NEW INSIGHTS INTO ADIPOKINETIC HORMONE SIGNALING

Flight activity of insects comprises one of the most intense biochemic al processes known in nature, and therefore provides an attractive mod el system to study the hormonal regulation of metabolism during physic al exercise. In long-distance flying insects, such as the migratory lo cust, both carbohydrate and lipid reserves are utilized as fuels for s ustained flight activity. The mobilization of these energy stores in L ocusta migratoria is mediated by three structurally related adipokinet ic hormones (AKHs), which are all capable of stimulating the release o f both carbohydrates and lipids from the fat body. To exert their effe cts intracellularly, these hormones induce a variety of signal transdu ction events, involving the activation of AKH receptors, GTP-binding p roteins, cyclic AMP, inositol phosphates and Ca2+. In this review, we discuss recent advances in the research into AKH signaling. This not o nly includes the effects of the three AKHs on each of the signaling mo lecules, but also crosstalk between signaling cascades and the degrada tion rates of the hormones in the hemolymph. On the basis of the obser ved differences between the three AKHs, we have tried to construct a p hysiological model for their action in locusts, in order to answer a f undamental question in endocrinology: why do several structurally and functionally related peptide hormones co-exist in locusts (and animals in general), when apparently one single hormone would he sufficient t o exert the desired effects? We suggest that the success of the migrat ory locust in performing long-distance flights is in part based on thi s neuropeptide multiplicity, with AKH-I being the strongest lipid-mobi lizing hormone, AKH-II the most powerful carbohydrate mobilizer and AK H-III, a modulatory entity that predominantly serves to provide the an imal with energy at rest. (C) 1998 Elsevier Science Ireland Ltd. All r ights reserved.