Summary The inositol pyrophosphate IP7 (5-diphosphoinositolpentakisphosphate), formed by a family of three inositol hexakisphosphate kinases (IP6Ks), modulates diverse cellular activities. We now report that IP7 is a physiologic inhibitor of Akt, a serine/threonine kinase which regulates glucose homeostasis and protein translation respectively via the GSK3β and mTOR pathways. Thus Akt, mTOR and GSK3β signaling are dramatically augmented in skeletal muscle, white adipose tissue, and liver of mice with targeted deletion of IP6K1. IP7 impacts this pathway by potently inhibiting the PDK1 phosphorylation of Akt, preventing its activation and thereby impacting insulin signaling. IP6K1 knockout mice manifest insulin sensitivity and are resistant to obesity elicited by high fat diet or aging. Inhibition of IP6K1 may afford a therapeutic approach to obesity and diabetes.

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 ...

SUMMARY mTOR Complex 1 (mTORC1; mammalian target of rapamycin (mTOR) in complex with raptor) is a key regulator of protein synthesis and cell growth in response to nutrient amino acids. Here we report that inositol polyphosphate multikinase (IPMK), which possesses both inositol phosphate kinase and lipid kinase activities, regulates amino acid signaling to mTORC1. This regulation is independent of IPMK's catalytic function, instead reflecting its binding with mTOR and raptor, which maintains the mTOR-raptor association. Thus, IPMK appears to be a physiologic mTOR cofactor, serving as a determinant of mTORC1 stability and amino acid-induced mTOR signaling. Substances that block IPMK-mTORC1 binding may afford therapeutic benefit in nutrient amino acid-regulated conditions such as obesity and diabetes.