Insect adipokinetic hormone stimulates inositol phosphate metabolism: roles for both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 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 [3H]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)P2, Ins(1,4)P2, Ins(3,4)P3, Ins(1,3,4)P3, Ins(1,4,5)P3 and Ins(1,3,4,5)P4. 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)P3 levels are substantially enhanced by AKH, which is indicative of hormonal activation of phospholipase C. In cell-free tissue preparations, Ins(1,4,5)P3 is metabolized through dephosphorylation as well as further phosphorylation. Ins(1,3,4,5)P4 is dephosphorylated primarily to Ins(1,3,4)P3, although the ability for its reconversion to Ins(1,4,5)P3 suggests that in vivo Ins(1,3,4,5)P4 may function as a rapidly mobilizable pool for Ins(1,4,5)P3 generation. Metabolic pathways for the conversion of InsPs to inositol in the locust fat body are proposed.