Discovery of InsP6-kinases As InsP6-dephosphorylating Enzymes Provides a New Mechanism of Cytosolic InsP6 Degradation Driven by the Cellular ATP/ADP Ratio

InsP6 (inositol hexakisphosphate), the most abundant inositol phosphate in metazoa, is pyrophosphorylated to InsP7 [5PP-InsP5 (diphosphoinositol pentakisphosphate)] by cytosolic and nuclear IP6Ks (InsP6 kinases) and to 1PP-InsP5 by another InsP6/InsP7 kinase family. MINPP1 (multiple inositol-polyphosphate phosphatase 1), the only known InsP6 phosphatase, is localized in the ER (endoplasmic reticulum) and lysosome lumina. A mechanism of cytosolic InsP6 dephosphorylation has remained enigmatic so far. In the present study, we demonstrated that IP6Ks change their kinase activity towards InsP6 at a decreasing ATP/ADP ratio to an ADP phosphotransferase activity and dephosphorylate InsP6. Enantio-selective analysis revealed that Ins(2,3,4,5,6)P5 is the main InsP5 product of the IP6K reaction, whereas the exclusive product of MINPP1 activity is the enantiomer Ins(1,2,4,5,6)P5. Whereas lentiviral RNAi-based depletion of MINPP1 at falling cellular ATP/ADP ratios had no significant impact on Ins(2,3,4,5,6)P5 production, the use of the selective IP6K inhibitor TNP [N2-(m-trifluorobenzyl),N6-(p-nitrobenzyl)purine] abolished the production of this enatiomer in different types of cells. Furthermore, by analysis of rat tissue and human blood samples all (main and minor) dephosphorylation products of InsP6 were detected in vivo. In summary, we identified IP6Ks as novel nuclear and cytosolic InsP6- (and InsP5-) dephosphorylating enzymes whose activity is sensitively driven by a decrease in the cellular ATP/ADP ratio, thus suggesting a role for IP6Ks as cellular adenylate energy 'sensors'.