Retinoids: Impact on Adiposity, Lipids and Lipoprotein Metabolism

Vitamin A is an important fat soluble vitamin with multiple physiological functions such as vision, growth, reproduction and gene regulation. Vitamin A exists in three physiologically active forms known as vitamers. These are retinol (alcohol), retinal (aldehyde) and retinoic acid (acid). The retinol and retinaldehyde (RALD) perform all the functions, whereas retinoic acid (RA) can perform all the functions of vitamin A except vision. Retinoids are a broad group of small vitamin A-derived natural and synthetic compounds with varied physiological functions. Vitamers of vitamin A and retinoids exert their action through the activation of transcription factors belonging to the retinoic acid receptors (RAR) and retinoid X receptors (RXR). Each of these receptors have three sub families namely α, β & γ, whose expressions vary in different tissues. Of late, RA is shown to play a role in adipocyte differentiation (adipogenesis) as well as adipose tissue loss through apoptosis, thereby determining adiposity in mammals. The importance of retinoids is mainly because of their therapeutic usage in several dermatological disorders and cancers. The most common side effect of this treatment is dyslipidemia. Hence, the purpose of this review is to bring about the molecular mechanisms involved in the regulation of adiposity/obesity, specifically by vitamin A (retinol & its metabolites) in normal and genetically obese rodent models and associated changes in lipid and lipoprotein metabolism by vitamin A and retinoids and also highlighting some of the important patents aimed at containing the problem of abnormal lipid metabolism.