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The Role of Adenosine Monophosphate Kinase in Remodeling White Adipose Tissue Metabolism

Gaidhu, Mandeep Pinky; Ceddia, Rolando Bacis

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Exercise and Sport Sciences Reviews: April 2011 - Volume 39 - Issue 2 - p 102-108
doi: 10.1097/JES.0b013e31820ac03e
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INTRODUCTION

Global rates of obesity are on the rise, as are related metabolic disorders, including type 2 diabetes, hypertension, and cardiovascular disease. Because obesity is characterized by an excessive accumulation of adipose tissue, treatment targeted toward reducing adiposity is critical in solving the global obesity epidemic. The general prescription for reducing adiposity is a regimen combining exercise and diet to induce a negative energy balance, a therapeutic approach that has been proven to be effective in reducing fat mass in the short-term. However, the activation of energy-sparing mechanisms under conditions of prolonged negative energy balance imposes a major obstacle for long-term continuous fat loss and/or maintenance of a reduced body mass (28). In fact, as obese patients start to lose weight through diet and exercise, they face a constant battle against biological mechanisms that oppose fat reduction, including decreased energy expenditure by lowered resting metabolic rate and up-regulation of signals promoting food intake. The biological adaptations that take place under conditions of chronic negative energy balance are believed to contribute greatly to the poor long-term success rate with regard to weight loss in these patients. Therefore, the discovery of nutritional or pharmacological strategies to overcome these energy-sparing mechanisms have become of great therapeutic interest for the treatment of obesity and its related metabolic disorders. In this context, the enzyme adenosine monophosphate (AMP) kinase (AMPK) has risen as a potential candidate for inducing fatty acid (FA) oxidation versus storage. Activation of AMPK generally promotes up-regulation of catabolic pathways (i.e., fatty acid oxidation, glycolysis) while suppressing anabolism. Furthermore, AMPK activation also increases the expression and activity of peroxisome proliferator-activated receptor-γ (PPAR-γ) coactivator-1α (PGC-1α) and promotes mitochondrial biogenesis (27,33). From a weight loss perspective, these features are very appealing because activation of AMPK in the white adipose tissue (WAT) could potentially shift adipocyte metabolism toward fat utilization instead of storage. However, although the ability of AMPK to induce FA oxidation and mitochondrial biogenesis were well established in skeletal muscle cells, very little was known about the effects of acute and chronic AMPK activation on adipocyte metabolism. We originally hypothesized that AMPK activation in WAT could deplete triglyceride (TG) content in adipocytes and ultimately lead to decreased adiposity. This could serve as a therapeutic approach to overcome energy-sparing mechanisms that are activated under conditions of chronic negative energy balance and potentiate long-term weight loss. To test this hypothesis, we have conducted a series of in vivo and in vitro studies that have given novel insight into how AMPK regulates glucose and lipid metabolism in the WAT (1,10-12). This is important because lipid handling by WAT has major implications for the determination of whole-body energy homeostasis under physiological and pathological conditions such as exercise and obesity, respectively. Thus, understanding the structural, molecular, and physiological characteristics of WAT and the role of AMPK in regulating glucose and lipid metabolism in this tissue is crucial to develop safe and effective strategies to reduce adiposity and treat or prevent obesity and its related metabolic disorders.

WHITE ADIPOSE TISSUE - STRUCTURE AND METABOLIC FUNCTION

The WAT is the major energy reservoir in mammals and plays a critical role in the maintenance of whole-body energy homeostasis. The WAT is composed of 35% to 75% white adipocytes, with the remaining being stromal vasculature tissue containing fibroblasts, endothelial cells, blood cells, macrophages, pericytes, and preadipocytes among others (8). White adipocytes are specialized and differentiated spherical cells with a great capacity to store TG for subsequent release of FA under conditions of high metabolic demand (i.e., exercise) or negative energy balance (i.e., food restriction). Once the adipocyte has reached a mature appearance of a single lipid droplet (unilocular adipocytes) surrounded by a rim of cytoplasm and offset nucleus, cell size can continue to increase from less than 10 μm to 130-150 μm in diameter, mostly by increases in TG storage (8). White adipocytes have a small number of thin and elongated mitochondria with randomly oriented cristae (8). In general, mitochondrial content and oxidative capacity reduce as adipocytes hypertrophy (8,10). Other organelles such as Golgi complex, rough endoplasmic reticulum, smooth endoplasmic reticulum, and lysosomes seem to be poorly developed (8). From a metabolic perspective, the main classical functions of white adipocytes are lipid synthesis and storage from a variety of substrates and TG breakdown (lipolysis) and exportation of FA (3,8). However, adipocytes also express and secrete various factors (adipokines) that exert autocrine, paracrine, and endocrine effects in the body (3). Of particular relevance is the ability of WAT to increase or reduce leptin secretion under conditions of positive and negative energy balance, respectively. In this context, leptin works as a signaling molecule that sends information to the central nervous system (CNS) regarding the content of fat stored in the WAT. Through this mechanism, the CNS can sense energy availability in the organism and make continuous adjustments in food intake and energy expenditure (22). Therefore, the WAT is currently viewed as a multifunctional organ that has the ability to regulate metabolic rate of various organs and tissues, as well as whole-body substrate metabolism and energy homeostasis (3,22).

STRUCTURAL CHARACTERISTICS OF AMPK AND REGULATION AND DISTRIBUTION IN THE WAT

AMPK is a heterotrimeric enzyme formed by a catalytic (α) and two regulatory subunits (β and γ). Multiple isoforms of each mammalian subunit (α1, α2, β1, β2, γ1-γ3) exist, each encoded by a different gene (32). Excluding possible alternative splice variants, these different subunits possibly can be arranged in 12 distinct trimeric (αβγ) complexes, which have been postulated to confer AMPK the ability to exert tissue-specific effects with regard to metabolic regulation (17). The α subunit houses a catalytic core in the N-terminus containing the threonine 172 residue targeted for phosphorylation, whereas the C-terminus region engages in autoregulatory function by binding the β and γ subunits. Both isoforms of the α subunit are expressed in rat adipose tissue (12). However, measurement of AMPK activity indicates that the α1 isoform accounts for most of the total activity of this kinase, whereas α2 contributes very little. Compatible with this are the observations that in isolated human adipocytes, AMPKα1 activity can be increased by 4-fold in cells exposed to 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) while no AMPKα2 activity was detected (23). The α isoforms share approximately 90% sequence homology with each other, yet differ in their subcellular localization. It has been shown that complexes containing α2 subunits preferentially are located to the nucleus, and this is thought to play a role in mediating alterations in gene expression (29). In fact, our work using isolated primary adipocytes exposed to AICAR for 15 h had increased AMPKα2 expression without any alteration in mRNA levels of the α1 isoform (11). The γ subunit is a critical regulator of AMPK activity and is composed of four cystathionine β synthase (CBS) domains in tandem pairs. The CBS domains bind nucleotides such as AMP and adenosine triphosphate (ATP) to gauge the cellular energy charge. When ATP levels drop causing a concomitant increase in AMP, the latter nucleotide binds to the γ subunit and causes allosteric activation of the AMPK complex. Allosteric activation only increases AMPK activity by 5- to 10-fold; however, it plays an important role in causing conformational change to AMPK, making it a better substrate for its upstream kinases such as the serine/threonine kinase 11 (LKB1), calcium/calmodulin-dependent protein kinase kinase-β, and transforming growth factor-β-activating kinase 1. Phosphorylation of the α subunit by these kinases increases the activity of AMPK by approximately 100-fold. The combined allosteric and covalent regulatory mechanisms can increase AMPK activity by up to approximately 1000-fold.

THE ROLE OF AMPK IN ADIPOCYTE LIPOLYSIS

Because a primary function of adipose tissue is delivery of substrate for ATP production in peripheral tissues, most studies investigating the role of AMPK in adipose tissue have focused on the effects of this enzyme on lipolysis. The initial evidence of a regulatory role for AMPK in adipocyte lipolysis came from the observation that hormone-sensitive lipase (HSL), an important intermediary enzyme involved in lipolysis, was phosphorylated by AMPK. Studies in vitro indicated that phosphorylation of HSL by AMPK occurs on a serine residue that antagonizes phosphorylation of HSL by cyclic AMP-dependent protein kinase (PKA) (14), causing suppression of β-adrenergic-stimulated lipolysis (Fig. 1). Based on these in vitro studies, it was proposed that AMPK activation would exert an antilipolytic effect in adipocytes. Although several studies (1,9,11) have reported an inhibitory effect of AMPK on adipocyte lipolysis, others (20,36) have found that activation of this kinase caused the opposite effect. In this scenario, whether AMPK exerted an antilipolytic or prolipolytic role in adipocytes became a controversial issue. This was particularly evident when considering that acute and chronic exercise bouts increase catecholamine release, lipolysis, and AMPK activation in the adipose tissue (20). Therefore, an antilipolytic role for AMPK seemed counterintuitive because during exercise, levels of FA are increased significantly and not reduced in the circulation. These disparities subsequently were reconciled by observations that AMPK was activated in adipocytes as a consequence of lipolysis and was proposed to function as a mechanism that limits TG breakdown in WAT to spare energy (15). The rationale for this is based on the fact that if FA released by lipolysis are not oxidized either within the adipocyte or in other tissues, they are reesterified into TG in the fat cells, creating an energy-consuming "futile cycle" (6) (Fig. 2). Therefore, AMPK activation as a consequence of lipolysis has been proposed to restrain energy depletion through its reciprocal antilipolytic effect in WAT. This is supported by studies in our laboratory showing that AICAR-induced AMPK activation potently suppresses glycerol release by isolated visceral and subcutaneous rat adipocytes, an effect that is prevented by pretreatment with the AMPK inhibitor compound C (1). Furthermore, adipocytes from AMPKα1-knockout mice exhibit increased lipolysis, indicating an antilipolytic role of this enzyme (9).

FIGURE 1
FIGURE 1:
Effects of adenosine monophosphate kinase (AMPK) on lipid metabolism in white adipose tissue (WAT). Acute and chronic activation of AMPK inhibits hormone-sensitive lipase (HSL) activity, which prevents hydrolysis of diacylglycerol (DAG) and subsequent release of glycerol and fatty acids (FA). Acute and chronic pharmacological AMPK activation powerfully suppresses FA uptake, conferring an antilipogenic role for AMPK in WAT. Furthermore, chronic AMPK activation up-regulates the expression of genes (PGC-1α, PPAR-α, PPAR-δ, CPT-1b) that increase the ability of the cell to dispose of FA intracellularly through β-oxidation. The adipose triglyceride lipase (ATGL) content also is increased with AMPK activation, which allows hydrolysis of one FA from triglyceride (TG), resulting in partial lipolysis. Altogether, our data indicate that chronic AMPK activation plays an antilipogenic and prooxidative role in WAT, promoting energy dissipation versus storage. and → denote stimulation; ⊝ and ⊣ denote inhibition. LKB1 indicates AMPK kinase; β-Ad, β-adrenergic receptor; FACS, fatty acyl-CoA synthase; Mito, mitochondria; PM, plasma membrane.
FIGURE 2
FIGURE 2:
The role of adenosine monophosphate kinase (AMPK) in fatty acid (FA) reesterification. Adipocytes rely on glucose and glyceroneogenesis for reesterification of FA into triglycerides (TG). Under conditions of higher energy demand, TG can undergo lipolysis and release the glycerol backbone and three FA. Under exercise and fasting conditions, a large portion of the FA return to the adipose tissue to be reesterified and stored as neutral TG. The process of reesterification is costly, as acylation of FA entering the cell requires adenosine triphosphate (ATP). The hydrolysis of ATP generates AMP and activates AMPK. In turn, AMPK potently suppresses glucose and FA uptake, thereby limiting reesterification and utilization of ATP. At the same time, chronic AMPK activation stimulates FA oxidation to generate ATP and reestablish the cellular energy charge. and → denote stimulation; ⊝ and ⊣ denote inhibition. FACS indicates fatty acyl-CoA synthase; PPi, pyrophosphate; glucose-6P, glucose-6-phosphate; glycerol-3P, glycerol-3-phosphate.

As originally proposed by Garton et al. (14), the mechanism by which AMPK inhibits lipolysis seems to be via suppression of PKA-mediated phosphorylation of key HSL serine residues. In fact, upon catecholamines binding to β-adrenergic receptors, a cascade of events leads to activation of PKA, which in turn phosphorylates HSL at Ser563, Ser660, and Ser659 residues, leading to activation of this lipase. It has been shown that phosphorylation of HSL at the Ser565 residue by AMPK prevents phosphorylation of the PKA-targeted serine residues, thereby impairing lipolysis (14) (Fig. 1). We have shown that both acute and prolonged AICAR-induced AMPK activation in rat adipocytes increases Ser565 phosphorylation, and this is accompanied by a potent suppression of phosphorylation at the HSL Ser563 and Ser660 residues under both basal and epinephrine-stimulated conditions (1,11). The inhibitory effect of AICAR on lipolysis and phosphorylation of Ser563 and Ser660 are reversed when cells were pretreated with compound C, suggesting that AMPK is inhibiting HSL activity by regulating key phosphorylation sites on this enzyme (1). In vivo studies also have indicated that acute AICAR infusions in diabetic rats decrease whole-body lipolysis (4), an effect that can be attributed to inhibition of HSL phosphorylation.

Although acute studies clearly show an inhibition of lipolysis, our studies provide evidence that prolonged AICAR-induced AMPK activation elicits time-dependent effects on this variable (11). In fact, acute and chronic AICAR-induced AMPK activation suppressed glycerol release under basal and epinephrine-stimulated conditions. However, when we assessed lipolysis by measuring nonesterified FA (NEFA) in the medium, we found that it initially was reduced after AICAR treatment but progressively increased in a time-dependent manner, reaching values 10- and 1.8-fold higher than controls after 15 h under basal and epinephrine-stimulated conditions, respectively. These findings also were reproduced in vivo when we determined the time course of NEFA in the plasma of AICAR-injected rats during an 8-h period (11). Measurement of HSL phosphorylation at Ser563 and Ser660 and activity at the 15-h time point indicated that AICAR treatment caused suppression of this lipase, which was compatible with the decrease in glycerol release. Interestingly, AICAR-induced AMPK activation increased the content of adipose triglyceride lipase (ATGL), an enzyme that specifically acts as a TG hydrolase (11) (Fig. 1). Recent data from other laboratories clearly demonstrate that ATGL is an important rate-limiting step in TG hydrolysis because studies with ATGL-knockout mice have shown that in the absence of this enzyme, adipose tissue accumulates (16). Furthermore, knockdown of ATGL and HSL showed that forskolin-stimulated lipolysis was blunted by approximately 90% and 53%, respectively (5). Because ATGL almost exclusively acts on TG molecules, up-regulation of this lipase by AICAR-induced AMPK activation appears to modulate lipolysis by cleaving a single FA from the TG molecule, forming diacylglycerol (DAG). However, because AMPK inhibits HSL activity, the subsequent breakdown of DAG is impaired, causing partial lipolysis and reduction of glycerol release (11). In keeping with the role of AMPK as a cellular energy sensor, modulation of HSL and ATGL in an antagonistic manner allows the adipocyte to prevent excessive release of NEFA into the circulation while still providing the substrate necessary for peripheral tissues to produce ATP for cellular processes (Fig. 1). In addition, partial lipolysis would facilitate a reduction in fat stores, which is seen with chronic AMPK activation in vivo (34) but invariably would result in accumulation of DAG in fat cells. However, after 15 h of AICAR treatment in vitro, we did not detect differences in DAG content relative to control cells (11). It may be that 15 h is not sufficient time to detect differences in DAG content or alternatively, other lipases may be up-regulated to breakdown DAG other than HSL. Currently, HSL is the only identified DAG lipase, therefore, investigation into other possible enzymes is warranted for future studies.

REGULATION OF GLUCOSE UPTAKE AND METABOLISM BY AMPK IN ADIPOCYTES

Studies in skeletal muscle have demonstrated that activation of AMPK through muscle contractions or by pharmacological agents leads to a significant increase in glucose uptake (24). This effect was demonstrated to be independent of insulin and mediated by increased plasma membrane glucose transporter 4 (GLUT4) content (18). Although the signaling mechanisms involved in the regulation of basal and insulin-stimulated glucose uptake are very similar in skeletal muscle cells and adipocytes, the role of AMPK activation in regulating this process in the latter is still controversial. Studies in differentiated 3T3-L1 adipocytes have shown that exposure of these cells to the AMPK agonist AICAR stimulated GLUT4 translocation to the plasma membrane. This also was accompanied by increased basal glucose uptake, which was prevented when cells were pretreated with the phosphatidylinositol-3 kinase inhibitor wortmannin, suggesting a crosstalk between the AMPK and insulin-signaling pathways (35). However, assessment of insulin-stimulated glucose uptake revealed that this variable was inhibited by AICAR in 3T3-L1 adipocytes, an effect that could not be prevented by preincubation of these cells with wortmannin. Furthermore, the phosphorylation states of the insulin receptor substrate 1 and of the downstream signaling target protein kinase B (PKB) were unaltered with AICAR treatment (30), suggesting that a signaling step downstream of PKB could be targeted by AMPK. Studies from our laboratory consistently have demonstrated that basal and insulin-stimulated glucose uptake potently was inhibited by AICAR-induced AMPK activation in primary rat adipocytes (12,13). These findings indicated that clear differences exist between fully differentiated primary adipocytes and 3T3-L1 cells with regard to the role of AMPK in the regulation of glucose uptake. The mechanisms underlying these differences still remain to be fully elucidated but could be caused by the presence of different AMPK isoforms in primary and 3T3-L1 adipocytes. These could exert distinct regulatory effects on specific steps of the signaling cascade involved in the regulation of basal and insulin-stimulated glucose uptake in primary and 3T3-L1 adipocytes.

A major step involved in the regulation of GLUT4 translocation in adipocytes that could affect the ability of these cells to uptake glucose is the downstream target of PKB known as Akt substrate of 160 kDa (AS160). In fact, it has been found in skeletal muscle that AS160 is a converging point of the AMPK and insulin signaling pathways. AS160 is a Rab-guanine triphosphosphatase activating protein (Rab-GAP), and when phosphorylated, it releases inhibition on GLUT4 vesicles in the intracellular compartment and allows translocation to the plasma membrane to facilitate glucose uptake (31) (Fig. 3). In vitro contraction and AICAR treatment of isolated extensor digitorum longus (EDL) muscle induced phosphorylation of AS160 in a wortmannin-insensitive manner (31). In addition, muscle incubations using the EDL and tibialis anterior from animals deficient in AMPK signaling showed that this kinase is required for contraction-induced AS160 phosphorylation (21). Therefore, we considered that AS160 could be distinctly regulated by AMPK in skeletal muscle and primary adipocytes because activation of this kinase leads to stimulation and inhibition of glucose uptake in the former and latter, respectively. In fact, recent studies from our laboratory provided evidence that inhibition of AS160 phosphorylation and reduction of plasma membrane GLUT4 occurred in primary rat adipocytes treated with AICAR (13) (Fig. 3). Importantly, overexpression of a kinase-dead mutant of AMPKα1 (KD-AMPKα1) in primary rat adipocytes was able to fully reverse the inhibitory effects of AICAR on insulin-stimulated AS160 phosphorylation. This was also accompanied by increases in plasma GLUT4 content and reestablishment of the glucose uptake response to insulin (13) (Fig. 3). Assessment of phosphorylation of other Akt targets such as glycogen synthase kinase-3α/β revealed that these were not affected by AICAR treatment, indicating that the effect of AMPK was specific to AS160. Interestingly, although basal glucose uptake was restored to control levels in adipocytes overexpressing the KD-AMPKα1 mutant, phosphorylation of AS160 was not recovered. This could be because distinct AMPK-mediated mechanisms govern glucose uptake under basal and insulin-stimulated conditions in adipocytes. In this context, previous studies have demonstrated that in skeletal muscle, at least two Rab-GAP (AS160 and tre-2/USP6, BUB2, cdc16 domain family member 1 (TBC1D1)) participate in the regulation of GLUT4 trafficking and glucose uptake in response to AMPK activation and insulin stimulation (7). However, because TBC1D1 expression is very low in fully differentiated 3T3-L1 adipocytes and undetectable by Western blot in mice WAT (7), it is unlikely that this Rab-GAP is a target of AMPK in adipose tissue. Future investigations are warranted to identify potential novel molecular targets by which basal and insulin-stimulated glucose uptake is regulated by AMPK in adipocytes.

FIGURE 3
FIGURE 3:
Regulation of glucose uptake by adenosine monophosphate kinase (AMPK) in primary adipocytes. When insulin binds to the insulin receptor (IR), it elicits a signaling cascade resulting in phosphorylation of Akt. Subsequently, Akt phosphorylates Akt substrate of 160 kDa (AS160), which under basal condition causes intracellular retention of glucose transporter 4 (GLUT4)-containing vesicles. Once phosphorylated, AS160 releases the brake on GLUT4 vesicles, which allows them to translocate to the plasma membrane and enhance glucose uptake. When AMPK is activated by 5-aminoimidazole-4-carboxamide1-β-D-ribofuranoside (AICAR) in adipocytes, basal and insulin-stimulated AS160 phosphorylation is inhibited, resulting in a reduction of GLUT4 at the plasma membrane and suppression of glucose uptake in these cells. Inhibition of AMPKα1 signaling in primary adipocytes reverses the suppressive effects of AICAR on plasma GLUT4 content and glucose uptake, indicating that the effect is indeed AMPK mediated. and → denote stimulation; ⊝ and ⊣ denote inhibition. IRS indicates insulin receptor substrate; PIP2, phosphatidylinositol 4,5-bisphosphate; PIP3, phosphatidylinositol-3,4,5-trisphosphate; PDK1, 3-phosphoinositide-dependent protein kinase 1; LKB1, AMPK kinase; glucose-6-P, glucose-6-phosphate; PM, plasma membrane.

From a physiological perspective, the inhibitory role of AMPK on adipocyte glucose uptake and metabolism may serve as a mechanism that conserves adipocyte energy content while providing substrate for energy production in peripheral tissues. The rationale is that the costly process of FA reesterification into TG in adipocytes relies on glucose metabolism for the production of glycerol 3-phosphate because glycerol kinase activity is negligible in WAT (26). Therefore, by suppressing glucose uptake and its metabolism, AMPK not only prevents energy consumption through FA reesterification, but also facilitates the exportation of FA to be used as substrate for energy production in other peripheral tissues (Fig. 2). This is particularly important under conditions of stress and increased energy demand, which also are known to induce AMPK activation in the WAT. This hypothesis is compatible with our observations that short- and long-term activation of AMPK markedly reduced glucose and FA uptake and the incorporation of glucose and palmitate into lipids in adipocytes (12,13).

THE ROLE OF AMPK IN WAT PLASTICITY

The classical view of the WAT is that it functions as an energy storage compartment. However, the fat cell possesses the biochemical oxidative machinery to dissipate energy within itself (8), without having to release FA to be oxidized in the liver, heart, or skeletal muscle. In fact, in vivo studies have demonstrated that when exposed to hyperleptinemia, white adipocytes rapidly can be transformed into very effective "fat-burning machines" (25). This remarkable increase in intra-adipocyte combustion of fat is characterized by elevated mitochondrial content and thermogenic proteins and decreased expression of lipogenic enzymes (25). In addition, WAT of mice exposed to cold acquires a brown adipose tissue (BAT) phenotype, a reversible phenomenon that is characterized by an increase in the amount of brown adipocytes, capillaries, and nerves in the adipose organ (2,8). Interestingly, it has been demonstrated that the molecular mechanisms mediating the acquisition of the BAT phenotype by WAT in cold-acclimatized mice involve up-regulated expression of PGC-1α and PRD1-BF-1-RIZ1 homologous domain containing 16 (PRDM16) in visceral and subcutaneous WAT in mice (2). Although up-regulation of expression and activity of PGC-1α promotes mitochondrial biogenesis, increased PRDM16 expression induces adipocyte differentiation toward the brown phenotype (19). Together, these transcriptional regulators can induce a nearly complete brown fat genetic program (19) and seem to play a major role in the process of WAT to BAT conversion (2) (Fig. 4). Also of interest are the observations that the switch toward a "brown-like" phenotype in white adipocytes of hyperleptinemic rats was accompanied by increases in PGC-1α, uncoupling protein 1 (UCP-1), and in AMPK activity in WAT (25). These findings suggested that induction of oxidative genes and acquisition of a brown adipocyte phenotype could be achieved through chronic pharmacological activation of AMPK in WAT. In fact, we recently demonstrated that prolonged (15 h) AICAR-induced AMPK activation caused increases in mRNA expression of PPAR-γ (∼3-fold), PPARα (3.7-fold), and PPARδ (∼2.5-fold) in isolated rat epididymal adipocytes (11). These cells also had approximately threefold and fourfold higher than control expression of PGC-1α and carnitine palmitoyl transferase-1b (CPT-1b) expression, respectively, which was accompanied by a twofold increase in palmitate oxidation (11). In fact, WAT metabolism was remodeled toward energy dissipation through chronic AICAR-induced AMPK activation (Fig. 1). Our data using AICAR to activate AMPK are supported by studies that also observe an up-regulation of PGC-1α mRNA in rat adipose tissue using exercise and adrenaline as physiological activators of AMPK (33). In fact, a recent study indicated that mice injected with AICAR showed increased UCP-1 expression and induced accumulation of brown adipocytes within the WAT (34). Altogether, these data support our hypothesis that AMPK could be a target for the conversion of WAT into a more "BAT-like phenotype." These findings provided evidence that adiposity can be reduced by specifically targeting AMPK in WAT, which opens up a new possibility for the treatment of obesity and its related metabolic disorders. Furthermore, because this approach directly can increase mitochondrial biogenesis and lipid oxidation in white adipocytes, it potentially could be used to overcome energy-sparing mechanisms that are activated under negative energy balance conditions and improve long-term success rate with more conventional weight loss strategies.

FIGURE 4
FIGURE 4:
Schematic representation of the plasticity of white adipose tissue (WAT) and acquirement of a "brown-like" phenotype. White adipocytes contain a large unilocular triglyceride-rich lipid droplet (LD) with an offset nucleus (N) and a few mitochondria within the limited cytoplasm (Cyt) area. Chronic activation of adenosine monophosphate kinase (AMPK) up-regulates the expression and activity of peroxisome proliferator-activated receptor-γ (PPAR-γ) coactivator-1α (PGC-1α) and PPAR-γ. This cascade of events promotes mitochondrial biogenesis, which in the presence of increased PRDM16 expression induces adipocyte differentiation toward the phenotype of brown adipocytes. Typical brown adipocytes have multiple LD (multilocular) and a much larger cytoplasm space, which is populated densely with mitochondria, rendering brown adipose tissue highly oxidative. We hypothesize that through chronic AMPK activation, white adipocytes shift metabolism toward energy dissipation versus storage. This appears to occur through the acquisition of metabolic characteristics that are typical of brown adipocytes, which could be of great relevance for the treatment of obesity and its related metabolic disorders.Arrows (→) denote stimulation.

SUMMARY

As a cellular energy sensor, AMPK has the ability to quickly regulate the activity of key enzymes involved in the control of glucose and lipid uptake and metabolism in a tissue-specific manner. This allows for short-term adjustments in substrate metabolism that maintain cellular ATP levels relatively constant despite oscillations in the energy demand imposed by the environment. AMPK also exerts important long-lasting effects by regulating the expression of specific genes involved in oxidative metabolism and energy production. In this context, activation of AMPK in adipocytes has been demonstrated to impact greatly glucose and FA metabolism in the WAT. In fact, our recent in vitro and in vivo studies provided evidence that prolonged activation of AMPK remodels adipocyte metabolism toward energy dissipation. These alterations are characterized by increased expression of markers involved in mitochondrial biogenesis and oxidative capacity, including PGC-1α, PPARγ/α/δ, CPT-1b, acetyl-CoA oxidase, and COX-6/8. These gene expression alterations are also accompanied by increased citrate synthase activity and the ability of adipocytes to oxidize endogenous and exogenous FA. As AMPK increases the oxidative capacity of the adipocyte, it simultaneously down-regulates lipogenesis and modulates the activity of major lipolytic enzymes (ATGL and HSL) in a time-dependent manner in the WAT. Altogether, these adaptations support our hypothesis that prolonged AMPK activation potentially could induce in white adipocytes metabolic characteristics that are typical of brown adipocytes (Fig. 4). These adaptations are of great relevance for the treatment of obesity and its related metabolic disorders because they have great potential to reduce fat mass. However, additional studies are necessary to investigate the impact of remodeling WAT through chronic AMPK activation on whole-body energy balance and metabolic partitioning. Of particular interest are the potential effects of chronic in vivo AMPK activation on the energy-sparing mechanisms that are engaged under conditions of reduced adiposity.

The authors apologize for not being able to cite the important works of other researchers because of reference limitations. This research was funded by operating grants from the Canadian Institute of Health Research (CIHR), the Natural Science and Engineering Research Council, and infrastructure grants from the Canada Foundation for Innovation and the Ontario Research Fund awarded to Rolando Bacis Ceddia. Rolando Bacis Ceddia also is a recipient of the CIHR New Investigator Award and the Early Research Award from the Ontario Ministry of Research and Innovation. Mandeep Pinky Gaidhu is supported by a CIHR Canada Graduate Scholarship-Doctoral Award and the Michael Smith Foreign Study Supplement.

References

1. Anthony NM, Gaidhu MP, Ceddia RB. Regulation of visceral and subcutaneous adipocyte lipolysis by acute AICAR-induced AMPK activation. Obesity (Silver Spring) 2009; 17:1312-7.
2. Barbatelli G, Murano I, Madsen L, et al. The emergence of cold-induced brown adipocytes in mouse white fat depots is determined predominantly by white to brown adipocyte transdifferentiation. Am. J. Physiol. Endocrinol. Metab. 2010; 298:E1244-53.
3. Bays HE, Gonzalez-Campoy JM, Bray GA, et al. Pathogenic potential of adipose tissue and metabolic consequences of adipocyte hypertrophy and increased visceral adiposity. Expert Rev. Cardiovasc. Ther. 2008; 6:343-68.
4. Bergeron R, Previs SF, Cline GW, et al. Effect of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion on in vivo glucose and lipid metabolism in lean and obese Zucker rats. Diabetes 2001; 50:1076-82.
5. Bezaire V, Mairal A, Ribet C, et al. Contribution of adipose triglyceride lipase and hormone-sensitive lipase to lipolysis in human hMADS adipocytes. J. Biol. Chem. 2009.
6. Brooks B, Arch JR, Newsholme EA. Effects of hormones on the rate of the triacylglycerol/fatty acid substrate cycle in adipocytes and epididymal fat pads. FEBS Lett. 1982; 146:327-30.
7. Chavez JA, Roach WG, Keller SR, Lane WS, Lienhard GE. Inhibition of GLUT4 translocation by Tbc1d1, a Rab-GTPase-activating protein abundant in skeletal muscle, is partially relieved by AMP-activated protein kinase activation. J. Biol. Chem. 2008; 283:9187-95.
8. Cinti S. Transdifferentiation properties of adipocytes in the adipose organ. Am. J. Physiol. Endocrinol. Metab. 2009.
9. Daval M, Diot-Dupuy F, Bazin R, et al. Anti-lipolytic action of AMP-activated protein kinase in rodent adipocytes. J. Biol. Chem. 2005; 280:25250-7.
10. Gaidhu MP, Anthony NM, Patel P, Hawke TJ, Ceddia RB. Dysregulation of lipolysis and lipid metabolism in visceral and subcutaneous adipocytes by high-fat diet: role of ATGL, HSL, and AMPK. Am. J. Physiol. Cell Physiol. 2010; 298:C961-C971.
11. Gaidhu MP, Fediuc S, Anthony NM, et al. Prolonged AICAR-induced AMP-kinase activation promotes energy dissipation in white adipocytes: novel mechanisms integrating HSL and ATGL. J. Lipid Res. 2009; 50:704-15.
12. Gaidhu MP, Fediuc S, Ceddia RB. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside-induced AMP-activated protein kinase phosphorylation inhibits basal and insulin-stimulated glucose uptake, lipid synthesis, and fatty acid oxidation in isolated rat adipocytes. J. Biol. Chem. 2006; 281:25956-64.
13. Gaidhu MP, Perry RL, Noor F, Ceddia RB. Disruption of AMPKalpha1 signaling prevents AICAR-induced inhibition of AS160/TBC1D4 phosphorylation and glucose uptake in primary rat adipocytes. Mol. Endocrinol. 2010; 24:1434-40.
14. Garton AJ, Yeaman SJ. Identification and role of the basal phosphorylation site on hormone-sensitive lipase. Eur. J. Biochem. 1990; 191:245-50.
15. Gauthier MS, Miyoshi H, Souza SC, et al. AMP-activated protein kinase is activated as a consequence of lipolysis in the adipocyte: potential mechanism and physiological relevance. J. Biol. Chem. 2008; 283:16514-24.
16. Haemmerle G, Lass A, Zimmermann R, et al. Defective lipolysis and altered energy metabolism in mice lacking adipose triglyceride lipase. Science 2006; 312:734-7.
17. Hardie DG. AMP-activated protein kinase as a drug target. Annu. Rev. Pharmacol. Toxicol 2007; 47:185-210.
18. Jessen N, Pold R, Buhl ES, Jensen LS, Schmitz O, Lund S. Effects of AICAR and exercise on insulin-stimulated glucose uptake, signaling, and GLUT-4 content in rat muscles. J. Appl. Physiol. 2003; 94:1373-9.
19. Kajimura S, Seale P, Spiegelman BM. Transcriptional control of brown fat development. Cell Metab. 2010; 11:257-62.
20. Koh HJ, Hirshman MF, He H, et al. Adrenaline is a critical mediator of acute exercise-induced AMP-activated protein kinase activation in adipocytes. Biochem. J. 2007; 403:473-81.
21. Kramer HF, Witczak CA, Fujii N, et al. Distinct signals regulate AS160 phosphorylation in response to insulin, AICAR, and contraction in mouse skeletal muscle. Diabetes 2006; 55:2067-76.
22. Leibel RL. Molecular physiology of weight regulation in mice and humans. Int. J. Obes. (Lond). 2008;32(Suppl. 7):S98-S108.
23. Lihn AS, Jessen N, Pedersen SB, Lund S, Richelsen B. AICAR stimulates adiponectin and inhibits cytokines in adipose tissue. Biochem. Biophys. Res. Commun. 2004; 316:853-8.
24. Musi N, Hayashi T, Fujii N, Hirshman MF, Witters LA, Goodyear LJ. AMP-activated protein kinase activity and glucose uptake in rat skeletal muscle. Am. J. Physiol. Endocrinol. Metab. 2001; 280:E677-84.
25. Orci L, Cook WS, Ravazzola M, et al. Rapid transformation of white adipocytes into fat-oxidizing machines. Proc. Natl. Acad. Sci. U S A 2004; 101:2058-63.
26. Reshef L, Olswang Y, Cassuto H, et al. Glyceroneogenesis and the triglyceride/fatty acid cycle. J. Biol. Chem. 2003; 278:30413-6.
27. Reznick RM, Shulman GI. The role of AMP-activated protein kinase in mitochondrial biogenesis. J. Physiol. 2006; 574:33-9.
28. Rosenbaum M, Leibel RL. The physiology of body weight regulation: relevance to the etiology of obesity in children. Pediatrics 1998; 101:525-39.
29. Salt I, Celler JW, Hawley SA, et al. AMP-activated protein kinase: greater AMP dependence, and preferential nuclear localization, of complexes containing the alpha2 isoform. Biochem. J. 1998; 334:177-87.
30. Salt IP, Connell JM, Gould GW. 5-Aminoimidazole-4-carboxamide ribonucleoside (AICAR) inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes. Diabetes 2000; 49:1649-56.
31. Sano H, Kane S, Sano E, et al. Insulin-stimulated phosphorylation of a Rab-GTPase-activating protein regulates GLUT4 translocation. J. Biol. Chem. 2003; 278:14599-602.
32. Stapleton D, Woollatt E, Mitchelhill KI, et al. AMP-activated protein kinase isoenzyme family: subunit structure and chromosomal location. FEBS Lett. 1997; 409:452-6.
33. Sutherland LN, Bomhof MR, Capozzi LC, Basaraba SA, Wright DC. Exercise and adrenaline increase PGC-1{alpha} mRNA expression in rat adipose tissue. J. Physiol. 2009; 587:1607-17.
34. Vila-Bedmar R, Lorenzo M, Fernandez-Veledo S. Adenosine 5′-monophosphate-activated protein kinase-mammalian target of rapamycin crosstalk regulates brown adipocyte differentiation. Endocrinology 2010.
35. Yamaguchi S, Katahira H, Ozawa S, et al. Activators of AMP-activated protein kinase enhance GLUT4 translocation and its glucose transport activity in 3T3-L1 adipocytes. Am. J. Physiol. Endocrinol. Metab. 2005; 289:E643-9.
36. Yin W, Mu J, Birnbaum MJ. Role of AMP-activated protein kinase in cyclic AMP-dependent lipolysis in 3T3-L1 adipocytes. J. Biol. Chem. 2003; 278:43074-80.
Keywords:

AMPK; adipose tissue; obesity; type 2 diabetes; glucose uptake; fatty acid oxidation

©2011 The American College of Sports Medicine