Angiotensinogen in essential hypertension: from genetics to nephrology

JTT-654, an 11-beta hydroxysteroid dehydrogenase type 1 inhibitor, improves hypertension and diabetic kidney injury by suppressing angiotensinogen production

11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) plays an important role in regulating the expression of glucocorticoid actions in target tissues. Overexpression of 11β-HSD1 in mouse adipose tissue causes a metabolic syndrome-like phenotype, leading to hypertension. Although, many 11β-HSD1 inhibitors have been studied, few have shown a clear ameliorative effect against hypertension. We investigated whether JTT-654, a novel 11β-HSD1 inhibitor, ameliorated hypertension and elucidated the underlying mechanisms. JTT-654 showed inhibitory effects on angiotensinogen production in cortisone-treated 3T3-L1 adipocytes and in a rat model. JTT-654 improved hypertension not only in cortisone-treated rats and spontaneously hypertensive rats (SHR), but also in SHR/NDmcr-cp rats. In the SHR study, JTT-654 and losartan showed the same degree of antihypertensive efficacy. In addition, JTT-654 ameliorated diabetic nephropathy by suppressing renal angiotensinogen production in SHR/NDmcr-cp rats. These effects of JTT-654 were independent of its insulin-sensitizing effects, and similar effects were not observed for pioglitazone, an insulin sensitizer. Moreover, JTT-654 did not affect normotension or hypothalamus-pituitary-adrenal (HPA) axis function in normal Sprague-Dawley rats. Our results indicate that JTT-654 ameliorates hypertension and diabetic nephropathy by inhibiting 11β-HSD1 in the adipose tissue, liver, and kidney.

Sirtuin 6 protects against podocyte injury by blocking the renin-angiotensin system by inhibiting the Wnt1/β-catenin pathway

Sirtuins (Sirts) are a family of nicotinamide adenine dinucleotide-dependent protein deacetylases that share diverse cellular functions. Increasing evidence shows that Sirts play a critical role in podocyte injury, which is a major determinant of proteinuria-associated renal disease. Membranous nephropathy (MN) is a typical glomerular disease in which podocyte damage mediates proteinuria development. In this study we investigated the molecular mechanisms underlying the regulatory roles of Sirt in podocyte injury in MN patients, rats with cationic bovine serum albumin (CBSA)-induced MN and zymosan activation serum (ZAS)-stimulated podocytes. Compared with healthy controls, MN patients showed significant reduction in intrarenal Sirt1 and Sirt6 protein expression. In CBSA-induced MN rats, significant reduction in intrarenal Sirt1, Sirt3 and Sirt6 protein expression was observed. However, only significant decrease in Sirt6 protein expression was found in ZAS-stimulated podocytes. MN patients showed significantly upregulated protein expression of Wnt1 and β-catenin and renin-angiotensin system (RAS) components in glomeruli. CBSA-induced MN rats exhibited significantly upregulated protein expression of intrarenal Wnt1 and β-catenin and their downstream gene products as well as RAS components. Similar results were observed in ZAS-stimulated podocytes. In ZAS-stimulated podocytes, treatment with a specific Sirt6 activator UBCS039 preserved the protein expression of podocin, nephrin and podocalyxin, accompanied by significant inhibition of the protein expression of β-catenin and its downstream gene products, including Snail1 and Twist; treatment with a β-catenin inhibitor ICG-001 significantly preserved the expression of podocyte-specific proteins and inhibited the upregulation of downstream β-catenin gene products accompanied by significant suppression of the protein expression of RAS components. Thus, we demonstrate that Sirt6 ameliorates podocyte injury by blocking RAS signalling via the Wnt1/β-catenin pathway. Sirt6 is a specific therapeutic target for the treatment of podocyte damage-associated renal disease.

Effect of Genetic and Nongenetic Factors on the Clinical Response to Mineralocorticoid Receptor Antagonist Therapy in Egyptians with Heart Failure

This prospective cohort study evaluated the association between the renin angiotensin aldosterone system genotypes and response to spironolactone in 155 Egyptian patients with heart failure with reduced ejection fraction (HFrEF). Genotype frequencies for AGT rs699 were: CC = 16%, CT = 48%, and TT = 36%. Frequencies for CYP11B2 rs1799998 were: TT = 33%, TC = 50%, and CC = 17%. After 6 months of spironolactone treatment, change in the left ventricular ejection fraction (LVEF) differed by AGT rs699 (CC, 14.6%; TC, 7.9%; TT, 2.7%; P = 2.1E‐26), and CYP11B2 rs1799998 (TT, 9.1%; TC, 8.7%; CC, 1.4%; P = 0.0006) genotypes. Multivariate linear regression showed that the AGT rs699 and CYP11B2 rs1799998 polymorphisms plus baseline serum potassium explained 71% of variability in LVEF improvement (P = 0.001), 63% of variability in serum potassium increase (P = 2.25E‐08), and 39% of the variability in improvement in quality of life (P = 2.3E‐04) with spironolactone therapy. These data suggest that AGT and CYP11B2 genotypes as well as baseline serum K are predictors of spironolactone response in HFrEF.

Study of the association between the donors and recipients angiotensin-converting enzyme insertion/deletion gene polymorphism and the acute renal allograft rejection

Background: Angiotensin converting enzyme (ACE) is involved in various pathophysiological conditions including renal function. ACE levels are under genetic control.

Objectives: This study was designed to investigate the association between the donors and recipients ACE-I/D gene polymorphism and risk of acute rejection outcome in renal allograft recipients.

Patients and Methods: ACE-I/D polymorphism was determined in 200 donor-recipient pairs who had been referred to Afzalipour hospital in Kerman. ACE-I/D polymorphism was detected using polymerase chain reaction (PCR). Acute rejection (AR) during at least six months post-transplantation was defined as a 20% increase in creatinine level from the postoperative baseline in the absence of other causes of graft dysfunction which responded to antirejection therapy.

Results: The observed allele frequencies were II 9.8%, ID 35.6% and DD 44.4% in donors and II 9.8%, ID 35.1% and DD 52.7% in recipients. There were no significant association between ACE genotypes and AR episodes (OR_ID=0.96 [0.18-5.00] and OR_DD: 1.24 [0.25-6.07] for the donors) and (OR_ID: 0.29 [0.06-1.45] and OR_DD: 0.75 [0.19-2.90] for the recipients).

Conclusions: It seems that donor and recipient ACE-I/D genotype might not be a risk factor for acute renal allograft rejection. However, due to conflicting results from this and other studies, multicenter collaborative studies with more participants and concomitant evaluation of ACE polymorphism with other polymorphisms in renin–angiotensin system (RAS) are suggested to determine whether ACE genotypes are significant predictors of renal allograft rejection.

Salt-Sensitive Hypertension: Perspectives on Intrarenal Mechanisms

Salt sensitive hypertension is characterized by increases in blood pressure in response to increases in dietary salt intake and is associated with an enhanced risk of cardiovascular and renal morbidity. Although researchers have sought for decades to understand how salt sensitivity develops in humans, the mechanisms responsible for the increases in blood pressure in response to high salt intake are complex and only partially understood. Until now, scientists have been unable to explain why some individuals are salt sensitive and others are salt resistant. Although a central role for the kidneys in the development of salt sensitivity and hypertension has been generally accepted, it is also recognized that hypertension is of multifactorial origin and a variety of factors can induce, or prevent, blood pressure responsiveness to the manipulation of salt intake. Excess salt intake in susceptible persons may also induce inappropriate central and sympathetic nervous system responses and increase the production of intrarenal angiotensin II, catecholamines and other factors such as oxidative stress and inflammatory cytokines. One key factor is the concomitant inappropriate or paradoxical activation of the intrarenal renin-angiotensin system, by high salt intake. This is reflected by the increases in urinary angiotensinogen during high salt intake in salt sensitive models. A complex interaction between neuroendocrine factors and the kidney may underlie the propensity for some individuals to retain salt and develop salt-dependent hypertension. In this review, we focus mainly on the renal contributions that provide the mechanistic links between chronic salt intake and the development of hypertension.

The influence of proximity to city parks on blood pressure in early pregnancy

This study investigated the effect of proximity to city parks on blood pressure categories during the first trimester of pregnancy. This cross-sectional study included 3,416 female residents of the city of Kaunas, Lithuania, who were enrolled in the FP7 PHENOTYPE project study. The women were classified into four blood pressure categories: optimal, normal, high-normal blood pressure, and hypertension. Multinomial regression models were used to investigate the association between three women's groups with respect to the residence distances from city parks (300, >300-1,000, and >1,000 m) and four blood pressure categories. When using the optimal blood pressure as the reference group, the crude and adjusted odds ratios (OR) for normal blood pressure and for high-normal blood pressure proved to be statistically significantly higher after the inclusion of the selected covariates into the regression analysis. The probability of normal blood pressure increased by 9%, and that of high-normal blood pressure-by 14% for every 300 m increase in the distance to green spaces. The findings of this study suggest a beneficial impact of nearby city parks on blood pressure amongst 20- to 45-year-old women. This relationship has important implications for the prevention of hypertension and the reduction of hypertension-related morbidity.

Evolving concepts on regulation and function of renin in distal nephron

Sustained stimulation of the intrarenal/intratubular renin-angiotensin system in a setting of elevated arterial pressure elicits renal vasoconstriction, increased sodium reabsorption, proliferation, fibrosis, and eventual renal injury. Activation of luminal AT(1) receptors in proximal and distal nephron segments by local Ang II formation stimulates various transport systems. Augmented angiotensinogen (AGT) production by proximal tubule cells increases AGT secretion contributing to increased proximal Ang II levels and leading to spillover of AGT into the distal nephron segments, as reflected by increased urinary AGT excretion. The increased distal delivery of AGT provides substrate for renin, which is expressed in principal cells of the collecting tubule and collecting ducts, and is also stimulated by AT(1) receptor activation. Renin and prorenin are secreted into the tubular lumen and act on the AGT delivered from the proximal tubule to form more Ang I. The catalytic actions of renin and or prorenin may be enhanced by binding to prorenin receptors on the intercalated cells or soluble prorenin receptor secreted into the tubular fluid. There is also increased luminal angiotensin converting enzyme in collecting ducts facilitating Ang II formation leading to stimulation of sodium reabsorption via sodium channel and sodium/chloride co-transporter. Thus, increased collecting duct renin contributes to Ang II-dependent hypertension by augmenting distal nephron intratubular Ang II formation leading to sustained stimulation of sodium reabsorption and progression of hypertension.

Exposure of luminal membranes of LLC-PK1 cells to ANG II induces dimerization of AT1/AT2 receptors to activate SERCA and to promote Ca2+ mobilization

ANG II is secreted into the lumens of proximal tubules where it is also synthesized, thus increasing the local concentration of the peptide to levels of potential physiological relevance. In the present work, we studied the effect of ANG II via the luminal membranes of LLC-PK(1) cells on Ca(2+)-ATPase of the sarco(endo)plasmic reticulum (SERCA) and plasma membrane (PMCA). ANG II (at concentrations found in the lumen) stimulated rapid (30 s) and persistent (30 min) SERCA activity by more than 100% and increased Ca(2+) mobilization. Pretreatment with ANG II for 30 min enhanced the ANG II-induced Ca(2+) spark, demonstrating a positively self-sustained stimulus of Ca(2+) mobilization by ANG II. ANG II in the medium facing the luminal side of the cells decreased with time with no formation of metabolites, indicating peptide internalization. ANG II increased heterodimerization of AT(1) and AT(2) receptors by 140%, and either losartan or PD123319 completely blocked the stimulation of SERCA by ANG II. Using the PLC inhibitor U73122, PMA, and calphostin C, it was possible to demonstrate the involvement of a PLC→DAG(PMA)→PKC pathway in the stimulation of SERCA by ANG II with no effect on PMCA. We conclude that ANG II triggers SERCA activation via the luminal membrane, increasing the Ca(2+) stock in the reticulum to ensure a more efficient subsequent mobilization of Ca(2+). This first report on the regulation of SERCA activity by ANG II shows a new mechanism for Ca(2+) homeostasis in renal cells and also for regulation of Ca(2+)-modulated fluid reabsorption in proximal tubules.

Blood pressure loci identified with a gene-centric array

Raised blood pressure (BP) is a major risk factor for cardiovascular disease. Previous studies have identified 47 distinct genetic variants robustly associated with BP, but collectively these explain only a few percent of the heritability for BP phenotypes. To find additional BP loci, we used a bespoke gene-centric array to genotype an independent discovery sample of 25,118 individuals that combined hypertensive case-control and general population samples. We followed up four SNPs associated with BP at our p < 8.56 × 10(-7) study-specific significance threshold and six suggestively associated SNPs in a further 59,349 individuals. We identified and replicated a SNP at LSP1/TNNT3, a SNP at MTHFR-NPPB independent (r(2) = 0.33) of previous reports, and replicated SNPs at AGT and ATP2B1 reported previously. An analysis of combined discovery and follow-up data identified SNPs significantly associated with BP at p < 8.56 × 10(-7) at four further loci (NPR3, HFE, NOS3, and SOX6). The high number of discoveries made with modest genotyping effort can be attributed to using a large-scale yet targeted genotyping array and to the development of a weighting scheme that maximized power when meta-analyzing results from samples ascertained with extreme phenotypes, in combination with results from nonascertained or population samples. Chromatin immunoprecipitation and transcript expression data highlight potential gene regulatory mechanisms at the MTHFR and NOS3 loci. These results provide candidates for further study to help dissect mechanisms affecting BP and highlight the utility of studying SNPs and samples that are independent of those studied previously even when the sample size is smaller than that in previous studies.

The association of the angiotensinogen gene with insulin sensitivity in humans: a tagging single nucleotide polymorphism and haplotype approach

The purpose of this study was to clarify the association of the angiotensinogen gene (AGT) with insulin sensitivity using single nucleotide polymorphism (SNP) and haplotype analyses in a white cohort. A candidate gene association study was conducted in white persons with and without hypertension (N = 449). Seventeen SNPs of the AGT gene and their haplotypes were analyzed for an association with homeostasis model assessment of insulin resistance (HOMA-IR). Multivariate regression model accounting for age, sex, body mass index, hypertension status, study site, and sibling relatedness was used to test the hypothesis. Nine of the 17 SNPs were significantly associated with lower HOMA-IR levels. Homozygous minor allele carriers of the most significant SNP, rs2493134 (GG), a surrogate for the gain-of-function mutation rs699 (AGT p.M268T), had significantly lower HOMA-IR levels (P = .0001) than heterozygous or homozygous major allele carriers (AG, AA). Direct genotyping of rs699 in a subset of the population showed similar results, with minor allele carriers exhibiting significantly decreased HOMA-IR levels (P = .003). Haplotype analysis demonstrated that haplotypes rs2493137A|rs5050A|rs3789678G|rs2493134A and rs2004776G|rs11122576A|rs699T|rs6687360G were also significantly associated with HOMA-IR (P = .0009, P = .02), and these results were driven by rs2493134 and rs699. This study confirms an association between the AGT gene and insulin sensitivity in white humans. Haplotype analysis extends this finding and implicates SNPs rs2493134 and rs699 as the most influential. Thus, AGT gene variants, previously shown to be associated with AGT levels, are also associated with insulin sensitivity; suggesting a relationship between the AGT gene, AGT levels, and insulin sensitivity in humans.

RAS polymorphisms in cancerous and benign breast tissue

Recent information has revealed new roles in the angiogenic processes linked to the rennin-angiotensin system. To date few studies have been done on the association between RAS genes and cancer and the majority focus mainly on angiotensin I-converting enzyme (ACE). For breast cancer there are three reports that include the angiotensin II receptor, subtype 1 (AGTR1), only one for angiotensinogen (AGT) and none for renin gene (REN). In the present study we investigate whether REN (Bgll), AGT (M235T), ACE (A245T, Indel), and AGTR1 (A1166C) are associated with breast cancer. Polymorphisms were analysed by PCR and RFPLs or sequence specific assay in three groups: breast cancer, benign breast disease (BBD) and general population. REN polymorphism shows that homozygous for A allele have an increased risk for BBD. Differences in M235T genotype frequencies were significant with less heterozygous in breast cancer. With different risk values ACE indel was associated with BBD and breast cancer. Association of AGTR1 was observed only in the breast cancer group, where C allele carriers present a reduced risk. Results of this work supports previous observations on the possible involvement of this system in breast cancer but it also suggests a role in benign disease.

Addressing the theoretical and clinical advantages of combination therapy with inhibitors of the renin-angiotensin-aldosterone system: antihypertensive effects and benefits beyond BP control

AIMS

This article reviews the importance of the renin-angiotensin-aldosterone system (RAAS) in the cardiometabolic continuum; presents the pros and cons of dual RAAS blockade with angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs); and examines the theoretical and practical benefits supporting the use of direct renin inhibitors (DRIs) in combination with ACEIs or ARBs.

MAIN METHODS

The author reviewed the literature for key publications related to the biochemical physiology of the RAAS and the pharmacodynamic effects of ACEIs, ARBs, and DRIs, with a particular focus on dual RAAS blockade with these drug classes.

KEY FINDINGS

Although ACEI/ARB combination therapy produces modest improvement in BP, it has not resulted in the major improvements predicted given the importance of the RAAS across the cardiorenal disease continuum. This may reflect the fact that RAAS blockade with ACEIs and/or ARBs leads to exacerbated renin release through loss of negative-feedback inhibition, as well as ACE/aldosterone escape through RAAS and non-RAAS-mediated mechanisms. Plasma renin activity (PRA) is an independent predictor of morbidity and mortality, even for patients receiving ACEIs and ARBs. When used alone or in combination with ACEIs and ARBs, the DRI aliskiren effectively reduces PRA. Reductions in BP are greater with these combinations, relative to the individual components alone.

SIGNIFICANCE

It is possible that aliskiren plus either an ACEI or ARB may provide greater RAAS blockade than monotherapy with ACEIs or ARBs, and lead to additive improvement in BP and clinically important outcomes.

From genetics to mechanism of disease liability

With each advance in genomic technology, new statistical methods have regularly emerged to test genetic hypotheses in complex inheritance, as evidenced throughout this book. Notwithstanding the approach used, the greatest challenge in the genetics of complex traits remains the identification of the gene(s) and the molecular variant(s) accounting for a genetic inference based on statistical testing. We take the example of quantitative trait locus (QTL) mapping for blood pressure (BP) and related phenotypes in rodents to review the current landscape. Traditional approaches to refined mapping are typically hampered by the small effect and the small proportion of the variance attached to individual QTLs. The alternative of functional screens in intact animals, whether by chemical mutagenesis or gene targeting, remains a daunting undertaking. Such limitations account for the slow progress to date of inferences from QTL to gene(s). We select a QTL for differential sodium sensitivity between two mouse inbred lines to propose an approach that can be used in relatively large genomic regions (1) by optimizing the selection of candidate genes and (2) by subjecting such genes to high-throughput functional screens. While this is still work in progress, we think it abundantly illustrates what is ahead of us in delineating genetic variation that underlie complex disease.

The Role of the Kidneys in Hypertension

The devastating long-term consequences of high blood pressure include stroke, heart disease, atherosclerosis, renal disease, and other end-organ damage. From a physiologic perspective, it is not apparent why the propensity for hypertension is so widespread in the general population. Clearly, an adequate arterial pressure is essential for perfusion of the tissues to provide adequate oxygenation and nutrition to the brain and other critical organs. Although the various microcirculatory beds have the capability to adjust vascular resistance to autoregulate blood flow, systemic arterial pressure is usually maintained at levels greater than required for requisite tissue perfusion. The myriad of neurohumoral mechanisms designed to protect against decreases in systemic arterial pressure provide a reserve capacity for increased perfusion when there are increased tissue demands. The unfortunate consequence of having these powerful physiologic control mechanisms is that they may be inappropriately activated in certain circumstances or by genetically determined traits, leading to hypertension and cardiovascular injury. Evidence continues to accumulate indicating that the kidney not only is victim to hypertension-related injury, but also contributes as a villain to the hypertensinogenic process.

Intrarenal renin-angiotensin system and counteracting protective mechanisms in angiotensin II-dependent hypertension

It is now well accepted that alterations in kidney function, due either to primary renal disease or to inappropriate hormonal influences on the kidney, are a cardinal characteristic in all forms of hypertension, and lead to a reduced ability of the kidneys to excrete sodium and the consequent development of elevated arterial pressures. However, it is also apparent that many extrarenal factors are important contributors to altered kidney function and hypertension. Central to many hypertensinogenic processes is the inappropriate activation of the renin-angiotensin system (RAS) and its downstream consequences by various pathophysiologic mechanisms. There may also be derangements in arachidonic acid metabolites, endothelium derived factors such as nitric oxide and carbon monoxide, and various paracrine and neural systems that normally interact with or provide a counteracting balance to the actions of the RAS. Thus, when the capacity of the kidneys to maintain sodium balance and extracellular fluid volume within appropriate ranges is compromised, increases in arterial pressure become necessary to re-establish normal balance.

Genetic susceptibility to essential hypertension: insight from angiotensinogen

Although progress in the genetics of essential hypertension may seem disappointing, it has considerable potential in defining research directions that will ultimately translate into clinical practice. The hypothesis that genetic variation at the angiotensinogen locus impacts on individual susceptibility to develop essential hypertension has motivated a substantial body of research by us and many others. We examine how analyses of the mechanisms by which variation in angiotensinogen expression may contribute to disease susceptibility and may have arisen in human populations have progressed in recent years. Although the objective of personalized medicine is still in the future, a genetic hypothesis based on human variation can uniquely empower functional genomics approaches to reach such an ultimate goal.

Polymerization of human angiotensinogen: insights into its structural mechanism and functional significance

In the present study, we have investigated the in vitro polymerization of human plasma AGT (angiotensinogen), a non-inhibitory member of the serpin (SERine Protease INhibitor) family. Polymerization of AGT is thought to contribute to a high molecular mass form of the protein in plasma that is increased in pregnancy and pregnancy-associated hypertension. The results of the present study demonstrate that the polymerization of AGT occurs through a novel mechanism which is primarily dependent on non-covalent linkages, while additional disulfide linkages formed after prolonged incubation are not essential for either formation or stability of polymers. We present the first analyses of AGT polymers by electron microscopy, CD spectroscopy, stability assays and sensitivity to proteinases and we conclude that their structure differs from the 'loop-sheet' polymers typical of inhibitory serpins. Histidine residues within the unique N-terminal extension of AGT appear to influence polymer formation, although polymer formation can still take place after their removal by renin. At a functional level, we show that AGT polymers are not substrates for renin, so polymerization of AGT in plasma would predictably lead to decreased formation of AngI (angiotensin I) with blood pressure lowering. Polymerization may therefore be an appropriate response to hypertension. The ability of AGT to protect its renin cleavage site through polymerization may explain why the AngI decapeptide has remained linked to the large and apparently inactive serpin body throughout evolution.

Expressional and epigenetic alterations of placental serine protease inhibitors: SERPINA3 is a potential marker of preeclampsia

Preeclampsia is the major pregnancy-induced hypertensive disorder. It modifies the expression profile of placental genes, including several serine protease inhibitors (SERPINs). The objective of this study was to perform a systematic expression analysis of these genes in normal and pathological placentas and to pinpoint epigenetic alterations inside their promoter regions. Expression of 18 placental SERPINs was analyzed by quantitative RT-PCR on placentas from pregnancies complicated by preeclampsia, intrauterine growth restriction, or both and was compared with normal controls. SERPINA3, A5, A8, B2, B5, and B7 presented significant differences in expression in >or=1 pathological situation. In parallel, the methylation status of the CpG islands located in their promoter regions was studied on a sample of control and preeclamptic placentas. Ten SERPIN promoters were either totally methylated or totally unmethylated, whereas SERPINA3, A5, and A8 presented complex methylation profiles. For SERPINA3, the analysis was extended to 81 samples and performed by pyrosequencing. For the SERPINA3 CpG island, the average methylation level was significantly diminished in preeclampsia and growth restriction. The hypomethylated CpGs were situated at putative binding sites for developmental and stress response (hypoxia and inflammation) factors. Our results provide one of the first observations of a specific epigenetic alteration in human placental diseases and provide new potential markers for an early diagnosis.

The molecular basis of pediatric hypertension

Blood pressure, the product of cardiac output and peripheral vascular resistance, follows a circadian rhythm and is altered by a host of circulating and local substances and by many physiologic events. The number of genes, signaling pathways, and systems involved in blood pressure regulation is enormous, and dissecting those factors that are most important in hypertension has proven challenging. This article discusses molecular mechanisms of hypertension in several conditions in which mutations in a single gene give rise to hypertension and then considers the contribution of these and other genes to essential hypertension.

Relationship of ENOS and RAS Gene Polymorphisms to Initial Peritoneal Transport Status in Peritoneal Dialysis Patients

BACKGROUND

Peritoneal membrane permeability is of major importance for adequate dialysis and fluid balance in peritoneal dialysis (PD) therapy. The peritoneal capillary endothelium plays a key role in peritoneal transport. Nitric oxide derived from endothelial cells is related to the maintenance of vascular permeability. We investigated the relationship between the endothelial nitric oxide synthase (ENOS) gene polymorphism, the renin-angiotensin system (RAS) gene polymorphisms, and initial peritoneal transport type in PD patients.

METHODS

This study included 74 incident continuous ambulatory PD patients. The ENOS gene polymorphism was identified at the 4a/b variable number of tandem repeats in intron 4. Genetic polymorphisms of the renin-angiotensin system were performed for the angiotensin-converting enzyme I/D, angiotensinogen M235T, and angiotensin II type 1 receptor A1166C and type 2 receptor C3123A by polymerase chain reaction. Patients were divided into two groups according to the initial peritoneal equilibration test results performed within 3 months of PD therapy: group 1 consisted of high/high average transporters (n = 41), and group 2 consisted of low/low average transporters (n = 33).

RESULTS

Demographic, clinical, and laboratory data were similar between the two groups (p > 0.05). Group 1 had a significantly higher prevalence of the ENOS b/b genotype than group 2 (78% vs. 48.5%, p < 0.008). In contrast, group 2 had a significantly greater prevalence of the ENOS a/a+a/b genotype than group 1 (51.5% vs. 22%, p < 0.008). Genetic polymorphisms of the renin-angiotensin system were not associated with initial peritoneal transport type (p > 0.05).

CONCLUSIONS

Modulation of the nitric oxide activity via the ENOS a/b polymorphism may have a considerable effect on the basal peritoneal permeability.

Renin–angiotensin system gene polymorphisms: association with susceptibility to Henoch–Schonlein purpura and renal involvement

The clinical course of Henoch-Schönlein Purpura (HSP) in children is variable, with some patients having a much more rapidly progressing course than others. We investigated whether polymorphisms of the renin-angiotensin system (RAS) genes are involved in HSP. Three RAS genotypes were examined in 114 children with HSP and in 164 healthy children: the angiotensin I converting enzyme (ACE) insertion/deletion polymorphism, the M235T mutation in the angiotensinogen gene (Agt), and the A1166C in the angiotensin II type I receptor (AT1R) gene. Significant differences were observed between HSP patients and control group in the frequency of ACE and Agt genotypes (p=0.004 and p=0.003, respectively). The TT genotype of Agt gene was associated with a 3.5-fold increased risk for Henoch-Schönlein nephritis (HSN) compared with the MM/MT genotype (odds ratio, 3.5; 95% confidence interval, 1.2-10.4). There was a trend to a higher prevalence of the TT genotype of the Agt gene among patients with nephrotic range proteinuria when compared to the patients with mild proteinuria, although the difference did not reach a statistical significance. The results of this study suggest that polymorphisms of ACE gene and Agt gene likely influence the risk of developing HSP. However, among the three genes of the RAS studies, only Agt gene was associated with the susceptibility to HSN. RAS gene polymorphisms studied are not associated with the presence of nephrotic range proteinuria. Additional studies are warranted to verify the correlation between RAS gene polymorphisms and susceptibility to HSP.

Renin-angiotensin system gene polymorphisms predict the progression to renal insufficiency among Asians with lupus nephritis

The renin-angiotensin system (RAS) is a strong candidate as a mediator for the development and progression of lupus nephritis (LN). We performed an ethnically stratified analysis of 642 systemic lupus erythematosus (SLE) patients to determine whether various functional RAS gene polymorphisms are associated with SLE renal outcomes. Patients were genotyped for two angiotensin-converting enzyme (ACE) gene polymorphisms: Alu insertion/deletion (I/D) and 23 949 (CT)(2/3), and for two angiotensinogen (Atg) gene polymorphisms: M235T and C-532T. Multivariate analyses demonstrated associations between the ACE I/D, ACE (CT)(2/3) and Atg C-532T functional polymorphisms and LN among Asians. In stratified analyses among LN cases according to high vs low glomerular filtration rate (GFR), associations remained significant for the ACE D (odds ratio (OR) 5.9, P=0.001) and (CT)(2) (OR 6.2, P=0.001) alleles among Asian subjects with low GFR. Lastly, we found allelic dose-dependent associations between the ACE I/D (P=0.003), ACE (CT)(2/3) (P=0.005) and Atg M235T (P=0.04) polymorphisms, and GFR analyzed as a continuous variable among Asians. These findings suggest a significant role for ACE and Atg gene sequence variation and severity of LN among Asians with SLE.

Association of the genetic polymorphisms of the renin-angiotensin system and endothelial nitric oxide synthase with chronic renal transplant dysfunction

BACKGROUND

Chronic allograft dysfunction (CAD) is a complex phenomenon caused by underlying kidney disease and superimposed environmental and genetic factors. We investigated the association of polymorphisms in the genes for angiotensin-converting enzyme (ACE), angiotensinogen (AGT), angiotensin II receptor type 1 (ATR1) and type 2 (ATR2), and endothelial nitric oxide synthase (ENOS) with the initiation of CAD.

METHODS

Genotyping was performed in 125 patients who underwent renal transplantation during a 5-year period for the ACE I/D, AGT M235T, ATR1 A1166C, ATR2 C3123A, and ENOS intron 4a/b gene polymorphisms. The following information was collected for each case: date of transplantation, age and sex of donor and recipient, donor type, cold ischemia time, number of human leukocyte antigen mismatches, number of acute rejection episodes, and laboratory findings at discharge from hospital and annual rechecks. Blood pressure was measured at yearly intervals throughout follow-up.

RESULTS

The proportions of the genotypes were ACE II/ID/DD 12%, 33.6%, 54.4%; AGT MM/MT/TT 33%, 65.2%, 1.9%; ATR1 AA/AC/CC 68.6%, 30.7%, 0.7%; ATR2 CC/CA/AA 57.9%, 27.5%, 14.4%; and ENOS aa/ab/bb 6.4%, 22%, 71.6%, respectively. Statistical analysis of the major risk factors for the initiation of CAD showed that ACE DD genotype, cadaveric donor type, and level of proteinuria at 1 year posttransplantation were associated with poorer renal function. The graft function was not affected by AGT, ATR1, ATR2, and ENOS gene polymorphisms.

CONCLUSIONS

These findings suggest that the DD variant of the ACE gene polymorphism is associated with increased risk of developing CAD.

Why are angiotensin concentrations so high in the kidney?

PURPOSE OF REVIEW

Recent studies have reported that intrarenal angiotensin II content and angiotensin II concentrations in the proximal tubular fluid and renal interstitial fluid are much greater than the circulating angiotensin II levels. These high intrarenal angiotensin II levels are responsible for regulating renal hemodynamics and tubular transport.

RECENT FINDINGS

Intrarenal angiotensin II levels have been assessed from total tissue contents as well as renal interstitial fluid and proximal tubular fluid concentrations. Total tissue contents expressed per gram of tissue weight are greater than plasma angiotensin II concentrations; tubular fluid concentrations and renal interstitial fluid concentrations are even greater in the range of 3-10 pmoles/ml. In hypertensive states, there is also an increased intracellular accumulation of angiotensin II mediated by angiotensin type 1 receptor-dependent endocytosis. The high intrarenal angiotensin II levels are also caused by the presence of angiotensinogen messenger RNA and protein in the proximal tubule cells. Furthermore, there is positive amplification by which increases in circulating angiotensin II stimulate increased production and secretion of angiotensinogen, which is also manifested as an increased urinary excretion rate.

SUMMARY

The ability of the kidney to generate high intratubular and interstitial concentrations allows the kidney to regulate intrarenal levels in accord with the homeostatic needs for the regulation of renal hemodynamics and tubular reabsorption and the regulation of sodium balance. When inappropriately stimulated, high intrarenal angiotensin II levels contribute to excessive salt and water retention, the development of hypertension, and long-term proliferative effects leading to renal injury.

Angiotensin-converting enzyme genotype is a predictive factor in the peak panel-reactive antibody response

The presence of a high panel-reactive antibody (PRA) level represents an independent risk factor for early graft failure and chronic allograft dysfunction. It has also been reported that patients with the ACE-DD and AGT-AA genotypes display poorer chronic allograft function. We investigated the effects of gene polymorphisms of the renin angiotensin system (RAS) on anti-HLA antibody production among renal transplant candidates. Genotyping was performed on 133 dialysis patients for the ACE (I/D) and AGT (M235T) as well as the type 1 (A1166C) and type 2 (A1223G) angiotensin II receptor genes. Patients with a peak PRA >/= 30% were considered to be positive for anti-HLA antibody (40.6% of 133 patients). Genetic polymorphisms of the RAS were not associated with anti-HLA antibody production at this PRA level. Another analysis comparing the 29 patients with a peak PRA >/=50% with the 104 patients with a peak PRA <50% showed that previous transplants, the presence of ACE-DD genotype, history of blood transfusions, and dialysis duration were all associated with the high levels of antibody production by univariant analysis. A multivariate analysis using a logistic regression model revealed previous transplants, the presence of ACE-DD genotype, and history of blood transfusions to be predictors of anti-HLA antibody production. The ACE-DD genotype is an important risk factor for higher PRA levels. This study suggests that genetic control of RAS activity correlates with production of anti-HLA antibodies, possibly explaining the relationship to chronic allograft outcome.

Transgenic amplification of glucocorticoid action in adipose tissue causes high blood pressure in mice

Obesity is closely associated with the metabolic syndrome, a combination of disorders including insulin resistance, diabetes, dyslipidemia, and hypertension. A role for local glucocorticoid reamplification in obesity and the metabolic syndrome has been suggested. The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) regenerates active cortisol from inactive 11-keto forms, and aP2-HSD1 mice with relative transgenic overexpression of this enzyme in fat cells develop visceral obesity with insulin resistance and dyslipidemia. Here we report that aP2-HSD1 mice also have high arterial blood pressure (BP). The mice have increased sensitivity to dietary salt and increased plasma levels of angiotensinogen, angiotensin II, and aldosterone. This hypertension is abolished by selective angiotensin II receptor AT-1 antagonist at a low dose that does not affect BP in non-Tg littermates. These findings suggest that activation of the circulating renin-angiotensin system (RAS) develops in aP2-HSD1 mice. The long-term hypertension is further reflected by an appreciable hypertrophy and hyperplasia of the distal tubule epithelium of the nephron, resembling salt-sensitive or angiotensin II-mediated hypertension. Taken together, our findings suggest that overexpression of 11beta-HSD1 in fat is sufficient to cause salt-sensitive hypertension mediated by an activated RAS. The potential role of adipose 11beta-HSD1 in mediating critical features of the metabolic syndrome extends beyond obesity and metabolic complications to include the most central cardiovascular feature of this disorder.

Intrarenal angiotensin II and hypertension

Elevations in intrarenal angiotensin II (Ang II) cause reductions in renal function and sodium excretion that contribute to progressive hypertension and lead to renal and vascular injury. Augmentation of intrarenal Ang II occurs by several processes, leading to levels much greater than can be explained from the circulating levels. In Ang II-dependent hypertension, Ang II is internalized via an AT1 receptor mechanism, but there is also sustained intrarenal production of Ang II. Ang II exerts a positive feedback action on intrarenal angiotensinogen (AGT) mRNA and protein. The increased intrarenal AGT production is associated with increased intrarenal and intracellular Ang II contents and urinary AGT excretion rates. The increased urinary AGT indicates spillover of AGT into distal nephron segments supporting enhanced distal Ang II formation and sodium reabsorption. The augmentation of intrarenal Ang II provides the basis for sustained actions on renal function, sodium excretion, and maintenance of hypertension.

Dopamine and the kidney: a role in hypertension?

PURPOSE OF REVIEW

Defective transduction of the dopamine receptor signal in the kidney has been shown to be important in the pathogenesis of hypertension This review will discuss the genetic mechanism for the defective renal dopaminergic function and the interaction with other gene variant products in the pathogenesis of salt sensitivity and essential hypertension.

RECENT FINDINGS

Single nucleotide polymorphisms of G protein-coupled receptor kinase type 4 (GRK4) phosphorylate, desensitize, and diminish the inhibitory action of D receptors on sodium transport in the kidney. Inhibition of GRK4 expression normalizes renal proximal tubule D receptor function in humans and rodents and ameliorates the hypertension in genetically hypertensive rats. Expression of the GRK4 variant, GRK4gammaA142V, produces hypertension and impairs the natriuretic effect of D receptor stimulation in mice. In humans, GRK4 single nucleotide polymorphisms are associated with essential hypertension, particularly salt sensitive hypertension. The prediction of the hypertensive phenotype is most accurate when elements of the renin-angiotensin system and GRK4 are included in the analysis.

SUMMARY

GRK4 single nucleotide polymorphisms, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic function of angiotensin II type 1 receptors to predominate, may be responsible for salt sensitivity. Hypertension develops with additional perturbations caused by the variants of other genes (e.g., alpha-adducin, angiotensin converting enzyme, angiotensinogen, angiotensin II type 1 receptor, aldosterone synthase, 11beta-hydroxysteroid dehydrogenase type 2), the quantitative interaction of which may vary depending upon the genetic background.

Paternal age and preeclampsia

BACKGROUND

Paternal aging is associated with premeiotic damage to spermatogonia, a mechanism by which new point mutations are introduced into the gene pool. We hypothesized that paternal age might contribute to preeclampsia.

METHODS

We studied the incidence of preeclampsia in 81,213 deliveries surveyed in 1964-1976 in the Jerusalem Perinatal Study. We controlled for maternal age, parity and other risk factors using logistic regression.

RESULTS

Preeclampsia was reported in 1303 deliveries (1.6%). Compared with fathers age 25-34 years, the odds ratios (ORs) for preeclampsia were 1.24 (95% confidence interval = 1.05-1.46) for age 35-44 and 1.80 (1.40-2.31) for age 45+. For fathers age <25, the OR was 1.25 (1.04-1.51). Although weaker than maternal age effects, paternal effects were consistent within subgroups of other variables.

CONCLUSIONS

These findings support the hypothesis that a modest proportion of preeclampsia might be explained by new mutations acquired from fathers and add to a growing body of evidence for paternal age effects in birth defects, neuropsychiatric disease and neoplasia.

Epithelial sodium channel and the control of sodium balance: interaction between genetic and environmental factors

The epithelial sodium channel (ENaC) expressed in aldosterone-responsive epithelial cells of the kidney and colon plays a critical role in the control of sodium balance, blood volume, and blood pressure. In lung, ENaC has a distinct role in controlling the ionic composition of the air-liquid interface and thus the rate of mucociliary transport. Loss-of-function mutations in ENaC cause a severe salt-wasting syndrome in human pseudohypoaldosteronism type 1 (PHA-1). Gain-of-function mutations in ENaC beta and gamma subunits cause pseudoaldosteronism (Liddle's syndrome), a severe form of salt-sensitive hypertension. This review discusses genetically defined forms of a salt sensitivity and salt resistance in human monogenic diseases and in animal models mimicking PHA-1 or Liddle's syndrome. The complex interaction between genetic factors (ENaC mutations) and the risk factor (salt intake) can now be studied experimentally. The role of single-nucleotide polymorphisms (SNPs) in determining salt sensitivity or salt resistance in general populations is one of the main challenges of the post-genomic era.