Intratubular Renin-Angiotensin System in Hypertension

Physiology and Pathophysiology of the Intrarenal Renin-Angiotensin System: An Update

The renin-angiotensin system (RAS) has a pivotal role in the maintenance of extracellular volume homeostasis and blood pressure through complex mechanisms. Apart from the well known systemic RAS, occurrence of a local RAS has been documented in multiple tissues, including the kidney. A large body of recent evidence from pharmacologic and genetic studies, particularly those using various transgenic approaches to manipulate intrarenal levels of RAS components, has established the important role of intrarenal RAS in hypertension. Recent studies have also begun to unravel the molecular mechanisms that govern intrarenal RAS activity. This local system is under the control of complex regulatory networks consisting of positive regulators of (pro)renin receptor, Wnt/β-catenin signaling, and PGE₂/PGE₂ receptor EP₄ subtype, and negative regulators of Klotho, vitamin D receptor, and liver X receptors. This review highlights recent advances in defining the regulation and function of intrarenal RAS as a unique entity separate from systemic angiotensin II generation.

The succinate receptor as a novel therapeutic target for oxidative and metabolic stress-related conditions

The succinate receptor (also known as GPR91) is a G protein-coupled receptor that is closely related to the family of P2Y purinoreceptors. It is expressed in a variety of tissues, including blood cells, adipose tissue, the liver, retina, and kidney. In these tissues, this receptor and its ligand succinate have recently emerged as novel mediators in local stress situations, including ischemia, hypoxia, toxicity, and hyperglycemia. Amongst others, the succinate receptor is involved in recruitment of immune cells to transplanted tissues. Moreover, it was shown to play a key role in the development of diabetic retinopathy. However, most prominently, the role of locally increased succinate levels and succinate receptor activation in the kidney, stimulating the systemic and local renin-angiotensin system, starts to unfold: the succinate receptor is a key mediator in the development of hypertension and possibly fibrosis in diabetes mellitus and metabolic syndrome. This makes the succinate receptor a promising drug target to counteract or prevent cardiovascular and fibrotic defects in these expanding disorders. Recent development of SUCNR1-specific antagonists opens novel possibilities for research in models for these disorders and may eventually provide novel opportunities for the treatment of patients.

Succinate receptors in the kidney

The G protein-coupled succinate and α-ketoglutarate receptors are closely related to the family of P2Y purinoreceptors. Although the α-ketoglutarate receptor is almost exclusively expressed in the kidney, its function is unknown. In contrast, the succinate receptor, SUCRN1, is expressed in a variety of tissues, including blood cells, adipose tissue, liver, retina, and the kidney. Recent evidence suggests SUCRN1 and its succinate ligand are novel detectors of local stress, including ischemia, hypoxia, toxicity, and hyperglycemia. Local levels of succinate in the kidney also activate the renin-angiotensin system and together with SUCRN1 may play a key role in the development of hypertension and the complications of diabetes mellitus, metabolic disease, and liver damage. This makes the succinate receptor a promising drug target to counteract an expanding number of interrelated disorders.

Regulation of angiotensin converting enzyme II by angiotensin peptides in human cardiofibroblasts

Numerous studies have suggested that angiotensin peptides modulate the expression of angiotensin converting enzyme II (ACE2) in the cardiovascular system, but the molecular mechanisms remain poorly understood. In the present study, human cardiac fibroblasts (HCF) were used to test the regulatory effects of angiotensin II (Ang II) and angiotensin-(1-7) [Ang-(1-7)] on ACE2 expression. The results show that Ang II upregulates ACE2 expression. This action is modulated through activation of Ang II type 1 receptor (AT1R). Ang II-mediated ACE2 upregulation was blocked by antagonists of AT1R and ERK-MAPK signaling pathways. Additionally, Ang-(1-7) increased ACE2 expression, and this upregulation was inhibited by Ang-(1-7) Mas receptor blockade. Our results further reveal that the activation of p-ERK1/2 proteins plays a critical role in upregulating ACE2 in Ang-(1-7)-stimulated HCF cells. This effect occurs independently of the Ang II-AT1R signaling pathway. In conclusion, we propose that Ang II-upregulated ACE2 may increase Ang-(1-7) formation from Ang II, and that ACE2 expression is further enhanced by Ang-(1-7) in a positive feedback loop.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Crucial role of Rho-nuclear factor-kappaB axis in angiotensin II-induced renal injury

This study was performed to determine the effectiveness of the Rho kinase inhibitor and NF-kappaB inhibitor in renal injury of ANG II-infused hypertensive rats. Male Sprague-Dawley rats, maintained on a normal diet, received either a sham operation (n = 7) or continuous ANG II infusion (120 ng/min) subcutaneously via minipumps. The ANG II-infused rats were further subdivided into three subgroups (n = 7 each) to receive one of the following treatments during the entire period: vehicle, Rho kinase inhibitor (fasudil; 3 mg.kg(-1).day(-1) ip), or NF-kappaB inhibitor (parthenolide; 1 mg.kg(-1).day(-1) ip). After 12 days of ANG II infusion, systolic blood pressure (BP; 208 +/- 7 vs. 136 +/- 3 mmHg), Rho kinase activity, NF-kappaB activity, renal ANG II contents (160 +/- 25 vs. 84 +/- 14 pg/g), monocytic chemotactic protein (MCP) 1 mRNA, interstitial macrophage infiltration, transforming growth factor-beta1 (TGF-beta1) mRNA, interstitial collagen-positive area, urinary protein excretion (43 +/- 6 vs. 11 +/- 2 mg/day), and urinary albumin excretion were significantly enhanced compared with the Sham group. While fasudil or parthenolide did not alter systolic BP (222 +/- and 190 +/- 21, respectively), both treatments completely blocked ANG II-induced enhancement of NF-kappaB activity, renal ANG II contents (103 +/- 11 and 116 +/- 21 pg/g, respectively), MCP1 mRNA, interstitial macrophage infiltration, TGF-beta1 mRNA, interstitial collagen-positive area, urinary protein excretion (28 +/- 6 and 23 +/- 3 mg/day, respectively), and urinary albumin excretion. Importantly, parthenolide did not alter ANG II-induced Rho kinase activation although fasudil abolished ANG II-induced Rho kinase activation. These data indicate that the Rho-NF-kappaB axis plays crucial roles in the development of ANG II-induced renal injury independently from BP regulation.