Adipose cell

FIGURE 24.1 Scanning electron micrograph of an adipose cell (fat cell). Globules of triacylglycerols occupy most of the volume of such cells. (Prof. P. Motta, Dept, of Anatomy, University La Sapienza, Rome/Science Photo Library/Photo Researchers, Inc.)... 

The pathways for liberation of fatty acids from triacylglycerols, either from adipose cells or from the diet, are shown in Figures 24.2 and 24.3. Fatty acids are mobilized from adipocytes in response to hormone messengers such as adren-... 

PPARy is a transcription factor which controls the expression of enzymes and proteins involved in fat and glucose metabolism. More importantly, stimulation of this receptor induces differentiation of preadipocytes to adipose cells. It is believed that the formation of additional, small fat cells lowers free fatty acids and hepatic triglycerides, thereby collecting insulin resistance. 

Of probably greater importance is the effect of local concentration gradients. For example, analysis for a given constituent in the entire meat mass does not reflect the real concentration at a given point. For example, DNA is localized in the nuclei and lipid is localized predominantly in the adipose cells. Another factor of potential influence in reaction schemes for nitrite is the fact that polar-nonpolar interfaces are present as a result of structural compartmentalization. In an adipose cell, the lipid is contained as the body of the cell, but it is surrounded by a thin layer of sarcoplasmic protein. Therefore, large surface areas are involved. 

Several of these morphological factors are illustrated in Figure 1. Figure lA is of the fat portion of bacon and has been stained for connective tissue. It is noted that fat tissue is not all lipid but has an extensive connective tissue component ranging from fairly thick layers to delicate layers defining each adipose cell. Figure IB is from a finely chopped emulsion. Connective tissue pieces are stained dark, the protein matrix is gray and the... 

Figure 2.2 Fat, or adipose, cells store extra energy from food. Adipose cells help insulate the body to keep it warm, cushion and protect the internal organs, and store extra energy for later use. When people consume more energy from food, the extra energy is stored as fat in adipose cells. Years ago, this stored energy could be used to avoid starvation in times of famine. Today, people never use this stored fat because famine does not exist in developed countries. These fat cells continue to accumulate and lead to weight gain and eventually to obesity.

The glycerol produced by the action of hormone-sensitive lipase in the adipose tissue cannot be utilized by adipose tissue itself. Adipose cells lack the enzyme glycerol kinase, which is necessary to convert glycerol to glycerol phosphate. 

Figure 1-12-1. Insulin Regulation of Glucose Transport in Muscle and Adipose Cells...

In addition to pulmonary toxicity, nausea/vomiting, lactic acidosis and transaminitis, UCN-01 induced insulin resistance during Phase I clinical trials. As shown recently with rat adipose cells, this effect may be due to UCN-01 inhibition of PKB Thr-308 phosphorylation—no effect on Ser-473 was observed in this study—and subsequent blockade of GLUT4 translocation in response to insulin [104]. If this mode of action is confirmed in the ongoing clinical trials and contrary to what was observed in the PDKl hypomorphic mice (vide supra) [100], insulin resistance may represent an important hurdle in the development of PDKl inhibitors and, in general, of any agent that blocks the PI3K/PKB pathway in adipose and muscle cells. 

Triacylglycerol. Triglyceride a compound consisting of three molecules of fatty acids esterified to glycerol. It is a neutral fat synthesized from carbohydrates for storage in animal adipose cells. On enzymatic hydrolysis, it releases free fatty acids in the blood. Tuberculosis. Any of the infectious diseases of man and animals caused by species Mycobacterium and characterized by the formation of tubercles and caseous necrosis in the tissues. 

The dose level of -agonists affects the response obtained, the optimum dose often varies for the different production parameters measured. The mode of action of -agonists is poorly understood, but their interaction with membrane-bound receptors increases lipolysis in adipose cells and stimulates hypertrophy in muscle fibers. Although -agonists are known to stimulate the secretion of growth hormone and insulin, there is no direct evidence that these hormones mediate the tissue responses to the drug. 

When lipids are required by the body for energy, adipose cell hormone-sensitive lipase (activated by epinephrine, and inhibited by insulin) initiates degradation of stored triacyl glycerol. 

Couillard C, Mauriege P, Imbeault P, Prud homme D, Nadeau A, Tremblay A, Bouchard C, Despres JP, 2000. Hyperleptinemia is more closely associated with adipose cell hypertrophy than with adipose tissue hyperplasia. Int J Obes Relat Metab Disord 24 782-788. 

Vanadate stimulates protein kinases in the cytosol, as demonstrated in adipose cells and extracts. The activation of a membrane and cytosolic protein tyrosine kinase have been demonstrated in adipocytes, and the membranous enzyme has been postulated to be a way to involve PI-3K actions without activation of insulin receptor substrate-1 (IRS-1) in the insulin signal transduction pathway [140], It is always difficult to determine if protein kinase activation is direct or the result of stimulation of a protein phosphatase. The fact that kinase stimulation was seen in isolated extracts after cell disintegration in this adipocyte cell system supports the idea that vanadium addition to cells could directly stimulate kinases via an as-yet-undetermined mechanism. In other experiments with 3T3-L1 adipocytes bis(acetylacetonato)oxovana-dium (IV) BMOV and bis(l-N-oxide-pyridine-2thiolato)oxovanadium (TV) caused increased tyrosine phosphorylation of both the insulin receptor and IRS-1 in a synergistic way with insulin, as measured by antibodies to phosphotyrosine residues [141]. 

The breakdown of fatty acids in (3-oxidation (see Topic K2) is controlled mainly by the concentration of free fatty acids in the blood, which is, in turn, controlled by the hydrolysis rate of triacylglycerols in adipose tissue by hormone-sensitive triacylglycerol lipase. This enzyme is regulated by phosphorylation and dephosphorylation (Fig. 5) in response to hormonally controlled levels of the intracellular second messenger cAMP (see Topic E5). The catabolic hormones glucagon, epinephrine and norepinephrine bind to receptor proteins on the cell surface and increase the levels of cAMP in adipose cells through activation of adenylate cyclase (see Topic E5). The cAMP allosterically activates... 

Inflammation—a state of tissue resulting from a protective tissue response to a chemical (but also mediators produced by sensitized adipose cells in response to allergen), physical injury, infection, or destruction of tissues, which serves to destroy, dilute, or wall off both the injurious agent and the injured tissues. The classical inflammation is manifested by pain, redness, swelling, heat, and functional disorders. 

Cushman, S.W. Wardzala, L.J. (1980). Potential mechanism of insulin action on glucose transport in the isolated rat adipose cell. Apparent translocation of intracellular transport systems to the plasma membrane. J. Biol. Chem. 255,4758-4762. 

Deep to the reticular layer of the dermis is the hypodermis (subcutis) consisting of very loose connective tissue with adipose cells. This layer helps to anchor the dermis to the underlying muscle or bone. This thermal barrier and mechanical cushion is sometimes considered to be a site that acts as a depot or reservoir for certain toxic compounds. 

Free fatty acids diffuse from adipose cells, combine with albumin in the blood and are thereby transported to other tissues. 

LPL catalyses the hydrolysis of triacylglycerols to hherate free fatty acids and glycerol. LPL is specifically found in endothelial cells lining the capillaries. LPL has different isoenzyme forms in different tissues that form found in adipocytes is activated hy insuhn (which helps to explain why adipose cells gain fat in a well-fed state). 

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