Our study establishes IPMK as a physiologic determinant of amino acid-induced mTORC1 signaling. Depletion of IPMK reduces amino acid-stimulated mTOR activation by about 60%, comparable to the effects of depletion of major components of the mTORC1 such as raptor (Kim et al., 2002
). A mutant IPMK that is devoid of catalytic activity stimulates mTOR signaling as effectively as wild-type IPMK. Thus, regulation of mTOR by IPMK is independent of IPMK's catalytic activity.
We elucidated mechanisms whereby mammalian IPMK regulates mTORC1 signaling. In MEFs, IPMK depletion selectively disrupts mTOR-raptor, but not mTOR-rictor or mTOR-mLST8 interactions. IPMK's stabilization of mTORC1 is dependent upon the amino terminus of IPMK, which is present only in the mammalian form of the enzyme. Overexpression of hIPMK-1–60 selectively disrupts the mTOR-raptor interaction in the presence of leucine and thereby dominant-negatively impairs amino acid-induced mTORC1 signaling. Mutant mouse IPMK lacking amino acids 1–31 fails to maintain mTOR-raptor binding and activation of mTORC1 in response to amino acids. Thus, our findings show that mammalian IPMK is a physiologic mTOR cofactor involved in maintaining the binding of mTOR with raptor, thereby influencing the activation of mTORC1 by nutrient amino acids (). As hIPMK-1–60 peptide does not alter mTORC1 stability under the leucine-deprived condition and IPMK depletion does not fully abolish the mTOR-raptor binding, we presume that other factors act together with IPMK.
Sabatini and associates proposed that mLST8, which occurs in both mTORC1 and mTORC2, maintains the integrity of mTORC2 (Guertin et al., 2006
). In MEFs null for mLST8, interactions of mTOR-rictor, but not mTOR-raptor, are lost, leading to depressed mTORC2 signaling. IPMK also associates with mTORC2, indicating that it can participate in the mTORC2 complex, and the decreased mTORC2 signaling toward Akt Ser473 phosphorylation in IPMK-depleted MEFs implies a functional role for IPMK.
The regulation of mTOR signaling by IPMK implies an important role in cellular protein synthesis that is critical for life. Thus genetic deletion of mTOR (Gangloff et al., 2004
; Murakami et al., 2004
; Guertin et al., 2006
) or raptor (Guertin et al., 2006
) leads to embryonic lethality at embryonic day 5.5–6.5 (e5.5–6.6) due to growth and proliferation defects. IPMK knockout mice also die early in development at around e9.5 due to severe growth and morphological defects, resembling the phenotype of mTOR or raptor knockout mice (Frederick et al., 2005
). This similarity is consistent with a role of IPMK in regulating mTORC1 function.
Obese patients frequently display increased plasma levels of amino acids (Um et al., 2006
; Dann et al., 2007
), which parallels reduced insulin sensitivity (Krebs, 2005
). Nutrient amino acid overload leads to hyperactivated mTORC1 signaling and insulin resistance (Tremblay et al., 2005
). Due to its role in protein synthesis and growth, mTOR has been a major target for the development of anti-cancer drugs. Interestingly, obesity is a risk factor for many cancers (Calle and Kaaks, 2004
), and anti-diabetic drugs such as metformin, which activates AMP-kinase and thereby inhibits mTORC1 signaling, inhibit cancer cell growth and metabolism (Dowling et al., 2007
). Accordingly, drugs that impair IPMK-mediated regulation of the mTORC1 stability and function may be useful in treating obesity, diabetes, and cancer.