Long-chain carboxychromanols are the major metabolites of tocopherols and tocotrienols in A549 lung epithelial cells but not HepG2 cells

Abstract

Human lung type II cell derived A549 epithelial cancer cells and HepG2 hepatocytes constitutively express cytochrome P4504F2, a P450 we previously identified as a tocopherol-ω-hydroxylase. To determine if A549 cells would metabolize tocochromanols via the ω-hydroxylase pathway, we compared the metabolism of tocopherols (α-, γ-, δ-TOH) and tocotrienols (α-, γ-, δ-T3) in these 2 cell lines. Cultures were incubated with α-, γ-, or δ-TOH, or the analogous T3s, and synthesis of their metabolites quantitated by GC-MS. A549 cells metabolized all tocochromanols 2-3 times more extensively than HepG2 cells (P < 0.001) except α-TOH, a difference not related to cell uptake of substrate but rather was reflective of greater microsomal TOH-ω-hydroxylase enzyme activity. Notably, 9′-carboxychromanols were the major metabolites of all γ- and δ-TOHs and T3s in A549 cultures, whereas 3′- and 5′-carboxychromanols predominated in HepG2 cultures. Accumulation of 9′-carboxychromanols in A549 cultures was due to their inefficient conversion to 7′-carboxychromanols relative to HepG2 cells. Sesamin inhibited tocochromanol metabolism in both cells types, and neither cell type exhibited evidence of alternative (sesamin-insensitive) pathways of metabolism. TOH-ω-hydroxylase activity was undetectable in rat primary lung type II cells, suggesting that expression of activity was associated with transformation of normal type II cells to cancer cells. Long-chain carboxychromanol metabolites of γ-TOH and other forms of vitamin E can be biosynthesized in A549 cultures for assessment of their biological activity, including their potential inhibition of synthesis of inflammatory mediators.