Lab Research Focus Lab Research Focus
During the past decade, there has been a great interest in biologically active retinoids, because these compounds contribute to pattern formation during development, exert multiple effects on cell differentiation with important clinical implications, and are essential components in vision. In vision, 11-cis-retinal serves as the chromophore of the visual pigments. Most other effects of vitamin A are exerted by all-trans- and 9-cis-retinoic acids, which serve as activating ligands for a family of nuclear retinoic acid and retinoid X receptors (RARs and RXRs). The intracellular levels of retinoic acid are tightly regulated, but the molecular mechanisms of this regulation are poorly understood.
|Figure 1. Metabolism of Retinoids.|
Humans and animals obtain the precursors of retinoic acid with diet in the form of retinyl esters or as provitamin A carotenoids, mainly as b-carotene. Dietary b-carotene absorbed in the small intestine is cleaved into two molecules of all-trans-retinaldehyde. All-trans-retinaldehyde appears to be positioned at the intersection of two opposite pathways, - activation and inactivation of retinoids, - because it can be either oxidized to bioactive retinoic acid by retinaldehyde dehydrogenases or reduced to inactive retinol by retinaldehyde reductases. The molecular identities of these enzymes have not yet established with certainty. We have identified two subfamilies of the human short-chain dehydrogenase/reductase (SDRs) superfamily of proteins that are active toward retinoids. Enzymes that belong to the first subfamily of SDRs (RoDH-like) prefer to oxidize retinol to retinaldehyde. Enzymes that belong to the second subfamily of SDRs (RalR1-like) prefer to reduce retinaldehyde back to retinol. Based on these findings, we propose a novel concept that the relative activities of these two groups of enzymes determine the overall rate of retinoic acid biosynthesis.
Both RoDH and RalR1-like SDR subfamilies consist of several isozymes. Our studies suggest that the isozymes have different substrate specificity, catalytic efficiency, and tissue distribution. Furthermore, their retinoid oxidoreductase activity in the cell cytosol can be influenced by cellular retinol binding proteins (CRBPs), which bind retinol and retinal with high affinity. RoDH-like enzymes are also very active toward androgens and neurosteroids, suggesting a potential cross-talk between the retinoid and steroid metabolic pathways. The overall goal of our laboratory is to determine how the isozymes of RoDH and RalR1-like SDRs work together under specific cellular conditions to control the rate of retinaldehyde and retinoic acid production in different cells.
To achieve our goals, we employ a wide variety of approaches and techniques such as recombinant protein expression and purification; in vitro assays of enzyme activity and analysis of reaction products using spectrophotometry, high-pressure liquid chromatography and thin-layer chromatography; stable and transient expression of enzymes in eukaryotic, bacterial and insect cell cultures; silencing of individual human genes using RNA interference; and gene knockout animal models.
Natalia Y. Kedishvili, Assistant Professor of Biochemistry and Molecular Genetics, received her M.S. degree in Biochemistry in 1982, and a Ph.D. degree in Biochemistry in 1987 from the Lomonosov?s Moscow State University, Russia. After completing her postdoctoral fellowship at Indiana University School of Medicine, she was recruited by the University of Missouri-Kansas City, where she developed a research program in retinoid and steroid metabolism. Dr. Kedishvili joined the UAB faculty in 2004.