MOLECULAR REGULATION OF LRAT AND CYP26 IN LIVER

视黄酸和相关的类视黄酸是两个配体激活的核受体家族RAR和RXR的有效的类视黄酸配体。视黄酸是由维生素A前体在多种细胞中合成的，在这些细胞中，视黄酸可能原位作用以诱导基因表达、控制生长并促进正常细胞差异。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。尽管在类维生素A受体生物学方面取得了许多进展，但我们对调节内源性类维生素A浓度的因素的理解仍然落后。了解类维生素A的产生和释放对于了解其受体介导的作用至关重要。待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P450 RA 1或CYP 26-分别作为视黄酸生物合成和催化剂的关键调节剂。最近，我们已经从大鼠和小鼠肝脏中克隆了LRAT cDNA。初步研究表明，LRAT和CYP 26基因表达在肝脏中受到强烈调控，实际上都是由外源性类维生素A和膳食维生素A长期调控。为了严格检验我们的假设，我们将进行4个具体目标。在目标1中，我们将研究类维生素A和饮食诱导的LRAT和CYP 26基因表达和类维生素A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CYP 26，并进一步测试我们的肝细胞和星状细胞中类维生素A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，并对来自肝脏组织采购和分配系统（LTPADS）的正常和患病肝脏标本中的LRAT和CYP 26表达进行分子研究。在目的4中，我们将研究LRAT和CYP 26基因的5'调控区，以确定它们在肝脏中对维甲酸反应性的分子基础。通过同时研究LRAT和CYP 26，我们期望获得对维甲酸生物合成和降解的分子和细胞类型特异性调节的新见解。这一信息可以揭示控制组织和血浆中维甲酸可用性的内源性因素，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.