Differentiation of pluripotent C3H10T1/2 cells rapidly elevates CYP1B1 through a novel process that overcomes a loss of Ah Receptor

Stimulation of C3H10T1/2 cells by an adipogenic hormonal mixture (IDM) consisting of insulin (I), dexamethasone (D), and methylisobutylxanthine (M) substantially induces cytochrome P450 (CYP) 1B1 expression. This stimulation represents up to 40% of the level produced by maximum activation of the arylhydrocarbon receptor (AhR) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dexamethasone and methylisobutylxanthine in combination produced near maximum elevation of CYP1B1 along with a subsequent decline in AhR that paralleled the rise in peroxisome proliferator-activated receptorgamma1 (PPARgamma1). Inhibitors of AhR activity, which block TCDD induction, did not affect this increase of CYP1B1 expression, which was, therefore, independent of AhR activity. These responses were unaffected by inhibition of DNA synthesis, which was required for PPARgamma1 induction and terminal differentiation. Induction of CYP1B1 mRNA was paralleled by increased CYP1B1 promoter-luciferase reporter activity. The initial 0.8kb of promoter region, which was sufficient for 24h near maximum stimulation, did not contain either the key AhR-responsive elements that mediate the TCDD response or CREB and SF1 elements that mediate cAMP stimulation of rat CYP1B1 in steroidogenic cells. This reporter response to IDM stimulation, but not to TCDD, was maintained in AhR-null fibroblasts. CYP1B1 expression, unlike TCDD induction, was stimulated by IDM in only about half the cells. CYP1B1 expression partially overlapped with PPARgamma expression, which was also inversely related in clonal sub-lines. CYP1B1 expression may, therefore, represent an early stage of differentiation that requires factors associated with DNA synthesis to subsequently generate PPARgamma1.     

Regulation of niemann-pick c1 gene expression by the 3'5'-cyclic adenosine monophosphate pathway in steroidogenic cells

The Niemann Pick-C1 (NPC-1) protein is essential for intracellular transport of cholesterol derived from low-density lipoprotein import in mammalian cells. The role of the protein kinase A (PKA) pathway in regulation of expression of the NPC-1 gene was investigated. NPC-1 promoter activity was induced by treatment with dibutryl cAMP (dbcAMP), alone or in combination with the cAMP response element (CRE) binding protein (CREB) overexpressed in adrenal Y-1 cells. When the catalytic subunit of PKA was overexpressed in Y-1 cells, there were similar increases in NPC-1 promoter activity in the presence of CREB. Responses were attenuated by blockade of the PKA pathway, and in the Kin-8 cell line deficient in PKA. Promoter deletion analysis revealed that this response was present in promoter fragments of 186 bp and larger but not present in the 121-bp fragment. Two promoter regions, one at -430 and one at -120 upstream of the translation initiation site, contained CRE consensus sequences. These bound recombinant CREB in EMSA, confirming their authenticity as CREB response elements. Promoters bearing mutations of both CRE displayed no response to dbcAMP. The orphan nuclear receptor, steroidogenic factor-1 (SF-1), was implicated in NPC-1 transactivation by the presence of SF-1 target sequence that formed a complex with recombinant SF-1 in EMSA. Furthermore, transfection of a plasmid that overexpressed SF-1 into ovarian granulosa cells increased promoter activity in response to dbcAMP, an effect abrogated by mutation of the SF-1 target sequence. Chromatin immunoprecipitation assays demonstrated that the CRE region of the endogenous and transfected NPC-1 promoter associated with both acetylated and phosphorylated histone H-3 and that this association was increased by dbcAMP treatment. Treatment with dbcAMP also increased the association of the CRE region of the promoter with CREB binding protein, which has histone acetyltransferase activity. Together, these results demonstrate a mechanism of regulation of NPC-1 expression by the cAMP-PKA pathway that includes PKA phosphorylation of CREB, recruitment of the coactivator CREB binding protein and the phosphorylation and acetylation of histone H-3 to transactivate the NPC-1 promoter.     

Biochemical and molecular biological analysis of different responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin in chick embryo heart and liver

We studied the mechanism of toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the chick embryo, which is an organism highly sensitive to TCDD. TCDD was injected into egg yolks prior to embryogenesis, and eggs were incubated for 12 or 18 days. In TCDD-exposed embryos, we observed increased heart wet weight and change in the color of the liver, with abnormal fatty vesicle formation. To determine whether these effects were mediated by the aryl hydrocarbon receptor (AhR), we examined expression levels of AhR, CYP1A4, and CYP1A5. AhR was expressed continuously in the heart and liver during embryogenesis, whereas induction of CYP1A4 and CYP1A5 by TCDD was detected only in the liver. In situ hybridization study of tissue sections revealed induction of CYP1A4 in the abnormal liver tissue in which color change was not observed. To determine whether these different responses to TCDD depended on the cell type, primary cultures of chick hepatocytes and cardiac myocytes were established and 7-ethoxyresorufin-O-deethylase (EROD) activity was measured. Induction of EROD activity following exposure to TCDD was detected in hepatocytes but not in cardiac myocytes. Although the heart is a principal target organ for TCDD toxicity and AhR is expressed throughout embryogenesis, induction of CYP1A was not observed in the chick heart. Thus, we conclude that defects in the heart induced by exposure to TCDD occur via a different pathway than that occurring in the liver.     

A novel induction mechanism of the rat CYP1A2 gene mediated by Ah receptor-Arnt heterodimer

We have identified an enhancer responsible for induction by 3-methylcholanthrene in the upstream region of the CYP1A2 gene. The enhancer does not contain the invariant core sequence of XREs that are binding sites for the Ah receptor (AhR) and Arnt heterodimer. The enhancer did not show any inducible expression in Hepa-1-derived cell lines, C4 and C12, deficient of Arnt and AhR, respectively. On the other hand, bacterially expressed AhR-Arnt heterodimer could not bind to the enhancer. Mutational analysis of the enhancer revealed that a repeated sequence separated by six nucleotides is important for expression. A factor binding specifically to the enhancer was found by using gel shift assays. Bacterially expressed AhR-Arnt heterodimer interacted with the factor. A dominant negative mutant of the AhR to XRE activated the enhancer. Collectively, these results demonstrate that a novel induction mechanism is present in which the AhR-Arnt heterodimer functions as a coactivator.     

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.