ReviewAMPK regulation of fatty acid metabolism and mitochondrial biogenesis: Implications for obesity
Highlights
► Skeletal muscle AMPK regulated fatty acid metabolism and mitochondrial biogenesis. ► AMPK, exercise, metabolism and implications in obesity. ► AMPK mediated-adaptations to exercise training. ► AMPK as a potential therapeutic target for obesity-induced insulin resistance.
Introduction
Insulin resistance in skeletal muscle is a major factor in the pathogenesis of type 2 diabetes (Yu et al., 2002). Chronic elevation of plasma free fatty acids is commonly associated with impaired insulin-mediated glucose uptake (Steiner et al., 1980, Frayn et al., 1993), and often co-exists with obesity and type 2 diabetes (Reaven and Chen, 1988). Skeletal muscle is the primary tissue contributing to whole-body energy expenditure, and is the major site for insulin-stimulated glucose disposal; therefore, responsiveness to insulin in this tissue greatly influences whole-body glucose homeostasis.
Insulin-mediated glucose uptake requires an intact signalling cascade that involves a number of spatially distinct phosphorylation events, which result in moving glucose transporters (GLUT4) to the plasma membrane, upregulating glucose transport into the cell. Specifically, in skeletal muscle, insulin mediates glucose uptake by binding to its tyrosine kinase receptor on the outside of the cell, causing further activation/phosphorylation of the insulin receptor substrate (IRS) proteins-1 and 2 inside the cell. Activation of IRS proteins trigger the activation of phosphatidlyinositol-3-kinase (PI3), which promotes the interaction between phosphoinositide-dependent kinase (PDK) and Akt, and the subsequent phosphorylation/activation of Akt/protein kinase B and inhibition of AS160. The latter results in the recruitment of the glucose transporter GLUT-4 to the plasma membrane and glucose uptake (Saltiel and Kahn, 2001). There are a number of defects within this signalling cascade that are associated with insulin resistance (Shulman, 2000, Steinberg, 2007); however, reduced IRS-phosphorylation in response to insulin is the earliest and most pronounced defect in the insulin signalling cascade (Steinberg, 2007). Therefore, an important research question is to address the mechanistic causes of insulin resistance.
A common observation in insulin resistant skeletal muscle is that intramuscular lipid levels are elevated in obesity, raising the possibility that alterations in lipid metabolism influence insulin signalling (Shulman, 2000, Bonen et al., 2004b, Steinberg, 2007). Studies in humans (Pan et al., 1997, Krssak et al., 1999, Kraegen et al., 2001) and mice (Kim et al., 2000, Kim et al., 2001) have demonstrated a strong relationship between increased intramuscular triacylglycerol (TAG) content and insulin resistance. In addition, acute elevations in plasma free fatty acid levels during lipid infusion reduce insulin-mediated glucose uptake in humans (Boden and Jadali, 1991, Kelley et al., 1993, Boden et al., 1994, Roden et al., 1996) and rats (Nolte et al., 1994, Jucker et al., 1997). In humans with obesity and type 2 diabetes, TAG accumulation is associated with increased rates of skeletal muscle fatty acid transport, and increased translocation of fatty acid transporter fatty acid translocase (FAT/CD36) (FAT is the rodent homolog of human CD36 (Oquendo et al., 1989)) to the plasma membrane (Bonen et al., 2004b, Aguer et al., 2010). Similarly, in obese and high-fat fed insulin-resistant rats, increased fatty acid uptake is associated with increased fatty acid transporter plasma membrane fatty acid binding protein (FABPPM) content and FAT/CD36 expression, respectively (Turcotte et al., 2001, Hegarty et al., 2002).
Collectively, these studies suggest an adaptation for enhanced fatty acid uptake during times of lipid-oversupply, which may contribute to accumulations of intramuscular lipids. However, recent studies suggest that TAGs, per se, may not be the problem since the accumulation of TAGs appears to be relatively innocuous, but instead insulin resistance is caused by an accumulation of lipid intermediates such as long-chain acyl CoA (LCACoA), diacylglycerol (DAG) and ceramide (Steinberg, 2007). The accumulation of these lipids within skeletal muscle, as a result of increased fatty acid uptake, triggers the activation of a serine/threonine kinase cascade involving the activation of protein kinase C (PKC) isoforms (Yu et al., 2002, Moeschel et al., 2004, Yi et al., 2007), IkappaB kinase-β (IKK-β) (Gao et al., 2002) and c-jun terminal amino kinase (JNK) (Aguirre et al., 2000), which inhibit IRS signalling and Akt phosphorylation. Suppressor of cytokine signalling 3 (SOCS3) also directly interacts with insulin receptor and IRS proteins to inhibit insulin signalling (Ueki et al., 2004, Steinberg et al., 2009). More importantly, in obese skeletal muscle, PKC (Bell et al., 2000, Itani et al., 2000, Kim et al., 2004a); IKK-β (Yuan et al., 2001, Arkan et al., 2005), JNK (Hirosumi et al., 2002) and SOCS3 (Steinberg et al., 2004b, Steinberg et al., 2006a, Watt et al., 2006) are elevated, and the genetic ablation of these proteins have been shown to be protective against obesity-induced insulin resistance. Taken together, these studies indicate that strategies that limit the accumulation of reactive intramuscular lipids may prevent the development of obesity induced insulin resistance.
AMPK is present in all tissues as a heterotrimeric complex consisting of a catalytic α subunit and regulatory β and γ subunits (Xiao et al., 2007, Witczak et al., 2008). Both β and γ subunits are required for optimal activity of the α-catalytic subunit (Chen et al., 1999). Multiple genes exist for each of the subunits (α1, α2; β1, β2; γ1, γ2, γ3), enabling the expression of 12 heterotrimer combinations, which are expressed in a tissue-specific manner (Mahlapuu et al., 2004). Alternative splice variants exist for α1 and γ2, which further add to the potential diversity of the AMPK αβγ heterotrimer.
In human skeletal muscle, the majority of AMPK complexes contain both α2 and β2 subunits, and of these α2/β2 complexes, 20% associate with γ3, whilst the remaining α1/β2 and α2/β2 associate with γ1 (Wojtaszewski et al., 2005). In agreement with these findings, AMPK β2 null mice have shown an essential role for this subunit in regulating heterotrimer formation in skeletal muscle (Steinberg et al. 2010). In mice, γ3- and γ2 AMPK is predominately expressed in fast-twitch glycolytic extensor digitroum longus (EDL) muscle compared to slow-twitch oxidative soleus muscle (Barnes et al., 2004, Mahlapuu et al., 2004), whereas in gastrocnemius muscle, γ1, γ2 and γ3 are evenly expressed (Mahlapuu et al., 2004).
Reversible phosphorylation at Thr172 within the activation loop of the α-subunit is the most potent activator of AMPK (>100-fold) (reviewed in Oakhill et al. (2012)). Besides phosphorylation, AMPK is also directly activated by both AMP and ADP, which bind to the γ-subunit (Sanders et al., 2007, Oakhill et al., 2011, Xiao et al., 2011). Myristoylation of the β-subunit is required for AMP and ADP to promote AMPK Thr172 phosphorylation and initiate AMPK signalling; however, allosteric activation by AMP (3- to 5-fold) does not require myristoylation (Oakhill et al., 2010). AMP and ADP binding to the γ-subunit is also thought to induce a conformational change in the kinase domain that protects AMPK Thr172 from dephosphorylation by the protein phosphatase 2 A and C (PP2A and C); therefore, important for maintaining AMPK activity (Sanders et al., 2007, Oakhill et al., 2011).
Two upstream kinases, LKB1 and Ca2+/CaM-dependent protein kinase kinase (CaMKK), have been shown phosphorylate AMPK Thr172 in mammalian cells. LKB1 is a heterotrimer complex with regulatory proteins STRAD and MO25 (Hawley et al., 2003, Woods et al., 2003). In skeletal muscle, studies in two independent models lacking LKB1 have shown that LKB1 is the major AMPK kinase in skeletal muscle. Skeletal muscle-specific deletion of LKB1 (LKB1-MKO) from mice, results in greatly reduced α2 AMPK T172 and ACC2 S221 phosphorylation following activation by aminoimidazole-4-carboxamide-1-β-d-ribonucleoside (AICAR- a cell-permeable adenosine analog that can be phosphorylated to form 5-aminoimidazoel4-carboxamide-1-d-riborfuronosil-5′monophosphate (ZMP)) or muscle contractions/exercise (Sakamoto et al., 2005, Koh et al., 2006, Thomson et al., 2007a); however, AMPK α1 activity does not appear to be substantially reduced. While CaMKK may also activate AMPK in response to elevated intracellular Ca2+ (Hawley et al., 2005, Hurley et al., 2005, Woods et al., 2005) there is currently no genetic evidence supporting the importance of this kinase in regulating skeletal muscle AMPK Thr172 phosphorylation.
Skeletal muscle is a highly dynamic tissue that can increase the rate of ATP turnover by >100-fold in response to exercise (Sahlin et al., 1998). Under such conditions, AMP and ADP levels are rapidly increased in an intensity-dependent manner and ATP levels decline only slightly. Given the sensitivity of AMPK to changes in nucleotides it is not surprising that AMPK is rapidly activated in response to muscle contractions (electrical stimulation) or during exercise (cycling exercise in humans and treadmill running in mice) (Winder and Hardie, 1996, Fujii et al., 2000, Wojtaszewski et al., 2000, Chen et al., 2003). The activation of skeletal muscle AMPK α1 and α2 is dependent on exercise intensity, with α2 AMPK activity increasing at moderate workloads starting at 40% of VO2 Max and increasing progressively with higher intensity exercise. In contrast, AMPK α1 activity only appears to be increased during high intensity tetanic muscle contractions equivalent to >100% VO2 Max (Chen and Hsieh, 2000, Fujii et al., 2000, Hayashi et al., 2000). The activation of AMPK in skeletal muscle by AICAR promotes glucose uptake and fatty acid oxidation in skeletal muscle, and this has led to the suggestion that AMPK may be the primary mechanism mediating the metabolic adaptations to exercise (Sabina et al., 1985, Sullivan et al., 1994, Merrill et al., 1997). Since muscle contractions bring about similar metabolic changes in skeletal muscle to AICAR (increased AMPK activity, fatty acid metabolism and glucose uptake), it is commonly believed that AMPK may mediate some of the effects of exercise on metabolism. In agreement with this idea, we have recently shown that skeletal muscle AMPK β subunits are critical for controlling exercise tolerance and glucose uptake during contractions (O’Neill et al., 2011). The focus of the current review is to discuss the role of AMPK in regulating fatty acid metabolism and mitochondrial biogenesis, findings which are summarized in Fig. 1.
Section snippets
AMPK and fatty acid uptake
Increased rates of long-chain fatty acid (LCFA) uptake have been observed in skeletal muscle of obese individuals (Bonen et al., 2004b), as well obese (Luiken et al., 2001, Coort et al., 2004, Han et al., 2007, Holloway et al., 2009a) and diabetic (Smith et al., 2007, Bonen et al., 2009) Zucker rats. This provides a plausible mechanism, in addition to the elevated levels of circulating plasma free fatty acids (Boden, 2003), to account for the intramuscular lipotoxic environment implicated in
Fatty acid handling (esterification and lipolysis)
Esterification (TAG synthesis) and lipolysis (breakdown of TAGs) are important processes involved in regulation of lipid levels in skeletal muscle, and disturbances in the balance between the two processes may contribute to insulin resistance due to a disproportionate increase in reactive lipid species such as DAG.
AMPK and fatty acid oxidation
Whether or not skeletal muscle fatty acid oxidation is altered in human obesity is of continuing debate with many studies demonstrating increased (Steinberg et al., 2002, Bucci et al., 2011), no change (Bonen et al., 2004b) or decreased (Kelley and Simoneau, 1994, Simoneau et al., 1999, Simoneau et al., 1995, Jong-Yeon et al., 2002, Hulver et al., 2003, Gaster et al., 2004) rates of skeletal muscle fatty acid oxidation in whole muscle. Interestingly, reductions in fatty acid oxidation may be
AMPK-independent regulation of mitochondrial fatty acid oxidation
Resting malonyl-CoA levels in both rat (Winder et al., 1989, Maclean and Winder, 1995, Chien et al., 2000) and human (Odland et al., 1996, Odland et al., 1998) skeletal muscles are substantially higher than the measured IC50 of CPT-I for malonyl-CoA (McGarry et al., 1983, Starritt et al., 2000), suggesting CPT-I activity and rates of fatty acid oxidation should be negligible in vivo. These data suggest that the regulation of fatty acid oxidation at the level of the mitochondria is not
AMPK and exercise-induced metabolic adaptations
It is well established that endurance exercise training is a major stimulus for increasing substrate utilization and mitochondrial content (Holloszy, 1967, Gollnick et al., 1973, Holloszy and Booth, 1976), and improving insulin sensitivity. Therefore, exercise is important for people with obesity and type 2 diabetes given mitochondrial content and metabolic inflexibility often accompanies these diseases. AMPK has been proposed as a key molecule in eliciting metabolic adaptations to exercise.
AMPK transcriptional regulation of mitochondrial oxidative genes
Early studies in purified rat liver showing AMPK α2 localises to the nucleus suggested that AMPK may play a direct role in regulating gene transcription (Salt et al., 1998). Since then, additional studies have shown similar findings in human skeletal muscle in response to exercise (McGee et al., 2003) and low glycogen availability (Steinberg et al., 2006c). Recently, Ju et al. (2011) have shown that skeletal muscle AMPK α1 can also translocate to the nucleas to potentiate neurogeneration in
AMPK as a potential therapeutic target
AMPK regulation of fatty acid metabolism and mitochondrial content in response to pharmacological activation as well as exercise has made AMPK a potential therapeutic target for treatment of obesity and type 2 diabetes. Although initial small trials found that skeletal muscle AMPK activity was normal with obesity (Steinberg et al., 2004a) and type 2 diabetes (Hojlund et al., 2004), a recent large scale genetic twin study found that both obesity and type 2 diabetes cause reductions in AMPK
Conclusion
The importance of AMPK in regulating fatty acid metabolism and mitochondrial biogenesis has been highlighted in this review. Given that disturbances in these pathways contribute to insulin resistance in people with obesity, it is critical to understand the underlying mechanisms eliciting these effects, in order to develop new strategies (exercise and pharmacological) that will combat the growing problem of insulin resistance and type 2 diabetes. AMPK has been implicated in regulation of fatty
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