The cardiometabolic syndrome (CMS) consists of a constellation of factors, including but not limited to the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) defining criteria of impaired glucose tolerance (≥110 mg/dL), elevated blood pressure (≥130/85 mm Hg), hypertriglyceridemia (≥150 mg/dL), low high-density lipoprotein (<40 mg/dL for men, <50 mg/dL for women), and visceral obesity (waist circumference >102 cm in men and >88 cm in women). Insulin resistance is considered a potential unifying mechanism by which these metabolic factors arise and contribute to cardiovascular disease. Although insulin resistance can occur in a variety of tissues including adipose, heart, liver, pancreas, brain, kidney, and vascular tissues, skeletal muscle is notable in that it uses >20% of the human body's energy stores and accounts for 70%–95% of insulin-mediated glucose disposal.1–3 Therefore, any change in muscle mass or response to insulin is critical to the body's ability to regulate energy stores. Moreover, it is important to note that the risk factors defining the CMS are all associated with decreased physical (skeletal muscle) activity.4,5 In this review we will discuss: 1) normal insulin signaling and actions in skeletal muscle; 2) known insulin signaling defects associated with the CMS; 3) mitochondrial oxidative capacity and intramuscular lipid accumulation; 4) oxidative stress as a mediator of insulin resistance; 5) adiposity and associated inflammatory cytokines; and 6) preventive and therapeutic considerations.
The cardiometabolic syndrome is associated with insulin resistance and a dysregulation of glucose and lipid metabolism that occurs in multiple tissues. Of these, skeletal muscle is the most abundant insulin-sensitive tissue, handling >40% of the postprandial glucose uptake, while consuming 20% of the body's energy. The inability to efficiently take up and store fuel, and to transition from fat to glucose as the primary source of fuel during times of plenty (increased insulin), has been termed metabolic inflexibility. This resistance to insulin is thought to be a major contributor to the whole-body metabolic dysregulation that leads to increased cardiovascular risk. Recent investigation has identified specific defects in postinsulin receptor signaling in skeletal muscle from resistant humans and animals. Potential mechanisms contributing to this reduced insulin signaling and action include decreases in mitochondrial oxidative capacity, increased intramuscular lipid accumulation, increased reactive oxygen species generation, and up-regulated inflammatory pathways. Future research is focused on understanding these and other potential mechanisms to identify therapeutic targets for reducing cardiometabolic syndrome risk.
Insulin Actions in Skeletal Muscle
Insulin-dependent skeletal muscle glucose transport is initiated when insulin binds to its sarcolemmal receptor and triggers a cascade of events that eventually lead to glucose uptake into the skeletal muscle (Figure 1). Insulin also acts to markedly suppress fatty acid (FA) oxidation in insulin-sensitive skeletal muscle.6 Signaling begins when circulating insulin binds to the α sub-unit of the insulin receptor within the sarcolemma. Insulin receptor binding increases the tyrosine kinase activity of the intracellular β subunits, which cause tyrosine autophosphorylation and phosphorylation of insulin receptor substrates (IRS-1, IRS-2). Tyrosine phosphorylation of IRS-1 and its association to the p85 regulatory subunit of phosphatidylinositol-3-kinase (PI3K) activates the p110 catalytic subunit, which increases phosphoinositides, particularly phosphatidylinositol 3,4,5-tri-sphosphate. This leads to the activation of phosphoinositide-dependent protein kinase and downstream protein kinase B (Akt) and/or atypical protein kinase C.7 Phosphorylation of Akt substrate 160 (AS160), which has a guanosine tri-phosphatase-activating domain (Rab4), allows translocation of the insulin-sensitive glucose transporter (GLUT4) to the sarcolemma to facilitate glucose diffusion into the cell. Intracellular glucose is rapidly phosphorylated by hexokinase and directed to oxidative or nonoxidative (glycogen synthesis) pathways. Although the capacity for insulin to increase skeletal muscle glucose transport is dependent on the GLUT4 protein content and the magnitude of the insulin signal, recent work indicates that the limiting step causing insulin resistance is the recruitment of GLUT4 to the sarcolemma.8 Glucose transport into skeletal muscle can also be stimulated independent of insulin (e.g., contractile activity, hypoxia), which appears to be mediated by the activation of 5′-adenosine monophosphate-activated kinase (AMPK), although additional pathways are likely to be important.9
The mechanism by which insulin suppresses skeletal muscle FA uptake and oxidation is less well understood; however, a decrease in lipoprotein lipase activity and increased malonyl coenzyme A (CoA) levels at high physiologic insulin concentrations have been documented.10 Insulin also acts on skeletal muscle to increase mitochondrial adenosine tri-phosphate (ATP) production and tricarboxylic acid cycle and electron transport chain enzyme activities, suggesting an increase in oxidative phosphorylation, a process that is decreased in insulin-resistant muscle11,12 (Figure 2).
As noted above, the metabolic effects of insulin on skeletal muscle consist primarily of increasing glucose uptake, utilization, and storage and suppressing FA oxidation. The ability of the human body to move seamlessly between the use of carbohydrates in times of plenty (fed, increased insulin) and lipids in times of starvation (fasting, decreased insulin) has recently been termed “metabolic flexibility.”13,14 A defect in this adaptation (metabolic inflexibility) is one of the earliest signs of the CMS—and skeletal muscle appears to be a major player in its development. For example, when lean, metabolically healthy individuals are challenged with high insulin and glucose levels (hyperinsulinemic—euglycemic clamps) they are capable of adjusting metabolically to increase storage and oxidation of glucose by approximately 10-fold compared with fasting conditions, while FA oxidation is simultaneously reduced.6 Consistent with this shift in substrate utilization, the respiratory quotient across skeletal muscle shifts from low values during prolonged fasting (0.82), which is consistent with FA oxidation, to higher values (1.0) during the clamp conditions, reflecting increased glucose oxidation. However, in insulin-resistant, obese individuals the hyperinsulinemic—euglycemic clamp fails to shift muscle to glucose oxidation and storage, while FA uptake and oxidation are relatively unchanged (respiratory quotient, 0.9). Interestingly, this trend is at least partially reversible with substantial weight loss6 or treatment with thiazolidinediones,14 which improve metabolic flexibility.
Insulin Signaling Defects
The defect in skeletal muscle response to insulin associated with the CMS is manifested at multiple levels. Many studies using animal models or humans with insulin resistance have demonstrated impaired signaling through the IRS-PI3K-Akt pathway15,16; however, impaired signaling through atypical protein kinase C may be of similar importance.17 In addition to insulin signaling steps, muscle content and translocation of GLUT4 transporters are reduced,18,19 which is associated with diminished glucose transport.18–20 Downstream control of glucose storage is also affected, as evidenced by decreased glycogen synthase activity,15 which is the rate-limiting step for nonoxidative glucose disposal.
Potential mechanisms contributing to impaired insulin signaling and action include decreased skeletal muscle perfusion due to increased vasoconstriction, reduced NO production, and vascular rarefaction.21–25 Increased production of reactive oxygen species (ROS),20,26 intramuscular lipid accumulation,27 decreased mitochondrial oxidative capacity,27–30 and/or low proportions of insulin-sensitive type-1 compared with less sensitive type-2 muscle fibers31 are also likely contributors. These factors will be explored in greater detail below.
Intramuscular Lipid Accumulation and Reduced Mitochondrial Oxidative Capacity
“Ectopic” lipid accumulation in nonadipose tissue (e.g., skeletal muscle, liver, kidney) is gaining increased attention as a contributor to the metabolic derangements of the CMS.32 Skeletal muscle triacylglycerol accumulation has been associated with increased insulin resistance and loss of metabolic flexibility.10,13,33 This would include fat interspersed between muscle fibers as sensitivity is observed in highly oxidative skeletal muscle, reflected by a greater proportion of red (type 1) to white (type 2) muscle fibers, while increased glycolytic type 2 fibers have been associated with components of the CMS.31,42 Alternatively, insulin has been shown to up-regulate genes encoding mitochondrial proteins and mitochondrial protein synthesis in skeletal muscle,12 while the ability of insulin to increase mitochondrial ATP production is reduced in insulin-resistant skeletal muscle (Figure 2).11,12 Therefore, it remains uncertain whether a reduction in skeletal muscle mitochondrial oxidative capacity is a cause or consequence of insulin resistance.
Oxidative Stress as a Mediator of Insulin Resistance
Oxidative stress occurs when the production of oxidation products, particularly ROS and reactive nitrogen species, exceeds the neutralizing capacity of the antioxidant systems. In insulin-resistant rodents that model aspects of the CMS, increased skeletal muscle oxidative stress is apparent by measuring protein carbonyl43 and superoxide levels44 and ROS production rates.20 Indeed, treatment of insulin-resistant rats with the antioxidants α-lipoic acid,43 or tempol20 (Figure 4) improved whole-body glucose tolerance and insulin-stimulated glucose transport into skeletal muscle.
Potential sources for ROS production include several pathways, including nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, xanthine oxidase, glucose autoxidation, and NO synthase and mitochondrial uncoupling. Although ROS effects on skeletal muscle insulin signaling are poorly understood at present, there is evidence that oxidative stress impairs the localization and activation of IRS-1 and PI3K. In addition, decreasing inducible NO synthase (iNOS) in insulin-resistant skeletal muscle seems to increase IRS-1 levels. Recent findings also suggest that lipid-induced insulin resistance is associated with ROS production from mitochondrial uncoupling, which causes mitochondrial matrix damage.33 Alternatively, up-regulation of uncoupling protein 3 appears to facilitate FA oxidation and minimizes ROS production in skeletal muscle cells.47 Therefore, uncoupling protein and iNOS may be important targets for reversing insulin resistance in muscle.
Recently, the NADPH oxidase enzyme complex has been described in skeletal muscle. While it is well established that angiotensin II increases NADPH oxidase activity and ROS in vascular tissues, less is known about its effects on skeletal muscle.20,48 Interestingly, inhibiting the renin—angiotensin system with angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin II type 1 receptors blockers (ARBs) has been shown to reduce muscle superoxide production while improving insulin signaling and insulin-dependent glucose uptake in insulin-resistant rodent skeletal muscle.15,20 Therefore, angiotensin II may represent an important factor contributing to both hypertension and impaired glucose tolerance in the CMS.
Adiposity and Associated Adipocytokines
Not only is abdominal girth a component of the CMS, but there is considerable evidence that visceral fat is closely linked to insulin resistance. Moreover, many cytokines and cytokine-like factors such as tumor necrosis factor-α (TNF-α), interleukin-6, interleukin-1β, leptin, adiponectin, visfatin, and resistin are affected by adipose tissue expansion, and several appear to have metabolic effects on skeletal muscle.49 It has been shown repeatedly that TNF-α impairs insulin action in animals and humans. Tis is likely through serine phosphorylation of IRS-1 causing inhibition of PI3K signaling and the reduction of AS160 phosphorylation.51 Pathways that might mediate this crosstalk between proinflammatory and metabolic signaling include c-Jun N-terminal kinase (JNK), I kappa β kinase (IKKβ), and nuclear factor-κ B (NFκB).50 Alternatively, adiponectin appears to have anti-inflammatory effects, improve insulin signaling, and increase FA oxidation in skeletal muscle.52 Moreover, adiponectin and leptin levels are reduced in insulin-resistant and obese individuals, while adiponectin receptors have recently been documented in skeletal muscle.52 However, decreased adiponectin effects in insulin-resistant skeletal muscle may be mediated by postreceptor reductions in AMPK activity and FA oxidation rather than receptor levels.53
Preventive and Therapeutic Considerations
Certainly, habitual skeletal muscle activity, diet, and weight control are mainstays for preventing and managing the CMS. Physical activity has been shown repeatedly to improve skeletal muscle and whole-body insulin sensitivity and to reverse the components of the CMS. Moreover, physical activity increases mitochondrial oxidative capacity and skeletal muscle capillarization and appears to elicit “antiinflammatory” effects which, in concert, likely mediate improvements in insulin signaling, glucose uptake, and metabolic flexibility.38,56,57 Diets low in saturated fat, high in fiber and low-glycemic index foods, and rich in fruits and vegetables have been recommended for improving CMS risk.54,58 Interestingly, a diet aimed at cardiovascular risk reduction was associated with a decrease in skeletal muscle TNF-α levels.59
Pharmacologic interventions also affect skeletal muscle insulin resistance and improve the CMS risk profile. Tiazolidinediones, ARBs and ACEIs (Figure 3) have all been shown to improve skeletal muscle insulin sensitivity.60,61 Putative mechanisms include increased glucose transporter levels, reductions in ROS, anti-inflammatory effects, decreased ectopic lipid accumulation, and improved insulin signaling through IRS-1 tyrosine phosphorylation. Furthermore, increases in plasma adiponectin have been reported with ACEI and ARB use, while ACEI effects on kininase II may improve skeletal muscle insulin signaling and blood flow by increasing bradykinin.62 Interestingly, several clinical trials have demonstrated that both ACEIs and ARBs reduce the incidence of new-onset diabetes.48,62 Other cellular targets for pharmacologic modulation of skeletal muscle insulin responses include glycogen synthase kinase-3, NO synthase, uncoupling protein 3, NF?β, and ROS.
Skeletal muscle is a large and metabolically active tissue. It is responsible for the majority of insulin-mediated glucose disposal and, therefore, maintaining sensitivity to insulin is essential for maintaining and utilizing energy stores. The CMS is associated with impaired insulin signaling that results in inflexibility in transitioning between lipid and carbohydrate fuels. Mechanisms by which insulin resistance develops include impaired mitochondrial function, increases in intramuscular lipids and ROS, and a shift toward a pro-inflammatory adipocytokine profile. Maintaining adequate physical activity and healthy weight is essential for limiting CMS risk. Pharmacologic interventions, including thiazolidinediones and ARBs, may also be beneficial.
This work was supported by Veterans Affairs VISN 15 and Advanced Research Career Development awards. Dr. Stump is supported by a University of Missouri Life Sciences Mission Enhancement grant.