Adenosine monophosphate (AMP)-activated protein kinase (AMPK) plays an important role in cellular energy homeostasis, helping in growth regulation and metabolic reprogramming in eukaryotes. The AMPK pathway is activated when there is a decrease in the production of adenosine triphosphate (ATP), as seen in heat shock, ischemia, and low blood glucose levels.
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Structure of AMPK and mechanism of activation of AMPK signaling
AMPK is an obligate heterotrimer comprising of a catalytic subunit α and two regulatory subunits, namely, β and γ. When the intracellular level of ATP decreases, adenosine diphosphate (ADP) or AMP directly binds to the γ unit of AMPK. A major upstream kinase, liver kinase B1 (LKB1), mediates the activation of AMPK by aiding phosphorylation of the threonine 172 (thr172) site of AMPK.
LKB1 is a tumor suppressor gene, supported byand m many studies that have suggested that AMPK plays a significant role in suppressing tumors. Other kinases involved in phosphorylation at thr172 are TGFβ activated kinase 1 (TAK1), and calcium/calmodulin-dependent kinase kinase 2 (CAKK2).
Intracellular calcium also activates AMPK through phosphorylation mediated by CAKK2. In addition, three phosphatases involved in phosphorylation at thr172 are Mg2+/Mn2- dependent protein phosphatase 1E, protein phosphatase 2C, and protein phosphatase 2A.
Physiological and metabolic functions of AMPK
AMPK positively regulates catabolic pathways such as fatty acid oxidation and autophagy when the concentration of intracellular ATP is low, while it negatively affects anabolic pathways such as glycogen synthesis, de novo synthesis of triglycerides, fatty acids, and cholesterol. AMPK regulates lipid metabolism by controlling the free fatty acids concentration by fatty acid and β-oxidation (FAO), and by inhibiting lipogenesis and lipolysis.
Studies have shown that AMPK inhibits gluconeogenesis by suppressing transcription factors that promote gluconeogenic enzymes such as glucose-6-phosphatase, hepatocyte nuclear factor 4, phosphoenolpyruvate carboxykinase, and CREB regulated transcription coactivator 2 (CRTC2). Another pathway of inhibiting gluconeogenesis is by phosphorylation of class IIa histone acetylates by AMPK.
AMPK inhibits protein synthesis by inhibition of cap-dependent translation and indirect inhibition of mammalian target of rapamycin complex 1 (mTORC1), a protein complex that controls protein synthesis. However, AMPK also stimulates cap-independent translation in response to energy stress to express genes that are important for cell survival.
To maintain cellular integrity, AMPK directly and indirectly activates the mammalian homolog of the autophagy protein ATG1, namely ULR1. Scientific evidence also shows that AMPK activates the transcription factor FOXO to regulate autophagy. In addition, AMPK plays an important role in the regulation of antioxidant defense during oxidative stress by up regulating many antioxidant genes and transcription factors.
Physiological factors that suppress AMPK signaling
Obesity, over nutrition, inflammation, and aging have an impact on AMPK signaling. High levels of glucose, amino acids, or fatty acids can suppress AMPK activity leading to lifestyle disorders such as obesity and type 2 diabetes.
During inflammation, pro-inflammatory cytokines such as tumor necrosis factor α (TNFα) inhibit AMPK. In addition, researchers point out suppression in AMPK activation with aging of tissues. However, many studies have also noted the anti-inflammatory effect of AMPK by FAO regulation.
AMPK in therapy
Use of AMPK activators in neurodegenerative diseases: There are two types of AMPK activators:, direct and indirect activators. Direct activators interact with a specific AMPK subunit bringing about a conformational change in the AMPK complex, thereby leading to its activation. Examples of direct activators are salicylates, benzimidazole, thienopyridone, and so on.
Indirect activators activate AMPK by bringing about accumulation of calcium or AMP. Examples of indirect activators are resveratrol, quercetin, berberin, curcumin, troglitazone, metformin, and so on. A number of such naturally occurring compounds are attracting the attention of researchers in preventing diseases through activation of AMPK.
The research industry has been focusing on understanding the molecular mechanisms of these activators, and learning whether if they carry the potential to treat human diseases. For instance, a study carried out on rodent models for Alzheimer’s disease and Huntington’s disease, demonstrated that administration of resveratrol, a sirtuin 1 activator, reduced neurodegeneration that can be partially attributed to AMPK activation.
Similarly, treatment of Parkinson’s disease (PD) with resveratrol showed a decrease in proinflammatory cytokines in model rats. The therapeutic effects of resveratrol were attributed to activation of AMPK through autophagy, antioxidant gene expression, and mitochondrial biogenesis.
However, further studies are being carried out to establish a clear understanding of the effect of resveratrol on slowing down neurodegeneration. Metformin, on the other hand, has been widely tested and researched on PD models. Metformin reduced the degeneration of dopaminergic neurons, enhanced autophagy, and reduced pro-inflammatory cytokines expression.
AMPK activation in cancer therapy: To enhance the efficacy of combination drugs used in cancer treatment, and to prevent multidrug resistance, a number of studies have been looking at using AMPK regulators for targeted therapy. For instance, epidemiological and preclinical studies showed that metformin, an anti-hyperglycemic agent, had an effect on reducing the incidences of certain types of cancer.
In non-small cell lung carcinoma (NSCLC), metformin inhibited cell proliferation and increased sensitivity of the cells to radiation therapy. Although the molecular mechanism is yet to be understood with certainty, metformin positively influences activation of AMPK and can be investigated further in cancer prevention and treatment.
A new class of non-steroidal anti-inflammatory drugs (NSAIDs) that inhibits cyclooxygenase-2 (COX2) has been developed as chemotherapeutic agents in the treatment of colorectal cancer. NSAIDs increase AMPK activation that also helps in reducing inflammation caused due to infection or injury by changing the activity of neutrophils, macrophages, as well as T-cells.
In addition, a number of natural products such as quercetin, resveratrol, and epigallocatechin are under study as chemotherapeutic agents for their low toxicity and proven effectiveness.
Disadvantages of AMPK activation
Although AMPK activation is being researched extensively for cancer therapy, activation of AMPK can sometimes promote the proliferation of cancer cells. Some studies have pointed out the tumor promoting capability of AMPK in addition to their tumor suppressing ability. In prostate cancer cases, AMPK activation will cause elevation of Ca2+/CaM-dependent protein kinase kinase β (CaMKKβ) expression thereby causing further proliferation of prostate cancer cells.
However, blocking of the CaMKK pathway or the AMPK pathway can arrest growth of prostate cancer cell. This shows the complexity involved in cancer therapy using AMPK activation where tumor suppression would depend on the type of pathway and cancer cells involved.
A number of substrates of AMPK are now known to be involved in maintaining activities such as growth, metabolism, autophagy, and cell polarity. What remains a challenge, and needs to be investigated further, are targets that bring about therapeutic effects upon activation of AMPK in neurodegenerative disorders, metabolic disorders, and cancer.
Currently, many research studies are focusing on identifying downstream regulators and the endogenous mechanisms involved to bring about therapeutic efficacy while maintaining cellular homeostasis.