Genetic analysis of the human type-1 angiotensin II receptor

Identification of a Novel Transcript and Regulatory Mechanism for Microsomal Triglyceride Transfer Protein

Microsomal triglyceride transfer protein (MTP) is essential for the assembly of triglyceride-rich apolipoprotein B-containing lipoproteins. Previous studies in our laboratory identified a novel splice variant of MTP in mice that we named MTP-B. MTP-B has a unique first exon (1B) located 2.7 kB upstream of the first exon (1A) for canonical MTP (MTP-A). The two mature isoforms, though nearly identical in sequence and function, have different tissue expression patterns. In this study we report the identification of a second MTP splice variant (MTP-C), which contains both exons 1B and 1A. MTP-C is expressed in all the tissues we tested. In cells transfected with MTP-C, protein expression was less than 15% of that found when the cells were transfected with MTP-A or MTP-B. In silico analysis of the 5’-UTR of MTP-C revealed seven ATGs upstream of the start site for MTP-A, which is the only viable start site in frame with the main coding sequence. One of those ATGs was located in the 5’-UTR for MTP-A. We generated reporter constructs in which the 5’-UTRs of MTP-A or MTP-C were inserted between an SV40 promoter and the coding sequence of the luciferase gene and transfected these constructs into HEK 293 cells. Luciferase activity was significantly reduced by the MTP-C 5’-UTR, but not by the MTP-A 5’-UTR. We conclude that alternative splicing plays a key role in regulating MTP expression by introducing unique 5’-UTRs, which contain elements that alter translation efficiency, enabling the cell to optimize MTP levels and activity.

LOX-1 and angiotensin receptors, and their interplay

The renin-angiotensin system (RAS) plays an important role in regulating blood pressure, water-salt balance and the pathogenesis of cardiovascular diseases. Angiotensin II (Ang II) is the physiologically active mediator and mediates the main pathophysiological actions in RAS. Ang II exerts the effects by activating its receptors, primarily type 1 (AT1R) and type 2 (AT2R). Most of the known pathophysiological effects of Ang II are mediated by AT1R activation. The precise physiological function of AT2R is still not clear. Generally, AT2R is considered to oppose the effects of AT1R. Lectin-like oxidized low-density lipoprotein scavenger receptor-1 (LOX-1) is one of the major receptors responsible for binding, internalizing and degrading ox-LDL. The activation of LOX-1 has been known to be related to many pathophysiological events, including endothelial dysfunction and injury, fibroblast growth, and vascular smooth muscle cell hypertrophy. Many of these alterations are present in atherosclerosis, hypertension, and myocardial ischemia and remodeling. A growing body of evidence suggests the existence of a cross-talk between LOX-1 and Ang II receptors. Their interplays are embodied in the reciprocal regulation of their expression and activity. Their interplays are involved in a series of signals. Recent studies suggests that reactive oxygen species (ROS), nitric oxide (NO), protein kinase C (PKC) and mitogen activated protein kinases (MAPKs) are important signals responsible for their cross-talk. This paper reviews these aspects of dyslipidemia and RAS activation.

Genetic polymorphisms of the RAS-cytokine pathway and chronic kidney disease

Chronic kidney disease (CKD) in children is irreversible. It is associated with renal failure progression and atherosclerotic cardiovascular (CV) abnormalities. Nearly 60% of children with CKD are affected since birth with congenital or inherited kidney disorders. Preliminary evidence primarily from adult CKD studies indicates common genetic risk factors for CKD and atherosclerotic CV disease. Although multiple physiologic pathways share common genes for CKD and CV disease, substantial evidence supports our attention to the renin angiotensin system (RAS) and the interlinked inflammatory cascade because they modulate the progressions of renal and CV disease. Gene polymorphisms in the RAS-cytokine pathway, through altered gene expression of inflammatory cytokines, are potential factors that modulate the rate of CKD progression and CV abnormalities in patients with CKD. For studying such hypotheses, the cooperative efforts among scientific groups and the availability of robust and affordable technologies to genotype thousands of single nucleotide polymorphisms (SNPs) across the genome make genome-wide association studies an attractive paradigm for studying polygenic diseases such as CKD. Although attractive, such studies should be interpreted carefully, with a fundamental understanding of their potential weaknesses. Nevertheless, whole-genome association studies for diabetic nephropathy and future studies pertaining to other types of CKD will offer further insight for the development of targeted interventions to treat CKD and associated atherosclerotic CV abnormalities in the pediatric CKD population.

Angiotensin II type I receptor gene polymorphism: anthropometric and metabolic syndrome traits

BACKGROUND

The renin angiotensin system is important in the regulation of vascular tone and fluid and electrolyte balance. The angiotensin converting enzyme gene (ACE) genotype has been shown to affect exercise response and glucose load response dependent on birth weight. Angiotensin II type I receptor (AGTR1) A1166C has previously been associated with the development of hypertension and coronary disease, but its metabolic effects have not been investigated.

METHOD

AGTR1 A1166C was genotyped by allele specific PCR in 378 individuals from Hertfordshire, UK, who had been characterised for metabolic syndrome traits.

RESULTS

Genotype counts were: AA, 183; AC, 170; CC, 25, consistent with Hardy-Weinberg equilibrium. The CC genotype was associated with significantly lower body mass index (by 1.7 units) in men (p = 0.03), and the same magnitude effect in women with significant lower weight in both genders (p = 0.01), also lower waist circumference and waist-hip ratio (p = 0.01) in men, with a trend for lower waist circumference in women also. Additionally, the CC genotype and/or C allele was associated with lower fasting glucose and insulin, and 30 and 120 min glucose in men (respectively, p = 0.08, 0.04, 0.01, 0.06). Lower means of systolic blood pressure, pulse pressure, cholesterol, and fasting triglyceride were also observed for the CC genotype in both genders though these were not statistically significant.

CONCLUSIONS

The AGTR1 1166 CC genotype appears to predispose to favourable anthropometric and metabolic traits, relative to cardiovascular risk.

Brain renin-angiotensin system dysfunction in hypertension: recent advances and perspectives

This review focuses on the dysfunction of the intrinsic brain renin-angiotensin system (RAS) in the pathogenesis of hypertension. Hyperactivity of the brain RAS plays a critical role in mediating hypertension in both humans and animal models of hypertension, including the spontaneously hypertensive rat (SHR). The specific mechanisms by which increased brain RAS activity results in hypertension are not well understood but include increases in sympathetic vasomotor tone and impaired arterial baroreflex function. We discuss the contribution of endogenous angiotensin (Ang) II actions on presympathetic vasomotor rostral ventrolateral medulla neurons to enhance sympathetic activity and maintain hypertension. In addition, we discuss Ang II-induced attenuation of afferent baroreceptor feedback within the nucleus tractus solitarius and its relevance to the development of hypertension. We also outline the cellular and molecular mechanisms of Ang II signal transduction that may be critical for the initiation and establishment of hypertension. In particular, we present evidence for a phosphoinositide-3-kinase-dependent signaling pathway that appears to contribute to hypertension in the SHR, possibly via augmented Ang II-induced increases in neuronal firing rate and enhanced transcriptional noradrenaline neuromodulation. Finally, we outline future directions in utilizing our understanding of the brain RAS dysfunction in hypertension for the development of improved therapeutic intervention in hypertension.

Genes of renin angiotensin system and coronary heart disease

Coronary constriction, proliferation of smooth muscle cells and arrhythmia are involved in the pathophysiology of coronary heart disease and its complications such as myocardial infarction and sudden death. All these effects are favoured by high angiotensin II levels. Angiotensin II is the main effector molecule of the renin angiotensin system and it acts through angiotensin II type receptors. Genetically determined differences in the expression of the components of this system could adversely affect angiotensin II concentration and subsequently heart. Consequently each component of this system represents a potential candidate in the etiology of cardiovascular disease. In this article we review the variation of the angiotensin I converting enzyme, angiotensin II type I receptor and angiotensinogen genes and their association with cardiovascular disease.

Murine double nullizygotes of the angiotensin type 1A and 1B receptor genes duplicate severe abnormal phenotypes of angiotensinogen nullizygotes

Rodents are the unique species carrying duplicated angiotensin (Ang) type 1 (AT1) receptor genes, Agtr1a and Agtr1b. After separately generating Agtr1a and Agtr1b null mutant mice by gene targeting, we produced double mutant mice homozygous for both Agtr1a and Agtr1b null mutation (Agtr1a-/-; Agtr1b-/-) by mating the single gene mutants. Agtr1a-/-, Agtr1b-/- mice are characterized by normal in utero survival but decreased ex utero survival rate. After birth they are characterized by low body weight gain, marked hypotension, and abnormal kidney morphology including delayed maturity in glomerular growth, hypoplastic papilla, and renal arterial hypertrophy. These abnormal phenotypes are quantitatively similar to those found in mutant mice homozygous for the angiotensinogen gene (Agt-/-), indicating that major biological functions of endogenous Ang elucidated by the abnormal phenotypes of Agt-/- are mediated by the AT1 receptors. Infusion of Ang II, AT1 blockers, or an AT2 blocker was without effect on blood pressure in Agtr1a-/-; Agtr1b-/- mice, indicating that AT2 receptor does not exert acute depressor effects in these mice lacking AT1 receptors. Also, unlike Agt-/- mice, some Agtr1a-/-; Agtr1b-/- mice have a large ventricular septum defect, suggesting that another receptor such as AT2 is functionally activated in Agtr1a-/-, Agtr1b-/- mice.

Mutation of Asp74 of the rat angiotensin II receptor confers changes in antagonist affinities and abolishes G-protein coupling

Aspartic acid in the second transmembrane domain is a highly conserved amino acid among the G protein-coupled receptors and is functionally important for agonist binding and G-protein coupling in beta 2-adrenergic and luteinizing hormone receptors. To determine whether this aspartic acid is also involved in the function of the rat vascular angiotensin II receptor subtype 1 (AT1a), Asp74 was replaced either by asparagine or by glutamic acid. When expressed in CHO cells, the two mutants and the wild-type receptor displayed similarly high affinities for the agonist [Sar1, Tyr(125I)4]angiotensin II [where Sar is sarcosine and Tyr(125I) is monoiodinated tyrosine] and the other agonists: ([Sar1]angiotensin II > angiotensin II > angiotensin III >> angiotensin I). However, the Asn74 mutant shows striking differences in its affinity for some antagonists when compared with the wild-type receptor: the affinity for DUP753 was decreased 10-fold, whereas it was increased 6-fold for [Sar1,Ala8]angiotensin II and 20-fold for CGP42112A. These pharmacological changes were associated with a major defect in transmembrane signaling, since angiotensin II was unable to stimulate inositol phosphate production and increase cytosolic Ca2+ concentration through the two mutated receptors, whereas a clear dose-dependent stimulation was observed in cells expressing the wild-type receptor. Angiotensin II was able to promote DNA synthesis through the wild type but not through the mutated receptors. These data indicate that the conserved Asp74 residue of the AT1a receptor is important for the binding of angiotensin II antagonists and is essential for the transmembrane signaling cascade.

Epstein-Barr virus-induced genes: first lymphocyte-specific G protein-coupled peptide receptors

Since Epstein-Barr virus (EBV) infection of Burkitt's lymphoma (BL) cells in vitro reproduces many of the activation effects of EBV infection of primary B lymphocytes, mRNAs induced in BL cells have been cloned and identified by subtractive hybridization. Nine genes encode RNAs which are 4- to > 100-fold more abundant after EBV infection. Two of these, the genes for CD21 and vimentin, were previously known to be induced by EBV infection. Five others, the genes for cathepsin H, annexin VI (p68), serglycin proteoglycan core protein, CD44, and the myristylated alanine-rich protein kinase C substrate (MARCKS), are genes which were not previously known to be induced by EBV infection. Two novel genes, EBV-induced genes 1 and 2 (EBI 1 and EBI 2, respectively) can be predicted from their cDNA sequences to encode G protein-coupled peptide receptors. EBI 1 is expressed exclusively in B- and T-lymphocyte cell lines and in lymphoid tissues and is highly homologous to the interleukin 8 receptors. EBI 2 is most closely related to the thrombin receptor. EBI 2 is expressed in B-lymphocyte cell lines and in lymphoid tissues but not in T-lymphocyte cell lines or peripheral blood T lymphocytes. EBI 2 is also expressed at lower levels in a promyelocytic and a histiocytic cell line and in pulmonary tissue. These predicted G protein-coupled peptide receptors are more likely to be mediators of EBV effects on B lymphocytes or of normal lymphocyte functions than are genes previously known to be up-regulated by EBV infection.