Shea-Eaton W K, Trinidad M J, Lopez D, Nackley A, McLean M P
Department of Obstetrics and Gynecology and Molecular Biology and Biochemistry, University of South Florida, College of Medicine, Tampa, Florida 33606, USA.
Endocrinology. 2001 Apr;142(4):1525-33. doi: 10.1210/endo.142.4.8075.
The binding of tropic hormones to their specific receptors in steroidogenic cells stimulates the cAMP second-messenger system in the presence of steroidogenic factor-1 (SF-1) to increase expression of steroidogenic acute regulatory (StAR) protein, facilitating the transfer of cholesterol to the inner mitochondrial membrane. The increased use of cholesterol in steroidogenesis triggers activation of sterol- sensitive genes through a second regulatory pathway involving the binding of sterol regulatory element (SRE)-binding proteins (SREBP) to SREs located in the promoter regions of these genes. A search of the rat StAR promoter revealed five potential SRE sites, which demonstrated specific binding with recombinant SREBP-1a. Overexpression of SREBP-1a, -1c or -2 in HTB-9 cells cotransfected with the rat StAR promoter resulted in an increase in promoter-driven luciferase activity. In addition, SREBP-1a was able to activate the StAR promoter through an E-box but only in a promoter construct lacking SREs. SREBPs are known to be weak transcriptional activators and require the presence of additional coactivators like Sp1 and nuclear factor-Y (NF-Y) to elicit maximum activation. Electrophoretic mobility shift assays demonstrated that Sp1, SF-1, and NF-Y enhanced SREBP-1a binding to SREs in the StAR promoter. There was a 4-fold increase in StAR promoter luciferase reporter gene expression when HTB-9 cells were cotransfected with expression vectors for SREBP-1a and NF-Y. In addition, the combined action of SREBP-1a and SF-1 increased both basal (1.6-fold) and cAMP-induced (3.5-fold) activation of the rat StAR promoter. Although Sp1 enhanced SREBP-1a binding to an SRE, Sp1 was not able to increase StAR promoter activity in the presence of SREBP-1a. These results suggest that SREBP-induced regulation of the rat StAR gene is responsive to selective combinations of transcriptional cofactors that could necessitate the convergence of multiple regulatory pathways to enhance gene transcription.
促性腺激素与其在类固醇生成细胞中的特异性受体结合，在类固醇生成因子-1（SF-1）存在的情况下刺激环磷酸腺苷（cAMP）第二信使系统，以增加类固醇生成急性调节（StAR）蛋白的表达，促进胆固醇向内线粒体膜的转运。类固醇生成过程中胆固醇利用的增加通过第二条调节途径触发固醇敏感基因的激活，该途径涉及固醇调节元件（SRE）结合蛋白（SREBP）与位于这些基因启动子区域的SRE结合。对大鼠StAR启动子的搜索揭示了五个潜在的SRE位点，这些位点与重组SREBP-1a表现出特异性结合。在与大鼠StAR启动子共转染的HTB-9细胞中过表达SREBP-1a、-1c或-2导致启动子驱动的荧光素酶活性增加。此外，SREBP-1a能够通过E盒激活StAR启动子，但仅在缺乏SRE的启动子构建体中。已知SREBP是弱转录激活因子，需要额外的共激活因子如Sp1和核因子-Y（NF-Y）的存在才能引发最大激活。电泳迁移率变动分析表明，Sp1、SF-1和NF-Y增强了SREBP-1a与StAR启动子中SRE的结合。当HTB-9细胞与SREBP-1a和NF-Y的表达载体共转染时，StAR启动子荧光素酶报告基因表达增加了4倍。此外，SREBP-1a和SF-1的联合作用增加了大鼠StAR启动子的基础激活（1.6倍）和cAMP诱导的激活（3.5倍）。尽管Sp1增强了SREBP-1a与SRE的结合，但在SREBP-1a存在的情况下，Sp1不能增加StAR启动子活性。这些结果表明，SREBP诱导的大鼠StAR基因调节对转录辅因子的选择性组合有反应，这可能需要多种调节途径汇聚以增强基因转录。
