Adipose Tissue Chapter PDF

ADIPOSE TISSUE OVERVIEW OF ADIPOSE TISSUE I 274 TRANSDIFFERENTIATION OF WHITE ADIPOSE TISSUE I 274 ADIPOSE TISSUE 1284 Function ofWhiteAdiposeTissue /274 Folder 9.1 Clinical Cor...

ADIPOSE TISSUE OVERVIEW OF ADIPOSE TISSUE I 274 TRANSDIFFERENTIATION OF WHITE ADIPOSE TISSUE I 274 ADIPOSE TISSUE 1284 Function ofWhiteAdiposeTissue /274 Folder 9.1 Clinical Correlation: Obesity /281 Differentiation of Adipocytes /275 Folder 9.2 Clinical Correlation: Adipose Tissue Structure of Adipocytes and Tumors/283 Adipose Tissue /276 Folder 9.3 Clinical Correlation: PET Scanning and Regulation of Adipose Tissue /277 BROWN ADIPOSE TISSUE 1279 HISTOIDGY 101/286 8 Brown Adipose Tissue Interference /285 OVERVIEW OF ADIPOSE TISSUE perfOrm other functions in addition to their role as fat-storage containers. They also regulate energy me- Adipose tissue is a specialized connective tissue that tabolism by secreting paracrine and endocrine substances. plays an important role in anergy homeostasis. Adipose tissue is considered a major endocrine organ. Individual fat cells, or adipocytes, and groups of adipocytes Considerable evidence links increased endocrine activity are fuund throughout loose connective tissue. TISSUes in which of adipocytes to the metabolic and cardiovascular compli- adipocytes are the primary cell type are designated adipose cations associated with obesity. tissue. Ad.ipocytes play a key role in energy homeostasis. Thera are two types of adipose tissue: white (unilocular) For its survival, the body needs to ensure a continuous and brown (multilocular)_ delivery ofenergy despite highly variable supplies of nutrients The two types of adipose tissue, white adipose tissue and from the external environment. To meet the body's energy brown adipose tissue, are so named because oftheir color demands when nutrient supplies are low, adipose tissue effi- in the living state: ciendy stores excess energy. The body has a limited capacity to store carbohydrate and protein; therefore, energy reserves White adipose tissue is the predominant type in adult are stored within lipid droplets of adipocyres in the form humans. of triglyceridaa. Triglycerides represent a dynamic form of Brown adipose tissue is present in large amounts in energy storage that is added to when fuod intake is greater humans during fetal life. It diminishes during the first than energy expenditure and is tapped when energy expendi- decade after binh but continues to be present in varied ture is greater than fuod intake. The energy stored in adipo- amounts, mainly around internal organs. cytes can be rapidly released for use at other sites in the body. Triglycerides are the most concentrated form of metabolic WHITE ADIPOSE TISSUE energy storage available to humans. Because triglyc.erides lack water, they have about twice the energy density ofcarbohydrates White (unilocular) adipose tissue represents at least and proteins. The energy density of triglyccrides is approxi- 10% of the body weight of a normal healthy individual. mately 37.7 kj/g (9 callg), whereas the density ofcarbohydrates and proteins is 16.8 kj/g (4 callg). In the event of food depri- Function of White Adipose Tissue vation, triglycerides are an essential source of water and energy. Functions of white adipose tissue include energy storage. Some animals can rely solely on metabolic water obtained from insulation. cushioning af vital argans. and sacration af oxidation fur maintenance oftheir water balance. For hannanas. instance, the hwnp ofa camd consists largc:ly of adipose tissue White adipose tissue forms a fatty layer of the subcuta- and is a source ofwater and energy fur this desert animal. neous (superficial) fascia called the panniculus adiposus 274 {Lat. panniculus, a little garmmt; t.ttlipatu.s, fony] in the con- A notable adipokine is leptin [Gr. leptos, thin], a 16 kDa nective tissue beneath the skin. Because the thermal conduc- peptide hormone discovered in 1994. Leptin is involved in 275 tivity ofadipose tissue is only about one-half that ofskeletal the regulation of energy homeostasis and is primary secreted muscle, the subcutaneous fascia provides a significant ther- by adipocytes. Small amounts of leptin are also produced mal insulation against cold by reducing the rate of heat loss. in other organs (e.g., stomach, placenta, mammary glands, Concentrations of adipose tissue are found in the connec- and ovaries). Leptin inhibits food intake and stimulates met- tive tissue under the skin of the abdomen, buttocks, axilla, abolic rate and loss of body weight. Thus, leptin fulfills the and thigh. Sex difFerences in the thickness of this fatty layer criteria for a circulating satiety factor that controls food in the skin ofdifferent parts of the body account, in part, for intake when the bodys store of energy is sufficient. Leptin the differences in body contour between females and males. also participates in an endocrine signaling pathway that In both sexes, the mammary fat pad is a preferential site communicates the energy state of adipose tissue ro brain for accumulation of adipose tissue; the nonlactating female centers that regulate food uptake. It acts on the central ner- mammary gland is composed primarily of this tissue. In vous system by binding to specific receptors, mainly in the the lactating female, the mammary fat pad plays an import- hypothalamus. In addition, leptin communicates the fuel ant role in supporting breast function. It provides lipids state of adipocytes from fat-storage sites to other metaboli- and energy for milk production, but it is also a site for the cally active tissues (i.e., from adipose tissue to muscle at a synthesis of growth factors that modulate responses to different site). steroid hormones and proteins that regulate mammary In addition to leptin, adipose tissue secretes a variety ofother gland function. adipokines such as adiponectin, resistin, retinol-binding Internally; adipose tissue is preferentially located in the protein 4 (RBP-4), visfatin, apelin, plasminogen activator greater omentum, mesentery; and retroperitoneal space and inhibiror-1 (PAI-l), tumor necrosis factors (TNFs), inter- is usually abundant around the kidneys. It is also found in leukin-6 (IL-6), monocyte chemotactic protein-1 (MCP-1), bone marrow and between other tissues, where it fills in and angiotensinogen (AGE). Leptin also regulates the spaces. Adipose tissue functions as a cushion in the palms of production of steroid hormones (testosterone, estrogens, the hands and the soles of the feet, beneath the visceral peri- and glucocorticoids). Some adipokines are also synthesized cardium (around the outside of the heart), and in the orbits in other tissues. For example, AGE is synthesized in the liver; around the eyeballs. It retains this structural function evan increased production of this peptide by adipose tissue con- during reduced caloric intake; when adipose tissue else- tributes to hypertension (elevation of blood pressure), which where becomes depleted of lipid, this structural adipose is a frequent complication of obesity. tissue remains undiminished. Adipocytes also help regulate the synthesis ofsex hormones While adipose tissue secretes a variety of adipokinn, and glucocorticoids. Specific enzymes expressed in adipocytes which include hormones. growth factors, and cytokines. convert the inactive fonns of these hormones to their active fonns. In this way, these enzymes influence the sex steroid Adlpocytes actively synthesize and secrete adrpokrnes, a profiles of obese individuals. Obesity-increased secretion group of biologically active substances, which include hor- of growth factora (tumor necrosis factor a. (TNf'.a.(, mones, growth factors, and cytokines (Fig. 9.1). For this transforming growth factor f5 [TGf'.!i], insulin-like growth reason, adipose tissue is regarded as an important player in factor 1 [JGF-1]), and cytokines (11.:6 and prostaglandins) energy homeostasis, adipogenesis, steroid metabolism, angio- may be linked to metabolic abnormalities and devel- genesis, and immune responses. opment of diabetes. Table 9.1 presents a summary of the most important molecules produced by adipocytes and their functions. leptin Differentiation of Adipocytes PGI2 PGF2a' l / adiponectin White adipocytes differentiate from mesenchymal stem cells under1he control of PPA&y/RXR transcription factors. During embryonic development, white adrpocytes de- IL-6 rive from und.Ufe.rentiated perivascular mesenchymal stem cells associated with the adventitia of small ve- TNF-a._..... _ _. resistin nules (Fig. 9.2). A transcription factor called peroxisome prollferator-actlvated receptor gamma {PPAR..,}, in IGF-1 complex with the retrnold X receptor (RXR), plays a crit- / ical role in adipocyte d.Ufe.rentiation and initiation of lipid angiotensinogen angiotensin II l apelin visfatin metabolism. This complex induces the maturation of earty lipoblasts (adipoblasts) or preadipocytes into mature fat cells of white adipose tissue. Most of the PPAR'Y target genes FIGURE 9.1. M1jor adipolines secreted by wbite adipose tislue. in adipose tissue influence lipogenic pathways and initiate the This schematic drawing shows various types of adipokines secreted storage of triglycerides. Therefore, PPAR1/RXR is regarded by white adipose tissue, including hormones (e.g., leptinl, cytokines {e.g., insulin-like growth factor 1), and other mole