AMPK
Basics
5′-adenosine monophosphate (AMP)-activated protein kinase (AMPK) is an enzyme that regulates cellular energy homeostasis, glucose, fatty acid uptake, and oxidation at low cellular ATP levels.
AMPK is a cellular energy sensor that when activated, stimulates catabolic processes that increase ATP synthesis, and concurrently inhibits anabolic processes that consume ATP. Nutritional or environmental stress, such as hypoglycemia, hypoxia, and/or muscle contraction, lead to an increase in the AMP:ATP ratio. (R3)
Structure of AMPK
The function of the enzyme is altered by the interaction of the AMPK subunits as conformational changes occur. AMPK is a heterotrimer consisting of one alpha catalytic subunit and two regulatory subunits, beta and gamma, and multiple isoforms of all subunits have been identified (α1, α2, β1 ,β2, γ1, γ2, γ3). AMPK complexes containing the α2 isoform are more sensitive to changes in AMP concentration than are complexes containing α1. (R3)
Genes of AMPK
alpha-subunit (catalytic) – PRKAA1, PRKAA2
beta-subunit (regulatory) – PRKAB1, PRKAB2
gamma-subunit (regulatory) - PRKAG1, PRKAG2, PRKAG3.
Cofactors
Magnesium (2+) is a cofactor.
Target enzymes of AMPK
Tuned by AMPK
Some proteins are neither inhibited nor activated by AMPK, but rather modified in a certain way to change their behaviour.
Nrf2 is phosphorylated by AMPK at three sites
Phosphorylation of Nrf2 by AMPK changes the extent of activation of the target genes of Nrf2.
MS-based analysis of immunoprecipitated Nrf2 revealed serine 374, 408 and 433 in human Nrf2 to be hyperphosphorylated as a function of activated AMPK. A direct phosphate-transfer by AMPK to those sites was indicated by in vitro kinase assays with recombinant proteins as well as interaction of AMPK and Nrf2 in cells, evident by co-immunoprecipitation. Mutation of serine 374, 408 and 433 to alanine did not markedly affect half-life, nuclear accumulation or induction of reporter gene expression upon Nrf2 activation with sulforaphane. However, some selected endogenous Nrf2 target genes responded with decreased induction when the identified phosphosites were mutated, whereas others remained unaffected.
Activated by AMPK
TBD
Inhibited by AMPK
AMPK phosphorylates and inactivates liver acetyl-CoA carboxylase (ACC), 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, glycogen synthase and creatine kinase, enzymes that control the synthesis of fatty acids, cholesterol, glycogen and phosphocreatine respectively.
GPAT
We provide direct evidence that AMPK inhibits the mitochondrial isoform of snglycerol-3-phosphate acyltransferase (GPAT), the enzyme that catalyses the initial and committed step in glycerolipid biosynthesis. GPAT appears to represent a novel target of AMPK, whose inhibition decreases the de novo synthesis of TAG and phospholipids. By inactivating both ACC and GPAT, AMPK would regulate the partitioning of fatty acid between oxidative and biosynthetic pathways, in both a co-ordinated and reciprocal manner. (R4)
Both DAG and TAG production are reduced:
These data suggest that AMPK decreases de novo the synthesis of TAG and its immediate precursor DAG, but without affecting the synthesis of phospholipid or cholesterol esters. (R4).
Regulation of AMPK
AMPK activators
- ATP depletion (increased AMP/ATP ratio)
- Exercise
- Starvation
- Hypoxia
- cellular pH
- redox status (GSH/GSSG ratio)
- Increased Creatine/Phosphocreatine ratio
- Hypoglycemia
- Glucocorticoids
- Thyroid hormones
- Cannabinoids
- Adiponectin
- Ghrelin
- Agouti-related peptide
- Sulforaphane (R6)
- Low level of fructose-1,6-bisphosphate (FBP)
AMPK was first known to be activated by ATP depletion (increased AMP/ATP ratio) and related stimuli (exercise, starvation, hypoxia, cellular pH and redox status, increased creatine/phosphocreatine ratio). However, AMPK is also activated by certain drugs, hormones, and cellular stressors that do not alter AMP/ATP ratio. (R5)
Signals of energy deficit that enhance hypothalamic AMPK activity to increase feeding include: hypoglycemia, glucocorticoids, thyroid hormones, cannabinoids, adiponectin, ghrelin, and agouti-related peptide (AgRP). (R5)
Activation by Aldolase not bound to FBP
Here, we uncover a mechanism that triggers AMPK activation via an AMP/ADP-independent mechanism sensing absence of FBP, with AMPK being progressively activated as extracellular glucose and intracellular FBP decrease. When unoccupied by FBP, aldolases promote the formation of lysosomal complexes containing the v-ATPase, Ragulator, AXIN, LKB1 and AMPK, previously shown to be required for AMPK activation.
Knockdown of aldolases activates AMPK even in cells with abundant glucose, while the catalysis-defective D34S aldolase mutant, which still binds FBP, blocks AMPK activation. (R9)
Aldolase
Aldolase A (ALDOA, or ALDA), also known as fructose-bisphosphate aldolase, is an enzyme that in humans is encoded by the ALDOA gene on chromosome 16. The protein encoded by this gene is a glycolytic enzyme that catalyzes the reversible conversion of fructose-1,6-bisphosphate to glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). Three aldolase isozymes (A, B, and C), encoded by three different genes, are differentially expressed during development. Aldolase A is found in the developing embryo and is produced in even greater amounts in adult muscle. Aldolase A expression is repressed in adult liver, kidney and intestine and similar to aldolase C levels in brain and other nervous tissue. Aldolase A deficiency has been associated with myopathy and hemolytic anemia. Alternative splicing and alternative promoter usage results in multiple transcript variants. (Wikipedia)
Chromium Picolinate increases AMPK activity and AMPK loss prevents CrPic’s beneficial action
myotubes cultured in either control or hyperinsulinemic conditions showed a CrPic-stimulated increase in AMPK activity, as evidenced by increased phosphorylation of the catalytic alpha subunit at Threonine residue 172. (R11)
AMPK Inhibitors
- Leptin
- Insulin
- Glucose
- Refeeding
- ATP
- Glycation of AMPK subunit
- GSK3b (see R14 below)
Minokoshi and colleagues showed that leptin injections into mediobasal hypothalamus (MBH) decreased AMPK activity in arcuate nucleus (ARC) and PVN. Other anorexigenic signals (insulin, glucose, refeeding) had broader effects, decreasing AMPK activity in samples from ventromedial/dorsomedial and lateral hypothalamus. (R5)
ATP promotes dephosphorylation of catalytic subunit, rendering the AMPK enzyme inactive. (Uniprot)
AMPK forms a complex with GSK3b
Here we report that glycogen synthase kinase 3 (GSK3) inhibits AMPK function.
Furthermore, the association between endogenous GSK3b and the endogenous AMPK complex was not dependent on cellular energy status, AMPK activity, or serum stimulation, indicating that GSK3b stably and constitutively associates with the AMPK complex. (R14)
Surprisingly, PI3K-Akt signaling, which is a major anabolic signaling and normally inhibits GSK3 activity, promotes GSK3 phosphorylation and inhibition of AMPK, thus revealing how AMPK senses anabolic environments in addition to cellular energy levels. Consistently, disrupting GSK3 function within the AMPK complex sustains higher AMPK activity and cellular catabolic processes even under anabolic conditions, indicating that GSK3 acts as a critical sensor for anabolic signaling to regulate AMPK. (R14)
Glycation of AMPK subunit
Results: The glycation level of γ2 subunit was significantly elevated in 3 × Tg mice as compared with control mice, meanwhile, the level of pT172-AMPK was obviously lower in 3 × Tg mice than that in control mice. Moreover, we found that arginine protects the γ2 subunit of AMPK from glycation, preserves AMPK function, and improves pathologies and cognitive deficits in 3 × Tg mice.
Conclusions: Arginine treatment decreases glycated γ2 subunit of AMPK and increases p-AMPK levels in 3 × Tg mice, suggesting that reduced glycation of the γ2 subunit could ameliorate AMPK function and become a new target for AD therapy in the future. (R8)
Fear extinction
our study provides insight into the role of hippocampal AMPK in regulation of fear renewal and indicates that increasing activity of hippocampal AMPK can prevent fear renewal, thus enhancing the potency of exposure therapy. (R1)
Chronic iron deficiency leads to chronic activation of AMPK
This study indicates that chronic iron deficiency causes a shift in the expression of AMPKα, β, and γ subunit composition. Iron deficiency also causes chronic activation of AMPK as well as an increase in AMPKα1 activity in exercised skeletal muscle. (R3)
Long term energy depletion leads to apoptosis
Long term energy depletion also induces apoptosis by mechanisms that are not well understood to date. Here we show that AMPK, activated by energy depletion, inhibited cell survival by binding to and phosphorylating IRS-1 at Ser-794.
Phosphorylation of IRS-1 at this site inhibited phosphatidylinositol 3-kinase/Akt signaling, suppressed the mitochondrial membrane potential, and promoted apoptosis. (R12)
AMPK and IRS-1
Acute activation of AMPK improves PI3K activity via phosphorylation of IRS-1:
We find that AMPK rapidly phosphorylates IRS-1 on Ser-789 in cell-free assays as well as in mouse C2C12 myotubes incubated with AICAR. In the C2C12 myotubes activation of AMPK by AICAR matched the phosphorylation of IRS-1 on Ser-789.
This phosphorylation correlates with a 65% increase in insulin-stimulated IRS-1-associated phosphatidylinositol 3-kinase activity in C2C12 myotubes preincubated with AICAR.
The binding of phosphatidylinositol 3-kinase to IRS-1 was not affected by AICAR. (R13)
Note that the residue Ser-789 is different than described in the previous section (Ser-794). It means that there should be an interaction with another protein that makes AMPK to phosphorylate another site.
AMPK and Obesity
Moreover, pharmacological chronic AMPK activation by A-769662 alleviated diet-induced obesity via promoting browning in inguinal WAT.
Overall, our findings indicate that AMPK plays a vital role in modulating WAT browning in response to thermal, nutritional and pharmacological cues, supporting chronic AMPK activation as a potentially effective approach for the treatment of obesity and related metabolic diseases through increasing thermogenesis. (R9)