Phosphoinositide-specific Inositol Polyphosphate 5-Phosphatase IV Inhibits Akt/Protein Kinase B Phosphorylation and Leads to Apoptotic Cell Death*

Phosphoinositide-specific inositol polyphosphate 5- phosphatase IV has the affinity for PI(3,4,5)P3 (K m = 0.65 μM) that is approximately 10-fold greater than the other inositol polyphosphate 5-phosphatases, which use this substrate including SHIP, OCRL, and 5ptase II, suggesting that it may be important in controlling intracellular levels of this metabolite. We created cell lines stably expressing the enzyme to study its effect on cell function. We found that overexpression of 5ptase IV in 293 cells caused the rapid depletion of both PI(4,5)P2 and PI(3,4,5)P3 in cells with corresponding increases in the products, PI(4)P and PI(3,4)P2, changing the balance of two phosphoinositol products of phosphoinositide 3-kinase, PI(3,4)P2 and PI(3,4,5)P3, in the cell. One of the targets of these phosphoinositides is the serine/threonine kinase Akt, which plays an important role in the control of apoptosis. We were able to address the relative roles of PI(3,4)P2 and PI(3,4,5)P3 in the activation of Akt by selective depletion of these phosphoinositides in cells stably transfected with 5ptase IV and inositol polyphosphate 4-phosphatase (4ptase I). In cells transfected with 4ptase I, the level of PI(3,4)P2 was reduced, and PI(3,4,5)P3 was increased. Expression of the two enzymes had the opposite effect on the phosphorylation of Akt in response to stimulation with growth factors or heat shock. Akt phosphorylation was inhibited in cells expressing 5ptase IV but increased in 4ptase I cells and correlated with the intracellular level of PI(3,4,5)P3 and not that of PI(3,4)P2. The inhibition of Akt phosphorylation in cells expressing 5ptase IV makes them highly susceptible to FAS-induced apoptosis, whereas overexpressing of the 4ptase I protects cells from apoptosis. Our results place 5ptase IV as a relevant biological regulator of PI3K/Akt pathway in cells.

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