Angela Woods

LKB1 is the upstream kinase in the AMP-activated protein kinase cascade.

Inactivating mutations in the protein kinase LKB1 lead to a dominantly inherited cancer in humans termed Peutz-Jeghers syndrome. The role of LKB1 is unclear, and only one target for LKB1 has been identified in vivo [3]. AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that plays a pivotal role in energy homeostasis. AMPK may have a role in protecting the body from metabolic diseases including type 2 diabetes, obesity, and cardiac hypertrophy. We previously reported the identification of three protein kinases (Elm1, Pak1, and Tos3 [9]) that lie upstream of Snf1, the yeast homologue of AMPK. LKB1 shares sequence similarity with Elm1, Pak1, and Tos3, and we demonstrated that LKB1 phosphorylates AMPK on the activation loop threonine (Thr172) within the catalytic subunit and activates AMPK in vitro [9]. Here, we have investigated whether LKB1 corresponds to the major AMPKK activity present in cell extracts. AMPKK purified from rat liver corresponds to LKB1, and blocking LKB1 activity in cells abolishes AMPK activation in response to different stimuli. These results identify a link between two protein kinases, previously thought to lie in unrelated, distinct pathways, that are associated with human diseases.

Characterization of the AMP-activated Protein Kinase Kinase from Rat Liver and Identification of Threonine 172 as the Major Site at Which It Phosphorylates AMP-activated Protein Kinase

We have developed a sensitive assay for the AMP-activated protein kinase kinase, the upstream component in the AMP-activated protein kinase cascade. Phosphorylation and activation of the downstream kinase by the upstream kinase absolutely requires AMP and is antagonized by high (millimolar) concentrations of ATP. We have purified the upstream kinase >1000-fold from rat liver; a variety of evidence indicates that the catalytic subunit may be a polypeptide of 58 kDa. The physical properties of the downstream and upstream kinases, e.g. catalytic subunit masses (63 versus 58 kDa) and native molecular masses (190 versus 195 kDa), are very similar. However, unlike the downstream kinase, the upstream kinase is not inactivated by protein phosphatases. The upstream kinase phosphorylates the downstream kinase at a single major site on the α subunit, i.e. threonine 172, which lies in the “activation segment” between the DFG and APE motifs. This site aligns with activating phosphorylation sites on many other protein kinases, including Thr177 on calmodulin-dependent protein kinase I. As well as suggesting a mechanism of activation of AMP-activated protein kinase, this finding is consistent with our recent report that the AMP-activated protein kinase kinase can slowly phosphorylate and activate calmodulin-dependent protein kinase I, at least in vitro (Hawley, S. A., Selbert, M. A., Goldstein, E. G., Edelman, A. M., Carling, D., and Hardie, D. G. (1995) J. Biol. Chem. 270, 27186-27191).

Ribosomal Protein S6 Kinase 1 Signaling Regulates Mammalian Life Span

Mimicking Caloric Restriction The extended life span and resistance to age-related diseases in animals exposed to caloric restriction has focused attention on the biochemical mechanisms that produce these effects. Selman et al. (p. 140; see the Perspective by Kaeberlein and Kapahi) explored the role of the mammalian ribosomal protein S6 kinase 1 (S6K1), which regulates protein translation and cellular energy metabolism. Female knockout mice lacking expression of S6K1 showed characteristics of animals exposed to caloric restriction, including improved health and increased longevity. The beneficial effects included reduced fat mass in spite of increased food intake. Thus, inhibition of signaling pathways activated by S6K1 might prove beneficial in protecting against age-related disease. A signaling pathway in mice mediates the effects of caloric restriction that protect against age-related diseases. Caloric restriction (CR) protects against aging and disease, but the mechanisms by which this affects mammalian life span are unclear. We show in mice that deletion of ribosomal S6 protein kinase 1 (S6K1), a component of the nutrient-responsive mTOR (mammalian target of rapamycin) signaling pathway, led to increased life span and resistance to age-related pathologies, such as bone, immune, and motor dysfunction and loss of insulin sensitivity. Deletion of S6K1 induced gene expression patterns similar to those seen in CR or with pharmacological activation of adenosine monophosphate (AMP)–activated protein kinase (AMPK), a conserved regulator of the metabolic response to CR. Our results demonstrate that S6K1 influences healthy mammalian life-span and suggest that therapeutic manipulation of S6K1 and AMPK might mimic CR and could provide broad protection against diseases of aging.

AMP-activated Protein Kinase Plays a Role in the Control of Food Intake

AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that acts as an intracellular energy sensor maintaining the energy balance within the cell. The finding that leptin and adiponectin activate AMPK to alter metabolic pathways in muscle and liver provides direct evidence for this role in peripheral tissues. The hypothalamus is a key regulator of food intake and energy balance, coordinating body adiposity and nutritional state in response to peripheral hormones, such as leptin, peptide YY-(3–36), and ghrelin. To date the hormonal regulation of AMPK in the hypothalamus, or its potential role in the control of food intake, have not been reported. Here we demonstrate that counter-regulatory hormones involved in appetite control regulate AMPK activity and that pharmacological activation of AMPK in the hypothalamus increases food intake. In vivo administration of leptin, which leads to a reduction in food intake, decreases hypothalamic AMPK activity. By contrast, injection of ghrelin in vivo, which increases food intake, stimulates AMPK activity in the hypothalamus. Consistent with the effect of ghrelin, injection of 5-amino-4-imidazole carboxamide riboside, a pharmacological activator of AMPK, into either the third cerebral ventricle or directly into the paraventricular nucleus of the hypothalamus significantly increased food intake. These results suggest that AMPK is regulated in the hypothalamus by hormones which regulate food intake. Furthermore, direct pharmacological activation of AMPK in the hypothalamus is sufficient to increase food intake. These findings demonstrate that AMPK plays a role in the regulation of feeding and identify AMPK as a novel target for anti-obesity drugs.

Characterization of the Role of AMP-Activated Protein Kinase in the Regulation of Glucose-Activated Gene Expression Using Constitutively Active and Dominant Negative Forms of the Kinase

ABSTRACT In the liver, glucose induces the expression of a number of genes involved in glucose and lipid metabolism, e.g., those encoding L-type pyruvate kinase and fatty acid synthase. Recent evidence has indicated a role for the AMP-activated protein kinase (AMPK) in the inhibition of glucose-activated gene expression in hepatocytes. It remains unclear, however, whether AMPK is involved in the glucose induction of these genes. In order to study further the role of AMPK in regulating gene expression, we have generated two mutant forms of AMPK. One of these (α1312) acts as a constitutively active kinase, while the other (α1DN) acts as a dominant negative inhibitor of endogenous AMPK. We have used adenovirus-mediated gene transfer to express these mutants in primary rat hepatocytes in culture in order to determine their effect on AMPK activity and the transcription of glucose-activated genes. Expression of α1312 increased AMPK activity in hepatocytes and blocked completely the induction of a number of glucose-activated genes in response to 25 mM glucose. This effect is similar to that observed following activation of AMPK by 5-amino-imidazolecarboxamide riboside. Expression of α1DN markedly inhibited both basal and stimulated activity of endogenous AMPK but had no effect on the transcription of glucose-activated genes. Our results suggest that AMPK is involved in the inhibition of glucose-activated gene expression but not in the induction pathway. This study demonstrates that the two mutants we have described will provide valuable tools for studying the wider physiological role of AMPK.

Insulin antagonizes ischemia-induced Thr172 phosphorylation of AMP-activated protein kinase alpha-subunits in heart via hierarchical phosphorylation of Ser485/491.

Previous studies showed that insulin antagonizes AMP-activated protein kinase activation by ischemia and that protein kinase B might be implicated. Here we investigated whether the direct phosphorylation of AMP-activated protein kinase by protein kinase B might participate in this effect. Protein kinase B phosphorylated recombinant bacterially expressed AMP-activated protein kinase heterotrimers at Ser485 of the α1-subunits. In perfused rat hearts, phosphorylation of the α1/α2 AMP-activated protein kinase subunits on Ser485/Ser491 was increased by insulin and insulin pretreatment decreased the phosphorylation of the α-subunits at Thr172 in a subsequent ischemic episode. It is proposed that the effect of insulin to antagonize AMP-activated protein kinase activation involves a hierarchical mechanism whereby Ser485/Ser491 phosphorylation by protein kinase B reduces subsequent phosphorylation of Thr172 by LKB1 and the resulting activation of AMP-activated protein kinase.

The α1 and α2 isoforms of the AMP-activated protein kinase have similar activities in rat liver but exhibit differences in substrate specificity in vitro

The AMP‐activated protein kinase (AMPK) is a heterotrimeric complex composed of a catalytic subunit (a) and two regulatory subunits (β and γ). Two isoforms of the catalytic subunit (αl and (α2) have been identified. We show here that the αl‐ and α2‐containing complexes contribute approximately equally to total AMPK activity in rat liver. Furthermore, expression of al or a2 with β and Y in mammalian cells demonstrates that both complexes have equal specific activity measured with the SAMS peptide. Using variant peptides, however, we show that al and a2 exhibit slightly different substrate preferences, which suggest that the two isoforms could play different physiological roles within the cell.

AMP-activated protein kinase: new regulation, new roles?

The hydrolysis of ATP drives virtually all of the energy-requiring processes in living cells. A prerequisite of living cells is that the concentration of ATP needs to be maintained at sufficiently high levels to sustain essential cellular functions. In eukaryotic cells, the AMPK (AMP-activated protein kinase) cascade is one of the systems that have evolved to ensure that energy homoeostasis is maintained. AMPK is activated in response to a fall in ATP, and recent studies have suggested that ADP plays an important role in regulating AMPK. Once activated, AMPK phosphorylates a broad range of downstream targets, resulting in the overall effect of increasing ATP-producing pathways whilst decreasing ATP-utilizing pathways. Disturbances in energy homoeostasis underlie a number of disease states in humans, e.g. Typexa02 diabetes, obesity and cancer. Reflecting its key role in energy metabolism, AMPK has emerged as a potential therapeutic target. In the present review we examine the recent progress aimed at understanding the regulation of AMPK and discuss some of the latest developments that have emerged in key areas of human physiology where AMPK is thought to play an important role.

CHARACTERIZATION OF AMP-ACTIVATED PROTEIN KINASE BETA AND GAMMA SUBUNITS: ASSEMBLY OF THE HETEROTRIMERIC COMPLEX IN VITRO

There is growing evidence that mammalian AMP-activated protein kinase (AMPK) plays a role in protecting cells from stresses that cause ATP depletion by switching off ATP-consuming biosynthetic pathways. The active form of AMPK from rat liver exists as a heterotrimeric complex and we have previously shown that the catalytic subunit is structurally and functionally related to the SNF1 protein kinase from Saccharomyces cerevisiae. Here we describe the isolation and characterization of the two other polypeptides, termed AMPKβ and AMPK, that together with the catalytic subunit (AMPKα) form the active kinase complex in mammalian liver. Sequence analysis of cDNA clones encoding these subunits reveals that they are related to yeast proteins that interact with SNF1, providing further evidence that the regulation and function of AMPK and SNF1 have been conserved throughout evolution. The amino acid sequence of the β subunit is most closely related to SIP2 (35% identity), while the amino acid sequence of the subunit is 35% identical with SNF4. We show that both AMPKβ and AMPK mRNA and protein are expressed widely in rat tissues. We show that AMPKβ interacts with both AMPKα and AMPK in vitro, whereas AMPKα does not interact with AMPK under the same conditions. These results suggest that AMPKβ mediates the association of the heterotrimeric AMPK complex in vitro, and will facilitate future studies aimed at investigating the regulation of AMPK in vivo.

Mammalian AMP-activated protein kinase: functional, heterotrimeric complexes by co-expression of subunits in Escherichia coli

The 5-AMP-activated protein kinase (AMPK) plays a critical role in the regulation of cellular energy homeostasis. AMPK is a heterotrimer composed of a catalytic subunit (alpha) and two regulatory subunits (beta and gamma). To date, purified AMPK has only been obtained in small, microgram quantities from tissues. Here, we describe an expression and purification system for production of functional AMPK in Escherichia coli. A plasmid carrying all three subunits of AMPK (alpha1, beta1, and gamma1) for T7 RNA polymerase-driven transcription of a single tricistronic messenger was constructed, allowing spontaneous formation of the heterotrimeric complex in the bacterial cytosol. AMPK was purified from the bacterial lysates by single-step nickel-ion chromatography, utilizing a poly-histidine tag fused to the N-terminus of the alpha-subunit. The recombinant AMPK complex was monodisperse, as shown by gel filtration chromatography with elution of a single peak at a Stokes radius of 52A. Bacterially expressed AMPK was entirely inactive, yet it could be activated by upstream kinase in the presence of AMP. Sufficient quantities of purified functional AMPK should prove to be an invaluable tool to solve many of the pertinent questions about its molecular structure and function, in particular facilitating protein crystallization for X-ray structure analysis.