Liver-Enriched Transcription Factors

hnf4α在控制胆汁酸合成和偶联中的作用。在hnf4α肝缺失小鼠（hnf4α - deltal）的许多有趣表型中，血清胆汁酸升高。与对照组小鼠（hnf4 α -F/F）相比，hnf4 α - deltal小鼠血清胆汁酸（BAs）水平显著升高。由于胆汁酸是由肝脏中的胆固醇产生的，许多参与胆汁酸生物合成的酶优先在肝脏中表达，因此研究了hnf4 α在BA产生中的作用。这部分是由于编码氧甾醇7 α -羟化酶（CYP7A1）、甾醇12 α -羟化酶（CYP8B1）和甾醇载体蛋白x的基因下调。CYP7A1 mRNA和蛋白仅在HNF4alphadeltaL小鼠的黑暗周期中减少，而在HNF4alphadeltaL和hnf4α -F/F小鼠的光周期中表达没有差异。无论光照周期还是黑暗周期，CYP8B1 mRNA和酶活性均降低。在小鼠Cyp8b1启动子中发现了一个能够指导HNF4alpha依赖性转录的HNF4alpha结合位点。令人惊讶的是，在这些小鼠的血清和胆囊中仍然观察到由CYP8B1活性产生的胆酸衍生的BAs。这些研究表明，HNF4alpha通过调节BA生物合成相关基因，包括体内胆固醇的羟基化和侧链β氧化，在BA稳态中发挥核心作用。hnf4 α - deltal小鼠也表现出极长链酰基辅酶a合成酶相关基因（VLACSR）的表达降低，也称为胆汁酸-辅酶a连接酶和胆汁酸-辅酶a：氨基酸n -酰基转移酶（BAT）。这与hnf4 α - deltal小鼠胆囊中未结合的和甘氨酸结合的胆汁酸水平显著升高有关。与这一体内发现一致，HNF4？也被发现直接结合小鼠VLACSR和BAT基因启动子，启动子活性依赖于hnf4α结合位点和hnf4α在体内的表达，直接调节VLACSR和BAT。这些研究表明，HNF4alpha在胆汁酸的合成和结合中起着核心作用，并解释了为什么HNF4alpha- deltal小鼠有异常的胆汁酸稳态。HNF4alpha在许多肝脏特异性功能的维持中起重要作用。采用肝脏特异性HNF4alpha缺失小鼠，测定肝脏HNF4alpha是否在体内调节血液凝固。这些小鼠表现出肝凝血因子V、IX、XI、XII和XIIIB的表达降低，活化的部分凝血活素时间延长，但凝血酶原时间没有延长。对小鼠FXII和FXIIIB基因进行启动子分析，以确定HNF4alpha是否直接调控编码这些凝血因子的基因。序列分析显示，小鼠FXII和FXIIIB基因中分别存在1个和2个HNF4alpha结合位点。通过小鼠FXII和FXIIIB启动子的瞬时转染和电泳迁移分析，我们确定了高水平的启动子活性依赖于HNF4alpha结合位点和HNF4alpha的表达。综上所述，HNF4alpha通过指导FXII和XIIIB基因的转录，在凝血稳态中起着至关重要的作用。最近的研究揭示了转录因子在禁食对基因代谢调节中的相互作用。辅激活因子PGC-1alpha和HNF4alpha通过位于其近端启动子的保守的hnf4反应元件直接调节组成型雄甾受体（CAR）基因的表达。细胞中pgc -1 α的表达增加了CAR的表达和不依赖配体的CAR活性。对HNF4alpha缺失小鼠的研究表明，HNF4alpha的肝脏表达是产生禁食诱导的CAR表达和活性所必需的。禁食通过肝脏中cAMP、pgc -1 α、hnf4 α、CAR和CAR靶基因的相互作用网络，使编码关键代谢酶和摄取转运蛋白的基因表达增加

Role of HNF4alpha in control of bile acid synthesis and conjugation. Among the many interesting phenotypes in the HNF4alpha liver null mice (HNF4alpha-deltaL) is an elevation in serum bile acids. HNF4alpha-deltaL mice have markedly increased levels of serum bile acids (BAs) compared with control floxed mice (HNF4alpha-F/F). Because bile acids are produced from cholesterol in liver and many enzymes involved in their biosynthesis are preferentially expressed in liver, the role of HNF4alpha in BA production was examined. This is due in part to the downregulation of genes encoding oxysterol 7alpha-hydroxylase (CYP7A1), sterol 12alpha-hydroxylase (CYP8B1), and sterol carrier protein x. CYP7A1 mRNA and protein were diminished only during the dark cycle in HNF4alphadeltaL mice, whereas expression in the light cycle was not different between HNF4alphadeltaL and HNF4alpha-F/F mice. CYP8B1 mRNA and enzyme activity was reduced regardless of light or dark cycle. An HNF4alpha binding site was found in the mouse Cyp8b1 promoter that was able to direct HNF4alpha-dependent transcription. Surprisingly, cholic acid-derived BAs, produced as a result of CYP8B1 activity, were still observed in the serum and gallbladder of these mice. These studies reveal that HNF4alpha plays a central role in BA homeostasis by regulation of genes involved in BA biosynthesis, including hydroxylation and side chain beta-oxidation of cholesterol in vivo. HNF4alpha-deltaL mice also exhibited decreased expression of the very long chain acyl-CoA synthase-related gene (VLACSR), also called bile acid-CoA ligase, and bile acid-CoA:amino acid N-acyltransferase (BAT). This was associated with markedly elevated levels of unconjugated and glycine-conjugated bile acids in gallbladder of the HNF4alpha-deltaL mice. In agreement with this in vivo finding, HNF4? was also found to bind directly to the mouse VLACSR and BAT gene promoters, and the promoter activities were dependent on HNF4alpha-binding sites and HNF4alpha expression by direct regulation of VLACSR and BAT in vivo. These studies indicate HNF4alpha plays a central role in bile acid synthesis and conjugation and explain why the HNF4alpha-deltaL mice have abnormal bile acid homeostasis. HNF4alpha plays an important role in the maintenance of many liver-specific functions. Liver-specific HNF4alpha-null mice were used to determine whether hepatic HNF4alpha regulates blood coagulation in vivo. These mice exhibited reduced expression of hepatic coagulation factors V, IX, XI, XII, and XIIIB and a prolonged activated partial thromboplastin time but not prothrombin time. Promoter analysis of the mouse FXII and FXIIIB genes was performed to determine whether HNF4alpha directly regulates the genes encoding these coagulation factors. Sequence analysis revealed the presence of one and two HNF4alpha binding sites in the mouse FXII and FXIIIB genes, respectively. Using transient transfection and electrophoretic mobility shift analyses with the mouse FXII and FXIIIB promoters, it was established that the high levels of promoter activity were dependent on HNF4alpha binding sites and the expression of HNF4alpha. In conclusion, HNF4alpha has a critical role in blood coagulation homeostasis by directing transcription of the FXII and XIIIB genes Recent studies have uncovered transcription factor interactions in the metabolic regulation of genes by fasting. The coactivator PGC-1alpha along with HNF4alpha directly regulates expression of the constitutive androstane receptor (CAR) gene expression through a conserved HNF4-response element located in its proximal promoter. Expression of PGC-1alpha in cells increases CAR expression and ligand-independent CAR activity. Studies in HNF4alpha-null mice revealed that hepatic expression of HNF4alpha is required to produce fasting-inducible CAR expression and activity. Fasting produces increased expression of genes encoding key metabolic enzymes and an uptake transporter protein through a network of interactions involving cAMP, PGC-1alpha, HNF4alpha, CAR, and CAR target genes in liver