Vitamin A metabolism in the human intestinal Caco-2 cell line

Publisher Website
Google Scholar
Abstract
The human intestinal Caco-2 cell line, described as enterocyte-like in a number of studies, was examined for its ability to carry out the metabolism of vitamin A normally required in the absorptive process. Caco-2 cells contained cellular retinol-binding protein II, a protein which is abundant in human villus-associated enterocytes and may play an important role in the absorption of vitamin A. Microsomal preparations from Caco-2 cells contained retinal reductase, acyl-CoA-retinol acyltransferase (ARAT), and lecithin-retinol acyltransferase (LRAT) activities, which have previously been proposed to be involved in the metabolism of dietary vitamin A in the enterocyte. When intact Caco-2 cells were provided with .beta.-carotene, retinyl lacetate, or retinol, synthesis of retinyl palmitoleate, oleate, palmitate, and small amounts of stearate resulted. However, exogenous retinyl palmitate or stearate was not used by Caco-2 cells as a source of retinol for ester synthesis. While there was a disproportionate synthesis of monoenoic fatty acid esters of retinol in Caco-2 cells compared to the retinyl esters typically found in human chylomicrons or the esters normally synthesized in rat intestine, the pattern was consistent with the substantial amount of unsaturated fatty acids, particularly 18:1 and 16:1, found in the sn-1 position of Caco-2 microsomal phosphatidylcholine, the fatty acyl donor for LRAT. Both ARAT and LRAT have been proposed to be responsible for retinyl ester synthesis in the enterocyte. When Caco-2 cells were pretreated with phenyl-methanesulfonyl fluoride (PMSF), previously shown to inhibit LRAT but not ARAT, cellular accumulation of exogenous retinol was unaffected whereas retinyl ester synthesis was reduced by > 90%, even at supraphysiological levels (30 .mu.M) of retinol exposure. Examination of microsomes from PMSF-treated cells confirmed the inhibition of LRAT activity and the retention ARAT activity as well as the ability to generate acyl-CoA. These data suggest that LRAT may be the physiologically important enzyme for the esterification of retinol in Caco-2 cells.