ATF-like element contributes to hepatic activation of human angiotensinogen promoter

Angiotensinogen gene transcription in pulmonary fibrosis

An established body of literature supports the hypothesis that activation of a local tissue angiotensin (ANG) system in the extravascular tissue compartment of the lungs is required for lung fibrogenesis. Transcriptional activation of the angiotensinogen (AGT) gene is believed to be a critical and necessary step in this activation. This paper summarizes the data in support of this theory and discusses transcriptional regulation of AGT, with an emphasis on lung AGT synthesis as a determinant of fibrosis severity. Genetic data linking AGT polymorphisms to the severity of disease in Idiopathic Pulmonary Fibrosis are also discussed.

A nuclear receptor, hepatocyte nuclear factor 4, differently contributes to the human and mouse angiotensinogen promoter activities

Angiotensinogen (AGT), mainly produced in the liver, is the precursor of angiotensin II, an important regulator of blood pressure and electrolyte homeostasis. We previously showed, in hepatoma-derived HepG2 cells that a hepatocyte nuclear factor 4 (HNF4) potentiated human AGT (hAGT) promoter activity and identified its binding sites (termed regions C and J) in the hAGT promoter region. We also showed in transgenic mouse (TgM) that the hAGT is abundantly expressed in the kidney where the level of endogenous mouse AGT (mAGT) expression is low. To elucidate molecular mechanisms of the AGT gene activation in the kidney, we first investigated the HNF4 and AGT expression in the mouse kidney. Northern blot, in situ hybridization and immunohistochemical analyses revealed that the hAGT and HNF4 were both expressed in the proximal tubular (PT) cells of the kidney. We then transfected the hAGT reporter constructs into immortalized mouse PT (mProx) cells and found that regions C and J contributed additively to the HNF4-potentiated hAGT promoter activity. Curiously, no obvious HNF4 binding motif was found in the corresponding region of the mAGT promoter and co-transfected HNF4 failed to activate this promoter in neither HepG2 nor mProx cells. These results suggest that the high-level hAGT expression in the TgM kidney is, at least in part, due to a presence of high-affinity HNF4 binding sites in its promoter.

Inhibitory effect of the small heterodimer partner on hepatocyte nuclear factor-4 mediates bile acid-induced repression of the human angiotensinogen gene

Bile acids function as transcriptional regulators for the genes important in bile acid synthesis and cholesterol homeostasis. In this study, we identified angiotensinogen (ANG), the precursor of vasoactive octapeptide angiotensin II, as a novel target gene of bile acids. In human ANG transgenic mice, administration of cholic acid resulted in the down-regulation of human ANG gene expression in the liver. ANG gene expression in HepG2 cells was also repressed by chenodeoxycholic acid. Because the expression of small heterodimer partner (SHP) mRNA was induced by chenodeoxycholic acid in HepG2 cells, we analyzed the effects of SHP on the human ANG promoter. Promoter mutation analysis demonstrated that SHP repressed human ANG promoter activity through the element, which has been previously determined as a binding site for hepatocyte nuclear factor-4 (HNF-4). SHP repressed human ANG promoter activity only when the HNF-4 expression vector was cotransfected in HeLa cells. Furthermore, we found that SHP bound to the HNF-4 N-terminal region including the DNA-binding domain and activation function-1 and that SHP prevented HNF-4 from binding to the human ANG promoter. These results suggest that bile acids negatively regulate the human ANG gene through the inhibitory effect of SHP on HNF-4.

Regulated expression of human angiotensinogen gene by hepatocyte nuclear factor 4 and chicken ovalbumin upstream promoter-transcription factor

We previously identified various upstream and downstream regulatory elements and factors important for hepatic expression of the human angiotensinogen (ANG) gene, the precursor of vasoactive octapeptide angiotensin II. In the present study, to further investigate the molecular mechanism of human ANG transcriptional regulation, we generated transgenic mice carrying the fusion gene composed of the 1. 3-kilobase promoter of the human ANG gene, its downstream enhancer, and the chloramphenicol acetyltransferase reporter gene. Because expression of the chloramphenicol acetyltransferase gene was observed strongly in the liver and weakly in the kidney, we suspected that hepatocyte nuclear factor (HNF) 4 with a tissue expression pattern similar to that of the reporter gene would regulate ANG transcription. In vitro assays indicated that HNF4 bound to the promoter elements and strongly activated the ANG transcription, but that chicken ovalbumin upstream promoter transcription factor (COUP-TF), a transcriptional repressor, dramatically repressed human ANG transcription through the promoter elements and the downstream enhancer core elements. Furthermore, COUP-TF dramatically decreased the human ANG transcription in the mouse liver by the Helios Gene Gun system in vivo. These results suggest that an interplay between HNF4 and COUP-TF could be important in hepatic human ANG transcription.

Transcriptional regulation of angiotensinogen gene expression

The renin--angiotensin system (RAS) is an extracellular hormonal system implicated in acute, homeostatic control of peripheral vascular resistance and electrolyte homeostasis. In this tightly regulated system, physiological regulators of blood pressure and fluid balance induce the production of the potent vasoactive angiotensin peptides by sequential proteolysis of the angiotensinogen (AGT) prohormone. AGT is the only known precursor of the angiotensin peptides, whose circulating concentrations influence the tonic activity of the RAS. AGT abundance is regulated at the transcriptional level through hormonal and cell-type specific regulators. In this review, we will discuss the identified mechanisms controlling AGT expression separately for the rodent and human genes. The most intensively investigated gene (rodent AGT) is regulated constitutively by multiple positive- and negative-acting cis factors that function in a cell-type dependent fashion. Inducible rodent AGT expression is mediated through a multihormone-inducible enhancer that integrates signals from steroid and cytokine hormones into AGT transcription. We review recent advances in understanding the mechanism of the nuclear factor-kappa B (NF-kappa B) family in mediating cytokine-induced AGT expression and our recent discoveries on the existence of differentially inducible pools of cytoplasmic NF-kappa B. Constitutive control of the human AGT gene will be discussed; there is surprisingly little information on the cis- and trans-acting regulators controlling inducible expression of human AGT. Finally, we will explore some of the recent developments in gene linkage studies where human AGT alleles have been associated with hypertensive phenotypes through a mechanism that may involve enhanced transcription. These studies have provided a molecular explanation for a subset of heritable hypertensive disorders in humans.

DBP binds to the proximal promoter and regulates liver-specific expression of the human angiotensinogen gene

Angiotensinogen is the glycoprotein precursor of one of the most potent vasoactive hormones, angiotensin-II. It has been shown recently that an ATF like element (ALE) located between bases -102 and -87 of the human angiotensinogen gene plays an important role in liver specific expression of this gene and binds to CREB/ATF family of transcription factors and a novel factor (ALF). We show here that this sequence binds to the liver enriched transcription factor DBP and cotransfection of expression vector CMV-DBP increases the expression of reporter constructs containing this sequence. In addition, we show that transcription factor C/EBP-delta binds to this sequence and an expression vector containing C/EBP-delta coding region increases the expression of reporter constructs containing this sequence. Since DBP is involved in circadian rhythm, our studies suggest that this sequence may be involved in circadian expression of the human angiotensinogen gene.