative phosphorylation and aerobic capacity are associated with decreased longevity, it is interesting that resveratrol treatment in mice was associated with an induction of genes for oxidative phosphorylation and mitochondrial biogenesis,151 which suggests that resveratrol could have endurance-enhancing activities. Because resveratrol prevented wasting after mechanical unloading in rats152 and muscle wasting in diabetic rats,153 sirtuin activation may be a novel treatment strategy to prevent protein-energy wasting and interrupt the urea cycle in CKD.154 The benefits of physical activity in preventing premature death have been established by many epidemiologic studies,155 effects that may be mediated by both improved cardiometabolic risk-factor profile and effects on telomere dynamics.156
Upregulating telomerase pharmacologically has been proposed as a way to slow the aging process in some diseases.126 However, these treatments inspire substantial concern about their possible carcinogenic effects. Interestingly, it has been suggested that the antiaging effects of statins are linked to their ability to inhibit telomere shortening.124 Also, estrogen therapy125 and higher vitamin D levels127 seem to be associated with longer telomeres. Although marine omega-3 fatty acids are associated with telomere aging in patients with coronary heart disease,157 there currently are not enough data to draw firm conclusions about how omega-3 fatty acids affect the aging metabolism.158 Because NF-κB inhibition delays DNA damage-induced senesence and aging in mice,128 there also is a rationale for testing antioxidative129 anti-inflammatory159 treatment strategies to delay uremic telomere attrition.
An interesting and novel concept to extend life span is the "gerosuppressant" drug rapamycin, which reduces insulin/IGF-1 signaling by inhibiting the mTOR pathway.160 Rapamycin has been regarded as a "double whammy"; that is, a drug that both increases life span and inhibits cancer in animal models.161 Because of the salutory effects of rapamycin on life span in animal models, its rejuvinating effects on stem cells,162 and the finding that Tor complex 1 controls telomere length,163 rapamycin has the potential to be used an antiaging drug in progeroid syndromes.164 However, given the magnitude of rapamycin-associated adverse effects, as well as the lack of human data, caution is advised regarding the routine use of rapamycin as an antiaging agent. Weaker mTOR inhibitors such as the antidiabetic drug metformin165 and reservatrol147 may be of more interest in this regard.
Given the role of klotho deficiency in premature aging, much interest has focused on interventions that increase systemic klotho expression.94 Because vitamin D receptor activators are the most potent klotho expression stimulators,166 it is possible that the survivial benefit associated with vitamin D therapy in patients with CKD may be attributed in part to an increase in klotho levels. Furthermore, the recent finding that the uremic toxins indoxyl sulfate and p-cresyl sulfate silence the klotho gene through hypermethylation88 opens a new possibility of reversing the aging process by manipulating the epigenome. PPAR-γ, which suppresses NF-κB and decreases the production of cytokines and chemokines, upregulates tissue klotho and presumably also its circulatory level,120 which implies that PPAR-γ agonists affect the aging process. Other interesting possibilities to increase klotho expression include thyroid hormones12 and angiotensin-converting enzyme inhibitors.122 Because a recent study implies crosstalk between FGF-23-klotho and the renin-angiotensin-aldosterone system,167 there is an opportunity for combined treatment regimens to improve survival.
Because acute postprandial hyperphosphatemia acutely impairs endothelial function,168 phosphate-lowering therapies may decrease cardiovascular risk and aging. Reducing phosphate levels in different animal models through means such as vitamin D receptor knockout, sodium/phosphate cotransporter knockout, low-phosphate diet, and low-vitamin D diet consistently rescue the premature aging phenotype of klotho and FGF-23 knockout mice. Thus, it has been suggested that phosphate per se may promote the aging process. Although the mechanisms by which phosphate promotes aging have not been elucidated, it can be speculated that increased oxidative stress, reduced mitochondrial respiration, and apoptosis contribute.169 Moreover, because phosphate is associated with both C-reactive protein and fetuin A-containing calciprotein particles (stable colloidal complexes with minerals that reflect a procalcified milieu) in CKD,170 it can be hypothesized that phosphate may promote aging through inflammatory pathways (Fig 1). No randomized controlled trial has yet proved that phosphate binders decrease mortality in dialysis patients. A recent randomized placebo-controlled pilot clinical trial in patients with moderate CKD showed that although phosphate binders significantly decrease phosphate levels and attenuate the progression of secondary hyperparathyroidism, they also promote the progression of vascular calcification.171 These findings may be explained by increased intestinal availability and systemic absorption of free calcium when the amount of intestinal phosphate and its complex binding to calcium is decreased. Alternate strategies to reduce phosphate burden without increasing calcium absorption are desirable. Because hyperphosphatemia affects the progression of aging in both klotho mutant mice118 and Drosophila,172 dietary phosphate restriction may be a novel nutritional intervention that affects the aging process. However, dietary phosphate restriction in patients with CKD provides a greater risk of protein-energy wasting and should be monitored carefully. Another promising strategy to decrease phosphate toxicity is pharmaceutical blocking of the intestinal phosphate transporter Npt2b.131
Conclusions
Recent data in the gerontology literature have shed new light on the complicated process of human aging. It is noteworthy that all the proposed mechanisms of premature aging and cellular senescence, such as DNA and mitochondrial instability, inflammation, free radical excess, telomere shortening, phosphate toxicity, and systemic klotho deficiency, seem to be affected in the uremic milieu. Thus, uremia could be considered a progeroid syndrome and a clinical model to study the aging process. Because the recent literature suggests that growth factor signaling has a pivotal role in the aging phenotype, interventions targeting the nutrient-sensing mTOR pathway, such as rapamycin and resveratrol, are of major interest in studies of uremic premature aging. Given the combined role of phosphate and klotho in uremic vascular disease, interventions that target these factors also should be studied. Gerontologists and nephrologists should collaborate to further elucidate the intriguing mechanisms by which decreased kidney function contributes to cellular senescence and a premature aging phenotype.
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