Modulation of Phosphorylation of Tocopherol and Phosphatidylinositol by hTAP1/SEC14L2-Mediated Lipid Exchange

The vitamin E derivative, alpha-tocopheryl phosphate (alpha TP), is detectable in cultured cells, plasma and tissues in small amounts, suggesting the existence of enzyme(s) with alpha-tocopherol (alpha T) kinase activity. Here, we characterize the production of alpha TP from alpha T and [gamma-P-32]-ATP in primary human coronary artery smooth muscle cells (HCA-SMC) using separation by thin layer chromatography (TLC) and subsequent analysis by Ultra Performance Liquid Chromatography (UPLC). In addition to alpha T, although to a lower amount, also gamma T is phosphorylated. In THP-1 monocytes, gamma TP inhibits cell proliferation and reduces CD36 scavenger receptor expression more potently than alpha TP. Both alpha TP and gamma TP activate the promoter of the human vascular endothelial growth factor (VEGF) gene with similar potency, whereas alpha T and gamma T had no significant effect. The recombinant human tocopherol associated protein 1 (hTAP1, hSEC14L2) binds both alpha T and alpha TP and stimulates phosphorylation of alpha T possibly by facilitating its transport and presentation to a putative alpha T kinase. Recombinant hTAP1 reduces the in vitro activity of the phosphatidylinositol-3-kinase gamma (PI3K gamma) indicating the formation of a stalled/inactive hTAP1/PI3K gamma heterodimer. The addition of alpha T, beta T, gamma T, delta dT or alpha TP differentially stimulates PI3K gamma, suggesting facilitated egress of sequestered PI from hTAP1 to the enzyme. It is suggested that the continuous competitive exchange of different lipophilic ligands in hTAPs with cell enzymes and membranes may be a way to make these lipophiles more accessible as substrates for enzymes and as components of specific membrane domains.