MOLECULAR REGULATION OF LRAT AND CYP26 IN LIVER

视黄酸及相关类视黄酸是两种配体激活核受体RAR和RXR家族的有效激素样配体。视黄酸是由多种细胞中的维生素A前体合成的，它可能在原位诱导基因表达、控制生长和促进正常细胞差异。这些作用使得类维生素a在癌症的原位化学预防中引起了极大的兴趣。尽管在类视黄醛受体生物学方面取得了许多进展，但我们对调节内源性类视黄醛浓度的因素的理解仍然落后。了解类维生素a的产生和分解代谢是了解其受体介导作用的关键。要验证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-分别作为视黄酸生物合成和分解代谢的关键调节因子。最近，我们从大鼠和小鼠肝脏中克隆了LRAT cDNA。初步研究表明，肝脏中LRAT和CYP26基因表达受到外源性类维生素a和长期膳食维生素a的强烈调节。为了严格检验我们的假设，我们将进行4个具体目标。在目的1中，我们将检查类视黄醇和饮食诱导的LRAT和CYP26基因表达和类视黄醇代谢在完整大鼠中的差异。在目标2中，我们将研究哪些肝细胞类型表达LRAT和CYP26，并进一步测试我们在肝细胞和星状细胞中的类视黄醇代谢模型。在目标3中，我们将对人类肝脏LRAT的同源cDNA进行测序，并在肝脏组织采购和分配系统（LTPADS）提供的正常和病变肝脏标本中进行LRAT和CYP26表达的分子研究。在目标4中，我们将研究LRAT和CYP26基因的5'调控区域，以确定它们对肝脏维甲酸反应的分子基础。通过同时研究LRAT和CYP26，我们希望获得维甲酸生物合成和降解的分子和细胞类型特异性调控的新见解。这一信息可以揭示控制维甲酸在组织和血浆中可用性的内源性因素，进而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.