Dynamic regulation of hepatic vitamin E secretion by the α‐tocopherol transfer protein (LB410)

Abstract

Vitamin E, a plant‐derived neutral lipid, is an essential nutrient for all vertebrates that scavenges free radicals in biological membranes, thereby preventing oxidative stress. Of the eight naturally‐occurring forms of vitamin E, α‐tocopherol is the most biologically active. This discrimination is achieved by the selective retention of α‐tocopherol by the hepatic α‐tocopherol transfer protein (α‐TTP), and by the selective degradation of other vitamin E isoforms by the hepatic cytochrome P450 CYP4F2. In hepatocytes, α‐TTP facilitates the secretion of α‐tocopherol to the circulation for uptake by extrahepatic target tissues. α‐TTP has also been shown to bind to phosphatidylinositol phosphates in vitro, which may constitute a mechanism for regulation of the protein’s activity. We aim to understand how the actions of α‐TTP are regulated in vivo. Specifically, we study how α‐tocopherol status affects the intracellular localization of α‐TTP, and whether phosphorylation of tyrosines in α‐TTP affects it activity. Using live‐cell fluorescence imaging, we found that localization of α‐TTP in hepatocytes is dynamic: in the absence of α‐tocopherol, the protein is found in a punctate perinuclear pattern, but upon addition of vitamin E, the protein redistributes to a diffuse cytosolic pattern. In addition, we found that tyrosine residue(s) of α‐TTP are phosphorylated, and that this modification is necessary for α‐TTP’s activity. These findings suggest that trans‐localization and tyrosine phosphorylation of α‐TTP are regulated under physiological conditions. Thus, dynamic and homeostatic mechanisms regulate the body‐wide distribution of α‐tocopherol.