Angiotensin II Type 1 Receptor-Associated Protein Regulates Kidney Aging and Lifespan Independent of Angiotensin

Oxidative stress and senescence in aging kidneys: the protective role of SIRT1

Aging leads to a gradual decline in kidney function, making the kidneys increasingly vulnerable to various diseases. Oxidative stress, together with cellular senescence, has been established as paramount in promoting the aging process of the kidney. Oxidative stress, defined as an imbalance between ROS formation and antioxidant defense mechanisms, has been implicated in the kidney's cellular injury, inflammation, and premature senescence. Concurrently, the accumulation of SCs in the kidney also exacerbates oxidative stress via the secretion of pro-inflammatory and tissue-damaging factors as the senescence-associated secretory phenotype (SASP). Recently, SIRT1, a nicotinamide adenine dinucleotide (NAD)-dependent deacetylase, has been pivotal in combating oxidative stress and cellular senescence in the aging kidney. SIRT1 acts as a potential antioxidant molecule through myriad pathways that influence diverse transcription factors and enzymes essential in maintaining redox homeostasis. SIRT1 promotes longevity and renal health by modulating the acetylation of cell cycle and senescence pathways. This review covers the complex relationship between oxidative stress and cellular senescence in the aging kidney, emphasizing the protective role of SIRT1. See also the graphical abstract(Fig. 1).

The central renin-angiotensin system: A genetic pathway, functional decoding, and selective target engagement characterization in humans

Accumulating evidence suggests that the brain renin angiotensin system (RAS) plays a pivotal role in the regulation of cognition and behavior as well as in the neuropathology of neurological and mental disorders. The angiotensin II type 1 receptor (AT1R) mediates most functional and neuropathology-relevant actions associated with the central RAS. However, an overarching comprehension to guide translation and utilize the therapeutic potential of the central RAS in humans is currently lacking. We conducted a comprehensive characterization of the RAS using an innovative combination of transcriptomic gene expression mapping, image-based behavioral decoding, and pre-registered randomized controlled discovery-replication pharmacological resting-state functional magnetic resonance imaging (fMRI) trials (N = 132) with a selective AT1R antagonist. The AT1R exhibited a particular dense expression in a subcortical network encompassing the thalamus, striatum, and amygdalo-hippocampal formation. Behavioral decoding of the AT1R gene expression brain map showed an association with memory, stress, reward, and motivational processes. Transient pharmacological blockade of the AT1R further decreased neural activity in subcortical systems characterized by a high AT1R expression, while increasing functional connectivity in the cortico-basal ganglia-thalamo-cortical circuitry. Effects of AT1R blockade on the network level were specifically associated with the transcriptomic signatures of the dopaminergic, opioid, acetylcholine, and corticotropin-releasing hormone signaling systems. The robustness of the results was supported in an independent pharmacological fMRI trial. These findings present a biologically informed comprehensive characterization of the central AT1R pathways and their functional relevance on the neural and behavioral level in humans.

Angiotensin II type 1 receptor-associated protein deletion combined with angiotensin II stimulation accelerates the development of diabetic kidney disease in mice on a C57BL/6 strain

The progress in the research field of diabetic kidney disease (DKD) has been disturbed by the lack of reliable animal models. Angiotensin II (Ang II) type 1 receptor (AT1R)-associated protein (ATRAP) promotes internalization of AT1R and selectively inhibits pathological AT1R signaling. In this study, we investigated whether overactivation of the renin-angiotensin system (RAS) through a combination of ATRAP deletion with Ang II stimulation developed a progressive DKD model in C57BL/6 mice, which are resistant to the development of kidney injury. Eight-week-old male systemic ATRAP-knockout mice on the C57BL/6 strain (KO) and their littermate wild-type mice (Ctrl) were divided into five groups: 1) Ctrl, 2) Ctrl-streptozotocin (STZ), 3) KO-STZ, 4) Ctrl-STZ-Ang II, and 5) KO-STZ-Ang II. Ang II was administered for 6 weeks from 4 weeks after STZ administration. At 10 weeks after STZ administration, mice were euthanized to evaluate kidney injuries. Neither ATRAP deletion alone nor Ang II stimulation alone developed a progressive DKD model in STZ-induced diabetic C57BL/6 mice. However, a combination of ATRAP deletion with Ang II stimulation accelerated the development of DKD as manifested by overt albuminuria, glomerular hypertrophy, podocyte loss, mesangial expansion, kidney interstitial fibrosis and functional insufficiency, concomitant with increased angiotensinogen and AT1R expression in the kidneys. In STZ-induced diabetic C57BL/6 mice that are resistant to the development of kidney injury, the combination of ATRAP deletion and Ang II stimulation accelerates the development of DKD, which may be associated with intrarenal RAS overactivation.

Enhancement of angiotensin II type 1 receptor-associated protein in the paraventricular nucleus suppresses angiotensin II-dependent hypertension

The renin-angiotensin system in the brain plays a pivotal role in modulating sympathetic nerve activity and contributes to the pathogenesis of hypertension. Angiotensin II (Ang II) type 1 receptor (AT1R)-associated protein (ATRAP) promotes internalization of AT1R while suppressing pathological overactivation of AT1R signaling. However, the pathophysiological function of ATRAP in the brain remains unknown. Therefore, this study aims to investigate whether ATRAP in the paraventricular nucleus (PVN) is involved in neurogenic hypertension pathogenesis in Ang II-infused rats. The ATRAP/AT1R ratio, which serves as an indicator of tissue AT1R hyperactivity, tended to decrease within the PVN in the Ang II group than in the vehicle group. This suggests an Ang II-induced hyperactivation of the AT1R signaling pathway in the PVN. Lentiviral vectors were generated to stimulate ATRAP expression. At 6 weeks of age, rats were microinjected with LV-Venus (Venus-expressing lentivirus) or LV-ATRAP (Venus-ATRAP-expressing lentivirus). The rats were then randomly divided into four groups: (1) Vehicle/LV-Venus, (2) Vehicle/LV-ATRAP, (3) Ang II/LV-Venus, and (4) Ang II/LV-ATRAP. Two weeks after microinjection, vehicle or Ang II was administered systemically for 2 weeks. In the Ang II/LV-ATRAP group, systolic blood pressure at 1 and 2 weeks following administration was significantly lower than that in the Ang II/LV-Venus group. Furthermore, urinary adrenaline levels tended to decrease in the Ang II/LV-ATRAP group than in the Ang II/LV-Venus group. These findings suggest that enhanced ATRAP expression in the PVN suppresses Ang II-induced hypertension, potentially by suppressing hyperactivation of the tissue AT1R signaling pathway and, subsequently, sympathetic nerve activity.

Thymus transplantation regulates blood pressure and alleviates hypertension-associated heart and kidney damage via transcription factors FoxN1 pathway

Previous studies have found that thymus is involved in the process of hypertension. However, whether thymus transplantation alleviates target organ damage in hypertensive mice remains unknown. The aim of this study was to evaluate the effects of thymus transplantation on blood pressure and target organ changes in mice with hypertension. Mice were randomly divided into normal control group (Con), hypertensive group (HTN) and thymus transplantation group (HTN + Trans). Thymus of neonatal mice was transplanted into the renal capsule of the transplantation group. After transplantation, the mouse tail noninvasive pressure was measured and heart function was evaluated weekly. Then mice were euthanized and organs or tissues were harvested at 4 weeks post-transplantation. The blood pressure of HTN + Trans group was lower than that in the HTN group. The expression of FoxN1, Aire, ATRAP, thymosin β4 and the content of sjTREC in thymus of HTN group was decreased and the number of naïve T cells in HTN group was lower compared with other two groups. The ratio of cTEC/mTEC in HTN group was higher than that in Con group and lower than that in HTN + Trans group. Cardiac pathology showed cardiac hypertrophy and fibrosis in HTN group whereas thymus transplantation improved heart function and structure. Altogether, our findings demonstrated thymus transplantation could improve thymus function of hypertensive mice, which increased the expression of thymus transcription factor FoxN1, affected the proportion of T cell subsets, and increased thymosin β4 thereby reducing blood pressure and reversing the progression of target organ damage.

Angiotensin II type-1 receptor-associated protein interacts with transferrin receptor-1 and promotes its internalization

Kidney fibrosis is a common pathway that leads to chronic kidney disease. Angiotensin II type-1 receptor (AT1R)-associated protein (ATRAP) was originally identified as an AT1R-binding protein. Previously, we reported that systemic knockout of ATRAP exacerbates kidney fibrosis in aged mice. Although these effects of ATRAP appeared to be AT1R-independent actions, the molecular mechanism remains poorly understood. To elucidate the molecular mechanism of ATRAP independent of AT1R, we explored novel ATRAP-interacting proteins. Mass spectrometric analysis of the immunoprecipitants of a Flag-tagged ATRAP complex revealed 376 candidate proteins that potentially interact with ATRAP. Gene ontology analysis revealed that proteins related to vesicle trafficking, membrane transport, and many membrane proteins, including transferrin receptor 1 (TfR1), were enriched. Because TfR1 promotes cellular iron uptake and iron is a key factor involved in kidney fibrosis, we focused on TfR1 and confirmed that it interacts with ATRAP. In addition, our findings revealed that enhanced ATRAP expression decreased cell-surface TfR1 expression without altering the overall cellular TfR1 expression levels. Furthermore, enhanced ATRAP expression attenuated cellular iron levels. Together, our results highlight the role of ATRAP as a suppressor of TfR1 that functions by facilitating TfR1 internalization, which affects iron metabolism and oxidative stress signaling.

Sirtuins as novel pharmacological targets in podocyte injury and related glomerular diseases

Podocyte injury is a major cause of proteinuria in kidney diseases, and persistent loss of podocytes leads to rapid irreversible progression of kidney disease. Sirtuins, a class of nicotinamide adenine dinucleotide-dependent deacetylases, can promote DNA repair, modify transcription factors, and regulate the cell cycle. Additionally, sirtuins play a critical role in renoprotection, particularly against podocyte injury. They also have pleiotropic protective effects on podocyte injury-related glomerular diseases, such as improving the immune inflammatory status and oxidative stress levels, maintaining mitochondrial homeostasis, enhancing autophagy, and regulating lipid metabolism. Sirtuins deficiency causes podocyte injury in different glomerular diseases. Studies using podocyte sirtuin-specific knockout and transgenic models corroborate this conclusion. Of note, sirtuin activators have protective effects in different podocyte injury-related glomerular diseases, including diabetic kidney disease, focal segmental glomerulosclerosis, membranous nephropathy, IgA nephropathy, and lupus nephritis. These findings suggest that sirtuins are promising therapeutic targets for preventing podocyte injury. This review provides an overview of recent advances in the role of sirtuins in kidney diseases, especially their role in podocyte injury, and summarizes the possible rationale for sirtuins as targets for pharmacological intervention in podocyte injury-related glomerular diseases.

Recent Advances in Epigenetics of Age-Related Kidney Diseases

Renal aging has attracted increasing attention in today’s aging society, as elderly people with advanced age are more susceptible to various kidney disorders such as acute kidney injury (AKI) and chronic kidney disease (CKD). There is no clear-cut universal mechanism for identifying age-related kidney diseases, and therefore, they pose a considerable medical and public health challenge. Epigenetics refers to the study of heritable modifications in the regulation of gene expression that do not require changes in the underlying genomic DNA sequence. A variety of epigenetic modifiers such as histone deacetylases (HDAC) inhibitors and DNA methyltransferase (DNMT) inhibitors have been proposed as potential biomarkers and therapeutic targets in numerous fields including cardiovascular diseases, immune system disease, nervous system diseases, and neoplasms. Accumulating evidence in recent years indicates that epigenetic modifications have been implicated in renal aging. However, no previous systematic review has been performed to systematically generalize the relationship between epigenetics and age-related kidney diseases. In this review, we aim to summarize the recent advances in epigenetic mechanisms of age-related kidney diseases as well as discuss the application of epigenetic modifiers as potential biomarkers and therapeutic targets in the field of age-related kidney diseases. In summary, the main types of epigenetic processes including DNA methylation, histone modifications, non-coding RNA (ncRNA) modulation have all been implicated in the progression of age-related kidney diseases, and therapeutic targeting of these processes will yield novel therapeutic strategies for the prevention and/or treatment of age-related kidney diseases.

Molecular mechanisms of dietary restriction promoting health and longevity

Dietary restriction with adequate nutrition is the gold standard for delaying ageing and extending healthspan and lifespan in diverse species, including rodents and non-human primates. In this Review, we discuss the effects of dietary restriction in these mammalian model organisms and discuss accumulating data that suggest that dietary restriction results in many of the same physiological, metabolic and molecular changes responsible for the prevention of multiple ageing-associated diseases in humans. We further discuss how different forms of fasting, protein restriction and specific reductions in the levels of essential amino acids such as methionine and the branched-chain amino acids selectively impact the activity of AKT, FOXO, mTOR, nicotinamide adenine dinucleotide (NAD+), AMP-activated protein kinase (AMPK) and fibroblast growth factor 21 (FGF21), which are key components of some of the most important nutrient-sensing geroprotective signalling pathways that promote healthy longevity.

ATRAP, a receptor-interacting modulator of kidney physiology, as a novel player in blood pressure and beyond

Pathological activation of kidney angiotensin II (Ang II) type 1 receptor (AT1R) signaling stimulates tubular sodium transporters, including epithelial sodium channels, to increase sodium reabsorption and blood pressure. During a search for a means to functionally and selectively modulate AT1R signaling, a molecule directly interacting with the carboxyl-terminal cytoplasmic domain of AT1R was identified and named AT1R-associated protein (ATRAP/Agtrap). We showed that ATRAP promotes constitutive AT1R internalization to inhibit pathological AT1R activation in response to certain stimuli. In the kidney, ATRAP is abundantly distributed in epithelial cells along the proximal and distal tubules. Results from genetically engineered mice with modified ATRAP expression show that ATRAP plays a key role in the regulation of renal sodium handling and the modulation of blood pressure in response to pathological stimuli and further suggest that the function of kidney tubule ATRAP may be different between distal tubules and proximal tubules, implying that ATRAP is a target of interest in hypertension.

Coordinated expression of the renin-angiotensin genes in the lumbar spinal cord: Lateralization and effects of unilateral brain injury

In spite of its apparent symmetry, the spinal cord is asymmetric in its reflexes and gene expression patterns including leftward expression bias of the opioid and glutamate genes. To examine whether this is a general phenomenon for neurotransmitter and neurohormonal genes, we here characterized expression and co-expression (transcriptionally coordinated) patterns of genes of the renin-angiotensin system (RAS) that is involved in neuroprotection and pathological neuroplasticity in the left and right lumbar spinal cord. We also tested whether the RAS expression patterns were affected by unilateral brain injury (UBI) that rewired lumbar spinal neurocircuits. The left and right halves of the lumbar spinal cord were analysed in intact rats, and rats with left- or right-sided unilateral cortical injury, and left- or right-sided sham surgery. The findings were (i) lateralized expression of the RAS genes Ace, Agtr2 and Ren with higher levels on the left side; (ii) the asymmetry in coordination of the RAS gene expression that was stronger on the right side; (iii) the decay in coordination of co-expression of the RAS and neuroplasticity-related genes induced by the right-side but not left-side sham surgery and UBI; and (iv) the UBI-induced shift to negative regulatory interactions between RAS and neuroplasticity-related genes on the contralesional spinal side. Thus, the RAS genes may be a part of lateralized gene co-expression networks and have a role in a side-specific regulation of spinal neurocircuits.

Role and mechanism of TXNIP in ageing-related renal fibrosis

Kidney ageing, which is always accompanied by renal fibrosis, drives the progression of renal fibrosis. Thioredoxin-interacting protein (TXNIP) is an endogenous suppressor of the reactive oxygen species-scavenging protein thioredoxin, which has been implicated in the ageing of some organs and is involved in renal fibrosis. However, the expression of TXNIP in ageing kidneys has not been examined, and the relationship between TXNIP and ageing-related renal fibrosis is unclear. We found that TXNIP expression was upregulated in aged mouse kidneys, and this upregulation was accompanied by ageing-related renal fibrosis phenotypes. We demonstrated that the ageing biomarkers were downregulated in TXNIP-knockout mice, and these effects resulted in the alleviation of renal fibrosis and impairments in kidney function. TXNIP overexpression in tubular cells upregulated senescence markers, promoted a profibrotic response and activated STAT3 signalling, and these parameters were inhibited by the silencing of TXNIP. Similarly, the TXNIP-mediated profibrotic response was significantly suppressed by a STAT3 inhibitor. By coimmunoprecipitation, we verified that TXNIP directly bound to STAT3, which suggested that TXNIP exacerbates renal tubular epithelial fibrosis by activating the STAT3 pathway. In summary, TXNIP plays an important role in age-related renal fibrosis and might be a therapeutic target for preventing ageing-associated renal fibrosis.

The Role of Sirtuins in Kidney Diseases

Sirtuins (SIRTs) are class III histone deacetylases (HDACs) that play important roles in aging and a wide range of cellular functions. Sirtuins are crucial to numerous biological processes, including proliferation, DNA repair, mitochondrial energy homeostasis, and antioxidant activity. Mammals have seven different sirtuins, SIRT1–7, and the diverse biological functions of each sirtuin are due to differences in subcellular localization, expression profiles, and cellular substrates. In this review, we summarize research advances into the role of sirtuins in the pathogenesis of various kidney diseases including acute kidney injury, diabetic kidney disease, renal fibrosis, and kidney aging along with the possible underlying molecular mechanisms. The available evidence indicates that sirtuins have great potential as novel therapeutic targets for the prevention and treatment of kidney diseases.

Complement C3 deficiency ameliorates aging related changes in the kidney

AIMS

Complement C3 (C3) has been shown to be involved in the aging process. However, the role of C3 in kidney aging has not been fully elucidated. This study aimed to investigate the effect of C3 on senescence related kidney disorders in mice.

MATERIALS AND METHODS

Two-, 8-, and 16-month-old C3-deficient male mice (KO) (n = 6) and age-, gender-, and strain- matched wild type (WT) C57BL/6 mice (n = 6) were selected to represent young, middle-aged and aging mice. Renal, blood and urine samples were collected. Hematoxylin-eosin (HE), Masson, and immunohistochemistry (IHC) staining as well as ELISA and Western blotting were used to explore the mechanisms involved in renal aging.

KEY FINDINGS

The level of C3 was upregulated during aging in WT mice. The glomerular sclerosis index and tubulointerstitial fibrosis index were increased significantly in WT mice during aging. Renal function was not significantly different between the young and aged groups. Compared with those in WT mice, the levels of inflammation and fibrosis were decreased, while the expression of CD31 was significantly increased in the KO group.

SIGNIFICANCE

Our data demonstrated that age-related changes in renal structure occur earlier than functional changes and that complement C3 is involved in aging-related kidney disorder.

Quercetin alleviates kidney fibrosis by reducing renal tubular epithelial cell senescence through the SIRT1/PINK1/mitophagy axis

Emerging evidence implicates accelerated renal tubular epithelial cell (RTEC) senescence in renal fibrosis progression. Mitophagy protects against kidney injury. However, the mechanistic interplay between cell senescence and mitophagy in RTECs is not clearly defined. The purpose of this study was to evaluate the inhibition of RTEC senescence and renal fibrosis by quercetin and explore the underlying mechanisms. We found that quercetin attenuated RTEC senescence induced by angiotensin II (AngII) in vitro and unilateral ureteral obstruction in vivo. Moreover, we demonstrated that mitochondrial abnormalities such as elevated reactive oxygen species, decreased membrane potential, and fragmentation and accumulation of mitochondrial mass, occurred in AngII-treated RTECs. Quercetin treatment reversed these effects. Furthermore, quercetin enhanced mitophagy in AngII-treated RTECs, which was markedly reduced by treatment with mitophagy-specific inhibitors. Sirtuin-1 (SIRT1) was involved in quercetin-mediated PTEN-induced kinase 1 (PINK1)/Parkin-associated mitophagy activation. Pharmacological antagonism of SIRT1 in AngII-treated RTECs blocked the effects of quercetin on mitophagy and cellular senescence. Finally, quercetin alleviated kidney fibrosis by reducing RTEC senescence via mitophagy. Collectively, the antifibrotic effect of quercetin involved inhibition of RTEC senescence, possibly through activation of SIRT1/PINK1/Parkin-mediated mitophagy. These findings suggest that pharmacological elimination of senescent cells and stimulation of mitophagy represent effective therapeutic strategies to prevent kidney fibrosis.

Age‑related changes in mineralocorticoid receptors in rat hearts

Age-related alterations in the renin-angiotensin-aldosterone system (RAAS) have been reported in the cardiovascular system; however, the detailed mechanism of the RAAS component mineralocorticoid receptors (MR) has not been elucidated. The present study aimed to investigate the associations between MR and cardiac aging in rats, as well as the regulatory effects of oxidative stress and mitochondrial abnormalities in the aging process. MR expression in the hearts of male Sprague-Dawley rats aged 3 months (young rats) and 24 months (old rats) was evaluated in vivo. In addition, in vitro, H9C2 cells were treated with a specific MR antagonist, eplerenone, in order to investigate the molecular mechanism underlying the inhibition of myocyte aging process. The results demonstrated that MR expression was significantly higher in 24-month-old rat hearts compared with in 3-month-old rat hearts. These changes were accompanied by increased p53 expression, decreased peroxisome proliferator-activated receptor γ coactivator-1α expression, decreased mitochondrial renewal as assessed by electron microscopy, increased oxidative stress and decreased superoxide dismutase. In vitro, selective antagonism of MR partially blocked H₂O₂-induced myocardial aging as assessed by p16, p21 and p53 expression levels and excessive reactive oxygen species (ROS) accumulation. These results indicated that increased MR expression may drive age-related cardiac dysfunction via mitochondrial damage, increased ROS accumulation and an imbalanced redox state.

Effect of renin-angiotensin system on senescence

The renin-angiotensin system (RAS) plays crucial roles in the control of blood pressure and sodium homeostasis. Moreover, RAS also acts as a key player in cell and organ senescence, mainly by activation of the classical axis of angiotensin (Ang) converting enzyme (ACE)/Ang II/Ang II type 1 receptor via overproduction of reactive oxygen species. Overactivation of the classical RAS axis induces organ dysfunction in the vasculature, brain, kidney and skeletal muscle, resulting in atherosclerosis, stroke, chronic kidney disease and sarcopenia. Moreover, RAS has been shown to regulate lifespan, using gene-modification models. Recently, mice lacking the Ang II type 1 receptor were shown to exhibit an increase in lifespan compared with control mice. Here, the effect of RAS on age-related tissue dysfunction in several organs is reviewed, including not only the classical axis but also protective functions of RAS such as the ACE2/Ang (1-7)/Mas axis. Geriatr Gerontol Int 2020; ••: ••-••.

The pathophysiological role of angiotensin receptor-binding protein in hypertension and kidney diseases: Oshima Award Address 2019

Excessive activation of the tissue renin–angiotensin system through angiotensin II (Ang II) type 1 receptor (AT1R) plays a pivotal role in the pathogenesis of hypertension and related organ injury. AT1R-associated protein (ATRAP/Agtrap) was identified as a molecule specifically interacting with the carboxyl- terminal domain of AT1R. The results of in vitro studies showed that ATRAP suppresses Ang II-mediated pathological responses in cardiovascular cells by promoting AT1R internalization. With respect to the tissue distribution and regulation of ATRAP expression in vivo, ATRAP is broadly expressed in many tissues as is AT1R including kidney. The results of in vivo study employing genetic engineered mice with modified ATRAP expression showed that ATRAP inhibits cardiovascular injuries provoked by Ang II-induced hypertension, along with preserving physiological AT1R signaling. In addition, we have shown that ATRAP functions as an endogenous modulator so as to prevent hypertension in response to pathological stimuli, by regulating renal sodium handling. Furthermore, ATRAP may have an AT1R-independent function of renal proximal tubule to protect aging and fibrosis. These results suggest the clinical potential benefit of an ATRAP activation strategy in the treatment of hypertension and cardiorenal and vascular diseases.

Recent Research Advances in Renin-Angiotensin-Aldosterone System Receptors

PURPOSE OF REVIEW

The renin-angiotensin-aldosterone system (RAAS) plays important roles in regulating blood pressure and body fluid, which contributes to the pathophysiology of hypertension and cardiovascular/renal diseases. However, accumulating evidence has further revealed the complexity of this signal transduction system, including direct interactions with other receptors and proteins. This review focuses on recent research advances in RAAS with an emphasis on its receptors.

RECENT FINDINGS

Both systemically and locally produced angiotensin II (Ang II) bind to Ang II type 1 receptor (AT1R) and elicit strong biological functions. Recent studies have shown that Ang II-induced activation of Ang II type 2 receptor (AT2R) elicits the opposite functions to those of AT1R. However, accumulating evidence has now expanded the components of RAAS, including (pro)renin receptor, angiotensin-converting enzyme 2, angiotensin 1-7, and Mas receptor. In addition, the signal transductions of AT1R and AT2R are regulated by not only Ang II but also its receptor-associated proteins such as AT1R-associated protein and AT2R-interacting protein. Recent studies have indicated that inappropriate activation of local mineralocorticoid receptor contributes to cardiovascular and renal tissue injuries through aldosterone-dependent and -independent mechanisms. Since the mechanisms of RAAS signal transduction still remain to be elucidated, further investigations are necessary to explore novel molecular mechanisms of the RAAS, which will provide alternative therapeutic agents other than existing RAAS blockers.

Effects of Rikkunshito treatment on renal fibrosis/inflammation and body weight reduction in a unilateral ureteral obstruction model in mice

Chronic kidney disease (CKD) progresses to end-stage renal failure via renal tubulointerstitial fibrosis. Malnutrition, inflammation, and arteriosclerosis interact to exacerbate the poor prognosis of CKD, and their effective management is thus essential. The traditional Japanese medicine Rikkunshito (RKT) exerts appetite-stimulating effects via ghrelin, which attenuates inflammation and fibrosis. We evaluated the therapeutic effect of RKT in unilateral ureter obstruction (UUO)-induced renal fibrosis/inflammation and body weight loss in mice. UUO and sham-operated mice were fed a standard diet or diet containing 3.0% RKT. Renal fibrosis was investigated by histopathology and macrophage infiltration was determined by immunohistochemistry. Expression levels of genes associated with fibrosis, inflammation, ghrelin, and mitochondrial function were determined by quantitative reverse transcription-polymerase chain reaction and western blot analyses. RKT treatment partially prevented UUO-induced weight loss but failed to attenuate renal fibrosis and inflammation. Renal expression of sirtuin 1, a ghrelin-downstream signalling molecule, and gene expression of peroxisome proliferator-activated receptor-γ coactivator 1α and Bcl-2/adenovirus E1B interacting protein 3 were unaffected by RKT. These results indicate that RKT inhibits weight loss but does not improve renal fibrosis or inflammation in a rapidly progressive renal fibrosis mouse model. RKT may have a protective effect on weight loss associated with CKD.

Angiotensin II type 1 receptor-associated protein deficiency attenuates sirtuin1 expression in an immortalised human renal proximal tubule cell line

The proximal tubule is a particularly important site for ageing-related kidney damage. Sirtuin 1 (SIRT1), an NAD⁺ (nicotinamide adenine dinucleotide)-dependent deacetylase in the proximal tubule, may be involved in renal injury associated with ageing. However, the mechanisms of SIRT1 regulation remain to be elucidated. We recently reported that angiotensin II type 1 receptor (AT1R)-associated protein (ATRAP)-deficient mice displayed age-associated renal function decline and tubulointerstitial fibrosis. Our data showed that SIRT1 protein expression was reduced in ATRAP-deficient mice, although the relationship between ATRAP deficiency and age-associated renal fibrosis is still not fully understood. It is, therefore, necessary to investigate how ATRAP affects SIRT1 protein expression to resolve ageing-associated kidney dysfunction. Here, since ageing studies are inherently lengthy, we used an ex vivo model of the proximal tubule to determine the role of ATRAP in SIRT1 protein expression. We first generated a clonal immortalised human renal proximal tubule epithelial cell line (ciRPTEC) expressing AT1R and ATRAP. Using this cell line, we demonstrated that ATRAP knockdown reduced SIRT1 protein expression in the ciRPTEC but did not alter SIRT1 mRNA expression. Thus, ATRAP likely mediates SIRT1 protein abundance in ciRPTEC.

Effects of rikkunshito on renal fibrosis and inflammation in angiotensin II-infused mice

The underlying pathogenesis of chronic kidney disease involves an activated renin-angiotensin system and systemic inflammation which ultimately develop renal injury. Rikkunshito (RKT) has been reported to exert anti-fibrotic and anti-inflammatory effects through enhancement of ghrelin signaling pathway. In this study, we investigated the effects of RKT on renal fibrosis and inflammation in angiotensin II (Ang II)-induced renal injury model. Ang II-infused mice exhibited hypertension, cardiac hypertrophy, increases in blood urea nitrogen and serum creatinine, moderate albuminuria and renal pathological changes such as mild urinary cast, interstitial macrophage infiltration and modest interstitial fibrosis. RKT had no evident effects on the Ang II-induced renal functional insufficiency and fibrosis, but attenuated renal interstitial macrophage infiltration. In addition, RKT significantly restored the Ang II-induced alteration in the expression of renal fibrosis- and inflammation-related genes such as type 3 collagen, transforming growth factor-β, monocyte chemoattractant protein-1 and interleukin-6. Furthermore, although RKT did not affect the expression of renal ghrelin receptor, an Ang II-induced decrease in renal sirtuin 1 expression, a critical down-stream pathway of the ghrelin receptor, was restored by RKT. These findings suggest that RKT potentially has a renal anti-inflammatory effect in the development of renal injury, and this effect could be mediated by the ghrelin signaling pathway.

Effects of ATRAP in Renal Proximal Tubules on Angiotensin-Dependent Hypertension

Background We have previously shown that ATRAP (angiotensin II receptor-associated protein; Agtrap) interacts with AT1R (angiotensin II type 1 receptor) and promotes constitutive internalization of AT 1R so as to inhibit hyperactivation of its downstream signaling. In response to angiotensin II , systemic ATRAP deficiency exacerbates angiotensin II -mediated hypertension via hyperactivation of renal tubular AT 1R. Although ATRAP expression is abundant in renal proximal tubules, little is known about the actual function of renal proximal tubule ATRAP in angiotensin-mediated hypertension. Methods and Results In this study, we examined the in vivo functional role of renal proximal tubule ATRAP in angiotensin-dependent hypertension. We succeeded in generating proximal tubule-specific ATRAP knockout ( PT - KO ) mice for the first time using the Cre/loxP system with Pepck-Cre. Detailed analysis of renal ATRAP expression in PT - KO mice estimated by immunohistochemical and laser-capture microdissection analysis revealed that ATRAP mRNA expression decreased by ≈80% in proximal regions of the nephron in PT - KO mice compared with wild-type ( WT ) mice. We compared blood pressure of PT - KO and WT mice using both tail-cuff and radiotelemetric methods. Blood pressure of PT - KO mice was comparable with that of WT mice at baseline. Moreover, no significant differences were noted in pressor response to angiotensin II (600 ng/kg per min or 1000 ng/kg per minute) infusion between PT - KO and WT mice. In addition, angiotensin II -mediated cardiac hypertrophy was identical between PT - KO and WT mice. Conclusions ATRAP deficiency in proximal tubules did not exacerbate angiotensin-dependent hypertension in vivo. The results indicate that renal proximal tubule ATRAP has a minor role in angiotensin-dependent hypertension in vivo.

Early Enhanced Leucine-Rich α-2-Glycoprotein-1 Expression in Glomerular Endothelial Cells of Type 2 Diabetic Nephropathy Model Mice

Abnormal angiogenesis plays a major role in the development of early stage diabetic nephropathy. Vascular endothelial growth factor (VEGF) is a classical proangiogenic factor that regulates abnormal glomerular angiogenesis linked to glomerular hypertrophy in the early stage of diabetic nephropathy. Leucine-rich α-2-glycoprotein-1 (LRG1) was recently reported as a novel proangiogenic factor that is expressed in endothelial cells and promotes angiogenesis by modulating the transforming growth factor-β signaling pathway. However, the pathophysiology of LRG1 in diabetic nephropathy remains largely unknown. In the present study, we investigated intrarenal expression of the novel proangiogenic factor LRG1 in diabetic db/db mice by immunohistochemistry and a laser capture microdissection method during the development of diabetic nephropathy. We hypothesized that glomerular LRG1 expression is increased earlier than VEGF expression under conditions of pathological angiogenesis in the early stage of diabetic nephropathy. Thus, we compared glomerular expression of VEGF and LRG1 in diabetic db/db mice at 16 and 24 weeks of age. At 16 weeks, diabetic db/db mice exhibited glomerular hypertrophy with abnormal angiogenesis characterized by endothelial cell proliferation, which was concomitant with an increase in LRG1 expression of glomerular endothelial cells. However, glomerular VEGF expression was not increased at this early stage. At 24 weeks, the features of early diabetic nephropathy in db/db mice had developed further, along with further enhanced glomerular LRG1 expression. At this late stage, glomerular VEGF and fibrosis-related-gene expression was also significantly increased compared with nondiabetic db/m mice. These results suggest that LRG1 plays a pivotal role in the initial development of diabetic nephropathy by promoting abnormal angiogenesis, thereby suggesting that LRG1 is a potential preemptive therapeutic target of diabetic nephropathy.

Sulfur dioxide improves endothelial dysfunction by downregulating the angiotensin II/AT1R pathway in D-galactose-induced aging rats

The aim of this study was to investigate the protective effects of sulfur dioxide (SO₂) on the endothelial function of the aorta in D-galactose (D-gal)-induced aging rats. Sprague Dawley rats were randomized into a D-gal group, a D-gal + SO₂ group and a control group, then injected with D-gal, D-gal + SO₂ donor or equivalent volumes of saline, respectively, for 8 consecutive weeks. After 8 weeks, the mean arterial pressure was significantly increased in the D-gal group, but was lowered by SO₂. SO₂ significantly ameliorated the endothelial dysfunction induced by D-gal treatment. The vasorelaxant effect of SO₂ was associated with the elevated nitric oxide levels and upregulated phosphorylation of endothelial nitric oxide synthase. In the D-gal group, the concentration of angiotensin II in the plasma was significantly increased, but was decreased by SO₂. Moreover, levels of vascular tissue hydrogen peroxide (H₂O₂) and malondialdehyde were significantly lower in SO₂-treated groups than those in the D-gal group. Western blot analysis showed that the expressions of oxidative stress-related proteins (the angiotensin II type 1 receptor (AT₁R), and nicotinamide adenine dinucleotide phosphate oxidase subunits) were increased in the D-gal group, while they were decreased after treatment with SO₂. In conclusion, SO₂ attenuated endothelial dysfunction in association with the inhibition of oxidative stress injury and the downregulation of the angiotensin II/AT₁R pathway in D-gal-induced aging rats.

Age-Related Expression of Human AT1R Variants and Associated Renal Dysfunction in Transgenic Mice

BACKGROUND

The contribution of single nucleotide polymorphisms in transcriptional regulation of the human angiotensin receptor type I (hAT1R) gene in age-related chronic pathologies such as hypertension and associated renal disorders is not well known. The hAT1R gene has single nucleotide polymorphisms in its promoter that forms 2 haplotypes (Hap), Hap-I and Hap-II. Hap-I of AT1R gene is associated with hypertension in Caucasians. We have hypothesized here that age will alter the transcriptional environment of the cell and will regulate the expression of hAT1R gene in a haplotype-dependent manner. This could likely make subjects with Hap-I increasingly susceptible to age-associated, AT1R-mediated complications.

METHOD

We generated transgenic (TG) mice with Hap-I and Hap-II. Adults (10-12 weeks) and aged (20-24 months) TG male mice containing either Hap-I or Hap-II were divided into 4 groups to study (i) the age-associated and haplotype-specific transcriptional regulation of hAT1R gene and (ii) their physiological relevance.

RESULTS

In aged animals, TG mice with Hap-I show increased expression of hAT1R and higher blood pressure (BP); suppression of antioxidant defenses (hemoxygenase, superoxide dismutase) and antiaging molecules (ATRAP, Klotho, Sirt3); increased expression of pro-inflammatory markers (IL-6, TNFα, CRP, NOX1); and increased insulin resistance. In vivo ChIP assay shows stronger binding of transcription factor USF2 to the chromatin of Hap-I mice.

CONCLUSION

Our results suggest that in aged animals, as compared with Hap-II, the TG mice with Hap-I overexpress hAT1R gene due to the stronger transcriptional activity, thus resulting in an increase in their BP and associated renal disorders.

Immunologic and endocrine functions of adipose tissue: implications for kidney disease

Excess adiposity can induce adverse sequelae in multiple cell types and organ systems. The transition from the lean to the obese state is characterized by fundamental cellular changes at the level of the adipocyte. These changes affect the local microenvironment within the respective adipose tissue but can also affect nonadipose systems. Adipocytes within fat pads respond to chronic nutrient excess through hyperplasia or hypertrophy, which can differentially affect interorgan crosstalk between various adipose depots and other organs. This crosstalk is dependent on the unique ability of the adipocyte to coordinate metabolic adjustments throughout the body and to integrate responses to maintain metabolic homeostasis. These actions occur through the release of free fatty acids and metabolites during times of energy need - a process that is altered in the obese state. In addition, adipocytes release a wide array of signalling molecules, such as sphingolipids, as well as inflammatory and hormonal factors (adipokines) that are critical for interorgan crosstalk. The interactions of adipose tissue with the kidney - referred to as the adipo-renal axis - are important for normal kidney function as well as the response of the kidney to injury. Here, we discuss the mechanistic basis of this interorgan crosstalk, which clearly has great therapeutic potential given the increasing rates of chronic kidney disease secondary to obesity and type 2 diabetes mellitus.