149 - Molecular Mechanisms of Vitamin E Transport in Hepatocytes

Abstract

Alpha-tocopherol (vitamin E) is an essential plant lipid that scavenges free radicals in biological membranes, thereby preventing oxidative stress. In the liver, the alpha-tocopherol transfer protein (TTP) facilitates the secretion of ingested α-tocopherol to the circulation for uptake by extra hepatic tissues. We aim to understand the molecular mechanisms that regulate TTP activity in hepatocytes, and report here on our findings regarding intracellular trafficking of the protein and its ligands. In the absence of α-tocopherol, TTP localizes to lysosomal vesicles. After treatment with α-tocopherol, the protein redistributes to a diffuse cytosolic pattern, and this re-distribution is altered in TTP mutants that cause ataxia with vitamin E deficiency (AVED) in humans. Kinetic microscopy experiments show that upon binding α-tocopherol, TTP is transported to cell periphery by vesicles derived from recycling endosomes. The directionality of this vectorial transport of TTP to the plasma membrane id directed by inverse concentration gradients of α-tocopherol and PI(4,5)P2. These data allow us to develop a mechanistic model for TTP action, its modulation by physiological triggers, and the impact of disease-causing mutations in the protein.