MOLECULAR REGULATION OF LRAT AND CYP26 IN LIVER

维甲酸和相关的维甲酸是两个配体激活的核受体RAR和RXR家族的有效激素样配体。维甲酸是由多种细胞中的维生素A前体合成的，在这些细胞中，它可能在原位作用于诱导基因表达，控制生长，并促进正常的细胞分化。这些作用使维甲酸在癌症的原位化学预防中受到极大的关注。尽管在维甲酸受体生物学方面取得了许多进展，但我们对调节内源性维甲酸浓度的因素的了解仍然滞后。了解类维甲酸的产生和分解代谢对于了解其受体介导的作用至关重要。需要检验的中心假设是，两种肝微粒体酶-卵磷脂：视黄醇酰基转移酶(LRAT)和细胞色素P450RA1(或CYP26)-分别是维甲酸生物合成和分解代谢的关键调节因子。最近，我们从大鼠和小鼠的肝脏中克隆了LRAT基因。初步研究表明，LRAT和CYP26基因在肝脏中的表达受到外源性维甲酸和饮食维生素A的长期调控。为了严格检验我们的假设，我们将进行四个特定的目标。在目标1中，我们将检验维甲酸和饮食诱导的完整大鼠LRAT和CYP26基因表达和维甲酸代谢的差异。在目标2中，我们将研究哪些肝细胞类型表达LRAT和CYP26，并进一步测试我们的模型在肝细胞和星状细胞中的维甲酸代谢。在目标3中，我们将对人肝LRAT的同源基因进行测序，并对来自肝组织采购和分配系统(LTPADS)的正常和病变肝脏标本中LRAT和CYP26的表达进行分子研究。在目标4中，我们将研究LRAT和CYP26基因的5‘调控区，以确定它们对肝脏中维甲酸反应的分子基础。通过同时研究LRAT和CYP26，我们期望对维甲酸生物合成和降解的分子和细胞类型的特定调控获得新的见解。这一信息可能为控制维甲酸在组织和血浆中的可获得性的内源性因素提供新的线索，这些因素反过来可能影响维甲酸的抗癌潜力。

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.