High levels of vitamin E affect retinol binding protein but not CYP26A1 in liver and hepatocytes from laying hens

This study was conducted to determine the influence of vitamin E on the retinol binding protein (RBP) and cytochrome p450 family 26 subfamily A polypeptide 1 (CYP26A1), which are specific transporters and catabolic enzymes of vitamin A, respectively. In the in vivo experiments, a total of 450 laying hens was fed 5 levels of vitamin E (0, 20, 80, 320, and 1,280 IU/kg of feed) supplementation. For the in vitro assays, hepatocytes from laying hens were cultured in 4 levels of α-tocopherol (0, 10, 50, and 100 μM). High dietary vitamin E increased the concentration of vitamin A in liver (P < 0.05). The RBP and its mRNA expression in liver and hepatocytes were markedly inhibited by dietary vitamin E (320 and 1,280 IU/kg) and α-tocopherol (100 μM) in culture medium (P < 0.05). However, CYP26A1 and its mRNA expression were not affected by vitamin E in both liver and hepatocytes (P > 0.05). The results indicate that excessive vitamin E could increase the concentration of vitamin A in liver by inhibiting RBP synthesis in hepatocytes.