Abstract
Retinoids play pivotal roles in the physiological action of three types of retinoid-binding proteins. It has been shown that cellular retinol- or cellular retinoic acid-binding proteins (CRBP, CRABP) enable retinol or retinoic acid to be transferred to the nucleus, suggesting that the action of vitamin A may be expressed via the nucleus. Recent studies have revealed the physiological roles of intestinal cellular retinol-binding protein type II (CRBPII), which is abundant in intestinal epithelial cells, and plays pivotal roles in intestinal absorption and metabolism of retinol and β-carotene. Intestinal CRBPII expression was enhanced under conditions where fat absorption was stimulated. Dietary fat (especially unsaturated fatty acids) increased the gene expression of CRBPII in rat jejunum. This increase resulted from induction of gene transcription through enhancement of peroxisome proliferator-activated receptor (PPAR) and its ligand levels, and binding of a PPARα-RXRα heterodimer to DR-1-type elements (RXRE and RE3) of the gene. It is concluded that CRBPII gene expression is regulated predominantly by dietary fatty acids, but only slightly by dietary retinoids.
