Cellular retinol-binding proteins are determinants of retinol uptake and metabolism in stably transfected Caco-2 cells

The mammalian small intestine contains two related cellular retinol- binding proteins, CRBP and CRBP II, which are thought to have distinct functions. The human intestinal cell line, Caco-2, was used as a model system for testing the hypothesis that intracellular levels of these proteins directly modulate the absorption and subsequent metabolism of retinol in enterocytes. Immunoblot and Northern blot hybridization demonstrated that Caco-2 cells express CRBP II and cellular retinoic acid-binding protein I in increasing amounts as the cells become more differentiated but do not express detectable quantities of CRBP. Stably transfected cloned Caco-2 cell lines that over-express CRBP II or coexpress CRBP and CRBP II and a control cell line that contains the expression vector without an insert were established. Retinol uptake and retinyl ester synthesis were increased up to 2-fold by coexpression of CRBP or over-expression of CRBP II. No significant differences were detected in the pattern of retinyl esters synthesized from exogenous [³H]retinol. This suggests that the fatty acid pools utilized for retinol esterification were the same despite differences in the CRBP and CRBP II phenotype of the cell lines. There were no differences between apical and basolateral [³H]retinol uptake or metabolism for filter grown transfected or wild type cell monolayers. Thus, neither CRBP II nor CRBP appear to preferentially interact with luminal- or plasma-derived retinol. Notably, in a cell line which over-expressed CRBP, endogenous CRBP II was reduced by 5- 10-fold compared with the wild type and control cell lines. These studies indicate that CRBP and CRBP II levels are determinants of intracellular retinol accumulation and esterification, and they suggest that CRBP-bound retinol or a metabolite can regulate the expression of CRBP II in the mammalian intestine.