Nonantioxidant Functions of α-Tocopherol in Smooth Muscle Cells

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Abstract
Most tocopherols and tocotrienols, with the exception of α-tocopherol, are not retained by humans. This suggests that α-tocopherol is recognized uniquely; therefore, it may exert an exclusive function. α-Tocopherol possesses distinct properties that are independent of its prooxidant, antioxidant or radical-scavenging ability. α-Tocopherol specifically inhibits protein kinase C, the growth of certain cells and the transcription of the CD36 and collagenase genes. Activation events have also been seen on the protein phosphatase 2A (PP2A) and on the expression of other genes (α-tropomyosin and connective tissue growth factor). Neither β-tocopherol nor probucol possessed the same specialty functions as α-tocopherol. Recently, we isolated a new ubiquitous cytosolic α-tocopherol binding protein (TAP). Its motifs suggest that it is a member of the hydrophobic ligand-binding protein family (CRAL-TRIO). TAP may also be involved in the regulation of cellular α-tocopherol concentration and α-tocopherol–mediated signaling.