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The AMP-activated protein kinase (AMPK) signaling pathway is one of the master regulators of cellular metabolism that is found in nearly all eukaryotes including plants, budding yeast, and mammals.View pathway
The AMP-activated protein kinase (AMPK) signaling pathway is one of the master regulators of cellular metabolism that is found in nearly all eukaryotes including plants, budding yeast, and mammals (Hardie et al., 1997). AMPK is sensitive to changes in AMP/ADP concentration as a result of ATP hydrolysis when used as the source of energy in cellular processes (Mihaylova & Shaw, 2011). Increased levels of AMP/ADP activate AMPK which promotes ATP biosynthesis while decreasing the rate of ATP consumption. Most ATP is produced in the mitochondria and involves processes that include the breakdown of glucose and lipids. Therefore, AMPK and the other proteins involved in the pathway are tightly linked to cellular metabolism, making them potential targets for therapeutics (Steinberg & Carling, 2019).
AMPK is a serine/threonine kinase, which means it can phosphorylate substrate proteins at either of these residues (Herzig & Shaw, 2018). Phosphorylation can either lead to upregulation or downregulation of the substrate proteins.
In vivo, AMPK is a heterotrimeric protein complex composed of α, β, and γ subunits (Mihaylova & Shaw, 2011). The α subunit is the catalytic domain, while both β and γ are regulatory and modulate activity. Several isoforms for each subunit have been identified in mammals: 2 of α (α1, α2), 2 of β (β1, β2), and 3 of γ (γ1, γ2, γ3). Each AMPK complex contains only one of each subunit, but all combinations can potentially occur, resulting in 12 possible combinations. In addition, expression of some of these isoforms has been found to be tissue specific.
AMP or ADP bind to the γ subunit and promote a conformational change that results in AMPK phosphorylation by LKB1 or CAMKKβ at threonine 172 in the AMPK activation loop, which is required for AMPK activity (Mihaylova & Shaw, 2011). Reports also show that phosphorylation of serine 108 and small molecule activators can also promote AMPK activation (Steinberg & Carling, 2019).
Metformin is a biguanide type molecule that inhibits complex I in the mitochondria and is used as a first-line medication for type 2 diabetes (Herzig & Shaw, 2018). AMPK can be indirectly activated by metformin through a LKB1 mediated mechanism (Zhang et al., 2016).
Other molecules including berberine and quercetin can also activate AMPK by inhibiting mitochondrial function (Steinberg & Carling, 2019). Resveratrol, a well-studied phenol found in grapes and peanuts, helps activate AMPK by inhibiting the F1F0 mitochondrial ATPase, among other mechanisms (Mihaylova & Shaw, 2011).
Efforts for making direct activators of AMPK have focused on developing molecules that mimic nucleotide-binding activation. Worthy of note is A-769662 developed by Abbott Laboratories, which activates AMPK by an allosteric mechanism (Steinberg & Carling, 2019).
|Biological Process or Disease||Discussion|
|Lipid and cholesterol biosynthesis||The acetyl-CoA carboxylases ACC1 and ACC2, both involved in the first step of de novo lipid synthesis are inhibited by phosphorylation by AMPK (Herzig & Shaw, 2018). Another enzyme, involved in cholesterol biosynthesis, called HMG-CoA reductase is also phosphorylated and inhibited by AMPK (Herzig & Shaw, 2018). Interestingly, HMG-CoA reductase is the target of the class of drugs called statins (Istvan & Deisenhofer, 2001).|
|Glucose metabolism||Activation of AMPK has been shown to increase glucose uptake in muscle and decrease levels of blood glucose in type 2 diabetes mice (Herzig & Shaw, 2018) . AMPK phosphorylates CRCT2 and class II histone deacetylases which results in the inhibition of gluconeogenesis (Mihaylova & Shaw, 2011). AMPK phosphorylates and activates PFKFB3, which is one of the most powerful activators of glycolysis, while also phosphorylating TXNIP13 and TBC1D1 which leads to increased plasma membrane localization of the GLUT1 and GLUT4 glucose transporters (Herzig & Shaw, 2018).|
|Appetite||AMPK in the hypothalamus is able to increase appetite in response to ghrelin (also known as the hunger hormone) (Steinberg & Carling, 2019). Conversely, leptin and insulin can lead to hypothalamic AMPK inactivation and appetite suppression.|
|Autophagy and cell growth||
mTOR is one of the key players in cellular growth. Under nutrient rich conditions mTOR is active, promoting protein synthesis and other anabolic pathways. When nutrients are scarce, AMPK phosphorylates RAPTOR, a mTOR subunit, which leads to inhibition of mTOR (Mihaylova & Shaw, 2011).
Autophagy can be triggered under low nutrient conditions to help maintain cellular homeostasis. Upon glucose starvation, AMPK activates the autophagy-initiation kinase Ulk1 (Kim et al., 2011). Conversely, mTOR inhibits Ulk1, representing a multi-protein system capable of sensing changes in intracellular nutrient levels.
|Neurodegenerative disease||Intracellular neurofibrillary tangles composed of hyper phosphorylated tau are thought to be a hallmark trait of Alzheimer’s disease (Congdon & Sigurdsson, 2018). In normal biology, tau is involved in modulating the stability of microtubules. AMPK has been shown to directly phosphorylate tau and is currently hypothesized to play a role in Alzheimer’s disease pathology (Cai et al., 2012).|
|Non-alcoholic liver disease, diabetes, and cardiovascular disease||Activation of AMPK has been shown to reduce non-alcoholic liver disease, liver insulin resistance, and liver fibrosis through mediation of ACC phosphorylation (Herzig & Shaw, 2018). Combined these benefits also decrease the risk for diabetes type 2, which apart from being detrimental in itself, accelerates thrombus formation superimposed on disrupted atherosclerosis plaques that can cause heart attacks and strokes (Steinberg & Carling, 2019).|
AMPK’s involvement in various anabolic and catabolic processes makes it an important player in virus survivability (Bhutta et al., 2021). For example, AMPK plays a role in autophagy, which is used by cells to clear various cargoes such as viruses. Although, not all viruses are affected in equal manner and what proves detrimental or beneficial to virus replication involves many factors, complicating the problem even further. Below are a few examples.
Replication of HBV can be stimulated by the AMPK/ mTOR-Ulk1 autophagy axis in low glucose conditions (Wang et al., 2020). On the other hand, AMPK can be activated by HBV-induced oxidative stress and viral replication decreased by promoting autophagic degradation (Xie et al., 2016).
Loss of activity of AMPK leads to reduced actin polymerization and decreased ebola virus uptake by macropinocytosis (Kondratowicz et al., 2013).
AMPK suppression favors Zika infection, while AMPK activation results in an enhanced innate antiviral response and inhibition of viral-induced glycolysis (Singh et al., 2020).
The earliest work on AMPK dates back to the early 1970s (Steinberg & Carling, 2019). Microsomal preparations of HMG-CoA reductase were shown to be inhibited by a cytosolic factor in an ATP or ADP dependent manner in 1975 (Brown et al., 1975). Later studies identified this protein as a kinase (Harwood et al., 1984). Another study in 1980 showed that ACC was inactivated by a protein kinase (Yeh et al., 1980). In 1987, it was proposed that the same kinase cascade inhibited both HMG-CoA reductase and ACC (Steinberg & Carling, 2019). The first time this kinase was referenced as AMPK was in 1988 (Munday et al., 1988). The naming is inspired by the fact that AMPK is allosterically activated by AMP.
It should not come as a surprise that the central role that AMPK plays in metabolism has made it an interesting target in various disease states. The current hypothesis is that AMPK activation is beneficial. In addition to the small molecule activators, exercise and restrictions on caloric intake are known to activate AMPK (Steinberg & Carling, 2019).