Adipocyte
Lipid-storing mesenchymal cells in three types: white (WAT — energy storage + endocrine via leptin/adiponectin), brown (BAT — UCP1 non-shivering thermogenesis), and beige (recruitable thermogenic within WAT by cold or β3-AR agonism). Central to whole-body energy homeostasis, insulin sensitivity, inflammation, and cardiovascular risk. WAT is the largest endocrine organ in the body.
Overview
The adipocyte is the lipid-storing cell of adipose tissue — a specialised mesenchymal cell capable of accumulating and mobilising triglycerides to match the body's moment-to-moment energy needs. Long regarded as a passive energy depot, adipose tissue is now recognised as a major endocrine organ secreting more than 600 bioactive proteins (adipokines) that regulate appetite, insulin sensitivity, inflammation, immune function, and cardiovascular risk.
Three functionally and developmentally distinct adipocyte types exist. White adipocytes (WAT) dominate body fat composition: subcutaneous (50–60%), visceral (intra-abdominal), and ectopic (hepatic, intramuscular) depots. A single giant unilocular lipid droplet occupies >90% of cell volume. Brown adipocytes (BAT) are present in paravertebral, supraclavicular, and perirenal depots in adult humans; they contain multilocular lipid droplets and dense mitochondria, and express UCP1 (uncoupling protein 1) for non-shivering thermogenesis. Beige (brite) adipocytes are recruited within WAT depots by cold exposure, β3-adrenergic receptor agonism, FGF21, or irisin — upregulating UCP1 and acquiring BAT-like thermogenic capacity.
Structure
| Feature | White Adipocyte | Brown Adipocyte |
|---|---|---|
| Diameter | 80–120 µm (among largest cells) | 25–40 µm (smaller, multilocular) |
| Lipid droplet | Unilocular; occupies >90% volume; phospholipid monolayer + TAG core | Multilocular (many small LDs — maximises lipase access) |
| Mitochondria | Sparse; thin cytoplasmic rim | Abundant, dense cristae; very high oxidative capacity (COX high) |
| LD coat protein | Perilipin-1 (PLIN1) — master LD coat; phosphorylated by PKA on lipolysis | PLIN1 + PLIN2; UCP1 in IMM |
| Key markers | PLIN1, FABP4/aP2, adiponectin, leptin, CD36, GLUT4 | UCP1, Cidea, Cox8b, PGC-1α, PRDM16; Tmem26 (beige) |
| Nucleus | Peripherally displaced, crescent-shaped | Central; round |
| Innervation | Sparse sympathetic; β3-AR present but few nerve fibres | Dense sympathetic (β1/β3-AR) — required for thermogenic activation |
Function — Lipolysis, Lipogenesis, and Thermogenesis
WHITE ADIPOCYTE — LIPID STORAGE AND MOBILISATION
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FED STATE (insulin-dominant) — LIPOGENESIS:
Insulin --> IRS-1/PI3K/Akt --> GLUT4 translocation (glucose uptake)
--> PDE3B --> cAMP degradation --> anti-lipolysis
LPL (endothelium, insulin-activated) cleaves VLDL/CM-TAG
--> FFA release --> CD36/FATP4 uptake
Intracellular: G-3-P + FFA --> DAG (DGAT1/DGAT2) --> TAG --> LD (PLIN1 coat)
FASTED / STRESS STATE (catecholamine-dominant) — LIPOLYSIS:
beta1/beta3-AR stimulus (NE, glucagon, ANP, GH)
--> Gs --> adenylyl cyclase --> cAMP --> PKA activation
PKA phospho PLIN1 (Ser492/522)
--> releases CGI-58/ABHD5 --> activates ATGL (adipose TAG lipase)
--> TAG --> DAG
PKA phospho HSL (Ser563/660)
--> active HSL translocates to LD --> DAG --> MAG
MGL (constitutive) --> MAG --> glycerol + FFA
Products: 3 FFAs + 1 glycerol --> albumin-bound in plasma
--> liver (beta-oxidation, ketogenesis, VLDL)
--> muscle/heart (beta-oxidation)
BROWN ADIPOCYTE — UCP1 THERMOGENESIS:
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Cold / beta3-AR --> cAMP --> PKA --> lipolysis (FFA as fuel + UCP1 activator)
Mitochondrial OXPHOS generates H+ gradient (proton-motive force)
UCP1 (inner mitochondrial membrane):
H+ re-enters matrix via UCP1 (bypassing ATP synthase)
--> proton-motive force DISSIPATED AS HEAT (not ATP)
Inhibition: purine nucleotides (GDP, ADP) bind UCP1 -- competitive
Activation: long-chain free fatty acids (LCFA) released by lipolysis
Net: O2 consumption maximal, ATP synthesis minimal
--> heat generation up to 300 W/kg BAT
BEIGE/BRITE ADIPOCYTE INDUCTION:
Cold, beta3-AR agonists (mirabegron), FGF21, irisin (muscle-derived FNDC5)
--> PGC-1alpha transcription --> UCP1, Cidea, Cpt1b upregulation
--> WAT depot acquires brown-like multilocular phenotype
Endocrine Function — Adipokines
| Adipokine | Source / Level | Receptor / Signalling | Key Effects |
|---|---|---|---|
| Leptin (LEP) | WAT; proportional to fat mass; ↑ postprandially; ↓ fasting | LepRb (long form) in hypothalamic ARC → JAK2/STAT3; ↓AgRP/NPY (orexigenic), ↑POMC/CART (anorexigenic) | Satiety, ↑energy expenditure, ↑sympathetic tone; pro-inflammatory at high levels; angiogenic (VEGF-like) |
| Adiponectin (ADIPOQ) | WAT; ~5–10 µg/mL; paradoxically ↓ with fat mass | AdipoR1 (muscle → AMPK); AdipoR2 (liver → PPARα) | ↑FA oxidation (AMPK→ACC→↓malonyl-CoA→↑CPT1), ↓gluconeogenesis, ↑insulin sensitivity; anti-inflammatory (↓NF-κB, ↑IL-10); ↓ in obesity/T2DM; ↑ with TZDs |
| Resistin | Primarily ATMs in humans (adipocyte in rodents); ↑ in obesity | TLR4 / serine kinase cascades | Pro-inflammatory; impairs insulin signalling; ↓ AMPK |
| PAI-1 | WAT (visceral > subcutaneous); ↑ in obesity | Inhibits tissue plasminogen activator (tPA) | Pro-thrombotic (impaired fibrinolysis); CVD risk marker in metabolic syndrome |
| CCL2/MCP-1 | Enlarged adipocytes, ATMs | CCR2 on monocytes | Monocyte chemoattractant → ATM infiltration → crown-like structures → inflammatory amplification |
Adipogenesis — Transcriptional Programme
Adipogenesis proceeds from mesenchymal progenitors through a committed pre-adipocyte stage to the mature lipid-laden adipocyte. The transcriptional programme is one of the best understood in cell biology:
Early Induction
Adipogenic stimulus (insulin, glucocorticoid, IBMX) → growth arrest → C/EBPβ and C/EBPδ (early TFs, hours) → activate PPARγ and C/EBPα promoters.
Master TF Activation
PPARγ (nuclear receptor) = obligatory master regulator of all adipogenesis. Ligands: PUFAs, 15d-PGJ₂; pharmacological agonists: TZDs (rosiglitazone, pioglitazone → ↑adiponectin, ↑insulin sensitivity). C/EBPα co-activates in positive feedback.
BAT Specification
PRDM16 + PPARγ + PGC-1α → brown adipocyte fate. BMP7 activates PRDM16. β3-AR/cAMP → PGC-1α → mitochondrial biogenesis, UCP1 transcription (via PGC-1α binding to PPAR response element in UCP1 enhancer).
Suppressors
Wnt/β-catenin (inhibits C/EBPα/PPARγ → maintains pre-adipocyte state), Pref-1/DLK1, GATA2/3, KLF2. TGF-β (anti-adipogenic in WAT). NE/sympathetic → PRDM16 → BAT differentiation (via β3-AR/cAMP).
Pathology
Obesity and Metabolic Syndrome
Adipocyte hypertrophy >100 µm → hypoxia (HIF-1α), ER stress, cell death → ATM infiltration (M1 polarisation via MCP-1/CCL2) → crown-like structures (CLSs) → ↑TNF-α, IL-6, IL-1β → systemic insulin resistance (IRS-1 Ser307 phosphorylation by JNK). ↑portal FFA delivery → hepatic lipid accumulation → NAFLD/NASH. Visceral fat > subcutaneous fat for metabolic risk (direct portal FFA delivery). BMI is a poor predictor; waist circumference and visceral fat mass better correlate with cardiometabolic risk.
Type 2 Diabetes Mellitus — Adipose Contribution
Obesity → adipocyte-derived TNF-α + excess FFAs → IRS-1 Ser307 phosphorylation (JNK) → uncoupling of insulin receptor → impaired GLUT4 translocation → adipocyte and systemic insulin resistance → ↑lipolysis → further FFA elevation → β-cell lipotoxicity + glucolipotoxicity → progressive β-cell failure. ↓Adiponectin in visceral obesity impairs AMPK/PPARα signalling in liver and muscle. Weight loss reverses adipose inflammation and partially restores insulin sensitivity.
Lipodystrophy
Congenital generalised lipodystrophy (CGL): AGPAT2, BSCL2/seipin, CAVEOLIN1, PTRF mutations → near-complete absence of adipose tissue → paradoxically severe insulin resistance, hypertriglyceridaemia (>1000 mg/dL), fatty liver, early T2DM. Familial partial lipodystrophy (FPLD2, Dunnigan type): LMNA mutations → loss of limb/gluteal adipose, truncal sparing → metabolic syndrome. HIV/ART lipodystrophy: NRTIs/PIs → mitochondrial dysfunction, ↓adipogenesis → lipoatrophy + central fat accumulation.
BAT Dysfunction in Obesity and Ageing
BAT mass and activity decline with age and in obesity (↓sympathetic innervation density, ↓UCP1, BAT whitening). Reduced BAT thermogenic capacity ↓energy expenditure contribution (estimated 20–30 kcal/day in lean young adults, 2–5 kcal/day in obese individuals). Therapeutic activation of BAT/beige adipocytes (mirabegron, β3-AR agonists; FGF21 analogues; BMP7) is an active clinical development area.
Liposarcoma
Most common soft tissue sarcoma in adults; retroperitoneal and extremity locations predominate. Well-differentiated (WDLS) / dedifferentiated (DDLS): MDM2 + CDK4 amplification on chromosome 12q13-15 (detected by FISH/IHC — diagnostic). Myxoid/round cell: DDIT3 (CHOP)-FUS or DDIT3-EWSR1 fusion → failure of terminal adipogenesis. Histologically: variable lipoblast differentiation + fat necrosis. Treatment: surgical resection ± radiotherapy; limited systemic options for DDLS (doxorubicin-based).
Cross-Atlas Connections
References
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- Scherer PE. The many secret lives of adipocytes: implications for diabetes. Diabetologia. 2019;62(2):223-232. doi:10.1007/s00125-018-4755-6 · PubMed 30471288
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