Na pump and plasma membrane structure in L-cell fibroblasts expressing rat liver fatty acid binding protein

Although the intracellular fatty acid binding proteins have been investigated for nearly two decades and purified proteins are now available, little is known regarding the function of these proteins in intact cells. Therefore, L-cell fibroblasts transfected with cDNA encoding for rat liver fatty acid binding protein (L-FABP) were examined as to whether L-FABP expression in intact cells modifies plasma membrane enzyme activities, fluidity, and lipids. Plasma membrane Na K-ATPase activity was 65.9 ± 18.7 and 38.6 ± 22.8 (P < 0.001) nmol/mg protein × min for control and high-expression transfected cells, respectively. Consistent with this observation, [³H] ouabain binding to whole cells was significantly decreased from 3.7 ± 0.3 to 2.0 ± 0.8 pmol ouabain bound/mg cell protein in control and high-expression cells, respectively, whereas the cell's affinity for ouabain was not significantly altered. Unexpectedly, Western blot analysis indicated that transfected cells had higher levels of Na⁺, K⁺-ATPase protein; in contrast, the activities of 5′-nucleotidase and Mg-ATPase were unaltered. The effects of L-FABP expression on plasma membrane Na K-ATPase function appeared to be mediated through alterations in plasma membrane lipids and/or structure. The plasma membrane cholesterol/phospholipid ratio decreased and the bulk plasma membrane fluidity increased in the high-expression cells. In conclusion, plasma membrane Na K-ATPase activity in L cells may be regulated in part through expression of cytosolic L-FABP.