Metabolic Pathways and Modulating Effects of Vitamin A

Vitamin A is an essential micronutrient in vertebrates throughout embryogenesis and fetal differentiation and growth, postnatal development, adulthood and aging, either during normal physiological conditions or in some pathological cases, principally in several neoplasias. Vitamin A is a generic designation for all compounds, natural or synthetic, with most, but not necessaily all, properties of all-trans-retinol compounds. In order to fulfill all of its functions, the different forms of retinoids must be metabolized to its principal active biological compound, retinoic acid, which functions as a ligand controlling retinoic acid pathway through retinoid acid receptors (RARs and RXRs), which binds, to DNA retinoid acid response elements (RAREs), thus regulating, directly or interacting with several others factors, the transcription of innumerous genes. The aim of this review is primarily to discuss and summarize the homeostasis of retinoids, from their absorption and storage, to the genetic signal transduction of retinoic acid. An attempt to integrate the most pertinent and recent information available from different studies, with an emphasis on new insights into the several metabolic pathways of retinoids and also in the role of retinoic acid as a regulatory factor of transcription, is made.