Cholesterol biosynthesic enzymes and inborn errors

本研究旨在确定人类角鲨烯环氧酶基因(SQLE)的染色体定位。聚合酶链式反应将人SqLE基因定位于8号染色体。由于先天异常的Langer-Giedion(LG)综合征基因(8q24.1)与人类SqLE(8q24.1)非常接近，SqLE被认为是LG综合征基因(S)的候选基因。人类Sqle基因全长约17kb，由87~3.2kb的11个外显子和10个内切片段组成。我们在人类SE基因启动子上鉴定了一个新的SREBP-1a和一个典型的核因子-Y结合部位。这两个位点在Sqle的转录水平上都有功能。在脂蛋白缺陷血清(LPDS)和氧化甾醇的作用下，研究了LXR反应元件(LXRE)和人SE启动子报告基因在LXRα转染的HeLa细胞中的转录。与LXRα转基因阴性细胞相比，LPDS对LXRE阳性细胞有明显的诱导作用，25-羟基胆固醇对LXRE的诱导作用更强。相反，在相同的条件下，SE被相互抑制。LxRα突变体不支持LXRE的诱导和SE的抑制。这些诱导和抑制完全依赖于LXRs的过度表达和具有特定结构的氧固醇配体。提示LXRs可能通过抑制LXRs内源性氧固醇配体的关键合成酶SE，通过抑制LXRs靶基因产物(S)的作用而参与胆固醇稳态。

The present study was undertaken to determine the chromosomal mapping of the human squalene epoxidase gene (SQLE). PCR evidence localizes human SQLE to chromosome 8 by hybrid panel as templete. Since the locus of the Langer-Giedion (LG) syndrome genes (8q24.1), a congenital abnormality, is close to the locus of human SQLE(8q24.1), SQLE is considered the candidate for the LG syndrome gene(s). The human SQLE consisted about 17 kb long and organized into 11 exons and 10 intorns ranging from 87 to 3.2 kb. We characterized a new SREBP-la and a typical NF-Y binding sites in the promoter of the human SE gene. Both sites are functional in the transcriptional levels of SQLE. Transcription of reporter genes of LXR responsive element (LXRE) and human SE promoter were studied with the effects of lipoprotein deficient serum (LPDS) and oxysterols in LXR α-transfected HeLa cells. Compared with LXR α transgene negative cells, LXRE was significantly induced by LPDS in positive cells and 25-hydroxycholesterol reinforced the induction. In contrast, SE was reciprocally suppressed under the same conditions. Transfection of LXR α mutant failed to support LXRE induction and SE suppression. Those induction and supporession were completely dependent on the LXRs overexpression and oxysterol ligands having specific structure. It is suggested that LXRs may involve in cholesterol homeostasis through the action of target gene product(s) of LXRs by suppressing SE, a key synthesizing enzyme of the endogenous oxysterol ligands for LXRs.