Mechanisms Of The Sensing Of Glucose Levels And Energy Stresses By AMPK

AMPK,a master regulator of organism metabolic homeostasis,is activated by nutrient shortage and following energy stress,and therefore remodels metabolic flux by phosphorylating its downstream targets,lowering energy expense while promoting energy production to maintain cellular metabolic homeostasis.The major energy source for most cells is glucose and its deprivation activates AMPK[1].But it is unclear whether this activation occurs solely via energy stress represented by increases in AMP or ADP levels,which is the classical mechanism of AMPK activation[2-5].This study reveals 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,aldolase promotes the formation of a lysosomal complex containing V-ATPase,Ragulator complex,AXIN,liver kinase B1(LKB1)and AMPK,which has been proved to be required for AMPK activation[6,7].Knockdown of aldolase activates AMPK even in cells with abundant glucose,whereas the 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.In some cell types AMP/ATP and ADP/ATP ratios remain unchanged during glucose starvation.These results establish that aldolase is a sensor of glucose availability that regulates AMPK.Besides,we also 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 treatment has differential effects on cellular energy levels of distinct cell types.In MEFs,HEK293T cells,and livers,low glucose has no significant influence on energy state represented by cellular AMP levels,while it causes moderate increases in intracellular AMP concentrations to about 60 μM in HEK293 cells.Under conditions where both glucose and glutamine are withdrawn or blood supply is deficient,AMP concentrations increase largely in cells as well as in livers.Low glucose treatment without changing cellular AMP levels activates AMPK exclusively through the AXIN-based pathway in lysosomes to phosphorylate targets such as ACC1 and SREBP1c,exerting anti-anabolic roles.Moderate increases in intracellular AMP further 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,resulting in phosphorylation of mitochondrion-localized ACC2 and mitochondrial fission factor(MFF).Our findings not only reveal a novel mechanism of AMPK activation which is independent on AMP,but also systemically determine how AMPK responds to different nutrient or energy stress.