AMP-activated Protein Kinase Suppresses Protein Synthesis in Rat Skeletal Muscle through Down-regulated Mammalian Target of Rapamycin (mTOR) Signaling

Abstract

AMP-activated protein kinase (AMPK) is viewed as an energy sensor that acts to modulate glucose uptake and fatty acid oxidation in skeletal muscle. Given that protein synthesis is a high energy-consuming process, it may be transiently depressed during cellular energy stress. Thus, the intent of this investigation was to examine whether AMPK activation modulates the translational control of protein synthesis in skeletal muscle. Injections of 5-aminoimidazole-4-carboxamide 1-␤-D-ribonucleoside (AICAR) were used to activate AMPK in male rats. The activity of ␣ 1 AMPK remained unchanged in gastrocnemius muscle from AICAR-treated animals compared with controls, whereas ␣ 2 AMPK activity was significantly increased (51%). AICAR treatment resulted in a reduction in protein synthesis to 45% of the control value. This depression was associated with decreased activation of protein kinases in the mammalian target of rapamycin (mTOR) signal transduction pathway as evidenced by reduced phosphorylation of protein kinase B on Ser 473 , mTOR on Ser 2448 , ribosomal protein S6 kinase on Thr 389 , and eukaryotic initiation factor eIF4E-binding protein on Thr 37 . A reduction in eIF4E associated with eIF4G to 10% of the control value was also noted. In contrast, eIF2B activity remained unchanged in response to AICAR treatment and therefore would not appear to contribute to the depression in protein synthesis. This is the first investigation to demonstrate changes in translation initiation and skeletal muscle protein synthesis in response to AMPK activation.Considerable attention has focused on understanding the role of AMP-activated protein kinase (AMPK) 1 in monitoring the energy status of the cell and mediating subsequent metabolic events. AMPK has been referred to as an energy-sensing/ signaling protein within the cell that responds to changes in the ratio of ATP/AMP as well as phosphocreatine/creatine (1, 2). Changes in the cellular energy state activate AMPK through various mechanisms involving allosteric regulation of AMPK, activation by an upstream AMPK kinase, and diminished activity of phosphatases (3). AMPK activation increases glucose uptake and fatty acid oxidation in muscle (4) as well as up-regulates expression of various metabolic genes (e.g. the glucose transporter, GLUT4, uncoupling protein-3, and cytochrome c) (5-7). Consequently AMPK serves as a sensor/modulator of intermediary metabolism by directing cellular events to increase energy availability and sustain high energy phosphate levels. Research using in vitro systems has shown that AMPK can be activated under artificial conditions such as treatment with high fructose or 2-deoxyglucose, heat shock, and inhibitors of oxidative phosphorylation (3). Pharmacological use of 5-aminoimidazole-4-carboxamide 1-␤-D-ribonucleoside (AICAR) has been commonly utilized to directly activate AMPK without altering cellular concentrations of ATP, ADP, and AMP (8). Additionally, starvation and endurance exercise result in increased activity of AMPK in skeletal muscle (9 -11). Ex...

Stress and mTORture signaling

“…The connection to mTOR became apparent when it was found that 5 0 -aminoimidazole-4-carboxamide riboside (ALCAR), a synthetic AMP analog, could activate AMPK, leading to decreased S6K Thr389 phosphorylation (Bolster et al, 2002) and that AMPK phosphorylated TSC2 on at least two residues leading to an enhanced activity of the latter (Inoki et al, 2003b). AMPK via TSC2 downregulates TOR/S6K activity in response to energy deprivation (Inoki et al, 2003b).…”

Mitochondrial DNA deletions inhibit proteasomal activity and stimulate an autophagic transcript

“…A second generally accepted consequence is the reduction of ATP synthesis [26,79]; and ATP depletion is known to activate the AMPK response [80] as a protective mechanism. Both amino acid depletion and AMPK activation can result in inhibition of mTOR [51,81] which in turn inhibits translation, protein synthesis, and cellular growth, and induces autophagy ( Figure 6). …”

Heat shock response and autophagy—cooperation and control

“…Additionally, AKT may also activate the MTOR pathway by the inhibition of AMPK-mediated phosphorylation of TSC2 47 and AMPK may inhibit MTOR directly by modulating its phosphorylation site. [60][61][62] Moreover, direct physical interaction between AMPK or MTOR and ULK1 (unc-51 like autophagy activating kinase 1) plays a crucial role in the regulation of mammalian autophagy. [63][64][65][66][67][68][69] Under conditions of nutrients abundance, the activated MTOR phosphorylates ULK1 and prevents its interaction with AMPK.…”