PIP4kγ is a substrate for mTORC1 that maintains basal mTORC1 signaling during starvation

Abstract

Phosphatidylinositol-5-phosphate 4-kinases (PIP4ks) are a family of lipid kinases that specifically use phosphatidylinositol 5-phosphate (PI-5-P) as a substrate to synthesize phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2). Suppression of PIP4k function in Drosophila results in smaller cells and reduced target of rapamycin complex 1 (TORC1) signaling. Here we showed that the γ isoform of PIP4k stimulated signaling through mammalian TORC1 (mTORC1). Knockdown of PIP4kγ reduced cell mass in cells in which mTORC1 is constitutively activated by Tsc2 deficiency. In Tsc2 null cells mTORC1 activation was partially independent of amino acids or glucose and glutamine. PIP4kγ knockdown inhibited the nutrient-independent activation of mTORC1 in Tsc2 knockdown cells and reduced basal mTORC1 signaling in wild-type cells. PIP4kγ was phosphorylated by mTORC1 and associated with the complex. Phosphorylated PIP4kγ was enriched in light microsomal vesicles, whereas the unphosphorylated form was enriched in heavy microsomal vesicles associated with the Golgi. Furthermore, basal mTORC1 signaling was enhanced by overexpression of unphosphorylated wild-type PIP4kγ or a phosphorylation-defective mutant and decreased by overexpression of a phosphorylation mimetic mutant. Together these results demonstrate that PIP4kγ and mTORC1 interact in a self-regulated feedback loop to maintain low and tightly regulated mTORC1 activation during starvation.

Deletion of the gene Pip4k2c , a novel phosphatidylinositol kinase, results in hyperactivation of the immune system

“…These results, along with a recent observation that the enzyme encoded by this gene is a substrate of mTORC1 (6), suggest a close relationship between mTORC1 and Pip4k2c. Moreover, the hyperimmune phenotype of the Pip4k2c −/− mice could be partially ameliorated by treatment with the mTORC1 inhibitor, rapamycin.…”

“…The fact that PI5P4Kγ has very low activity compared with PI5P4Kα and PI5P4Kβ but forms heterodimeric complexes with these active enzymes suggests that its major role is to regulate the localization and/or activity of PI5P4Kα and PI5P4Kβ. As discussed in the introduction, it was reported by Mackey et al (6) that PI5P4Kγ is phosphorylated by mTORC1 on S324 and S328. Interestingly, Mackey et al (6) found that expression of a S324A/ S328A double-mutant form of PI5P4Kγ in HeLa cells enhanced mTORC1 signaling, whereas expression of a phosphomimetic mutant (S324D/S328D) suppressed mTORC1 signaling.…”

“…As discussed in the introduction, it was reported by Mackey et al (6) that PI5P4Kγ is phosphorylated by mTORC1 on S324 and S328. Interestingly, Mackey et al (6) found that expression of a S324A/ S328A double-mutant form of PI5P4Kγ in HeLa cells enhanced mTORC1 signaling, whereas expression of a phosphomimetic mutant (S324D/S328D) suppressed mTORC1 signaling. These results are consistent with a model in which PI5P4Kγ can facilitate mTORC1 signaling (perhaps by recruiting the more active enzymes, PI5P4Kα and/or PI5P4Kβ to lysosomes where mTORC1 is activated) and that phosphorylation of PI5P4Kγ at S324 and S328 by mTORC1 provides a negative feedback loop to shut off mTORC1 signaling (perhaps by preventing recruitment of PI5P4Kα and PI5P4Kβ to lysosomes).…”

“…Mackey et al argued that PI5P4Kγ was negatively regulated by mTORC1 through direct phosphorylation at serine 324 and serine 328 (6). On the other hand, it was reported that knocking out the only PI5P4K isoform in Drosophila resulted in lower body weight and that this correlated with decreased mTORC1 signaling (7).…”

The Mechanistic Target of Rapamycin: The Grand ConducTOR of Metabolism and Aging

“…While the 4E-BPs are responsible for many of the effects of mTOR on protein translation, the La related protein 1 (LARP1) is a recently discovered mTORC1 effector which also represses the translation of TOP mRNAs (Fonseca et al, 2015). Another novel mTORC1 substrate discovered recently is the phosphatidylinositol-5-phosphate 4-kinase γ (PIP4Kγ) (Mackey et al, 2014). Finally, a major new transcriptional effector of TORC1 signaling was identified this year in Drosophila melanogaster , REPTOR (Tiebe et al, 2015).…”