Proapoptotic effects of long chain vitamin E metabolites in HepG2 cells are mediated by oxidative stress

While the metabolism of vitamin E has been extensively studied in cell culture, animals and humans, biochemical analyses of intermediate metabolites are scare. We here describe synthesis and pro-apoptotic properties of long-chain metabolites of alpha- and delta-tocopherol. Several long-chain vitamin E metabolites, namely 13-hydroxy- and 13-carboxychromanols, were synthesized from garcinoic acid, a delta-tocotrienol derivative extracted from the African bitter nut Garcinia kola. Both alpha- and delta-13-carboxychromanol induced cell death in HepG2 cells at EC50 of 13.5 and 6.5 µM, respectively. Apoptosis was quantified by Annexin-V/7-AAD staining and flow cytometry analysis. By immunoblot analyses, we observed activation of both caspase-3 and -9 as well as PARP-1 cleavage. Parameters of mitochondrial dysfunction including reduced mitochondrial membrane potential (m) and increased intracellular and intra-mitochondrial reactive oxygen species (ROS) formation were observed after metabolite treatment. Lastly, long chain hydroxychromanols were readily metabolized to the corresponding carboxychromanols when exposed to HepG2 cells. Taken together, these results indicate that long-chain metabolites may be responsible for anti-proliferative properties of vitamin E vitamers.