Adipogenesis and Adipose Tissue Physiology Study Guide

Course Introduction to Adipogenesis

Instructor: Sheila Jacobi
Course: ANIMSCI 3100 – Animal Growth & Development
Institution: The Ohio State University (College of Food, Agricultural, and Environmental Sciences)

Overview of Adipogenesis and Cell Biology

Adipogenesis is the biological process of commitment, proliferation, differentiation, and maturation of stem-like cells into functional adipocytes. These mature cells are capable of lipogenesis (lipid storage) and lipolysis (lipid mobilization).

Molecular Signaling and Cellular Markers

The development of adipocytes involves a complex interplay of signaling molecules and transcription factors. Key markers and regulators include:

Adipocyte Commitment Markers: $PREF1$ , $Sca1$ , $Lin$ , $Ly6a$ , $a7$ , $PDGFR$ .
Stem Cell Markers: $CD24$ , $CD34$ .
Extracellular Signals: Bone Morphogenetic Proteins ( $BMPs$ ), $FGF1$ , $FGF2$ , and mechanical signals related to cell geometry.
Inhibitory Factors: $IL17$ , $ECM$ , Cilia, $LIF$ , $WNT$ , Hedgehog ( $HH$ ).
Transcription Factor Regulators:
- Pro-adipogenic: $PPAR\gamma$ , $C/EBP$ family, $ZFP423$ , $KLF4$ , $KLF5$ , $EGR2$ .
- Anti-adipogenic: $GATA-binding\,protein$ , $p53$ , $RB$ (Retinoblastoma protein), $SOX9$ , $KLF2$ , $KLF3$ .
Chromatin Modifiers: Histone acetyltransferases ( $HATs$ ) and Histone deacetylases ( $HDACs$ ).

The Mature Adipocyte Structure

Mature adipocytes contain a large, unilocular lipid droplet and an active Endoplasmic Reticulum ( $ER$ ) for protein and lipid synthesis. They function as endocrine cells, secreting adipokines such as $ADIPOQ$ (Adiponectin) and $LEP$ (Leptin).

The Biology and Physiology of Adipose Tissue

Adipose tissue is not merely a passive energy storage depot; it is a dynamic organ with multiple roles:

Essential Fatty Acids: Provides components necessary for body functions.
Energy Production: High-density storage; $1\,g$ of fat yields $9\,kcal$ , compared to $4\,kcal$ for protein or carbohydrates.
Insulation and Protection: Provides thermal regulation and physical cushioning.
Endocrine Function: Secretes hormones (e.g., Leptin, discovered in 1994) that regulate metabolism and homeostasis.
Storage Density: No water is associated with fat storage, making it a very dense energy source.

Adipose Categories and Regulation

Poultry/Livestock Specifics: Unlike humans, insulin is not the primary driver of adipose development in poultry; other growth hormones, such as $IGF-1$ , have a greater impact.
Adipokines: Involved in various processes including hemostasis and whole-body metabolism.

Types of Adipose Tissue

White Adipose Tissue (WAT)

The primary site for energy storage, characterized by a single large lipid droplet ( $LD$ ).

Brown Adipose Tissue (BAT)

Characteristics: Contains multiple small lipid droplets and a significantly higher density of large mitochondria compared to white fat.
Function: Responsible for non-shivering thermogenesis through Uncoupling Protein 1 ( $UCP1$ ), which uncouples oxidation from $ATP$ production to generate heat.
Presence: Exists at birth and is vital for neonate heat generation.
Species Exceptions: Pigs and poultry generally lack functional brown fat/ $UCP1$ for thermoregulation at birth.

Economic Impact and Research Drivers in Food Animals

Research programs focus on adipose accretion because excess fat reduces production efficiency.

Historical Data (Swine): In the late 1960s, a swine carcass could have $4.04\,kg$ ( $8.9\,lbs$ ) of lard.
    - Cost to produce fat: $1.70\,USD$ (live animal basis).
    - Value of lard: $0.52\,USD$ .
    - Net loss: $1.18\,USD$ per pig, totaling a $98\,million\,USD$ loss annually in the 1960s.
Beef Industry Impact: In the US, excess fat costs approximately $4.4\,billion\,USD$ annually ( $2\,billion\,USD$ in production + $2.4\,billion\,USD$ in removal and shipping).
Production Efficiency: Feed costs constitute $65-75\%$ of production costs. Nutrients diverted to excess fat are considered wasteful.
Consumer Demand: Modern consumers prefer leaner products, requiring researchers to balance lean growth with eating quality (sensory characteristics).

Anatomical Location of Adipose Depots

Adipose development occurs in specific body sites called depots:

Visceral Fat: Includes mesenteric fat, lace/caul fat, and perirenal (Kidney, Pelvic, Heart - $KPH$ ) fat.
Subcutaneous Fat: Located under the skin.
    - Outer Layer: First to develop; provides insulation.
    - Middle Layer: Second to develop; typically the thickest postnatally and the most metabolically active.
    - Inner Layer: Last to develop; usually thin and hard to detect in lean animals.
    - Note: These layers are most clearly organized and defined in pigs.
Intermuscular Fat (Seam Fat): Located between individual muscles; associated with the epimysium.
Intramuscular Fat (Marbling): The last depot to develop. It is associated with the perimysium surrounding muscle fiber bundles and is a key determinant of meat eating quality.

Fat Distribution by Species

Beef: $30\%$ Subcutaneous, $42\%$ Intermuscular, $15\%$ Intramuscular, $13\%$ Kidney.
Lamb: $44\%$ Subcutaneous, $34\%$ Intermuscular, $9\%$ Intramuscular, $13\%$ Kidney.
Pork: $70\%$ Subcutaneous, $15\%$ Intermuscular, $10\%$ Intramuscular, $5\%$ Kidney.

Avian Adipose Depots

Adipose depots make up approx. $20\%$ of total body fat content.
Major Pads: Neck, thigh, back, abdominal, and gizzard fat pads. The abdominal fat pad is the largest and positively correlates with total body weight.
Maturation Order: Neck < gizzard < abdominal < mesenteric.
Correlation: Genetic selection for lean mass is negatively correlated with abdominal fat pad size.

Biological Development and Cellularity

Adipogenesis Timeline

Gastrulation (approx. Day 16): Epiblast cells prolifereate and migrate to form three germ layers. The Mesoderm gives rise to adipose precursor cells.
Steps of Cell Formation:
    1. Mesenchymal Stem Cell
    2. Adipoblast
    3. Preadipocyte (multilocular lipids start appearing)
    4. Mature Adipocyte (unilocular lipid droplet formed)

Hyperplasia and Hypertrophy

Hyperplasia: Increase in cell number. Thought to occur mostly prenatally, though postnatal recruitment of cells is possible.
Hypertrophy: Increase in cell size (diameter and volume).
- Adipoblast size: < 20\,\mu m diameter.
- Mature Adipocyte size: $120-300\,\mu m$ diameter.

Species Variations at Birth

Cattle & Sheep: Most adipocytes are already unilocular at birth.
Pigs: Adipocytes are mostly multilocular at birth; coalescence into unilocular droplets occurs postnatally.

Hormonal and Molecular Control

Key Hormones of Adipogenesis

$IGF-1$ and $Insulin$ .
Epidermal Growth Factor ( $EGF$ ).
Transforming Growth Factor ( $TGF\alpha$ and $TGF\beta$ ).
Glucocorticoids (e.g., Cortisol).
Retinoic Acid.

Transcription Factors and Molecular Mechanisms

Terminal differentiation is controlled by specialized transcription factors:

$PPAR\gamma$ : Peroxisome proliferator-activated receptor gamma. It heterodimerizes with $RXR\alpha$ (Retinoid X receptor alpha) to bind the $PPRE$ (PPAR Response Element) in gene promoters.
$C/EBP$ Family: $C/EBP\alpha, \beta, \delta$ . $\beta$ and $\delta$ typically increase first to turn on $PPAR\gamma$ .
$ADD1/SREBP1$ : Maintains $PPAR\gamma$ expression and promotes differentiation.

Genes Expressed in Adipocyte Maturation

Lipogenic Genes: $Stearoyl-CoA\,desaturase$ , Fatty Acid Synthetase ( $FAS$ ), Acetyl-CoA Carboxylase ( $ACC$ ), $GLUT-4$ .
Lipolytic Genes: Hormone-Sensitive Lipase ( $HSL$ ), Adipose Triglyceride Lipase ( $ATGL$ ), Monoacylglycerol Lipase ( $MGL$ ).

Adipose Tissue Metabolism

Lipogenesis (Lipid Synthesis)

Site of synthesis:
- Cattle, sheep, pigs: Adipose tissue and mammary glands.
- Poultry: Primarily the liver (Hepatic synthesis).
Substrates:
- Non-ruminants: Dietary Fatty Acids and de novo synthesis from glucose.
- Ruminants: De novo synthesis primarily from Acetate; dietary fats are changed via biohydrogenation (unsaturated to saturated).

Lipolysis (Lipid Breakdown)

The hydrolysis of Triglycerides ( $TG$ ) into glycerol and three fatty acids.
Regulated by $G\,protein-coupled\,receptors$ .
Protein Kinase A ( $PKA$ ) phosphorylates $HSL$ , which then acts on the lipid droplet.

Factors Affecting Composition and Adiposity

Age: Lipid percentage increases with age; highly correlated with adipocyte hypertrophy.
Breed: Variations exist (e.g., Berkshire vs. Yorkshire pigs).
Sex:
    - General rule: Intact males < Castrate males < Intact females.
    - Exception in Swine: Gilts (females) are leaner than Barrows (castrate males).
    - Poultry: Females (pullets) have a higher fat-to-bodyweight ratio than males (cockerels).
Nutrition: Feed restriction reduces adipocyte diameter at a constant age, though cells look similar when compared at a constant weight. Supplementation (e.g., Asiaticoside/ $Asi$ ) can reduce abdominal fat weight by increasing lipolysis.

Modern Genetic Trends and Technology

Performance Shifts (1950 to Present)

Broilers: Significant reduction in proportion of fat-pad weight to total body weight due to selection for lean growth (2005 birds are much larger and leaner than 1957 birds).
Lambs/Steers/Hogs: All show trends of increased market weight, better feed efficiency (lower $F:G$ ratio), decreased fat thickness, and increased Loin Muscle Area ( $LMA$ ).

Genome Editing (CRISPR/Cas9)

This technology allows for precise removal or alteration of DNA sections.
Case Study: Pigs lack functional $UCP1$ . Researchers used CRISPR/Cas9 to create $UCP1$ adipose-specific Knock-In (KI) pigs.
- Results: KI pigs had improved thermoregulation during cold exposure.
- Body Composition: KI pigs showed no difference in weight or feed conversion but produced a leaner carcass with less backfat and higher muscle percentage compared to Wild Type ( $WT$ ) pigs.