Both in animals in vivo and cells in culture, protein synthesis is regulated in part through a signaling pathway involving a protein kinase referred to as the mammalian target of rapamycin (mTOR) (1,12). In this regard, activation of mTOR stimulates both the initiation and elongation phases of mRNA translation. In addition, mTOR activation is associated with increased cell-cycle progression, selective enhancements in genetranscription, and increased ribosome biogenesis. Thus, signaling through mTOR upregulates many energy-consuming processes.
In contrast to mTOR signaling, activation of AMP-activated protein kinase (AMPK) results in a repression of energy-consuming processes (e.g., fatty acid synthesis) while simultaneously enhancing energy-producing processes (e.g., glycolysis) (6,16,25). Because mRNA translation is one of the most energy-intensive processes in a cell, it might be expected that activation of AMPK would repress protein synthesis. However, that possibility was unexplored until recently. The purpose of this review is to summarize recent findings defining the mechanisms through which AMPK represses signaling through mTOR, with a focus on the regulation of mRNA translation.
REGULATION OF mRNA TRANSLATION BY mTOR
The process of mRNA translation is functionally divided into three phases: initiation, during which the 40S and 60S ribosomal subunits bind to mRNA and locate the AUG start codon; elongation, during which the ribosome moves along the mRNA, translating stored information into a growing peptide chain; and termination, a process resulting in release of the completed protein from the ribosome (26). Because the majority of the examples of translational regulation occur at the initiation phase, this article will focus on the regulation of initiation by mTOR and AMPK.
The initiation phase of mRNA translation is a complicated process that results in the binding of mRNA to the 40S ribosomal subunit (26). One of the first steps in this process is the association of a protein referred to as eukaryotic initiation factor (eIF)4E with the m7GTP cap structure at the 5′-end of the mRNA. The eIF4E-mRNA complex then binds to a binary complex consisting of two other initiation factors, eIF4A and eIF4G, and the resulting eIF4F-mRNA complex associates with the 40S ribosomal subunit. The mRNA binding step in initiation is regulated through the reversible association of eIF4E with one of three eIF4E binding proteins (4E-BP). Of the 4E-BP, 4E-BP1 is the best characterized. Both 4E-BP1 and eIF4G bind to the same site on eIF4E. Consequently, binding of 4E-BP1 to eIF4E prevents its association with eIF4G and thereby decreases the binding of mRNA to the 40S ribosomal subunit. The binding of 4E-BP1 to eIF4E is regulated through phosphorylation of 4E-BP1 on multiple residues, whereby unphosphorylated and hypophosphorylated forms of the protein bind to eIF4E, but hyperphosphorylated forms do not. Phosphorylation of 4E-BP1 is controlled in large part through activation of the mTOR signaling pathway (Fig. 1). mTOR phosphorylates 4E-BP1 on at least two residues, threonine 37 and threonine 46. Phosphorylation on those residues permits subsequent phosphorylation events by other, unidentified protein kinases, ultimately resulting in release of the protein from eIF4E.
In addition to 4E-BP1, mTOR also phosphorylates the 70-kDa ribosomal protein (rp)S6 kinase, S6K1 (27). Phosphorylation of S6K1 on threonine 389 by mTOR creates a docking site for another protein kinase, the phosphoinositide-dependent protein kinase (PDK)1, which phosphorylates S6K1 on serine 229 and activates the kinase (3,13). S6K1 phosphorylates multiple downstream targets relevant to mRNA translation including eIF4B (19,41), eukaryotic elongation factor (eEF)2 protein kinase (42), and rpS6 (11). Phosphorylation of eIF4B enhances its association with the eIF4F complex, resulting in increased translation of mRNAs with long, highly structured 5′-untranslated regions (41). Phosphorylation of eEF2 kinase by S6K1 inhibits it, resulting in dephosphorylation and, consequently, activation of eEF2 (55). Early studies suggested that phosphorylation of rpS6 by S6K1 promoted a preferential increase in the translation of mRNAs bearing a terminal oligopyrimidine (TOP) tract at the 5′-end of the mRNA (36). However, recent studies have shown that TOP mRNA translation is regulated normally, both in cells lacking both isoforms of S6K1 and also in cells in which the wild-type rpS6 was replaced by a variant lacking the S6K1 phosphorylation sites, suggesting that phosphorylation of rpS6 is not required for modulation of TOP mRNA translation (39,45). Thus, the effect of rpS6 phosphorylation on mRNA translation, if any, remains to be established.
REGULATION OF mTOR
mTOR forms at least two complexes in mammalian cells that are referred to as mTOR complexes 1 and 2 (mTORC1 and mTORC2, respectively). The mTORC1 complex is distinguished from mTORC2 in part by its subunit composition. Thus, in addition to mTOR, mTORC1 contains a protein referred to as the regulatory associated protein of mTOR (raptor), whereas mTORC2 contains the rapamycin-insensitive companion of mTOR (rictor) (24,47). The two complexes are also unique in their downstream targets. For example, mTORC1 phosphorylates 4E-BP1 and S6K1 (4,38,49), whereas mTORC2 phosphorylates protein kinase Cα (47) and protein kinase B (PKB, also known as Akt) (48). Because only mTORC1 has been shown to regulate mRNA translation, only it will be further considered here. In addition to mTOR and raptor, at least two other proteins comprise the mTORC1 complex, including G-protein β-subunit-like protein (GβL) and the ras homolog enriched in brain (Rheb) (Fig. 1). Raptor binds to a domain present on both 4E-BP1 and S6K1 referred to as an mTOR signaling (TOS) motif and thereby recruits them to the mTORC1 complex, permitting their phosphorylation by mTOR (38,49). Thus, raptor serves as a molecular bridge linking mTOR with its substrates. The association of raptor with mTOR is regulated by hormones such as insulin and IGF-1 and by nutrients such as leucine through an unknown mechanism. However, it is clear that raptor is required for optimal phosphorylation of 4E-BP1 and S6K1 because reducing raptor expression severely attenuates hormone- and amino acid-induced phosphorylation of both proteins (27,28). Like raptor, GβL also enhances signaling through mTOR (28). However, the mechanism through which it functions is unclear. Rheb is a GTPase that binds both GDP and GTP (14,21,33). The GTP-bound form of Rheb activates mTOR, whereas Rheb-GDP inhibits it (Fig. 2). The GTPase activity of Rheb is stimulated by the GTPase-activator protein (GAP) tuberin (also known as TSC2) functioning in a complex with a second protein, hamartin (also known as TSC1) (31,32). Tuberin GAP activity is modulated in response to phosphorylation by several protein kinases including AMPK (discussed further in the next section) (23), PKB (9,22,35,40), the extracellularly regulated protein kinases (ERK) 1 and 2 (34), and p90rsk (44). Phosphorylation by either PKB, ERK1/ERK2, or p90rsk inhibits tuberin GAP activity, resulting in an increase in the proportion of Rheb associated with GTP, and increased mTOR signaling. In part, phosphorylation of tuberin may repress its function by promoting dissociation of the tuberin-hamartin complex (2).
REGULATION OF AMPK
AMPK is a heterotrimeric complex composed of a catalytic subunit (AMPKα) and two regulatory subunits (AMPKβ and AMPKγ) (6,16,25). The γ-subunit binds two AMP molecules with positive cooperativity. Thus, the binding of one molecule of AMP enhances the binding of a second, thereby increasing the sensitivity of the kinase to small changes in AMP concentration. Binding of AMP to the kinase is thought to alter its conformation, permitting subsequent phosphorylation of the α-subunit on threonine 172 by upstream protein kinases (Fig. 3). The combination of AMP binding to the γ-subunit and phosphorylation of the α-subunit results in activation of the kinase. In contrast, binding of ATP prevents phosphorylation of the α-subunit and therefore attenuates its activation. Thus, AMPK responds not only to changes in AMP concentration but also to changes in the ratio of AMP to ATP.
The upstream kinase(s) that phosphorylates AMPKα on threonine 172 was unknown until a seminal study performed in the laboratories of Drs. Dario R. Alessi and D. Grahame Hardie identified the tumor suppressor LKB1 as an AMPK kinase (17). In that study, LKB1 in a complex with two other proteins, STRAD and MO25, was shown to be required for AMPKα phosphorylation in cells treated with the AMP mimetic 5-aminoimidazole-4-carboxamide 1-β-D-ribonucleoside (AICAR). AICAR does not stimulate LKB1 kinase activity; instead, it binds to AMPK and permits its phosphorylation. In addition, a complex of the three proteins was sufficient for phosphorylation of AMPKα on threonine 172 in in vitro studies. A subsequent study (51) confirmed the results of the initial report and extended them to show that LKB1 is required for AMPK phosphorylation in response to a variety of cellular stressors that promote energy depletion. Two more recent studies show that, under a variety of conditions, LKB1 is the primary AMPK kinase in skeletal muscle (46) and liver (52). In both studies, a portion of the wild-type LKB1 gene was replaced with a construct consisting of the wild-type sequence flanked by loxP Cre-excision sequences. In the study by Sakamoto et al. (46), expression of Cre recombinase specifically in muscle resulted in a reduction in LKB1 mRNA and protein to undetectable levels. In such mice, both activation of AMPKα2 activity and AMPK phosphorylation on threonine 172 are dramatically repressed in muscles either treated with AICAR or stimulated to contract, suggesting that LKB1 is the principal AMPK kinase activated under these conditions. Similarly, liver-specific reduction in LKB1 to less than 5% of the wild-type level dramatically attenuates AMPK phosphorylation on threonine 172 (52). Importantly, in that study, both basal mTOR signaling and the feeding-induced increase in mTOR signaling were significantly enhanced in LKB1-deficient liver.
In addition to LKB1, two recent studies have shown that AMPKα is also phosphorylated on threonine 172 by calcium/calmodulin-dependent protein kinase kinase-β (CaMKKβ) (18,56). Thus, in LKB1-deficient cells, activation of CaMKKβ promotes phosphorylation of AMPK independently of changes in adenine nucleotide levels (18). Moreover, recombinant CaMKKβ expressed in E. coli phosphorylates and activates AMPK in in vitro reactions (56). Interestingly, calcium ionophores promote AMPK phosphorylation and activation in cells lacking LKB1, and this effect is blocked by CaMKK inhibitors or knockdown of the protein by RNAi (18). Overall, the results of the two studies strongly suggest that AMPK activity is regulated by changes in intracellular calcium concentrations through changes in CaMKKβ activity. Future studies will hopefully address the question of whether CaMKKβ might mediate the small residual increase in contraction-stimulated AMPK activity observed in LKB1-deficient skeletal muscle (46).
REPRESSION OF mTOR SIGNALING THROUGH ACTIVATION OF AMPK
The first study linking activation of AMPK with a repression of signaling through mTOR showed that in skeletal muscle of rats treated with AICAR, AMPK phosphorylation and activity were increased concomitantly with dephosphorylation of 4E-BP1 and S6K1 (5). In particular, phosphorylation of 4E-BP1 on threonine 37 and S6K1 on threonine 389 were significantly decreased 1 h after AICAR administration. Because both phosphorylation sites are thought to be directly phosphorylated by mTOR, the results suggest that activation of AMPK promotes inactivation of mTOR. A caveat to this finding is that because AICAR affects many metabolic pathways, its administration in vivo can alter blood concentrations of a number of metabolites and hormones. Thus, the observed changes in mTOR signaling could partly have been a result of changes in circulating insulin concentration. More direct evidence linking activation of AMPK with reduced mTOR signaling was presented in two studies that immediately followed publication of the initial report. In those studies (10,30), activation of AMPK using AICAR or agents that decrease intracellular ATP levels (e.g., fructose, glycerol, or oligomycin) in cells in culture was associated with decreased phosphorylation of S6K1, indicating that mTOR signaling was repressed. Moreover, in both studies, activation of AMPK by AICAR overcame the amino acid-induced activation of mTOR, providing the first evidence that negative regulation of mTOR caused by activation of AMPK is dominant to the positive input from amino acids. In contrast to the results of the three studies described above supporting a link between AMPK and mTOR, a fourth study published in the same year showed that activation of AMPK was associated with decreased rates of protein synthesis concomitant with increased phosphorylation of eEF2, but no change in phosphorylation of 4E-BP1 (20). The authors of that study conclude that the inhibition of protein synthesis caused by activation of AMPK occurred independently of changes in mTOR signaling. However, in that study, there was a noticeable shift in 4E-BP1 distribution from the hyperphosphorylated γ-form to the lesser phosphorylated β-form, a result that is consistent with reduced mTOR signaling. The finding that S6K1 phosphorylation, as assessed by changes in migration during SDS-polyacrylamide gel electrophoresis, remains elevated in cells treated with 2-deoxyglucose, may be a consequence of other signaling pathways impinging on S6K1. Alternatively, because phosphorylation of S6K1 on threonine 389 by mTOR is one of a series of phosphorylation events leading to activation of the kinase, changes in phosphorylation of a single residue may not have been detectable by the gel-shift analysis used in that study. Thus, in a variety of cell lines, activation of AMPK results in decreased S6K1 activity as assessed by an in vitro kinase assay (29).
Activation of AMPK has been reported to repress mTOR signaling through at least two mechanisms. In the first case, AMPK has been shown to directly phosphorylate tuberin on multiple residues (23). In that study, eight potential AMPK phosphorylation sites were identified on tuberin, each was subsequently mutated to an unphosphorylatable alanine, and the variant protein was expressed in cells. All of the individual mutants except two (where threonine 1227 and serine 1345 were changed to alanine) exhibited a wild-type pattern of phosphorylation in response to activation of AMPK, suggesting that AMPK phosphorylates tuberin on threonine 1227 and serine 1345 (Fig. 4). The finding that AMPK directly phosphorylates threonine 1227 and serine 1345 in an in vitro kinase assay provides further support for this conclusion. Importantly, expression of a tuberin variant with the two AMPK phosphorylation sites changed to alanine attenuates the decrease in 4E-BP1 and S6K1 phosphorylation caused by 2-deoxyglucose-mediated activation of AMPK. A more recent study (53) provides further evidence for a role for tuberin in AMPK signaling to mTOR. In that study, phosphorylation of 4E-BP1 was dramatically elevated in cells lacking tuberin compared with wild-type cells. Activation of AMPK with AICAR attenuated 4E-BP1 phosphorylation in wild-type cells but not in cells lacking tuberin. Overall, the results suggest that one mechanism through which AMPK represses mTOR signaling is through phosphorylation and activation of tuberin.
A second mechanism through which AMPK may regulate mTOR signaling involves phosphorylation of mTOR on threonine 2446 by AMPK. In a study by Cheng et al. (7), activation of AMPK using either AICAR or dinitrophenol was associated with enhanced phosphorylation of mTOR on threonine 2446. A 111-amino acid peptide containing the sequence surrounding threonine 2446 is phosphorylated by AMPK in an in vitro kinase assay, which further suggests that AMPK directly phosphorylates mTOR on that residue. It is not known whether phosphorylation of mTOR on threonine 2446 by AMPK alters it catalytic activity or its interaction with mTOR regulatory proteins such as raptor or Rheb. However, it is interesting that phosphorylation of mTOR on threonine 2446 is inversely correlated with phosphorylation of an adjacent residue, serine 2448 (7). Thus, phosphorylation of threonine 2446 is attenuated when serine 2448 is already phosphorylated, and PKB-mediated phosphorylation of serine 2448 is repressed when threonine 2446 is mutated to an acidic residue to mimic phosphorylation. Serine 2446 on mTOR has been reported to be phosphorylated by either PKB (37,50) or S6K1 (8). Phosphorylation of serine 2448 typically correlates with enhanced mTOR signaling (43). Therefore, phosphorylation of threonine 2446 may interfere with mTOR function by preventing phosphorylation of serine 2448. However, this possibility must be viewed with caution because mutation of serine 2448 to alanine has no detectable effect on mTOR signaling to 4E-BP1 and S6K1 (50).
In addition to phosphorylating tuberin and mTOR, AMPK also phosphorylates Raf on serine 621 and enhances phosphorylation of ERK1 and ERK2 on threonine 202 and tyrosine 204 (54). Recent studies have shown that tuberin is phosphorylated both by ERK1/ERK2 (34) and by a downstream target of ERK1/ERK2 signaling, p90rsk (44). In contrast to the activation of tuberin GAP activity caused by phosphorylation by AMPK, phosphorylation of the protein by ERK1/ERK2 or p90rsk is associated with decreased tuberin GAP activity and enhanced mTOR signaling (34,44). Thus, by activating ERK1/ERK2 and p90rsk, AMPK would be expected to activate rather than repress mTOR signaling. However, it has been observed that in C2C12 myotubes, activation of AMPK by treatment with AICAR is associated with enhanced phosphorylation of ERK1/ERK2 and decreased phosphorylation of 4E-BP1 and S6K1, suggesting that the repressive action of AMPK on mTOR signaling is dominant to the stimulatory action of ERK1/ERK2 (57). In that study, the decrease in mTOR signaling was associated with enhanced association of tuberin with hamartin, suggesting that AMPK may increase tuberin GAP activity by promoting assembly of the tuberin-hamartin complex. Similarly, a recent study (15) reports that activation of AMPK attenuates insulin-induced PKB activation and represses mTOR signaling. In skeletal muscle of mice run on a treadmill, both AMPK and ERK1/ERK2 phosphorylation increase in conjunction with decreased phosphorylation of 4E-BP1 and S6K1 (58). Therefore, a possible mechanism through which AMPK might repress mTOR signaling despite the potential opposing actions of ERK1/ERK2 and/or p90rsk would involve enhanced binding of tuberin to hamartin (Fig. 4).
The finding that activation of AMPK results in decreased mTOR signaling and inhibition of protein synthesis perhaps isn't surprising, considering the known role of AMPK in repressing energy-consuming metabolic processes and the large energy expenditure devoted to protein synthesis in cells. Once the link between AMPK and mTOR was identified, a number of studies quickly provided evidence for at least two mechanisms through which the regulation might occur. Together, the results of these studies provide support for a model in which phosphorylation of tuberin by AMPK would enhance its association with hamartin, resulting in an increase in Rheb GTPase activity and in an increased association of Rheb with GDP. The resulting Rheb-GDP complex would inhibit mTOR activity toward downstream targets including 4E-BP1 and S6K1. In this model, phosphorylation of tuberin by AMPK would overcome positive input from other kinases such as PKB, ERK1/ERK2, and p90rsk. However, despite the growing number of studies examining the mechanism(s) through which AMPK represses mTOR signaling, a number of questions remain unanswered. For example, what role, if any, does phosphorylation of mTOR on threonine 2446 have in the regulation of mTOR by AMPK? How does phosphorylation of tuberin by AMPK overcome positive signaling through PKB, ERK1/ERK2, and p90rsk? What role does CaMMKβ play in the regulation of AMPK in animals in vivo? These and many other questions remain to be answered in future studies.
I wish to thank Dr. Leonard S. Jefferson for critical reading of the manuscript. The studies described here that were performed in the author's laboratory were supported by NIH grant DK-15658.
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