AMPK (AMP-activated protein kinase)
Protein Phosphorylation

obesity and AMPK

obesity and AMPK

AMPK and inflammation

AMPK and inflammation

ampk and TNF-α

AMPK and IL-6

Adenosine monophosphate-activated protein kinase: a central regulator of metabolism with roles in diabetes, cancer, and viral infection. 2011

• Abstract
  Adenosine monophosphate-activated protein kinase (AMPK) is a cellular energy sensor activated by metabolic stresses that inhibit catabolic ATP production or accelerate ATP consumption. Once activated, AMPK switches on catabolic pathways, generating ATP, while inhibiting cell growth and proliferation, thus promoting energy homeostasis. AMPK is activated by the antidiabetic drug metformin, and by many natural products including "nutraceuticals" and compounds used in traditional medicines. Most of these xenobiotics activate AMPK by inhibiting mitochondrial ATP production. AMPK activation by metabolic stress requires the upstream kinase, LKB1, whose tumor suppressor effects may be largely mediated by AMPK. However, many tumor cells appear to have developed mechanisms to reduce AMPK activation and thus escape its growth-restraining effects. A similar phenomenon occurs during viral infection. If we can establish how down-regulation occurs in tumors and virus-infected cells, there may be therapeutic avenues to reverse these effects.

AMP-activated protein kinase: maintaining energy homeostasis at the cellular and whole-body levels. 2014

Abstract
The adenosine monophosphate (AMP)-activated protein kinase (AMPK) signaling pathway arose early during evolution of eukaryotic cells, when it appears to have been involved in the response to glucose starvation and perhaps also in monitoring the output of the newly acquired mitochondria. Due to the advent of hormonal regulation of glucose homeostasis, glucose starvation is a less frequent event for mammalian cells than for single-celled eukaryotes. Nevertheless, the AMPK system has been preserved in mammals where, by monitoring cellular AMP:adenosine triphosphate (ATP) and adenosine diphosphate (ADP):ATP ratios and balancing the rates of catabolism and ATP consumption, it maintains energy homeostasis at a cell-autonomous level. In addition, hormones involved in maintaining energy balance at the whole-body level interact with AMPK in the hypothalamus. AMPK is activated by two widely used clinical drugs, metformin and aspirin, and also by many natural products of plants that are either derived from traditional medicines or are promoted as "nutraceuticals."

Emodin regulates glucose utilization by activating AMP-activated protein kinase. 2013

Thalidezine, a novel AMPK activator, eliminates apoptosis-resistant cancer cells through energy-mediated autophagic cell death. 2017

Berberine, a natural plant product, activates AMP-activated protein kinase with beneficial metabolic effects in diabetic and insulin-resistant states. 2006

Regulation of AMP-activated protein kinase by natural and synthetic activators. 2016

• Abstract
  The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status that is almost universally expressed in eukaryotic cells. While it appears to have evolved in single-celled eukaryotes to regulate energy balance in a cell-autonomous manner, during the evolution of multicellular animals its role has become adapted so that it also regulates energy balance at the whole body level, by responding to hormones that act primarily on the hypothalamus. AMPK monitors energy balance at the cellular level by sensing the ratios of AMP/ATP and ADP/ATP, and recent structural analyses of the AMPK heterotrimer that have provided insight into the complex mechanisms for these effects will be discussed. Given the central importance of energy balance in diseases that are major causes of morbidity or death in humans, such as type 2 diabetes, cancer and inflammatory disorders, there has been a major drive to develop pharmacological activators of AMPK. Many such activators have been described, and the various mechanisms by which these activate AMPK will be discussed. A particularly large class of AMPK activators are natural products of plants derived from traditional herbal medicines. While the mechanism by which most of these activate AMPK has not yet been addressed, I will argue that many of them may be defensive compounds produced by plants to deter infection by pathogens or grazing by insects or herbivores, and that many of them will turn out to be inhibitors of mitochondrial function.

AMP-activated protein kinase signaling activation by resveratrol modulates amyloid-beta peptide metabolism. 2010

• Abstract
  Alzheimer disease is an age-related neurodegenerative disorder characterized by amyloid-beta (Abeta) peptide deposition into cerebral amyloid plaques. The natural polyphenol resveratrol promotes anti-aging pathways via the activation of several metabolic sensors, including the AMP-activated protein kinase (AMPK). Resveratrol also lowers Abeta levels in cell lines; however, the underlying mechanism responsible for this effect is largely unknown. Moreover, the bioavailability of resveratrol in the brain remains uncertain. Here we show that AMPK signaling controls Abeta metabolism and mediates the anti-amyloidogenic effect of resveratrol in non-neuronal and neuronal cells, including in mouse primary neurons. Resveratrol increased cytosolic calcium levels and promoted AMPK activation by the calcium/calmodulin-dependent protein kinase kinase-beta. Direct pharmacological and genetic activation of AMPK lowered extracellular Abeta accumulation, whereas AMPK inhibition reduced the effect of resveratrol on Abeta levels. Furthermore, resveratrol inhibited the AMPK target mTOR (mammalian target of rapamycin) to trigger autophagy and lysosomal degradation of Abeta. Finally, orally administered resveratrol in mice was detected in the brain where it activated AMPK and reduced cerebral Abeta levels and deposition in the cortex. These data suggest that resveratrol and pharmacological activation of AMPK have therapeutic potential against Alzheimer disease.

The inhibitory effect in Fraxinellone on oxidative stress-induced senescence correlates with AMP-activated protein kinase-dependent autophagy restoration. 2017
As a natural metabolite of limonoids from Dictamnus dasycarpus, fraxinellone has been reported to be neuroprotective and anti-inflammatory. However, its influence on cellular metabolism remains largely unknown. In the present study, we investigated the effect of fraxinellone on cellular senescence-induced by oxidative stress and the potential mechanism. We found that fraxinellone administration caused growth arrest and certainly repressed the activity of senescence associated β-galactosidase as well as the expression of senescence-associated-genes. Interestingly, this effect of fraxinellone is closely correlated with the restoration of impaired autophagy and the activation of AMPK. Notably, fraxinellone reacts in an AMPK-dependent but mTORC1-independent manner. Together, our study demonstrates for the first time that fraxinellone has the effect on senescence inhibition and AMPK activation, and supports the notion that autophagic mechanism is important for aging prevention. These findings expanded the list of natural compounds and will be potentially utilized for aging decay and/or AMPK activation.

Edoardo Giai Via
Eleonora Orsucci

DEFINITION

AMP-activated protein kinase or AMPK is an enzyme that plays a role in cellular energy homeostasis.
The net effect of AMPK activation is:

• stimulation of hepatic fatty acid oxidation and ketogenesis
• inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis
• inhibition of adipocyte lipolysis and lipogenesis
• stimulation of skeletal muscle fatty acid oxidation and muscle glucose uptake
• modulation of insulin secretion by pancreatic beta-cells

CHEMICAL STRUCTURE AND IMAGES

The heterotrimeric protein AMPK is formed by α, β, and γ subunits. Each of these three subunits takes on a specific role in both the stability and activity of AMPK. Specifically, the γ subunit includes four particular Cystathionine beta synthase (CBS) domains giving AMPK its ability to sensitively detect shifts in the AMP:ATP ratio.
The four CBS domains create two binding sites for AMP commonly referred to as Bateman domains. Binding of one AMP to a Bateman domain cooperatively increases the binding affinity of the second AMP to the other Bateman domain. As AMP binds both Bateman domains the γ subunit undergoes a conformational change which exposes the catalytic domain found on the α subunit. It is in this catalytic domain where AMPK becomes activated when phosphorylation takes place at threonine-172 by an upstream AMPK kinase (AMPKK).
The α, β, and γ subunits can also be found in different isoforms: the γ subunit can exist as either the γ1, γ2 or γ3 isoform; the β subunit can exist as either the β1 or β2 isoform; and the α subunit can exist as either the α1 or α2 isoform.
Although the most common isoforms expressed in most cells are the α1, β1, and γ1 isoforms.
The α2 isoform is the subunit of AMPK found predominantly within skeletal and cardiac muscle, whereas, approximately equal distribution of both the α1 and α2 isoforms are present in hepatic AMPK.
the β subunits of AMPK have a glycogen-binding domain, GBD
Within pancreatic islet β-cells the α1 isoform predominates.
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The N-terminal half of the α subunits contains a typical serine/threonine kinase catalytic domain. Interaction with the β and γ subunits occurs via the C-terminal half of the α subunits.
The yeast AMPK β subunits are lipid modified with myristic acid. Myristoylation may account for the membrane association of mammalian AMPK. The core of the β subunits have a glycogen-binding domain (GBD). This domain is closely related to the isoamylase N domain subfamily and weakly related to domains in the glycogen-targeting phosphatase subunits and several starch-binding proteins. The close proximity of AMPK to cellular glycogen stores allows it to rapidly effect changes in glycogen metabolism in response to changes in metabolic demands.
The γ subunits of AMPK have been shown to contain nucleotide binding sites with similarity to cystathionine β-synthase (CBS) domains. Indeed, direct AMP-binding studies have shown that AMP is bound to the γ subunits by a pair of CBS domains.

h3. CELLULAR FUNCTIONS

It is expressed in a number of tissues, including the liver, brain, and skeletal muscle.
The signaling cascades initiated by the activation of AMPK exert effects on glucose and lipid metabolism, gene expression and protein synthesis. These effects are most important for regulating metabolic events in the liver, skeletal muscle, heart, adipose tissue, and pancreas.

REGULATION

In the presence of AMP the activity of AMPK is increased approximately 5-fold. However, more importantly is the role of AMP in regulating the level of phosphorylation of AMPK. An increased AMP to ATP ratio leads to a conformational change in the γ-subunit leading to increased phosphorylation and decreased dephosphorylation of AMPK. The phosphorylation of AMPK results in activation by at least 100-fold. AMPK is phosphorylated by at least three different upstream AMPK kinases (AMPKKs). Phosphorylation of AMPK occurs in the α subunit at threonine 172 (T172) which lies in the activation loop.
As the name implies, AMPK is also regulated by AMP. The effects of AMP are two-fold: a direct allosteric activation and making AMPK a poorer substrate for dephosphorylation. Because AMP affects both the rate of AMPK phoshorylation in the positive direction and dephosphorylation in the negative direction, the cascade is ultrasensitive. This means that a very small rise in AMP levels can induce a dramatic increase in the activity of AMPK. The activity of adenylate kinase, catalyzing the reaction shown below, ensures that AMPK is highly sensitive to small changes in the intracellular [ATP]/[ADP] ratio. 2 ADP ——> ATP + AMP

One kinase activator of AMPK is Ca2+-calmodulin-dependent kinase kinase β (CaMKKβ) which phosphorylates and activates AMPK in response to increased calcium. The distribution of CaMKKβ expression is primarily in the brain with detectable levels also found in the testes, thymus, and T cells. As described for the Ca2+-mediated regulation of glycogen metabolism, increased release of intracellular stores of Ca2+ create a subsequent demand for ATP. Activation of AMPK in response to Ca fluxes provides a mechanism for cells to anticipate the increased demand for ATP.

Evidence has also demonstrated that the serine-threonine kinase, LKB1(also called serine-threonine kinase 11, STK11) which is encoded by the Peutz-Jeghers syndrome tumor suppressor gene, is required for activation of AMPK in response to stress.
Unlike the limited distribution of CaMKKβ, LKB1 is widely expressed, thus making it the primary AMPK-regulating kinase. Loss of LKB1 activity in adult mouse liver leads to near complete loss of AMPK activity and is associated with hyperglycemia (The kinase LKB1 mediates glucose homeostasis in liver and therapeutic effects of metformin, 2005). The hyperglycemia is, in part, due to an increase in the transcription of gluconeogenic genes. Of particular significance is the increased expression of the peroxisome proliferator-activated receptor-γ (PPAR-γ) coactivator 1α (PGC-1α) which drives gluconeogenesis. Reduction in PGC-1α activity results in normalized blood glucose levels in LKB1-deficient mice.
The third AMPK phosphorylating kinase is transforming growth factor-β-activated kinase 1 (TAK1). However, the normal physiological conditions under which TAK1 phosphorylates AMPK are currently unclear.

Negative allosteric regulation of AMPK also occurs and this effect is exerted by phosphocreatine. As indicated above, the β subunits of AMPK have a glycogen-binding domain, GBD.
In muscle, a high glycogen content represses AMPK activity and this is likely the result of interaction between the GBD and glycogen, although this has not been shown directly. As suggested above, the GBD of AMPK allows association of the enzyme with the regulation of glycogen metabolism by placing AMPK in close proximity to one of its substrates glycogen synthase.
AMPK has also been shown to be activated by receptors that are coupled to phospholipase C-γ (PLC-γ) and by hormones secreted by adipose tissue (termed adipokines) such as leptin and adiponectin

Whereas, stress and exercise are powerful inducers of AMPK activity in skeletal muscle, additional regulators of its activity have been identified.
Insulin-sensitizing drugs of the thiazolidinedione family (activators of PPAR-γ, see below) as well as the hypoglycemia drug metformin exert a portion of their effects through regulation of the activity of AMPK. As indicated above, the activity of the AMPK activating kinase, LKB1, is critical for regulation of gluconeogenic flux and consequent glucose homeostasis. The action of metformin in reducing blood glucose levels requires the activity of LKB1 in the liver for this function. Also, several adipokines (hormones secreted by adipocytes) either stimulate or inhibit AMPK activation: leptin and adiponectin have been shown to stimulate AMPK activation, whereas, resistin inhibits AMPK activation.

EFFECTS

• AMPK blocks mTOR-raptor (mTORC1). mTOR is a grow factor (over expressed in hyperlipemic diet-relations between diet and cancer) and contributes to the development of insulin resistance. Physical exercise raises the level of AMPK and acts against insulin resistance. Lipid-induced mTOR activation in rat skeletal muscle reversed by exercise and 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside, 2009

• AMPK blocks ACC (acetilCoa carboxylase). ACC normally produces malonilCoa which is an inhibitor of carnitine acetiltranferase (CPT-1). This impairs the synthesis of fatty acids . The regulation of beta oxidation consists in the entry or not of acetilCoa (carnitine shunt) in the mitochondria. The beta oxidation occurs in mitochondria, the synthesis of fatty acids in the cytosol. Also an hyperactivity of the malonilcoa decarboxylase contribute even more to lower the concentration of malonilCoa. Coordinate regulation of malonyl-CoA decarboxylase, sn-glycerol-3-phosphate acyltransferase, and acetyl-CoA carboxylase by AMP-activated protein kinase in rat tissues in response to exercise

• AMPK increases eNOS activity, phosphorylating it in serin 1177. Cardiac effects exerted by activation of AMPK also include phosphorylation of endothelial nitric oxide synthase, eNOS in cardiac endothelium. AMPK-mediated phosphorylation of eNOS leads to increased activity and consequent NO production and provides a link between metabolic stresses and cardiac function. In platelets, insulin action leads to an increase in eNOS activity that is due to its phosphorylation by AMPK. Activation of NO production in platelets leads to a decrease in thrombin-induced aggregation, thereby, limiting the pro-coagulant effects of platelet activation. The response of platelets to insulin function clearly indicates why disruption in insulin action is a major contributing factor in the development of the metabolic syndrome Endothelial nitric oxide synthase phosphorylation in treadmill-running mice: role of vascular signalling kinases,2009.

• AMPK raises the expression of glut4 transporter in two ways: insulin mediated mechanism and non-insulin mediated mechanism. Evidence for 5' AMP-activated protein kinase mediation of the effect of muscle contraction on glucose transport, 1998

• AMPK blocks HMGCoA reductase, which is the key enzyme of cholesterol synthesis. Some studies suggest that statins increase eNOS by activating AMKT. Simvastatin increases the activity of endothelial nitric oxide synthase via enhancing phosphorylation,2009.

• T3 activates AMPK. Furthermore the activation of AMPK in the hypothalamus leads to hyperphagia. Thyroid hormone activates adenosine 5'-monophosphate-activated protein kinase via intracellular calcium mobilization and activation of calcium/calmodulin-dependent protein kinase kinase-beta, 2008 ; Triiodothyronine (T3) stimulates food intake via enhanced hypothalamic AMP-activated kinase activity