Cellular lipid binding proteins: expression, function, and nutritional regulation

The membrane transport and cytosolic solubilization of hydrophobic ligands, including sterols, fatty acids, retinoids, and certain hydrophobic carcinogens, are facilitated by a group of similar low molecular weight proteins: plasma membrane transport protein, fatty acid binding proteins, sterol carrier protein, and retinoid binding proteins. The cellular content of these proteins, which establishes the capacity of a cell to utilize the various ligands, is determined by events regulating transcription and translation, e.g., the mRNA abundance of liver- and gut-type FABPs is increased by dietary fat, and translation of hepatic FABP appears to be stimulated by insulin. Functions attributable to these lipid binding proteins remain unclear, but data are presented that indicate physiological roles in 1) fatty acid transport, esterification, and oxidation, 2) steroidogenesis, and 3) retinoid uptake, retinaldehyde reduction, and retinol esterification. An exciting and novel prospect for cellular trafficking proteins is the role they may play in regulating gene expression. In this respect, cellular lipid binding proteins, e.g., retinoid binding proteins, may deliver their ligands to nuclear trans-acting proteins, and thereby modulate genes coding for key proteins involved in lipid metabolism or differentiation. Even though the functions of these proteins still need to be unequivocally established, it is clear that they are important in the overall homeostasis of lipid metabolism.