Retinoic acid and related retinoids are potent hormone-like ligand for two families of ligand-activated nuclear receptors, RAR and RXR. Retinoic acid is synthesized from vitamin A precursors in a variety of cells where it potentially acts in situ to induce gene expression, control growth, and promote normal cellular differentness. These actions make retinoids a great interest in situ chemoprevention of cancer. Despite many advances in retinoid receptor biology, our understanding of the factors that regulate endogenous retinoid concentrations has lagged behind. Understanding the production and catabolism of retinoids is critical to understanding their receptor-mediated actions. The central hypothesis to be tested is that two liver microsomal enzymes - lecithin: retinol acyltransferase, LRAT, and cytochrome P450RA1, or CYP26- serve as key regulators of Retinoic acid biosynthesis and catabolism, respectively. Recently we have cloned LRAT cDNA from rat and mouse liver. Preliminary studies are presented in which LRAT and CYP26 gene expression was strongly regulated in liver, both actually by exogenous retinoids and chronically by dietary vitamin A. To critically test our hypothesis we will conduct 4 specific aims.
In aim 1 we will examine retinoid- and diet- induced differences in LRAT and CYP26 gene expression and retinoid metabolism in intact rats.
In aim 2 we will investigate which liver cell types express LRAT and CYP26 and further test our model of retinoid metabolism in hepatocytes and stellate cells.
In aim 3, we will sequence the homologous cDNA for human liver LRAT and conduct molecular studies of LRAT and CYP26 expression in normal and diseased liver specimens available from the Liver Tissue Procurement and Distribution System (LTPADS).
In aim 4, we will study the 5' regulatory regions of the LRAT and CYP26 genes to determine the molecular basis for their responsiveness to Retinoic acid in liver. By investigating both LRAT and CYP26 simultaneously we expect to obtain novel insights into the molecular and cell-type specific regulation of Retinoic acid biosynthesis and degradation. This information could shed new light on the endogenous factors that control the availability of Retinoic acid in tissues and plasma which, in turn, are likely to affect Retinoic acid's anticarcinogenic potential.
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