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AMPK Signaling Pathway

Figure 1. AMPK Signaling Pathway.

Overview of AMPK

Adenylate-activated protein kinase (AMPK) plays a key role in the regulation of cellular energy homeostasis. This kinase is activated by a response to stress factors that deplete the supply of cellular ATP, including hypoglycemia, hypoxia, ischemia, and heat shock. AMPK can be found as a heterotrimeric complex containing a catalytic alpha subunit and regulatory beta and gamma subunits. Upon binding of AMP to the gamma subunit, the variable construct activates the complex, making it a more phosphorylated substrate for the threonine 172 site, and more susceptible to phosphorylation by the major upstream AMPK kinase LKB1 in the activation loop of the alpha subunit. AMPK can also be directly phosphorylated by CAMKK2 at the threonine 172 site, a response caused by changes in intracellular calcium levels stimulated by metabolic hormones such as adiponectin and leptin. As a cellular energy receptor, AMPK responds to low levels of ATP and positively regulates the signal transduction pathways that complement the cellular ATP supply, including fatty acid oxidation and autophagy. AMPK has a negative regulatory effect on ATP-consuming biosynthesis processes, including gluconeogenesis, lipid and protein synthesis. AMPK can achieve its negative regulation by direct phosphorylation of a series of enzymes in these processes, or by transcriptional regulation of metabolism by phosphorylated transcription factors, synergistic activators, and synergistic inhibitors. AMPK is a major regulator of lipid metabolism and glucose metabolism and is a potential therapeutic target for type 2 diabetes, obesity, and cancer. In many species, AMPK interacts with mTOR and sirtuins as important regulators of aging.

AMPK family

AMPK is in the form of α, β, γ -subunits. Sestrin1 interacts with Thr172 to directly activate AMPK. This interaction has been confirmed in cardiomyocytes and protects cardiovascular system function when AMPK is activated. AMPK-α1 is mainly distributed in the kidney, heart, liver, lungs and brain; AMPK-α2 is mostly distributed in the heart, liver, skeletal muscle and brain plexus. AMPK-β1 is mainly present in the liver, while AMPK-β2 is mainly distributed in skeletal muscle and myocardium. The β subunit is mainly associated with glycogen synthase and participates in the regulation of AMPK by glycogen. The activity of γ subunit is mainly affected by AMPK-linked AMP. AMPK-γ1 and γ2 are distributed in liver, kidney, heart and lung, skeletal muscle and pancreas. AMPK-γ3 is only distributed in skeletal muscle, regulating glucose uptake and mitochondria functions in cells.

AMPK signaling pathway

  1. AMPK signaling pathway cascade
    he AMPK signaling pathway cascade can be divided into activation of AMPK signaling pathway and signal transduction. In the activation of AMPK signaling pathway, AMPK can be activated by upstream AMPK kinase (AMPKK), and their sites of action are phosphorylated AMPKα subunit 172-sulphide amino acid. The upstream kinases of AMPK mainly include AMP. There are three main mechanisms for AMP to activate AMPK signaling pathway. AMPK can directly act on AMPK and change the conformation of AMPK to activate AMPK. Binding of AMP and AMPK makes it a good substrate for its upstream kinase AMPK kinase and a poor substrate for protein phosphatase. In addition, AMP directly allosterically activates AMPKK, which activates AMPK through phosphorylation. LKB1 was first discovered in the study of pigmented polyposis syndrome. It is a tumor suppressor gene in human cells with LKB1/STK11 as its encoded protein and a member of the serine/threonine protein family encoded by the 1Kb gene It activates AMPK by directly phosphorylating 172 threonines on the AMPKα subunit. LKB1 must also have the coexistence of two accessory proteins: STRAD and MO25, which are LKB1-specific linker proteins and substrates. MO25 is mainly involved in the regulation of LKB1, which functions to stabilize the STRAD and LKB1 complexes by binding to the carboxyl terminus of STRAD. Experiments have shown that LKB1 not only activates AMPK but also activates multiple kinases of the AMPK subfamily. Accordingly, these kinases are involved in mediating the phosphorylation of LKB1, including its tumor suppressor function. TAK1, also known as TGF-β-activated kinase-1, is widely recognized as a MAPKK-kinase-7. Further research has shown that it has a central regulatory role in the AMPK activation pathway. Studies have shown that TAK1 and TAK1-related proteins in vitro can phosphorylate Thr172 and activate the AMPK signaling pathway, which is a novel AMPKK. Calmodulin-dependent protein kinase also activates AMPK, which is mainly found in the nervous system. Its phosphorylation of Thr-172 is independent of the increase in AMP concentration but activates AMPK by increasing Ca concentration. Its regulation is initiated by an increase in intracellular calcium ion concentration. Acetyl-CoA carboxylase (ACC) and hydroxymethylglutaryl-CoA reductase play key roles in the synthesis of fatty acids and cholesterol, respectively. Both ACC and HMGR are target molecules of AMPK. Activated AMPK can phosphorylate and inhibit their function, thereby inhibiting the synthesis of glycosidic acid and cholesterol. Overexpression of recombinantly activated AMPKa can negatively regulate the activity of ACC, reduce the lipid content in stem cells, thereby inhibiting AMPK and increasing hepatocytes. Medium-high sugar-induced lipid aggregation. Fatty acid oxidation is the main source of energy for muscle tissue. Muscle contraction and exercise can activate AMPK, then inhibit ACC by phosphorylation, reduce the synthesis of malonyl-CoA, enhances the activity of CPT-1 and the oxidation of fatty acids. In fat cells, AMPK agonists not only inhibit the formation of fat by ACC phosphorylation but also inhibit lipogenesis by phosphorylation, thereby inhibiting lipolysis induced by isoproterenol. In adipocytes, AMPK enhances glucose transport and GLUT4 translocation, which is not the same as insulin signaling. Activation of the translocation of the adipocyte GLUT4 increases glucose uptake. Activation of AMPK can cause phosphorylation of fructose-2-kinase, and stimulate the production of fructose 2,6-diphosphate, thereby promoting glycolysis and produce more ATP. There are four isomers of fructose 2,6-diphosphate. Only cardiac and inducible isomers are targets of AMPK. Activation of AMPK in hepatocytes not only inhibits glucose degradation by a 6-phosphate fructose-2 kinase, L-pyruvate kinase (L-PK), but also inhibits gluconeogenesis by inhibiting fructose 1,6-bisphosphatase. And it inhibits glycogen synthesis by phosphorylation of glycogen synthase. Studies have shown that AMPK is associated with the mTOR signaling pathway, and activation of the AMPK signal will inhibit mTOR and its effectors. AMPK can regulate protein synthesis and decreases the phosphorylation of mTOR by its effector eukaryotic promoter factor 4E binding protein, 70KD s6 kinase, ribosomal protein S6 kinase and eukaryotic promoter factor 4G, thereby inhibiting protein synthesis and reducing capacity consumption. According to research, AMPK may activate eEF-2 by activating eEF-2 kinase, resulting in inhibition of protein synthesis. In addition, AMPK phosphorylates Ser89, a transcriptional activator of P300, and regulates its ability to mediate nuclear receptor transcriptional activity, i.e., reducing P300 and nuclear receptors (such as hydrogen peroxide proliferator-activated receptors, thyroid receptors, and visual receptors). The xanthate receptor interacts with the retinoid X receptor but does not affect the interaction of P300 with non-nuclear receptors. Hypoxia-inducible factor I is an important transcription factor that regulates cellular hypoxia response genes. Activation of AMPK can act as an energy receptor in hypoxia that can activate the AMPK signaling pathway, thereby promoting HIF-1 expression to initiate metabolic adaptation. Activation of AMPK induces apoptosis of pancreatic beta cells via c-myc or c-Jun N-terminal kinase and subsequent caspase pathway, and apoptosis of neuroblastoma is induced by caspase pathway. In addition, it has been found that in neuroblastoma, AICAR promotes oxidative stress-induced apoptosis by activating nuclear factor-kB after activation of AMPK. AMPK activates endothelial nitric oxide synthase by phosphorylation, produces nitric oxide, and regulates vascular tone.
  2. Pathway regulation
    Due to the complexity and importance of the AMPK signaling pathway, regulation of the AMPK signaling pathway can serve as a potential therapeutic target for many diseases. The following describes several common modes of regulation of the AMPK signaling pathway. The most important regulation of the AMPK signaling pathway in the body is by AMP/ATP ratio regulation, the activity of AMPK is mainly regulated by an increase in the ratio of AMP/ATP in the cells and the ratio of creatine/phosphocreatine. Under physiological conditions, to maintain stable metabolic requirements, high levels of ATP are maintained in the cells. In most eukaryotic cells, the ATP/ADP ratio is about 10:1 and varies within a small range. Under the action of adenosine kinase, ADP and ATP can be transformed into each other. When the cells are reduced ATP, the AMP/ATP ratio is increased, and the AMPK is activated when the consumption is increased. Therefore, AMP is the key to regulate AMPK, but the specific regulatory mechanism is currently unclear. Leptin is also a regulator of the AMPK signaling pathway. According to recent studies, leptin can activate the AMPK α2 subunit in skeletal muscle. Early activation is a role for leptin in skeletal muscle bytes, and skeletal muscle leptin can directly activate AMPK activity and inhibit acetyl-coenzyme carboxylase (ACC) activity, while accelerating the oxidation of fatty acids and reducing triglyceride deposition. The regulation of glycolipids in the liver is also regulated by AMPK and depends on the network. The kinase pathway in insulin secretion is still different, but it is certain that the activation of AMPK can inhibit insulin release. In the hypothalamus, leptin inhibits the AMPK signaling pathway, presumably due to the expression of each subunit of AMPK in different tissues and the differential expression of upstream kinase LKB1 and calmodulin kinase. Adiponectin is an anti-diabetic insulin resistance and anti-atherosclerotic cytokine produced by fat cells. Adiponectin activates AMPK, which activates its downstream target MAPK signaling pathway, enhances PPARα transcriptional activity and expression of target genes, promotes oxidation of skeletal muscle fatty acids, reduces accumulation of fat in skeletal muscle, and reduces free fatty acids into the liver and improves liver insulin resistance. It can act on insulin target cells such as fat, liver and skeletal muscle, and promote glycogen output and lead to insulin resistance. The current study found that the role of resistin is mainly mediated by AMPK, but the current regulatory mechanisms are still unclear. The decrease in AMPK activity caused by the increase of inflammatory factor TNF-α can inhibit insulin signaling. At present, in vitro studies have confirmed that AICAR can only inhibit the release of cytokines secreted by IL-6, TNFα, and other adipose tissue, thereby protecting the myocardium.
  3. Relationship with diseases
    Diabetes:
    During the onset of type 2 diabetes, the level of plasma adiponectin is paralleled by the development of the disease. Adiponectin stimulates ACC phosphorylation, fatty acid oxidation, glucose uptake, and lactate production in muscle cells, are related to the AMPK signaling pathway. According to studies, the AMPK signaling pathway is upstream of the signaling pathway for insulin release from islet B cells, which in turn affects the severity of diabetes.
    Leukemia
    The study found that in leukemia, AMPK signaling pathway plays a very important role in the occurrence and development of leukemia, and the regulation of AMPK signaling pathway has a certain effect on the treatment of leukemia. The specific mechanism remains to be further studied.

References

  1. Qi D, Young L H. AMPK: energy sensor and survival mechanism in the ischemic heart. Trends in Endocrinology & Metabolism. 2015, 26(8):422-429.
  2. Monteverde T, Muthalagu N, Port J, et al. Evidence of cancer-promoting roles for AMPK and related kinases. Febs Journal. 2015, 282(24):4658-4671.
  3. Burkewitz K, Zhang Y, Mair W B. AMPK at the nexus of energetics and aging. Cell Metabolism. 2014, 20(1):10.
  4. Liu X, Chhipa R R, Nakano I, et al. The AMPK inhibitor compound C is a potent AMPK-independent antiglioma agent. Molecular Cancer Therapeutics. 2014, 13(3):596.
  5. Hardie D G. AMPK: positive and negative regulation, and its role in whole-body energy homeostasis. Current Opinion in Cell Biology. 2015, 33:1-7.

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