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

Pro-Arrhythmic Potential of Accumulated Uremic Toxins Is Mediated via Vulnerability of Action Potential Repolarization

Chronic kidney disease (CKD) is represented by a diminished filtration capacity of the kidneys. End-stage renal disease patients need dialysis treatment to remove waste and toxins from the circulation. However, endogenously produced uremic toxins (UTs) cannot always be filtered during dialysis. UTs are among the CKD-related factors that have been linked to maladaptive and pathophysiological remodeling of the heart. Importantly, 50% of the deaths in dialysis patients are cardiovascular related, with sudden cardiac death predominating. However, the mechanisms responsible remain poorly understood. The current study aimed to assess the vulnerability of action potential repolarization caused by exposure to pre-identified UTs at clinically relevant concentrations. We exposed human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and HEK293 chronically (48 h) to the UTs indoxyl sulfate, kynurenine, or kynurenic acid. We used optical and manual electrophysiological techniques to assess action potential duration (APD) in the hiPSC-CMs and recorded IKr currents in stably transfected HEK293 cells (HEK-hERG). Molecular analysis of KV11.1, the ion channel responsible for IKr, was performed to further understand the potential mechanism underlying the effects of the UTs. Chronic exposure to the UTs resulted in significant APD prolongation. Subsequent assessment of the repolarization current IKr, often most sensitive and responsible for APD alterations, showed decreased current densities after chronic exposure to the UTs. This outcome was supported by lowered protein levels of KV11.1. Finally, treatment with an activator of the IKr current, LUF7244, could reverse the APD prolongation, indicating the potential modulation of electrophysiological effects caused by these UTs. This study highlights the pro-arrhythmogenic potential of UTs and reveals a mode of action by which they affect cardiac repolarization.

Uremic toxins in chronic kidney disease highlight a fundamental gap in understanding their detrimental effects on cardiac electrophysiology and arrhythmogenesis

Chronic kidney disease (CKD) and cardiovascular disease (CVD) have an estimated 700-800 and 523 million cases worldwide, respectively, with CVD being the leading cause of death in CKD patients. The pathophysiological interplay between the heart and kidneys is defined as the cardiorenal syndrome (CRS), in which worsening of kidney function is represented by increased plasma concentrations of uremic toxins (UTs), culminating in dialysis patients. As there is a high incidence of CVD in CKD patients, accompanied by arrhythmias and sudden cardiac death, knowledge on electrophysiological remodeling would be instrumental for understanding the CRS. While the interplay between both organs is clearly of importance in CRS, the involvement of UTs in pro-arrhythmic remodeling is only poorly investigated, especially regarding the mechanistic background. Currently, the clinical approach against potential arrhythmic events is mainly restricted to symptom treatment, stressing the need for fundamental research on UT in relation to electrophysiology. This review addresses the existing knowledge of UTs and cardiac electrophysiology, and the experimental research gap between fundamental research and clinical research of the CRS. Clinically, mainly absorbents like ibuprofen and AST-120 are studied, which show limited safe and efficient usability. Experimental research shows disturbances in cardiac electrical activation and conduction after inducing CKD or exposure to UTs, but are scarcely present or focus solely on already well-investigated UTs. Based on UTs data derived from CKD patient cohort studies, a clinically relevant overview of physiological and pathological UTs concentrations is created. Using this, future experimental research is stimulated to involve electrophysiologically translatable animals, such as rabbits, or in vitro engineered heart tissues.

Indole-3-acetic acid correlates with monocyte-to-high-density lipoprotein (HDL) ratio (MHR) in chronic kidney disease patients

PURPOSE

Indole-3-acetic acid is a protein-bound indolic uremic toxin deriving from tryptophan metabolism. Increased levels are associated with higher thrombotic risk and both cardiovascular and all-cause mortality. An emerging biomarker of cardiovascular disease is the monocyte-to-high-density lipoprotein ratio (MHR). The main purpose of this study was to investigate the association of indole-3-acetic acid with MHR and other markers of cardiovascular risk in patients with chronic kidney disease (CKD).

METHODS

We enrolled 61 non-dialysis CKD patients and 6 dialysis patients. Indole-3-acetic acid levels were measured with ELISA technique.

RESULTS

In the whole cohort of 67 patients, indole-3-acetic acid was directly related to Ca × P (ρ = 0.256; P = 0.0365) and MHR (ρ = 0.321; P = 0.0082). In the 40 patients with previous cardiovascular events, indole-3-acetic acid correlated with uric acid (r = 0.3952; P = 0.0116) and MHR (ρ = 0.380; P = 0.0157). MHR was related with fibrinogen (ρ = 0.426; P = 0.0010), arterial hypertension (ρ = 0.274; P = 0.0251), C-reactive protein (ρ = 0.332; P = 0.0061), gender (ρ = - 0.375; P = 0.0017; 0 = male, 1 = female), and CKD stage (ρ = 0.260; P = 0.0337). A multiple regression analysis suggested that indole-3-acetic acid might be an independent predictor of MHR.

CONCLUSION

This study shows a significant association between indole-3-acetic acid and MHR. Prospective studies are required to evaluate if decreasing indole-3-acetic acid concentrations may reduce MHR levels and cardiovascular events and improve clinical outcomes.

Cardiorenal syndrome: long road between kidney and heart

Almost 200 years ago, the first evidence described by Robert Bright (1836) showed the strong interaction between the kidneys and heart and, since then, the scientific community has dedicated itself to better understanding the mechanisms involved in the kidney-heart relationship, known in recent decades as cardiorenal syndrome (CRS). This syndrome includes a wide clinical variety that affects the kidneys and heart, in an acute or chronic manner. Moreover, it is well established in the literature that the immune system, the sympathetic nervous system, the renin-angiotensin-aldosterone, and the oxidative stress actively play a strong role in the cellular and molecular processes present in CRS. More recently, uremic molecules and epigenetic factors have been also shown to be key mediators in the development of syndrome. The present review intends to present the state of the art regarding CRS and to show the paths known, until now, in the long road between the kidneys and heart.

The effect of dihydroceramide desaturase 1 inhibition on endothelial impairment induced by indoxyl sulfate

Protein-bound uremic toxins (PBUTs) have adverse effects on vascular function, which is imperative in the progression of cardiovascular and renal diseases. The role of sphingolipids in PBUT-mediated vasculo-endothelial pathophysiology is unclear. This study assessed the therapeutic potential of dihydroceramide desaturase 1 (Des1) inhibition, the last enzyme involved in de novo ceramide synthesis, to mitigate the vascular effects of the PBUT indoxyl sulfate (IS). Rat aortic rings were isolated and vascular reactivity was assessed in organ bath experiments followed by immunohistochemical analyses. Furthermore, cultured human aortic endothelial cells were assessed for phenotypic and mechanistic changes. Inhibition of Des1 by a selective inhibitor CIN038 (0.1 to 0.3 μM) improved IS-induced impairment of vasorelaxation and modulated immunoreactivity of oxidative stress markers. Des1 inhibition also reversed IS-induced reduction in endothelial cell migration (1.0 μM) by promoting the expression of angiogenic cytokines and reducing inflammatory and oxidative stress markers. These effects were associated with a reduction of TIMP1 and the restoration of Akt phosphorylation. In conclusion, Des1 inhibition improved vascular relaxation and endothelial cell migration impaired by IS overload. Therefore, Des1 may be a suitable intracellular target to mitigate PBUT-induced adverse vascular effects.

Sphingolipid imbalance and inflammatory effects induced by uremic toxins in heart and kidney cells are reversed by dihydroceramide desaturase 1 inhibition

Non-dialysable protein-bound uremic toxins (PBUTs) contribute to the development of cardiovascular disease (CVD) in chronic kidney disease (CKD) and vice versa. PBUTs have been shown to alter sphingolipid imbalance. Dihydroceramide desaturase 1 (Des1) is an important gatekeeper enzyme which controls the non-reversible conversion of sphingolipids, dihydroceramide, into ceramide. The present study assessed the effect of Des1 inhibition on PBUT-induced cardiac and renal effects in vitro, using a selective Des1 inhibitor (CIN038). Des1 inhibition attenuated hypertrophy in neonatal rat cardiac myocytes and collagen synthesis in neonatal rat cardiac fibroblasts and renal mesangial cells induced by the PBUTs, indoxyl sulfate and p-cresol sulfate. This is at least attributable to modulation of NF-κB signalling and reductions in β-MHC, Collagen I and TNF-α gene expression. Lipidomic analyses revealed Des1 inhibition restored C16-dihydroceramide levels reduced by indoxyl sulfate. In conclusion, PBUTs play a critical role in mediating sphingolipid imbalance and inflammatory responses in heart and kidney cells, and these effects were attenuated by Des1 inhibition. Therefore, sphingolipid modifying agents may have therapeutic potential for the treatment of CVD and CKD and warrant further investigation.

Apoptosis signal-regulating kinase 1 inhibition reverses deleterious indoxyl sulfate-mediated endothelial effects

AIMS

Indoxyl sulfate (IS), a protein-bound uremic toxin, is implicated in endothelial dysfunction, which contributes to adverse cardiovascular events in chronic kidney disease. Apoptosis signal regulating kinase 1 (ASK1) is a reactive oxygen species-driven kinase involved in IS-mediated adverse effects. This study assessed the therapeutic potential of ASK1 inhibition in alleviating endothelial effects induced by IS.

MAIN METHODS

IS, in the presence and absence of a selective ASK1 inhibitor (GSK2261818A), was assessed for its effect on vascular reactivity in rat aortic rings, and cultured human aortic endothelial cells where we evaluated phenotypic and mechanistic changes.

KEY FINDINGS

IS directly impairs endothelium-dependent vasorelaxation and endothelial cell migration. Mechanistic studies revealed increased production of reactive oxygen species-related markers, reduction of endothelial nitric oxide synthase and increased protein expression of tissue inhibitor of matrix metalloproteinase 1 (TIMP1). IS also increases angiopoietin-2 and tumour necrosis factor α gene expression and promotes transforming growth factor β receptor abundance. Inhibition of ASK1 ameliorated the increase in oxidative stress markers, promoted autocrine interleukin 8 pro-angiogenic signalling and decreased anti-angiogenic responses at least in part via reducing TIMP1 protein expression.

SIGNIFICANCE

ASK1 inhibition attenuated vasorelaxation and endothelial cell migration impaired by IS. Therefore, ASK1 is a viable intracellular target to alleviate uremic toxin-induced impairment in the vasculature.

Gut microbiome and cardiovascular disease

PURPOSE OF REVIEW

This review aims to highlight the association between gut microbiome and cardiovascular disease (CVD) with emphasis on the possible molecular mechanisms by which how gut microbiome contributes to CVD.

RECENT FINDINGS

Increasingly, the roles of gut microbiome in cardiovascular health and disease have gained much attention. Most of the investigations focus on how the gut dysbiosis contributes to CVD risk factors and which gut microbial-derived metabolites mediate such effects.

SUMMARY

In this review, we discuss the molecular mechanisms of gut microbiome contributing to CVD, which include gut microbes translocalization to aortic artery because of gut barrier defect to initiate inflammation and microbial-derived metabolites inducing inflammation-signaling pathway and renal insufficiency. Specifically, we categorize beneficial and deleterious microbial-derived metabolites in cardiovascular health. We also summarize recent findings in the gut microbiome modulation of drug efficacy in treatment of CVD and the microbiome mechanisms by which how physical exercise ameliorates cardiovascular health. Gut microbiome has become an essential component of cardiovascular research and a crucial consideration factor in cardiovascular health and disease.

Forensic interrogation of diabetic endothelitis in cardiovascular diseases and clinical translation in heart failure

Diabetic heart disease (DHD) can be classified as a primary consequence from several pathophysiological manifestation of diabetes mellitus (DM) on cardiac tissues or secondarily in extracardiac tissues and is encountered as either primary or secondary complications of DM. Endothelitis is inflammation of the vascular endothelium and is likely to be seen in the majority of patients who start to manifest an end organ complication of DM in this case DHD. Diabetes is a leading cause for many cardiovascular syndromes and diseases including congestive heart failure (CHF) however much remains unknown about the transition from diagnosed DM to clinical state and the contribution of the various mechanical and counterregulatory systems in the manifested complaint. Diastolic heart failure or heart failure with preserved ejection fraction (DHF/HFpEF), accounts for half of all CHF presentations, has DM as a major contributor, however, there remain large gaps in clinical and pathophysiological understanding. This review aims to explore the microscopic aspects in diabetic endothelitis and provide a clinical link to with context to HFpEF.

Cardiorenal syndrome: Multi-organ dysfunction involving the heart, kidney and vasculature

Cardiorenal syndrome (CRS) is a multi-organ disease, encompassing heart, kidney and vascular system dysfunction. CRS is a worldwide problem, with high morbidity, mortality, and inflicts a significant burden on the health care system. The pathophysiology is complex, involving interactions between neurohormones, inflammatory processes, oxidative stress and metabolic derangements. Therapies remain inadequate, mainly comprising symptomatic care with minimal prospect of full recovery. Challenges include limiting the contradictory effects of multi-organ targeted drug prescriptions and continuous monitoring of volume overload. Novel strategies such as multi-organ transplantation and innovative dialysis modalities have been considered but lack evidence in the CRS context. The adjunct use of pharmaceuticals targeting alternative pathways showing positive results in preclinical models also warrants further validation in the clinic. In recent years, studies have identified the involvement of gut dysbiosis, uraemic toxin accumulation, sphingolipid imbalance and other unconventional contributors, which has encouraged a shift in the paradigm of CRS therapy.

Quantification of free and protein bound uremic toxins in human serum by LC-MS/MS: Comparison of rapid equilibrium dialysis and ultrafiltration

The objectives of this study were (1) to develop a method for the determination of 10 uremic toxins (3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), hippuric acid, indole-3-acetic acid, indoxyl sulfate, kynurenic acid, kynurenine, p-cresyl glucuronide, p-cresyl sulfate, phenylacetylglutamine and trimethylamine N-oxide (TMAO)), and 3 precursors (tyrosine, phenylalanine, tryptophan) in serum and (2) to compare two separation methods to determine the free serum fraction: rapid equilibrium dialysis (RED) and ultrafiltration (UF). The method was developed on a liquid chromatography system coupled to a tandem mass spectrometer. Fifty µL of serum sample were precipitated with methanol after addition of internal standard. The two separation methods were compared using serum samples from patients suffering from renal impairment (n = 30). The method has been validated according to the European Medicines Agency (EMA) guidelines. Calibration curves were linear from 1 to 50 ng/mL up to 10,000-50,000 ng/mL according to the compounds. The comparison between the two separation methods produced similar results for all compounds except kynurenine, tryptophan (around 30% more with UF) and indole-3-acetic acid (around 30% more with RED). This study has allowed the development and validation of a sensitive and robust assay for the quantification of free and total concentrations of 10 uremic toxins and 3 precursors in human serum.

Inhibition of CYP1B1 ameliorates cardiac hypertrophy induced by uremic toxin

Cardiovascular disease is the predominant complication and leading cause of mortality in patients with chronic kidney disease (CKD). Previous studies have revealed that uremic toxins, including indoxyl sulfate (IS), participate in cardiac hypertrophy. As a heme‑thiolate monooxygenase, cytochrome P450 family 1 subfamily B member 1 (CYP1B1) is able to metabolize arachidonic acid into hydroxyeicosatetraenoic acids, which are thought to serve a central function in the pathophysiology of the cardiovascular system. However, whether CYP1B1 is involved in cardiac hypertrophy induced by uremic toxins remains unknown. The present study revealed that the expression of the CYP1B1 gene was significantly (P<0.05, CKD or IS vs. control) upregulated by CKD serum or IS at the transcriptional and translational level. Furthermore, IS treatment resulted in the nuclear translocation of aryl hydrocarbon receptor (AhR), an endogenous ligand of IS. Binding of AhR in the promoter region of CYP1B1 was confirmed using a chromatin immunoprecipitation assay in the cardiomyoblast H9c2 cell line. In addition, knockdown of AhR or CYP1B1 reversed the production of cardiac hypertrophy markers. The in vivo injection of a CYP1B1 inhibitor significantly (P<0.05, Inhibitor vs. control) attenuated cardiac hypertrophy in mice. The data from the present study clearly demonstrated that CYP1B1 was involved in cardiac hypertrophy induced by uremic toxins.