Metabolic Functions

The fat body participates in myriad metabolic activities and functions. Absorption from hemolymph and buildup of intracellular storage nutrients in the form of lipid droplets, carbohydrate (glycogen) deposits, and protein granules during the immature stages are aimed at accumulating reserves for later stages, and primarily to serve adult activities. Fat body cells, having homeostatic functions related to metabolism, respond to nutritional and hormonal cues that regulate and modulate blood sugars, lipids, and proteins at larval and mature stages.

As in vertebrates, the oxidative metabolism is mediated via the tricarboxylic acid (TCA) cycle and the electron transport enzyme systems. The fat body contains enzymes mediating the gluconeogenesis process as well as enzymatic systems with a detoxification role to manage harmful endogenous metabolites and toxic xenobiotic compounds. Detoxifying enzyme systems include microsomal mixed function oxidases, in which the cytochrome P450 is predominant, various hydrolytic enzymes (esterases, phosphoesterases), and conjugating systems.

The cells synthesize the various blood proteins (lipoproteins, glycolipoproteins), which include juvenile hormone (JH) carrier proteins (protecting JH from degradation), diglyceride carrier proteins, diapause proteins, and, particularly at the adult stage, production of vitellogenins (yolk proteins) that are absorbed by the maturing oocytes. Fat body cells also synthesize JH esterase, which regulates levels of JH in the insect blood, and enzymes involved in purine metabolism. Generally, proteins released into the insect blood during larval development are sequestered by the adipocytes, forming large intracellular granules until their use during metamorphosis. Triglycerides, which are the major form of stored lipids, are mobilized when needed and released into the hemolymph in the form of diglycerides accompanied by the production of specific carrier proteins. Trehalose, produced by the fat body, constitutes the major disaccharide in the insect blood. Glycogen, which is the principal form of stored carbohydrates, is mainly present in the peripheral fat body adipocytes. Glycogen is synthesized (by glycogen synthase) and hydrolyzed (by glycogen phosphorylase), by these enzymes active in the fat body cells. The hydrolytic products are mobilized at molting and metamorphosis to serve as precursors required for chitin synthesis and formation of the new cuticle.

of trehalose is regulated by a corpora cardiaca neurohormone. The adipokinetic hormone from the corpora cardiaca stimulates the adipocyes to release diglycerides and the accompanied lipoprotein carrier, and enhances lipid oxidation to fuel flight in favor of carbohydrate oxidation. Synthesis and release of vitellogenins by the female fat body cells usually are under the control of JH, although in certain insect species also the molting hormone is involved.

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