Laura Stan
Complex homeostatic mechanisms, including those originating in adipose tissue, regulate energy balance maintenance. The primary function of adipose tissue is to store excess metabolic energy as fat. During periods of energy deprivation, the energy stored as fat can be mobilized (hunger, fasting, diseases). Adipose tissue also plays a homeostatic role, regulating energy balance and acting as an endocrine organ, secreting substances that regulate body homeostasis. White and Brown Adipose Tissues (WAT and BAT) with distinct phenotypes, functions, and regulation have been identified. WAT stores energy, whereas BAT releases it as heat. Brown and white adipocytes have distinct ontogenetic origins and lineages, and distinct WAT and BAT markers have been identified. WAT has been found to contain "brite" or beige adipose tissue, which shares some properties with BAT. Thyroid hormones have pleiotropic effects, regulating differentiation in many tissues, including adipose tissue. Adipogenesis is the process by which mature adipocytes are produced. It is regulated by several transcription factors (c/EBPs, PPARs) that coordinately activate specific genes, resulting in the adipocyte phenotype. T3 controls many genes involved in lipid mobilization and storage, as well as thermogenesis. Thyroid hormones regulate genes important for WAT and BAT function, including lipogenesis, lipolysis, thermogenesis, mitochondrial function, transcription factors, and nutrient availability. T3 regulates transcription factors directly through specific TREs in gene promoters. T3 availability is regulated by the deiodinases D3, D2, and D1. D3 is activated during proliferation, whereas D2 is associated with the adipocyte differentiation programme, supplying T3 for lipogenesis and thermogenesis.