Vitamin E Transport: Dual Pathways Contribute to Intestinal Absorption

Abstract

It is known that dietary vitamin E is taken up by enterocytes and is secreted with chylomicrons. However, molecular details of this absorption process are poorly understood. We studied mechanisms involved in vitamin E absorption utilizing Caco- 2 cells, primary enterocytes, and mice. Incubation of Caco-2 cells with vitamin E resulted in its accumulation in the microsomal membranes. Oleic acid (OA) treatment resulted in the secretion of vitamin E along with apoB-lipoproteins and HDL, whereas, taurocholate treatment alone resulted in the secretion of vitamin E only with HDL particles. A specific inhibitor for microsomal triglyceride transfer protein (MTP) obliterated the apoB-lipoprotein mediated secretion, whereas an ABCA1 inhibitor, glyburide, decreased the HDL associated secretion of vitamin E. Treatment of the cells with exogenous HDL increased vitamin E secretion with HDL. These data indicate that vitamin E transport by these cells involves two independent mechanisms. Primary enterocytes treated with OA secreted vitamin E along with apoB lipoproteins and this secretion was significantly decreased in the presence of an MTP inhibitor and in enterocytes from Mttp deleted mice. In addition, small amounts of vitamin E were secreted in the high-density range and exogenous HDL further increased this secretion. In vivo absorption studies in mice treated with P407 revealed that the majority of vitamin E was in apoB lipoproteins, whereas a small amount was in the HDL fraction. These studies indicate dual pathways for vitamin E absorption by the intestine. The first pathway involves apoB lipoproteins and is dependent on MTP. The second pathway involves high-density lipoproteins and is partially dependent on ABCA1.