Adiponectin and Its Role in Metabolism

Lecture 5: Adiponectin

Overview of Adipose Tissue

  • Not only storage of lipids

    • Serves as an important endocrine organ.

  • Integration of endocrine, metabolic, and inflammatory signals for the control of energy homeostasis.

  • Bioactive proteins secreted from adipose tissue are termed adipocytokines or adipokines.

Adipokines

  • List of key adipokines:

    • Leptin

    • Tumor necrosis factor (TNF-α)

    • Plasminogen-activator inhibitor type I (PAI-I)

    • Adipsin

    • Resistin

    • Adiponectin:

    • Cytokine produced by adipose tissue that plays a crucial role in regulating glucose levels and fatty acid breakdown.

    • It is the most abundant peptide secreted from adipocytes.

    • Produced from white adipose tissue when you make more adipocytes (adipocyte differentiation)

Adiponectin

  • Characteristics:

    • Decreased adiponectin levels observed in obese individuals.

    • Associations:

    • Low adiponectin is linked to insulin resistance.

    • Exhibits anti-diabetic effects.

    • Demonstrates anti-inflammatory properties.

    • Acts as an anti-atherogenic factor.

    • Functions as an insulin sensitizer.

Structure of Adiponectin

  • Forms:

    • Full Length Adiponectin

    • Globular Adiponectin

  • Structural features:

    • Amino acid sequence from NH₂ (1) to COOH (244).

  • Composed of:

    • Collagen-like region (107 residues)

    • C1q-like domain

  • Forms multimers:

    • Monomers, Trimers, and Hexamers.

Mechanism of Action

  • FFA Oxidation:

    • Adiponectin influences the oxidation of free fatty acids (FFA) in adipose tissue.

  • Impact on metabolism:

    • Enhances insulin sensitivity.

    • Reduces triglyceride (TG) levels in circulation.

  • Interactions with Insulin:

    • Stimulates glucose uptake and fatty acid oxidation through signaling pathways involving insulin receptors and adiponectin receptors (AdipoR1 and AdipoR2).

      • AdipoR1

        • Found in all tissues, but very abundant in skeletal muscle

        • Mostly binds to globular form

      • AdipoR2

        • Found predominantly in liver

Biological Effects of Adiponectin

Effects in Muscle

  • Mechanisms:

    • Increased glucose uptake via GLUT4 transporters.

    • Enhanced fatty acid oxidation.

    • Decreased ceramide levels, which results in enhanced insulin sensitivity.

Effects in Adipose Tissue

  • Mechanisms:

    • Promotes increased differentiation of adipocytes.

    • Favors formation of smaller adipocytes instead of fewer larger ones, which contribute to reduced inflammation.

Effects in Liver

  • Mechanisms:

    • Increased activity of carnitine acyltransferase I (CAT1) and Carnitine palmitoyltransferase I (CPT1).

    • Enhanced fatty acid oxidation and decreased gluconeogenesis and glycogenolysis.

    • Decreased activity of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS).

Effects on Vascular Endothelium

  • Protective effects:

    • Protects endothelial cells, smooth muscle cells, and cardiac myocytes.

    • Reduces arterial wall thickening in injured arteries.

    • Increases nitric oxide (NO) production.

    • Reduces cell adhesion processes.

Adipocyte Hypertrophy and Inflammation

  • Lipid accumulation leads to adipocyte hypertrophy, which results in:

    • Cellular stress states and activation of pro-inflammatory pathways.

    • Increased production of pro-inflammatory adipokines that recruit macrophages.

    • Upregulation of endothelial adhesion by activated macrophages, increasing the secretion of these adipokines.

    • Increased lipolysis from hypertrophied adipocytes which further contributes to inflammation.

Relationship Between Fat Mass and Inflammation

  • Increased adipose tissue mass correlates with increased levels of monocyte chemoattractant protein-1 (MCP-1), promoting macrophage migration into adipose tissue and increases in adipokines such as IL-6 and TNFα.

  • Obese adipose tissue is characterized by reduced adiponectin production, leading to decreased glucose uptake in muscle cells and enhanced glycogenolysis and gluconeogenesis in the liver, both of which contribute to insulin resistance and diabetes.

Comparison of Visceral and Subcutaneous Adipose Tissue

  • Subcutaneous fat:

    • Produces more adiponectin than visceral fat.

  • Visceral fat:

    • Enhanced lipolysis contributes to hepatic insulin resistance.

    • Increases the output of free fatty acids (FA) and glucose from the liver.

  • Exercise:

    • An acute bout of exercise increases plasma adiponectin levels in individuals with abdominal obesity.