Vitamin E analogues as inducers of apoptosis: Structure-function relation

Birringer M, EyTina JH, Salvatore BA, Neuzil J.

Br J Cancer. 2003 Jun 16;88(12):1948-55.

Recent results show that alpha-tocopheryl succinate (alpha-TOS) is a proapoptotic agent with antineoplastic activity. As modifications of the vitamin E (VE) molecule may affect its apoptogenic activity, we tested a number of newly synthesised VE analogues using malignant cell lines. Analogues of alpha-TOS with lower number of methyl substitutions on the aromatic ring were less active than alpha-TOS. Replacement of the succinyl group with a maleyl group greatly enhanced the activity, while it was lower for the glutaryl esters. Methylation of the free succinyl carboxyl group on alpha-TOS and delta-TOS completely prevented the apoptogenic activity of the parent compounds. Both Trolox and its succinylated derivative were inactive. alpha-tocotrienol (alpha-T3 H) failed to induce apoptosis, while gamma-T3 H was apoptogenic, and more so when succinylated. Shortening the aliphatic side chain of gamma-T3 by one isoprenyl unit increased its activity. Neither phytyl nor oleyl succinate caused apoptosis. These findings show that modifications of different functional moieties of the VE molecule can enhance apoptogenic activity. It is hoped that these observations will lead to the synthesis of analogues with even higher apoptogenic and, consequently, antineoplastic efficacy.

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Tocopherols and tocotrienols are metabolized by side chain degradation initiated by cytochrome P450 (CYP)-catalyzed omega-hydroxylation followed by beta-oxidation. Whereas alpha-tocopherol is only poorly metabolized, high amounts of the final products, carboxyethyl hydroxychroman (CEHC), are found from other tocols in HepG2 cells and in human urine. CYP3A4 and CYP4F2 were suggested to be involved in tocopherol degradation. CYP3A4 metabolizes most of the drugs and is induced by many of its substrates via the activation of the pregnane X receptor (PXR). Also tocopherols and in particular tocotrienols induce the expression of a PXR-driven reporter gene and the expression of endogenous CYP3A4 and CYP3A5 which is supported by sporadic publications spread over the last 30 years. The potential interference of vitamin E with drug metabolism is discussed in the light of related complications evoked by herbal remedies.

Inhibition of THP-1 cell adhesion to endothelial cells by alpha-tocopherol and alpha-tocotrienol is dependent on intracellular concentration of the antioxidants

Noguchi N, Hanyu R, Nonaka A, Okimoto Y, Kodama T.

Free Radic Biol Med. 2003 Jun 15;34(12):1614-20.

Vitamin E analogs such as alpha-tocopherol and alpha-tocotrienol have been shown to reduce endothelial expression of adhesion molecules. The reactivity of alpha-tocopherol and alpha-tocotrienol in inhibiting lipid peroxidation in vitro was essentially identical but the inhibition of adhesion of THP-1 cells, a monocytic-“like” cell line, to endothelial cells differs substantially. To determine the mechanism underlying this response, human umbilical vein endothelial cells (HUVECs) were assessed for their ability to accumulate vitamin E analogs. alpha-Tocotrienol accumulated in HUVECs to levels approximately 10-fold greater than that of alpha-tocopherol. The decrease in expression of vascular cell adhesion molecule-1 (VCAM-1) and the adhesion of THP-1 cells to HUVECs by alpha-tocopherol and alpha-tocotrienol was also determined. Both alpha-tocopherol and alpha-tocotrienol suppressed VCAM-1 expression and adhesion of THP-1 cells to HUVECs in a concentration-dependent manner. The efficacy of tocotrienol for reduction of VCAM-1 expression and adhesion of THP-1 cells to HUVECs was also 10-fold higher than that of tocopherol. The inhibitory effects of vitamin E analogs on the adhesiveness of endothelial cells clearly correlated with their intracellular concentrations. The data demonstrated that, in assessing the biological responses of antioxidants, intracellular accumulation and metabolism were additional important factors that must be considered.

Tocotrienols induce IKBKAP expression: a possible therapy for familial dysautonomia

Anderson SL, Qiu J, Rubin BY.

Biochem Biophys Res Commun. 2003 Jun 20;306(1):303-9.

Familial dysautonomia (FD), a neurodegenerative genetic disorder primarily affecting individuals of Ashkenazi Jewish descent, is caused by mutations in the IKBKAP gene which encodes the IkappaB kinase complex-associated protein (IKAP). The more common or major mutation causes aberrant splicing, resulting in a truncated form of IKAP. Tissues from individuals homozygous for the major mutation contain both mutant and wild-type IKAP transcripts. The apparent leaky nature of this mutation prompted a search for agents capable of elevating the level of expression of the wild-type IKAP transcript. We report the ability of tocotrienols, members of the vitamin E family, to increase transcription of IKAP mRNA in FD-derived cells, with corresponding increases in the correctly spliced transcript and normal protein. These findings suggest that in vivo supplementation with tocotrienols may elevate IKBKAP gene expression and in turn increase the amount of functional IKAP protein produced in FD patients.