Abstract
In recent years, the importance of vitamin A in adipose tissue biology, obesity and type II diabetes has become apparent. This review focuses on recent developments within the area of vitamin A and adipose tissue biology. Adipose tissue has an active vitamin A metabolism as it not only stores vitamin A but retinol is also converted to its active metabolite retinoic acid. Several mouse models point to a relationship between vitamin A metabolism and the development of adiposity. Similarly, in vitro studies provide new molecular mechanisms for the function of different forms of vitamin A and retinol- or retinoic acid-binding proteins in adipose tissue.
Highlights
Vitamin A must be obtained from the diet by intake of either food containing preformed vitamin A or provitamin A carotenoids
Retinol bound to retinol-binding protein in the circulation is one source used by adipose tissue
The question that arises is what are the potential mechanisms of action by which retinoids and retinoid binding proteins affect adipose tissue biology and/or energy metabolism? Adipogenesis is governed by a complex regulatory cascade that has been extensively studied in committed clonal cell lines including 3T3-L1 cells
Summary
Vitamin A must be obtained from the diet by intake of either food containing preformed vitamin A (e.g., red meats) or provitamin A carotenoids (e.g., carrots, green leafy vegetables). Retinaldehyde is irreversibly oxidized to retinoic acid. The enzyme class of cytosolic retinaldehyde dehydrogenases (RALDH) has been clearly identified to catalyze this step [4]. In addition to its oxidation to retinoic acid, retinol can be esterified and stored as retinyl ester in hepatic stellate cells or, to a smaller extent, in extrahepatic tissues including adipose tissue (see below). Retinol is re-esterified to retinyl ester by lecithin:retinol acyltransfrase (LRAT) and stored in hepatic stellate cells, the major storage site for vitamin A in the body [7]. Retinol bound to retinol-binding protein (RBP; RBP4) is secreted from the liver to maintain serum vitamin A levels and to deliver retinol to extrahepatic target tissues for intracellular retinoic acid synthesis [8]. This review will focus on recent new developments within the area of retinoids and adipose tissue biology
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