Role of CREB in transcriptional regulation of CCAAT/enhancer-binding protein beta gene during adipogenesis

The proximal promoter of the C/EBPbeta gene possesses dual cis regulatory elements (TGA1 and TGA2), both of which contain core CREB binding sites. Comparison of the activities of C/EBPbeta promoter-reporter genes with 5'-truncations or site-directed mutations in the TGA elements showed that both are required for maximal promoter function. Electrophoretic mobility shift and chromatin immunoprecipitation (ChIP) analyses with antibodies specific to CREB and ATF1 showed that these CREB family members associate with the proximal promoter both in vitro and ex vivo. Immunoblotting and ChIP analysis revealed that other CREB family members, CREM and ATF1, are up-regulated and associate with the proximal C/EBPbeta promoter in mouse embryonic fibroblasts (MEFs) from CREB(-/-) mice. ChIP analysis of wild-type MEFs and 3T3-L1 preadipocytes revealed that interaction of phospho-CREB, the active form of CREB, with the C/EBPbeta gene promoter occurs only after induction of differentiation of 3T3-L1 preadipocytes and MEFs. Consistent with the interaction of CREB and ATF1 at the TGA regulatory elements, expression of constitutively active CREB strongly activated C/EBPbeta promoter-reporter genes, induced expression of endogenous C/EBPbeta, and caused adipogenesis in the absence of the hormonal inducers normally required. Conversely, expression of a dominant-negative CREB blocked promoter-reporter activity, expression of C/EBPbeta, and adipogenesis. When subjected to the standard adipocyte differentiation protocol, wild-type MEFs differentiate into adipocytes at high frequency, whereas CREB(-/-) MEFs exhibit greatly reduced expression of C/EBPbeta and differentiation. The low level of expression of C/EBPbeta and differentiation in CREB(-/-) MEFs appears to be due to up-regulation of other CREB protein family members, i.e. ATF1 and CREM.     

细胞动粒蛋白Hec1启动子的结构及功能分析

细胞的分裂是一个严格调控,高度有序的过程.为了将复制后的染色体均匀、准确地传递给两个子细胞,细胞在分裂中后期受到纺锤体检验点的严格监控.Hec1定位于动粒,是纺锤体检验点调控的关键蛋白之一,它通过螺旋 螺旋结构域与其他动粒蛋白相互作用调节姐妹染色体的精确分离.为研究Hec1转录水平的调控机理,采用BLAST工具,从GenBank 中搜索到了人Hec1基因上游的序列,并利用在线工具http://mbs.cbrc.jp/research/db/TFSEARCH.html提供的转录因子结合位点搜索引擎,对其5′启动子调节区段进行了分析.分析结果表明：在Hec1基因上游-200～-1序列内,存在E2F、ATF4和cAMP应答元件结合蛋白（CREB）等转录因子调控元件.在结构分析的基础上,提取HeLa细胞基因组DNA,用PCR方法克隆了Hec1基因启动子,并构建了多个含启动子不同区段的pGL3荧光素酶报告基因表达质粒.瞬时转染HeLa细胞后的结果表明,-70～－63以及－155～－144之间的启动子区对维持荧光素酶活性最为关键.凝胶迁移实验证明,这两个区段分别能够和转录因子CREB以及ATF4结合.随后,采用野生型的以及含有133位磷酸化位点突变的CREB转染HeLa细胞,通过荧光定量PCR实验发现,Hec1的表达水平分别出现明显上升和下降.该结果表明,Hec1表达的调控是通过CREB的活化来完成的.