Uptake of vitamin A in macrophages from physiologic transport proteins: role of retinol-binding protein and chylomicron remnants

Abstract

Vitamin A plays an important role in reducing infectious disease morbidity and mortality by enhancing immunity, an effect that is partly mediated by macrophages. Thus, knowing how these cells take up vitamin A is important. The results in the present study demonstrate that J774 macrophages efficiently take up chylomicron remnant retinyl esters and retinol-binding protein (retinol-RBP) bound retinol by specific and saturable mechanisms. The binding of 125I-RBP to plasma membrane vesicles demonstrated that the macrophage receptor had a similar binding affinity, as was discovered previously for other cells. The B max for the macrophages was smaller than the values reported for placenta, bone marrow, and kidney, but larger than that reported for liver. The J774 cells also bound and took up [ 3H]retinol-RBP. Approximately 50 to 60% of the uptake may compete with excess unlabeled retinol-RBP and approximately 30 to 40% with excess transtyrethin. Following the uptake of [ 3H]retinol-RBP, an extensive esterification occurred: After 5 hours of incubation, 77.8 ± 3.9% (SD; n = 3) of the cellular radioactivity was recovered as retinyl esters. The J774 cells also demonstrated saturable binding of chylomicron remnant [ 3H]retinyl esters, and a continuous uptake at 37°C followed by an extensive hydrolysis of the retinyl esters. Binding could be inhibited by approximately 50% by excess unlabeled low density lipoprotein (LDL). In addition, lipoprotein lipase increased the binding of chylomicron remnant [ 3H]retinyl esters by approximately 30% and the uptake of chylomicron remnant [ 3H]retinyl ester by more than 300%. Furthermore, because sodium chlorate reduced binding with 40% and uptake with 55%, the results suggest that proteoglycans are involved in the uptake. Thus, the results suggest that both LDL receptor and LDL-related protein are involved in the uptake of chylomicron remnant [ 3H]retinyl ester in macrophages.