Interacting NAD + and Cell Senescence Pathways Complicate Antiaging Therapies

Abstract

During human aging, decrease of NAD⁺ levels is associated with potentially reversible dysfunction in the liver, kidney, skeletal and cardiac muscle, endothelial cells, and neurons. At the same time, the number of senescent cells, associated with damage or stress that secretes proinflammatory factors (SASP or senescence-associated secretory phenotype), increases with age in many key tissues, including the kidneys, lungs, blood vessels, and brain. Senescent cells are believed to contribute to numerous age-associated pathologies and their elimination by senolytic regimens appears to help in numerous preclinical aging-associated disease models, including those for atherosclerosis, idiopathic pulmonary fibrosis, diabetes, and osteoarthritis. A recent report links these processes, such that decreased NAD⁺ levels associated with aging may attenuate the SASP potentially reducing its pathological effect. Conversely, increasing NAD⁺ levels by supplementation or genetic manipulation, which may benefit tissue homeostasis, also may worsen SASP and encourage tumorigenesis at least in mouse models of cancer. Taken together, these findings suggest a fundamental trade-off in treating aging-related diseases with drugs or supplements that increase NAD⁺. Even more interesting is a report that senescent cells can induce CD38 on macrophages and endothelial cells. In turn, increased CD38 expression is believed to be the key modulator of lowered NAD⁺ levels with aging in mammals. So, accumulation of senescent cells may itself be a root cause of decreased NAD⁺, which in turn could promote dysfunction. On the contrary, the lower NAD⁺ levels may attenuate SASP, decreasing the pathological influence of senescence. The elimination of most senescent cells by senolysis before initiating NAD⁺ therapies may be beneficial and increase safety, and in the best-case scenario reduce the need for NAD⁺ supplementation.

Introduction: Aging, Senescence, and Loss of NAD⁺

Evidence is building for a key role for epigenetic changes in driving the biological processes that constitute aging. However, the interplay among various molecular and cellular systems that underlie the observed age-associated dysfunction is in the process of being elucidated. Two key changes that are potentially treatable are loss of function associated with decreased NAD⁺ levels and with increased numbers of senescent cells in mammals. The relationship between these aging-associated phenotypes has been murky.

NAD⁺ in aging

NAD⁺ levels and NAD⁺/NADH ratio decrease with organismal age in eukaryotes with effects on life span and vigor reported in organisms from baker's yeast (Saccharomyces cerevisiae) to fruit flies (Drosophila melanogaster) and mice. NAD⁺ plays key roles in metabolism and cellular signaling/regulation, including in glycolysis, oxidative phosphorylation, citric acid cycle, and poly-ADP-ribose polymerases (PARPs), sirtuins, and CD38/157 ectoenzymes.¹ PARPs play important roles in DNA repair. Sirtuins are protein deacetylases that are involved in cell survival, cell cycle, apoptosis, mitochondrial biogenesis and homeostasis, and stem cell function. It is not surprising that lower NAD⁺ levels in aging are associated with many hallmarks of aging pathophysiologies, including DNA damage, mitochondrial dysfunction, reduced autophagy, loss of proteostasis, deregulated nutrient sensing, stem exhaustion, and epigenetic alterations.²

Although reduction of the NAD⁺/NADH ratio and NAD⁺ levels with age appears evolutionarily conserved and there is strong evidence for reduced NAD⁺ in old mammalian muscle, liver, fat, brain, pancreas, spleen, heart, kidney, lung, and cerebrospinal fluid,³ it is unclear that the exact mechanisms underlying the alteration of NAD⁺ levels with age are conserved beyond a general sense that there are reduced biosynthesis and increased consumption. In aging mammals, decreased levels of nicotinamide phosphoribosyltransferase (NAMPT), a rate limiting enzyme for the conversion of nicotinamide (NAM) into nicotinamide mononucleotide (NMN) a direct precursor of NAD⁺, are observed in many tissues.^4,5 Increased NAD⁺ consumption may arise from increased PARP activity with age, which may result from cells with greater amounts of accumulated DNA damage, as inhibition of PARP can rescue age-associated phenotypes.^6
–8 Increased consumption in mammals also results from increased levels and activity of CD38/CD157 with age, which cleaves NAD⁺ into NAM and ADP-ribose. Beyond DNA damage, what other mechanisms are responsible for the reduction in NAD^±?

While the cause of NAD⁺ decline is murky, it is likely that altered activity of key regulators such as sirtuins and PARPs in these varied organisms and tissues, as well as altered rates of glycolysis and oxidative phosphorylation, may explain how low NAD⁺ levels cause aging-associated dysfunction. That NAD^± and its precursor NMN have a significant extracellular presence suggests that they may play an important role in systemic aging-related effects⁹ and altered NAD^± levels could help explain the organism side changes observed in aging.

Of great interest is that some aging and other aging-associated phenotypes can be partially ameliorated by the addition of NAD⁺, or NAD precursors such as NMN¹⁰ or nicotinamide riboside (NR), or by the inhibition of consumer enzymes such as CD38 or PARP.³ These include restoring insulin sensitivity, improved glucose tolerance/homeostasis, slowing cognitive decline, increased mitochondrial biogenesis, stimulated unfolded protein response, improved skeletal muscle metabolism and endurance, improved grip strength, reduced DNA damage and tumor development, increased survival time in heart failure, reversal of fatty liver, including reduced fibrosis, neuropathy protection, improved recovery from cardiotoxin-induced muscle injury in mice, and improved neurological function in Alzheimer's disease model and cerebral ischemic mice reviewed by Yoshino et al.³

Senescence in aging

The role played by cellular senescence in aging and mammalian diseases associated with aging is among the most exciting active fields of investigation. Cell senescence is actually a catchall for a set of states, in which damaged cells stop dividing and then exhibit some elements of a set of phenotypes that include the following: secretion of proinflammatory factors by senescence-associated secretory phenotype (SASP), resistance to apoptosis, altered morphology, senescence-associated heterochromatic foci (SAHF), DNA segments with chromatin alterations reinforcing senescence, expression of senescence-associated beta-galactosidase, expression of cyclin-dependent kinase inhibitors p16 and p21, activation of p53, expression of high-mobility group A (HMGA) proteins, and reduced lamin B1 among others. There is significant variation in the senescent phenotype dependent on cell type and how senescence is induced. The best-characterized induction routes are telomere shortening associated with somatic cell proliferation (Hayflick limit), expression of activated oncogenes (oncogene-induced senescence or OIS), radiation or other agents that induce DNA damage, and stress associated with reactive oxygen species or p38 mitogen-activated protein kinase (MAPK).^11,12

Cell senescence is not a random cell state; it likely was created by evolutionary processes to play defined roles in tissue formation during development, wound healing and repair, and prevention of tumors. The normal role of senescent cells appears to be as beacons of damage to delineate where and when repair/regeneration occurs. In such scenarios, the senescent cells are removed. However, senescent cells accumulate during aging and in numerous pathological conditions, including idiopathic pulmonary fibrosis, atherosclerosis, osteoarthritis,^13,14 and Alzheimer's disease among others. The continued presence of senescent cells can induce senescence in nearby cells and promote tumor development in nascent tumors.¹⁴ The bottom line is that abnormalities result when senescent cells are not removed by the immune system, or are continually created by nearby cells expressing SASP or by being subject to chronic injury.

Senescence has been postulated to both inhibit and stimulate tumorigenesis by preventing cells from becoming tumors early in the process, but then stimulating tumor growth by SASP later.¹⁵ Cell senescence is a complex phenomenon that seems to vary by tissue type and route of induction. One key common factor is that it is a response to cell stress with an intimate connection to repair processes.¹⁶ It is apparent that the specific form of senescence plays a key role in the way surrounding tissue responds and whether the response is appropriate or pathological.

Senescent cells wreak havoc on neighboring cells via SASP. The expression of SASP factors is temporally regulated and complex. The first wave of SASP factors, which include transforming growth factor (TGF)-β1 and TGF-β3, is typically immunosuppressive. By contrast, the second wave consists of proinflammatory factors such as interleukin (IL)-1β, IL-6, and IL-8.¹⁷

NAD⁺ Accentuates the Proinflammatory SASP

Nacarelli et al.¹⁷ have made a key discovery tying NAD⁺ levels to the expression of the SASP. Nacarelli et al.¹⁷ used a transcriptomic and chromatin immunoprecipitation (cHIP)-based bioinformatic approach on cultured human primary fibroblasts that carry an inducible mutated Ras oncogene capable of causing OIS to identify NAMPT as a key target of HMGA1, a member of the HMGA proteins known to promote tumorigenesis and to promote senescence, including SAHF.¹⁸ The upregulation of NAMPT was dependent on either HMGA1 or related HMGA2 proteins since knockdown with RNAi against either of the HMGAs inhibited the increase in NAMPT and the senescent phenotype.¹⁷

Knockdown of HMGA1 or NAMPT by RNAi or inhibiting NAMPT activity with the drug FK866 at the time of Ras induction in the human fibroblasts prevented OIS as assessed by multiple senescence biomarkers, including beta-galactosidase and p16, showing that HMGA1 and its downstream effector NAMPT are necessary for the establishment of OIS. However, once OIS is established, knockdown of HMGA1 or NAMPT does not reverse senescence, as most biomarkers remain expressed. Instead inhibition of HMGA1 or NAMPT only blocks the second wave of proinflammatory SASP factors, such as IL-1β, IL-6, and IL-8, but not the immunosuppressive first wave of SASP, which includes TGF-β1 and TGF-β3. The mitochondrial dysfunction-associated senescence secretory phenotype, which includes IL-10 and tumor necrosis factor-alpha, is also increased in OIS dependent on active HMGA1 and NAMPT. Interestingly, ectopic expression of NAMPT, but not a mutant enzymatically inactive NAMPT, restores proinflammatory SASP in OIS cells with HMGA1 knockdown. Nacarelli et al.¹⁷ conclude that active NAMPT is necessary for proinflammatory SASP in OIS or senescence induced by chemotherapeutic agent (DNA damaged) human fibroblasts.

Are NAD⁺ levels involved? The answer is affirmative. Liquid chromatography with tandem mass spectrometry analysis reveals that intracellular NMN, NAD⁺ levels, and the NAD⁺/NADH ratio all increase in OIS. Moreover there is a temporal correlation between increased NAD⁺ and increased SASP expression. As might be expected, ectopic expression of HMGA1 or NAMPT in normal fibroblasts itself has similar effects on NAD metabolism. Nacarelli et al.¹⁷ conclude that NAMPT is downstream of HMGA1 and increases SASP by increasing NAD⁺ levels and the NAD⁺/NADH ratio.

Because senescence has been hypothesized to act to promote developing tumors, Nacarelli et al.¹⁷ were interested in determining whether NAD⁺ levels have an effect on cancer progression using systems where senescence has been observed to be involved tumorigenesis. Enhanced growth of an ovarian cancer cell line cocultured with OIS human fibroblasts was dependent on not reducing HMGA1 or NAMPT levels or activity, suggesting NAD levels were important. Presumably, just adding NAD⁺ or compounds that can be converted to NAD⁺ such as NMN or NR would not be sufficient if added to normal fibroblasts cocultured with the ovarian cancer cell line. However, does NAMPT or NAD⁺ levels play a role in tumorigenesis in vivo?

In an engineered mouse model of pancreatic cancer, pancreatic intraepithelial neoplasias, precursors to malignant tumors, develop adjacent to senescent cells. Injection of young animals with NMN, which is converted to NAD⁺ intracellularly, daily for 2 weeks, results in an increase in the number of precancerous and cancerous cells and an increased amount of fibrotic tissue around the tumors, which also exhibited increased SASP factors such as IL-1β, IL-6, and IL-8. Molecular analysis showed that NAMPT levels correlated with the expression of proinflammatory SASP factors, although levels of more general senescence biomarkers, such as p16 or beta-galactosidase, were unaffected by NMN. Interestingly, inhibition of NAMPT by FK866 did not inhibit tumor progression more than a negative control, indicating that SASP could make things worse, but decreased SASP could not block a background level of tumor progression. One difference from the cultured experiments in human cells is that in the mouse model, FK866, the NAMPT inhibitor, significantly reduced all biomarkers of senescence examined, suggesting that the senescent phenotype of mouse pancreatic cells is more sensitive to NAMPT inhibition or that at 8 weeks, senescence was still not established in the tumors. In a xenograft mouse model of ovarian cancer in which tumor cells were engineered so that NAMPT levels could be reduced by doxycycline, injection of NAM, an NAD⁺ precursor, resulted in a significant boost in tumor growth over 18 days. Blocking NAMPT, which makes NAD|, blocked the effect. The conclusion is that NAD⁺ can stimulate tumor progression, likely through proinflammatory signaling associated with SASP.¹⁷

How does NAD⁺ stimulate SASP? The authors hypothesize that the effects are metabolic. High NAD⁺ levels are needed to maintain glycolysis. When the NAD⁺/NADH levels drop, the rate of glycolysis is reduced, which the authors observed in OIS cells with decreased NAMPT. Under these conditions, AMP kinase (AMPK) sensing an increased AMP/ATP ratio becomes activated, phosphorylates p53, in turn suppressing p38 MAPK, which when active controls inflammatory signaling pathways by stimulating transcription of nuclear factor-kappa B (NF-κB). When NAD⁺ levels are high, AMPK and p53 are not active, allowing p38 MAPK to stimulate NF-κB, which in turn drives the expression of proinflammatory cytokines (Fig. 1). Consistent with this model, inhibition of AMPK by the drug C25 rescued the inhibition of SASP by NAMPT inhibition by FK688 or knockdown by RNAi.¹⁷

FIG. 1.

NAD⁺ metabolism drives the proinflammatory SASP. Top: Under high proinflammatory conditions, HMGA proteins drive NAMPT expression converting dietary nicotinamide into NMN. NMN is converted into NAD⁺ by NMNAT. NMN can be obtained directly from dietary supplement or from the action of NRK on popular dietary supplement, NR. Bottom: Normal cells do not express SASP and p38 MAPK is not activated. In OIS, high NAD⁺ levels prevent activation of AMPK, allowing p38 MAPK to stimulate SASP expression via NF-κB. A low proinflammatory SASP accompanies replicative senescence, in which p38 MAPK activation and downstream SASP are attenuated by low NAD⁺ levels that activate AMPK and p53, which moderately decreases p38 MAPK signaling. AMPK, AMP kinase; HMGA, high-mobility group A; NAM, nicotinamide; NAMPT, nicotinamide phosphoribosyltransferase; NF-κB, nuclear factor-kappa B; NMN, nicotinamide mononucleotide; NMNAT, NMN adenyltransferase; NR, nicotinamide riboside; NRK, nicotinamide riboside kinase; SASP, senescence-associated secretory phenotype.

Increased NAD⁺ levels exacerbate OIS, but what about replicative senescence (RS)? Here, the story grows more interesting. During RS, NAMPT and NAD⁺ levels decrease. In fact, it is known that inhibition of NAMPT can actually drive cells into early RS, in a sense the opposite effect observed than that observed with OIS. Nacarelli et al.¹⁷ hypothesize that actually this is consistent with their central hypothesis. How? RS expresses much lower levels of SASP, an effect that they confirmed in their ovarian cancer coculture model. Indeed, senescence induced by inhibition of NAMPT also expressed low levels of SASP. They hypothesize that senescence associated with DNA damage, such as RS, has intrinsically less SASP than OIS, which has relatively lower levels of DNA damage. The key experiment they performed is to supplement RS human fibroblasts in culture with exogenous NMN, which increased the NAD⁺/NADH ratio, enhanced proinflammatory SASP factors, and drove tumorigenesis in the ovarian cell coculture system.¹⁷ It would be amiss for us not to point out that there seems to be a goldilocks effect with NAD⁺ and senescence: too little NAD^± and cells can become senescent; too much and SASP in existing senescent cells is worsened with all the implied negative consequences.

Senescent State Can Reduce NAD⁺

The mechanisms that underlie NAD⁺ level decrease remain a fundamental question. A recent report from Chini et al.¹⁹ suggests that SASP from senescent cells can induce CD38 in endothelial cells and bone marrow-derived macrophages. CD38 is a major consumer of NAD⁺. Upregulation of CD38 has been hypothesized to be sufficient to explain the loss of extracellular NAD⁺ levels, as CD38 knockout mice or mice treated with 78c, a CD38 inhibitor,^20,21 maintain normal levels of extracellular NAD⁺ throughout their lives. Youthful levels of glucose tolerance and youthful exercise capacity are maintained in these knockout animals.

These results suggest that cellular senescence, which may result from cell stress and damage, itself could contribute to decreasing NAD^± levels, which then in turn degrade organismal function via dysregulated NAD^± metabolism. On the contrary, this also might constitute a somewhat protective loop, as lower NAD⁺ levels should attenuate the SASP response with the downside that should the local NAD⁺ concentration fall too low, increased senescence would result.

Medical Implications

Decreased NAD⁺ levels with age are well documented in numerous studies, as are the associated negative effects demonstrated in numerous tissue types. It logically follows that supplementation with substances such as NR or NMN to increase NAD⁺ levels may counter some of the loss of function that accompanies aging. Numerous studies in rodents confirm this hypothesis (reviewed by Yoshino et al.).³ One interesting exception reported a marginal decrease in exercise capacity in young rats given NR supplementation, suggesting that excess NAD⁺ may not be helpful and even counterproductive in young healthy animals or humans.^22,23

In fact, a cottage industry has sprung around this logical narration. However, controversy has arisen over how effective the various methods to raise NAD⁺ levels are. Many of the rodent studies involve injected NAD⁺ or its precursors. However, oral supplementation of various precursors has been reported to raise NAD⁺ levels in mice and humans. For example, NAM has been reported to raise NAD⁺ levels in humans, although oral NR appears more bioavailable,²⁴ relatively high doses of 1000 mg of NR were needed to achieve physiologically relevant levels. NAM also has the drawback of inhibiting many key NAD⁺ target enzymes, including sirtuins and PARPs. Until recently it was believed that NMN has to be converted to uncharged NR by CD73 to enter cells, but the recent identification of Slc12a8 as an NMN transporter in the gut of mice²⁵ and widely expressed in many tissues (Human Protein Atlas) suggests that oral NMN could be directly absorbed by the gut and subsequently most tissues. Although expression of Slc12a8 is near ubiquitous, it should be noted that bone marrow and heart cells do not express detectable Slc12a8, which may make absorption in these cells dependent on conversion to NR.

Data supporting the benefits of supplementation of NAD⁺ via its precursors in humans are still pending, but clinical trials (NCT NCT03151239, NCT03432871, NCT03423342, NCT03501433, and NCT02835664) are in progress. The results from one human study showed modest improvement in exercise capacity in old people (a mean age of 71 years), although they did not approach the results for young people in their 20s.²⁶ However, large-scale trials of nicotinic acid (NA), another NAD⁺ precursor) for cardiovascular disease (NCT00461630 and NCT00880178), showed some efficacy but with adverse side effects. NA can be converted into NAD⁺ by the sequential actions of enzymes NAPRT, NMNAT, and NADS.

Given that NAD⁺/NADH are found in compartmentalized pools, Kulkarni and Brookes make the point that increasing NAD⁺ levels in the plasma may not restore localized pools of NAD⁺ because of restricted access to a particular compartment by external NAD⁺.²⁷ Future work may clarify how serious a potential problem this is.

But what about the implications of the reports connecting NAD⁺ to SASP?

That increased NAD⁺ can drive cells that have become senescent, due to incipient tumor formation, into secreting more SASP and promoting oncogenesis needs to be carefully weighed against the potential benefits of NAD⁺ supplementation on aging. Stimulation of SASP is potentially more serious with increasing age, as older people are more likely to have mutated incipient precancerous tumors. Moreover, NAD⁺ supplementation is likely to exacerbate pathological changes associated with senescence via increased SASP, while at the same time ameliorating phenotypes due to reduced NAD⁺ levels. Perhaps that explains the only very modest 5% increase in life span reported in one rodent study on the effects of increasing NAD⁺ levels in old age.²⁸

The results from Nacarelli et al.¹⁷ apply to oncogenic and RS, but what about senescence potentially caused by low NAD⁺ levels? Clearly, should such senescent cells actually occur in vivo, NAD⁺ supplementation via NR or NMN may be prophylactic as there are multiple recent studies suggesting that NAD⁺-dependent sirtuins such as SIRT1-SIRT6 counter the induction of senescence.

Are there any other cancer-related concerns? Indeed, as might be expected, NMN treatment can increase glycolysis and lactate-mediated acidosis and in fact this effect benefited ischemic perfusion injuries of the heart, but increased glycolysis may also act to promote tumorigenesis by the Warburg effect.²⁹ Increased NAMPT levels are associated with several cancers,³⁰ and a positive feedback loop between NAMPT SIRT1 and c-myc expression in gastric cancer has been found in obese mice.³¹

Are there any other potential caveats to increasing NAD⁺?

SIRT1 plays a key role in the regulation of autoimmunity by suppressing regulatory T cells that protect against autoimmunity and activating T helper cells that can contribute to autoimmunity.^32
–34 Perhaps NAD⁺-induced activation of SIRT1 increases the risk of autoimmune disease, although the same data could support a role for NAD⁺ in stimulating useful humoral immune response against disease.

Both SIRT1 and SIRT2 play a critical role in regulating neurodegeneration, but apparently their activities oppose each other. Ectopic SIRT1 overexpression increases neuronal survival in Alzheimer's disease, amyotrophic lateral sclerosis, and Huntington's disease.^35,36 On the contrary, SIRT2 activity can promote neuronal death. For example, inhibition of SIRT2 rescues a rodent Parkinson's disease model based on α-synuclein toxicity.³⁷

An obvious strategy that awaits the availability of proven senolytic drugs would be to ensure that senescent cells are removed before the initiation of NAD⁺-based therapeutic regimens to replace deficient NAD⁺ levels that occur with aging.

An important related question: Does NAD⁺ supplementation sensitize cells to senolytics or does it protect senescent cells from destruction? One hint comes from a study in which NAD⁺ supplementation increases the mitochondrial protecting master regulator NRF2.³⁸ NRF2 helps to protect cells from cell death, suggesting that NAD⁺ supplementation may make it more difficult to apply senolytic therapies, but this hypothesis requires rigorous testing in a variety of cell types.

Finally, are there any benefits of using increasing NAD⁺ to increase SASP? Perhaps. Given the role of senescent cells in repair processes such as wound healing, increasing NAD⁺/SASP may increase the rate of healing if applied with correct timing. This hypothesis should be tested in preclinical models of wound healing.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

References

Johnson

, Imai

. NAD + biosynthesis, aging, and disease. F1000Res, 2018; 7:132.

Aman

, Qiu

, Tao

, Fang

. Therapeutic potential of boosting NAD+ in aging and age-related diseases. Transl Med Aging, 2018; 2:30–37.

Yoshino

, Baur

, Imai

. NAD+ intermediates: The biology and therapeutic potential of NMN and NR. Cell Metab, 2018; 27:513–528.

Yoshino

, Mills

, Yoon

, Imai

. Nicotinamide mononucleotide, a key NAD+ intermediate, treats the pathophysiology of diet- and age-induced diabetes in mice. Cell Metab, 2011; 14:528–536.

Stein

, Imai

. Specific ablation of NAMPT in adult neural stem cells recapitulates their functional defects during aging. EMBO J, 2014; 33:1321–1340.

Bai

, Cantó

, Oudart

, et al. PARP-1 inhibition increases mitochondrial metabolism through SIRT1 activation. Cell Metab, 2011; 13:461–468.

Mouchiroud

, Houtkooper

, Moullan

, et al. The NAD+/Sirtuin pathway modulates longevity through activation of mitochondrial UPR and FOXO signaling. Cell, 2013; 154:430–441.

Fang

, Scheibye-Knudsen

, Brace

, Kassahun

, SenGupta

, Nilsen

, Mitchell

, Croteau

, Bohr

. Defective mitophagy in XPA via PARP-1 hyperactivation and NAD(+)/SIRT1 reduction. Cell, 2014; 157:882–896.

Clement

, Wong

, Poljak

, Sachdev

, Braidy

. The plasma NAD+ metabolome is dysregulated in “normal” aging. Rejuvenation Res, 2019; 22:121–130.

10.

Mills

, Yoshida

, Stein

, et al. Long-term administration of nicotinamide mononucleotide mitigates age-associated physiological decline in mice. Cell Metab, 2016; 24:795–806.

11.

Nacarelli

, Liu

, Zhang

. Epigenetic basis of cellular senescence and its implications in aging. Genes, 2017; 8:E343.

12.

Noren Hooten

, Evans

. Techniques to induce and quantify cellular senescence. J Vis Exp JoVE, 2017; 123:55533.

13.

Jeon

, Kim

, Laberge

R-M

, et al. Local clearance of senescent cells attenuates the development of post-traumatic osteoarthritis and creates a pro-regenerative environment. Nat Med, 2017; 23:775–781.

14.

Deursen

JM van.

Senolytic therapies for healthy longevity. Science, 2019; 364:636–637.

15.

Campisi

. Aging, cellular senescence, and cancer. Annu Rev Physiol, 2013; 75:685–705.

16.

Ritschka

, Storer

, Mas

, et al. The senescence-associated secretory phenotype induces cellular plasticity and tissue regeneration. Genes Dev, 2017; 31:172–183.

17.

Nacarelli

, Lau

, Fukumoto

, et al. NAD+ metabolism governs the proinflammatory senescence-associated secretome. Nat Cell Biol, 2019; 21:397–407.

18.

Narita

, Narita

, Krizhanovsky

, Nuñez

, Chicas

, Hearn

, Myers

, Lowe

. A novel role for high-mobility group a proteins in cellular senescence and heterochromatin formation. Cell, 2006; 126:503–14.

19.

Chini

, Hogan

, Warner

, et al. The NADase CD38 is induced by factors secreted from senescent cells providing a potential link between senescence and age-related cellular NAD+ decline. Biochem Biophys Res Commun, 2019; 513:486–493.

20.

Camacho-Pereira

, Tarragó

, Chini

CCS

, et al. CD38 dictates age-related NAD decline and mitochondrial dysfunction through a SIRT3-dependent mechanism. Cell Metab, 2016; 23:1127–1139.

21.

Tarragó

, Chini

CCS

, Kanamori

, et al. A potent and specific CD38 inhibitor ameliorates age-related metabolic dysfunction by reversing tissue NAD+ decline. Cell Metab, 2018; 27:1081.e10–1095.e10.

22.

Kourtzidis

, Dolopikou

, Tsiftsis

, et al. Nicotinamide riboside supplementation dysregulates redox and energy metabolism in rats: Implications for exercise performance. Exp Physiol, 2018; 103:1357–1366.

23.

Kourtzidis

, Stoupas

, Gioris

, et al. The NAD+ precursor nicotinamide riboside decreases exercise performance in rats. J Int Soc Sports Nutr, 2016; 13:32.

24.

Trammell

SAJ

, Schmidt

, Weidemann

, et al. Nicotinamide riboside is uniquely and orally bioavailable in mice and humans. Nat Commun, 2016; 7:12948.

25.

Grozio

, Mills

, Yoshino

, et al. Slc12a8 is a nicotinamide mononucleotide transporter. Nat Metab, 2019; 1:47.

26.

Dolopikou

, Kourtzidis

, Margaritelis

, et al. Acute nicotinamide riboside supplementation improves redox homeostasis and exercise performance in old individuals: A double-blind cross-over study. Eur J Nutr, 2019 [Epub ahead of print]; DOI: 10.1007/s00394-019-01919-4.

27.

Kulkarni

, Brookes

. Cellular compartmentation and the redox/nonredox functions of NAD. Antioxid Redox Signal, 2019 [Epub ahead of print]; DOI: 10.1089/ars.2018.7722.

28.

Zhang

, Ryu

, Wu

, et al. NAD⁺ repletion improves mitochondrial and stem cell function and enhances life span in mice. Science, 2016; 352:1436–1443.

29.

Nadtochiy

, Wang

, Nehrke

, Munger

, Brookes

. Cardioprotection by nicotinamide mononucleotide (NMN): Involvement of glycolysis and acidic pH. J Mol Cell Cardiol, 2018; 121:155–162.

30.

Shackelford

, Mayhall

, Maxwell

, Kandil

, Coppola

. Nicotinamide phosphoribosyltransferase in malignancy. Genes Cancer, 2013; 4:447–456.

31.

H-J

, Che

X-M

, Zhao

, et al. Diet-induced obesity promotes murine gastric cancer growth through a NAMPT/SIRT1/c-Myc positive feedback loop. Oncol Rep, 2013; 30:2153–2160.

32.

Kwon

H-S

, Lim

, Wu

, Schnölzer

, Verdin

, Ott

. Three novel acetylation sites in the Foxp3 transcription factor regulate the suppressive activity of regulatory T cells. J Immunol Baltim Md 1950, 2012; 188:2712–2721.

33.

Lim

, Kang

, Ryu

, et al. SIRT1 deacetylates RORγt and enhances Th17 cell generation. J Exp Med, 2015; 212:607–617.

34.

Loosdregt

J van

, Vercoulen

, Guichelaar

, et al. Regulation of Treg functionality by acetylation-mediated Foxp3 protein stabilization. Blood, 2010; 115:965–974.

35.

, Yokota

, Gama

, et al. Bax-inhibiting peptide protects cells from polyglutamine toxicity caused by Ku70 acetylation. Cell Death Differ, 2007; 14:2058–2067.

36.

Kim

, Nguyen

, Dobbin

, et al. SIRT1 deacetylase protects against neurodegeneration in models for Alzheimer's disease and amyotrophic lateral sclerosis. EMBO J, 2007; 26:3169–3179.

37.

Outeiro

, Kontopoulos

, Altmann

, et al. Sirtuin 2 inhibitors rescue alpha-synuclein-mediated toxicity in models of Parkinson's disease. Science, 2007; 317:516–519.

38.

Zhang

, Hong

, Cao

, Yin

, Shi

, Ying

. SIRT2, ERK and Nrf2 mediate NAD⁺ treatment-induced increase in the antioxidant capacity of PC12 cells under basal conditions. Front Mol Neurosci, 2019; 12:108.

Interacting NAD + and Cell Senescence Pathways Complicate Antiaging Therapies

Abstract

Introduction: Aging, Senescence, and Loss of NAD+

NAD+ in aging

Senescence in aging

NAD+ Accentuates the Proinflammatory SASP

Senescent State Can Reduce NAD+

Medical Implications

But what about the implications of the reports connecting NAD+ to SASP?

Are there any other potential caveats to increasing NAD+?

Footnotes

Author Disclosure Statement

References

Introduction: Aging, Senescence, and Loss of NAD⁺

NAD⁺ in aging

NAD⁺ Accentuates the Proinflammatory SASP

Senescent State Can Reduce NAD⁺

But what about the implications of the reports connecting NAD⁺ to SASP?

Are there any other potential caveats to increasing NAD⁺?