Adipose tissue triacylglycerol storage

Fatty acids, whether synthesized by the liver or taken in through the diet, must be transported through the blood, usually for storage in the adipose tissue. Triacylglycerols have relatively low solubility in the plasma, and therefore are transported as lipoproteins (Fig. 15.6). 

Lipotrophic factors are those required for transportation of triacylglycerol from the liver to the adipose tissue for storage. These factors are those that cannot be synthesized from nonlipotrophic components of the diet. The major role of lipotrophic factors is the formation of phosphatidylcholine, which is critical in VLDL formation. One of the lipotrophic factors obviously would be choline, which can be incorporated into phosphatidylcholine. Two other lipotrophic factors are related to the potential de novo synthesis of choline. The first and foremost is methionine, which can be used to donate the methyl groups for choline formation in the absence of dietary choline, thus allowing lipids to be moved from liver to adipose tissue (Fig. 18.7). 

The liver converts fatty acids to triacylglycerols, phospholipids, or cholesterol and its esters, for transport as plasma lipoproteins to adipose tissue for storage. Fatty acids can also be oxidized to yield ATP or to form ketone bodies, which are circulated to other tissues. 

Once the glycogen reserves are replenished then any surplus glucose will be converted under the influence of insuhn to triacylglycerol (TAG) as illustrated opposite in Fig. 21.2. This is not normally stored in the liver otherwise steatosis results, as in the case of unfortunate geese that are force-fed so their livers become paid de foie gras. Instead, the TAG is transported from the liver in very low density hpoproteins (Chapter 40) to adipose tissue for storage. 

Fat absorbed from the diet and lipids synthesized by the liver and adipose tissue must be transported between the various tissues and organs for utilization and storage. Since lipids are insoluble in water, the problem of how to transport them in the aqueous blood plasma is solved by associating nonpolar lipids (triacylglycerol and cholesteryl esters) with amphipathic hpids (phospholipids and cholesterol) and proteins to make water-miscible hpoproteins. 

Triacylglycerol is the main storage Hpid in adipose tissue. Upon mobihzation, free fatty acids and glycerol are released. Free fatty acids are an important fuel source. 

Adipose tissue Storage and breakdown of triacylglyc-erol Esterification of fatty acids and lipolysis lipogenesis Glucose, lipoprotein triacylglycerol Free fatty acids, glycerol Lipoprotein lipase, hormone-sensitive lipase... 

In the normal fed state, the enzyme is active in adipose tissue, so that the fatty acids in the triacylglycerol are taken up by adipocytes and then esterified for storage. 

Under conditions of nutritional excess, fatty acids are absorbed by adipose tissue where they are converted to storage lipids in the form of triacylglycerols. The triacylglycerols can be mobilized at a later time, when the carbohydrate... 

Chylomicrons are synthesized in the intestine and transport dietary triacylglycerols to skeletal muscle and adipose tissue, and dietary cholesterol to the liver. At these target tissues the triacylglycerols are hydrolyzed by lipoprotein lipase on the surface of the cells and the released fatty acids are taken up either for metabolism to generate energy or for storage. The resulting cholesterol-rich chylomicron remnants are transported in the blood to the liver where they are taken up by receptor-mediated endocytosis. 

The storage of triacylglycerols in adipose tissue is mediated by insulin, which stimulates adipose cells to secrete lipoprotein lipase and to take up glucose, the source of glycerol for triacylglycerol synthesis. 

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