Sex differences in intracellular fatty acid transport: role of cytoplasmic binding proteins.

Female liver clears long-chain fatty acids from plasma more rapidly than male liver, and yet the basis for this sex difference is poorly understood. We tested the hypothesis that cytosolic fatty acid binding protein (FABP), which is more concentrated in female liver, may enhance fatty acid utilization by increasing the rate of transport through the cytoplasm. We modified the technique of fluorescence recovery after laser photobleaching to measure the cytoplasmic diffusion rate of the fluorescent long-chain fatty acid 12-N-methyl-(7-nitrobenz-2-oxa-1,3-diazol)aminostearate (NBD-stearate) in cultured hepatocytes from female and male rats. NBD-stearate was used because its hepatic handling is similar to natural fatty acids. After uptake, NBD-stearate distributed uniformly in the cytoplasm but was excluded from the nucleus. Intracellular transport occurred by diffusion with no detectable convective flux. The cytoplasmic diffusion rate at 37 degrees C was 65% greater in female cells than in male cells (mean +/- SE, 5.03 +/- 0.37 vs. 3.05 +/- 0.21 x 10(-9) cm2/s respectively; P < 0.001) and was two to three orders of magnitude slower than for either unbound NBD-stearate or FABP in water. A correspondingly greater fraction of cellular NBD-stearate was found in the aqueous cytosol in females (35.1 +/- 7.0 vs. 18.2 +/- 2.7%), suggesting that FABP reduces binding of NBD-stearate to immobile cytoplasmic membranes. These data indicate that intracellular transport of NBD-stearate, a typical amphipathic molecule, is slowed by binding to cytoplasmic membranes. The primary function of soluble binding proteins such as FABP may be to enhance the diffusive fluxes of their ligands by reducing membrane binding. If cytoplasmic transport of rapidly metabolized fatty acids such as palmitate is similarly slow, substantial concentration gradients could develop within the cytoplasm of hepatocytes at steady state. By catalyzing these diffusive fluxes, FABP may regulate fatty acid metabolism.