AMPK: The Energy Sensor at the Crossroads of Aging and Cancer

Abstract

AMP-activated protein kinase (AMPK) is a protein kinase that plays versatile roles in response to a variety of physiological stresses, including glucose deprivation, hypoxia, and ischemia. As a kinase with pleiotropic functions, it plays a complex role in tumor progression, exhibiting both tumor-promoting and tumor-suppressing activities. On one hand, AMPK enhances cancer cell proliferation and survival, promotes cancer metastasis, and impairs anti-tumor immunity. On the other hand, AMPK inhibits cancer cell growth and survival and stimulates immune responses in a context-dependent manner. Apart from these functions, AMPK plays a key role in orchestrating aging and aging-related disorders, including cardiovascular diseases (CVD), Osteoarthritis (OA), and Diabetes. In this review article, we summarized the functions of AMPK pathway based on its oncogenic and tumor-suppressive roles and highlighted the importance of AMPK pathway in regulating cellular aging. We also spotlighted the significant role of various signaling pathways, activators, and inhibitors of AMPK in serving as therapeutic strategies for anti-cancer and anti-aging therapy.

1. Introduction

The global population of 60 years and older is rapidly increasing, from 1 billion in 2020 to an estimated 1.4 billion in 2030 and 2.1 billion by 2050 [1]. This poses a significant challenge for every country in providing adequate care and social security to this growing aged population [1]. Aging can be caused by the natural physiological changes that lead to senescence, a loss of biological functioning, the organism’s ability to respond to metabolic stress, and other inevitable responses. In 2013, López-Otín and colleagues listed nine hallmarks of aging, including genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication to provide the relevant guidance for future research [2]. Because these hallmarks are insufficient to serve as a causal paradigm for aging, another group has recently suggested additional characteristics, which include compromised autophagy, dysregulation in RNA splicing, inflammation, loss of cytoskeleton integrity, and disturbance of the microbiome [3]. These novel aging characteristics may enhance our knowledge of aging and diseases related to age and offer insights into diagnostic and therapeutic studies to benefit aged life. Aging, as an identifiable risk factor, has been associated with several human diseases, including cardiovascular diseases, neurodegenerative diseases, lipid metabolic disorders, various types of cancers, and unknown aging-related diseases [4].

Tumorigenesis is the process by which normal cells convert into cancer cells by preventing them from executing apoptosis, a programmed cell death, and causing them to proliferate uncontrollably. Cancer cells acquire particular capabilities, such as sustaining proliferative signaling, evading growth suppressors, resisting cell death, enabling replicative immortality, inducing angiogenesis, and activating invasion and metastasis during their development [5]. Aging can cause cancer or promote its progression through various mechanisms, including the accumulation of mutations, epigenetic alterations, a failure to control chronic inflammation, changes in the gut microbiome, compromised tissue repair, and disabled autophagy, ultimately contributing to the development and progression of cancer [6]. According to a report from the National Cancer Institute (NCI), cancer cases were significantly elevated among people older than 60 years, with an incidence rate of 1,000 cases per 100,000 people, compared to 350 cases per 100,000 people in the 45–49 age group, indicating a higher cancer incidence among older adults [7]. Aging is not only a risk factor for cancer but also for various human diseases mentioned above. During the aging process, various hallmarks of aging are induced, leading to the accumulation of gene mutations and alterations of gene expression and gene functions, which collectively contribute to the development of aging-related disorders.

AMPK is a stress-responsible kinase that maintains cellular homeostasis by engaging in energy production pathways while inhibiting energy consumption pathways to balance energy in the cells. It modulates a variety of cellular phenomena, such as autophagy, tissue anti-stress capability, oxidative stress, endoplasmic reticulum stress, inflammatory response suppression, and aging [8, 9]. As AMPK activation decreases during the progression of age [10], contributing to age-related complications such as cancer and other diseases, activating AMPK has been shown to offer a promising therapy for aging and its consequential diseases [10, 11].

In this review, we first described AMPK’s dual role in cancer, elaborating on recent findings for its oncogenic and tumor-suppressive activities. Next, we provided information on the functions and molecular mechanisms of AMPK in aging, shedding light on its complex actions in these processes. Furthermore, we discussed various signaling pathways, activators, and inhibitors of AMPK that can be used as therapeutic targets of AMPK in cancer and aging.

2. AMPK in Tumorigenesis

2.1. An overview of AMPK

AMPK is a heterotrimeric protein complex consisting of three subunits: AMPKα, AMPKβ, and AMPKγ. AMPKα is expressed in two isoforms, AMPKα1 and AMPKα2, and these isoforms are encoded by the genes PRKAA1 and PRKAA2, respectively. Similarly, AMPKβ is also expressed in two isoforms, AMPKβ1 and AMPKβ2, and these isoforms are encoded by the genes PRKAB1 and PRKAB2, respectively. However, AMPKγ is expressed in three isoforms, AMPKγ1, AMPKγ2, and AMPKγ3, which are encoded by the genes PRKAG1, PRKAG2, and PRKAG3 respectively (Figure-1A) [12]. The various AMPK isoforms can form distinct complexes, leading to diverse functional roles and overlapping functions in regulating cellular energy balance, metabolism, exercise adaptation, cancer, and aging [9]. The specific functions of each isoform largely depend on their cell and tissue-specific expression patterns and localization. For instance, AMPKα1, β1, and γ1 are widely expressed and involved in general cellular energy regulation, whereas AMPKα2 and γ3 are predominantly expressed in skeletal muscle and heart, playing a crucial role in exercise-induced glucose uptake [13]. Similarly, the role of AMPK isoforms in cancer is context-dependent, with specific isoforms being more critical in certain cancer types. Furthermore, research has shown that the AMPKα1 isoform, but not AMPKα2, impacts cognitive and synaptic functions related to aging [14]. Overall, the biological significance of these different isoforms is still being explored and requires further in-depth investigation. AMPK activation is regulated by three upstream kinases, namely Liver Kinase B1 (LKB1) [15], Calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) [16], and Transforming growth factor β (TGFβ) activated kinase 1 (TAK1) [17], which phosphorylate AMPKα1/α2 at Thr183/Thr172, thereby activating it. Conversely, phosphatases such as protein phosphatase 2A (PP2A), protein phosphatase 2C (PP2C), and Mg2⁺/Mn2⁺ dependent protein phosphatase 1E (PPM1E) dephosphorylate these sites (Figure-1B) [18], resulting in AMPK inactivation. Since AMPK is a cellular energy sensor that maintains the cellular energy status by promoting energy-synthesizing pathways like β-oxidation and suppressing energy-consuming pathways like fatty acid synthesis, it affects cancer cell properties as a tumor suppressor or oncogene largely dependent on the contexts [9]. Hence, in separate sections, we have discussed AMPK’s dual roles as a tumor suppressor and oncogene.

Figure-1: Functional domains of AMPK and its regulation.

A: AMPKα is expressed as AMPKα1 and AMPKα2, and these isoforms have Kinase Domain (KD), autoinhibitory domain (AID), α-regulatory subunit interacting motifs (α-RIM) in linker peptide (α-linker) and α-subunit carboxy-terminal domain (α-CTD) which binds to the β subunit. AMPKβ is expressed as AMPKβ1 and AMPKβ2, and these isoforms have carbohydrate-binding module (CBM), β-subunit C-terminal domain (β-CTD), which binds to the α and γ subunits. AMPKγ is expressed as AMPKγ1, AMPKγ2, and AMPKγ3, having tandem repeats of CBS motifs (CBS1, CBS2, CBS3 and CBS4). AMPK forms a functional heterotrimeric protein complex consisting of three subunits: AMPKα, AMPKβ, and AMPKγ.B: AMPK is activated by Liver Kinase B1 (LKB1), Calcium/calmodulin-dependent protein kinase kinaseβ (CaMKKβ), and TGFβ-activated kinase 1 (TAK1), which phosphorylates AMPKα1/α2 at Thr183/Thr172. Phosphatases such as protein phosphatase 2A (PP2A), protein phosphatase 2C (PP2C), and Mg2⁺/Mn2⁺ dependent protein phosphatase 1E (PPM1E) dephosphorylate these sites thereby inactivates AMPK.

2.2. AMPK as an oncogene

Cancer cell survival depends on various factors and tumor microenvironment. Cancer cells adopt specific processes to cope with metabolic stresses or unfavorable conditions to maintain their survival. Cancer cells exhibit various adaptations to survive under adverse conditions, including metabolic reprogramming, enhanced mitochondrial biogenesis, autophagy, overexpression of specific oncogenes, downregulation of tumor suppressor genes, evasion of immune responses, and immune suppression [19]. AMPK’s function in cancer cells is paradoxical, displaying a context-dependent dual role that can have both tumor-suppressive and tumor-promoting effects. Some studies explain its oncogenic role, whereas others depict its tumor-suppressive role [8, 9]. This review updates the most recent information about its oncogenic (Figure-2) and tumor-suppressive roles (Figure-3) in various cance types.

Figure-2: Role of AMPK as an oncogene and its regulation in tumorigenesis.

AMPK regulates various cellular signaling pathways that support tumor growth and metastasis by modulating metabolic pathways, including glycolysis, TCA cycle, glutaminolysis, lipolysis, and cellular phenomena such as autophagy, anti-ferroptosis, and anti-host immunity, leading to tumor immune evasion. AMPK’s oncogenic effects are mediated by its downstream targets, including mTORC1, NF-κB, ZDHHC8, INF2, and PTEN. Post-translational modifications (PTMs) on upstream kinases that phosphorylate and activate AMPK and PTMs directly on AMPK modulate its context-dependent oncogenic role.

Figure-3: AMPK as a tumor suppressor and its role in tumorigenesis.

AMPK exerts anti-tumor effects by phosphorylating diverse substrates, thereby regulating key cellular processes such as mitochondrial dynamics, autophagy, apoptosis, cancer stem cell maintenance, chromatin remodeling, and anti-tumor immune responses, ultimately suppressing tumor progression.

AMPK significantly promotes cancer cell migration by inducing metabolic rewiring of certain pathways like the tricarboxylic acid (TCA) cycle and Glutaminolysis. Under metabolic stress conditions, AMPK phosphorylates Pyruvate Dehydrogenase E1 Subunit Alpha (PDHA), one of the components in the Pyruvate dehydrogenase complex (PDHC), and then activates the PDHC to promote the TCA cycle. This increased TCA cycle generates energy and promotes cell adaptation to the metastatic microenvironments composed of diverse metabolic and oxidative stresses [20, 21]. Research has shown that AMPK-mediated upregulation of Solute carrier family 1 member 5 (SLC1A5) and Glutaminase (GLS) genes, driven by Hematopoietic PBX-Interacting Protein (HPIP)/Avian Myelocytomatosis Viral Oncogene Homolog (MYC) transcriptional activity under metabolic stress, facilitates glutaminolysis in cancer cells by increasing the expression of these genes, exemplifying the metabolic reprogramming strategies employed by cancer cells to maintain their growth and survival [22]. Cancer cells adapt under metabolic stress conditions, such as glutamine deprivation, by shifting their metabolic reliance to alternative sources, including lipolysis. This adaptive response is mediated by AMPK, which phosphorylates and activates choline kinase alpha 2 (CHKα2) at serine 279, enabling the cell to survive under these challenging conditions [23, 24]. Under metabolic stress, AMPK is crucial in maintaining energy homeostasis by suppressing energy-consuming and inducing energy-producing pathways. For instance, AMPK regulates nucleotide synthesis, an energy-investing process, by phosphorylating phosphoribosyl pyrophosphate synthetase 1 (PRPS1) at S180 and PRPS2 at S183. This phosphorylation event promotes the conversion of PRPS hexamers to monomers, thereby inhibiting PRPS1/2 activity and nucleotide synthesis [25]. AMPK phosphorylates and activates 6-phosphofructo-2-kinase (PFK-2) at Ser466, inducing glycolysis during ischemia and oxidative stress conditions, which can support the high energy demands of rapidly proliferating cells, including cancer cells, thereby potentially promoting oncogenic actions [26, 27]. In addition, cancer cells respond to metabolic stress by activating AMPK, which in turn stabilizes and transactivates HIF1α, leading to the upregulation of key enzymes involved in the de novo serine synthesis pathway, including phosphoglycerate dehydrogenase (PHGDH), phosphoserine aminotransferase 1 (PSAT1), and phosphoserine phosphatase (PSPH). This adaptive response enhances glucose uptake and glycolytic gene expression, ultimately fueling the serine synthesis pathway [28]. In another study, AMPK was found to directly phosphorylate Zinc Finger DHHC-Type Palmitoyltransferase 8 (ZDHHC8) at Ser299, leading to increased interaction between ZDHHC8 and solute carrier family 7 member 11 (SLC7A11), resulting in S-palmitoylation and deubiquitination of Solute Carrier Family 7 Member 11 (SLC7A11), and ultimately inducing ferroptosis resistance during glioblastoma multiforme (GBM) tumorigenesis [29]. In human glioblastomas (GBMs), overexpression of hypoxanthine phosphoribosyl transferase 1 (HPRT1) enables the conversion of AICA, a metabolic byproduct of Temozolomide (TMZ) drug to AICAR, thereby activating AMPK and conferring resistance to TMZ therapy [30].

AMPK also shows its oncogenic functions by regulating mitochondrial dynamics. AMPK-mediated phosphorylation of the Mitochondrial fission factor (MFF) under metabolic stress increases mitochondrial fission [31]. Research has revealed that inositol, a metabolite produced by Inositol Monophosphatase (IMPA1/2), plays a dual role as both a secondary messenger and a structural component. Notably, inositol also functions as a natural inhibitor of AMPK to regulate the mitochondrial dynamics by phosphorylating the MFF, making it the first identified metabolite to bind and inhibit AMPK activity directly [32]. In endometrial cancer cells under metabolic stress, AMPK phosphorylates the oncogene Inverted Formin 2 (INF2) at Ser1077, leading to actin polymerization and recruitment of Dynamin-Related Protein 1 (DRP1) to Endoplasmic Reticulum (ER). This process promotes mitochondrial fission and induces the oncogenic activity of INF2 through AMPK activation [33].

Autophagy is a vital regulatory mechanism in maintaining cellular homeostasis, which AMPK regulates. While AMPK activation can promote autophagy and contribute to cancer progression in specific contexts, the proto-oncogene Neurensin-2 (NRSN2) activates the AMPK/Unc-51-like autophagy-activating kinases 1 (ULK1) pathway, leading to autophagy and the progression of Human papillomavirus (HPV)-infected laryngeal carcinoma (LC) [34]. Phosphorylation of acetyl-CoA synthetase 2 (ACSS2) at S659 by AMPK triggers its nuclear translocation and enhances acetate availability for histone acetylation leading to promoting the transcription of autophagy and lysosomal genes. This regulatory axis is crucial for maintaining energy homeostasis and modulating tumor development under metabolic stress conditions [35]. Enhanced autophagy via AMPK pathway can contribute to drug resistance in specific cancers. Recent studies have shown that calcium-binding protein 39 (CAB39) directly interacts with LKB1, leading to AMPK activation and subsequent autophagy and promoting cisplatin resistance in invasive bladder cancer [36]. In another study on hepatocellular carcinoma (HCC), nuclear receptor subfamily 0 Group B member (NR0B1) was found to induce autophagy by activating AMPK pathway, leading to sorafenib resistance [37].

Most cancer cells express a wide range of oncogenes, including MYC, Kirsten rat sarcoma viral oncogene homolog (KRAS), epidermal growth factor receptor (EGFR), Hypoxia-inducible factor 1-alpha (HIF1α), and AMPK. This expression impacts various cellular pathways, including cell proliferation and phenotypic plasticity, ultimately affecting the tumor microenvironments and metastasis [38–40]. Tripartite motif-containing protein 11 (TRIM11) increases AMPK expression and activation in non-small cell lung cancer (NSCLC) cells to enhance survival and cell proliferation by suppressing ferroptosis [41]. Notably, AMPK expression has been reported to be elevated in several cancer types, including breast, lung, and esophageal cancers [9, 18, 22]. AMPK activation leads to reduced expression of tumor suppressor genes such as tuberous sclerosis 2 (TSC2), a negative regulator of the mechanistic target of rapamycin kinase (mTOR) [42]. Research has shown that phosphatase and tensin homolog (PTEN)-deficient prostate cancer cells utilize AMPK-dependent macropinocytosis to survive and proliferate under nutrient-stress conditions . Therefore, targeting this pathway by developing AMPK inhibitors holds great promise as a potential therapeutic strategy.

Inhibition of the host tumor immune response is one of the main obstacles to immunotherapy. Recent reports suggest that AMPK acts against host immunity by maintaining the protein stability of STIP1 homology and U-Box containing protein 1 (STUB1/CHIP), an E3 ligase required to degrade the forkhead box P3 (FOXP3), one of the well-known tumor immune suppressors in tumor-infiltrating Treg cells [43, 44]. AMPK also regulates anti-tumor immunity by downregulating the programmed cell death protein 1 (PD-1) by the 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR)/p38 signaling pathway in T cells [45]. The tumor microenvironment comprises various cell types, such as fibroblasts, macrophages, T cells, and other immune cells. Cell polarization is the process in which immune cells adopt specific signals and pathways in response to the challenge of pathogens or tumors. This process is crucial for the immune cells to maintain their designated functions. For example, M1 macrophages exhibit pro-inflammatory and cytotoxic effects on tumor cells, whereas M2 macrophages promote tumor progression by suppressing inflammation and fostering angiogenesis after the cell polarization. Besides maintaining cellular homeostasis, AMPK significantly influences the M2 macrophage polarization to promote cancer [46]. In addition, AMPK can inhibit the activity of anti-tumor immune cells, such as cytotoxic T cells and natural killer cells, to exert its oncogenic functions. Yan Ouyang et al. reported that AMPKα2 expression is higher in liver hepatocellular carcinoma cells, promoting tumor immune escape by reducing the cluster of differentiation 8⁺ (CD8⁺) cells and promoting the CD4⁺ Treg cells [47]. The same study reported that AMPK promotes the production of immune suppressive cytokines, such as TGF-β [47] and Interleukin 10 (IL-10) [48]. AMPK can also inhibit the production of pro-inflammatory cytokines, such as tumor necrosis factor (TNF-α) and Interleukin 1β (IL-1β) [46]. Collectively, AMPK can function as an oncogene to promote tumor growth and progression in specific cellular contexts (Figure-2).

2.3. AMPK as a Tumor Suppressor

Differential expressions of multiple proteins have been shown to influence cancer cell growth and metastasis. In triple-negative breast cancer cells (TNBC), AMPK activation, triggered by metabolic stress or metformin treatment, phosphorylates Snail at Ser11, leading to Snail ubiquitination and degradation by F-box protein 11 (FBXO11) and subsequent migration inhibition [49]. In ovarian cancer cells, the oncogene hepatitis B virus X-associated protein 8 (XTP8) promotes cancer cell growth and metastasis by inhibiting AMPK [50]. AMPK activation by Metformin or LKB1 in lung cancer cells suppresses tumor invasion or metastasis by increasing RNA binding motif single stranded interacting protein 3 (RBMS3) expression [51].

Autophagy-mediated cell death serves as a cancer-suppressive pathway. AMPK is known to induce autophagy-mediated cell death in various cancer types. Recently, reports have suggested that some of the proteins, like phospholipase C-like protein 1 (PLCL1), function as a suppressor of renal cell carcinoma (RCC) progression by activating the AMPK/mTOR pathway, which interacts with decidual protein induced by progesterone (DEPP) and initiates autophagy to induce apoptosis [52]. Pseudokinase mixed lineage kinase domain-like (MLKL) is an essential regulator of AMPK that bridges the AMPK with its phosphatase PPM1B and reduces the AMPK activation to promote HCC progression [53].

AMPK activation executes its tumor-suppressive role by inhibiting cell proliferation and metastasis. AMPK activation depends on its upstream kinases like LKB1, CAMKKβ, and TAK1, whose activation is regulated by diverse post-translational modifications and signaling pathways. Recent reports show that deubiquitinating enzymes are essential regulators of the upstream kinase-like LKB1. BRCA1-associated protein-1 (BAP1), one of the deubiquitinating enzymes for LKB1, inhibits its ubiquitination and degradation, thereby affecting AMPK activation and downstream mTOR activity [54]. AMPK can be regulated through its upstream kinases and downstream substrates. AMPK has been shown to regulate certain ubiquitinating and deubiquitinating enzymes, which play a crucial role in attaching or detaching ubiquitin molecules, resulting in influencing post-translational modifications. In colorectal cancer cells (CRC), AMPK phosphorylates the ubiquitin-specific peptidase 10 (USP10), a deubiquitinating enzyme, and stabilizes the Axin1 for suppressing the Wnt/β-Catenin pathway leading to cancer cell proliferation and self-renewal inhibition [55]. Tripartite motif-containing 22 (TRIM22), an E3 ligase, degrades Nuclear factor erythroid 2-related factor 2 (NRF2) independently of Kelch-like ECH-associated protein 1 (KEAP1), leading to the activation of reactive oxygen species (ROS) mediated AMPK/mTOR/autophagy signaling for autophagic cell death in osteosarcoma [56]. Acetylation is another post-translational modification that regulates cancer cell growth in most tumor cells. Recent reports suggest that Metformin-mediated AMPK activation drives histone H3K9 acetylation dependent on P300/CBP-associated factor (PCAF), leading to chromatin remodeling in cervical cancer cells [57]. This H3K9 methylation leads to the transcriptional upregulation of specific tumor suppressors [57].

Metformin and other drugs can activate AMPK and suppress certain cancers. For example, Aspirin activates AMPK for subsequent NRF2 activation, which then induces the expression of tumor suppressor microRNAs like miR-34a/b/c for colorectal cancer suppression [58]. The mutation in AMPK encoding genes reflects the tumor-suppressive role of AMPK. For example, the PRKAA2 gene (encoding the α2 isoform of the catalytic subunit) is often subject to missense mutations in most cancers, particularly in melanoma and non-melanoma skin cancers. The PRKAA2 gene mutations are accompanied mainly by the tumor suppressor Neurofibromin 1 (NF1). Knockout of PRKAA2 in these cancer cells deficient in NF1 increased anchorage-independent cell growth in vitro and tumor volume in immunodeficient mice in vivo, suggesting the tumor suppressor role of AMPK-α2 [59]. In another study, melanoma cells with mutated AMPKα2 exhibited increased brain metastasis [60].

AMPK-mediated activation of forkhead box O3 (FOXO3a) inhibits paclitaxel-resistant NSCLC cancer stemness. Metformin activates AMPK and suppresses the protein kinase B (PKB, also known as AKT) and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (MEK), which are critical drivers for cancer cell proliferation and survival [61]. AMPK also exerts its tumor suppressive functions by downregulating specific oncogenes or proto-oncogenes. For example, tumor protein D52 (TPD52) is a proto-oncogene highly expressed in various cancers. TPD52 promotes oncogenesis by directly interacting with LKB1 and inhibiting its tumor-suppressive effect by reducing the interaction with AMPK. 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR), an AMPK activator, increases the activation of AMPK by disrupting the interaction between LKB1 and TPD52, leading to apoptosis in prostate cancer cells [62]. Nucleotide-binding oligomerization domain 2 (NOD2), a potent immune activator against various pathogens and cancers, directly binds to the AMPK-LKB1 complex and promotes AMPK activation, which in turn suppresses the mTOR pathway to induce autophagy-mediated cell death in HCC [63, 64]. Interestingly, AMPK expression in cytotoxic T cells and natural killer cells is crucial for its effective anti-tumor immune cell functions. Recent studies indicate that the deficiency of AMPK in CD8⁺ T cells suppresses their anti-tumor activity [65], while AMPK expression is required for the cell viability of these T cells [66]. AMPK exerts its tumor suppressor functions by phosphorylating programmed death ligand-1 (PDL-1) at S195, resulting in PDL-1 degradation through an Endoplasmic reticulum-associated degradation (ERAD) pathway [67, 68]. Mechanistically, phosphorylation of PDL-1 recruits HMG-CoA Reductase Degradation 1 Homolog-1 (HRD-1) as an ERAD E3 ligase of PDL-1 into the PDL-1&ERAD complex, leading to proteasomal degradation of PDL-1 [67, 68]. In another study, under nutrient deprivation conditions, AMPK phosphorylates PDL-1 at S283 and disrupts PDL-1 interaction with CKLF-like MARVEL transmembrane domain containing 4 (CMTM4), leading to PDL-1 degradation [69]. AMPK also phosphorylates enhancer of zeste homolog 2 (EZH2) under metabolic stress conditions, which disrupts the polycomb repressive complex (PRC2) complex and enhances interferon (IFN) and antigen presentation gene expression to help immune-checkpoint blockade therapy, such as anti-cytotoxic T-lymphocyte associated protein 4 (anti-CTLA4) immunotherapy, to combat cancer [69]. AMPK can inhibit the activity of immune stimulatory molecules, such as the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), leading to immune suppression [70]. Interestingly, AMPK can promote innate immunity by directly phosphorylating TANK Binding Kinase 1 (TBK1) at Ser511 [71] and regulating the stimulator of interferon genes protein (STING) pathways, either directly or indirectly [72], depending on cellular contexts. Collectively, AMPK functions as a tumor suppressor dependent on the specific molecular mechanisms and cellular environments involved (Figure-3).

3. AMPK in Cellular aging

AMPK is a well-known kinase that regulates cellular energy status by promoting energy-producing pathways while suppressing energy-consuming pathways, thereby maintaining cellular energy homeostasis [9]. Aging is a process of resilient and long-term changes characterized by a constant decline in functioning and homeostasis, with significant negative consequences for an organism’s healthy life [2, 3]. Recent reports suggest that activation or the expression of AMPK declines with age [73, 74], but the mechanisms underlying AMPK decline and inactivation are currently unclear. We speculate that the decline in AMPK expression and/or activation during aging may be attributed to diverse mechasmins resulting from a complex interplay of diverse factors. For instance, DNA damage and chronic inflammation may disrupt AMPK signaling, hence leading to a reduction in its expression. Epigenetic changes and mutations in AMPK genes or upstream kinases like LKB1 may also impair gene function, leading to decreased AMPK activity. Given that LKB1 regulates AMPK activation and considering the observed decline in LKB1 expression during aging-related complications, such as retinal synaptic dysfunction, we suspect that decline in expression of LKB1, which plays a crucial role in anti-aging mechanisms, may be also involved [75]. Hormonal shifts, such as decreased adiponectin and insulin levels, may also contribute to reduced AMPK activity, with higher adiponectin levels potentially linked to increased longevity [76]. Furthermore, alterations in metabolic pathways leading to changes in specific metabolites, such as inositol, may suppress AMPK activation or expression.

AMPK activation is required to support cell integrity and functions, and compounds or drugs that increase AMPK activation or expression pose a significant role in anti-aging therapy [10]. Increased ROS, defects in mitochondrial function, changes in epigenetic modifications, changes in hormones such as decreased adiponectin, impaired nutrient sensing, gene mutations, and loss of physical activity commonly observed in aged organisms can lead to reduced AMPK activation [10]. This decline in activation or expression is due to abnormal changes at downstream signaling pathways and/or the upstream kinase and phosphatases that regulate the AMPK [77]. As a cellular energy sensor, AMPK is ubiquitously expressed in all mammalian cells, playing a crucial role in maintaining energy balance by orchestrating various metabolic pathways that support the overall health and well-being of the organism [74]. Apart from energy homeostasis, AMPK also plays a significant role in different signaling pathways, including autophagy, mitochondrial dynamics, and DNA repair, to regulate aging in multicellular organisms (Figure-4A) [74].

Figure-4: AMPK’s role in aging and its mechanistic actions.

A. AMPK expression declines during aging and influences aging hallmarks such as autophagy, hormonal imbalance, mitochondrial dysfunction, cellular senescence, genome instability and mutations, deregulated nutrient sensing, epigenetic alterations, and loss of proteostasis.

B. AMPK regulates various metabolic pathways involved in aging, such as glycolysis and the TCA cycle. Excess calorie intake reduces AMPK activity and influences other pathways, such as mitochondrial dynamics and biogenesis. mTOR and SIRT1/PGC1α mediated autophagy, decreased expression of AMPK in lung fibrosis conditions, and calorie restriction mimetic induced AMPK regulate aging.

AMPK expression or activation is required to maintain various cellular metabolic pathways, including glycolysis, TCA cycle, and ATP synthesizing pathways in the aged organisms. AMPK activation by multiple drugs, calorie restriction, and strenuous exercise regulates these metabolic pathways and impacts the downstream signaling cascades implicated in aging [78]. Interestingly, the glycolysis end-product lactate exhibits a negative impact on aging. Acute lactate accumulation after exercise leads to muscle fatigue, and chronic lactate accumulation due to lactate dehydrogenase-B (LDHB) deficiency is associated with an aging phenotype [79]. Osmotin, an adipoR1 natural agonist, induces AMPK activation but reduces oxidative stress to alleviate neurodegeneration in LDHB^−/− mice. In a recent study, lactate dehydrogenase A (LDHA), the enzyme that converts the pyruvate to lactate, is targeted by the extracts from Sanguisorba officinalis L, which showed a significant reduction in LDHA and increased expression of AMPK [80]. Lung fibrosis is another risk factor for aging [81] and is associated with an increased risk of lung damage and infection. Soo Jung Cho et al. reported that Glucose transporter type 1 (GLUT1) dependent increased glycolysis is observed in mouse and human lung fibrosis samples, and aged lungs exhibit decreased AMPK activation compared to young lungs [82]. Some of the TCA cycle metabolites, such as α-Ketoglutarate, extend the lifespan of Drosophila [83] and C. elegans [84] by increasing the activation of AMPK but suppressing the mTOR pathway, leading to increased autophagy in these models. Other metabolites, such as oxaloacetate, were also shown to increase the lifespan of C. elegans by AMPK/FOXO3a mediated pathway [85].

AMPK activation by strenuous exercise, calorie restriction (CR) [86], or activators or compounds like Metformin plays a significant role in inducing autophagy, antioxidants, vital mitochondrial physiology, and reduced inflammation, contributing to an increased lifespan of various model organisms, including nematodes (C. elegans), Drosophila (Drosophila melanogaster) and mice [87]. Autophagy is a cellular process that removes damaged organelles and debris to promote healthy cellular functions. A recent study in C. elegans model organisms suggests that CR promotes autophagy by inducing the expression of protein kinase casein kinase 2 (CK2). Increased CK2 activates SIRT1, a NAD+-dependent deacetylase enzyme that regulates cellular stress resistance, metabolism, and longevity by deacetylating and activating key targets like AMPK and p53, thereby mediating the activation of AMPK and FOXO3a and inhibiting the PI3K/AKT/mTORC1 pathway [88]. This study also reported that along with CK2 activation, the miR-186, miR-216b, miR-337–3p, and miR-760, which are the anti-sense inhibitors, were increased to regulate the aging process.

Excessive calorie intake causes the decline of AMPK levels or activation and induces aging. In a recent C. elegans study, overexpression of Glycerol-3-phosphate phosphatase (G3PP), an enzyme that converts glycerol-3-phosphate to glycerol, reduced glycogen levels. This reduction in glycogen levels activates three longevity factors: AMPK, Helix loop helix-30 (HLH-30, a TFEB homolog), and autophagy [89]. Mitochondrial dynamics are essential to prevent cellular aging in various organisms. Improving mitochondrial efficacy or biogenesis is achieved through AMPK activation. Lulin Nie et al. recently synthesized Tetramethylpyrazine nitrone (TBN) and found that treating D-galactose-induced aging mouse models with TBN activated the AMPK signaling pathway to restore mitochondrial integrity but suppressed NF-κB-Murf1/Atrogin-1-mediated muscle protein degradation, ultimately improving age-related motor deficits [90]. In another study, researchers found that D-glucosamine (GlcN), a commonly used dietary supplement, displayed anti-aging properties. Mechanistically, GlcN activates the AAK-2 (AMPK isoform in C. elegans) [91], which induces mitochondrial biogenesis through the increased reactive oxygen species (ROS) in the mitochondria and amino acid-transporter 1 (aat-1) gene. Similarly, 2-deoxyglucose (2DG) is another CRMs that improves the lifespan of mice and rats by inducing AMPK-mediated mitochondrial biogenesis [64, 91]. In essence, AMPK is phosphorylated at T172/T183 on the AMPKα2/α1 subunit by the upstream kinases like LKB1, CaMKKβ, and TAK1 under metabolic stress conditions. However, AKT phosphorylates AMPK at S496 under hyperinsulinemia, leading to the reduction in AMPK activation and its association with the MFF-DRP1 signaling complex for mitochondrial fission and the increase in hyperglycemia in aged and obese patients [92]. Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) is a well-known master regulator of mitochondrial biogenesis. AMPK directly phosphorylates PGC-1α at T177 and S538, and such phosphorylation is required to maintain glucose uptake, fatty acid oxidation, and mitochondrial biogenesis in skeletal muscle [93, 94]. Proper maintenance of skeletal muscle mass and functioning regulates healthy aging. Myonectin, a factor secreted by muscles, plays a significant role in muscle health. Myonectin induces AMPK/PGC-1α-mediated mitochondrial biogenesis in muscle atrophy models induced by aging, sciatic denervation, or dexamethasone (DEX). Hence, myonectin could represent a therapeutic target of muscle atrophy, an aging paradigm [95].

Histone deacetylases (HDAC) are enzymes that remove the acetyl groups from the histones or other proteins to regulate their activity through epigenetic changes. These HDACs are involved in various cellular processes and aging-related disorders, such as sarcopenia and neurodegenerative disorders [96]. SIRT1 plays an important role in various cellular processes, including cell cycle regulation, DNA repair, apoptosis, inflammation, autophagy, and aging, where it promotes cellular homeostasis and longevity by modulating these processes [97]. SIRT1 and AMPK have a reciprocal relationship in that AMPK regulates SIRT1 activity by increasing NAD⁺ levels. Activated SIRT1 deacetylates the PGC1α, a key regulator of mitochondrial biogenesis, promoting mitochondrial function [98]. Furthermore, SIRT1 also deacetylates LKB1, leading to LKB1 and AMPK activation (Figure-4B) [99]. Therefore, activating AMPK using various molecules such as CRMs, strenuous exercise, calorie restriction, AMPK phosphomimetic peptides, and drugs like Metformin and resveratrol may be a possible anti-aging therapy strategy. Further research is needed to validate this concept.

Several hallmarks of aging, including altered metabolic activity, genomic instability, epigenetic alterations, and chronic inflammation, are commonly observed in aged patients with cancer [100, 101]. Age-related changes disrupt AMPK signaling, contributing to the development of aging-induced cancer malignancies by disturbing these hallmarks. For instance, metabolic dysregulation during aging promotes prostate cancer development, and research by Penfold et al. has demonstrated that AMPK activation can protect against prostate cancer by inducing catabolic reprogramming [102]. As UV-induced DNA damage, prevalent in aged individuals exposed to sunlight, contributes to skin cancer development, decline in AMPK expression in response to UV-induced DNA damage may potentially lead to skin cancer development. The supporting evidence came from the observation that AMPK activators like AICAR and metformin have been shown to prevent skin tumorigenesis [103]. Furthermore, the anti-inflammatory effect of AMPK decreases with age, and AMPK-mediated inflammatory responses can facilitate immune cell recruitment into the tumor microenvironment, hindering tumorigenesis [104]. Accumulated evidence undersocres the significance of AMPK, as a potential therapeutic target, in preventing and treating age-related malignancies. Nevertheless, additional studies are warranted to fully elucidate the therapeutic potential of targeting AMPK in elderly individuals with cancer.

4. Therapeutic targeting of AMPK in cancer and aging

Anti-cancer

As aforementioned, AMPK displays either tumor-suppressive or tumor-promoting roles in cancer development, largely dependent on distinct cell contexts. For cancers such as NSCLC, where AMPK acts as a tumor suppressor, it will be possible to induce AMPK activation to fight against those cancers. Indeed, many groups using various tumor models identified that diterpenoids (Pseudolaric acid B), saponins (Ginsenoside Rk1, Gitogenin), Capsaicin (CAP), β-carotenoids (β-ionone), Resveratrol (Resv) could act as an anti-tumor effect through regulation of AMPK activation and autophagy-mediated cell death [105–109]. Another study revealed that nicotinamide mononucleotide (NMN) could activate AMPK and subsequent mTOR inactivation, leading to increased autophagy/ferroptosis, apoptosis, and necrosis in HCC tissues [108, 110]. AMPK-activating compounds like ASP4132, NT-1044, and Metformin have been shown to suppress cancer growth in various cell lines and xenograft models [111–113]. Moreover, GSK621, an AMPK activator, can enhance the immunotherapy efficacy in murine and human AML cells [114].

Under metabolic stress conditions, cancer cells frequently upregulate alternative metabolic pathways, including serine synthesis (SSP) and glutaminolysis, in addition to glycolysis, to sustain their growth and survival. Moreover, they can rely on metabolites, such as glutamine, lipids, acetate, and ketone bodies, as alternative energy sources when glucose is scarce or under metabolic stress. AMPK activation is indcued under metabolic stress conditions and promotes these metabolic pathways [22, 28, 115–117]. Hence, targeting these compensatory pathways may offer a promising therapeutic strategy for various cancers that rely on these metabolic adaptations. AMPK activation also causes drug resistance in specific cancers. A recent study revelated that HPRT1-mediated AMPK activation contributes to Temozolomide (TMZ) drug resistance in glioblastomas (GBMs), but this resistance could be overcome by co-treating with 6-Mercaptopurine (6-MP), which enhances TMZ efficacy and improves GBM treatment outcomes [30]. Other research has shown that treatment with Penfluridol (PF), an FDA-approved antipsychotic drug with anti-cancer properties, increases glucose consumption in gallbladder cancer (GBC) by activating the AMPK/PFKFB3 signaling pathway and increasing glycolytic activity. This finding suggests that combining AMPK inhibition with PF treatment may represent a novel therapeutic strategy for combating GBC [118].

On the contrary, in those cancers where AMPK acts as an oncogene and its expression/activation is elevated, applying AMPK inhibitors to reduce AMPK activation would be logical for targeting these cancers. Compound C (Dorsomorphin), a potent inhibitor of AMPK, has been shown to induce apoptosis in various cancer cells, including breast, ovarian, colorectal, glioma, hematological cancer, skin cancer, and melanoma [119–123]. Despite its great efficacy in exerting cell death in various cancer cell models, the compound C cannot be used as a cancer-targeting agent due to its high toxicity resulting from its multiple effects on other kinases and poor pharmacokinetics (PK) properties [124, 125]. Recent research has led to the development of various cancer screening methodologies that have identified several compounds that regulate AMPK activation or inhibition as a potential therapeutic target. The details of these compounds and mechanistic actions are summarized in Table 1.

Table-1:

Direct and indirect AMPK activators and inhibitors, their mechanisms of action, and effects on various cancer types

Activator	Mechanism of action	Role in Cancer	Reference
AICAR	Activate AMPK and induce anti-Warburg effect	Breast cancer (MDAMB-231) cell viability is reduced and increased apoptosis.	[126]
Aspirin	Activate AMPK and reduce prostaglandin E2 (PGE2)	Inhibit the Melanoma cell motility and selective tumor growth	[127]
D561–0775	AMPK activation and inhibition of lipid metabolism	Inhibit NSCLC cell growth and sensitization of Gefitinib	[128]
Dihydroartemisinin (DHA)	Induce apoptosis and autophagy, ROS/AMPK/mTOR signaling pathway	Induces apoptosis and autophagy in 22Rv1 cells	[129]
Ginsenoside Rk1	AMPK/mTOR mediated autophagy-dependent apoptosis	Suppress hepatocellular carcinoma	[106]
GSK621	AMPK activator	Improve the immunotherapy in Murine and human AML cells, Inhibit melanoma cell growth	[114, 130]
JCI-20679	Activation of AMPK	Suppression of proliferation	[131]
Pseudolaric acid B	ROS/AMPK/mTOR/autophagy-induced cell death	Suppress NSCLC progression	[105]
Metformin	Activates AMPK	Inducing immunogenic cell death in ovarian cancers, Suppress PDL-1 expression	[132, 133]
MK8722	Pan AMPK activator	Inhibits pancreatic cancer cell cycle arrest and Epithelial ovarian cancer	[134, 135]
Nicotinamide mononucleotide (NMN)	Induce autophagy and ferroptosis via AMPK/mTOR signaling	Inhibit HCC progression	[110]
Polyphyllin I (PPI)	PPI directly binds with AMPKa and stabilizes	Induce autophagic cell death in NSCLC	[136]
Sakurasosaponin	Autophagy induction via AMPK activation	Anti-proliferative effects on NSCLC cells	[137]
Thalidezine	Activating AMPK pathway	Induce autophagic cell death	[138]
TQFL12	TQFL12 directly binds with AMPKa and stabilizes	Inhibit TNBC cell metastasis and invasion	[139, 140]
Wedelolactone	Activates AMPK and reduces AKT activation	Inhibit the Melanoma cell growth	[141]
Inhibitor	Mechanism of action	Role in Cancer	Reference
Choline	Attenuate the AMPK/mTOR pathway and suppresses the autophagy	Reduce the HCC cell growth	[142]
Fructose	Generates high amount of ATP and inhibits AMPK-mediated autophagic cell death	Promotes pancreatic cancer cell growth	[143]
PF-3758309	Acts as an AMPK inhibitor	Sensitizes ferroptosis inducers	[144]

Anti-Aging:

AMPK orchestrates the aging process and related disorders by promoting cellular homeostasis, survival, autophagy, and mitochondrial functions while reducing oxidative stress [145–148]. As AMPK expression and activation decline during the progression of aging [10], targeting AMPK activation may be a promising therapeutic strategy for anti-aging therapy. AMPK activation can be achieved via calorie restriction (CR), reduced glucose and fructose consumption, and strenuous exercise, which may represent potential strategies for targeting aging and its related disorders. Exercise offers numerous anti-aging benefits, including enhanced cellular health, improved mitochondrial function, and reduced inflammation. Regular physical activity promotes telomere maintenance, supports epigenetic regulation, and preserves muscle mass and strength. Additionally, exercise improves cardiovascular health, cognitive function, and immune response while reducing stress levels. By incorporating exercise into one’s lifestyle, individuals can potentially slow aging and reduce the risk of age-related diseases [149]. Apart from the CR, certain drugs can act as calorie restriction mimetics (CRMs), which can be used as anti-aging agents by inducing autophagy or other cellular processes [150]. Sodium-glucose cotransporter-2 inhibitors (SGLT2i) can be used as CRMs and show promising results in various aging research models [151]. These CRMs work by AMPK-dependent or independent to regulate multiple signaling pathways involved in various cellular processes, including autophagy, skeletal muscle fibrosis, and cellular proteostasis. For example, Empagliflozin, a potent CRM, inhibits skeletal muscle fibrosis in naturally aging male mice through the AMPKα/MMP9/TGF-β1/Smad pathway [152]. These CRMs primarily act through AMPK-mediated phosphorylation of mTOR and deacetylation of SIRT1 to promote autophagy, which is essential for anti-aging therapy [153].

It has been reported that high blood glucose seen in diabetic patients displays detrimental effects on repressing AMPK activation and causes various aging-related complications [154]. Thus, maintaining normal blood glucose levels will be critical for AMPK activation to prevent aging-related complications. Notably, treatment with AMPK activators, such as Metformin and AICAR, promoted the degradation of miR146a and inhibited cellular senescence, suggesting that AMPK activation may be required to prevent cellular senescence [155]. AMPK activators, including Metformin, Resveratrol, Berberine, and Curcumin, have been demonstrated to exhibit anti-aging properties in various preclinical models, such as Caenorhabditis elegans and Drosophila, as well as in models of aging-related conditions, including neurocognitive impairment, fibrosis, and osteoarthritis, suggesting their potential therapeutic role in promoting healthy aging and preventing age-related diseases [156–161]. Quercetin, a flavonoid found in many fruits, vegetables, leaves, and seeds, is one of the activators of the AMPK/SIRT1 pathway that has been shown to display anti-aging properties using yeast and human fibroblast cell models [162]. Another study employed proteomic analysis to investigate the effects of quercetin on Simocephalus vetulus and found that treatment with quercetin at a concentration of 1 mg/L led to increased lifespan and reproductive capacity, likely through multiple pathways, including metabolic pathways, sphingolipid metabolism, and the AMPK pathway [163]. While the anti-aging effects of these compounds have been verified in animal models and some human cell line studies, their anti-aging effects in humans have not been verified. It will be interesting to explore whether these agents can show anti-aging effects in a human setting.

In conclusion, the accumulating evidence suggests that AMPK is a promising therapeutic target for aging and age-related diseases. Activation of AMPK has been shown to improve various aging-related phenotypes, including metabolic dysfunction, cellular stress, and mitochondrial impairment. While some natural compounds and small molecules have been identified as AMPK activators, further research is needed to develop more potent and specific AMPK-targeting therapies.

5. Conclusions and Future Directions

AMPK is a cellular energy sensor activated by various cellular stresses, including glucose deprivation, hypoxia, and ischemia, and critically regulates cancer and aging. Interestingly, AMPK expression and activation alterations are observed in various cancer types and aging conditions. Thus, maintaining AMPK expression and activation in the normal range will be essential for cancer and aging control. Several outstanding questions and issues remain to be addressed. As AMPK plays distinct roles in different tissues and cancer cell types [164], addressing its role in cancer will be necessary before considering applying the AMPK activator or inhibitor for cancer targeting. Since no potent and specific AMPK inhibitors with great Pharmacokinetics (PK) properties are currently available, developing such agents will be urgently needed when considering applying the AMPK inhibitor for cancer targeting. Another crucial consideration in exploiting AMPK’s potential as a cancer therapy is also the need to selectively target cancer cells while sparing normal cells, as AMPK’s multifaceted roles in various signaling pathways pose a significant risk of off-target effects, thereby presenting a major limitation in its therapeutic applications. Targeting AMPK with optimal dosage and treatment duration to achieve efficacious AMPK activation without off-target effects should also be considered. The absence of reliable biomarkers for AMPK-mediated cancer progression necessitates the identification of surrogate markers to monitor AMPK activity better and predict treatment response.

Moreover, the poor bioavailability and pharmacokinetic limitations of AMPK modulators, including low solubility, poor absorption, and rapid clearance, must be overcome to ensure efficient delivery and optimal therapeutic efficacy. Developing targeted delivery strategies is essential to enhance tissue-specific uptake and minimize systemic toxicity. Addressing these challenges is crucial to harnessing the therapeutic potential of AMPK activation.

While targeting AMPK activation has shown promise in anti-aging research, some limitations and potential drawbacks must be overcome. For instance, calorie restriction, exercise, and ketogenic diet-induced AMPK activation may not be suitable for everyone, as such an approach may lead to weight loss and negative health consequences in some individuals. Pharmacological AMPK activators can disrupt gut homeostasis and microbiota, either positively or negatively, as the microbiota can influence drug metabolism and efficacy. Conversely, these drugs can alter the gut microbiota composition by favoring the growth of beneficial or harmful microorganisms [165]. Moreover, context-dependent AMPK activation may not always be helpful, and more research is needed to understand its effects fully. While animal studies (C. elegans, Drosophila, mice) have suggested a role for AMPK activation by using AMPK activators in anti-aging, further human clinical trials are necessary to validate these findings. Despite substantial research efforts, the intricate regulation of AMPK activity remains incompletely understood, thus hindering the development of effective therapeutic strategies. Additionally, more studies are required to fully elucidate the mechanisms by which AMPK influences aging and to explore its potential as a therapeutic target for various age-related diseases. With continued research and development, AMPK-targeting therapies are promising to improve lifespan and treat age-related diseases and cancer.

Data availability

No data was used for the research described in the article.