The pathway for the production of inositol hexakisphosphate in human cells

Abstract

The yeast and Drosophila pathways leading to the production of inositol hexakisphosphate (InsP6) have been elucidated recently. The in vivo pathway in humans has been assumed to be similar. Here we show that overexpression of Ins(1,3,4)P3 5/6-kinase in human cell lines results in an increase of inositol tetrakisphosphate (InsP4) isomers, inositol pentakisphosphate (InsP5) and InsP6, whereas its depletion by RNA interference decreases the amounts of these inositol phosphates. Expression of Ins(1,3,4,6)P4 5-kinase does not increase the amount of InsP5 and InsP6, although its depletion does block InsP5 and InsP6 production, showing that it is necessary for production of InsP5 and InsP6. Expression of Ins(1,3,4,5,6)P5 2-kinase increases the amount of InsP6 by depleting the InsP5 in the cell, and depletion of 2-kinase decreases the amount of InsP6 and causes an increase in InsP 5. These results are consistent with a pathway that produces InsP6 through the sequential action of Ins(1,3,4)P3 5/6-kinase, Ins(1,3,4,6)P4 5-kinase, and Ins(1,3,4,5,6)P5 2-kinase to convert Ins(1,3,4)P3 to InsP6. Furthermore, the evidence implicates 5/6-kinase as the rate-limiting enzyme in this pathway.