Supernatant protein factor and tocopherol-associated protein: an unexpected link between cholesterol synthesis and...

Supernatant protein factor (SPF) is a recently cloned member of a family of cytosolic lipid-binding proteins that includes Sec14p, alpha-tocopherol transfer protein, and cellular retinal-binding protein. SPF stimulates the conversion of squalene to lanosterol in the downstream pathway for cholesterol biosynthesis, and overexpression of cloned SPF in hepatoma cells increases cholesterol synthesis. The mechanism of this stimulation has yet to be defined, but SPF appears to facilitate the transfer of squalene into and between intracellular membranes. The recent identification of SPF as alpha-tocopherol-associated protein (TAP) has called into question its long-standing association with cholesterol biosynthesis. TAP binds alpha-tocopherol, but not other isomers of tocopherol, with high affinity; in the presence of alpha-tocopherol TAP translocates to the nucleus and activates reporter gene transcription. Given the ability of alpha-tocopherol to down-regulate the expression of two scavenger lipoprotein receptors, SR-A and CD36, these observations raise some interesting questions regarding the role of SPF/TAP and vitamin E in cholesterol metabolism.