Weight Loss, Health, Longevity: Activating AMPK is Key (Part 1)

For readers who follow anti-aging research, AMPK is likely a familiar term, even if you can’t recall its full name. We often mention that the anti-aging mechanism behind certain measures or substances is activating AMPK. But what exactly is AMPK? Why does activating it promote anti-aging? Beyond classic methods like calorie restriction, exercise, the "old wonder drug" aspirin, and the "new wonder drug" metformin, what other ways can activate it? Today, we’ll dive into AMPK and its signaling pathways. It might get a bit technical, but it’s a crucial piece of the anti-aging puzzle—enjoy!

AMPK stands for AMP-activated protein kinase. It exists in most eukaryotic cells, widely expressed from yeast, nematodes, and plants to mammals. Yeast’s Pasteur effect, nematodes’ survival and longevity under starvation, plants’ circadian light responses, and animal cells’ metabolic stress or hypoxia—all are closely linked to AMPK function.

As a serine/threonine protein kinase, AMPK senses changes in cellular energy metabolism and maintains energy balance by regulating multiple aspects of cellular material metabolism. When intracellular ATP levels drop, AMPK reduces ATP consumption by inhibiting glycogen, fat, and cholesterol synthesis; simultaneously, it increases ATP production by promoting fatty acid oxidation and glucose transport. Thus, AMPK is regarded as the switch regulating cellular energy metabolism.

AMPK is tightly regulated by various metabolic signals through multiple mechanisms. Overall, the AMPK pathway is activated under conditions of low nutrients and low energy.

A decrease in ATP is always accompanied by increased AMP and ADP levels, maintained by adenylate kinase via the reaction 2ADP → ATP + AMP. Under normal physiological conditions, the intracellular ATP/AMP ratio in eukaryotes is approximately 100:1, with very low AMP levels, leaving AMPK inactive.

AMPK is activated when:
① Cells face glucose or oxygen deprivation;
② Cellular respiratory chain or ATP synthesis is inhibited by drugs;
③ ATP consumption increases (e.g., during muscle contraction from exercise).

These metabolic stresses—whether inhibiting ATP production or accelerating ATP consumption—reduce intracellular ATP levels and increase the AMP/ATP ratio, thereby activating AMPK[3] (some studies use ADP/ATP ratio).

The rise in AMP/ATP ratio triggers LKB1-mediated phosphorylation of AMPK. Adenylate kinase amplifies this ratio; AMP binds to the γ subunit of AMPK, enhancing phosphorylation by upstream kinase LKB1, reducing dephosphorylation by phosphatases, and inducing further allosteric activation of phosphorylated AMPK. These three mechanisms ensure AMPK’s high sensitivity to even small increases in AMP, boosting its activity by over 100-fold.

After cellular stress subsides, AMP dissociates from AMPK, losing allosteric activation and inactivating AMPK[4].

Once activated, AMPK primarily regulates four major metabolic processes in mammals, as well as autophagy and mitochondrial homeostasis—covering almost all physiological activities of living organisms.

AMPK’s downstream targets mainly regulate energy metabolism, shifting the body’s energy balance toward ATP production. Thus, AMPK activation weakens anabolic and energy-consuming processes while enhancing ATP-generating and catabolic activities.

① Regulation of fat metabolism:
Fatty acid oxidation is a key energy source for muscle tissue—normally, 60%-80% of cardiac energy comes from fatty acids. During myocardial ischemia, intense exercise, or skeletal muscle electrical stimulation, AMPK is activated. It phosphorylates and inhibits acetyl-CoA carboxylase (ACC), reducing malonyl-CoA synthesis, which relieves inhibition of CPT-1 and enhances fatty acid oxidation. This regulatory pathway is called the "AMPK-ACC-malonyl-CoA axis."

Additionally, since ACC is the rate-limiting enzyme for fat synthesis in the liver and adipocytes, AMPK activation not only increases fatty acid oxidation but also inhibits fat synthesis[5].

② Regulation of glucose metabolism:
AMPK is activated during skeletal muscle contraction, regulating glucose uptake by promoting glucose transporter translocation and increasing transporter expression. Moreover, Halse’s study on isolated skeletal muscle cells found that AMPK also enhances glycolysis[6].

③ Regulation of cholesterol metabolism:
HMG-CoA reductase is the key enzyme in cholesterol synthesis. When ATP levels drop, activated AMPK phosphorylates HMG-CoA reductase at Ser871, inactivating it and reducing cholesterol synthesis.

Autophagy primarily degrades large cellular structures (e.g., via the ubiquitin-proteasome system) and recycles amino acids to maintain cellular function, enabling cell survival under starvation.

The LKB1-AMPK pathway is the main pathway activated during energy stress. When intracellular energy declines, LKB1 phosphorylates and activates AMPK. Activated AMPK inhibits mTORC1 activity by directly phosphorylating the tumor suppressor TSC2 and RAPTOR (a key subunit of mTORC1), thereby inducing autophagy. Meanwhile, AMPK directly activates ULK1/2 and Beclin-1 VPS34, promoting early PAS formation in autophagy[7].

Studies show AMPK is a key regulator of mitochondrial synthesis. Activated AMPK activates mitophagy via ULK1 to clear dysfunctional mitochondria; simultaneously, it transcriptionally regulates the production of new mitochondria through PGC-1α. This dual regulation replaces defective mitochondria with new functional ones, achieving mitochondrial "purification" and playing a key role in many physiological and pathological processes[8].

This concludes our brief introduction to AMPK’s structure, activity regulation, and functions. Beyond being a cellular energy switch, AMPK is critical for countless potential anti-aging drugs. Tomorrow, we’ll explore whether activating AMPK can truly delay aging—stay tuned!

As early as 1930, scientists found that AMP participates in glycogen phosphorylation but attributed it to contaminants. It wasn’t until they discovered that the key enzymes inactivating fatty acid and cholesterol synthesis (acetyl-CoA carboxylase and HMG-CoA reductase) are downstream targets of the same molecule that they recognized AMP activates a single protein.

In 1973, Berg and Carlson reported a protein kinase related to HMG-CoA reductase. Subsequent studies showed its activity is regulated by AMP. In 1987, Professor Hardie from the University of Dundee’s School of Life Sciences first identified and named it AMPK, publishing the findings in European Journal of Biochemistry. In 2012, he further discovered that salicylate (aspirin’s metabolite) directly activates the AMPK-β subunit, revealing aspirin’s mechanism of action in the human body[1,2].

Human AMPK is a heterotrimeric protein composed of α, β, and γ subunits. The α subunit contains the main catalytic domain, transferring phosphate from ATP to target proteins. The β and γ subunits are regulatory: the β subunit acts as a scaffold, with α and γ subunits embedded in its KIS and ASC regions; the γ subunit likely links AMPK to AMP.

There are 2 isoforms for α (α1, α2) and β (β1, β2), and 3 for γ (γ1, γ2, γ3), resulting in 12 possible combinations. Isoform distribution varies across species (e.g., humans vs. mice) and tissues (e.g., cardiac vs. skeletal muscle). For example, γ2 is expressed in adult cardiac and skeletal muscle, while γ3 is specific to skeletal muscle. Understanding isoform distribution is crucial for designing AMPK activators or inhibitors.

References
[1] Kemp, B.E., et al., Dealing with energy demand: the AMP-activated protein kinase. (0968-0004 (Print)).
[2] O'Neill, L.A. and D.G. Hardie, Metabolism of inflammation limited by AMPK and pseudo-starvation. (1476-4687 (Electronic)).
[3] Hwang, J.T., et al., Resveratrol Induces Apoptosis in Chemoresistant Cancer Cells via Modulation of AMPK Signaling Pathway. Annals of the New York Academy of Sciences, 2010. 1095(1): p. 441-448.
[4] Hardie, D.G., New roles for the LKB1→AMPK pathway. Current Opinion in Cell Biology, 2005. 17(2): p. 167-173.
[5] Sambandam, N. and G.D. Lopaschuk, AMP-activated protein kinase (AMPK) control of fatty acid and glucose metabolism in the ischemic heart. (0163-7827 (Print)).
[6] Halse, R., et al., Regulation of Glycogen Synthase by Glucose and Glycogen: A Possible Role for AMP-Activated Protein Kinase. Diabetes, 2003. 52(1): p. 9-15.
[7] Di Malta C, Siciliano D, Calcagni A, et al. Transcriptional activation of RagD GTPase controls mTORC1 and promotes cancer growth[J]. Science, 2017,356(6343):1188-1192.
[8] Mihaylova, M.M. and R.J. Shaw, The AMPK signalling pathway coordinates cell growth, autophagy and metabolism. (1476-4679 (Electronic)).