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By the end, learners will be able to:
  - Explain the historical shift from 'fat storage' to 'endocrine organ'.
  - Differentiate white adipose tissue, brown adipose tissue, and beige adipose tissue and their depot-specific functions.
  - Describe major classes of adipose-derived endocrine factors and their systemic targets.
  - Connect adipose tissue dysfunction to insulin resistance/T2DM, cardiovascular disease (CVD), endocrine disorders, and other pathologies.

Adipose tissue (AT): Composes adipocytes and stromal vascular fraction (SVF) cells.
Adipokines: AT-derived signaling proteins (e.g., leptin, adiponectin).
SVF: Contains preadipocytes, fibroblasts, endothelial/vascular cells, and immune cells.
Depots:
  - Subcutaneous white adipose tissue (sWAT)
  - Visceral white adipose tissue (vWAT)
  - Brown adipose tissue (BAT)
Metaflammation: Chronic low-grade inflammation driven by metabolic stress.

In the twentieth century, adipose tissue (AT) was primarily considered a passive energy storage site in the form of triglycerides.
Advances in biochemistry, such as electron microscopy and improved cell isolation techniques, revealed complex details about AT’s anatomy and functions beyond energy storage.
The development of radioimmunoassay (RIA) in the 1950s by Rosalyn Sussman Yalow and Solomon Berson enabled measurement of hormones like insulin in AT.
A key breakthrough occurred in 1994 when Jeffrey M. Friedman discovered leptin, highlighting AT's active endocrine role and prompting extensive research.

Early 1900s: Viewed as a passive depot.
1959-1960: Radioimmunoassay (RIA) developed.
1994: Discovery of leptin.
2000s: Advances in omics technologies, allowing detailed study of cellular components.
2009: Identification of significant amounts of brown adipose tissue (BAT) in adult humans, opening therapeutic possibilities against obesity.

AT is recognized as an active endocrine organ that secretes numerous factors that influence lipid and glucose metabolism, inflammation, and vascular health.
It maintains continuous communication with other organs via a complex network of nerves and blood vessels, playing a crucial role in homeostasis.

Adipose tissue-derived signals influence multiple organs, and these organs signal back to AT, creating an intricate feedback loop.

Adipose Tissue
  - Hypothalamus
  - Immune System
  - Liver
  - Pancreas
  - Endothelium
  - Skeletal Muscle

Small amount present in adults; located mainly in the upper back, around vertebrae.
Functions: thermogenesis and energy expenditure via UCP1-mediated oxidative phosphorylation, converting FFAs into ATP.
Endocrine-active, secreting factors like FGFs and irisin (produced during exercise, with potential therapeutic effects).

Found within WAT, shares characteristics with both WAT and BAT; adaptable to stimuli; potential target for metabolic interventions.

sWAT:
- Higher leptin/adiponectin ratios; less pro-inflammatory.
vWAT:
- More inflammatory cytokines, higher FFA release; higher cardiometabolic risk.
BAT:
- Thermogenic; contributes to energy expenditure.

Adipokines: (e.g., leptin, adiponectin, resistin)
- Functions in appetite regulation, insulin sensitivity, inflammation, and endothelial effects.
Immune factors: (e.g., TNF, IL-6, MCP-1)
- Inflammation and endothelial effects.
Growth factors: (e.g., VEGF, FGFs, TGFβ)
- Angiogenesis and remodeling.
Metabolic regulators: (e.g., ATGL, HSL, FABP4)
- Lipolysis and fatty acid handling.
Steroid hormones: (e.g., estrogens, cortisol)
- Intracrine/paracrine regulation.

Increased ffA oxidation leads to a proton gradient increase.
UCP1 causes a proton leak, increasing heat production through energy dissipation.

AT contributes to systemic inflammation through factors such as cytokines (e.g., TNF, IL-6, IL-1β), MCP-1, and PAI-1. These factors can:
  - Impair insulin signaling
  - Recruit immune cells
  - Promote endothelial dysfunction

Adiponectin is a key player, enhancing insulin sensitivity and supporting metabolic balance.
Maintaining a balance between pro-inflammatory and anti-inflammatory factors is vital for metabolic health.

AT plays a critical role in the regulation of innate and adaptive immune responses, impacting overall systemic inflammation and metabolic health.
The dynamics of immune cell profiles change during obesity, contributing to a state of chronic inflammation known as metaflammation.

Insulin Resistance: Adipose tissue dysfunction, driven by obesity-induced hypertrophy, impacts insulin sensitivity.
Cardiovascular Diseases: Dysfunction is linked to CVDs via mechanisms such as systemic inflammation and impaired lipid metabolism.
Endocrine Disorders: AT can disrupt hormonal balances impacting reproductive health, thyroid function, and metabolic processes.
Neurodegenerative Diseases: Leptin and other factors from AT can influence neuroprotective pathways or exacerbate neuroinflammation.
Cancer Risk: Chronic inflammation and dysregulated hormone signaling from AT can promote tumor growth and progression.

Further studies are needed to explore the detailed endocrine functions of AT and its interactions with other organs, particularly in metabolic disorders.
Research into gut microbiome interactions, obesity treatment mechanisms, and potential pharmacological targets for improving metabolic health is crucial.
Understanding the cellular mechanisms and signaling pathways of adipokines can help in developing targeted therapies for obesity-related complications.

The understanding of adipose tissue has transitioned from viewing it merely as a fat storage site to recognizing it as an active endocrine organ that plays a pivotal role in overall metabolic health and disease. Its interactions and signaling capacities make it an important focus for further research concerning obesity, diabetes, cardiovascular health, and other metabolic disorders.