In summary, IP7 generation by IP6K1 is enhanced by insulin. Moreover, IP7 is a physiologic inhibitor of Akt signaling, diminishing insulin sensitivity and protein translation via the GSK3β and mTOR signaling pathways which are associated with insulin resistance and weight gain (). Insulin activation of Akt stimulates protein translation as well as glucose uptake and glycogen formation (). Aging or high fat diet increases IP7 levels which interfere with Akt activation leading to insulin resistance and weight gain ().
Model depicting insulin and IP6K1 regulation of Akt and sequelae
IP7 inhibits Akt by acting at the PH domain of Akt to prevent its phosphorylation and activation by PDK1 both in vitro and in vivo. IP7's regulation of Akt phosphorylation by PDK1 is selective, as the catalytic activity of PDK1 toward artificial substrates is not affected by IP7. IP7 exerts this action with marked potency, with its IC50
of 20 nM being several orders of magnitude lower than the IC50
values for other reported actions of inositol pyrophosphates such as inhibition of cyclin-CDK activity by 1/3-IP7 (Lee YS, 2007
) and similar to the Kd (35 nM) for PIP3 binding to the PH domain of Akt (Currie RA, 1999
). Even in the presence of 1 μM PIP3, the physiologic activator of Akt, IP7 inhibits PDK1's influences on Akt at equimolar concentration, comparable to endogenous levels of IP7 (Bennett M, 2006
). Effects of IP7 are highly selective with other inositol phosphates being substantially less potent. The diphosphate in IP7 differentiates it from IP6 and has been shown to alter the protonation state of the molecule (Hand CE, 2007
). Thus, IP7 binds the clathrin assembly protein AP3 with 5–10 fold greater affinity than IP6 (Ye W, 1995
The physiologic relevance of these findings is buttressed by the increased Akt signaling, decreased GSK3β phosphorylation and augmented protein translation in IP6K1 knockouts. Phosphorylation of GSK3β inhibits its catalytic activity leading to increased glycogen levels and reduced adipogenesis (Kaidanovich O, 2002
) predicting that deletion of IP6K1 should lead to insulin hypersensitivity, as observed in IP6K1 KO mice. Insulin hypersensitivity of IP6K1 KO mice protects them from the impaired glucose tolerance and hyperinsulinemia associated with age or high fat diet consumption. Thus, IP7 synthesized by IP6K1 appears to mediate obesity and insulin resistance in mice at least in part by inhibiting Akt and increasing GSK3β activity.
Genetic models of insulin hypersensitivity, such as murine mutants of protein phosphatase 1B, PPARγ, S6K1, and JNK mutants, are resistant to HFD-induced obesity (Elchebly M, 1999
; Hirosumi J, 2002
; Izumiya Y, 2008
; Jones JR, 2005
; Um SH, 2004
). Akt activation is a common feature of these diverse models of increased insulin sensitivity. These models support the notion that the sustained insulin sensitivity of IP6K1 KO mice conveys resistance to weight gain. Both reduced obesity and increased Akt signaling may elicit the improved glucose tolerance and insulin sensitivity of the IP6K1 mutants.
Akt has lipogenic effects. Akt 1 and Akt 2 double knockout mice display reduced adipose mass and skeletal muscle atrophy (Peng XD, 2003
). Akt 2 deletion in ob/ob mice reduces fat accumulation with insulin resistance and hyperglycemia (Leavens KF, 2009
). On the other hand, high fat diet induced hepatic steatosis is correlated with decreased Akt phosphorylation upon insulin treatment (Pinto Lde F, 2010
). Skeletal muscle specific overexpression of Akt 1 reduces fat accumulation, while increasing fatty acid oxidation in the liver with less steatosis (Izumiya Y, 2008
). Akt/mTOR mediated skeletal muscle hypertrophy (Rommel C, 2001
) leading to increased insulin sensitivity (Harrison BC, 2008
; Izumiya Y, 2008
) may be physiologically associated with the alterations in insulin sensitivity of IP6K1 deleted mice. Moreover, GSK3β is adipogenic so that its inhibition in IP6K1 mutants may contribute to their leanness (Ross SE, 2000
). Thus, the role of Akt in lipogenesis is complex and may reflect isoform and tissue specific effects.
Overexpression of Akt can be tumorigenic (Manning BD, 2007
). IP6K1 knockouts do not display spontaneous tumors in their lifetime (data not shown), though we have not exhaustively explored possible tumorigenicity.
We observe increased IP6K activity in the skeletal muscle of HFD mice and older mice. Moreover, leptin receptor deficient obese `pound mice' display increased IP6K protein levels (Chakraborty and Snyder unpublished observation). These findings are consistent with age dependent increases in IP7 levels leading to insulin resistance and obesity.
Our findings imply that selective inhibitors of IP6K1 will have therapeutic potential in treating Type-2 diabetes associated with obesity and insulin resistance. The risk of adverse effects from such treatment can be inferred from the phenotype of IP6K1 knockouts. IP6K1 mutants weigh about 15% less than controls due to less fat deposition, but otherwise appear normal. Males manifest decreased sperm formation, but potential infertility of males may not represent a major problem in typical elderly Type-2 diabetics.