The major energy source for most cells is glucose, from which ATP is generated via glycolysis and/or oxidative metabolism. Glucose deprivation activates AMP-activated protein kinase (AMPK), but it is ...unclear whether this activation occurs solely via changes in AMP or ADP, the classical activators of AMPK. Here, we describe an AMP/ADP-independent mechanism that triggers AMPK activation by sensing the absence of fructose-1,6-bisphosphate (FBP), with AMPK being progressively activated as extracellular glucose and intracellular FBP decrease. When unoccupied by FBP, aldolases promote the formation of a lysosomal complex containing at least v-ATPase, ragulator, axin, liver kinase B1 (LKB1) and AMPK, which has previously been shown to be required for AMPK activation. Knockdown of aldolases activates AMPK even in cells with abundant glucose, whereas the catalysis-defective D34S aldolase mutant, which still binds FBP, blocks AMPK activation. Cell-free reconstitution assays show that addition of FBP disrupts the association of axin and LKB1 with v-ATPase and ragulator. Importantly, in some cell types AMP/ATP and ADP/ATP ratios remain unchanged during acute glucose starvation, and intact AMP-binding sites on AMPK are not required for AMPK activation. These results establish that aldolase, as well as being a glycolytic enzyme, is a sensor of glucose availability that regulates AMPK.
Metformin, the most prescribed antidiabetic medicine, has shown other benefits such as anti-ageing and anticancer effects
. For clinical doses of metformin, AMP-activated protein kinase (AMPK) has a ...major role in its mechanism of action
; however, the direct molecular target of metformin remains unknown. Here we show that clinically relevant concentrations of metformin inhibit the lysosomal proton pump v-ATPase, which is a central node for AMPK activation following glucose starvation
. We synthesize a photoactive metformin probe and identify PEN2, a subunit of γ-secretase
, as a binding partner of metformin with a dissociation constant at micromolar levels. Metformin-bound PEN2 forms a complex with ATP6AP1, a subunit of the v-ATPase
, which leads to the inhibition of v-ATPase and the activation of AMPK without effects on cellular AMP levels. Knockout of PEN2 or re-introduction of a PEN2 mutant that does not bind ATP6AP1 blunts AMPK activation. In vivo, liver-specific knockout of Pen2 abolishes metformin-mediated reduction of hepatic fat content, whereas intestine-specific knockout of Pen2 impairs its glucose-lowering effects. Furthermore, knockdown of pen-2 in Caenorhabditis elegans abrogates metformin-induced extension of lifespan. Together, these findings reveal that metformin binds PEN2 and initiates a signalling route that intersects, through ATP6AP1, the lysosomal glucose-sensing pathway for AMPK activation. This ensures that metformin exerts its therapeutic benefits in patients without substantial adverse effects.
AMPK and mTOR play principal roles in governing metabolic programs; however, mechanisms underlying the coordination of the two inversely regulated kinases remain unclear. In this study we found, most ...surprisingly, that the late endosomal/lysosomal protein complex v-ATPase-Ragulator, essential for activation of mTORC1, is also required for AMPK activation. We also uncovered that AMPK is a residential protein of late endosome/lysosome. Under glucose starvation, the v-ATPase-Ragulator complex is accessible to AXIN/LKB1 for AMPK activation. Concurrently, the guanine nucleotide exchange factor (GEF) activity of Ragulator toward RAG is inhibited by AXIN, causing dissociation from endosome and inactivation of mTORC1. We have thus revealed that the v-ATPase-Ragulator complex is also an initiating sensor for energy stress and meanwhile serves as an endosomal docking site for LKB1-mediated AMPK activation by forming the v-ATPase-Ragulator-AXIN/LKB1-AMPK complex, thereby providing a switch between catabolism and anabolism. Our current study also emphasizes a general role of late endosome/lysosome in controlling metabolic programs.
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•Ragulator is essential for starvation-induced AMPK activation•LKB1-dependent activation of AMPK takes place on late endosome/lysosome•V-ATPase-Ragulator provides docking sites for AXIN/LKB1 endosomal translocation•V-ATPase-Ragulator is a switch between anabolism and catabolism
AMPK and mTOR regulate cellular balance between catabolism and anabolism. Zhang et al. show that the endosomal v-ATPase-Ragulator complex, required for mTORC1 activation when nutrients are abundant, is also essential in LKB1-mediated AMPK activation in response to energy stress, thus acting as a dual energy sensor.
AMPK, a master regulator of metabolic homeostasis, is activated by both AMP-dependent and AMP-independent mechanisms. The conditions under which these different mechanisms operate, and their ...biological implications are unclear. Here, we show that, depending on the degree of elevation of cellular AMP, distinct compartmentalized pools of AMPK are activated, phosphorylating different sets of targets. Low glucose activates AMPK exclusively through the AMP-independent, AXIN-based pathway in lysosomes to phosphorylate targets such as ACC1 and SREBP1c, exerting early anti-anabolic and pro-catabolic roles. Moderate increases in AMP expand this to activate cytosolic AMPK also in an AXIN-dependent manner. In contrast, high concentrations of AMP, arising from severe nutrient stress, activate all pools of AMPK independently of AXIN. Surprisingly, mitochondrion-localized AMPK is activated to phosphorylate ACC2 and mitochondrial fission factor (MFF) only during severe nutrient stress. Our findings reveal a spatiotemporal basis for hierarchical activation of different pools of AMPK during differing degrees of stress severity.
Increased lipogenesis has been linked to an increased cancer risk and poor prognosis; however, the underlying mechanisms remain obscure. Here we show that phosphatidic acid phosphatase (PAP) lipin-1, ...which generates diglyceride precursors necessary for the synthesis of glycerolipids, interacts with and is a direct substrate of the Src proto-oncogenic tyrosine kinase. Obesity-associated microenvironmental factors and other Src-activating growth factors, including the epidermal growth factor, activate Src and promote Src-mediated lipin-1 phosphorylation on Tyr398, Tyr413 and Tyr795 residues. The tyrosine phosphorylation of lipin-1 markedly increases its PAP activity, accelerating the synthesis of glycerophospholipids and triglyceride. Alteration of the three tyrosine residues to phenylalanine (3YF-lipin-1) disables lipin-1 from mediating Src-enhanced glycerolipid synthesis, cell proliferation and xenograft growth. Re-expression of 3YF-lipin-1 in PyVT;Lpin1
mice fails to promote progression and metastasis of mammary tumours. Human breast tumours exhibit increased p-Tyr-lipin-1 levels compared to the adjacent tissues. Importantly, statistical analyses show that levels of p-Tyr-lipin-1 correlate with tumour sizes, lymph node metastasis, time to recurrence and survival of the patients. These results illustrate a direct lipogenesis-promoting role of the pro-oncogenic Src, providing a mechanistic link between obesity-associated mitogenic signaling and breast cancer malignancy.
In metazoans, cells depend on extracellular growth factors for energy homeostasis. We found that glycogen synthase kinase-3 (GSK3), when deinhibited by default in cells deprived of growth factors, ...activates acetyltransferase TIP60 through phosphorylating TIP60-Ser⁸⁶, which directly acetylates and stimulates the protein kinase ULK1, which is required for autophagy. Cells engineered to express TIP60 S86A that cannot be phosphorylated by GSK3 could not undergo serum deprivation-induced autophagy. An acetylation-defective mutant of ULK1 failed to rescue autophagy in ULK1 -/- mouse embryonic fibroblasts. Cells used signaling from GSK3 to TIP60 and ULK1 to regulate autophagy when deprived of serum but not glucose. These findings uncover an activating pathway that integrates protein phosphorylation and acetylation to connect growth factor deprivation to autophagy.
unc‐51‐like autophagy activating kinase 1 and 2 (Ulk1/2) regulate autophagy initiation under various stress conditions. However, the physiological functions of these Ser/Thr kinases are not well ...characterized. Here, we show that mice with liver‐specific double knockout (LDKO) of Ulk1 and Ulk2 (Ulk1/2 LDKO) are viable, but exhibit overt hepatomegaly phenotype. Surprisingly, Ulk1/2 LDKO mice display normal autophagic activity in hepatocytes upon overnight fasting, but are strongly resistant to acetaminophen (APAP)‐induced liver injury. Further studies revealed that Ulk1/2 are also dispensable for APAP‐induced autophagy process, but are essential for the maximum activation of c‐Jun N‐terminal kinase (JNK) signaling both in vivo and in isolated primary hepatocytes during APAP treatment. Mechanistically, APAP‐induced inhibition of mechanistic target of rapamycin complex 1 releases Ulk1 from an inactive state. Activated Ulk1 then directly phosphorylates and increases the kinase activity of mitogen‐activated protein kinase kinase 4 and 7 (MKK4/7), the upstream kinases and activator of JNK, and mediates APAP‐induced liver injury. Ulk1‐dependent phosphorylation of MKK7 was further confirmed by a context‐dependent phosphorylation antibody. Moreover, activation of JNK and APAP‐induced cell death was markedly attenuated in Mkk4/7 double knockdown hepatocytes reconstituted with an Ulk1‐unphosphorylatable mutant of MKK7 compared to those in cells rescued with wild‐type MKK7. Conclusion: Together, these findings reveal an important role of Ulk1/2 for APAP‐induced JNK activation and liver injury, and understanding of this regulatory mechanism may offer us new strategies for prevention and treatment of human APAP hepatotoxicity. (Hepatology 2018;67:2397‐2413).
Fiber‐based structures are highly desirable for wearable electronics that are expected to be light‐weight, long‐lasting, flexible, and conformable. Many fibrous structures have been manufactured by ...well‐established lost‐effective textile processing technologies, normally at ambient conditions. The advancement of nanotechnology has made it feasible to build electronic devices directly on the surface or inside of single fibers, which have typical thickness of several to tens microns. However, imparting electronic functions to porous, highly deformable and three‐dimensional fiber assemblies and maintaining them during wear represent great challenges from both views of fundamental understanding and practical implementation. This article attempts to critically review the current state‐of‐arts with respect to materials, fabrication techniques, and structural design of devices as well as applications of the fiber‐based wearable electronic products. In addition, this review elaborates the performance requirements of the fiber‐based wearable electronic products, especially regarding the correlation among materials, fiber/textile structures and electronic as well as mechanical functionalities of fiber‐based electronic devices. Finally, discussions will be presented regarding to limitations of current materials, fabrication techniques, devices concerning manufacturability and performance as well as scientific understanding that must be improved prior to their wide adoption.
Fiber‐based electronic structures have great potential to be light‐weight, long‐lasting, flexible, and comfortable. It is highly feasible to build electronic functions directly on the surface or inside of single fibers by cost‐effective manufacturing technologies. This article presents a critical review of the state of the art with respect to materials, fabrication techniques, structural design of devices as well as applications of the fiber‐based wearable electronic products.
The utilization of waste activated sludge (WAS) to recover energy in the form of methane or short-chain fatty acids (SCFAs) is generally restricted by the low energy density of products and poor ...degradability of WAS. Herein, this study reported a novel alternative WAS fermentation technology to produce high-energy medium-chain fatty acids (MCFAs) from WAS in a one-stage anaerobic fermentation system using ethanol as electron donor. The MCFAs production and WAS degradation at different ethanol levels were investigated. The increased ethanol levels resulted in increasing MCFAs production (from 1875 to 6115 mg chemical oxygen demand (COD)/L) and selectivity (from 30.3 to 56.2%). The main MCFAs products were n-caproate and n-caprylate at lower level of ethanol, while n-caproate was the sole MCFA product at higher level of ethanol with longer chain alcohol (i.e., n-hexanol) produced as well. The ethanol markedly increased WAS degradation, with the greatest degradation (0.72 g COD/g volatile solids (VS)) being 1.9 times of that without ethanol (0.38 mg COD/mg VS, at 0 mmol/L), which was ascribed to the advancement of sludge solubilization, hydrolysis and acidification. Microbial community revealed that the ethanol participation induced the community shift to the favorable direction for hydrolysis-acidification and chain elongation in anaerobic WAS fermentation.
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•MCFAs production from WAS and enhanced WAS degradation were simultaneously achieved.•Sludge solubilization, hydrolysis and acidification were enhanced by ethanol.•MCFAs production and selectivity were positively related to the ethanol addition.•Hydrolytic, acidification and CE microbes were enriched for MCFAs production.
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