PMC

Proposed mechanisms by which AMPK both activates SIRT1 and cooperates with it in enhancing the ability of PGC-1α to stimulate mitochondrial biogenesis and function. This schema, based on studies carried out primarily in skeletal muscle and cultured myocytes, assumes that the activation of AMPK and the phosphorylation of PGC-1α are early events and that activation of SIRT1 and PGC-1α deacetylation occur later. It has been suggested that the phosphorylation of PGC-1α by AMPK makes it more susceptible to deacetylation by SIRT1 () and enhances its ability to activate its own promoter (). Whether Nampt activation is pivotal for SIRT1 activation by AMPK is unclear (cf. Refs. and ).

Commonalities between AMPK and SIRT1. Both AMPK and SIRT1 are activated in vivo in many tissues by caloric restriction and exercise as well as treatment with resveratrol and 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (see below). In addition, they have many common target molecules and biological actions (, , ). A case in point: the transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), which in muscle () and other tissues is a master regulator of mitochondrial biogenesis and function, is illustrated in . FOXO, forkhead box-containing protein.

The hypothetical SIRT1/AMPK cycle and its significance. A decrease in energy state or activation of AMPK by other means leads to activation of SIRT1, perhaps by increasing NAD⁺ or the NAD/NADH ratio () and/or the activity of Nampt (). SIRT1 then deacetylates and activates LKB1, which in turn activates AMPK. Alternatively, these events could be set in motion by factors that primarily increase SIRT1. The joint activation of SIRT1 and AMPK allows for the concurrent deacetylation and phosphorylation of the listed target molecules and presumably others. The predicted result would be a decreased susceptibility to metabolic syndrome-associated disorders and possibly delayed aging. Not shown is that a primary downregulation of SIRT1 or AMPK would presumably have opposite effects and predispose to the metabolic syndrome and accelerated aging.

Proposed mechanism for activation of LKB1 and LKB1 target molecules by SIRT1 Activation of SIRT1 by genetic or pharmacological means in human embryonic kidney-293T cells (and presumably others) leads to deacetylation of Lys⁴⁸ and possibly other key lysine residues on LKB1. This in turn enhances LKB1 binding to STE20-related adaptor protein (STRAD) and mouse embryo scaffold protein (MO25), which activates its kinase activity and leads to the phosphorylation of AMPK. The scheme assumes that SIRT1 is primarily nuclear and that LKB1 acetylation occurs in the nucleus and in some way enhances its movement to the cytoplasm (where it binds to STRAD). Since under some circumstances SIRT1 may be found in the cytoplasm, it is also possible that LKB1 acetylation could be an extranuclear event. In addition to AMPK, LKB1 phosphorylates and activates MARK1 and 12 other AMPK-related kinases (ARKs). CaMK kinase (CaMKK), which phosphorylates and activates AMPK even in the absence of LKB1, presumably would not activate the ARKs unless the increase in AMPK activity activated SIRT1 and, secondarily, LKB1 (adapted from Ref. ).

The metabolic syndrome: pathogenetic factors and associated diseases. A combination of overnutrition, inactivity, and genetic and other factors interact to produce a state of metabolic susceptibility that we are proposing leads to dysregulation of AMPK and SIRT1. This in turn could lead to pathogenetic factors for the metabolic syndrome, such as insulin resistance, hyperinsulinemia, and mitochondrial dysfunction, and abnormalities in cellular lipid metabolism. The latter is reflected typically by modest increases in plasma triglycerides and ectopic lipid deposition in muscle and liver. Systemic evidence of inflammation is often a later event (). The transition from this preclinical stage to the clinically diagnosed metabolic syndrome to overt disease may take many years, with the rate of progression determined by environmental factors such as diet and exercise, genetic predisposition, and pharmacotherapy. Presumably, treatments (Rx) targeted at AMPK and SIRT1 would be useful at all stages but would be most effective early before abnormalities in tissues and cells such as oxidative modification of proteins and DNA, plaque formation in arteries, and capillary rarefaction in muscle and other tissues occur. ER, endoplasmic reticulum; ASCVD, atherosclerotic cardiovascular disease; NAFLD, nonalcoholic fatty liver disease.

Regulation of SIRT1 (). SIRT1 is an NAD⁺-dependent histone/protein deacetylase whose activity is regulated by nutrient availability. It has been proposed that nutrient deprivation (shown in the figure) increases SIRT1 activity by increasing the abundance of NAD⁺ and decreasing the abundance of nicotinamide (NAM), a product of the reaction, and NADH, both of which inhibit SIRT1. NAM phosphoribosyltransferase (Nampt) catalyzes the conversion of NAM to NAD⁺; therefore, it activates SIRT1 both by increasing cellular NAD⁺ and diminishing NAM. Exercise has been shown to increase Nampt activity in human muscle (). Nutrient excess appears to have opposite effects on SIRT1 activity and these regulatory factors. Nampt, sometimes referred to as visfatin, is also found in the circulation and has been reported to increase insulin sensitivity (). AMPK may mediate the activation of SIRT1 caused by fuel deprivation and other stimuli by its effects on these molecules (, ). SIRT1 can also be activated directly by binding the nuclear protein to active regulator of SIRT1 (AROS) () and inhibited by interaction with DBC1 (deleted in breast cancer; an inhibitor SIRT1 deacetylation) () and binding to SUMO1/snetrin-specific peptidase (SENP1) (), which catalyzes its desumoylation. SIRT1 expression and abundance may also be upregulated by endothelial nitric oxide synthase (eNOS)/nitric oxide, the mRNA binding protein HuR (), and a p53:FOXO3a complex and downregulated by p53 and a H1C1:CtBP corepressor complex (not shown). Of the latter molecules, AMPK has been demonstrated to influence eNOS, p53, and FOXO activity by causing their phosphorylation (); however, the physiological relevance of this with regard to SIRT1 regulation is unclear. Finally, SIRT1 can be found in both the nucleus and the cytoplasm, depending on cell type and conditions, and movement between these compartments could be another determinant of its actions (). Lys and Ac.

AMP-activated protein kinase (AMPK) activation and its regulation (). AMPK is a heterotrimer consisting of a catalytic α-subunit (α1, α2) and regulatory β- and λ-subunits (β1, β2, α1, α2, and α3), all of which are required for its activity. Heterotrimers containing the α1 subunit are exclusively cytoplasmic; however, α2-containing AMPK is also found in the nucleus, where its presence may increase after exercise (). The γ-subunit contains several cystathione β-synthase domains that under baseline conditions predominantly bind ATP. When a cell is energy stressed and the AMP/ATP ratio increases, AMP replaces ATP on 2 of these domains (). This results in a conformational change that 1) causes a modest increase in AMPK activity (2- to 10-fold) and 2) enhances the phosphorylation of Thr¹⁷² on the α-subunit, which results in a much greater activation of the enzyme. The active enzyme then phosphorylates multiple molecules (enzymes, transcriptional activators, and coactivators), with the end result a restoration of the cell's energy state. Serine-threonine liver kinase B1 (LKB1) is required for this phosphorylation; however, it is less clear whether changes in LKB1 activity specifically regulate it. Thus studies, predominantly in skeletal muscle, have suggested that LKB1 is constitutively active and that the conformational change induced by an increase in the AMP/ATP ratio increases phosphorylation at Thr¹⁷² by making AMPK resistant to the action of phosphatases (). On the other hand, as is described in the text, activation of LKB1 and subsequently AMPK has been observed in various cells and tissues when silent information regulator T1 (SIRT1) is activated, and conversely, decreases in SIRT1 are associated with diminished LKB1 and AMPK activity (, , ). Not shown in the figure is that nutrient and O₂ deprivation, increased energy expenditure (exercise), and various hormones typically initiate AMPK activation, whereas nutrient excess (e.g., high glucose) and other hormones (e.g., glucocorticoids) downregulate AMPK, leading to decreases in fatty acid oxidation and increases in lipid and protein synthesis (). CaMKKβ, calcium/calmodulin kinase kinase-β; PP2A and PP2C, protein phosphatases 2A and 2C, respectively.