CREB, NF-κB, and NADPH oxidase coordinately upregulate indoxyl sulfate-induced angiotensinogen expression in proximal tubular cells

In laying hens, chronic heat stress-induced renal fibrosis is potentially promoted by indoxyl sulfate

Indoxyl sulfate (IS), a uremic toxin, is a harmful factor that damages kidneys. Chronic heat stress in laying hens causes renal injury; however, whether IS accumulation is involved in this injury remains unknown. We selected 20 Boris brown laying hens (27 weeks old) and randomly assigned them to two groups (n = 10), one group was exposed to chronic heat stress (32 °C for 4 weeks), whereas the other was maintained at 24 °C. Chronic heat exposure significantly increased plasma and renal IS concentrations (P < 0.05). Exposure to heat also increased renal expression of the aryl hydrocarbon receptor (AhR) and its target genes (CYP1A4 and CYP1B1). Furthermore, chronic heat exposure tended to increase the 2-thiobarbituric acid reactive substances content (P = 0.08) and significantly decreased the antioxidant capacity in the kidney, while increasing the transcription levels of nuclear factor κB and fibrosis-related genes (COLA1A1, αSMA, TGF-β, Smad3, and VCAM-1) and the area of renal fibrosis. Our results indicate that chronic heat exposure induces systemic and renal IS accumulation in laying hens. This accumulated IS may activate the AhR pathway and chronically disrupt the oxidative stress status and antioxidant activity, thus promoting renal fibrosis and dysfunction in laying hens.

Increased Prorenin Expression in the Kidneys May Be Involved in the Abnormal Renal Function Caused by Prolonged Environmental Exposure to Microcystin-LR

Toxic algae in eutrophic lakes produce cyanotoxic microcystins. Prior research on the effect of microcystin-LR in the kidney utilized intraperitoneal injections, which did not reflect natural exposure. Oral microcystin-LR research has focused on renal function and histopathology without examining the molecular mechanisms. The present study aimed to evaluate the mechanism of microcystin-LR in the kidneys via oral administration in WKAH/HkmSlc rats over 7 weeks, alongside stimulation of the proximal tubular cells. Although there were no differences in the concentrations of plasma albumin, blood urea nitrogen, and creatinine, which are parameters of renal function, between the control and microcystin-LR-administrated rats, prorenin expression was significantly increased in the renal cortex of the rats administered microcystin-LR and the microcystin-LR-treated proximal tubular cells. The expression levels of (pro)renin receptor (PRR), transforming growth factor-β1 (TGFβ1), and α-smooth muscle actin (α-SMA) in the renal cortex did not differ significantly between the control and microcystin-LR-administered rats. However, the expression levels of prorenin were significantly positively correlated with those of PRR, TGFβ1, and α-SMA in the renal cortex of rats administered microcystin-LR. Additionally, a significant positive correlation was observed between the expression levels of TGFβ1 and α-SMA. Collectively, increased prorenin expression caused by the long-term consumption of microcystin-LR may initiate a process that influences renal fibrosis and abnormal renal function by regulating the expression levels of PRR, TGFβ1, and α-SMA.

Aryl hydrocarbon receptor impairs circadian regulation in Alzheimer's disease: Potential impact on glymphatic system dysfunction

Circadian clocks maintain diurnal rhythms of sleep-wake cycle of 24 h that regulate not only the metabolism of an organism but also many other periodical processes. There is substantial evidence that circadian regulation is impaired in Alzheimer's disease. Circadian clocks regulate many properties known to be disturbed in Alzheimer's patients, such as the integrity of the blood-brain barrier (BBB) as well as the diurnal glymphatic flow that controls waste clearance from the brain. Interestingly, an evolutionarily conserved transcription factor, that is, aryl hydrocarbon receptor (AhR), impairs the function of the core clock proteins and thus could disturb diurnal rhythmicity in the BBB. There is abundant evidence that the activation of AhR signalling inhibits the expression of the major core clock proteins, such as the brain and muscle arnt-like 1 (BMAL1), clock circadian regulator (CLOCK) and period circadian regulator 1 (PER1) in different experimental models. The expression of AhR is robustly increased in the brains of Alzheimer's patients, and protein level is enriched in astrocytes of the BBB. It seems that AhR signalling inhibits glymphatic flow since it is known that (i) activation of AhR impairs the function of the BBB, which is cooperatively interconnected with the glymphatic system in the brain, and (ii) neuroinflammation and dysbiosis of gut microbiota generate potent activators of AhR, which are able to impair glymphatic flow. I will examine current evidence indicating that activation of AhR signalling could disturb circadian functions of the BBB and impair glymphatic flow and thus be involved in the development of Alzheimer's pathology.

Indoxyl sulphate-initiated activation of cardiac fibroblasts is modulated by aryl hydrocarbon receptor and nuclear factor-erythroid-2-related factor 2

In the last decade, extensive attention has been paid to the uremic toxin indoxyl sulphate (IS) as an inducer of cardiac fibroblast (cFib) activation and cardiac fibrosis in chronic kidney disease. At cellular level, IS engages aryl hydrocarbon receptor (AhR) and regulates many biological functions. We analysed how AhR inhibition by CH-223191 (CH) and overexpression of non-functional (dominant negative, DN) nuclear factor-erythroid-2-related factor 2 (NRF2), a transcription factor recruited by AhR, modulate the response of neonatal mouse (nm) cFib to IS. We also evaluated nm-cardiomyocytes after incubation with the conditioned medium (CM) of IS±CH-treated nm-cFib. IS induced activation, collagen synthesis, TLR4 and-downstream-MCP-1, and the genes encoding angiotensinogen, angiotensin-converting enzyme, angiotensin type 1 receptor (AT1r) and neprilysin (Nepr) in nm-cFib. CH antagonized IS-initiated nm-cFib activation, but did not affect or even magnified the other features. IS promoted NRF2 nuclear translocation and expression the NRF2 target Nqo1. Both pre-incubation with CH and transfection of DN-NRF2 resulted in loss of NRF2 nuclear localization. Moreover, DN-NRF2 overexpression led to greater TLR4 and MCP-1 levels following exposure to IS. The CM of IS-primed nm-cFib and to a larger extent the CM of IS+CH-treated nm-cFib upregulated AT1r, Nepr and TNFα and myostatin genes in nm-cardiomyocytes. Hence, IS triggers pro-inflammatory activation of nm-cFib partly via AhR, and AhR-NRF2 counteract it. Strategies other than AhR inhibition are needed to target IS detrimental actions on cardiac cells.

Uremic toxins mediate kidney diseases: the role of aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AhR) was originally identified as an environmental sensor that responds to pollutants. Subsequent research has revealed that AhR recognizes multiple exogenous and endogenous molecules, including uremic toxins retained in the body due to the decline in renal function. Therefore, AhR is also considered to be a uremic toxin receptor. As a ligand-activated transcriptional factor, the activation of AhR is involved in cell differentiation and senescence, lipid metabolism and fibrogenesis. The accumulation of uremic toxins in the body is hazardous to all tissues and organs. The identification of the endogenous uremic toxin receptor opens the door to investigating the precise role and molecular mechanism of tissue and organ damage induced by uremic toxins. This review focuses on summarizing recent findings on the role of AhR activation induced by uremic toxins in chronic kidney disease, diabetic nephropathy and acute kidney injury. Furthermore, potential clinical approaches to mitigate the effects of uremic toxins are explored herein, such as enhancing uremic toxin clearance through dialysis, reducing uremic toxin production through dietary interventions or microbial manipulation, and manipulating metabolic pathways induced by uremic toxins through controlling AhR signaling. This information may also shed light on the mechanism of uremic toxin-induced injury to other organs, and provide insights into clinical approaches to manipulate the accumulated uremic toxins.

The AKI-to-CKD Transition: The Role of Uremic Toxins

After acute kidney injury (AKI), renal function continues to deteriorate in some patients. In a pro-inflammatory and profibrotic environment, the proximal tubules are subject to maladaptive repair. In the AKI-to-CKD transition, impaired recovery from AKI reduces tubular and glomerular filtration and leads to chronic kidney disease (CKD). Reduced kidney secretion capacity is characterized by the plasma accumulation of biologically active molecules, referred to as uremic toxins (UTs). These toxins have a role in the development of neurological, cardiovascular, bone, and renal complications of CKD. However, UTs might also cause CKD as well as be the consequence. Recent studies have shown that these molecules accumulate early in AKI and contribute to the establishment of this pro-inflammatory and profibrotic environment in the kidney. The objective of the present work was to review the mechanisms of UT toxicity that potentially contribute to the AKI-to-CKD transition in each renal compartment.

S-Nitrosylation of Tissue Transglutaminase in Modulating Glycolysis, Oxidative Stress, and Inflammatory Responses in Normal and Indoxyl-Sulfate-Induced Endothelial Cells

Circulating uremic toxin indoxyl sulfate (IS), endothelial cell (EC) dysfunction, and decreased nitric oxide (NO) bioavailability are found in chronic kidney disease patients. NO nitrosylates/denitrosylates a specific protein's cysteine residue(s), forming S-nitrosothios (SNOs), and the decreased NO bioavailability could interfere with NO-mediated signaling events. We were interested in investigating the underlying mechanism(s) of the reduced NO and how it would regulate the S-nitrosylation of tissue transglutaminase (TG2) and its substrates on glycolytic, redox and inflammatory responses in normal and IS-induced EC injury. TG2, a therapeutic target for fibrosis, has a Ca²⁺-dependent transamidase (TGase) that is modulated by S-nitrosylation. We found IS increased oxidative stress, reduced NADPH and GSH levels, and uncoupled eNOS to generate NO. Immunoblot analysis demonstrated the upregulation of an angiotensin-converting enzyme (ACE) and significant downregulation of the beneficial ACE2 isoform that could contribute to oxidative stress in IS-induced injury. An in situ TGase assay demonstrated IS-activated TG2/TGase aminylated eNOS, NFkB, IkBα, PKM2, G6PD, GAPDH, and fibronectin (FN), leading to caspases activation. Except for FN, TGase substrates were all differentially S-nitrosylated either with or without IS but were denitrosylated in the presence of a specific, irreversible TG2/TGase inhibitor ZDON, suggesting ZDON-bound TG2 was not effectively transnitrosylating to TG2/TGase substrates. The data suggest novel roles of TG2 in the aminylation of its substrates and could also potentially function as a Cys-to-Cys S-nitrosylase to exert NO's bioactivity to its substrates and modulate glycolysis, redox, and inflammation in normal and IS-induced EC injury.

Indoxyl Sulfate Promotes Metastatic Characteristics of Ovarian Cancer Cells via Aryl Hydrocarbon Receptor-Mediated Downregulation of the Mas Receptor

Although platinum-combination chemotherapy shows a high response rate at the primary site, epithelial ovarian cancer (EOC) treatment remains challenging because of tumor recurrence and metastasis. Recent studies have revealed that chemotherapy paradoxically promotes cancer cell survival, proliferation, and metastasis, although the reason for this remains unclear. The underlying molecular mechanisms that contribute to chemotherapy-induced metastasis need to be elucidated to establish effective therapeutic strategies. Acute kidney injury is a known side effect of cisplatin treatment, and kidney dysfunction results in the accumulation of uremic toxins in the serum. The present study aimed to investigate whether indoxyl sulfate (IS), a representative uremic toxin, affects the pathophysiology of EOC. In this study, IS reduced the expression of Mas receptor (MasR) in cultured human EOC cells. Both knockdown of the aryl hydrocarbon receptor (AhR), which is an intracellular IS receptor, and inhibition of AhR function suppressed IS-mediated downregulation of MasR in SK-OV-3 cells. IS induced the phosphorylation of signal transducer and activator of transcription 3 (STAT3) in an AhR-dependent manner. Inhibition of the STAT3 pathway or reactive oxygen species production suppressed the IS-mediated reduction of MasR. IS stimulated cell migration and invasion of SK-OV-3 cells in an AhR-dependent manner. Cisplatin-nephropathy model mice exhibited elevated levels of serum IS accompanied by elevated levels of blood urea nitrogen and serum creatinine. Furthermore, intraperitoneal administration of IS in mice promoted tumor growth and metastasis. Finally, we found that the MasR agonist Ang-(1-7) suppressed the IS-mediated effects on cell proliferation, migration, and invasion of SK-OV-3 cells. However, the knockdown of MasR expression by specific small interfering RNA in the absence of IS resulted in only minimal promotion of cell migration and invasion. These findings demonstrate that IS promotes malignancy in ovarian cancer via AhR-mediated downregulation of MasR function, whereas Ang-(1-7) attenuates this effect, thereby suggesting that Ang-(1-7) could provide a future treatment strategy for this cancer type.

Activation of aryl hydrocarbon receptor (AhR) in Alzheimer's disease: role of tryptophan metabolites generated by gut host-microbiota

Gut microbiota in interaction with intestinal host tissues influences many brain functions and microbial dysbiosis has been linked with brain disorders, such as neuropsychiatric conditions and Alzheimer's disease (AD). L-tryptophan metabolites and short-chained fatty acids (SCFA) are major messengers in the microbiota-brain axis. Aryl hydrocarbon receptors (AhR) are main targets of tryptophan metabolites in brain microvessels which possess an enriched expression of AhR protein. The Ah receptor is an evolutionarily conserved, ligand-activated transcription factor which is not only a sensor of xenobiotic toxins but also a pleiotropic regulator of both developmental processes and age-related tissue degeneration. Major microbiota-produced tryptophan metabolites involve indole derivatives, e.g., indole 3-pyruvic acid, indole 3-acetaldehyde, and indoxyl sulfate, whereas indoleamine and tryptophan 2,3-dioxygenases (IDO/TDO) of intestine host cells activate the kynurenine (KYN) pathway generating KYN metabolites, many of which are activators of AhR signaling. Chronic kidney disease (CKD) increases the serum level of indoxyl sulfate which promotes AD pathogenesis, e.g., it disrupts integrity of blood-brain barrier (BBB) and impairs cognitive functions. Activation of AhR signaling disturbs vascular homeostasis in brain; (i) it controls blood flow via the renin-angiotensin system, (ii) it inactivates endothelial nitric oxide synthase (eNOS), thus impairing NO production and vasodilatation, and (iii) it induces oxidative stress, stimulates inflammation, promotes cellular senescence, and enhances calcification of vascular walls. All these alterations are evident in cerebral amyloid angiopathy (CAA) in AD pathology. Moreover, AhR signaling can disturb circadian regulation and probably affect glymphatic flow. It seems plausible that dysbiosis of gut microbiota impairs the integrity of BBB via the activation of AhR signaling and thus aggravates AD pathology. KEY MESSAGES: Dysbiosis of gut microbiota is associated with dementia and Alzheimer's disease. Tryptophan metabolites are major messengers from the gut host-microbiota to brain. Tryptophan metabolites activate aryl hydrocarbon receptor (AhR) signaling in brain. The expression of AhR protein is enriched in brain microvessels and blood-brain barrier. Tryptophan metabolites disturb brain vascular integrity via AhR signaling. Dysbiosis of gut microbiota promotes inflammation and AD pathology via AhR signaling.

Uremic toxins activate CREB/ATF1 in endothelial cells related to chronic kidney disease

Uremic toxins, such as p-cresyl sulfate (PCS) and indoxyl sulfate (IS), contribute to endothelial dysfunction in chronic kidney disease (CKD). This process is mediated by several cellular pathways, but it is unclear whether cAMP-responsive element-binding protein (CREB) and activating transcription factor 1 (ATF1) participate in endothelial dysfunction in uremic conditions despite playing roles in inflammatory modulation. This study aimed to evaluate the expression, activation, and transcriptional activity of CREB/ATF1 in endothelial cells exposed to PCS, IS, and uremic serum (US). In vitro, ATF1 protein levels were increased by PCS and IS, whereas CREB levels were enhanced only by IS. Activation through CREB-Ser¹³³ and ATF1-Ser⁶³ phosphorylation was induced by PCS, IS, and US. We evaluated the CREB/ATF1 transcriptional activity by analyzing the expression of their target genes, including ICAM1, PTGS2, NOX1, and SLC22A6, which are related to endothelial dysfunction through their roles in vascular inflammation, oxidative stress, and cellular uptake of PCS and IS. The expression of ICAM1, PTGS2 and NOX1 genes was increased by PCS, IS, and US, whereas that of SLC22A6 was induced only by IS. KG-501, a CREB inhibitor, restored the inductive effects of PCS on ICAM1, PTGS2, and NOX1 expression; IS on ICAM1, PTGS2 and SLC22A6 expression; and US on NOX1 expression. The presence of CREB and ATF1 was observed in healthy arteries and in arteries of patients with CKD, which were structurally damaged. These findings suggest that CREB/ATF1 is activated by uremic toxins and may play a relevant role in endothelial dysfunction in CKD.

PbAc Triggers Oxidation and Apoptosis via the PKA Pathway in NRK-52E Cells

This study aimed to investigate the mechanism of the lead exposure-induced oxidative stress and apoptosis of renal tubular epithelial cells. We explored the effects of lead acetate (PbAc) on the oxidation and apoptosis of renal proximal tubular cells (NRK-52E) through in vitro experiments. Results showed that PbAc induced dose-dependent reactive oxygen species (ROS) accumulation in NRK-52E cells, and the activities of superoxide dismutase (SOD) and glutathione (GSH) decreased, whereas the malondialdehyde (MDA) content increased. Under the exposure of 40 and 80 μM PbAc, the mRNA level of B cell lymphoma-2 (Bcl-2) in the cells decreased, the mRNA levels of Bcl-2-associated X protein (Bax) and caspase-3 increased, and apoptosis was obvious. Furthermore, the nicotinamide adenine dinucleotide phosphate oxidase 4 (Nox4) activity was enhanced by PbAc in a dose-dependent manner. The mRNA levels of protein kinase A (PKA) were upregulated by PbAc. H-89, a PKA inhibitor, suppressed PKA activation, ROS accumulation, and Nox4 activity in NRK-52E cells. Our results indicated that PbAc potentially stimulated oxidative stress and apoptosis in NRK-52E cells by increasing Nox4-dependent ROS production via the PKA signaling pathway.

Indoxyl-Sulfate-Induced Redox Imbalance in Chronic Kidney Disease

The accumulation of the uremic toxin indoxyl sulfate (IS) induces target organ damage in chronic kidney disease (CKD) patients, and causes complications including cardiovascular diseases, renal osteodystrophy, muscle wasting, and anemia. IS stimulates reactive oxygen species (ROS) production in CKD, which impairs glomerular filtration by a direct cytotoxic effect on the mesangial cells. IS further reduces antioxidant capacity in renal proximal tubular cells and contributes to tubulointerstitial injury. IS-induced ROS formation triggers the switching of vascular smooth muscular cells to the osteoblastic phenotype, which induces cardiovascular risk. Low-turnover bone disease seen in early CKD relies on the inhibitory effects of IS on osteoblast viability and differentiation, and osteoblastic signaling via the parathyroid hormone. Excessive ROS and inflammatory cytokine releases caused by IS directly inhibit myocyte growth in muscle wasting via myokines’ effects. Moreover, IS triggers eryptosis via ROS-mediated oxidative stress, and elevates hepcidin levels in order to prevent iron flux in circulation in renal anemia. Thus, IS-induced oxidative stress underlies the mechanisms in CKD-related complications. This review summarizes the underlying mechanisms of how IS mediates oxidative stress in the pathogenesis of CKD’s complications. Furthermore, we also discuss the potential role of oral AST-120 in attenuating IS-mediated oxidative stress after gastrointestinal adsorption of the IS precursor indole.

Protein-Bound Uremic Toxins and Immunity

Protein-bound uremic toxins (PBUTs) are bioactive microbiota metabolites originated exclusively from protein fermentation of the bacterial community resident within the gut microbiota, whose composition and function is profoundly different in the chronic kidney disease (CKD) population. PBUTs accumulate in the later stages of CKD because they cannot be efficiently removed by conventional hemodialysis due to their high binding affinity for albumin, worsening their toxic effects, especially at the cardiovascular level. The accumulation of uremic toxins, along with oxidative stress products and pro-inflammatory cytokines, characterizes the uremic status of CKD patients which is increasingly associated to a state of immune dysfunction including both immune activation and immunodepression. Furthermore, the links between immune activation and cardiovascular disease (CVD), and between immunodepression and infection diseases, which are the two major complications of CKD, are becoming more and more evident. This review summarizes and discusses the current state of knowledge on the role of the main PBUTs, namely indoxyl sulfate and p-cresyl sulfate, as regulators of immune response in CKD, in order to understand whether a microbiota modulation may be useful in the management of its main complications, CVD, and infections. Summarizing the direct effects of PBUT on immune system we may conclude that PCS seemed to be associated to an immune deficiency status of CKD mainly related to the adaptative immune response, while IS seemed to reflect the activation of both innate and adaptative immune systems likely responsible of the CKD-associated inflammation. However, the exact role of IS and PCS on immunity modulation in physiological and pathological state still needs in-depth investigation, particularly in vivo studies.

Gut microbiome - A potential mediator of pathogenesis in heart failure and its comorbidities: State-of-the-art review

Gut microbiome (GMB) has been increasingly recognized as a contributor to development and progression of heart failure (HF), immune-mediated subtypes of cardiomyopathy (myocarditis and anthracycline-induced cardiotoxicity), response to certain cardiovascular drugs, and HF-related comorbidities, such as chronic kidney disease, cardiorenal syndrome, insulin resistance, malnutrition, and cardiac cachexia. Gut microbiome is also responsible for the "gut hypothesis" of HF, which explains the adverse effects of gut barrier dysfunction and translocation of GMB on the progression of HF. Furthermore, accumulating evidence has suggested that gut microbial metabolites, including short chain fatty acids, trimethylamine N-oxide (TMAO), amino acid metabolites, and bile acids, are mechanistically linked to pathogenesis of HF, and could, therefore, serve as potential therapeutic targets for HF. Even though there are a variety of proposed therapeutic approaches, such as dietary modifications, prebiotics, probiotics, TMAO synthesis inhibitors, and fecal microbial transplant, targeting GMB in HF is still in its infancy and, indeed, requires further preclinical and clinical evidence. In this review, we aim to highlight the role gut microbiome plays in HF pathophysiology and its potential as a novel therapeutic target in HF.

Standalone or combinatorial phenylbutyrate therapy shows excellent antiviral activity and mimics CREB3 silencing

Herpesviruses are ubiquitous human pathogens that tightly regulate many cellular pathways including the unfolded protein response to endoplasmic reticulum (ER) stress. Pharmacological modulation of this pathway results in the inhibition of viral replication. In this study, we tested 4-phenylbutyrate (PBA), a chemical chaperone-based potent alleviator of ER stress, for its effects on herpes simplex virus (HSV) type 1 infection. Through in vitro studies, we observed that application of PBA to HSV-infected cells results in the down-regulation of a proviral, ER-localized host protein CREB3 and a resultant inhibition of viral protein synthesis. PBA treatment caused viral inhibition in cultured human corneas and human skin grafts as well as murine models of ocular and genital HSV infection. Thus, we propose that this drug can provide an alternative to current antivirals to treat both ocular HSV-1 and genital HSV-2 infections and may be a strong candidate for human trials.

Uremic Apelin and Leucocytic Angiotensin-Converting Enzyme 2 in CKD Patients

Apelin peptides (APLN) serve as second substrates for angiotensin-converting enzyme 2 (ACE2) and, in contrast to angiotensin II (AngII), exert blood-pressure lowering and vasodilatation effects through binding to G-coupled APLN receptor (APLNR). ACE2-mediated cleavage of the APLN may reduce its vasodilatory effects, but decreased ACE2 may potentiate the hypotensive properties of APLN. The role of APLN in uremia is unclear. We investigated the correlations between serum-APLN, leucocytic APLNR, and ACE2 in 32 healthy controls (NP), 66 HD, and 24 CKD3-5 patients, and the impact of APLN peptides on monocytic behavior and ACE2 expression under uremic conditions in vitro. We observed that serum APLN and leucocytic APLNR or SLCO2B1 were significantly elevated in uremic patients and correlated with decreased ACE2 on uremic leucocytes. APLN-treated THP-1 monocytes revealed significantly increased APLNR and ACE2, and reduced TNFa, IL-6, and MCSF. Uremic toxins induced a dramatic increase of miR-421 followed by significant reduction of ACE2 transcripts, partially counteracted with APLN-13 and -36. APLN-36 triggered the most potent transmigration and reduction of endothelial adhesion. These results suggest that although APLN peptides may partly protect against the decay of monocytic ACE2 transcripts, uremic milieu is the most dominant modulator of local ACE2, and likely to contribute to the progression of atherosclerosis.

Indoxyl Sulfate, a Tubular Toxin, Contributes to the Development of Chronic Kidney Disease

Indoxyl sulfate (IS), a uremic toxin, causes chronic kidney disease (CKD) progression via its tubulotoxicity. After cellular uptake, IS directly induces apoptotic and necrotic cell death of tubular cells. Additionally, IS increases oxidative stress and decreases antioxidant capacity, which are associated with tubulointerstitial injury. Injured tubular cells are a major source of transforming growth factor-β1 (TGF-β1), which induces myofibroblast transition from residual renal cells in damaged kidney, recruits inflammatory cells and thereby promotes extracellular matrix deposition in renal fibrosis. Moreover, IS upregulates signal transducers and activators of transcription 3 phosphorylation, followed by increases in TGF-β1, monocyte chemotactic protein-1 and α-smooth muscle actin production, which participate in interstitial inflammation, renal fibrosis and, consequently, CKD progression. Clinically, higher serum IS levels are independently associated with renal function decline and predict all-cause mortality in CKD. The poor removal of serum IS in conventional hemodialysis is also significantly associated with all-cause mortality and heart failure incidence in end-stage renal disease patients. Scavenging the IS precursor by AST-120 can markedly reduce tubular IS staining that attenuates renal tubular injury, ameliorates IS-induced oxidative stress and rescues antioxidant glutathione activity in tubular epithelial cells, thereby providing a protective role against tubular injury and ultimately retarding renal function decline.

(-)-Epicatechin and the colonic metabolite 2,3-dihydroxybenzoic acid protect against high glucose and lipopolysaccharide-induced inflammation in renal proximal tubular cells through NOX-4/p38 signalling

Chronic hyperglycaemia and inflammation are present in diabetes and both processes have been related to the pathogenesis of diabetic kidney disease. Epicatechin (EC) and main colonic phenolic acids derived from flavonoid intake, such as 2,3-dihydroxybenzoic acid (DHBA), 3,4-dihydroxyphenylacetic acid (DHPAA) and 3-hydroxyphenylpropionic acid (HPPA), have been suggested to exert beneficial effects in diabetes. This study was aimed at investigating whether the mentioned compounds could prevent inflammation in renal proximal tubular NRK-52E cells induced by high glucose and lipopolysaccharide (LPS). Pre-treatment of cells with EC and DHBA (5 μM) reverted the enhanced levels of pro-inflammatory cytokines, such as tumour necrosis factor-α (TNF-α), interleukin-6 (IL-6) and monocyte chemoattractant protein 1 (MCP-1), activated by high glucose and LPS. Additionally, EC and DHBA pre-incubation reduced the increased values of adhesion molecules, namely, intercellular cell adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), as well as those of mitogen-activated protein kinases (MAPKs) [extracellular signal-regulated kinase (ERK), -c-jun N-terminal kinase (JNK) and -p38 protein kinase (p38)] activated by the high glucose and LPS challenge. Thus, in EC and DHBA pre-treated cells ICAM-1, p-ERK and p-JNK were returned to control values, and VCAM-1 and p-p38 levels were reduced by ∼20 and 25%, respectively, when compared to high glucose plus LPS-stimulated cells. Likewise, pre-treatment with EC and DHBA protected against high glucose plus LPS-triggered oxidative stress by preventing increased ROS and NADPH oxidase 4 (NOX-4) levels (∼25 and 45% reduction, respectively). By using specific inhibitors of p38 and NOX-4, the participation of both proteins in EC- and DHBA-mediated protection against inflammation and associated oxidative stress was shown. Taken together, EC and DHBA exert beneficial effects in renal proximal tubular cells, as they contribute to preventing the inflammatory-induced milieu and the accompanying redox imbalance, playing NOX-4/p38 a crucial role.

A review on the efficacy and mechanism of action of Shenkang injection against chronic kidney disease

Chronic kidney disease (CKD) is one of the most common conditions which significantly increases the risk for serious health outcomes. Epidemiological investigations have shown that CKD has become a serious global health problem. At present, there are no treatments for CKD, thus the need for an effective and safe treatment for this condition. Shenkang Injection (SKI), which is an herbal medication in Chinese Medicine, has been used in the management and treatment of CKD and has achieved favorable therapeutic effects. The purpose of this paper is to review the clinical efficacy, mechanism of action, and safety profile of SKI when used in CKD, and to provide comprehensive potential evidence for its clinical application.

Plasma indoxyl sulfate levels predict cardiovascular events in patients with mild chronic heart failure

Indoxyl sulfate (IS) is associated with either chronic kidney disease or renal failure, which may predict cardiovascular events via cardiorenal syndrome. The present study aimed to elucidate whether the plasma levels of IS can predict the occurrence of cardiovascular events in patients with chronic heart failure (CHF) and investigate which causes of CHF leading to cardiovascular events are highly influenced by plasma IS levels. We measured the plasma IS levels in 165 patients with CHF [valvular disease: 78, dilated cardiomyopathy: 29, hypertrophic cardiomyopathy (HCM): 25 and others: 33] admitted to our hospital in 2012, and we followed up these patients for more than 5 years (the median follow-up period: 5.3 years). We measured the plasma IS level in 165 patients with CHF, and Kaplan–Meier analyses showed that high plasma IS levels (≥ 0.79 µg/mL, the median value) could predict the occurrence of cardiovascular events, i.e., cardiovascular death or rehospitalization due to the worsening of CHF. The sub-analyses showed that the high IS level could predict cardiovascular events in patients with CHF due to HCM and that the plasma IS levels were closely associated with left ventricular (LV) dimension, LV systolic dysfunction, and plasma B-type natriuretic peptide levels, rather than LV diastolic dysfunction. Plasma IS level predicts cardiovascular events in patients with CHF, especially those with HCM along with cardiac dysfunction. Besides, IS may become a proper biomarker to predict cardiovascular events in patients with CHF.

Hippuric Acid Promotes Renal Fibrosis by Disrupting Redox Homeostasis via Facilitation of NRF2-KEAP1-CUL3 Interactions in Chronic Kidney Disease

Chronic kidney disease (CKD) is characterized by the accumulation of protein-bound uremic toxins (PBUTs), which play a pathophysiological role in renal fibrosis (a common pathological process resulting in CKD progression). Accumulation of the PBUT hippuric acid (HA) is positively correlated with disease progression in CKD patients, suggesting that HA may promote renal fibrosis. Oxidative stress is the most important factor affecting PBUTs nephrotoxicity. Herein, we assessed the ability of HA to promote kidney fibrosis by disrupting redox homeostasis. In HK-2 cells, HA increased fibrosis-related gene expression, extracellular matrix imbalance, and oxidative stress. Additionally, reactive oxygen species (ROS)-mediated TGFβ/SMAD signaling contributed to HA-induced fibrotic responses. HA disrupted antioxidant networks by decreasing the levels of nuclear factor erythroid 2-related factor 2 (NRF2), leading to ROS accumulation and fibrotic responses, as evidenced by NRF2 activation and knockdown. Moreover, NRF2 levels were reduced by NRF2 ubiquitination, which was regulated via increased interactions of Kelch-like ECH-associated protein 1 with Cullin 3 and NRF2. Finally, renal fibrosis and redox imbalance promoted by HA were confirmed in rats. Importantly, sulforaphane (NRF2 activator) reversed HA-promoted renal fibrosis. Thus, HA promotes renal fibrosis in CKD by disrupting NRF2-driven antioxidant system, indicating that NRF2 is a potential therapeutic target for CKD.

Therapeutic Targeting of Aristolochic Acid Induced Uremic Toxin Retention, SMAD 2/3 and JNK/ERK Pathways in Tubulointerstitial Fibrosis: Nephroprotective Role of Propolis in Chronic Kidney Disease

The nephrotoxicity of aristolochic acids (AAs), p-cresyl sulfate (PCS) and indoxyl sulfate (IS) were well-documented, culminating in tubulointerstitial fibrosis (TIF), advanced chronic kidney disease (CKD) and fatal urothelial cancer. Nonetheless, information regarding the attenuation of AAs-induced nephropathy (AAN) and uremic toxin retention is scarce. Propolis is a versatile natural product, exerting anti-oxidant, anti-cancer and anti-fibrotic properties. We aimed to evaluate nephroprotective effects of propolis extract (PE) in a murine model. AAN was developed to retain circulating PCS and IS using C57BL/6 mice, mimicking human CKD. The kidney sizes/masses, renal function indicators, plasma concentrations of PCS/IS, tissue expressions of TIF, α-SMA, collagen IaI, collagen IV and signaling pathways in transforming growth factor-β (TGF-β) family were analyzed among the control, PE, AAN, and AAN-PE groups. PE ameliorated AAN-induced renal atrophy, renal function deterioration, TIF, plasma retention of PCS and IS. PE also suppressed α-SMA expression and deposition of collagen IaI and IV in the fibrotic epithelial-mesenchymal transition. Notably, PE treatment in AAN model inhibited not only SMAD 2/3-dependent pathways but also SMAD-independent JNK/ERK activation in the signaling cascades of TGF-β family. Through disrupting fibrotic epithelial-mesenchymal transition and TGF-β signaling transduction pathways, PE improves TIF and thereby facilitates renal excretion of PCS and IS in AAN. In light of multi-faced toxicity of AAs, PE may be capable of developing a new potential drug to treat CKD patients exposed to AAs.

Focus on Early Events: Pathogenesis of Pulmonary Arterial Hypertension Development

Significance: Pulmonary arterial hypertension (PAH) is a progressive disease of the lung vasculature characterized by the proliferation of all vascular wall cell types, including endothelial, smooth muscle, and fibroblasts. The disease rapidly advances into a form with extensive pulmonary vascular remodeling, leading to a rapid increase in pulmonary vascular resistance, which results in right heart failure. Recent Advances: Most current research in the PAH field has been focused on the late stage of the disease, largely due to an urgent need for patient treatment options in clinics. Further, the pathobiology of PAH is multifaceted in the advanced disease, and there has been promising recent progress in identifying various pathological pathways related to the late clinical picture. Critical Issues: Early stage PAH still requires additional attention from the scientific community, and although the survival of patients with early diagnosis is comparatively higher, the disease develops in patients asymptomatically, making it difficult to identify and treat early. Future Directions: There are several reasons to focus on the early stage of PAH. First, the complexity of late stage disease, owing to multiple pathways being activated in a complex system with intra- and intercellular signaling, leads to an unclear picture of the key contributors to the pathobiology. Second, an understanding of early pathophysiological events can increase the ability to identify PAH patients earlier than what is currently possible. Third, the prompt diagnosis of PAH would allow for the therapy to start earlier, which has proved to be a more successful strategy, and it ensures better survival in PAH patients.

Serum metabolic profile characteristics of offspring rats before and after birth caused by prenatal caffeine exposure

Epidemiological investigations have confirmed that prenatal caffeine intake could increase the incidence rate of intrauterine growth retardation (IUGR) and multiple diseases after birth. Based on liquid chromatography-mass spectrometry, we analyzed serum metabolic profiles of offspring rats before and after birth in IUGR model induced by prenatal caffeine exposure (PCE). We discovered that differential metabolites in PCE fetuses mainly manifested as amino acids and lipid metabolism. In adulthood, PCE offspring showed less and inconsistent types of differential metabolites compared to those in utero, which still exhibited gender differences. The main differential metabolites induced by PCE, including phospholipids, platelet-activating factor, arachidonic acid, bile acid, sphingosine-1-phosphoric acid, indoxyl sulfuric acid, and cortexolone, may participate in the pathological and physiological processes of organ toxicities. This study demonstrated the short- and long-term developmental toxicity and gender differences of caffeine, providing new ideas for exploring the early warning and drug intervention targets of IUGR offspring.

Crosstalk Between Connexin32 and Mitochondrial Apoptotic Signaling Pathway Plays a Pivotal Role in Renal Ischemia Reperfusion-Induced Acute Kidney Injury

Aims: Perioperative acute kidney injury (AKI) resulting from renal ischemia reperfusion (IR) is not conducive to the postoperative surgical recovery. Our previous study demonstrated that reactive oxygen species (ROS) transmitted by gap junction (GJ) composed of connexin32 (Cx32) contributed to AKI. However, the precise underlying pathophysiologic mechanisms were largely unknown. This study focuses on the underlying mechanisms related to ROS transmitted by Cx32 responsible for AKI aggravation. Results: In a set of in vivo studies, renal IR was found to cause severe impairment in renal tissues with massive ROS generation, which occurred contemporaneously with activation of NF-κB/p53/p53 upregulated modulator of apoptosis (PUMA)-mediated mitochondrial apoptosis pathways. Cx32 deficiency alleviated renal IR-induced AKI, and simultaneously attenuated ROS generation and distribution in renal tissues, which further inhibited NF-κB/p53/PUMA-mediated mitochondrial apoptotic pathways. Correspondingly, in a set of in vitro studies, hypoxia reoxygenation (HR)-induced cellular injury, and cell apoptosis in both human kidney tubular epithelial cells (HK-2s) and rat kidney tubular epithelial cells (NRK52Es) were significantly attenuated by Cx32 inhibitors or Cx32 gene knockdown. More importantly, Cx32 inhibition not only decreased ROS generation and distribution in human or rat kidney tubular epithelial cells but also inhibited its downstream NF-κB/p53/PUMA-mediated mitochondrial apoptotic pathway activation. Innovation and Conclusion: This is the first identification of the underlying mechanisms of IR-induced renal injury integrally which demonstrates the critical role played by Cx32 in IR-induced AKI. Moreover, GJ composed of Cx32 manipulates ROS generation and distribution between neighboring cells, and alters activation of NF-κB/p53/PUMA-mediated mitochondrial apoptotic pathways. Both inhibiting Cx32 function and scavenging ROS effectively reduce mitochondrial apoptosis and subsequently attenuate AKI, providing effective strategies for kidney protection.

Indoxyl Sulfate Induces Renal Fibroblast Activation through a Targetable Heat Shock Protein 90-Dependent Pathway

Indoxyl sulfate (IS) accumulation occurs early during chronic kidney disease (CKD) progression and contributes to renal dysfunction by inducing fibrosis, inflammation, oxidative stress, and tissue remodeling. Renal toxicity of high IS concentrations (250 μM) has been widely explored, particularly in resident tubular and glomerular cells, while the effect of a moderate IS increase on kidneys is still mostly unknown. To define the effects of IS accumulation on renal fibroblasts, we first analyzed kidneys of C57BL/6 mice receiving IS (0.1%) in drinking water for 12 weeks. As a next step, we treated renal fibroblasts (NRK-49F) with IS (20 μM) with or without the HSP90 inhibitor 17-AAG (1 μM). In mouse kidneys, IS increased the collagen deposition and HSP90 and α-SMA expression (immunohistochemistry) in interstitial fibroblasts and caused tubular necrosis (histological H&E and picrosirius red staining). In NRK-49F cells, IS induced MCP1, TGF-β, collagen I, α-SMA, and HSP90 gene/protein expression and Smad2/3 pathway activation. IS had no effects on fibroblast proliferation and ROS production. 17-AAG counteracted IS-induced MCP1, TGF-β, collagen I, and α-SMA expression and Smad2/3 phosphorylation. Our study demonstrates that the IS increase promotes renal fibroblast activation by a HSP90-dependent pathway and indicates HSP90 inhibition as a potential strategy to restrain IS-induced kidney inflammation and fibrosis in CKD.

AST-120, an Adsorbent of Uremic Toxins, Improves the Pathophysiology of Heart Failure in Conscious Dogs

PURPOSE

Several lines of evidence suggest that renal dysfunction is associated with cardiovascular toxicity through the action of uremic toxins. The levels of those uremic toxins can be reportedly reduced by the spherical carbon adsorbent AST-120. Because heart failure (HF) causes renal dysfunction by low cardiac output and renal edema, the removal of uremic toxins could be cardioprotective.

METHOD

To determine whether blood levels of the uremic toxin indoxyl sulfate (IS) increase in HF and whether AST-120 can reduce those levels and improve HF. We induced HF in 12 beagle dogs by 6 weeks of rapid right ventricular pacing at 230 beats per min. We treated six dogs with a 1-g/kg/day oral dosage of AST-120 for 14 days from week 4 after the start of rapid ventricular pacing. The other six dogs did not receive any treatment (control group).

RESULTS

In the untreated dogs, IS levels increased as cardiac function deteriorated. In contrast, plasma IS levels in the treated dogs decreased to baseline levels, with both left ventricular fractional shortening and pulmonary capillary wedge pressure also improving when compared with untreated dogs. Finally, AST-120 treatment was shown to reduce both myocardial apoptosis and fibrosis along with decreases in extracellular signal-regulated kinase phosphorylation, the Bax/Bcl-2 ratio, and TGF-β1 expression and increases in AKT phosphorylation.

CONCLUSIONS

IS levels are increased in HF. AST-120 treatment reduces the levels of IS and improves the pathophysiology of HF in a canine model. AST-120 could be a novel candidate for the treatment of HF.

Molecular mechanisms of protein-bound uremic toxin-mediated cardiac, renal and vascular effects: underpinning intracellular targets for cardiorenal syndrome therapy

Cardiorenal syndrome (CRS) remains a global health burden with a lack of definitive and effective treatment. Protein-bound uremic toxin (PBUT) overload has been identified as a non-traditional risk factor for cardiac, renal and vascular dysfunction due to significant albumin-binding properties, rendering these solutes non-dialyzable upon the state of irreversible kidney dysfunction. Although limited, experimental studies have investigated possible mechanisms in PBUT-mediated cardiac, renal and vascular effects. The ultimate aim is to identify relevant and efficacious targets that may translate beneficial outcomes in disease models and eventually in the clinic. This review will expand on detailed knowledge on mechanisms involved in detrimental effects of PBUT, specifically affecting the heart, kidney and vasculature, and explore potential effective intracellular targets to abolish their effects in CRS initiation and/or progression.

Skatole regulates intestinal epithelial cellular functions through activating aryl hydrocarbon receptors and p38

Intestinal bacteria produce skatole (3-methylindole) from tryptophan in dietary proteins and ingesting large quantities of animal protein is associated with increased fecal skatole concentrations. Although possibly associated with disrupted intestinal homeostasis, the influence of skatole on intestinal epithelial cellular function has not been characterized in detail. The present study aimed to determine whether skatole induces intestinal epithelial cell (IEC) dysfunction. We found that skatole dose-dependently caused IEC death and time-dependently induced IEC apoptosis. Since skatole directly interacts with aryl hydrocarbon receptors (AhR), we investigated whether these receptors influence the skatole-induced death of IEC. In addition to increased AhR transcriptional activity induced by skatole, the AhR antagonist CH223191 partially suppressed of skatole-induced IEC death. Extracellular signal-related kinase (ERK), p38 and c-Jun N-terminal kinase (JNK) are mitogen-activated protein kinases (MAPK) induced by skatole. None of them were repressed by CH223191, whereas the p38 inhibitor SB203580 promoted skatole-induced IEC death. These findings together indicated that skatole induces both AhR-dependent activation pathways and the AhR-independent activation of p38, consequently regulating the amount of IEC death. Accumulating evidence indicates that consuming large amounts of animal protein is associated with the pathogenesis and progression of inflammatory bowel diseases (IBD). Thus, intestinal skatole production induced by large amounts of dietary animal protein might be associated via IEC death with intestinal pathologies such as IBD.

G Protein-Coupled Estrogen Receptor Protects From Angiotensin II-Induced Increases in Pulse Pressure and Oxidative Stress

Our previous work showed that the G protein-coupled estrogen receptor (GPER) is protective in the vasculature and kidneys during angiotensin (Ang) II-dependent hypertension by inhibiting oxidative stress. The goal of the current study was to assess the impact of GPER deletion on sex differences in Ang II-induced hypertension and oxidative stress. Male and female wildtype and GPER knockout mice were implanted with radiotelemetry probes for measurement of baseline blood pressure before infusion of Ang II (700 ng/kg/min) for 2 weeks. Mean arterial pressure was increased to the same extent in all groups, but female wildtype mice were protected from Ang II-induced increases in pulse pressure, aortic wall thickness, and Nox4 mRNA. In vitro studies using vascular smooth muscle cells found that pre-treatment with the GPER agonist G-1 inhibited Ang II-induced ROS and NADP/NADPH. Ang II increased while G-1 decreased Nox4 mRNA and protein. The effects of Ang II were blocked by losartan and Nox4 siRNA, while the effects of G-1 were inhibited by adenylyl cyclase inhibition and mimicked by phosphodiesterase inhibition. We conclude that during conditions of elevated Ang II, GPER via the cAMP pathway suppresses Nox4 transcription to limit ROS production and prevent arterial stiffening. Taken together with our previous work, this study provides insight into how acute estrogen signaling via GPER provides cardiovascular protection during Ang II hypertension and potentially other diseases characterized by increased oxidative stress.

Modulation of leucocytic angiotensin-converting enzymes expression in patients maintained on high-permeable haemodialysis

Background

High mortality of haemodialysis patients is associated with systemic chronic inflammation and overactivation of the renin-angiotensin system (RAS). Insufficient elimination of pro-inflammatory immune mediators, especially in the molecular weight range of 15-45 kDa, may be one of the reasons for this. Employment of haemodialysis membranes with increased permeability was shown to ameliorate the inflammatory response and might modulate the effects of local RAS. In this study, we tested the impact of high cut-off (HCO), medium cut-off (MCO) and high-flux (HF) dialysis on leucocytic transcripts of angiotensin-converting enzymes (ACE and ACE2). Additionally, the impact of HCO, MCO and HF sera and dialysates on local ACEs and inflammation markers was tested in THP-1 monocytes.

Methods

Patients' leucocytes were obtained from our recent clinical studies comparing HCO and MCO dialysers with HF. The cells were subjected to quantitaive polymerase chain reaction (qPCR) analyses with TaqMan probes specific for ACE, ACE2 and angiotensin II (AngII) and Ang1-7 receptors. Sera and dialysates from the clinical trials as well as samples from in vitro dialysis were tested on THP-1 monocytic cells. The cells were subjected to qPCR analyses with TaqMan probes specific for ACE, ACE2, interleukin-6 and tumour necrosis factor α and immunocytochemistry with ACE and ACE2 antibodies.

Results

Leucocytes obtained from patients treated with HCO or MCO demonstrated decreased transcript expression of ACE, while ACE2 was significantly upregulated as compared with HF. Receptors for AngII and Ang1-7 remained unchanged. THP-1 monocytes preconditioned with HCO and MCO patients' or in vitro dialysis sera reflected the same expressional regulation of ACE and ACE2 as those observed in HCO and MCO leucocytes. As a complementary finding, treatment with HCO and MCO in vitro dialysates induced a pro-inflammatory response of the cells as demonstrated by elevated messenger RNA expression of tumour necrosis factor α and interleukin-6, as well as upregulation of ACE and decreased levels of ACE2.

Conclusions

Taken together, these data demonstrate that employment of membranes with high permeability eliminates a spectrum of mediators from circulation that affect the RAS components in leucocytes, especially ACE/ACE2.

ALDH2 protects against alcoholic cardiomyopathy through a mechanism involving the p38 MAPK/CREB pathway and local renin-angiotensin system inhibition in cardiomyocytes

BACKGROUND

Angiotensin II (Ang II) in the local cardiac renin-angiotensin system (RAS) is closely associated with alcoholic cardiomyopathy (ACM). Inhibition of local cardiac RAS has great significance in the treatment of ACM. Although aldehyde dehydrogenase 2 (ALDH2) has been demonstrated to protect against ACM through detoxification of aldehydes, the precise mechanisms are largely unknown. In the present study, we determined whether ALDH2 improved cardiac damage by inhibiting the local RAS in ACM and investigated the related regulatory mechanisms.

METHODS AND RESULTS

Adult male mice were fed with 5% ethanol or a control diet for 2months, with or without the ALDH2 activator Alda-1. Heavy ethanol consumption induced cardiac damage, increased angiotensinogen (AGT) and Ang II and decreased myocardial ALDH2 activity in hearts. ALDH2 activation improved ethanol-induced cardiac damage and decreased AGT and Ang II in hearts. In vitro, ALDH2 activation or overexpression decreased AGT and Ang II in cultured cardiomyocytes treated with 400mmol/L ethanol for 24h. Furthermore, p38 MAP kinase (p38 MAPK)/cyclic adenosine monophosphate response element-binding protein (CREB) pathway activation by ethanol increased AGT and Ang II in cardiomyocytes. In addition, ALDH2 activation or overexpression inhibited the p38 MAPK/CREB pathway leading to decreased AGT and Ang II in cardiomyocytes. We also found that p38 MAPK activation effectively mitigated Alda-1-decreased AGT and Ang II, the effect of which was reversed by inhibition of CREB.

CONCLUSIONS

ALDH2 decreased AGT and Ang II in the local cardiac RAS via inhibiting the p38 MAPK/CREB pathway in ACM, thus improving ethanol-induced cardiac damage.

Generation, clearance, toxicity, and monitoring possibilities of unaccounted uremic toxins for improved dialysis prescriptions

Current dialysis-dosing calculations provide an incomplete assessment of blood purification. They exclude clearances of protein-bound uremic toxins (PB-UTs), such as polyamines, p-cresol sulfate, and indoxyl sulfate, relying solely on the clearance of urea as a surrogate for all molecules accumulating in patients with end-stage renal disease (ESRD). PB-UTs clear differently in dialysis but also during normal renal function. The kidney clears PB toxins via the process of secretion, whereas it clears urea through filtration. Herein, we review the clearance, accumulation, and toxicity of various UTs. We also suggest possible methods for their monitoring toward the ultimate goal of a more comprehensive dialysis prescription. A more inclusive dialysis prescription would retain the kidney-filtration surrogate, urea, and consider at least one PB toxin as a surrogate for UTs cleared through cellular secretion. A more comprehensive assessment of UTs that includes both secretion and filtration is expected to result in a better understanding of ESRD toxicity and consequently, to reduce ESRD mortality.

The double-edged sword of endoplasmic reticulum stress in uremic sarcopenia through myogenesis perturbation

BACKGROUND

Sarcopenia is the age-related degeneration characterized with the decline of skeletal muscle mass, strength, and function. The imbalance of protein synthesis and degradation which jeopardizes immune, hormone regulation, and muscle-motor neuron connection is the main cause of sarcopenia. There is limited knowledge regarding molecular mechanism of sarcopenia. As the endoplasmic reticulum is the control centre of the protein syntheses and degradation, we hypothesized that endoplasmic reticulum stress and unfolded protein response (UPR) play an important in the development of sarcopenia. Understanding the sarcopenia molecular mechanisms may benefit the therapeutic diagnosis and treatment in the future.

METHODS

Mouse myoblast C2C12 cells are exposed to designated time and concentration of indoxyl sulfate (IS), a uremic toxin of chronic kidney disease. The proliferation, differentiation, and the expression of atrogin 1 are examined. The protein and mRNA expression of IS treated-C2C12 cells are inspected to distinguish the role of ER stress and oxidative stress underlying the sarcopenia.

RESULTS

Indoxyl sulfate inhibits myoblast differentiation. We demonstrate that as the number of multi-nuclei myotube decreased, the differentiation markers including myoD, myoG, and myosin heavy chain are also suppressed. Indoxyl sulfate inhibits myoblast proliferation and induces the myotubular atrophy marker atrogin-1 protein expression. Indoxyl sulfate stimulates eIF2α phosphorylation and XBP1 mRNA splicing in UPR. Interestingly, the oxidative stress is related to eIF2α phosphorylation but not XBP1 mRNA splicing. The eIF2α phosphorylation triggered by IS reduces myoD, myoG, and myosin heavy chain protein expression, which represents the anti-myogenic modulation on the early differentiation event. The XBP1 mRNA splicing induced by IS, however, is considered the adaptive response to restore the myogenic differentiation.

CONCLUSIONS

Our studies indicated that the ER stress and UPR modulation are critical in the chronic kidney disease uremic toxin-accumulated sarcopenia model. We believe that UPR-related signals showed great potential in clinical application.

Monocytic angiotensin-converting enzyme 2 relates to atherosclerosis in patients with chronic kidney disease

Background: Increased levels of monocytic angiotensin-converting enzyme (ACE) found in haemodialysis (HD) patients may directly participate in the pathogenesis of atherosclerosis. We demonstrated recently that uremia triggers the development of highly pro-atherogenic monocytes via an angiotensin II (AngII)–dependent mechanism. Opposing actions of the AngII-degrading ACE2 remain largely unknown. We examined the status of both ACEs and related receptors in circulating leukocytes of HD, not-dialyzed CKD and healthy individuals. Furthermore, we tested the possible impact of monocytic ACEs on atherogenesis and behaviour of the cells under conditions mimicking chronic renal failure.

Methods: Expression of ACE, ACE2, AT1R, AT2R and MASR was investigated on circulating leukocytes from 71 HD (62 ± 14 years), 24 CKD stage 3–5 (74 ± 10 years) patients and 37 healthy control subjects (53 ± 6 years) and isolated healthy monocytes treated with normal and uremic serum. Analyses of ACE, ACE2, ICAM-1, VCAM-1, MCSF and endothelial adhesion were tested on ACE-overexpressing THP-1 monocytes treated with captopril or losartan. ACE2-overexpressing monocytes were subjected to transmigration and adhesion assays and investigated for MCP-1, ICAM-1, VCAM-1, MCSF, AT1R and AT2R expression.

Results: The ACE mRNA level was significantly increased in HD and CKD stage 3–5 leukocytes. Correspondingly, ACE2 was downregulated and AngII as well as MAS receptor expression was upregulated in these cells. Healthy monocytes preconditioned with uremic serum reflected the same expressional regulation of ACE/ACE2, MAS and AngII receptors as those observed in HD and CKD stage 3–5 leukocytes. Overexpression of monocytic ACE dramatically decreased levels of ACE2 and induced a pro-atherogenic phenotype, partly reversed by AngII-modifying treatments, leading to an increase in ACE2. Overexpression of ACE2 in monocytes led to reduced endothelial adhesion, transmigration and downregulation of adhesion-related molecules.

Conclusions: HD and not-dialyzed CKD stage 3–5 patients show enhanced ACE and decreased ACE2 expression on monocytes. This constellation renders the cells endothelial adhesive and likely supports the development of atherosclerosis.

Microbiota metabolites: Pivotal players of cardiovascular damage in chronic kidney disease

In chronic kidney disease (CKD), cardiovascular (CV) damage is present in parallel which leads to an increased risk of CV disease. Both traditional and non-traditional risk factors contribute to CV damage in CKD. The systemic role of the microbiota as a central player in the pathophysiology of many organs is progressively emerging in the literature: the microbiota is indeed involved in a complex, bi-directional network between many organs, including the kidney and heart connection, although many of these relationships still need to be elucidated through in-depth mechanistic studies. The aim of this review is to provide evidence that microbiota metabolites influence non-traditional risk factors, such as inflammation and endothelial dysfunction in CKD-associated CV damage. Here, we report our current understanding and hypotheses on the gut-kidney and gut-heart axes and provide details on the potential mechanisms mediated by microbial metabolites. More specifically, we summarize some novel hypotheses linking the microbiota to blood pressure regulation and hypertension. We also emphasise the idea that the nutritional management of CKD should be redesigned and include the new findings from research on the intrinsic plasticity of the microbiota and its metabolites in response to food intake. The need is felt to integrate the classical salt and protein restriction approach for CKD patients with foods that enhance intestinal wellness. Finally, we discuss the new perspectives, especially the importance of taking care of the microbiota in order to prevent the risk of developing CKD and hypertension, as well as the still not tested but very promising CKD innovative treatments, such as postbiotic supplementation and bacteriotherapy. This interesting area of research offers potential complementary approaches to the management of CKD and CV damage assuming that the causal mechanisms underlying the gut-kidney and gut-heart axes are clarified. This will pave the way to the design of new personalized therapies targeting gut microbiota.

Indole-3-propionic acid suppresses indoxyl sulfate-induced expression of fibrotic and inflammatory genes in proximal tubular cells

Indoxyl sulfate (IS) induces fibrosis and inflammation in kidneys via oxidative stress through the induction of transforming growth factor-β1 (TGF-β1) and monocyte chemotactic protein-1 (MCP-1). Furthermore, IS is a potent endogenous agonist for aryl hydrocarbon receptor (AHR), which regulates the transcription of genes such as cytochrome P450 (CYP) 1A1. Indole-3-propionic acid (IPA) is an antioxidant and has been reported to be neuroprotective. We determined whether IPA suppresses IS-induced expression of AHR, CYP1A1, TGF-β1, and MCP-1 in proximal tubular cells. The effects of IS on the expression of AHR, CYP1A1, TGF-β1, and MCP-1 were studied using normotensive rats and hypertensive rats. The effects of IPA on IS-induced expression of AHR, CYP1A1, TGF-β1, and MCP-1 were studied using proximal tubular cells (HK-2). Furthermore, the effects of IPA on IS-induced expression and phosphorylation of signal transducer and activator of transcription 3 (Stat3) were studied in HK-2 cells. Administration of IS induced the expression of AHR, CYP1A1, TGF-β1, and MCP-1 in the tubular cells of rat kidneys. IPA significantly suppressed IS-induced mRNA and protein expression of AHR, CYP1A1, TGF-β1, and MCP-1 in HK-2 cells. IPA suppressed the IS-induced expression and phosphorylation of Stat3 in HK-2 cells. Furthermore, knockdown of Stat3 inhibited the IS-induced mRNA and protein expression of AHR, CYP1A1, TGF-β1, and MCP-1 in HK-2 cells. In conclusion, IPA suppressed the IS-induced expression of AHR, CYP1A1, TGF-β1, and MCP-1 through suppression of Stat3 in proximal tubular cells. Thus, IPA suppresses IS-induced expression of fibrotic and inflammatory genes in proximal tubular cells.

Influence of AHRR Pro189Ala polymorphism on kidney functions

The function of aryl hydrocarbon receptor repressor (AHRR) in the kidney is unclear. The present study investigated associations between AHRR Pro189Ala polymorphism and estimated glomerular filtration rates (eGFR), serum creatinine, and hemoglobin levels in 2775 Japanese adults without diabetes. In addition, we examined whether AHRR expression levels in the kidney of control and chronic kidney disease (CKD) rats were changed. Multiple linear regression analyses showed that carriers of the Ala allele had increased eGFR and lower concentrations of serum creatinine and hemoglobin (p &lt; 0.05). Immunohistochemical analysis showed that the expression of AHRR was upregulated in the kidneys of rats with CKD. These findings suggest that AHRR plays distinct roles in kidney functions and hemoglobin values. The effects of the AHRR polymorphism might be intensified in the kidneys of patients with CKD.

Kidney–brain axis inflammatory cross-talk: from bench to bedside

Epidemiologic data suggest that individuals at all stages of chronic kidney disease (CKD) have a higher risk of developing neuropsychiatric disorders, cognitive impairment, and dementia. This risk is generally explained by the high prevalence of both symptomatic and subclinical ischemic cerebrovascular lesions. However, other potential mechanisms, including cytokine/chemokine release, production of reactive oxygen species (ROS), circulating and local formation of trophic factors and of renin–angiotensin system (RAS) molecules, could also be involved, especially in the absence of obvious cerebrovascular disease. In this review, we discuss experimental and clinical evidence for the role of these mechanisms in kidney–brain cross-talk. In addition, we hypothesize potential pathways for the interactions between kidney and brain and their pathophysiological role in neuropsychiatric and cognitive changes found in patients with CKD. Understanding the pathophysiologic interactions between renal impairment and brain function is important in order to minimize the risk for future cognitive impairment and to develop new strategies for innovative pharmacological treatment.

Corticotropin releasing hormone receptor 2 exacerbates chronic cardiac dysfunction

Prognosis of patients with chronic heart failure remains poor, emphasizing the need to identify additional pathophysiological factors. Tsuda et al. show that Crhr2 activation causes cardiac dysfunction and suggest Crhr2 blockade is a promising therapeutic strategy for chronic heart failure.

Heart failure occurs when the heart is unable to effectively pump blood and maintain tissue perfusion. Despite numerous therapeutic advancements over previous decades, the prognosis of patients with chronic heart failure remains poor, emphasizing the need to identify additional pathophysiological factors. Here, we show that corticotropin releasing hormone receptor 2 (Crhr2) is a G protein–coupled receptor highly expressed in cardiomyocytes and continuous infusion of the Crhr2 agonist, urocortin 2 (Ucn2), reduced left ventricular ejection fraction in mice. Moreover, plasma Ucn2 levels were 7.5-fold higher in patients with heart failure compared to those in healthy controls. Additionally, cardiomyocyte-specific deletion of Crhr2 protected mice from pressure overload-induced cardiac dysfunction. Mice treated with a Crhr2 antagonist lost maladaptive 3′-5′-cyclic adenosine monophosphate (cAMP)–dependent signaling and did not develop heart failure in response to overload. Collectively, our results indicate that constitutive Crhr2 activation causes cardiac dysfunction and suggests that Crhr2 blockade is a promising therapeutic strategy for patients with chronic heart failure.

Indoxyl sulfate potentiates endothelial dysfunction via reciprocal role for reactive oxygen species and RhoA/ROCK signaling in 5/6 nephrectomized rats

Accumulative indoxyl sulfate (IS) retained in chronic kidney disease (CKD) can potentiate vascular endothelial dysfunction, and herein, we aim at elucidating the underlying mechanisms from the perspective of possible association between reactive oxygen species (ROS) and RhoA/ROCK pathway. IS-treated nephrectomized rats are administered with antioxidants including NADPH oxidase inhibitor apocynin, SOD analog tempol, and mitochondrion-targeted SOD mimetic mito-TEMPO to scavenge ROS, or ROCK inhibitor fasudil to obstruct RhoA/ROCK pathway. First, we find in response to IS stimulation, antioxidants treatments suppress increased aortic ROCK activity and expression levels. Additionally, ROCK blockade prevent IS-induced increased NADPH oxidase expression (mainly p22phox and p47phox), mitochondrial and intracellular ROS (superoxide and hydrogen peroxide) generation, and decreased Cu/Zn-SOD expression in thoracic aortas. Apocynin, mito-TEMPO, and tempol also reverse these markers of oxidative stress. These results suggest that IS induces excessive ROS production and ROCK activation involving a circuitous relationship in which ROS activate ROCK and ROCK promotes ROS overproduction. Finally, ROS and ROCK depletion attenuate IS-induced decrease in nitric oxide (NO) production and eNOS expression levels, and alleviate impaired vasomotor responses including increased vasocontraction to phenylephrine and decreased vasorelaxation to acetylcholine, thereby preventing cardiovascular complications accompanied by CKD. Taken together, excessive ROS derived from NADPH oxidase and mitochondria coordinate with RhoA/ROCK activation in a form of positive reciprocal relationship to induce endothelial dysfunction through disturbing endothelium-dependent NO signaling upon IS stimulation in CKD status.

Indoxyl sulphate and kidney disease: Causes, consequences and interventions

In the last decade, chronic kidney disease (CKD), defined as reduced renal function (glomerular filtration rate (GFR) < 60 mL/min per 1.73 m(2) ) and/or evidence of kidney damage (typically manifested as albuminuria) for at least 3 months, has become one of the fastest-growing public health concerns worldwide. CKD is characterized by reduced clearance and increased serum accumulation of metabolic waste products (uremic retention solutes). At least 152 uremic retention solutes have been reported. This review focuses on indoxyl sulphate (IS), a protein-bound, tryptophan-derived metabolite that is generated by intestinal micro-organisms (microbiota). Animal studies have demonstrated an association between IS accumulation and increased fibrosis, and oxidative stress. This has been mirrored by in vitro studies, many of which report cytotoxic effects in kidney proximal tubular cells following IS exposure. Clinical studies have associated IS accumulation with deleterious effects, such as kidney functional decline and adverse cardiovascular events, although causality has not been conclusively established. The aims of this review are to: (i) establish factors associated with increased serum accumulation of IS; (ii) report effects of IS accumulation in clinical studies; (iii) critique the reported effects of IS in the kidney, when administered both in vivo and in vitro; and (iv) summarize both established and hypothetical therapeutic options for reducing serum IS or antagonizing its reported downstream effects in the kidney.

Indoxyl Sulfate Downregulates Mas Receptor via Aryl Hydrocarbon Receptor/Nuclear Factor-kappa B, and Induces Cell Proliferation and Tissue Factor Expression in Vascular Smooth Muscle Cells

BACKGROUND/AIM

Angiotensin converting enzyme-related carboxypeptidase 2/angiotensin (Ang)-(1-7)/Mas receptor axis is protective in the development of chronic kidney disease and cardiovascular disease. This study is aimed at investigating whether indoxyl sulfate (IS) affects Mas receptor expression, cell proliferation and tissue factor expression in vascular smooth muscle cells, and if Ang-(1-7), an activator of Mas receptor, counteracts the IS-induced effects.

METHODS

IS was administered to normotensive and hypertensive rats. Human aortic smooth muscle cells (HASMCs) were cultured with IS.

RESULTS

IS reduced the expression of Mas receptor in the aorta of normotensive and hypertensive rats. IS downregulated the Mas receptor expression in a time- and dose-dependent manner in HASMCs. Knockdown of aryl hydrocarbon receptor (AhR) and nuclear factor-kappa B (NF-x03BA;B) inhibited IS-induced downregulation of Mas receptor. Further, IS stimulated cell proliferation and tissue factor expression in HASMCs. Ang-(1-7) attenuated IS-induced cell proliferation and tissue factor expression in HASMCs. Ang-(1-7) suppressed phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and NF-x03BA;B in HASMCs.

CONCLUSION

IS downregulated the expression of Mas receptor via AhR/NF-x03BA;B, and induced cell proliferation and tissue factor expression in HASMCs. Ang-(1-7) inhibited IS-induced cell proliferation and tissue factor expression by suppressing the phosphorylation of ERK1/2 and NF-x03BA;B p65.

The balance of powers: Redox regulation of fibrogenic pathways in kidney injury

Oxidative stress plays a central role in the pathogenesis of diverse chronic inflammatory disorders including diabetic complications, cardiovascular disease, aging, and chronic kidney disease (CKD). Patients with moderate to advanced CKD have markedly increased levels of oxidative stress and inflammation that likely contribute to the unacceptable high rates of morbidity and mortality in this patient population. Oxidative stress is defined as an imbalance of the generation of reactive oxygen species (ROS) in excess of the capacity of cells/tissues to detoxify or scavenge them. Such a state of oxidative stress may alter the structure/function of cellular macromolecules and tissues that eventually leads to organ dysfunction. The harmful effects of ROS have been largely attributed to its indiscriminate, stochastic effects on the oxidation of protein, lipids, or DNA but in many instances the oxidants target particular amino acid residues or lipid moieties. Oxidant mechanisms are intimately involved in cell signaling and are linked to several key redox-sensitive signaling pathways in fibrogenesis. Dysregulation of antioxidant mechanisms and overproduction of ROS not only promotes a fibrotic milieu but leads to mitochondrial dysfunction and further exacerbates kidney injury. Our studies support the hypothesis that unique reactive intermediates generated in localized microenvironments of vulnerable tissues such as the kidney activate fibrogenic pathways and promote end-organ damage. The ability to quantify these changes and assess response to therapies will be pivotal in understanding disease mechanisms and monitoring efficacy of therapy.

Increased Proinflammatory Cytokine Production and Decreased Cholesterol Efflux Due to Downregulation of ABCG1 in Macrophages Exposed to Indoxyl Sulfate

One of the possible causes of enhanced atherosclerosis in patients with chronic kidney disease (CKD) is the accumulation of uremic toxins. Since macrophage foam cell formation is a hallmark of atherosclerosis, we examined the direct effect of indoxyl sulfate (IS), a representative uremic toxin, on macrophage function. Macrophages differentiated from THP-1 cells were exposed to IS in vitro. IS decreased the cell viability of THP-1 derived macrophages but promoted the production of inflammatory cytokines (IL-1β, IS 1.0 mM: 101.8 ± 21.8 pg/mL vs. 0 mM: 7.0 ± 0.3 pg/mL, TNF-α, IS 1.0 mM: 96.6 ± 11.0 pg/mL vs. 0 mM: 15.1 ± 3.1 pg/mL) and reactive oxygen species. IS reduced macrophage cholesterol efflux (IS 0.5 mM: 30.3% ± 7.3% vs. 0 mM: 43.5% ± 1.6%) and decreased ATP-binding cassette transporter G1 expression. However, lipid uptake into cells was not enhanced. A liver X receptor (LXR) agonist, T0901317, improved IS-induced production of inflammatory cytokines as well as reduced cholesterol efflux. In conclusion, IS induced inflammatory reactions and reduced cholesterol efflux in macrophages. Both effects of IS were improved with activation of LXR. Direct interactions of uremic toxins with macrophages may be a major cause of atherosclerosis acceleration in patients with CKD.

Uric acid, indoxyl sulfate, and methylguanidine activate bulbospinal neurons in the RVLM via their specific transporters and by producing oxidative stress

Patients with chronic renal failure often have hypertension, but the cause of hypertension, other than an excess of body fluid, is not well known. We hypothesized that the bulbospinal neurons in the rostral ventrolateral medulla (RVLM) are stimulated by uremic toxins in patients with chronic renal failure. To investigate whether RVLM neurons are sensitive to uremic toxins, such as uric acid, indoxyl sulfate, or methylguanidine, we examined changes in the membrane potentials (MPs) of bulbospinal RVLM neurons of Wister rats using the whole-cell patch-clamp technique during superfusion with these toxins. A brainstem-spinal cord preparation that preserved the sympathetic nervous system was used for the experiments. During uric acid, indoxyl sulfate, or methylguanidine superfusion, almost all the RVLM neurons were depolarized. To examine the transporters for these toxins on RVLM neurons, histological examinations were performed. The uric acid-, indoxyl sulfate-, and methylguanidine-depolarized RVLM neurons showed the presence of urate transporter 1 (URAT 1), organic anion transporter (OAT)1 or OAT3, and organic cation transporter (OCT)3, respectively. Furthermore, the toxin-induced activities of the RVLM neurons were suppressed by the addition of an anti-oxidation drug (VAS2870, an NAD(P)H oxidase inhibitor), and a histological examination revealed the presence of NAD(P)H oxidase (nox)2 and nox4 in these RVLM neurons. The present results show that uric acid, indoxyl sulfate, and methylguanidine directly stimulate bulbospinal RVLM neurons via specific transporters on these neurons and by producing oxidative stress. These uremic toxins may cause hypertension by activating RVLM neurons.