Regulation of gene expression by vitamin A: the role of nuclear retinoic acid receptors.

Over the past five years, a wealth of information has accumulated concerning the molecular mechanisms mediating RA effects on gene expression. The molecular cloning of the 3 retinoic acid receptors (RARs), of their 16 or so different isoforms, and of the 3 retinoid X receptors (RXRs) as well as the identification of at least 2 different active isomers of RA (all trans- and 9-cis-RA) and of several different CRABPs and CRBPs now provide the essential tools to explain the pleiotropy that has become associated with RA effects. In the years to come, a concentrated effort to delineate the complex interactions between the various components of the retinoid signal transduction system should shed light on the mechanisms underlying pattern formation during vertebrate development and point to new ways in which retinoids can be exploited therapeutically.