METABOLISM OF STORED RESERVES IN INSECT FAT-BODY - HORMONAL SIGNAL-TRANSDUCTION IMPLICATED IN GLYCOGEN MOBILIZATION AND BIOSYNTHESIS OF THELIPOPHORIN SYSTEM

The mobilization of carbohydrate and lipid reserves from the insect fa t body as fuels for migratory flight activity is controlled by adipoki netic hormone (AKH), of which in Locusta migratoria three different fo rms occur: AXH-I, -II and -III. In fat body in vitro, each AKH is capa ble of activating glycogen phosphorylase and of stimulating cAMP produ ction, but only in the presence of extracellular Ca2+. The hormones st imulate both the influx and the efflux of Ca2+, the higher influx prob ably causing an increase in intracellular [Ca2+]. AKH enhances the pro duction of inositol phosphates among which inositol 1,4,5-triphosphate may mediate the mobilization of Ca2+ from intracellular stores. Evide nce is presented in favor of the occurrence of a capacitative calcium entry mechanism. Results suggest that transduction of the AKH signal o ccurs through stimulatory G protein-coupled receptor(s). A tentative m odel is presented for the interactions between the AKH signaling pathw ays in the locust fat body cell. AKH-induced lipid mobilization during flight requires the presence in the insect blood of high-density lipo phorin (HDLp) particles and apolipophorin III (apoLp-III). Both protei n components are synthesized in the fat body. In the locust, the two i ntegral, nonexchangeable HDLp apolipophorins (apoLp-I and -II) were sh own to originate from a common precursor; an mRNA of 10.3 kb seems to code for this precursor protein. The models proposed for lipophorin as sembly and secretion in a number of insects are not in agreement, The exchangeable apoLp-III may occur in two or more isoforms; locust apoLp -III is secreted from the fat body as one of the two isoforms and in t he hemolymph converted into the truncated second one. The rationale fo r this process is as yet unknown. (C) 1997 Elsevier Science Inc.