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

Vitamin E is a fat-soluble vitamin with antioxidant properties. Tocopherols are the predominant form of vitamin E found in the diet and in supplements and have garnered interest for their potential cancer therapeutic and preventive effects, such as the dephosphorylation of Akt, a serine/threonine kinase with a pivotal role in cell growth, survival, and metabolism. Dephosphorylation of Akt at Ser(473) substantially reduces its catalytic activity and inhibits downstream signaling. We found that the mechanism by which alpha-tocopherol and gamma-tocopherol facilitate this site-specific dephosphorylation of Akt was 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. 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 recruitment of these proteins to membranes containing tocopherols. Molecular modeling revealed the structural determinants of the interaction with the PH domain of Akt that may inform strategies for continued structural optimization. By describing a mechanism by which tocopherols facilitate the dephosphorylation of Akt at Ser(473), we provide 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.