Regulation of vitamin A transport into cultured stellate cells of rat liver: studies by anchored cell analysis and sorting system.

Stellate cells (SC) in the liver store the most retinoid in the body, but the mechanisms of specific retinoid transport into SC remain to be elucidated. In this study, to analyze the retinoid content of cultured SC, we employed an anchored cell analysis and sorting system (ACAS), which provides fluorescence analysis of single cultured cells under the phase-contrast microscope by utilizing a laser. First, we examined the effect of retinol binding protein (RBP) on retinol transport into cultured SC of rat liver. Rat holo-RBP added to the medium inhibited retinol uptake into SC. We also prepared RBP-free human serum by affinity chromatography using conjugated anti-human RBP IgG and compared retinoid fluorescence of SC cultured in human serum with or without RBP. No significant difference in retinoid fluorescence intensity was observed between SC cultured with and without holo-RBP. Second, the removal of cellular retinol by esterification may be important for the continued uptake of retinol. Retinyl esters are stored in lipid droplets of SC. Therefore we examined the relationship between the lipid droplet number and the retinoid fluorescence intensity in SC which were cultured in medium containing retinol for 1-3 days. The increases in lipid droplet number and in retinoid fluorescence in SC were almost parallel. Progesterone, previously shown to increase the esterification of retinol by lecithin:retinol acyltransferase (LRAT) in vitro, was added to the SC medium; progesterone facilitated retinol uptake in cultured SC. In conclusion, RBP did not facilitate specific retinol transport into SC. However, the specific transport of retinol is likely to be dependent on the intracellular esterification of retinol by LRAT in SC.