Na+-Retaining Action of COX-2 (Cyclooxygenase-2)/EP1 Pathway in the Collecting Duct via Activation of Intrarenal Renin-Angiotensin-Aldosterone System and Epithelial Sodium Channel

Principle role of the (pro)renin receptor system in cardiovascular and metabolic diseases: An update

(Pro)renin receptor (PRR), along with its soluble form, sPRR, functions not only as a crucial activator of the local renin-angiotensin system but also engages with and activates various angiotensin II-independent signaling pathways, thus playing complex and significant roles in numerous physiological and pathophysiological processes, including cardiovascular and metabolic disorders. This article reviews current knowledge on the intracellular partners of the PRR system and explores its physiological and pathophysiological impacts on cardiovascular diseases as well as conditions related to glucose and lipid metabolism, such as hypertension, heart disease, liver disease, diabetes, and diabetic complications. Targeting the PRR system could emerge as a promising therapeutic strategy for treating these conditions. Elevated levels of circulating sPRR might indicate the severity of these diseases, potentially serving as a biomarker for diagnosis and prognosis in clinical settings. A comprehensive understanding of the functions and regulatory mechanisms of the PRR system could facilitate the development of novel therapeutic approaches for the prevention and management of cardiovascular and metabolic diseases.

Kidney Renin-Angiotensin System: Lost in a RAS Cascade

Renin was discovered more than a century ago. Since then, the functions of the renin-angiotensin system in the kidney have been the focus of intensive research revealing its importance in regulation of renal physiology and in the pathogenesis of heart, vascular, and kidney diseases. Inhibitors of renin-angiotensin system components are now foundational therapies for a range of kidney and cardiovascular diseases from hypertension to heart failure to diabetic nephropathy. Despite years of voluminous research, emerging studies continue to reveal new complexities of the regulation of the renin-angiotensin system within the kidney and identification of nonclassical components of the system like the prorenin receptor (PRR) and ACE2 (angiotensin-converting enzyme 2), with powerful renal effects that ultimately impact the broader cardiovascular system. With the emergence of a range of novel therapies for cardiovascular and kidney diseases, the importance of a detailed understanding of the renin-angiotensin system in the kidney will allow for the development of informed complementary approaches for combinations of treatments that will optimally promote health and longevity over the century ahead.

Extra-adrenal aldosterone: a mini review focusing on the physiology and pathophysiology of intrarenal aldosterone

PURPOSE

Accumulating evidence has demonstrated the existence of extra-adrenal aldosterone in various tissues, including the brain, heart, vascular, adipocyte, and kidney, mainly based on the detection of the CYP11B2 (aldosterone synthase, cytochrome P450, family 11, subfamily B, polypeptide 2) expression using semi-quantitative methods including reverse transcription-polymerase chain reaction and antibody-based western blotting, as well as local tissue aldosterone levels by antibody-based immunosorbent assays. This mini-review highlights the current evidence and challenges in extra-adrenal aldosterone, focusing on intrarenal aldosterone.

METHODS

A narrative review.

RESULTS

Locally synthesized aldosterone may play a vital role in various physio-pathological processes, especially cardiovascular events. The site of local aldosterone synthesis in the kidney may include the mesangial cells, podocytes, proximal tubules, and collecting ducts. The synthesis of renal aldosterone may be regulated by (pro)renin receptor/(pro)renin, angiotensin II/Angiotensin II type 1 receptor, wnt/β-catenin, cyclooxygenase-2/prostaglandin E2, and klotho. Enhanced renal aldosterone release promotes Na+ reabsorption and K+ excretion in the distal nephron and may contribute to the progress of diabetic nephropathy and salt-related hypertension.

CONCLUSIONS

Inhibition of intrarenal aldosterone signaling by aldosterone synthase inhibitors or mineralocorticoid receptor antagonists may be a hopeful pharmacological technique for the therapy of diabetic nephropathy and saltrelated hypertension. Yet, current reports are often conflicting or ambiguous, leading many to question whether extra-adrenal aldosterone exists, or whether it is of any physiological and pathophysiological significance.

Tiliroside Protects against Lipopolysaccharide-Induced Acute Kidney Injury via Intrarenal Renin-Angiotensin System in Mice

Tiliroside, a natural flavonoid, has various biological activities and improves several inflammatory diseases in rodents. However, the effect of Tiliroside on lipopolysaccharide (LPS)-induced acute kidney injury (AKI) and the underlying mechanisms are still unclear. This study aimed to evaluate the potential renoprotective effect of Tiliroside on LPS-induced AKI in mice. Male C57BL/6 mice were intraperitoneally injected with LPS (a single dose, 3 mg/kg) with or without Tiliroside (50 or 200 mg/kg/day for 8 days). Tiliroside administration protected against LPS-induced AKI, as reflected by ameliorated renal dysfunction and histological alterations. LPS-stimulated renal expression of inflammatory cytokines, fibrosis markers, and kidney injury markers in mice was significantly abolished by Tiliroside. This flavonoid also stimulated autophagy flux but inhibited oxidative stress and tubular cell apoptosis in kidneys from LPS-injected mice. Mechanistically, our study showed the regulation of Tiliroside on the intrarenal renin-angiotensin system in LPS-induced AKI mice. Tiliroside treatment suppressed intrarenal AGT, Renin, ACE, and Ang II, but upregulated intrarenal ACE2 and Ang1-7, without affecting plasma Ang II and Ang1-7 levels. Collectively, our data highlight the renoprotective action of Tiliroside on LPS-induced AKI by suppressing inflammation, oxidative stress, and tubular cell apoptosis and activating autophagy flux via the shift towards the intrarenal ACE2/Ang1-7 axis and away from the intrarenal ACE/Ang II axis.

Differential regulation of autophagy on urine-concentrating capability through modulating the renal AQP2 expression and renin-angiotensin system in mice

Autophagy, a cellular process of "self-eating," plays an essential role in renal pathophysiology. However, the effect of autophagy on urine-concentrating ability in physiological conditions is still unknown. This study aimed to determine the relevance and mechanisms of autophagy for maintaining urine-concentrating capability during antidiuresis. The extent of the autophagic response to water deprivation (WD) was different between the renal cortex and medulla in mice. Autophagy activity levels in the renal cortex were initially suppressed and then stimulated by WD in a time-dependent manner. During 48 h WD, the urine-concentrating capability of mice was impaired by rapamycin (Rapa) but not by 3-methyladenine (3-MA), accompanied by suppressed renal aquaporin 2 (AQP2), V2 receptor (V2R), renin, and angiotensin-converting enzyme (ACE) expression, and levels of prorenin/renin, angiotensin II (ANG II), and aldosterone in the plasma and urine. In contrast, 3-MA and chloroquine (CQ) suppressed the urine-concentrating capability in WD72 mice, accompanied by downregulation of AQP2 and V2R expression in the renal cortex. 3-MA and CQ further increased AQP2 and V2R expression in the renal medulla of WD72 mice. Compared with 3-MA and CQ, Rapa administration yielded completely opposite results on the above parameters in WD72 mice. In addition, 3-MA and CQ abolished the upregulation of prorenin/renin, ANG II, and aldosterone levels in the plasma and urine in WD72 mice. Taken together, our study demonstrated that autophagy regulated urine-concentrating capability through differential regulation of renal AQP2/V2R and ACE/ANG II signaling during WD.NEW & NOTEWORTHY Autophagy exhibits a double-edged effect on cell survival and plays an essential role in renal pathophysiology. We for the first time reported a novel function of autophagy that controls the urine-concentrating capability in physiological conditions. We found that water deprivation (WD) differentially regulated autophagy in the kidneys of mice in a time-dependent manner and autophagy regulates the urine-concentrating capability mainly by regulating AQP2/V2R and ACE/ANG II signaling in the renal cortex in WD mice.

The Epithelial Sodium Channel-An Underestimated Drug Target

Epithelial sodium channels (ENaC) are part of a complex network of interacting biochemical pathways and as such are involved in several disease states. Dependent on site and type of mutation, gain- or loss-of-function generated symptoms occur which span from asymptomatic to life-threatening disorders such as Liddle syndrome, cystic fibrosis or generalized pseudohypoaldosteronism type 1. Variants of ENaC which are implicated in disease assist further understanding of their molecular mechanisms in order to create models for specific pharmacological targeting. Identification and characterization of ENaC modifiers not only furthers our basic understanding of how these regulatory processes interact, but also enables discovery of new therapeutic targets for the disease conditions caused by ENaC dysfunction. Numerous test compounds have revealed encouraging results in vitro and in animal models but less in clinical settings. The EMA- and FDA-designated orphan drug solnatide is currently being tested in phase 2 clinical trials in the setting of acute respiratory distress syndrome, and the NOX1/ NOX4 inhibitor setanaxib is undergoing clinical phase 2 and 3 trials for therapy of primary biliary cholangitis, liver stiffness, and carcinoma. The established ENaC blocker amiloride is mainly used as an add-on drug in the therapy of resistant hypertension and is being studied in ongoing clinical phase 3 and 4 trials for special applications. This review focuses on discussing some recent developments in the search for novel therapeutic agents.

Revisiting the relationship between (Pro)Renin receptor and the intrarenal RAS: focus on the soluble receptor

PURPOSE OF REVIEW

The (pro)renin receptor (PRR), also termed as ATPase H+ transporting accessory protein 2 (ATP6AP2), was originally cloned as a specific receptor for prorenin and renin [together called (pro)renin]. Given the wide tissue distribution of PRR, PRR was further postulated to act as a regulator of tissue renin. However, assigning a physiological role of PRR within the renin-angiotensin system (RAS) has been challenging largely due to its pleotropic functions in regulation of embryogenesis, autophagy, and H+ transport. The current review will summarize recent advances in understanding the roles of sPPR within the intrarenal RAS as well as those outside this local system.

RECENT FINDINGS

Site-1 protease (S1P) is a predominant source of sPPR at least in the kidney. So far most of the known physiological functions of PRR including renal handling of electrolytes and fluid and blood pressure are mediated by sPRR. In particular, sPRR serves as a positive regulator of collecting duct renin to activate the intrarenal RAS during water deprivation or angiotensin-II (AngII) infusion. However, PRR/sPRR can act in renin-independent manner under other circumstances.

SUMMARY

S1P-derived sPRR has emerged as a key regulator of kidney function and blood pressure and its relationship with the intrarenal RAS depends on the physiological context.