Metabolic Syndrome (1)

Dr. Yousef M. Elshrek

• The metabolic syndrome is a cluster of the most dangerous heart attack risk factors: diabetes and raised fasting plasma glucose, abdominal obesity, high cholesterol and high blood pressure.

• In recent years, there has been a greater

concern about the presence of obesity

and metabolic syndrome in children and

adolescents.

• However, there is no consensus

regarding the diagnosis of metabolic

syndrome in children and adolescents.

• It is evident that each component of

the syndrome must be identified as

early as possible in order to prevent

definitive lesions.

• The question is how to do this and

which cut-offs must be adopted for

this diagnosis.

• Pediatric Metabolic Syndrome

(PMETs) does not have a clear

definition.

• Most institutions agree that it can be

diagnosed when a patient has two or

more of the following:

1. insulin resistance and impaired glucose

tolerance,

2. elevated cholesterol and triglycerides,

3. low circulating amounts of high-

density lipoproteins,

4. obesity (especially around the

abdomen),

5. and/or high blood pressure.

Insulin Resistance and Glucose

Intolerance • the world die from complications associated

with diabetes.

• In countries with a high diabetes incidence, such as those in the Pacific and the Middle East, as many as one in four deaths in adults aged between 35 and 64 years is due to the disease, (e. g. Obesity distribution in Libya reached 41% among women, and 21% among men, at the same time the body mass index reached 26.4% for males and 29.0% for females. Main while, overweight reached 57.5% and 69.8% for males and female respectively).

• Type 2 diabetes, which accounts

for 90 per cent of all diabetes, has

become one of the major causes of

premature illness and death,

mainly through the increased risk

of CVD which is responsible for up

to 80 per cent of these deaths.6b,

• Insulin is the most potent anabolic hormone

in the body, exerting a plethora of effects on

lipid and protein metabolism, ion an amino

acid transport, cell cycle proliferation, cell

differentiation and nitric oxide synthesis .

• The majority of persons with Metabolic

Syndrome also have insulin resistance.

• Insulin resistance and/or associated

hyperinsulinemia are believed to be the

direct cause of other Metabolic Syndrome

risk factors .

• Insulin resistance is accepted as the most unifying pathophysiologic mechanism underlying the cluster of characteristics of Metabolic Syndrome and is usually caused by a defect in insulin action within target organs and tissues that results in compensatory hyperinsulinemia

• Several possible mechanisms of insulin resistance have been proposed: pre-receptor, receptor and post-receptor mechanism.

• The most studied pathway that appears to be absolutely necessary for mediating metabolic effects of insulin involves the phosphorylation of the insulin receptor substrate (IRS) 1 and 2 and activation of phosphatidylinosital (PI) 3-kinase.

• This pathway also contributes to the mitogenic (Causing mitosis or transformation ) aspects of insulin activity.

• Under normal conditions in endothelial cells (A tissue consisting of a single layer of cells that lines the blood and lymph vessels, heart, and some other cavities ) , insulin is antiatherogenic (Protects against atherogenesis and Atherogenic means tending to promote the formation of fatty plaques in the arteries ) , and stimulates nitric oxide (a potent vasodilator*) production and decreases the expression of adhesion molecules thereby protecting cells from excessive interaction with circulating monocytes**.

• In the insulin resistant state, the PI 3-kinase pathway is impaired and insulin is no longer antiatherogenic

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• *Vasodilators are medicines that act directly on muscles in blood vessel walls to make blood vessels widen (dilate).

• ** A large phagocytic white blood cell with a simple oval nucleus and clear, grayish cytoplasm.

• A second pathway involves the phosphorylation activation of Ras, Raf, MEK* and mitogen activated protein (MAP) kinases (Erk 1 and 2)**.

• This second pathway contributes solely to the nuclear and mitogenic effects of insulin and does not convey the metabolic action of insulin.

_______________________________________*It will be explained latter in detail

**It will be also explained latter in detail

• This pathway is unimpaired in insulin resistance

and is more strongly activated by compensatory

hyperinsulinemia leading to increased activity of

growth promoting agents.

• Over stimulation of this pathway is perhaps the

source of the proatherogenic mechanism.

• Insulin resistance is enhanced by excess adipose

tissue, in particular abdominal adiposity.

• Excess adipose tissue releases non - esterfied

fatty acids (NEFA).

• A high NEFA level overloads muscle and liver

with lipid and enhances insulin resistance.

• Free fatty acids are also produced through the lipolysis of lipoproteins by the action of lipoprotein lipase, the stimulation of which is influenced by insulin.

• Insulin also inhibits lipolysis in adipose tissue.

• When insulin resistance develops, increased lipolysis in adipose tissue produces more fatty acids, further inhibiting the antilipolytic effect of insulin and creates additional lipolysis

• Over production of toxic metabolites that contribute to defective insulin signaling and insulin resistance can result from intracellular accumulation of free fatty acids.

• High plasma levels of free fatty acids

accompanied by fatty acid overloaded muscle

cells contributes to the development of fatty

liver as excess free fatty acids are directed to

the liver.

• Hyperinsulinemia may increase the production

of very low-density lipoprotein triglycerides

and thus raise triglycerides. Insulin resistance

can raise blood pressure.

• Insulin resistance generally rises with body fat

content, but a broad range of insulin resistance

exists at any given body fat level .

• Visceral or omental( See the figures) fat appears to be the most detrimental and contributes most to the development of lipotoxicity in peripheral tissues by the secretion of adipocytokines.

• Several of these adipocytokines*: adiponectin, resisting, leptin, tumor necrosis factor-alpha and interlukin-6 are implicated in insulin resistance.

• Leptin is secreted by adipocytes and secretion is regulated by the size of fat stores.

• Leptin receptors are located mostly in the hypothalamus and the brain stem and signals through these receptors controls satiety, energy expenditure and neuroendocrine function.

• Most overweight and obese individuals have elevated levels of leptin that do not suppress appetite, or in other words, leptin resistance. Leptin resistance is thought to be a fundamental pathology in obesity.

• For example, circulating levels of adiponectin ** seem to

correlate with hyperinsulinemia and insulin resistance

• Adiponectin is an anti-inflammatory cytokine that is

produced by adipocytes. Adiponectin not only enhances

insulin sensitivity, but also inhibits several steps in the

inflammatory process.

• ______________________________________________________________

*Adipocytokines, are soluble mediators derived mainly from adipocytes (fat cells), in the interaction between adipose tissue, inflammation and immunity. The adipocytokines adiponectin and leptin have emerged as the most abundant adipocyte products, thereby redefining adipose tissue as a key component not only of the endocrine system, but also of the immune system.

• **Adiponectin (also referred to as GBP-28, apM1, AdipoQ and Acrp30) is a protein which in humans is encoded by the ADIPOQ gene. It is involved in regulating glucose levels as well as fatty acid breakdown.

• It also inhibits hepatic gluconeogenic enzymes

and the rate of endogenous glucose production

in the liver.

• It increases glucose transport in muscle and

enhances fatty acid oxidation.

• There is an association between insulin

resistance and asymmetric dimethylarginine

(ADMA is an endogenous inhibitor of nitric

oxide synthase and there is relationship

between insulin resistance and endothelial

dysfunction.

• ADMA is known to be elevated in

syndromes associated with vascular

diseases.

• Also, ADMA is known to correlate with NO

mediated vasodilation and with adherence

of monocytes to the endothelium.

• Measurements of ADMA and insulin

sensitivity via the insulin suppression test

indicate that ADMA concentrations are

elevated in insulin sensitive individuals,

both normal and hypertensive.

• This relationship was further validated

when pharmacological intervention

with rosiglitazone enhanced insulin

sensitivity and reduced ADMA

concentrations.

• ADMA may be yet another metabolic

contributor to endothelial damage and

increased risk for cardiovascular

disease in insulin resistance.

• In the post-menopausal women with Metabolic Syndrome, leptin and resistin will be elevate where as adiponectin is decreased.

• In addition, BMI correlated strongly with markers of insulin resistance and adipocytokine levels.

• The relation between impaired fasting glucose or impaired glucose tolerance and insulin resistance is well supported.

• To compensate for defects in insulin activity, insulin secretion or clearance needs to be modified to sustain normal glucose levels.

• Hyperglycemia is the end result if these mechanisms fail

• Hyperglycemic induced endothelium damage takes place by five major molecular mechanisms all of which result in overproduction of superoxide by the mitochondrial electron transport chain.

• The formation of these reactive oxygen species can lead to endothelial dysfunction and decreased nitric oxide and prostacyclin production.

• Reactive oxygen species can increase the formation of vasoconstrictor prostanoids* and endothelin and promote atherosclerotic plague formation.

________________________________________ • *Prostanoids are a subclass of eicosanoids consisting of the prostaglandins (mediators of inflammatory

and anaphylactic reactions), the thromboxanes (mediators of vasoconstriction), and the prostacyclins (active in the resolution phase of inflammation.)

• Increased vascular permeability is also a consequence of hyperglycemia induced reactive oxygen species formation and is responsible for the expression of endothelial mitogen vascular endothelial growth factor, which promotes diabetic microangiopathy.*

__________________________________________________________

• *Microangiopathy (or microvascular disease, or small vessel disease) is an angiopathy (i.e. disease of blood vessels) affecting small blood vessels in the body.[1] It can be contrasted to macroangiopathy. The term cerebral small vessel disease refers to a group of pathological processes with various aetiologies that affect the small arteries, arterioles, venules, and capillaries of the brain. Age-related and hypertension-related small vessel diseases and cerebral amyloid angiopathy are the most common forms.

• Since insulin resistance increases a person's risk for developing cardiovascular disease and Type 2 diabetes, several researchers have proposed measures of insulin resistance in obese individuals with and without Metabolic Syndrome.

• Some investigators found that plasma triglyceride concentration, the ratio of triglyceride to HDL cholesterol and insulin concentrations were the most useful metabolic markers for determining insulin resistance.

• Tumor necrosis factor-alpha has also been implicated in the development of obesity and insulin resistance.

• Elevated levels of tumor necrosis factor-alpha are positively correlated with insulin resistance and chronic exposure of tumor necrosis factor-alpha induces insulin resistance.

• Tumor necrosis factor-alpha may also impair insulin receptor tyrosine kinase activity and lead to impaired downstream insulin signaling .

• Tumor necrosis factor-alpha also impairs insulin signaling by increasing serum non-esterfied fatty acids, which can induce insulin resistance in many tissues

• Interleukin-6 is also associated with insulin resistance and obesity and its expression and concentration are positively correlated with obesity, impaired glucose tolerance and insulin resistance.

• Interleukin 6 (IL-6) is an interleukin that acts as both a pro-inflammatory cytokine and an anti-inflammatory myokine.

• In humans, it is encoded by the IL6 gene.

• In addition, plasma concentrations of interleukin-6 predict the development of type 2 diabetes

• Interleukin-6 decreases insulin signaling in peripheral tissues by decreasing the expression of insulin receptor signaling components .

• Interleukin-6 has recently been revealed to be an important factor in the chronic inflammatory state and hepatic insulin resistance in obesity

• Plasminogen activator inhibitor-1 is a regulator protein in the coagulation cascade and elevated levels in obese states are a known risk factor for thrombosis, as it decreases the generation of plasmin and thus decreases fibrinolysis.

• High levels of plasminogen activator inhibitor-1 along with obesity-induced increases in clotting factors and platelet activation create a hypercoagulable state, atherogenesis and increase cardiovascular risk.

• Plasminogen activator inhibitor-1 has also been implicated in the accumulation of visceral fat

• Resistin (resistance to insulin) expression is 15-

fold greater in visceral as compared to

subcutaneous fat in rodents and is potentially

linked to obesity with insulin resistance.

• However, numerous epidemiological studies in

humans have failed to link resistin expression

in adipose tissue or circulating resistin levels

with adiposity or insulin resistance

Ras-Raf-MEK-ERK-MAPK pathway Transcription Factors

• Introduction • Cellular growth and differentiation are controlled

by multiple extracellular signals, many of which activatedthe Ras/mitogen-activated protein (MAP or ERK) kinase cascade.

• This pathway is conserved trough evolution and transfer information from the environment (growth factors, mitogens and antigen receptors, by GPCR activation, by stress and inflammatory stimulus, by UV, FASL activation) to the nucleus through a three levels pathway that involves the sequential phosphorylation of three kinases: MAPK kinase kinase (MAPKKK), MAPK kinase (MAPKK) and MAPK:

• This pathway is conserved trough evolution and transfer information from the environment (growth factors, mitogens and antigen receptors, by GPCR activation, by stress and inflammatory stimulus, by UV, FASL activation) to the nucleus through a three levels pathway that involves the sequential phosphorylation of three kinases:

1. MAPK kinase kinase (MAPKKK),

2. MAPK kinase (MAPKK)

3. and MAPK:

Yeast

Vertebrate

MAPKKK • Ras directly interacts with and activates Raf.

• Raf phosphorylates and activates MEK, which in turn phosphorylates and activates ERKs.

• The receptor tyrosine kinase effector, Raf, named for Rapidly Accelerated Fibrosarcoma, was discovered over two decades ago by two groups independently as a retroviral oncogene, v-Raf or v-MIL, possessing a serine/threonine kinase activity.

• This serine/threonine kinase was later found to function in the linear Ras-Raf-MEK-ERK mitogen activated protein kinase (MAPK) pathway with an intricate regulation.

• The Ras-Raf-MEK-ERK mitogen activated protein kinase (MAPK) pathway is activated by growth factors, mitogens and antigen receptors, by GPCR activation, by stress and inflammatory stimulus, by UV, FASL activation directly and by the activation of G-coupled receptor that switches on Calmoduline signal Ca++-citoplasmatic-dependent induced by PLC.

• For these reasons the activation of MAPK pathway is an indicator of a "good state" of the cell because it is involved in pro-proliferation and pro-survival response.

Raf kinases functions

• Besides their established role in tumorigenesis, Raf proteins and the MAPK pathway have been shown to play key roles in various “normal” physiological processes 1. as diverse as cellular metabolism,

2. cell cycle progression,

3. cell death and neurological function.

Raf kinase signaling

• RAS GTPases are activated by the majority of growth factor receptors and bind and recruit Raf to the cell membrane upon activation.

• The central components of Raf, MEK and ERK signaling are then sequentially phosphorylated and activated by each other.

• More than 70 nuclear and non-nuclear effector molecules of the mitogenic cascade have been identified so far.

• In addition, Raf kinase signaling in a cascade-independent fashion has been described.

• This includes the activation of the NF-kB transcription factor, the prevention of apoptosis by antagonizing proapoptotic factors such as

1. MST2, the mammalian sterile 20-like kinase, ASK1, the apoptosis signal-regulating kinase1,

2. and BAD, the BCL-2-antagonist of cell death, and finally the positive regulation of cell migration via the Rho effector kinase Rok-a.

• The regulation of Raf kinase activity is quite complex, far from being fully understood.

• The key feature involves assembly of the cascade at the membrane from preexisting modules (Ras module, Raf module, KSR module).

• This process is paralleled by an intricate pattern of phosphorylation and dephosphorylation events leading to conformational changes of signaling molecules.

• Within this signaling zoo along the mitogenic cascade, there is still more room for novel players.

• They are definitely more than just additional signaling proteins and contribute significantly to our understanding how Raf kinase signaling really works.

• Homo- and heterodimerization of Raf kinases clearly exist, and that heterodimerization can be Ras induced.

• The kinetics of this process depends on the presence of individual Raf isoenzymes and on the engagement of various positive and negative feedback loops.

• Primarily the phosphorylation status and the localization of Raf kinases determine the association with interacting partners, such as chaperones, other kinases, prolyl isomerases, phosphatases, scaffolding proteins and also lipids and vice versa.

• In addition, it was shown that Raf heterodimerization is regulated by 14-3-3 proteins, mitogens and the Mixed-lineage kinase 3 and is also stabilized by MEK inhibition.

• Several authors described that heterodimerization is involved in the activation of C-RAF by B-RAF, but that wild-type and mutant B-RAF use different activation mechanisms

• Whereas wild-type B-RAF activates C-RAF via RAS-induced heterodimerization, mutant B-RAF heterodimerizes with and activates C-RAF in a RAS-independent manner, thereby generating a novel B-RAF > C-RAF > MEK > ERK pathway that is active in normal as well as in transformed cells.

• Whether signaling via this pathway just reflects some sort of crossactivation of Raf isoforms or whether it leads to a different set of effector functions in comparison to the classical RAF > MEK > ERK cascade remains to be determined in future.

Role of Raf in Cancer

• Since its first isolation as a potential cellular oncogene, many studies involving Raf were focused on its role in cancer.

• These included examining both its direct role in cancer and its involvement in mediating transformation by its upstream effectors, especially Ras and growth factor receptors.

Raf Regulation

• Most of understanding of Raf regulation comes from studies using C-Raf, though several fundamental studies using B-Raf have also provided significant input to view of this complex process.

• C-Raf is a 648 amino acid protein migrating on SDS/PAGE as a ~75 kDa polypeptide.

• There are several regions conserved among all Raf proteins designated CR1, CR2, and CR3.

• This interaction is stabilized by the binding of a 14-3-3 dimer at two C-Raf phosphorylated sites, S259 and S621.

• As to the Raf activation process following the interaction with active Ras, the common thinking is that C-Raf undergoes a series of phosphorylation and dephosphorylation events that result in a stably active form.

• This aspect of Raf regulation turned out to be a highly challenging task to tackle and remains only partially resolved.

• The transforming v-Raf form contains a deletion of CR1 and CR2, resulting in a constitutively active form of Raf.

• Thus, the N-terminal half of Raf is considered to be a negative regulatory domain which helps in maintaining Raf in an inactive state in the absence of stimulation.

• The common view is that the catalytic domain of Raf is folded and bound to the N-terminal regulatory domain.