AMPK: a nutrient and energy sensor that maintains energy homeostasis

Glucose-Based Regulation of miR-451/AMPK Signaling Depends on the OCT1 Transcription Factor

PAS kinase (PASK) is a nutrient sensor that is highly conserved throughout evolution. PASK-deficient mice reveal a metabolic phenotype similar to that described in S6 kinase-1 S6K1-deficient mice that are protected against obesity. Hypothalamic metabolic sensors, such as AMP-activated protein kinase (AMPK) and the mammalian target of rapamycin (mTOR), play an important role in feeding behavior, the homeostasis of body weight, and energy balance. These sensors respond to changes in nutrient levels in the hypothalamic areas involved in feeding behavior and in neuroblastoma N2A cells, and we have recently reported that those effects are modulated by the anorexigenic peptide glucagon-like peptide-1 (GLP-1). Here, we identified PASK in both N2A cells and rat VMH and LH areas and found that its expression is regulated by glucose and GLP-1. High levels of glucose decreased Pask gene expression. Furthermore, PASK-silenced N2A cells record an impaired response by the AMPK and mTOR/S6K1 pathways to changes in glucose levels. Likewise, GLP-1 effect on the activity of AMPK, S6K1, and other intermediaries of both pathways and the regulatory role at the level of gene expression were also blocked in PASK-silenced cells. The absence of response to low glucose concentrations in PASK-silenced cells correlates with increased ATP content, low expression of mRNA coding for AMPK upstream kinase LKB1, and enhanced activation of S6K1. Our findings indicate that, at least in N2A cells, PASK is a key kinase in GLP-1 actions and exerts a coordinated response with the other metabolic sensors, suggesting that PASK might play an important role in feeding behavior.

The AMP-activated protein kinase (AMPK) is a cellular energy sensor that is conserved throughout the eukaryotic domain. AMPK exists as complexes comprising catalytic α subunits and regulatory β and γ subunits, which are activated >100-fold by phosphorylation of the α subunit at Thr-172 by upstream kinases. Binding of AMP to one site on the Y subunit causes 10-fold allosteric activation by AMP, while binding of either AMP or ADP to a second site promotes phosphorylation and inhibits dephosphorylation of Thr-172. Since both effects are antagonized by ATP, the kinase is activated by falling cellular energy status, when it phosphorylates downstream targets that switch on catabolic pathways generating ATP, while switching off processes consuming ATP. Catabolic processes switched on including uptake and metabolism of glucose and fatty acids, and mitochondrial biogenesis. ATP-consuming processes switched off include biosynthetic pathways for lipids (fatty acids, phospholipids, cholesterol), carbohydrates (gluconeogenesis, glycogen synthesis), proteins and ribosomal RNA, and progress through the cell cycle. By switching on glucose and fatty acid uptake and metabolism and switching off gluconeogenesis, AMPK should reverse the key metabolic defects in insulin resistance and type 2 diabetes. Consistent with this, AMPK is a target, although probably not the only target, for the anti-diabetic drug metformin. Because AMPK activation inhibits most biosynthetic pathways and causes cell cycle arrest, it should also have a cytostatic, tumor suppressor effect. The discovery in 2003 that the major upstream kinase that phosphorylates Thr-172 is the tumor suppressor kinase, LKB1, strengthened this view. The finding also led to retrospective analyses suggesting that metformin protects against the development of cancer. If AMPK is a tumor suppressor, one would expect that it would be down-regulated in many tumors. This indeed occurs through loss-of-function mutations in the upstream kinase LKB1, particularly in non-small cell lung cancer and cervical cancer. The majority of cases of breast cancer also appear to have defects in AMPK activation, although the mechanism is not known. We have found that AMPK is down-regulated in cells infected with hepatitis C virus (HCV). HCV may switch off AMPK to prevent its inhibitory effects on lipid and protein synthesis, required for viral replication. HCV infection activates the PKB/Akt pathway, leading to phosphorylation of the α1 subunit of AMPK on Ser-485, thus inhibiting AMPK activation via phosphorylation at Thr-172. It is possible that this contributes to the elevated risk of hepatocellular carcinoma observed in humans with chronic HCV infection. Finally, we have found that AMPK is directly activated by salicylate, and have evidence that at least some of the metabolic changes induced by the drug in vivo are mediated by AMPK. Salicylate is a natural product of plants whose medicinal use was described in ancient manuscripts. Its use has largely been replaced by derivatives such as salsalate and aspirin, but both are rapidly broken down to salicylate in vivo. It is possible that some of the beneficial effects of salicylate derivatives, including amelioration of insulin resistance and protection against colon cancer, might be mediated by AMPK.

AMP-activated protein kinase (AMPK) serves as an energy sensor and is considered a promising drug target for treatment of type II diabetes and obesity. A previous report has shown that mammalian AMPK alpha1 catalytic subunit including autoinhibitory domain was inactive. To test the hypothesis that small molecules can activate AMPK through antagonizing the autoinhibition in alpha subunits, we screened a chemical library with inactive human alpha1(394) (alpha1, residues 1-394) and found a novel small-molecule activator, PT1, which dose-dependently activated AMPK alpha1(394), alpha1(335), alpha2(398), and even heterotrimer alpha1beta1gamma1. Based on PT1-docked AMPK alpha1 subunit structure model and different mutations, we found PT1 might interact with Glu-96 and Lys-156 residues near the autoinhibitory domain and directly relieve autoinhibition. Further studies using L6 myotubes showed that the phosphorylation of AMPK and its downstream substrate, acetyl-CoA carboxylase, were dose-dependently and time-dependently increased by PT1 with-out an increase in cellular AMP:ATP ratio. Moreover, in HeLa cells deficient in LKB1, PT1 enhanced AMPK phosphorylation, which can be inhibited by the calcium/calmodulin-dependent protein kinase kinases inhibitor STO-609 and AMPK inhibitor compound C. PT1 also lowered hepatic lipid content in a dose-dependent manner through AMPK activation in HepG2 cells, and this effect was diminished by compound C. Taken together, these data indicate that this small-molecule activator may directly activate AMPK via antagonizing the autoinhibition in vitro and in cells. This compound highlights the effort to discover novel AMPK activators and can be a useful tool for elucidating the mechanism responsible for conformational change and autoinhibitory regulation of AMPK.

AMP-activated protein kinase in the grass carp Ctenopharyngodon idellus: Molecular characterization, tissue distribution and mRNA expression in response to overwinter starvation stress.

Structural Insight into AMPK Regulation: ADP Comes into Play

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."

metabolic syndrome is a cluster of conditions including insulin resistance, dyslipidemia, hypertension, and central obesity, and it results in an increased risk of type 2 diabetes mellitus and cardiovascular diseases such as atherosclerosis. These conditions are rising year to year and are a leading

AMP-activated protein kinase (AMPK) is an important enzyme that plays a critical role in cellular energy metabolism. By activating AMPK through diet, lifestyle, and supplement strategies, individuals can improve their glucose and lipid metabolism, promote mitochondrial function and autophagy, and protect against oxidative stress and inflammation through activities of AMPK. However, it is important to point out that while AMPK activation can be beneficial for health, it should not be viewed as the only solution to prevent or treat disease. A comprehensive approach to health includes a healthy diet, regular exercise, stress management, and regular medical checkups. These are necessary to achieve good health and wellbeing. The aim of this review is to provide a broad overview of some of the more recent developments within the AMP-activated protein kinase (AMPK) field. Here, a number of recent breakthroughs in the mechanistic understanding of AMPK function have been reviewed. The AMP-activated protein kinase (AMPK) acts as a cellular metabolic sensor. When activated by increases in cellular AMP, it promotes energy restoration and inhibits energy consumption. It coordinates the changes in the activity of enzymes of lipid metabolism and regulates the partitioning of fatty acid between oxidative and biosynthetic pathways. When activated, AMPK initiates metabolic changes like increased glucose uptake, fatty acid oxidation, and glycogen breakdown to generate ATP while inhibiting processes like protein synthesis and lipid synthesis that consume energy. Furthermore, the review suggested that there are several diet, lifestyle, and supplement strategies that can help to better activate AMPK. A low-carbohydrate diet has been shown to activate AMPK and improve glucose and lipid metabolism. Obesity and the metabolic syndrome are a major health problem in Western and developing countries. Keeping in view the role of AMPK in regulating energy balance at both the cellular and whole-body levels, this kinase occupies a central position in studies regarding obesity and diabetes. The study concluded that due to its role in regulating lipid metabolism, AMPK is a potential therapeutic target for conditions like obesity and metabolic syndrome, where dysregulation of lipid metabolism contributes to disease progression. Further research suggests that AMPK may play a role in protecting neurons from stress-induced damage, making it a potential target for neurodegenerative conditions.

The adenosine monophosphate (AMP)–activated protein kinase (AMPK) was initially identified as an enzyme acting as an “energy sensor” in maintaining energy homeostasis via serine/threonine phosphorylation when low cellular adenosine triphosphate (ATP) level was sensed. AMPK participates in catabolic and anabolic processes at the molecular and cellular levels and is involved in appetite-regulating circuit in the hypothalamus. AMPK signaling also modulates energy metabolism in organs such as adipose tissue, brain, muscle, and heart, which are highly dependent on energy consumption via adjusting the AMP/ADP:ATP ratio. In clinics, biguanides and thiazolidinediones are prescribed to patients with metabolic disorders through activating AMPK signaling and inhibiting complex I in the mitochondria, leading to a reduction in mitochondrial respiration and elevated ATP production. The role of AMPK in mediating skeletal development and related diseases remains obscure. In this review, in addition to discuss the emerging advances of AMPK studies in energy control, we will also illustrate current discoveries of AMPK in chondrocyte homeostasis, osteoarthritis (OA) development, and the signaling interaction of AMPK with other pathways, such as mTOR (mechanistic target of rapamycin), Wnt, and NF-κB (nuclear factor κB) under OA condition.

Heat shock factor 1 (HSF1), a master transcriptional regulator of the evolutionarily conserved proteotoxic stress response, preserves proteomic stability upon environmental insults. HSF1 is also a potent pro-oncogenic factor, suggesting that proteomic stability enables oncogenesis. AMP-activated protein kinase (AMPK), a key cellular metabolic sensor and downstream effector of the tumor suppressor LKB1, activates catabolic pathways but suppresses anabolic pathways, including fatty acid and cholesterol biosynthesis, thereby maintaining energy homeostasis. We previously showed that upon activation by metabolic stressors, AMPK interacts with and phosphorylates HSF1 to inactivate it and cause proteomic instability, leading to tumor suppression. Intriguingly, we also discovered that HSF1 suppresses AMPK and controls body fat mass. This study is to investigate the mechanisms by which HSF1 reciprocally suppresses AMPK and the roles of this suppression in lipid metabolism and tumorigenesis. By utilizing recombinant proteins, HSF1 peptide libraries and transcription-deficient HSF1 mutants, our in vitro studies reveal that through physical interactions HSF1 imposes multilayer regulations on AMPK, including blocking AMP binding to γ subunits, impairing LKB1-mediated Thr172 phosphorylation, promoting Thr172 de-phosphorylation by PP2A, and impeding ATP binding to catalytic α subunits. Results from circular dichroism spectroscopy reveal that HSF1 induces global AMPK conformational changes. Biologically, Hsf1 deficiency suppresses lipogenesis and decreases lipid content via AMPK activation. Moreover, Hsf1-deficient cells and mice display reduced cholesterol levels. Interestingly, this defect leads to impaired cholesteroylation of sonic hedgehog (SHH). Consequently, the oncogenic SHH signaling is impaired in HSF1-deficient cells. In vivo, Hsf1 deficiency reduces body fat mass in mice, which can be markedly rescued by either pharmacological or genetic inhibition of AMPK. Importantly, the transcription-deficient HSF1 mutant, through AMPK suppression, enhances the lipid content and SHH cholesteroylation, and promotes the in vivo growth of xenografted human melanomas. In conclusion, HSF1 is a direct AMPK antagonist. This transcription-independent interaction of HSF1 with AMPK epitomizes a reciprocal kinase-substrate regulation whereby lipid metabolism and proteomic stability intertwine and promotes cancer anabolism and oncogenesis. Citation Format: Kuo-Hui Su, Siyuan Dai, Zijian Tang, Meng Xu, Chengkai Dai. Heat shock factor 1 directly suppresses AMP-activated protein kinase to promote tumorigenesis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 293.

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) serves as an energy sensor and master regulator of metabolism. In general, AMPK inhibits anabolism to minimize energy consumption and activates catabolism to increase ATP production. One of the mechanisms employed by AMPK to regulate metabolism is protein acetylation. AMPK regulates protein acetylation by at least five distinct mechanisms. First, AMPK phosphorylates and inhibits acetyl-CoA carboxylase (ACC) and thus regulates acetyl-CoA homeostasis. Since acetyl-CoA is a substrate for all lysine acetyltransferases (KATs), AMPK affects the activity of KATs by regulating the cellular level of acetyl-CoA. Second, AMPK activates histone deacetylases (HDACs) sirtuins by increasing the cellular concentration of NAD+, a cofactor of sirtuins. Third, AMPK inhibits class I and II HDACs by upregulating hepatic synthesis of α-hydroxybutyrate, a natural inhibitor of HDACs. Fourth, AMPK induces translocation of HDACs 4 and 5 from the nucleus to the cytoplasm and thus increases histone acetylation in the nucleus. Fifth, AMPK directly phosphorylates and downregulates p300 KAT. On the other hand, protein acetylation regulates AMPK activity. Sirtuin SIRT1-mediated deacetylation of liver kinase B1 (LKB1), an upstream kinase of AMPK, activates LKB1 and AMPK. AMPK phosphorylates and inactivates ACC, thus increasing acetyl-CoA level and promoting LKB1 acetylation and inhibition. In yeast cells, acetylation of Sip2p, one of the regulatory β-subunits of the SNF1 complex, results in inhibition of SNF1. This results in activation of ACC and reduced cellular level of acetyl-CoA, which promotes deacetylation of Sip2p and activation of SNF1. Thus, in both yeast and mammalian cells, AMPK/SNF1 regulate protein acetylation and are themselves regulated by protein acetylation.

Adenosine monophosphate-activated protein kinase (AMPK) is a key player in maintaining energy homeostasis in response to metabolic stress. Beyond diabetes and metabolic syndrome, there is a growing interest in the therapeutic exploitation of the AMPK pathway in cancer treatment in light of its unique ability to regulate cancer cell proliferation through the reprogramming of cell metabolism. Although many studies support the tumor-suppressive role of AMPK, emerging evidence suggests that the metabolic checkpoint function of AMPK might be overridden by stress or oncogenic signals so that tumor cells use AMPK activation as a survival strategy to gain growth advantage. These findings underscore the complexity in the cellular function of AMPK in maintaining energy homeostasis under physiological versus pathological conditions. Thus, this review aims to provide an overview of recent findings on the functional interplay of AMPK with different cell metabolic and signaling effectors, particularly histone deacetylases, in mediating downstream tumor suppressive or promoting mechanisms in different cell systems. Although AMPK activation inhibits tumor growth by targeting multiple signaling pathways relevant to tumorigenesis, under certain cellular contexts or certain stages of tumor development, AMPK might act as a protective response to metabolic stresses, such as nutrient deprivation, low oxygen, and low pH, or as downstream effectors of oncogenic proteins, including androgen receptor, hypoxia-inducible factor-1α, c-Src, and MYC. Thus, investigations to define at which stage(s) of tumorigenesis and cancer progression or for which genetic aberrations AMPK inhibition might represent a more relevant strategy than AMPK activation for cancer treatment are clearly warranted.

AMP-activated protein kinase (AMPK) β subunits (β1 and β2) provide scaffolds for binding α and γ subunits and contain a carbohydrate-binding module important for regulating enzyme activity. We generated C57Bl/6 mice with germline deletion of AMPK β2 (β2 KO) and examined AMPK expression and activity, exercise capacity, metabolic control during muscle contractions, aminoimidazole carboxamide ribonucleotide (AICAR) sensitivity, and susceptibility to obesity-induced insulin resistance. We find that β2 KO mice are viable and breed normally. β2 KO mice had a reduction in skeletal muscle AMPK α1 and α2 expression despite up-regulation of the β1 isoform. Heart AMPK α2 expression was also reduced but this did not affect resting AMPK α1 or α2 activities. AMPK α1 and α2 activities were not changed in liver, fat, or hypothalamus. AICAR-stimulated glucose uptake but not fatty acid oxidation was impaired in β2 KO mice. During treadmill running β2 KO mice had reduced maximal and endurance exercise capacity, which was associated with lower muscle and heart AMPK activity and reduced levels of muscle and liver glycogen. Reductions in exercise capacity of β2 KO mice were not due to lower muscle mitochondrial content or defects in contraction-stimulated glucose uptake or fatty acid oxidation. When challenged with a high-fat diet β2 KO mice gained more weight and were more susceptible to the development of hyperinsulinemia and glucose intolerance. In summary these data show that deletion of AMPK β2 reduces AMPK activity in skeletal muscle resulting in impaired exercise capacity and the worsening of diet-induced obesity and glucose intolerance.

Mitogen-activated protein kinase (MAPK) pathways are involved in the regulation of cellular responses, including cell proliferation, differentiation, cell growth, and apoptosis. Because these responses are tightly related to cellular energy level, AMP-activated protein kinase (AMPK), which plays an essential role in energy homeostasis, has emerged as another key regulator. In the present study, we demonstrate a novel signal network between AMPK and MAPK in HCT116 human colon carcinoma. Glucose deprivation activated AMPK and three MAPK subfamilies, extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38 MAPK. Under these conditions, inhibition of endogenous AMPK by expressing a dominant-negative form significantly potentiated ERK activation, indicating that glucose deprivation-induced AMPK is specifically antagonizing ERK activity in HCT116 cells. Moreover, we provide novel evidence that AMPK activity is critical for p53-dependent expression of dual-specificity phosphatase (DUSP) 1 & 2, which are negative regulators of ERK. Notably, ERK exhibits pro-apoptotic effects in HCT116 cells under glucose deprivation. Collectively, our data suggest that AMPK protects HCT116 cancer cells from glucose deprivation, in part, via inducing DUSPs, which suppresses pro-apoptotic ERK, further implying that a signal network between AMPK and ERK is a critical regulatory point in coupling the energy status of the cell to the regulation of cell survival.