In the present study, we show that Akt1-mediated growth of muscle can regress obesity and resolve metabolic disorders in obese mice. Muscle-specific Akt1 transgene expression led to hypertrophy of fast/glycolytic muscle fibers that was accompanied by atrophy of visceral adipocytes and regression of hepatic steatosis in diet-induced obese mice. Akt1-mediated muscle growth in obese mice also led to improved insulin sensitivity and reductions in blood glucose, insulin, and leptin levels. These effects occurred despite a reduction in physical activity following transgene induction and were achieved with no changes in food intake.
Accumulating evidence indicates that the Akt1 signaling pathway plays an important role in skeletal muscle hypertrophy in vitro (Bodine et al., 2001
; Pallafacchina et al., 2002
; Rommel et al., 2001
; Takahashi et al., 2002
). Previously, Lai et al. (2004)
demonstrated that skeletal muscle-specific Akt1
transgenic mice exhibit robust muscle growth, but the functional and physiological aspects of this muscle growth were not examined. In the present study, DOX-inducible Akt1
transgene expression under the control of the 1256 [3Emut] MCK promoter fragment led to the hypertrophy of type IIb, but not I or IIa, muscle fibers. Transgene expression and fiber hypertrophy were detected in some but not all skeletal muscle tissues, likely due to the heterogeneous pattern of transgene expression from the MCK promoter fragment (Grill et al., 2003
). Muscle hypertrophy in this model was reversible, and myofibers regressed to their noninduced size when DOX was removed from the drinking water. Activation of the Akt1
transgene led to an increase in grip strength but a reduction in endurance on a treadmill test. Transcript profiling of skeletal muscle showed that transgene activation led to the induction of genes involved in glycolysis but decreased expression of genes associated with mitochondrial biogenesis and fatty acid oxidation. Furthermore, Akt1
transgene-induced muscle growth led to an increase in glucose uptake in muscle and an increase in circulating levels of lactate, indicative of Cori cycle activity. The increase in glucose uptake occurred in the absence of detectable Akt2 activation, and these findings are in contrast to those reported previously by Cleasby et al. (2007)
. Collectively, these data indicate that Akt1
transgene expression promotes the growth of muscle that can be characterized as glycolytic/fast rather than oxidative/slow.
Using the inducible transgenic system, we show that Akt1-mediated hypertrophy of fast/glycolytic muscle fibers results in the regression of fat mass and improved metabolic homeostasis in mice that had previously been made obese through a high-calorie diet. These reductions in fat mass were reversible. Transgene inactivation led to a regression of muscle hypertrophy that was associated with accelerated gains in fat deposition and overall body weight, and large increases in circulating leptin and insulin levels indicating metabolic dysfunction. Moreover, rapamycin treatment blocked Akt1-mediated muscle hypertrophy and its salutary effects on body composition. The growth of fast/glycolytic muscle in obese mice also reversed hepatic steatosis, normalized responses to exogenously administered glucose and insulin, and lowered circulating levels of glucose, insulin, and leptin. These data illustrate the importance of fast/glycolytic muscle in the control of obesity and whole-body metabolism. The metabolic improvements were observed between 4 and 6 weeks after transgene activation, when the increase in muscle mass had stabilized. Thus, this model probably reflects the metabolic regulation that is associated with the maintenance phase of a strength training program rather than the initial period of muscle growth.
In contrast to the model described here, a number of studies have focused on the role of oxidative muscle fibers in metabolic homeostasis using mice that chronically express UCP1 (Li et al., 2000
), calcineurin (Ryder et al., 2003
), or PPARδ (Luquet et al., 2003
; Wang et al., 2004
) transgenes from skeletal muscle-specific promoters. For example, PPAR
δ transgenic mice display higher levels of type I fibers relative to type II fibers, increased running performance, and resistance to diet-induced obesity. Conversely, mice that are deficient in PGC-1α display abnormal oxidative fiber growth and develop an increase in body fat (Leone et al., 2005
). These studies indicate that an increase in the “energy burn” in muscle can protect against weight gain and metabolic dysfunction. In addition, myostatin-deficient mice are resistant to diet-induced obesity (McPherron and Lee, 2002
), but it is not clear whether this metabolic effect results from changes in type I or type II fibers or from the action of myostatin on adipose tissue (Feldman et al., 2006
In the present study, we show that Akt1-mediated type II muscle fiber growth leads to an increase in whole-body energy expenditure in obese mice independent of physical activity levels. Transgene-induced changes in energy balance are indicated by an increase in circulating glucagon levels and the production of ketone bodies. Akt1-mediated muscle growth in obese mice also led to a marked decrease in circulating levels of leptin, which serves as an indicator of energy stores. The metabolic changes in the Akt1
transgenic mice probably result from increased energy expenditure as a consequence of building and maintaining additional myofibrillar structures. Empirical data show that energy expenditure corresponds to the 3/4 power of lean body mass (Reitman, 2002
). However, the overall increase in muscle mass was relatively small in the Akt1
transgenic mice, and no statistically significant increase in body mass occurred as a result of transgene induction in animals fed a normal chow diet. In contrast, myostatin-deficient mice exhibit large increases in skeletal muscle mass throughout the body, leading to a doubling of body weight (McPherron et al., 1997
). Thus, the results from the current study indicate that modest increases in type IIb skeletal muscle mass can have a profound systemic effect on whole-body metabolism and adipose mass.
Akt1-mediated growth of fast/glycolytic muscle in obese mice increases whole-body oxygen consumption, consistent with an increase in energy expenditure. Furthermore, these mice displayed a reduction in the respiratory exchange ratio, which reflects an increase in fatty acid β-oxidation. However, we could detect no increase in fatty acid oxidation in gastrocnemius muscle following transgene induction in obese or normal chow-fed mice (data not shown). In contrast, an increase in fatty acid oxidation could be observed in livers excised from these mice. Fat redistribution from adipose tissue to the liver is one of the features of obesity, and hepatic lipid deposition leads to insulin resistance and contributes to the pathogenesis of type 2 diabetes (Browning and Horton, 2004
). Our study found that diet-induced hepatic steatosis was dramatically resolved following transgene-induced growth of type II muscle, indicating that the role of the liver was converted from lipid storage to lipid oxidation. Transcript profile analysis showed that an increase in fast/glycolytic fiber growth had numerous effects on hepatic metabolism. Muscle growth increased the expression of genes specifically associated with fatty acid oxidation and mitochondrial biogenesis and reversed the effects of the high-calorie diet on 67% of the 1281 transcript expression changes in liver. Akt1-mediated muscle growth led to a reduction in the hepatic expression of SCD1, which catalyzes the conversion of saturated fatty acids to monosaturated fatty acids and is an important target of leptin action (Cohen et al., 2002
). The phenotype of the SCD1-deficient mouse is similar to that seen following Akt1-mediated skeletal muscle growth in obese mice. Both are lean and hypermetabolic and display reduced hepatic steatosis. Thus, the metabolic improvement resulting from type II skeletal muscle growth may be partly due to the downregulation of hepatic SCD1.
Our study found that an increase in fast/glycolytic muscle appeared to increase the animal’s tolerance to excess adiposity. While Akt1-mediated skeletal muscle growth in mice fed the high-calorie diet resulted in a reduction in fat-pad mass, these animals retained significantly more adipose tissue than control animals on a normal chow diet, although they appeared metabolically normal and free of hepatic steatosis. To investigate this issue in greater detail, control mice were assessed after being fed the high-calorie diet for a shorter 4-week period. These mice displayed abnormal liver histology, insulin resistance, and markedly elevated leptin levels although they were matched in body weight and fat mass to mice fed the high-calorie diet for 12 weeks with 4 weeks of muscle transgene induction. These data indicate that the metabolic improvement observed following transgene induction is not the result of fat reduction per se, but that the increased type II muscle mass attenuates the pathological consequences of excess adiposity on whole-body metabolism. The metabolic improvement in this model may result from the increase in glucose uptake by muscle, which leads the pancreas to secrete less insulin and more glucogen. These conditions would favor hepatic β-oxidation, ketogenesis, and gluconeogenesis and promote lipolysis in adipocytes. In addition, it is tempting to speculate that type II skeletal muscle allows the organism to cope with excess adipose tissue through the production of hormonal factors released by muscle that act on adipose, hepatic, or central nervous system tissues. As such, these muscle-secreted factors, or myokines (Pedersen et al., 2007
), may oppose the actions of adipose-derived cytokines that promote metabolic disorders. However, the levels of IL-6, a myokine candidate, did not change upon muscle growth in our model, suggesting that other as yet unidentified factors produced by type IIb muscle coordinate these metabolic activities.
A number of parallels can be drawn between obese mice undergoing Akt1-mediated muscle growth and obese mice on a ketogenic diet. A ketogenic diet leads to reductions in body weight and fat-pad mass that are accompanied by an increase in energy expenditure (Kennedy et al., 2007
). These effects are associated with reductions in insulin and leptin levels and numerous changes in the hepatic transcript expression profile, including a marked reduction in the expression of SCD1. Similar to Akt1
transgenic mice, myostatin-deficient mice are resistant to the development of obesity, but these changes appear to occur independently of detectable changes in O2
consumption and respiratory exchange ratio (McPherron and Lee, 2002
). Surprisingly, administration of myostatin to mice leads to reductions in both muscle and fat mass (Zimmers et al., 2002
), indicating that the regulation of fat mass by myostatin-mediated changes in muscle growth is complex and incompletely understood. Furthermore, a direct action of myostatin on adipose tissue has been reported (Feldman et al., 2006
In summary, we have created a conditional transgenic mouse model that specifically expresses Akt1 in skeletal muscle, leading to functional type II skeletal muscle growth and regression in a sequential manner. In a model of diet-induced obesity, reversible Akt1-mediated type II muscle growth led to the reversible reduction of fat mass and overall body mass independent of changes in physical activity and food intake. Muscle growth also led to an improvement in whole-body metabolism that was associated with an increase in fatty acid β-oxidation in liver, the production of ketone bodies, and the resolution of hepatic steatosis. These changes in liver function were accompanied by marked changes in the expression of hepatic genes that control energy balance. The metabolic improvement in this model cannot be entirely explained by a reduction in fat-pad mass, indicating that type II muscle counteracts the actions of excess adipose tissue on whole-body metabolism. These findings indicate that type II muscle has a previously unappreciated role in regulating whole-body metabolism through its ability to alter the metabolic properties of remote tissues. These data also suggest that strength training, in addition to the widely prescribed therapy of endurance training, may be of particular benefit to overweight individuals.