EXPRESSION OF RAT L-FABP IN MOUSE FIBROBLASTS - ROLE IN FAT-ABSORPTION

Fatty acid-binding proteins (FABP) are abundant cytosolic proteins who se level is responsive to nutritional, endocrine, and a variety of pat hological states. Although FABPs have been investigated in vitro for s everal decades, little is known of their physiological function. Liver L-FABP binds both fatty acids and cholesterol. Competitive binding an alysis and molecular modeling studies of L-FABP indicate the presence of two ligand binding pockets that accomodate one fatty acid each. One fatty acid binding site is identical to the cholesterol binding site. To test whether these observations obtained in vitro were physiologic ally relevant, the cDNA encoding L-FABP was transfected into L-cells, a cell line with very low endogenous FABP and sterol carrier proteins. Uptake of both ligands did not differ between control cells and low e xpression clones. In contrast, both fatty acid uptake and cholesterol uptake were stimulated in the high expression cells. In high expressio n cells, uptake of fluorescent cis-parinaric acid was enhanced more th an that of trans-parinaric acid. This is consistent with the preferent ial binding of cis-fatty acids to L-FABP but in contrast to the prefer ential binding of trans-parinaric acid to the L-cell plasma membrane f atty acid transporter (PMFABP). These data show that the level of cyto solic fatty acids in intact cells can regulate both the extent and spe cificity of fatty acid uptake. Last, sphingomyelinase treatment of L-c ells released cholesterol from the plasma membrane to the cytoplasm an d stimulated microsomal acyl-CoA: cholesteryl acyl transferase (ACAT). This process was accelerated in high expression cells. These observat ions show for the first time in intact cells that L-FABP, a protein mo st prevalent in liver and intestine where much fat absorption takes pl ace, may have a role in fatty acid and cholesterol absorption.