Inositol polyphosphate multikinase is a physiologic PI3-kinase that activates Akt/PKB

Abstract

The second messenger phosphatidylinositol (3,4,5)-trisphosphate (PIP 3 ), formed by the p110 family of PI3-kinases, promotes cellular growth, proliferation, and survival, in large part by activating the protein kinase Akt/PKB. We show that inositol polyphosphate multikinase (IPMK) physiologically generates PIP 3 as well as water soluble inositol phosphates. IPMK deletion reduces growth factor-elicited Akt signaling and cell proliferation caused uniquely by loss of its PI3-kinase activity. Inhibition of p110 PI3-kinases by wortmannin prevents IPMK phosphorylation and activation. Thus, growth factor stimulation of Akt signaling involves PIP 3 generation through the sequential activations of the p110 PI3-kinases and IPMK. As inositol phosphates inhibit Akt signaling, IPMK appears to act as a molecular switch, inhibiting or stimulating Akt via its inositol phosphate kinase or PI3-kinase activities, respectively. Drugs regulating IPMK may have therapeutic relevance in influencing cell proliferation.signal transduction | cancer A large family of inositol phosphates serves multiple functions, with inositol 1,4,5-trisphosphate (IP 3 ) being well known as a second messenger releasing intracellular calcium (1). Inositol diphosphates, incorporating an energetic pyrophosphate bond, display numerous physiological roles, including pyrophosphorylation of a variety of protein targets (2-4). These inositol pyrophosphates are synthesized by a family of IP 6 kinase enzymes (5). Recently, novel isomers of inositol pyrophosphates have been described that are synthesized by a distinct inositol phosphate kinase enzyme designated Vip1 (6, 7).Inositol polyphosphate multikinase (IPMK) is a member of the IP 6 kinase family of enzymes but is not primarily associated with the formation of inositol pyrophosphates. Instead it generates several inositol phosphates, converting IP 3 to IP 4 and IP 4 to IP 5 , with its primary physiologic role in this pathway being to form the bulk of IP 5 in cells (5,(8)(9)(10)(11). IPMK also possesses phosphatidylinositol 3-kinase (PI3K) activity in vitro (12), specifically phosphorylating phosphatidylinositol(4,5)-bisphosphate (PIP 2 ) to generate phosphatidylinositol (3,4,5)-trisphosphate (PIP 3 ), a second messenger known to promote cellular growth, proliferation, survival, and migration (13). The physiologic role of this activity has not heretofore been established. The principal PI3K activity in cells has been attributed to a family of enzymes identified by Cantley and associates (reviewed in ref. 14), whose catalytic subunits are designated p110. PIP 3 generated by p110 in response to extracellular stimuli, such as growth factors, is a principal stimulus of the Akt/mammalian target of rapamycin (mTOR) signaling pathway, which in turn regulates protein synthesis and plays a role in some cancers (15)(16)(17).We wondered whether the PI3K activity of IPMK contributes to the generation of PIP 3 under physiologic conditions to influence Akt signaling and cell growth. There is good reason to assume that IPMK is ...

Inositol pyrophosphates promote tumor growth and metastasis by antagonizing liver kinase B1

“…In vitro cell migration and invasion assays were performed as described (43). Anchorage-independent growth was assessed as described (10).…”

“…IP6K1 and IP6K2 are widely distributed, whereas IP6K3 is expressed primarily in the brain (9). A related enzyme, inositol polyphosphate multikinase (IPMK), converts IP3 to IP4 and IP5, displays physiologic PI3 kinase activity (10), and noncatalytically acts as a transcriptional coactivator (11) and stabilizer of the mTOR complex-1 (12). Whether IP6Ks and IPMK play direct roles in tumor progression remains unexplored.…”

AMP-activated protein kinase is physiologically regulated by inositol polyphosphate multikinase

“…Other actions of IPMK may also increase mTOR pathways. Thus, IPMK, via its PI3-kinase activity, stimulates Akt kinase activity, which leads to augmented mTOR signaling (10). In a kinase-independent fashion, IPMK binds to mTOR, stabilizing its binding interactions with raptor and also increasing mTOR signaling (11).…”

“…Cells were seeded at a density of 2 × 10 6 cells per 10-cm dish and then labeled with 200 μCi (1 Ci = 37 GBq) [ 3 H]inositol (PerkinElmer Life Sciences) for 4 d. Soluble inositol phosphates were extracted from labeled cells as described previously (10). Inositol incorporated into lipids were measured by extracting the remaining cell pellet with 0.1 M NaOH and 0.1% Triton X-100 overnight at room temperature with shaking and counting a fraction of the solubilized material in a liquid scintillation counter.…”

“…It is the principal physiologic generator of inositol pentakisphosphate (IP5) and, thus, is rate-limiting in the formation of the inositol pyrophosphates such as diphosphoinositol pentakisphosphate (IP7) (7)(8)(9). IPMK also possesses physiologic PI3-kinase activity, which regulates Akt signaling (10). Acting in a noncatalytic fashion, IPMK binds to mTOR (mammalian target of rapamycin), stabilizing the mTORraptor complex and enhancing protein synthesis (11).…”