Expression analysis of α-TTP, PI-TP and SPF genes in H₂O₂-induced HUVECs and neuronal cells supplemented with α-tocopherol and Tocotrienol-Rich Fraction

Vitamin E has 8 isoforms namely; α, β, γ, δ -tocopherols (TCP) and α, β, γ, δ -tocotrienols (TCT). Natural α-tocopherol (α-TCP) but not TCT is preferentially retained in the human body. Studies showed that α-tocopherol transfer protein (α-TTP) is responsible to bind α-TCP for cellular uptake. However, α-TTP has strong specificity and high affinity for α-TCP and poorly binds to α-tocotrienol. Despite of the nature of α-TTP discriminating tocotrienol, population with palm oil as primary source of lipid consisting of 75% TCT and 25% TCP which is taken daily, however has no alarming deficiency reported. Therefore, interest on mechanism of uptake of vitamin E is addressed in this study. The purposes of this study were to examine the modification of α-TTP together with other vitamin E binding related genes in regulating vitamin E uptake in neuronal cell and HUVECs under resting and oxidative stress. Oxidative stress was induced with H2O2 for one hour followed by supplementation with different ratios of α-TCP and Tocotrienol Rich Fraction (TRF) for 4 hours. Likewise, both cells were treated with vitamin E without oxidative stress. Real-time PCR was used to determine expression levels of the genes. The cellular levels of vitamin E were quantified by HPLC as the index of cell bioavailability. The study showed that expression levels of genes encoding the vitamin E binding proteins, including α-tocopherol transfer protein (α-TTP/TTPA), Supernatant protein factor (SPF/SEC14L2) and Phosphatidyl inositol transfer protein (PI-TP/PI-TPNA) in 0% α-TCP positively correlated to the cellular levels of vitamin E in resting neuronal cells and HUVECs under oxidative stress. The expression levels of all genes examined were different in the two cells under oxidative stress, which may contribute to cellular vitamin E content. However, in resting neuronal cells and HUVECs cells the levels were similar. Between the two cells, HUVECs was more sensitive to oxidative stress, which induced gene expressions of TTPA, SEC14L2, and PI-TPNA. Altogether, these results suggest that the regulation of TTPA, SEC14L2 and PI-TPNA genes in the HUVECs and the neurons, affects the distribution of vitamin E in endothelial and neuronal cells. Furthermore, it is reasonable to postulate that under conditions of oxidative stress, increased gene levels would cause increased α-TCP secretion from the neuronal cells or HUVECs thereby proteins could be modified and in the absence of α-TCP they may switch to take up TCT. Generally, our data suggests that probably the expression levels of vitamin E transport proteins might influence cellular concentrations of vitamin E levels in neuronal cells and HUVECs.

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