Insect adipokinetic hormones: release and integration of flight energy metabolism

• Published in: Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology Vol. 136; no. 2; pp. 217 - 226
• Main Author: Van der Horst, Dick J.
• Format: Book Review Journal Article
• Language: English
• Published: England Elsevier Inc 01.10.2003
• Subjects: Animals; Apolipophorin III; Carrier Proteins - metabolism; Energy Metabolism - drug effects; Exercise; Flight, Animal; Insect Hormones - genetics; Insect Hormones - metabolism; Insect Hormones - pharmacology; Insect Hormones - secretion; Insecta - cytology; Insecta - drug effects; Insecta - metabolism; Lipid Metabolism; Lipid shuttle; Lipophorin; Lipoprotein; Lipoproteins - metabolism; Locusta migratoria; Neuropeptides; Oligopeptides - genetics; Oligopeptides - metabolism; Oligopeptides - pharmacology; Oligopeptides - secretion; Pyrrolidonecarboxylic Acid - analogs & derivatives; Secretory granules; ICG, intracisternal granule; Neuropeptides; Lipoprotein; Secretory granules; Lom-TK, locustatachykinin; AKH, adipokinetic hormone; Exercise; HDLp, high-density lipophorin; FFA, free fatty acids; iLR, insect lipophorin receptor; Q-TOF, quadrupole time of flight; Lipophorin; apoLp, apolipophorin; AAP, AKH-associated peptide; LTP, lipid transfer particle; CCAP, crustacean cardio active peptide; DAG, diacylglycerol; Locusta migratoria; Apolipophorin III; LDLp, low-density lipophorin; TAG, triacylglycerol; HSL, hormone-sensitive lipase; JP, joining peptide; Lipid shuttle; DMPC, dimyristoylphosphatidylcholine; APRP, AKH-precursor-related peptide
• ISSN: 1096-4959; 1879-1107
• DOI: 10.1016/S1096-4959(03)00151-9

Insect flight involves mobilization, transport and utilization of endogenous energy reserves at extremely high rates. Peptide adipokinetic hormones (AKHs), synthesized and stored in neuroendocrine cells, integrate flight energy metabolism. The complex multifactorial control mechanism for AKH release in the locust includes both stimulatory and inhibitory factors. The AKHs are synthesized continuously, resulting in an accumulation of AKH-containing secretory granules. Additionally, secretory material is stored in large intracisternal granules. Although only a limited part of these large reserves appears to be readily releasable, this strategy allows the adipokinetic cells to comply with large variations in secretory demands; changes in secretory activity do not affect the rate of hormone biosynthesis. AKH-induced lipid release from fat body target cells has revealed a novel concept for lipid transport during exercise. Similar to sustained locomotion of mammals, insect flight activity is powered by oxidation of free fatty acids derived from endogenous reserves of triacylglycerol. However, the transport form of the lipid in the circulatory system is diacylglycerol (DAG) that is delivered to the flight muscles associated with lipoproteins. While DAG is loaded onto the multifunctional insect lipoprotein, high-density lipophorin (HDLp) and multiple copies of the exchangeable apolipoprotein III (apoLp-III) associate reversibly with the expanding particle. The resulting low-density lipophorin (LDLp) specifically shuttles DAG to the working muscles. Following DAG hydrolysis by a lipophorin lipase, apoLp-III dissociates from the particle, regenerating HDLp that is re-utilized for lipid uptake at the fat body cells, thus functioning as an efficient lipid shuttle mechanism. Many structural elements of the lipoprotein system of insects appear to be similar to their counterparts in mammals; however, the functioning of the insect lipoprotein in energy transport during flight activity is intriguingly different.