Mechanism of RRR-alpha-tocopherol transport mediated by RRR-alpha-tocopherol transfer protein in liver cells

Vitamin E (RRR-alpha-tocopherol) is a fat-soluble antioxidant that is transported by plasma lipoproteins in the body. RRR-alpha-Tocopherol taken up by the liver with lipoprotein is thought to be re-secreted into the plasma in very low density lipoprotein (VLDL). RRR-alpha-Tocopherol transfer protein (alpha-TTP), which was recently identified as a product of the causative gene for familial isolated vitamin E deficiency, is a cytosolic liver protein and plays an important role in the efficient recycling of plasma vitamin E. To elucidate the mechanism of alpha-TTP-mediated RRR-alpha-tocopherol transfer in the liver cell, we devised an assay system using the hepatoma cell line McARH7777. Using this system, we found that the secretion of RRR-alpha-tocopherol was more efficient in cells expressing alpha-TTP than in matched cells lacking alpha-TTP. Brefeldin A, which effectively inhibits VLDL secretion by disrupting the Golgi apparatus, had no effect on RRR-alpha-tocopherol secretion, indicating that alpha-TTP-mediated RRR-alpha-tocopherol secretion is not coupled to VLDL secretion. Among other agents tested, only 25-hydroxycholesterol, a modulator of cholesterol metabolism, inhibited RRR-alpha-tocopherol secretion. This inhibition is most likely mediated by oxysterol binding protein. These results suggest that alpha-TTP present in the liver cytosol functions to stimulate secretion of cellular RRR-alpha-tocopherol into the extracellular medium and that the reaction utilizes a novel non-Golgi mediated pathway which may be linked to cellular cholesterol metabolism and/or transport.