Insect adipokinetic hormone stimulates inositol phosphate metabolism: Roles for both Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4 in signal transduction?

Adipokinetic hormones (AKHs) control the mobilization of energy reserves from the insect fat body as fuels for flight activity. As a part of our investigations on AKH signal transduction, we demonstrate in this study that the inositol lipid cycle may be involved in the action of AKH-I on fat body of the migratory locust. We show that [H-3]inositol is incorporated into fat body phosphoinositides in vitro, whose hydrolysis leads to the formation of the following inositol phosphates (InsPs): Ins(1 and/or 3)P, Ins(4)P, Ins(1,3)P-2, Ins(1,4)P-2, Ins(3,4)P-2, Ins(1,3,4)P-3, Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4. AKH stimulates the formation of these isomers, eliciting an increase in radioactivity of total InsPs already after 1 min. Mass measurements show that Ins(1,4,5)P-3 levels are substantially enhanced by AKH, which is indicative of hormonal activation of phospholipase C. In cell-free tissue preparations, Ins(1,4,5)P-3 is metabolized through dephosphorylation as well as further phosphorylation. Ins(1,3,4,5)P-4 is dephosphorylated primarily to Ins(1,3,4)P-3, although the ability for its reconversion to Ins(1,4,5)P-3 suggests that in vivo Ins(1,3,4,5)P-4 may function as a rapidly mobilizable pool for Ins(1,4,5)P-3 generation. Metabolic pathways for the conversion of InsPs to inositol in the locust fat body are proposed.