Retinoids: transport, metabolism, and mechanisms of action.

The retinoids comprise a family of polyisoprenoid lipids that includes vitamin A (retinol) and structurally related compounds. The biological activity of retinoids can be modified, for example, by changes in the molecules' state of oxidation and cis/trans isomerization. Their activity is also dependent on the levels of specific types of retinoid-binding proteins which exist in extracellular, cytosolic and nuclear compartments. The role of retinoids in gene expression represents an important biological function for this family of molecules. Retinoid-dependent modulation of gene expression is critical for normal cell and tissue function in mature as well as developing animals. Despite significant advances in the understanding of retinoid biological activity, important questions remain concerning aspects of retinoid metabolism, cellular uptake, intracellular trafficking and regulation of gene transcription. The purpose of this review is to present these topics as a compendium of retinoid endocrinology.