Endoplasmic reticulum stress associated apoptosis as a novel mechanism in indoxyl sulfate‑induced cardiomyocyte toxicity

Chronic Kidney Disease Is Associated With Increased Cardiac Corin Expression But Decreased Proatrial Natriuretic Peptide Conversion Activity

Background

Chronic kidney disease (CKD) is associated with an increased risk of cardiovascular disease. Corin converts proatrial natriuretic peptide into its active form after being activated by PCSK6 (proprotein convertase subtilisin/kexin type 6) protease. It remains unknown whether the PCSK6/corin/atrial natriuretic peptide pathway plays a role in CKD‐induced cardiomyopathy.

Methods and Results

Serum corin, left ventricular mass index, and corin–left ventricular mass index correlation were compared between outpatients with versus without CKD. Cardiac corin expression and activity as well as serum corin were compared between 5/6 nephrectomy CKD animal models and sham controls. The effects of indoxyl sulfate, a uremic toxin, on cardiomyocytes were examined in vitro in H9c2 cells. A total of 543 patients were enrolled in this study. Serum corin levels were elevated in patients with CKD compared with levels in patients without CKD. Serum corin levels correlated negatively with left ventricular mass index in participants without CKD, but not in patients with CKD. Compared with sham controls, CKD mice had higher serum corin levels and increased cardiac expression of corin but reduced cardiac corin conversion activity. Indoxyl sulfate stimulated corin expression while suppressing serine protease activity in H9c2 cardiomyoblasts. Lower PCSK6 expression in CKD mouse hearts and indoxyl sulfate–treated H9c2 cardiomyoblasts may explain, at least partly, the observed CKD‐associated reduction in corin activity.

Conclusions

In CKD, cardiac and serum levels of corin are increased, yet corin activity is suppressed. The latter may be attributable to reduced PCSK6 expression. These findings suggest that corin dysfunction may play a significant role in the pathogenesis of CKD‐associated cardiomyopathy.

The Role of Gut-Derived, Protein-Bound Uremic Toxins in the Cardiovascular Complications of Acute Kidney Injury

Acute kidney injury (AKI) is a frequent disease encountered in the hospital, with a higher incidence in intensive care units. Despite progress in renal replacement therapy, AKI is still associated with early and late complications, especially cardiovascular events and mortality. The role of gut-derived protein-bound uremic toxins (PBUTs) in vascular and cardiac dysfunction has been extensively studied during chronic kidney disease (CKD), in particular, that of indoxyl sulfate (IS), para-cresyl sulfate (PCS), and indole-3-acetic acid (IAA), resulting in both experimental and clinical evidence. PBUTs, which accumulate when the excretory function of the kidneys is impaired, have a deleterious effect on and cause damage to cardiovascular tissues. However, the link between PBUTs and the cardiovascular complications of AKI and the pathophysiological mechanisms potentially involved are unclear. This review aims to summarize available data concerning the participation of PBUTs in the early and late cardiovascular complications of AKI.

Establishing Cell Models to Understand Cellular Toxicity: Lessons Learned from an Unconventional Cell Type

The syndrome of uremic toxicity comprises a complex toxic milieu in-vivo, as numerous uremic substances accumulate and harm the organ systems. Among these substances, toxic and non-toxic players differently interfere with human cells. However, results from animal experiments are not always compatible with the expected reactions in human patients and studies on one organ system are limited in capturing the complexity of the uremic situation. In this narrative review, we present aspects relevant for cellular toxicity research based on our previous establishment of a human spermatozoa-based cell model, as follows: (i) applicability to compare the effects of more than 100 uremic substances, (ii) detection of the protective effects of uremic substances by the cellular responses towards the uremic milieu, (iii) inclusion of the drug milieu for cellular function, and (iv) transferability for clinical application, e.g., hemodialysis. Our technique allows the estimation of cell viability, vitality, and physiological state, not only restricted to acute or chronic kidney toxicity but also for other conditions, such as intoxications of unknown substances. The cellular models can clarify molecular mechanisms of action of toxins related to human physiology and therapy. Identification of uremic toxins retained during acute and chronic kidney injury enables further research on the removal or degradation of such products.

The gut-cardiovascular connection: new era for cardiovascular therapy

Our gut microbiome is constituted by trillions of microorganisms including bacteria, archaea and eukaryotic microbes. Nowadays, gut microbiome has been gradually recognized as a new organ system that systemically and biochemically interact with the host. Accumulating evidence suggests that the imbalanced gut microbiome contributes to the dysregulation of immune system and the disruption of cardiovascular homeostasis. Specific microbiome profiles and altered intestinal permeability are often observed in the pathophysiology of cardiovascular diseases. Gut-derived metabolites, toxins, peptides and immune cell-derived cytokines play pivotal roles in the induction of inflammation and the pathogenesis of dysfunction of heart and vasculature. Impaired crosstalk between gut microbiome and multiple organ systems, such as gut-vascular, heart-gut, gut-liver and brain-gut axes, are associated with higher cardiovascular risks. Medications and strategies that restore healthy gut microbiome might therefore represent novel therapeutic options to lower the incidence of cardiovascular and metabolic disorders.

Uremic toxin indoxyl sulfate promotes proinflammatory macrophage activation by regulation of β-catenin and YAP pathways

Evidence has been shown that indoxyl sulfate (IS) could impair kidney and cardiac functions. Moreover, macrophage polarization played important roles in chronic kidney disease and cardiovascular disease. IS acts as a nephron-vascular toxin, whereas its effect on macrophage polarization during inflammation is still not fully elucidated. In this study, we aimed to investigate the effect of IS on macrophage polarization during lipopolysaccharide (LPS) challenge. THP-1 monocytes were incubated with phorbol 12-myristate-13-acetate (PMA) to differentiate into macrophages, and then incubated with LPS and IS for 24 h. ELISA was used to detect the levels of TNFα, IL-6, IL-1β in THP-1-derived macrophages. Western blot assay was used to detect the levels of arginase1 and iNOS in THP-1-derived macrophages. Percentages of HLA-DR-positive cells (M1 macrophages) and CD206-positive cells (M2 macrophages) were detected by flow cytometry. IS markedly increased the production of the pro-inflammatory factors TNFα, IL-6, IL-1β in LPS-stimulated THP-1-derived macrophages. In addition, IS induced M1 macrophage polarization in response to LPS, as evidenced by the increased expression of iNOS and the increased proportion of HLA-DR+ macrophages. Moreover, IS downregulated the level of β-catenin, and upregulated the level of YAP in LPS-stimulated macrophages. Activating β-catenin signaling or inhibiting YAP signaling suppressed the IS-induced inflammatory response in LPS-stimulated macrophages by inhibiting M1 polarization. IS induced M1 macrophage polarization in LPS-stimulated macrophages via inhibiting β-catenin and activating YAP signaling. In addition, this study provided evidences that activation of β-catenin or inhibition of YAP could alleviate IS-induced inflammatory response in LPS-stimulated macrophages. This finding may contribute to the understanding of immune dysfunction observed in chronic kidney disease and cardiovascular disease.

Molecular Mechanisms Underlying the Cardiovascular Toxicity of Specific Uremic Solutes

Mounting evidence strongly suggests a causal link between chronic kidney disease (CKD) and cardiovascular disease (CVD). Compared with non-CKD patients, patients with CKD suffer disproportionately from CVD and derive suboptimal benefits from interventions targeting conventional CVD risk factors. Uremic toxins (UTs), whose plasma levels rapidly rise as CKD progresses, represent a unique risk factor in CKD, which has protean manifestations on CVD. Among the known UTs, tryptophan metabolites and trimethylamine N-oxide are well-established cardiovascular toxins. Their molecular mechanisms of effect warrant special consideration to draw translational value. This review surveys current knowledge on the effects of specific UTs on different pathways and cell functions that influence the integrity of cardiovascular health, with implication for CVD progression. The effect of UTs on cardiovascular health is an example of a paradigm in which a cascade of molecular and metabolic events induced by pathology in one organ in turn induces dysfunction in another organ. Deciphering the molecular mechanisms underlying such cross-organ pathologies will help uncover therapeutic targets to improve the management of CVD in patients with CKD.