Nutrient Sensing and Metabolic Disturbances

Pennington Biomedical Research Center

Division of Education

Publication # 33

Potential Causes of the Metabolic Syndrome & Insulin Resistance

Ectopic fat/Impaired fat oxidation

Intrinsic defects in substrate oxidation/mitochondrial biogenesis

“Locking” fat in the fat cell/lipolysis

Adipose tissue as an endocrine tissue

Nutrient/energy sensors

Smith, S. Metabolic Syndrome Targets. Current Drug Target. 2004:3;431-439. Pennington Biomedical Research Center

Ectopic Fat/Impaired Fat Oxidation

Defect in fat oxidation may be a precursor to obesity and the metabolic syndrome.

Early studies demonstrated that the “pre-obese” individuals have increased carbohydrate oxidation and impaired fat oxidation.

This increase in carbohydrate oxidation leads to storage of lipid energy as fat leading to obesity and the metabolic syndrome.

Intervention causing an increase in fat oxidation should improve the clinical features of metabolic syndrome.

Intrinsic Defects in Substrate Oxidation/Mitochondrial Biogenesis

Mitochondrial Biogenesis Several recent studies have demonstrated

that mitochondrial biogenesis and mitochondrial function are impaired in aging, diabetes, and in individuals with insulin resistance.

These defects show a reduction in the number, location and morphology of mitochondria and are strongly associated with insulin resistance.

In skeletal muscles, exercise is an effective strategy to increase mitochondrial number.

Intrinsic Defects in Substrate Oxidation/Mitochondrial Biogenesis

Mitochondrial Biogenesis Exercise also switches fiber type from glycolytic to oxidative.

Modest physical activity has been shown to reduce the common phenotypes of the metabolic syndrome, i.e. triglycerides decrease, insulin action improves, and waist circumference decreases.

Therefore, exercise looks to be an effective method in reducing the effects of metabolic syndrome.

Intrinsic Defects in Substrate Oxidation/Mitochondrial Biogenesis

Lipid Metabolism

Lipid is stored in two main compartments: Adipose tissue Intracellular compartments in peripheral tissues

(skeletal muscle, liver)

The presence of lipid in the adipose tissue is important for providing fuel during overnight fasting and starvation.

Excess lipid delivery to skeletal muscle and liver during periods of energy excess leads to an accumulation of lipid in the muscle.

This accumulation of lipid in the liver and muscle is associated with insulin resistance.

Adipose tissue

Intrinsic Defects in Substrate Oxidation/Mitochondrial Biogenesis

Lipid Metabolism

Although these intracellular stores may not be the cause of the insulin resistance, they are good markers of underlying cellular defects such as: activation of PKC, and increases in ceramides or long chain CoA’s.

Efforts to increase lipid flux into oxidation (and hence away from the generation of “toxic” intermediates) in skeletal muscle and the liver are likely to decrease signaling through these aforementioned pathways.

PPAR-α and ß are two examples of nuclear transcription factors that should produce beneficial effects on insulin action by increasing fat oxidation.

PPARα

PPAR-α and ß

Peroxisome proliferator-activated receptor (PPARα) is a ligand activated transcription factor that plays a key role in the regulation of genes involved in carbohydrate, lipid, and lipoprotein metabolism.

PPARα is highly expressed in tissues with high mitochondrial and peroxisomal β-oxidation activities, such as liver, heart, kidney, and skeletal muscle (2-5).

In humans, treatment with PPARα agonists, i.e. fibrates, results in decreased Plasma levels of triglycerides and increased plasma HDL cholesterol levels.

Locking Fat in the Fat Cells/Lipolysis

Obesity is associated with increases in whole body lipid turnover and elevated free fatty acid (FFA) concentrations in the blood.

One way that PPARγ agonists improve the lipid and insulin phenotype of the metabolic syndrome is by sequestering lipid within the triglyceride droplet in adipose tissue.

It is believed that this will protect the skeletal muscle, liver, and beta cells (from the pancreas) from excess lipid supply.

Some of the evidence supporting PPARγ agonists effectiveness include the observation of: Decreased free fatty acids (FFA) in the blood Increased insulin stimulated lipid storage

Locking Fat in the Fat Cells/Lipolysis

Of the available PPARγ agonists, it is still not fully understood how pioglitazone, but not rosiglitazone, lowers triglycerides.

With the development of “cleaner” PPARγ agonists, antagonists which act specifically on one area of the body, a better understanding of whether or not activation of lipid storage (sequestration) is an effective therapeutic strategy should be able to be determined.

“Locking” Fat in the Fat Cell/Lipolysis

Lipolytic Pathways

During exercise, both circulating catecholamines and lipolysis increases.

Other hormones and growth factors increase during exercise as well, including brain natriuretic peptide.

It has been recently demonstrated that natriuretic peptides are potent lipolytic agents which support exercise mediated lipolysis through activating cGMP mediated lipolysis in adipose tissue.

This pathway seems to provide a potential avenue to augment lipolysis.

However, if this lipolysis is not balanced by increased uptake and oxidation by muscle and liver, the peripheral effects (lipotoxicity) could be deadly.

“Locking” Fat in the Fat Cell/Lipolysis

Lipolytic Pathways

If these hormones and growth factors do increase fatty acid utilization similar to catecholamines, then either the ANP/BNP receptor or the cGMP/PDE system might have therapeutic relevance in the metabolic syndrome.

Further research in animal models is unlikely, since the adipocyte cGMP system is primate specific and not present in rodents.

Adipose Tissue as an Endocrine Organ

Adipocytokines

With the recognition of the adipocyte as an endocrine organ and the realization that the adipocyte plays a critical role in the metabolic syndrome, the discovery of several “adipocytokines” came about.

Adipocytokines influence peripheral metabolism and regulate CNS function.

Adiponectin is an adipocyte derived hormone also known as ACRP 30.

Adipose Tissue as an Endocrine Organ

Adipocytokines

Recent evidence suggests that Adiponectin is an important target for metabolic syndrome for several reasons:

1. Receptors for Adiponectin are all in the right places: liver, skeletal muscle, beta cells, and the brain.

2. Plasma concentrations of adiponectin are decreased in obesity and insulin resistance states making replacement therapy possible.

3. Adiponectin is an activator of the AMPK cellular energy sensor and AMPK plays a key role in the regulation of fat oxidation, mitochondrial biogenesis, glucose uptake, and other cellular functions.

Adipose Tissue as an Endocrine Organ

Adipocytokines

Another adipocytokine, known as Resistin or FIZZ3, has been suggested as a therapeutic target in the metabolic syndrome.

Resistin blocks adipocyte differentation in vitro and might contribute to the metabolic syndrome by increasing ectopic fat accumulation in peripheral tissues.

However, at the Endocrine society’s 86th Annual Meeting, it was concluded that because there are only modest relationships between resistin and the metabolic syndrome phenotype, resistin is actually a less desirable therapeutic target.

Nutrient Sensors

Overview

Energy and nutrient sensors effect how cells ultimately respond to energy excess.

In general, systems that detect energy excess will shunt energy into storage and dissipate energy by increasing energy expenditure and consuming ATP.

In contrast, systems sensing energy deficits will increase fuel utilization in order to increase ATP production, decreasing carbohydrate oxidation in an effort to preserve glycogen stores.

Some of these pathways will be examined in more detail because of: Their potential to either attenuate or

intensify the features of metabolic syndrome

Nutrient Sensors

AMPK

AMPK is an energy sensor which can activate or inactivate a variety of cellular systems in order to restore the ATP versus AMP balance within a cell.

When AMP levels rise, AMPK is activated.

This leads to a series of cellular events that serve to increase fat oxidation.

Long-term activation of AMPK may have other effects that are undesirable such as: a.) decreased protein synthesis and b.) increases in food intake.

These concerns contrast with animal studies that clearly demonstrate that activation of AMPK improves the negative effects of the metabolic syndrome.

Nutrient Sensors

CHREBP/X-5-P/PP2A

In the liver, carbohydrate excess leads to de novo synthesis of lipids from carbohydrate

In humans, de novo lipogenesis contributes to overall fat balance.

It was thought that insulin and glucagon were primary regulators of this system

Recent discoveries have illustrated the hexose monophosphate shunt pathways involvement.

Inhibition of PP2A is a therapeutic target to decrease lipid synthesis of triglycerides and increase fat oxidation in the liver.

CHREBP/X-5-P/PP2A Pathway

Overview

1.Carbohydrate flux increasing intracellular Xyulose-5-phosphate

concentrations 2. Leads to the activation of Protein phosphatasePP2A

3. This causes dephosphorylation of

the 3 subunits of PP2A

4. Leads to the activation of

carbohydrate response element binding protein

(CREBP)

5. Decreased fatty acid oxidation occurs via CREBP’s regulation over

fructose 2,6 bisophoshate levels

Nutrient Sensors

Glucosamine/GFAT

The Glucosamine/Glucosamine Fructose Amido-Transferase (GFAT) pathway is another cellular sensor of energy excess believed to lead to insulin resistance.

Increased carbohydrate flux into muscle cells leads to the formation of UDP-glucosamine via conversion by the enzyme GFAT.

Although the mechanism is unclear, increased glucosamine inhibits insulin action, which is an undesirable affect for any individual.

The contribution that this pathway might play in the metabolic syndrome in vivo is still uncertain, as specific inhibitors have not been described.

Nutrient Sensors

Long Chain AcylCoA’s/Ceramides

Increases in fatty acid flux lead to increases in the intracellular concentrations of Long chain AcylCoA’s and other intracellular molecules such as ceramides.

Evidence shows that these molecules drive insulin secretion and peripheral insulin resistance.

These pathways are difficult to use as candidates for the treatment of metabolic syndrome, since there is an absence of any specific downstream molecular targets.

Other Potential Therapeutic Targets

1. Inhibition of myostatin. Myostatin is a TGF-like growth factor that suppresses skeletal muscle protein synthesis/accumulation. In myostatin knock out animals, huge skeletal muscle mass and decreased adipose tissue have been observed. This is presumably due to “repartitioning” energy into muscle, decreasing lipid synthesis in adipose tissue, and/or increasing basal energy expenditure.

2. Inhibition of GSK-3. Glycogen synthase kinase 3 (GSK-3) is upregulated in insulin resistance and diabetes.

GSK-3 inhibitors actually mimic insulin, leading to reduced insulin levels and improved glycemic control in preclinical models. It is currently unknown as to whether or not this approach will reduce the other features of metabolic syndrome.

3. Inhibition of ACC. Acetyl-CoA Carboxylase (ACC) catalyzes the carboxylation of acetyl CoA to form

malonylCoA. MalonylCoA is a potent inhibitor of CPT-1 mediated fatty acid entry into mitochondria for oxidation. It is believed that by inhibiting ACC, this will allow for increased fat oxidation.

Other Potential Therapeutic Targets

4. Administration of anti-inflammatory salicylates. There is some evidence that treatment with salicylates will improve insulin

action and the metabolic syndrome. One downside observed with this treatment is that the pathways inhibited are necessary for the normal immune response to infectious agents. Therefore, an adverse effect of this treatment may be increased infections.

5. Inactivation of the glucocorticoid receptor in adipose tissue.

Systemic cortisol excess, known as Cushing’s syndrome, has long been known to increase visceral abdominal fat and lead to the development of diabetes and features of the metabolic syndrome. Cortisol has potent effects on adipocyte function leading to the differentiation of adipocytic precursors and lipid storage. These effects are mediated via the nuclear hormone receptor for cortisol: the glucocorticoid receptor. Therefore, inactivation of the glucocorticoid receptor is currently believed to be a rational target.

Heli J. Roy, PhD, RDShanna Lundy, BS

Division of EducationPhillip Brantley, PhD, Director

Pennington Biomedical Research CenterSteven Heymsfield, MD, Executive Director

Division of Education

Edited : October 2009

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References

Smith S. Metabolic syndrome targets. Current Drug Targets. 2004;3: 431-439. Mayo Clinic: Metabolic syndrome. Available at: http://www.mayoclinic.com .

The American Heart Association: Metabolic Syndrome. Available at:

http://www.americanheart.org