Micellar Cholesterol Esterification and its Secretion by a Well-differentiated Human Intestine Cell Line (CaCo-2): Effect of Insulin on Cholesterol Absorption

Abstract

The effects of insulin on cholesterol esters accumulation and its transepithelial secretion (from the upper to the lower medium) in human intestinal cell line CaCo-2, cultured on membrane filters as an in vitro model of the small intestine, were studied during stimulation of the rate limiting enzyme of cholesterol esterification, acyl CoA: cholesterol acyltransferase (ACAT), with 25-OH cholesterol to investigate the mechanism of increased cholesterol absorption efficiency in diabetes mellitus. In the presence of oleic acid (500 itM) in the upper medium, 25-OH cholesterol (10 μg/ml) enhanced [¹⁴C] cholesterol esterification in the cells 3 folds (1770±21 vs 561±8 pmol/well/48 hours) and resulted in a fold increase in secretion of esterified [¹⁴C] cholesterol (78.9±2.6 vs 39.1±2.1 pmol/well/48 hours). In the absence of oleic acid, howerer, 25-OH cholesterol did not change esterified [¹⁴C] cholesterol secretion into the lower medium (26.1±1.5 vs 26.5±0.5 pmol/well/48 hours), although [¹⁴C] cholesterol esterification in the cells was increased by 5 fold (301±16 vs 61.3±2.6 pmol/well/48 hours). Insulin (5 μ/m/) added to the lower medium inhibited the secretion of esterified [¹⁴C] cholesterol into lower medium both in the presence and absence of oleic acid. These effects were observed when cholesterol esterification in the cells was enhanced by 25 OH cholesterol. Insulin inhibited micellar [14C] cholesterol esterification in the cells and the transepithelial secretion dose-dependently, and the insulin concentration for a half maximal effect was approximately 5-10 ng/ml. These results indicated that the transepithelial cholesterol ester secretory activity of CaCo-2 cells is regulated not only by intracellular ester accumulation but by other mechanisms, including triglyceride formation and/or the assembling of lipoproteins, and suggest that the increased cholesterol absorption efficiency in diabetes is not explained solely by increased ACAT activity, but by increased lipoprotein secretory activity in the gut associated with insulin deficiency.