Vitamin E Facilitates the Inactivation of the Kinase Akt by the Phosphatase PHLPP1

Abstract

Vitamin E is a fat-soluble vitamin that includes isomers of tocopherols and tocotrienols which are known for their antioxidant properties. Tocopherols are the predominant form encountered in the diet and through supplementation, and have garnered interest for their potential cancer therapeutic and chemopreventive effects, which include the dephosphorylation of Akt, a serine/threonine kinase that plays a pivotal role in important cellular processes, such as cell growth, survival, metabolism and motility. Full catalytic activation of Akt requires phosphorylation at both Thr308 and Ser473. Dephosphorylation of Ser473 drastically reduces Akt catalytic activity and the number of downstream substrates it can regulate. The mechanism by which α- and γ-tocopherol facilitate the selective dephosphorylation of the kinase Akt at Ser473 was investigated. We showed that this site-specific Akt dephosphorylation was mediated through the pleckstrin homology (PH) domain-dependent recruitment to the plasma membrane of Akt and PHLPP1 (PH domain leucine-rich repeat protein phosphatase, isoform 1), a phosphatase that dephosphorylates Akt at Ser473. The ability of α- and γ-tocopherol to induce PHLPP-mediated Akt inhibition established PHLPP as a “druggable” target. We structurally optimized these tocopherols to obtain derivatives with greater in vitro potency and in vivo tumor-suppressive activity in two prostate xenograft tumor models. Binding affinities for the PH domains of Akt and PHLPP1 were greater than for other PH domain-containing proteins, which may underlie the preferential membrane recruitment of these proteins. Molecular modeling revealed the structural determinants of the interaction with the PH domain of Akt that may inform strategies for continued structural optimization. These findings describe a mechanism by which tocopherols facilitate the dephosphorylation of Akt at Ser473, thereby providing insights into the mode of antitumor action of tocopherols and a rationale for the translational development of tocopherols into novel PH domain-targeted Akt inhibitors.

α-Tocopherol and Hippocampal Neural Plasticity in Physiological and Pathological Conditions

“…The control of the phosphorylative activation of some kinases appears to be a distinctive mechanism of action for the main forms of vitamin E in human tissues, which appears to depend on protein phosphatase (PP) enzyme regulation. For instance, α-Tocopherol but not β-tocopherol, activates protein phosphatase 2A, decreases protein kinase C activity and attenuates smooth muscle cell proliferation at physiological concentrations, and in a similar fashion α-T was proposed to inhibit the protein kinase C (PKC)-dependent assembly of nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase complex at the plasma membrane and superoxide production stimulating PP (1 or 2A form) pathways of microglial cells and Akt-PKB dephosphorylation is prevented by α-T, but not γ-T [12], pretreatment in murine xenograft of human prostate carcinomas through the site-specific dephosphorylation of Akt, a process mediated through the pleckstrin homology (PH) domain-dependent recruitment of Akt and PHLPP1 (PH domain leucine-rich repeat protein phosphatase, isoform 1) to the plasma membrane [25]. PHLPP1 could also be involved in the α-T and γ-T phosphate dependent regulation of VEGF (vascular endothelial growth factor) expression in HEK293 cells, a response that occurs by the coordinated signaling of phosphatidylinositol-3-kinase γ (PI3Kγ) and PKB [10].…”

Two Faces of Vitamin E in the Lung

“…a-Tocopherol and g-tocopherol, at equal molar concentrations, have a relatively similar capacity to scavenge reactive oxygen species (ROS) during lipid peroxidation in vitro and in cells (18,19) and a relatively similar capacity to inhibit activation of protein kinase B (Akt) in cancer cells in vitro (20). Thus, because a-tocopherol is 10-fold higher in tissues than g-tocopherol, there is 10-fold more total ROS scavenging by a-tocopherol than g-tocopherol.…”

Vitamin E: A Role in Signal Transduction

“…The binding of PI to hTAP1 is reversed by αT, and more so by α-tocopheryl phosphate (αTP), and leads to stimulation of PI3Kγ activity, which suggests that αT and αTP promote dissociation of the inactive complex and/or the release of sequestered PI from hTAP1 for subsequent presentation to the kinase by means of a heterotypic lipid exchange mechanism (275,279). Thus, modulation of PI3K by hTAPs may affect gene expression in a vitamin E-dependent manner, e.g., through an impact on the PI3K/PKB signaltransduction pathway by transporting these ligands to specific enzymes such as cytosolic PI3Kγ or to membrane sites accessible for regulating PI3K/PKB/PHLPP1 (Figure 7) (88,89,153,278).…”

“…A recent study established that the phosphatase PHLPP1 dephosphorylates and inactivates PKB in cancer cells. PHLPP1 binds via the PH-domain to αT and γT and translocates to the plasma membrane, leading to dephosphorylation and inactivation of PKB (Figure 6b) (88). High concentrations of vitamin E also trigger the translocation of PDK1 and PKB to the plasma membrane (Figure 6c), which suggests that the overall PKB activity is the result of a balance of membrane translocation and activation of PDK1/2, PKB, and PHLPP1.…”