Highlights

  • A micro RNA – a tiny molecule that can bind RNAs to modulate protein synthesis – called​​ miR-146a impedes the anti-aging effect of a cell survival pathway related to metformin, NAD+, and sirtuins.
  • Metformin stops miR-146a from repressing NAMPT, the enzyme that leads to the production of NAD+.
  • These findings suggest that inhibiting miR-146a could help slow aging and treat aging-related diseases.

A rapidly developing aging theory called “metabolic imbalance theory” states that the ability of a cell to maintain metabolic balance when challenged with stress can slow the aging process. Using this theory as an experimental framework, backed by animal evidence showing that caloric restriction delays aging and prolongs lifespan, relevant molecules and signaling pathways have been discovered. Among them are the AMP-activated protein kinase (AMPK) and nicotinamide adenine dinucleotide (NAD+)-dependent sirtuin pathways, which significantly influence the aging process. However, the relationship between the AMPK and sirtuin pathways has not been fully elucidated.

Published in Signal Transduction and Targeted Therapy on March 4, 2022, research from Sichuan University shows that a tiny RNA molecule called a microRNA (miRNA) works as a cog between the AMPK, NAD+, and sirtuin pathways to affect aging. miRNA-146a works by blocking the enzyme NAMPT, which generates the NAD+ precursor NMN. The activity of miRNA-146a  increases with senescence – an age-related process in which cells no longer grow and replicate. Gong and colleagues show that a notable AMPK activator metformin hinders miR-146a activity, which derepresses NAMPT to promote increases in NAD+ levels and sirtuin activity. The authors conclude, “​​​​Our understanding of the molecular connections between AMPK and sirtuin and provide new insight into miRNA-mediated NAD+/sirtuin regulation and an intervention point for the prevention of aging and age-related diseases.”

Are Metformin and NAD+ Levels Linked?

If you reach back into the caverns of your memory into the days of high school biology, you may be able to recall that NAD+ and perhaps NADH (reduced form of NAD+) sit at the helm of many indispensable cell processes, notably cellular metabolism. NAD+ fuels some of its many essential cell functions, including DNA repair and maintenance,  by enabling the activation of the sirtuin family of enzymes, which have been implicated in anti-aging. Both NAD+ synthesis and sirtuin activity are regulated by an enzyme called nicotinamide phosphoribosyltransferase (NAMPT), which produces NMN – a direct precursor to NAD+.

Previous studies have shown that NAMPT levels are reduced in senescent cells. This isn’t good for senescent cells because elevated NAMPT levels increase cell lifespan, making NAMPT important foraging.  Additionally, recent studies show that NAMPT can be regulated by miRNAs under various physiological and pathological conditions. Another molecule, AMPK exerts an anti-aging effect by activating NAD+ and sirtuins. Thus, one hypothesis is that AMPK can increase the levels of NAD+ by increasing the levels of NAMPT. However, the link between AMPK and NAMPT to drive activation of NAD+ and sirtuins has remained elusive.

A MicroRNA Impedes Metformin’s Anti-aging Effect by Inhibiting NAD+ Levels

The AMPK linked to NAMPT hypothesis prompted Gong and colleagues to examine miRNAs that may be implicated in AMPK-driven NAMPT and NAD+/sirtuin activation.​​To establish a connection between miRNAs and AMPK activation in senescence, the Sichuan University researchers used a miRNA screening technique to find which miRNAs are differentially altered in senescent cells and AMPK activated cells. AMPK was activated by a diabetes drug called metformin. The miRNA screening identified miR-146a as being active in senescent cells, making it a potential link between AMPK and NAMPT.

In another experiment, miR-146a was also shown to be increased in the lung tissue of prematurely aged (progeria) mice, but miR-146a was reduced by metformin treatment. Metformin played a protective role against aging characteristics, such as weakened physical power (running endurance and grip force), in the aged mice, but increasing levels of miR-146a blocked the protective effect of metformin. miR-146a levels were also elevated in the liver, lung, and heart tissues of naturally aged mice, which was accompanied by aging-related characteristics, such as organ scarring and heart defects. Collectively, these findings suggest that increased levels of miR-146a are associated with aging or aging-related disease, but can be suppressed by metformin. Moreover, this data suggest that miR-146a could function as a pro-aging factor.


(Gong et al., 2022 | Signal Transduction and Targeted Therapy) miR-146a blocks the effect of metformin on the alleviation of aging in mice. Mice treated with D-gal to induce premature aging show reduced physical strength as measured by (top left) time to exhaustion and (top middle) grip strength. These mice also had (top right) increased levels of senescence (p16), and (bottom) increased levels of lung scarring (fibrosis). Metformin treatment reversed these conditions of premature aging: recovering time before exhaustion and grip strength while blocking senescence and lung scarring. Increasing levels of miR-146a blocked the effects of metformin.

Next, Gong and colleagues found that miR-146a directly targets NAMPT mRNA, blocking NAMPT from becoming a functional enzyme. This ultimately decreased cellular NAD+ levels and subsequently suppressed sirtuin-mediated anti-aging. These results suggest that AMPK increases NAMPT and NAD+ via repressing miR-146a, which promotes the sirtuin-driven anti-aging process. This study further elucidates the interplay between AMPK signaling and miRNA-mediated NAMPT activity in the anti-aging process.

(Gong et al., 2022 | Signal Transduction and Targeted Therapy) miR-146a reduces the effect of metformin on NAMPT, NAD+, and SIRT1 levels. Mice treated with D-gal to induce premature aging show reduced (far left) sirtuin (SIRT1) and (middle left) NAD+ levels. Metformin treatment reversed the actions of premature aging on sirtuin and NAD+ levels. Increasing levels of miR-146a with a mimic blocked the effects of metformin. Levels of Nampt mRNA were (middle right) reduced by a molecule that mimicked miR-146a and (far right) increased by a molecule that blocked miR-146a.

What’s the Difference Between Metformin and NMN?

Together with the data involving changes in NAD+ levels and sirtuin activity, this study expands our understanding of miRNA-related mechanisms involved in the aging process and suggests that miR-146a may be of value for slowing aging and treating aging-related diseases. ​​The mutual inhibitory relationship between miR-146a and AMPK enriches our understanding of the molecular connections between AMPK and sirtuins and provides new insight into miRNA-mediated NAD+/sirtuin regulation and an intervention point for the prevention of aging and age-related diseases.

This research raises the question of whether it’s better to take metformin or NMN to protect against or treat aging-related conditions. While metformin has an effect on NAD+ levels by influencing the production of NMN, metformin has effects on other pathways to influence inflammation, metabolism, and regulation of insulin and glucose sensitivity. The question becomes whether cell types throughout the body have different uptake potentials for metformin and NMN. This understanding will be crucial for informing whether metformin and NMN can work synergistically to affect aging and age-related conditions.