GW BGZ 2024 Case 6 - Fat storage in adipose and non-adipose tissues; ectopic fat

Where and in what form is fat stored in the human body?

Fat is primarily stored in adipose tissue in the form of triacylglycerols (TAGs) within specialized cells called adipocytes.

At the cellular level:

• Adipocytes contain a large central lipid droplet that occupies most of the cell volume.

• This droplet stores TAGs in a hydrophobic core, allowing efficient energy storage without water.

• TAGs consist of three fatty acids esterified to a glycerol backbone, making them highly energy-dense (~9 kcal/g).

Fat storage occurs mainly in lipid droplets, which are dynamic organelles regulated by proteins such as perilipin that control access to stored fat.

What are the different adipose tissue depots and how do they differ metabolically?

Adipose tissue is distributed in distinct depots with different metabolic implications:

1. Subcutaneous adipose tissue (SAT):

   • Located under the skin.

   • Especially prominent in gluteo-femoral (lower body) regions.

   • Generally considered metabolically protective.

   • Better capacity for lipid storage and expansion via hyperplasia.

2. Visceral adipose tissue (VAT):

   • Located around internal organs.

   • Associated with metabolic risk (“bad fat”).

   • More metabolically active and releases fatty acids directly into portal circulation (to the liver).

3. Upper body (abdominal) vs lower body fat:

   • Upper body fat (apple shape) → linked to metabolic disease.

   • Lower body fat (pear shape) → protective.

Adipose tissue can constitute 5–60% of total body mass, depending on body composition.

What is subcutaneous adipose tissue (SAT) and where is it located?

Subcutaneous adipose tissue (SAT) is fat located directly beneath the skin, especially in:

• Abdomen

• Thighs

• Hips

• Buttocks

It is the largest fat depot, accounting for ~80–90% of total body fat.

What are the functions and characteristics of SAT?

SAT has several key roles:

1. Energy storage

   • Stores excess energy as TAGs for long-term use

2. Thermal insulation

   • Reduces heat loss and helps maintain body temperature

3. Mechanical protection

   • Cushions the body and protects underlying tissues

4. Metabolic role

   • Acts as a metabolically safer fat depot

   • Stores lipids without strongly disrupting metabolic processes

Compared to visceral fat, SAT:

• Releases fewer free fatty acids (FFAs)

• Produces fewer inflammatory cytokines

• Has a lower association with metabolic disease

What is visceral adipose tissue (VAT) and where is it located?

Visceral adipose tissue (VAT) is fat located within the abdominal cavity, surrounding internal organs such as:

• Liver

• Intestines

• Pancreas

It drains into the portal circulation, meaning released fatty acids go directly to the liver.

Why is VAT metabolically harmful?

VAT is more metabolically active and harmful due to:

1. High lipolytic activity

   • Releases large amounts of FFAs

2. Portal delivery to liver

   • FFAs directly affect hepatic metabolism → promotes insulin resistance

3. Inflammatory profile

   • Produces pro-inflammatory cytokines (TNF-α, IL-6)

4. Endocrine dysfunction

   • Alters adipokine secretion

What diseases are associated with excess VAT?

High VAT is strongly linked to:

• Insulin resistance

• Type 2 diabetes

• Cardiovascular disease

• Metabolic syndrome

What is ectopic fat deposition, where is it and why is it harmful?

Ectopic fat refers to the accumulation of fat in non-adipose tissues when adipose storage capacity is exceeded.

• Liver → hepatic steatosis (fatty liver)

• Skeletal muscle

• Pancreas

• Heart

Ectopic fat causes:

• Lipotoxicity

• Insulin resistance

• Organ dysfunction

• Increased risk of metabolic disease

What are gender differences in fat distribution?

• Men:

  • More visceral fat (VAT)

  • More fat around organs and heart

• Women:

  • More subcutaneous fat (SAT)

  • Less visceral and ectopic fat

How does obesity affect fat distribution?

With increasing BMI:

• Both SAT and VAT increase

• VAT increases proportionally more

• Leads to:

  • Higher fatty acid turnover

  • Increased ectopic fat deposition

How does adipose tissue play a role in energy storage?

Adipose tissue stores excess energy as triacylglycerols (TAGs).

Mechanism:

• Dietary lipids are transported as chylomicrons and liver-derived lipids as VLDL

• Lipoprotein lipase (LPL) hydrolyzes TAGs into FFAs

• FFAs enter adipocytes and are re-esterified into TAGs

• Stored in lipid droplets

Storage occurs via:

• Hypertrophy (cell enlargement)

• Hyperplasia (formation of new adipocytes via PPARγ)

When storage capacity is exceeded → lipid spillover → ectopic fat → lipotoxicity

How does adipose tissue play a role in energy mobilization?

During fasting or increased energy demand:

• TAGs are broken down via lipolysis

• Enzymes involved:

  • ATGL (TAG → DAG)

  • HSL (DAG → MAG)

  • MGL (MAG → glycerol + FFA)

Products:

• FFA → transported bound to albumin → used in β-oxidation

• Glycerol → used in liver for gluconeogenesis

This ensures continuous energy supply during fasting and exercise.

What is the endocrine function of adipose tissue?

Adipose tissue secretes adipokines, which regulate systemic metabolism.

Key adipokines:

• Leptin

  • Signals satiety to the hypothalamus

  • Increases with fat mass

  • Obesity → leptin resistance

• Adiponectin

  • Enhances insulin sensitivity

  • Increases fatty acid oxidation

  • Anti-inflammatory

  • Decreases in obesity

• Pro-inflammatory cytokines (TNF-α, IL-6, IL-1β)

  • Promote insulin resistance

  • Produced mainly by infiltrating macrophages

These molecules regulate:

• Appetite

• Glucose metabolism

• Lipid metabolism

• Inflammation

How does the adipose tissue communicate with other organs?

Adipose tissue communicates with:

• Liver

  • Excess FFAs → fatty liver → insulin resistance

• Muscle

  • FFAs impair glucose uptake via PKC/ceramide pathways

• Pancreas

  • Lipotoxicity impairs β-cell function

• Brain

  • Leptin regulates appetite and energy expenditure

What is lipid buffering and what are the structural roles of adipose tissue?

Lipid Buffering

Healthy adipose tissue acts as a buffer:

• Stores excess lipids safely

• Prevents ectopic fat accumulation

Dysfunction leads to:

• Lipid overflow

• Insulin resistance

• Metabolic disease

Structural Roles

• Thermal insulation

• Mechanical protection of organs

What happens with fat during the postprandial (fed) state?

Hormonal environment

• High insulin

• Low catecholamines

Mechanisms of fat storage

1. Fatty acid uptake

   • Insulin stimulates LPL

   • TAGs from chylomicrons/VLDL → FFAs

   • FFAs enter adipocytes

2. Glucose uptake

   • Insulin → GLUT4 translocation

   • Glucose enters adipocyte

   • Provides glycerol-3-phosphate for TAG synthesis

3. TAG synthesis

   • FA + glycerol → TAG

   • Stored in lipid droplets

4. Inhibition of lipolysis

   • Insulin activates PDE3B

   • ↓ cAMP → ↓ PKA

   • Lipolytic enzymes inactive

Outcome

• Increased lipogenesis

• Decreased lipolysis

• Net fat storage

What happens in adipocytes during the postprandial (fed) state?

In the fed state, adipocytes actively store energy:

1. Triglyceride-rich lipoproteins (chylomicrons) circulate in blood.

2. Insulin levels rise due to carbohydrate intake.

3. Insulin stimulates:

   • Lipoprotein lipase (LPL):

     • Moves to capillary walls.

     • Hydrolyzes triglycerides into free fatty acids (FFA).

   • FFA uptake into adipocytes via transporters.

4. Insulin also stimulates:

   • GLUT4 translocation → increased glucose uptake.

5. Inside the adipocyte:

   • Glucose is converted to glycerol-3-phosphate.

   • FFA + glycerol backbone → triglyceride synthesis.

6. Triglycerides are stored in the lipid droplet.

Overall: Energy storage is maximized under insulin influence.

What happens with fat during the postprandial (fed) state?

Hormonal environment

• Low insulin

• High:

  • Catecholamines

  • Glucagon

  • Cortisol

  • Growth hormone

Mechanisms of lipolysis

1. Catecholamine signaling

   • Bind β-adrenergic receptors

   • ↑ cAMP → activates PKA

2. Activation of lipolytic machinery

   • PKA phosphorylates:

     • Perilipin → opens lipid droplet

     • HSL → activates enzyme

3. TAG breakdown

   • ATGL → HSL → MGL cascade

Fate of products

• FFA

  • Travel bound to albumin

  • Used for β-oxidation in muscle/liver

• Glycerol

  • Used in liver for gluconeogenesis

Role of cortisol

• Enhances lipolysis (long-term)

• Increases sensitivity to catecholamines

• Supports gluconeogenesis

Outcome

• Increased lipolysis

• Release of energy substrates

• Shift from glucose → fat metabolism

What happens in adipocytes during fasting?

During fasting, adipocytes shift to energy release:

• Low insulin levels remove inhibition of lipolysis.

• Catecholamines (e.g., adrenaline) stimulate fat breakdown.

• Lipolysis releases:

  • Free fatty acids (FFA) → used by muscles and liver.

  • Glycerol → used for gluconeogenesis in the liver.

Thus, adipose tissue becomes a fuel supplier for the body.

How is lipolysis regulated at the molecular level?

Lipolysis is tightly regulated by hormonal signaling pathways:

Stimulation (lipolysis ON):

• Catecholamines bind to Gs-coupled receptors →

• ↑ cAMP →

• Activation of protein kinase A (PKA) →

• Activation of lipases.

Alternative pathway:

• Natriuretic peptides →

• ↑ cGMP →

• Activation of protein kinase G (PKG).

Inhibition (lipolysis OFF):

• Insulin:

  • Decreases cAMP levels.

  • Prevents activation of PKA.

  • Suppresses lipolysis.

Key proteins:

• Perilipin 1: Protects lipid droplet in basal state.

• ABHD5 (CGI-58): Activates ATGL when released.

• ATGL: TAG → DAG

• HSL: DAG → MAG

• MGL: MAG → glycerol

PKA phosphorylation removes perilipin protection, allowing lipases access.

What is white adipose tissue and what is its structure?

White adipose tissue (WAT) is the primary fat storage tissue in humans and plays central roles in energy storage, endocrine regulation, and metabolic homeostasis.

Structure

WAT consists of:

1. Adipocytes

   • Large cells with a single lipid droplet

   • Store TAGs

   • Secrete adipokines

2. Stromal-Vascular Fraction (SVF)

   • Immune cells (macrophages, lymphocytes)

   • Endothelial cells

   • Fibroblasts

   • Preadipocytes (can differentiate into adipocytes)

What are the functions of WAT and what happens in obesity?

Functions

1. Energy storage

   • Stores TAGs as long-term energy reserve

2. Energy mobilization

   • Releases FFAs during fasting

3. Endocrine regulation

   • Secretes adipokines (leptin, adiponectin, cytokines)

4. Metabolic regulation

   • Controls insulin sensitivity

   • Regulates lipid and glucose metabolism

5. Protection and insulation

   • Cushions organs

   • Maintains body temperature

WAT dysfunction in obesity

• Adipocyte hypertrophy

• Hypoxia and poor vascularization

• Chronic inflammation

• Macrophage infiltration

• Insulin resistance

This contributes to:

• Type 2 diabetes

• Cardiovascular disease

• Metabolic syndrome

What is obesity and what causes it?

• Obesity is a condition characterized by excess body fat accumulation that impairs health.

• It is typically defined using BMI ≥ 30 kg/m², but metabolic health depends more on fat distribution (especially visceral fat) than total weight alone.

• Obesity results from a chronic positive energy balance, where energy intake exceeds energy expenditure over time.

• Excess energy is stored as triacylglycerols in adipose tissue.

How does obesity affect adipose tissue morphology?

Obesity primarily causes adipocyte hypertrophy (cell enlargement) rather than hyperplasia (increase in number), especially in upper body fat depots.

Consequences:

• Enlarged adipocytes become:

  • Hypoxic (due to insufficient blood supply)

  • Stressed and dysfunctional

• Increased risk of:

  • Cell death

  • Inflammation

• Reduced ability to store additional fat safely

This leads to adipose tissue dysfunction, a key driver of metabolic disease.

What changes occur in adipose tissue metabolism in obesity?

In obesity, several dysfunctions occur:

• Reduced blood flow → impaired nutrient delivery.

• Decreased LPL activity → reduced triglyceride clearance.

• Reduced glucose uptake → insulin resistance.

• Impaired fat storage capacity.

• Altered fatty acid release dynamics.

Result:

• Adipose tissue cannot properly buffer excess nutrients.

How does adipose tissue expand in obesity?

Adipose tissue expands through:

1. Hypertrophy (increase in adipocyte size)

• Existing fat cells enlarge

• Most common in adults

• Leads to:

  • Reduced insulin sensitivity

  • Increased inflammation

  • Higher FFA release

2. Hyperplasia (increase in adipocyte number)

• New adipocytes form from precursor cells

• Regulated by transcription factors like PPARγ

• Considered metabolically healthier because:

  • Smaller adipocytes function better

  • Less lipid spillover occurs

If adipose tissue expands mainly via hypertrophy, it becomes dysfunctional, increasing the risk of metabolic disease.

What happens in adipose tissue during obesity?

During obesity, adipose tissue undergoes major changes:

• Adipocyte enlargement (hypertrophy)

• Reduced blood supply, leading to hypoxia

• Cellular stress and dysfunction

• Increased release of pro-inflammatory cytokines (TNF-α, IL-6)

Additionally:

• Immune cells (especially macrophages) infiltrate the tissue

• Dead or stressed adipocytes are surrounded by macrophages, forming crown-like structures

These changes transform adipose tissue from a healthy energy storage organ into a pro-inflammatory, insulin-resistant tissue, contributing to systemic metabolic dysfunction.

Why does hypoxia occur in obese adipose tissue and what are the consequences?

As adipose tissue expands rapidly in obesity, its blood supply cannot increase sufficiently, leading to reduced oxygen availability (hypoxia).

Hypoxia causes:

• Activation of stress pathways

• Increased production of inflammatory cytokines

• Impaired adipocyte function

• Increased cell death

This contributes to:

• Recruitment of immune cells

• Chronic inflammation

• Development of insulin resistance

Thus, hypoxia is a key trigger for adipose tissue dysfunction.

How does inflammation develop in obesity and what are its effects?

Inflammation in obesity is a result of:

• Adipocyte stress and death

• Hypoxia

• Recruitment of immune cells

Adipose tissue begins to produce pro-inflammatory cytokines, including:

• TNF-α

• IL-6

• IL-1β

Macrophages shift from:

• M2 (anti-inflammatory) → M1 (pro-inflammatory)

This chronic low-grade inflammation:

• Disrupts insulin signaling

• Promotes insulin resistance

• Contributes to metabolic diseases such as type 2 diabetes

• Release of inflammatory cytokines.

• Impaired insulin signaling.

• Promotion of systemic insulin resistance.

How does obesity cause insulin resistance?

In obesity, chronic inflammation activates intracellular signaling pathways such as:

• JNK

• IKK

• PKR

Mechanism:

• Insulin normally promotes glucose uptake via GLUT4.

• In insulin resistance:

  • Signaling pathways are impaired due to phosphorylation of insulin receptor substrates (IRS)

  • Reducing the effectiveness of insulin

  • GLUT4 translocation is reduced.

  • Glucose remains in blood → hyperglycemia.

This is a central feature of type 2 diabetes, and ~80% of patients are overweight or obese.

What is lipotoxicity and how does it contribute to metabolic disease?

Lipotoxicity occurs when adipose tissue can no longer store excess fat, causing lipids to accumulate in non-adipose tissues such as:

• Liver

• Skeletal muscle

• Pancreas

These tissues are not specialized for fat storage, so lipid accumulation causes:

• Cellular dysfunction

• Impaired insulin signaling

• Increased oxidative stress

Consequences include:

• Fatty liver disease

• Muscle insulin resistance

• β-cell dysfunction in the pancreas

Lipotoxicity is a major driver of type 2 diabetes and metabolic complications.

How does lipid accumulation affect skeletal muscle?

In skeletal muscle:

• Excess fatty acids → accumulation of lipid intermediates (e.g., DAG).

• DAG activates protein kinase C (PKC).

• PKC interferes with insulin signaling (PI3K pathway).

• Result:

  • Reduced GLUT4 translocation.

  • Decreased glucose uptake.

  • Muscle insulin resistance.

How does obesity affect the liver?

Obesity often leads to non-alcoholic fatty liver disease (NAFLD):

• Fat accumulates in liver cells (hepatic steatosis).

• Strongly associated with insulin resistance.

Effects:

• Impaired insulin signaling.

• Increased glucose production.

• Secretion of harmful:

  • Lipids

  • Hepatokines (liver-derived signaling proteins)

These factors worsen whole-body metabolic dysfunction.

Why is visceral fat more strongly linked to disease than subcutaneous fat?

Visceral fat is more harmful because it is:

• More metabolically active

• More lipolytic (releases more FFAs)

• Drains into the portal circulation, directly affecting the liver

This leads to:

• Increased liver fat accumulation

• Hepatic insulin resistance

• Increased glucose production

Additionally, visceral fat produces more inflammatory cytokines, further worsening metabolic health.

How does lipid toxicity affect the pancreas?

Lipid accumulation in the pancreas leads to:

• Impaired beta-cell function.

• Reduced insulin secretion.

• Progression toward type 2 diabetes.

Examples:

• Lipodystrophy (impaired fat storage) → severe insulin resistance.

• Familial hyperlipidemia → increased cardiovascular risk.

How do adipokines change in obesity?

In obesity, adipose tissue secretes altered levels of adipokines:

• Leptin

  • Normally suppresses appetite

  • Increased in obesity, but causes leptin resistance

• Adiponectin

  • Improves insulin sensitivity

  • Decreases in obesity

• Pro-inflammatory cytokines (TNF-α, IL-6)

  • Increase significantly

  • Promote insulin resistance

This imbalance leads to:

• Increased appetite

• Reduced insulin sensitivity

• Chronic inflammation

How does liver fat contribute to systemic insulin resistance?

Fatty liver alters metabolic signaling:

• Releases abnormal levels of:

  • Lipids

  • Hepatokines

• These affect:

  • Muscle insulin sensitivity

  • Adipose tissue function

Importantly:

• Liver fat is a strong predictor of insulin resistance, even independent of BMI.

What are the key differences between lean and obese individuals in lipid metabolism?

Compared to lean individuals, obese individuals show:

• Similar circulating fatty acids.

• Higher triglyceride levels.

• Higher insulin levels (compensatory).

• Reduced:

  • Adipose tissue blood flow

  • Fat storage efficiency

  • Glucose uptake

• Increased insulin resistance.

What are metabolic phenotypes in obesity?

Not all obesity leads to the same metabolic outcome. Two main phenotypes:

1. Metabolically unhealthy obesity (MUO):

   • High visceral fat.

   • Insulin resistance.

   • Inflammation.

   • Higher disease risk.

2. Metabolically healthy obesity (MHO):

   • More subcutaneous fat.

   • Better fat storage capacity.

   • Lower inflammation.

   • Preserved insulin sensitivity.

How does obesity lead to type 2 diabetes?

• Excess adipose tissue releases large amounts of free fatty acids (FFAs).

• FFAs accumulate in non-adipose tissues such as:

  • Liver

  • Skeletal muscle

  • Pancreas

• FFAs are converted into harmful lipid intermediates:

  • Diacylglycerol (DAG)

Effects of these metabolites:

• Activate protein kinase C (PKC)

• Disrupt insulin receptor signaling

Consequences:

• Reduced GLUT4-mediated glucose uptake

• Development of insulin resistance

Progression:

• Pancreatic β-cells become overworked

• β-cell dysfunction and failure occur

• Blood glucose rises → T2DM develops

How does obesity increase the risk of cardiovascular disease (CVD)?

Obesity increases the risk of cardiovascular disease through interconnected metabolic disturbances, many of which overlap with type 2 diabetes.

The key drivers are:

• Increased visceral fat

• Development of insulin resistance

• Accumulation of fat in the liver (NAFLD)

• Dyslipidemia (abnormal lipid levels)

• Chronic inflammation

These changes lead to:

• Damage to blood vessels (endothelial dysfunction)

• Formation of atherosclerotic plaques

• Increased risk of coronary artery disease, stroke, and heart failure

Overall, obesity creates a metabolic environment that strongly promotes atherosclerosis and vascular damage.

Why is visceral fat particularly important in cardiovascular disease?

Visceral fat is a key driver of CVD because it is highly metabolically active and releases harmful substances directly into circulation.

It:

• Releases large amounts of free fatty acids (FFAs)

• Produces pro-inflammatory cytokines (e.g., TNF-α, IL-6)

• Drains into the portal vein, affecting the liver directly

These effects lead to:

• Increased liver fat and VLDL production

• Chronic low-grade inflammation

• Promotion of atherosclerosis

Because of these mechanisms, visceral fat is a strong predictor of cardiovascular risk, more so than total body fat.

What is the role of insulin resistance in cardiovascular disease?

In obesity, excess fat (especially visceral fat) leads to insulin resistance, where cells respond poorly to insulin.

This causes:

• Reduced glucose uptake (↓ GLUT4 activity)

• Increased blood glucose levels

• Compensatory hyperinsulinemia

These changes contribute to CVD by:

• Causing endothelial dysfunction (damage to blood vessels)

• Increasing blood pressure

• Promoting inflammation and abnormal lipid metabolism

Thus, insulin resistance is a central link between obesity and cardiovascular disease.

What factors contribute to obesity besides diet?

besity is influenced by multiple factors:

• Physical inactivity → reduced energy expenditure

• Genetics (e.g., FTO gene) → affects appetite and metabolism

• Hormones:

  • Cortisol → increases visceral fat

  • Leptin resistance → increased food intake

• Environment:

  • Easy access to high-calorie food

  • Sedentary lifestyle

• Gut microbiome:

  • Affects energy extraction and inflammation

These factors interact to promote long-term weight gain.

What is the difference between apple-shaped and pear-shaped fat distribution?

Apple shape (upper body obesity):

• More visceral fat.

• Higher metabolic risk.

• Associated with insulin resistance and T2D.

Pear shape (lower body obesity):

• More subcutaneous fat.

• Lower metabolic risk.

• Protective effect.

This distribution is partly influenced by sex hormones (estrogen).

How does adipose tissue expansion differ between upper and lower body fat?

• Upper body fat:

  • Expands mainly via hypertrophy.

  • Leads to dysfunction and insulin resistance.

• Lower body fat:

  • Expands via hyperplasia (new adipocytes).

  • Safer storage of fat.

  • More metabolically healthy.

Summarize how obesity leads to metabolic dysfunction across organs.

Obesity causes a cascade of dysfunction:

1. Adipose tissue:

   • Hypertrophy, hypoxia, inflammation.

   • Reduced storage capacity.

2. Lipid spillover:

   • Excess fat accumulates in other organs.

3. Skeletal muscle:

   • Lipid intermediates → insulin resistance.

4. Liver:

   • Fat accumulation → NAFLD.

   • Disrupted metabolic signaling.

5. Pancreas:

   • Lipotoxicity → impaired insulin secretion.

6. Systemic effects:

   • Chronic inflammation.

   • Hyperglycemia.

   • Development of type 2 diabetes.

Overall: obesity transforms adipose tissue from a safe energy storage organ into a driver of systemic metabolic disease.