NAD (nicotinamide adenine dinucleotide) and autophagy are key regulators of metabolic homeostasis and essential for alleviating cellular stress. With aging, both NAD levels and autophagic activity decline—making them closely linked to age-related diseases.
Enhancing NAD levels or autophagic activity can extend lifespan and may slow the progression of age-related diseases. However, research over the past decade has primarily focused on how NAD regulates autophagy, with little attention to the reverse: how autophagy affects NAD levels. Recent studies have confirmed that autophagy also influences NAD levels, exerting a significant impact on aging and lifespan. NAD and autophagy are indeed intertwined in a mutually regulatory relationship.
Recently, the top academic journal Cell published a review co-authored by teams from the University of Newcastle (UK) and the University of Birmingham. This review thoroughly explains the bidirectional regulatory mechanism between NAD and autophagy, as well as its potential as a therapeutic target for combating age-related diseases and promoting longevity[1].
1. NAD and Autophagy: Basic Concepts
Before exploring the interaction mechanism between NAD and autophagy, let’s first clarify their basic definitions.
NAD: A Ubiquitous Metabolite for Cellular Vitality
NAD is ubiquitous in the human body. As an essential metabolite involved in energy metabolism, it is critical for maintaining cellular viability. The redox pair between its oxidized form (NAD⁺) and reduced form (NADH) allows NAD to act as a cofactor for oxidoreductases. Additionally, NAD serves as a substrate for a range of NAD⁺-consuming enzymes, including PARPs (poly(ADP-ribose) polymerases) and SIRTs (sirtuins).
Autophagy: Cellular "Self-Eating" for Metabolic Homeostasis
Autophagy, simply defined as the process of cellular self-eating, is a catabolic pathway that engulfs and degrades intracellular components. By recycling subcellular materials, it maintains metabolic homeostasis. Dysfunctional autophagy leads to cellular degeneration and accumulation of toxic protein aggregates—hallmarks of age-related diseases. Due to its catabolic nature and connection to nutrient status, autophagy strongly regulates cellular metabolism.
senescence. Enhancing either autophagic activity or NAD levels can significantly extend the healthspan and lifespan of animals, while restoring normal cellular metabolic activity.
Previous studies have shown that NAD⁺-consuming enzymes directly regulate autophagy and mitochondrial quality control, and NAD metabolism promotes the autophagic clearance of damaged mitochondria.
Latest research reveals that autophagy also maintains NAD levels by regulating cellular stress; impaired autophagy in respiring cells triggers NAD depletion and cell death. In short, NAD and autophagy mutually regulate each other to optimize cellular function.
The link between NAD, autophagy, and aging is evident—but how does their bidirectional feedback regulation open new avenues for anti-aging interventions?
2. NAD Regulates Autophagy
Studies have demonstrated that supplementing NAD enhances autophagy and mitochondrial quality control, thereby promoting tissue rejuvenation and extending organismal lifespan. The drivers of this process are NAD⁺-consuming enzymes.
Beyond their roles in energy metabolism, the primary function of NAD⁺-consuming enzymes is to utilize NAD⁺. These include the sirtuin family of deacetylases (SIRTs), poly(ADP-ribose) polymerases (PARPs), and cyclic ADP-ribose (cADPR) synthases (CD157 and CD38). Among these, CD38 and PARP1 are the most significant intracellular consumers of NAD⁺.
As a common substrate for these key enzymes, NAD⁺ participates in numerous vital biological processes. Below, we focus on how two major enzyme families—SIRTs and PARPs—regulate autophagy.
2.1 SIRT Family: Regulating Autophagy Through Deacetylation
SIRT1, a member of the sirtuin family, is primarily localized in the nucleus and has the closest association with autophagy regulation.
Previous studies have shown that SIRT1 promotes autophagy by post-translationally modifying autophagy-initiating proteins and activating autophagy-related transcription factors in a deacetylation-dependent manner.
Numerous studies also emphasize that mitochondrial quality control (executed via mitophagy) is linked to SIRT1 activity.
SIRT3, SIRT4, and SIRT5—localized within mitochondria—are well-suited to regulate autophagy through metabolism and NAD⁺ consumption. These sirtuins coordinate metabolic responses to oxidative stress and fluctuating energy demands by directly deacetylating mitochondrial proteins, a process associated with many age-related diseases.
Mitochondrial SIRTs are also involved in autophagy activation. Research suggests their regulation of autophagy is mostly a secondary result of metabolic modulation. However, it remains unclear whether and how NAD⁺ levels—especially mitochondrial NAD⁺ levels—integrate into the dual roles of these enzymes in autophagy.
Figure Note: Biological processes involving the sirtuin family
2.2 PARP Family: Dual Roles in Autophagy Depending on Activation Level
PARPs sense DNA damage and play roles in DNA repair and other cellular processes by catalyzing the transfer of ADP-ribose (derived from NAD⁺ hydrolysis) to target proteins.
PARP1 can promote cytoprotective autophagy in response to DNA damage and short-term metabolic collapse. However, the same study showed that excessive exposure to reactive oxygen species (ROS) induces overactivation of PARP1 and subsequent cell death.
Whether PARP1 activity leads to cytoprotective autophagy or parthanatos (PARP1-dependent cell death) depends on the specific threshold of ROS exposure: Mild PARP activation promotes cytoprotective autophagy, while sustained PARP activation causes NAD⁺ depletion and cell death.
2.3 SARM1: Linking NAD Consumption to Mitophagy
SARM1 (sterile alpha and TIR motif-containing protein 1) is a key driver of axonal degeneration—a pathological feature of age-related neurodegenerative diseases.
Constitutively active SARM1 exerts its pro-neurodegenerative effects through intrinsic NAD⁺-consuming enzyme activity, catalyzing NAD⁺ depletion that leads to metabolic failure and axonal fragmentation.
While the mechanism by which SARM1 participates in autophagy remains unclear, studies have revealed a connection between SARM1 and mitophagy: SARM1 binds to and stabilizes PINK1 (PTEN-induced kinase 1) on the surface of damaged mitochondria, thereby activating PINK1/Parkin-dependent mitophagy—a process that aids in SARM1 self-clearance.
Research on NAD as a regulator of autophagy is relatively thorough: Different types of NAD⁺-consuming enzymes regulate autophagy by participating in NAD⁺ metabolism. Conversely, autophagy can also influence NAD homeostasis.
3. Autophagy Regulates NAD Metabolism
Although autophagy plays a role in nucleotide homeostasis, few studies have investigated whether autophagy supports cell survival by directly regulating NAD levels.
3.1 Autophagy Deficiency Triggers NAD Depletion
In a study of autophagy-deficient mouse embryonic fibroblasts, significant NAD⁺ depletion was observed—indicating that autophagy deficiency exhausts the total NAD pool.
This NAD depletion is driven by a cascade of events: Loss of mitophagy → Increased mitochondrial ROS → DNA damage → Overactivation of NAD⁺-consuming enzymes (e.g., PARPs, SIRTs) → Uncontrolled NAD⁺ consumption →ultimately cell apoptosis.
In another study, autophagy-deficient human neurons differentiated from autophagy-deficient human embryonic stem cells exhibited basal NAD depletion and apoptosis. Notably, increasing NAD levels with NAM (nicotinamide), NR (nicotinamide riboside), or NMN (nicotinamide mononucleotide) improved viability in autophagy-deficient models.
For example, neurons differentiated from induced pluripotent stem cells of patients with Niemann-Pick type C1 disease (characterized by severe neurodegeneration with autophagic dysfunction) showed NAD⁺/NADH depletion and apoptosis—defects that were significantly ameliorated by NAD supplementation.
These findings suggest that increasing total NAD levels may offer a potential treatment for diseases associated with impaired autophagy.
3.2 Autophagy-Inducing Drugs Impact NAD Metabolism
Similar to the autophagy-deficient models described above, well-known anti-aging drugs (e.g., rapamycin, resveratrol, metformin) that promote autophagic flux can be used to explore autophagy’s role in NAD⁺/NADH homeostasis.
Experiments have shown that these autophagy-inducing anti-aging drugs significantly alter NAD metabolism in vitro:
- Rapamycin may influence the aging process by increasing NAD⁺ availability, which can be utilized by NAD⁺-consuming enzymes to maintain healthy metabolic status. In a study of immortalized mice, rapamycin reduced NADH levels in mouse myoblasts, increased the NAD⁺/NADH ratio, and shifted the NAD⁺/NADH redox state toward oxidation (NAD⁺) in the muscles of aged mice.
- Resveratrol, an autophagy inducer, stimulated NADH oxidation via mitochondrial complex I, increasing the mitochondrial NAD⁺/NADH ratio in mouse livers and human hepatocellular carcinoma HepG2 cell lines.
- Metformin also altered the mitochondrial NAD⁺/NADH state in a biphasic manner: In mouse hepatocytes, low-dose treatment shifted the redox state toward oxidized NAD⁺, while high-dose metformin shifted the mitochondrial NAD pool toward a reduced state.
Despite their involvement in longevity-related molecular pathways, it remains unclear whether these drugs’ effects on NAD metabolism are (or are partially) mediated by enhanced autophagy.
3.3 Autophagic Degradation of NAD⁺-Consuming Enzymes
While NAD⁺-consuming enzymes regulate autophagy, the autophagic degradation of these key enzymes represents another layer of autophagy’s impact on NAD homeostasis. Beyond their roles in autophagy regulation, SIRTs are also subject to autophagic degradation in potential feedback loops. During oxidative stress, autophagy degrades all human SIRT proteins.
Reports indicate that SIRT1 undergoes autophagic degradation only in senescent cells—a process dependent on the SIRT1-LC3 interaction. Whether autophagy also selectively degrades other NAD⁺-consuming enzymes remains to be studied, which would provide deeper insights into the potential molecular effects of exogenous NAD supplementation therapies.
References
[1] Wilson, N., Kataura, T., Korsgen, M. E., Sun, C., Sarkar, S., & Korolchuk, V. I. (2023). The autophagy–NAD axis in longevity and disease. Trends in Cell Biology. https://doi.org/10.1016/j.tcb.2023.02.004