AMPK对线粒体功能的调节

5’单磷酸腺苷活化蛋白激酶（AMP—activated protein kinase,AMPK）是细胞的能量感受器,调节细胞能量代谢,在正常细胞和癌细胞中均发挥重要的生物功能,它的激活有助于纠正代谢紊乱,使细胞代谢趋向生理平衡。在细胞应急反应中,细胞感受到能量危机,ATP浓度下降,AMP浓度上升,细胞内AMP／ATP比例上升,AMPK被激活：而在病理状态下,如代谢综合征、肿瘤等,常伴随能量代谢紊乱和AMPK激活抑制,因此,AMPK被视为治疗代谢性疾病与肿瘤的潜在作用靶点。然而,AMPK对能量代谢的调节与线粒体的功能密不可分,线粒体作为细胞的能量工厂,在健康与疾病中也发挥着重要的作用。越来越多的研究表明,线粒体能影响AMPK的活性,同时AMPK也通过多方面对线粒体进行调节,线粒体相关疾病与AMPK的调节有着密切的关系。该文主要针对AMPK是如何对线粒体的合成、线粒体自噬、内源性凋亡及线粒体相关疾病等方面进行综述。     

AMPK phosphorylation of raptor mediates a metabolic checkpoint

AMPK is a highly conserved sensor of cellular energy status that is activated under conditions of low intracellular ATP. AMPK responds to energy stress by suppressing cell growth and biosynthetic processes, in part through its inhibition of the rapamycin-sensitive mTOR (mTORC1) pathway. AMPK phosphorylation of the TSC2 tumor suppressor contributes to suppression of mTORC1; however, TSC2-deficient cells remain responsive to energy stress. Using a proteomic and bioinformatics approach, we sought to identify additional substrates of AMPK that mediate its effects on growth control. We report here that AMPK directly phosphorylates the mTOR binding partner raptor on two well-conserved serine residues, and this phosphorylation induces 14-3-3 binding to raptor. The phosphorylation of raptor by AMPK is required for the inhibition of mTORC1 and cell-cycle arrest induced by energy stress. These findings uncover a conserved effector of AMPK that mediates its role as a metabolic checkpoint coordinating cell growth with energy status.     

The role of AMPK and mTOR in nutrient sensing in pancreatic beta-cells

The AMP-activated protein kinase (AMPK) is a central regulator of the energy status of the cell, based on its unique ability to respond directly to fluctuations in the ratio of AMP:ATP. Because glucose and amino acids stimulate insulin release from pancreatic beta-cells by the regulation of metabolic intermediates, AMPK represents an attractive candidate for control of beta-cell function. Here, we show that inhibition of AMPK in beta-cells by high glucose inversely correlates with activation of the mammalian Target of Rapamycin (mTOR) pathway, another cellular sensor for nutritional conditions. Forced activation of AMPK by AICAR, phenformin, or oligomycin significantly blocked phosphorylation of p70S6K, a downstream target of mTOR, in response to the combination of glucose and amino acids. Amino acids also suppressed the activity of AMPK, and this at a minimum required the presence of leucine and glutamine. It is unlikely that the ability of AMPK to sense both glucose and amino acids plays a role in regulation of insulin secretion, as inhibition of AMPK by amino acids did not influence insulin secretion. Moreover, activation of AMPK by AICAR or phenformin did not antagonize glucose-stimulated insulin secretion, and insulin secretion was also unaffected in response to suppression of AMPK activity by expression of a dominant negative AMPK construct (K45R). Taken together, these results suggest that the inhibition of AMPK activity by glucose and amino acids might be an important component of the mechanism for nutrient-stimulated mTOR activity but not insulin secretion in the beta-cell.     

Involvement of AMP-activated protein kinase and p38 mitogen-activated protein kinase in 8-Cl-cAMP-induced growth inhibition

8-Cl-cAMP (8-chloro-cyclic AMP), which induces differentiation, growth inhibition and apoptosis in various cancer cells, has been investigated as a putative anti-cancer drug. Although we reported that 8-Cl-cAMP induces growth inhibition via p38 mitogen-activated protein kinase (MAPK) and a metabolite of 8-Cl-cAMP, 8-Cl-adenosine mediates this process, the action mechanism of 8-Cl-cAMP is still uncertain. In this study, it was found that 8-Cl-cAMP-induced growth inhibition is mediated by AMP-activated protein kinase (AMPK). 8-Cl-cAMP was shown to activate AMPK, which was also dependent on the metabolic degradation of 8-Cl-cAMP. A potent agonist of AMPK, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) could also induce growth inhibition and apoptosis. To further delineate the role of AMPK in 8-Cl-cAMP-induced growth inhibition and apoptosis, we used two approaches: pharmacological inhibition of the enzyme with compound C and expression of a dominant negative mutant (a kinase-dead form of AMPKalpha2, KD-AMPK). AICAR was able to activate p38 MAPK and pre-treatment with AMPK inhibitor or expression of KD-AMPK blocked this p38 MAPK activation. Cell growth inhibition was also attenuated. Furthermore, p38 MAPK inhibitor attenuated 8-Cl-cAMP- or AICAR-induced growth inhibition but had no effect on AMPK activation. These results demonstrate that 8-Cl-cAMP induced growth inhibition through AMPK activation and p38 MAPK acts downstream of AMPK in this signaling pathway.     

Activators of the energy sensing kinase AMPK inhibit random cell movement and chemotaxis in U937 cells

AMP-activated protein kinase (AMPK) is a key energy sensor, known to regulate energy metabolism in diverse cell types. Hypoxia is encountered frequently in the microenvironments of inflammatory lesions and is a critical regulator of function in inflammatory cells. Energy deficiency is one of the consequences of hypoxia, but its potential role in modulating leucocyte function has received little attention. Using micropore chemotaxis assays to assess migratory responses of the monocyte-like cell line U937, it was found that the AMPK activators AICAR and phenformin rapidly reduced random migration (chemokinesis) as well as chemotaxis due to stromal cell-derived factor (SDF)1alpha. There was an approximate 50% reduction in both chemokinesis and chemotaxis following 30 min preincubation with both AICAR and phenformin (P < 0.01), and this continued with up to 24 h preincubation. The binding of SDF1alpha to its receptor CXCR4 was unaltered, suggesting AMPK was acting on downstream intracellular signalling pathways important in cell migration. As AMPK and statins are known to inhibit HMG CoA reductase, and both reduce cell migration, the effect of mevastatin on U937 cells was compared with AMPK activators. Mevastatin inhibited cell migration but required 24 h preincubation. As expected, the inhibitory effect of mevastatin was associated with altered subcellular localization of the Rho GTPases, RhoA and cdc42, indicating decreased prenylation of these molecules. Although the effect of AMPK activation was partially reversed by mevalonate, this was not associated with altered subcellular localization of Rho GTPases. The data suggest that activation of AMPK has a general effect on cell movement in U937 cells, and this is not due to inhibition of HMG CoA reductase. These are the first data to show an effect of AMPK on cell movement, and suggest a fundamental role for energy deficiency in regulating cellular behaviour.     

Regulation of AMP-activated protein kinase by multisite phosphorylation in response to agents that elevate cellular cAMP

The AMP-activated protein kinase (AMPK) and cAMP signaling systems are both key regulators of cellular metabolism. In this study, we show that AMPK activity is attenuated in response to cAMP-elevating agents through modulation of at least two of its alpha subunit phosphorylation sites, viz. alpha-Thr(172) and alpha1-Ser(485)/alpha2-Ser(491), in the clonal beta-cell line INS-1 as well as in mouse embryonic fibroblasts and COS cells. Forskolin, isobutylmethylxanthine, and the glucose-dependent insulinotropic peptide inhibited AMPK activity and reduced phosphorylation of the activation loop alpha-Thr(172) via inhibition of calcium/calmodulin-dependent protein kinase kinase-alpha and -beta, but not LKB1. These agents also enhanced phosphorylation of alpha-Ser(485/491) by the cAMP-dependent protein kinase. AMPK alpha-Ser(485/491) phosphorylation was necessary but not sufficient for inhibition of AMPK activity in response to forskolin/isobutylmethylxanthine. We show that AMPK alpha-Ser(485/491) can be a site for autophosphorylation, which may play a role in limiting AMPK activation in response to energy depletion or other regulators. Thus, our findings not only demonstrate cross-talk between the cAMP/cAMP-dependent protein kinase and AMPK signaling modules, but also describe a novel mechanism by which multisite phosphorylation of AMPK contributes to regulation of its enzyme activity.     

AMPK: a metabolic gauge regulating whole-body energy homeostasis

AMP-activated protein kinase (AMPK) is the downstream component of a kinase cascade that acts as a gauge of cellular energy levels. Over the last few years, accumulating evidence has demonstrated that AMPK is also involved in the regulation of energy balance at the whole-body level by responding to hormones and nutrient signals, which leads to changes in energy homeostasis. The physiological relevance of this new role of AMPK is demonstrated by the fact that impairment of AMPK function is associated with metabolic alterations, insulin resistance, obesity, hormonal disorders and cardiovascular disease. Here, we summarize the role of AMPK in the regulation of energy homeostasis. Understanding this key enzyme and its tissue-specific regulation will provide new targets for the treatment of metabolic disorders.     

A role for AMPK in the inhibition of glucose-6-phosphate dehydrogenase by polyunsaturated fatty acids

Both polyunsaturated fatty acids and AMPK promote energy partitioning away from energy consuming processes, such as fatty acid synthesis, towards energy generating processes, such as β-oxidation. In this report, we demonstrate that arachidonic acid activates AMPK in primary rat hepatocytes, and that this effect is p38 MAPK-dependent. Activation of AMPK mimics the inhibition by arachidonic acid of the insulin-mediated induction of G6PD. Similar to intracellular signaling by arachidonic acid, AMPK decreases insulin signal transduction, increasing Ser³⁰⁷ phosphorylation of IRS-1 and a subsequent decrease in AKT phosphorylation. Overexpression of dominant-negative AMPK abolishes the effect of arachidonic acid on G6PD expression. These data suggest a role for AMPK in the inhibition of G6PD by polyunsaturated fatty acids.     

Development of protein kinase activators: AMPK as a target in metabolic disorders and cancer

AMP-activated protein kinase (AMPK) is a cellular energy sensor activated by metabolic stresses that either inhibit ATP synthesis or accelerate ATP consumption. Activation of AMPK in response to an increase in the cellular AMP:ATP ratio results in inhibition of ATP-consuming processes such as gluconeogenesis and fatty acid synthesis, while stimulating ATP-generating processes, including fatty acid oxidation. These alterations in lipid and glucose metabolism would be expected to ameliorate the pathogenesis of obesity, type 2 diabetes and other metabolic disorders. Recently, AMPK has also been identified as a potential target for cancer prevention and/or treatment. Cell growth and proliferation are energetically demanding, and AMPK may act as an “energy checkpoint” that permits growth and proliferation only when energy reserves are sufficient. Thus, activators of AMPK could have potential as novel therapeutics both for metabolic disorders and for cancer, which together constitute two of the most prevalent groups of diseases worldwide.     

AMP-Activated Kinase Restricts Rift Valley Fever Virus Infection by Inhibiting Fatty Acid Synthesis

The cell intrinsic innate immune responses provide a first line of defense against viral infection, and often function by targeting cellular pathways usurped by the virus during infection. In particular, many viruses manipulate cellular lipids to form complex structures required for viral replication, many of which are dependent on de novo fatty acid synthesis. We found that the energy regulator AMPK, which potently inhibits fatty acid synthesis, restricts infection of the Bunyavirus, Rift Valley Fever Virus (RVFV), an important re-emerging arthropod-borne human pathogen for which there are no effective vaccines or therapeutics. We show restriction of RVFV both by AMPK and its upstream activator LKB1, indicating an antiviral role for this signaling pathway. Furthermore, we found that AMPK is activated during RVFV infection, leading to the phosphorylation and inhibition of acetyl-CoA carboxylase, the first rate-limiting enzyme in fatty acid synthesis. Activating AMPK pharmacologically both restricted infection and reduced lipid levels. This restriction could be bypassed by treatment with the fatty acid palmitate, demonstrating that AMPK restricts RVFV infection through its inhibition of fatty acid biosynthesis. Lastly, we found that this pathway plays a broad role in antiviral defense since additional viruses from disparate families were also restricted by AMPK and LKB1. Therefore, AMPK is an important component of the cell intrinsic immune response that restricts infection through a novel mechanism involving the inhibition of fatty acid metabolism.     

腺苷酸活化蛋白激酶在脂联素心血管保护效应中的作用

脂联素是主要由白色脂肪组织分泌的一种活性多肽,具有调节脂肪酸和葡萄糖代谢、抗炎、减轻动脉粥样硬化等多种生物学功能,血浆脂联素含量降低参与了代谢性疾病及心血管疾病的发生、发展。腺苷酸活化蛋白激酶（AMP．activated protein kinase,AMPK）是脂联素信号通路中的关键信号分子,本文就其在脂联素心血管保护效应中的作用作一综述,介绍脂联素改善糖、脂代谢紊乱、动脉粥样硬化、心力衰竭及心肌缺血,再灌注损伤作用机制的新进展。     

Activation of adenosine monophosphate activated protein kinase inhibits growth of multiple myeloma cells

The role of adenosine monophosphate activated protein kinase (AMPK) in regulating multiple myeloma (MM) cell growth is not yet clear. In this study, we show that the AMPK activators 5-aminoimidazole-4-carboxamide riboside (AICAr) and D942 inhibit cell growth in MM cell lines. AICAr also induced an S-phase cell cycle arrest in all four tested cell lines and led to phosphorylation and thus activation of AMPK. Furthermore, the inhibition of a nucleoside transporter by nitrobenzyl-thio-9-beta-d-ribofuranosylpurine (NBTI), inhibition of the adenosine kinase by iodotubericidine and inhibition of AMPK by AMPKI Compound C reversed AICAr effects, indicating that the cellular effects of AICAr were mediated by AMPK. Activation of AMPK inhibited basal extracellular signal-regulated kinase (ERK), mammalian target of rapamycin (mTOR) and P70S6 kinase (P70S6K) as well as AKT phosphorylation, and blocked IL-6, IGF-1, and HS-5 stromal cell conditioned medium-induced increase of cell growth. Troglitazone, which has previously been shown to activate AMPK, similarly inhibited MM cell growth, activated AMPK, and decreased ERK and P70S6K phosphorylation. Our results suggest that activation of AMPK inhibits MM cell growth despite stimulation with IL-6, IGF-1, or HS-5 stromal cell conditioned medium and represents a potential new target in the therapy of MM.     

AMPK-Activated Protein Kinase Suppresses Ccr2 Expression by Inhibiting the NF-κB Pathway in RAW264.7 Macrophages

C-C chemokine receptor 2 (Ccr2) is a key pro-inflammatory marker of classic (M1) macrophage activation. Although Ccr2 is known to be expressed both constitutively and inductively, the full regulatory mechanism of its expression remains unclear. AMP-activated protein kinase (AMPK) is not only a master regulator of energy homeostasis but also a central regulator of inflammation. In this study, we sought to assess AMPK’s role in regulating RAW264.7 macrophage Ccr2 protein levels in resting (M0) or LPS-induced M1 states. In both M0 and M1 RAW264.7 macrophages, knockdown of the AMPKα1 subunit by siRNA led to increased Ccr2 levels whereas pharmacologic (A769662) activation of AMPK, attenuated LPS-induced increases in Ccr2 expression in an AMPK dependent fashion. The increases in Ccr2 levels by AMPK downregulation were partially reversed by NF-κB inhibition whereas TNF-a inhibition had minimal effects. Our results indicate that AMPK is a negative regulator of Ccr2 expression in RAW264.7 macrophages, and that the mechanism of action of AMPK inhibition of Ccr2 is mediated, in part, through the NF-κB pathway.     

AMPK, an active player in the control of metabolism

Impairment in the regulation of energy homeostasis and imbalance between energy intake and energy expenditure lead to many metabolic disorders and diseases such as obesity and type 2 diabetes. AMP-activated protein kinase (AMPK) is considered as a "fuel-gauge" in the cell and plays a key role in the regulation of energy metabolism. Activated by an increase in the AMP/ATP ratio, AMPK switches on catabolic pathways such as fatty acid oxidation and switches off anabolic pathways such as lipogenesis or gluconeogenesis. Insulin-sensitizing adipokines (leptin and adiponectin) and anti-diabetic drugs (thiazolidinediones and biguanides) are acting in part through the activation of AMPK. More recent findings indicate that AMPK plays also a major role in the control of whole body energy homeostasis by integrating, at the hypothalamus level, nutrient and hormonal signals that regulate food intake and energy expenditure. AMPK provides therefore a potential target for the treatment of metabolic diseases such as obesity and type II diabetes.     

Melanocortin-induced PKA activation inhibits AMPK activity via ERK-1/2 and LKB-1 in hypothalamic GT1-7 cells

α-Melanocyte-stimulating hormone (α-MSH)-induced activation of the melanocortin-4 receptor in hypothalamic neurons increases energy expenditure and inhibits food intake. Active hypothalamic AMP-activated protein kinase (AMPK) has recently been reported to enhance food intake, and in vivo experiments suggested that intrahypothalamic injection of melanocortins decreased food intake due to the inhibition of AMPK activity. However, it is not clear whether α-MSH affects AMPK via direct intracellular signaling cascades or if the release of paracrine factors is involved. Here, we used a murine, hypothalamic cell line (GT1-7 cells) and monitored AMPK phosphorylation at Thr(172), which has been suggested to increase AMPK activity. We found that α-MSH dephosphorylated AMPK at Thr(172) and consequently decreased phosphorylation of the established AMPK substrate acetyl-coenzyme A-carboxylase at Ser(79). Inhibitory effects of α-MSH on AMPK were blocked by specific inhibitors of protein kinase A (PKA) or ERK-1/2, pointing to an important role of both kinases in this process. Because α-MSH-induced activation of ERK-1/2 was blunted by PKA inhibitors, we propose that ERK-1/2 serves as a link between PKA and AMPK in GT1-7 cells. Furthermore, down-regulation of liver kinase B-1, but not inhibition of calcium-calmodulin-dependent kinase kinase-β or TGFβ-activated kinase-1 decreased basal phosphorylation of AMPK and its dephosphorylation induced by α-MSH. Thus, we propose that α-MSH inhibits AMPK activity via a linear pathway, including PKA, ERK-1/2, and liver kinase B-1 in GT1-7 cells. Given the importance of the melanocortin system in the formation of adipositas, detailed knowledge about this pathway might help to develop drugs targeting obesity.     

Activation of AMP-activated protein kinase inhibits protein synthesis associated with hypertrophy in the cardiac myocyte

A necessary mediator of cardiac myocyte enlargement is protein synthesis, which is controlled at the levels of both translation initiation and elongation. Eukaryotic elongation factor-2 (eEF2) mediates the translocation step of peptide-chain elongation and is inhibited through phosphorylation by eEF2 kinase. In addition, p70S6 kinase can regulate protein synthesis by phosphorylating eEF2 kinase or via phosphorylation of ribosomal protein S6. We have recently shown that eEF2 kinase is also controlled by phosphorylation by AMP-activated protein kinase (AMPK), a key regulator of cellular energy homeostasis. Moreover, the mammalian target of rapamycin has also been shown to be inhibited, indirectly, by AMPK, thus leading to the inhibition of p70S6 kinase. Although AMPK activation has been shown to modulate protein synthesis, it is unknown whether AMPK could also be a regulator of cardiac hypertrophic growth. Therefore, we investigated the role of AMPK activation in regulating protein synthesis during both phenylephrine- and Akt-induced cardiac hypertrophy. Metformin and 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside were used to activate AMPK in neonatal rat cardiac myocytes. Activation of AMPK significantly decreased protein synthesis induced by phenylephrine treatment or by expression of constitutively active Akt. Activation of AMPK also resulted in decreased p70S6 kinase phosphorylation and increased phosphorylation of eEF2, suggesting that inhibition of protein synthesis involves the eEF2 kinase/eEF2 axis and/or the p70S6 kinase pathway. Together, our data suggest that the inhibition of protein synthesis by pharmacological activation of AMPK may be a key regulatory mechanism by which hypertrophic growth can be controlled.