Human AT1 receptor gene regulation

Translation of the human angiotensin II type 1 receptor mRNA is mediated by a highly efficient internal ribosome entry site

Activation of the angiotensin II type 1 receptor (AT1R) is closely involved in the pathogenesis of cardiovascular disease. The human AT1R (hAT1R) mRNA splice variants have long 5'-untranslated regions (5'-UTRs) ranging from 272 to 414 bp that have the potential to form stable secondary structures. In this study, we show that the 5'-UTR of hAT(1)R mRNAs contains an internal ribosome entry site (IRES) located within the first 40 bp of the proximal end of exon 1. Experiments utilizing the hAT1R 5'-UTR as a molecular decoy demonstrate a reduction in IRES activity of approximately 50%. This inhibition is most efficient for the hAT1R IRES suggesting that a defined set of trans-factors are required to initiate translation through this cis-element. Translation initiation from the hAT1R IRES appears to be physiologically relevant since IRES activity was maintained during serum starvation, a cellular stress known to inhibit cap-dependent translation. These results suggest that cap-independent translation initiation by internal ribosome entry may represent an important mechanism for the regulation of hAT1R expression.

Physiological genomic analysis of the brain renin-angiotensin system

The brain renin-angiotensin system (RAS) has long been considered pivotal in cardiovascular regulation and important in the pathogenesis of hypertension and heart failure. However, despite more than 30 years of study, the brain RAS continues to defy explanation. Our lack of understanding of how the brain RAS is organized at the cellular and regional levels has made it difficult to resolve long-sought questions of how ANG II is produced in the brain and the precise mechanisms by which it exerts its actions. A major reason for this is the difficulty in experimentally dissecting the brain RAS at the regional, cellular, and whole organism levels. Recently, we and others developed a series of molecular tools for selective manipulation of the murine brain RAS, in parallel with technologies for integrative analysis of cardiovascular and volume homeostasis in the conscious mouse. This review, based in part on a lecture given in conjunction with the American Physiological Society Young Investigator Award in Regulatory and Integrative Physiology (Water and Electrolyte Homeostasis Section), outlines the physiological genomics strategy that we have taken in an effort to unravel some of the complexities of this system. It also summarizes the principles, progress, and prospects for a better understanding of the brain RAS in health and disease.

The transcription factors Sp1 and Sp3 are required for human angiotensin II type 1 receptor gene expression in H295-R cells

The peptide hormone angiotensin II regulates a variety of physiological responses which are mediated by its interaction with high affinity G protein-coupled receptors localized on the surface of target cells. Our previous studies have demonstrated that a 145 bp sequence within the promoter region was required for basal level expression of the human angiotensin II type 1 receptor (hAT(1)R) gene. In the present study, deletional analysis of the hAT(1)R promoter localized the major regulatory sequence to two overlapping GC boxes harbored within the -105 to -85 bp region relative to the transcription start site in H295-R cells. Electrophoretic mobility shift assays (EMSAs) using a double-stranded (ds) oligonucleotide corresponding to this region and H295-R cell nuclear extract resulted in five specific DNA-protein complexes. EMSAs performed with competitive ds-oligonucleotides which harbored the consensus binding site for Sp1 prevented the formation of the DNA-protein complexes. Supershift EMSAs also demonstrated that Sp1 and Sp3 could bind to the GC boxes present within the -105 to -85 bp region of the hAT(1)R promoter. Transactivation experiments utilizing Drosophila SL2 cells, which lack endogenous Sp family transcription factors, demonstrated that Sp1 and Sp3 activated the hAT(1)R promoter and that maximal activation was only achieved when both GC boxes were present. Taken together, these findings suggest that Sp1 and Sp3 are necessary for the expression of the hAT(1)R gene in H295-R cells.

Basal level transcriptional regulation of the human angiotensin II type 1 receptor gene

The peptide hormone angiotensin II regulates a variety of physiological responses which are mediated by its interaction with high affinity G protein-coupled receptors localized on the surface of target cells. To gain insights into the transcriptional regulation of the human angiotensin II type 1 receptor (hAT(1)R) gene, we have isolated 1 kb of the 5'-flanking sequence of this gene. Expression constructs containing various 5'-deletions of the hAT(1)R promoter region, fused upstream to the luciferase reporter gene, were transiently transfected into H295-R, HEC-1B and A549 cells. It was demonstrated that a 145 bp sequence within the promoter region was required for basal level expression of the hAT(1)R gene in all of the three cell lines investigated. Computer analysis indicated the existence of numerous putative transcription factor binding sites in this region. Further detailed deletion data suggested essential transcription factor binding sites between -98 and -79 bp. Electrophoretic mobility shift assays revealed that four protein-DNA complexes were formed within the -98 to -79 bp region of the hAT(1)R gene when incubated with H295-R cell nuclear extract. Site-directed mutagenesis experiments showed that a putative Sp1 binding site was critical for the basal level expression of the hAT(1)R gene.