| Inositol hexakisphosphate kinase-2 (IP6K2) was shown to modulate a variety of cellular signaling events, the best-studied of which is the enhancement of apoptosis. The mechanisms by which IP6K2 and its role in cell death are regulated remain poorly understood. We demonstrate that the oncogene kinase CK2 promotes cell survival by regulating IP6K2. CK2 physiologically phosphorylates IP6K2 at serines 347 and 356 within a PEST consensus site for ubiquitination. Pharmacologic inhibition of CK2 activity prevents ubiquitnylation of IP6K2. Mutation of 5347 and 5356 on IP6K2 extends the half-life of IP6K2 from 90 minutes to 8 hours. Cell death in the colon cancer line, HCT116, elicited by CK2 inhibitors is diminished in cells depleted of IP6K2. These studies suggest a role for IP6K2 in suppressing CK2-mediated tumorigenesis. The mechanism by which IP6K2 ubiquitinylation is regulated may also involve a novel binding partner, USP9x, a deubiquitinylating enzyme. USP9x depletion causes IP6K2 levels to decrease, and USP9x interaction with IP6K2 is regulated by protein kinase C activity.;Next, we show that IP6K2 interacts with the tumor suppressor, PML. Similar to IP6K2, PML is a pro-apoptotic protein, which is already known to be phosphorylated and degraded by CK2 under conditions of osmotic stress. We show that the enzymatic product of IP6K2, 5-PP inositolpentakisphosphate (IP7) can inhibit PML phosphorylation by CK2 in vitro and in vivo. When IP6K2 is depleted acutely, PML ubiquitinylation increases while PML protein levels decrease. Overexpression of kinase active IP6K2 stabilizes PML in HeLa cells subjected to osmotic stress, whereas kinase inactive IP6K2 does not. These results demonstrate a role for IP7 in PML stability and another pathway by which IP6K2 may exert proapoptotic activity. |
