Activation of TGR5 restores AQP2 expression via the HIF pathway in renal ischemia-reperfusion injury

The role of hypoxia-inducible factors 1 and 2 in the pathogenesis of diabetic kidney disease

According to the 10th edition of the IDF Diabetes Atlas, 537 million people suffered from diabetes in 2021, and this number will increase by 47% by 2045. It is estimated that even 30-40% of these individuals may develop diabetic kidney disease (DKD) in the course of diabetes. DKD is one of the most important complications of diabetes, both in terms of impact and magnitude. It leads to high morbidity and mortality, which subsequently impacts on quality of life, and it carries a high financial burden. Diabetic kidney disease is considered a complex and heterogeneous entity involving disturbances in vascular, glomerular, podocyte, and tubular function. It would appear that hypoxia-inducible factors (HIF)-1 and HIF-2 may be important players in the pathogenesis of this disease. However, their exact role is still not fully investigated. In this article, we summarize the current knowledge about HIF signaling and its role in DKD. In addition, we focus on the possible effects of nephroprotective drugs on HIF expression and activity in various tissues.

Role of G protein coupled receptors in acute kidney injury

Acute kidney injury (AKI) is a clinical condition characterized by a rapid decline in kidney function, which is associated with local inflammation and programmed cell death in the kidney. The G protein-coupled receptors (GPCRs) represent the largest family of signaling transduction proteins in the body, and approximately 40% of drugs on the market target GPCRs. The expressions of various GPCRs, prostaglandin receptors and purinergic receptors, to name a few, are significantly altered in AKI models. And the role of GPCRs in AKI is catching the eyes of researchers due to their distinctive biological functions, such as regulation of hemodynamics, metabolic reprogramming, and inflammation. Therefore, in this review, we aim to discuss the role of GPCRs in the pathogenesis of AKI and summarize the relevant clinical trials involving GPCRs to assess the potential of GPCRs and their ligands as therapeutic targets in AKI and the transition to AKI-CKD.

'Aquaporin-omics': mechanisms of aquaporin-2 loss in polyuric disorders

Animal models of a variety of acquired nephrogenic diabetes insipidus (NDI) disorders have identified a common feature: all such models are associated with the loss of aquaporin-2 (AQP2) from collecting duct principal cells, explaining the associated polyuria. To discover mechanisms of AQP2 loss, previous investigators have carried out either transcriptomics (lithium-induced NDI, unilateral ureteral obstruction, endotoxin-induced NDI) or proteomics (hypokalaemia-associated NDI, hypercalcaemia-associated NDI, bilateral ureteral obstruction), yielding contrasting views. Here, to address whether there may be common mechanisms underlying loss of AQP2 in acquired NDI disorders, we have used bioinformatic data integration techniques to combine information from all transcriptomic and proteomic data sets. The analysis reveals roles for autophagy/apoptosis, oxidative stress and inflammatory signalling as key elements of the mechanism that results in loss of AQP2. These processes can cause AQP2 loss through the combined effects of repression of Aqp2 gene transcription, generalized translational repression, and increased autophagic degradation of proteins including AQP2. Two possible types of stress-sensor proteins, namely death receptors and stress-sensitive protein kinases of the EIF2AK family, are discussed as potential triggers for signalling processes that result in loss of AQP2. KEY POINTS: Prior studies have shown in a variety of animal models of acquired nephrogenic diabetes insipidus (NDI) that loss of the aquaporin-2 (AQP2) protein is a common feature. Investigations of acquired NDI using transcriptomics (RNA-seq) and proteomics (protein mass spectrometry) have led to differing conclusions regarding mechanisms of AQP2 loss. Bioinformatic integration of transcriptomic and proteomic data from these prior studies now reveals that acquired NDI models map to three core processes: oxidative stress, apoptosis/autophagy and inflammatory signalling. These processes cause loss of AQP2 through translational repression, accelerated degradation of proteins, and transcriptional repression.

Activation of TGR5 Increases Urine Concentration by Inducing AQP2 and AQP3 Expression in Renal Medullary Collecting Ducts

Introduction

G protein-coupled bile acid receptor (TGR5), the first G protein-coupled receptor for bile acids identified, is capable of activating a variety of intracellular signaling pathways after interacting with bile acids. TGR5 plays an important role in multiple physiological processes and is considered to be a potential target for the treatment of various metabolic diseases, including type 2 diabetes. Evidence has emerged that genetic deletion of TGR5 results in an increase in basal urine output, suggesting that it may play a critical role in renal water and salt reabsorption. The present study aims to elucidate the effect and mechanism of TGR5 activation on urine concentration.

Methods

Mice were treated with TGR5 agonists (LCA and INT-777) for 3 days. The 24-h urine of mice was collected and analyzed for urine biochemical parameters. The mRNA expressions were detected by real-time PCR, and the protein expressions were detected by western blot. Immunohistochemistry and immunofluorescence were performed to examine the cellular location of proteins. The cultured primary medullary collecting duct cells were pretreated with H89 (a PKA inhibitor) for 1 h, followed by 12-h treatment of LCA and INT-777. Luciferase reporter assays were used to detect the effect of CREB on the gene transcription of AQPs. Gel electrophoretic mobility shift assays were used to analyze DNA-protein interactions.

Results

Treatment of mice with the TGR5 agonist LCA and INT-777 markedly reduced urine output and increased urine osmolality, accompanied by a marked increase in AQP2 and AQP3 protein expression and membrane translocation. In cultured primary medullary collecting duct cells, LCA and INT-777 dose-dependently upregulated AQP2 and AQP3 expression in a cAMP/PKA-dependent manner. Mechanistically, both AQP2 and AQP3 gene promoter contains a putative CREB-binding site, which can be bound and activated by CREB as assessed by both gene promoter-driven luciferase and gel shift assays.

Conclusion

Collectively, our findings demonstrate that activation of TGR5 can promote urine concentration by upregulation of AQP2 and AQP3 expression in renal collecting ducts. TGR5 may represent an attractive target for the treatment of patients with urine concentration defect.

TGR5 signalling in heart and brain injuries: focus on metabolic and ischaemic mechanisms

The heart and brain are the core organs of the circulation and central nervous system, respectively, and play an important role in maintaining normal physiological functions. Early neuronal and cardiac damage affects organ function. The relationship between the heart and brain is being continuously investigated. Evidence-based medicine has revealed the concept of the "heart- brain axis," which may provide new therapeutic strategies for certain diseases. Takeda protein-coupled receptor 5 (TGR5) is a metabolic regulator involved in energy homeostasis, bile acid homeostasis, and glucose and lipid metabolism. Inflammation is critical for the development and regeneration of the heart and brain during metabolic diseases. Herein, we discuss the role of TGR5 as a metabolic regulator of heart and brain development and injury to facilitate new therapeutic strategies for metabolic and ischemic diseases of the heart and brain.

Bile acid receptors and renal regulation of water homeostasis

The kidney is the key organ responsible for maintaining the body's water and electrolyte homeostasis. About 99% of the primary urine filtered from the Bowman's capsule is reabsorbed along various renal tubules every day, with only 1-2 L of urine excreted. Aquaporins (AQPs) play a vital role in water reabsorption in the kidney. Currently, a variety of molecules are found to be involved in the process of urine concentration by regulating the expression or activity of AQPs, such as antidiuretic hormone, renin-angiotensin-aldosterone system (RAAS), prostaglandin, and several nuclear receptors. As the main bile acid receptors, farnesoid X receptor (FXR) and membrane G protein-coupled bile acid receptor 1 (TGR5) play important roles in bile acid, glucose, lipid, and energy metabolism. In the kidney, FXR and TGR5 exhibit broad expression across all segments of renal tubules, and their activation holds significant therapeutic potential for numerous acute and chronic kidney diseases through alleviating renal lipid accumulation, inflammation, oxidative stress, and fibrosis. Emerging evidence has demonstrated that the genetic deletion of FXR or TGR5 exhibits increased basal urine output, suggesting that bile acid receptors play a critical role in urine concentration. Here, we briefly summarize the function of bile acid receptors in renal water reabsorption and urine concentration.

Aquaporin-1 inhibition exacerbates ischemia-reperfusion-induced lung injury in mouse

BACKGROUND

Ischemia-reperfusion injury (IRI), which involves severe inflammation and edema, is an inevitable feature of the lung transplantation process and leads to primary graft dysfunction (PGD). The activation of aquaporin 1 (AQP1) modulates fluid transport in the alveolar space. The current study investigated the role of AQP1 in ischemia-reperfusion (IR)-induced lung injury.

METHODS

A mouse model of lung IR was established by clamping the left lung hilar for 1 h and released for reperfusion for 24 h. The AQP1 inhibitor acetazolamide (AZA) was administered 3 days before lung ischemia with a dose of 100 mg/kg per day via gavage. Lung injury was evaluated using the ratio of wet-to-dry weight, peripheral bronchial epithelial thickness, degree of angioedema, acute lung injury score, neutrophil infiltration, and cytokine concentrations in bronchoalveolar lavage fluid.

RESULTS

Compared with sham treatment, ischemia with no reperfusion (IR 0h) and ischemia with reperfusion for 24 h (IR 24 h) significantly upregulated AQP1 expression, increased the wet/dry weight ratio, angioedema, neutrophil infiltration and cytokine production (interleukin -6 and tumor necrosis factor -α) and thickened the peripheral bronchial epithelium. AZA exacerbated inflammation and pulmonary edema.

CONCLUSION

AQP1 may exert a protective effect against IR-induced lung injury, which could be attributed to alleviating pulmonary edema and inflammation. AQP1 upregulation might be a potential application to alleviate lung IRI and decrease the incidence of PGD.

The hypoxia-inducible factor-α prolyl hydroxylase inhibitor FG4592 ameliorates renal fibrosis by inducing the H3K9 demethylase JMJD1A

The transcription factors hypoxia-inducible factor-1α and -2α (HIF-1α/2α) are the major regulators of the cellular response to hypoxia and play a key role in renal fibrosis associated with acute and chronic kidney disease. Jumonji domain-containing 1a (JMJD1A), a histone H3 lysine 9 (H3K9) demethylase, is reported to be an important target gene of HIF-α. However, whether JMJD1A and H3K9 methylation status play a role in renal fibrosis is unclear. Here, we investigated the involvement of HIF-α, JMJD1A, and monomethylated/dimethylated H3K9 (H3K9me1/H3K9me2) levels in unilateral ureteral obstruction (UUO)-induced renal fibrosis in mice. Intraperitoneal administration of FG4592, an inhibitor of HIF-α prolyl hydroxylase, which controls HIF-α protein stability, significantly attenuated renal fibrosis on days 3 and 7 following UUO. FG4592 concomitantly increased JMJD1A expression, decreased H3K9me1/me2 levels, reduced profibrotic gene expression, and increased erythropoietin expression in renal tissues of UUO mice. The beneficial effects of FG4592 on renal fibrosis were inhibited by the administration of JMJD1A-specific siRNA to mice immediately following UUO. Incubation of normal rat kidney-49F and/or -52E cells with transforming growth factor-β1 (TGF-β1) in vitro resulted in upregulated expression of α-smooth muscle actin and H3K9me1/me2, and these effects were inhibited by cotreatment with FG4592. In contrast, FG4592 treatment further enhanced the TGF-β1-stimulated upregulation of JMJD1A but had no effect on TGF-β1-stimulated expression of the H3K9 methyltransferase euchromatic histone-lysine N-methyltransferase 2. Collectively, these findings establish a crucial role for the HIF-α1/2-JMJD1A-H3K9me1/me2 regulatory axis in the therapeutic effect of FG4592 in renal fibrosis.NEW & NOTEWORTHY Using a mouse model of renal fibrosis and transforming growth factor-β1-stimulated rat cell lines, we show that treatment with FG4592, an inhibitor of hypoxia-inducible factor-1α and -2α (HIF-1α/2α) prolyl hydroxylase decreases renal fibrosis and concomitantly reduces methylated lysine 9 of histone H3 (H3K9) levels via upregulation of Jumonji domain-containing 1a (JMJD1A). The results identify a novel role for the HIF-1α/2α-JMJD1A-H3K9 regulatory axis in suppressing renal fibrosis.

Recent advances in modulation of cardiovascular diseases by the gut microbiota

The gut microbiota has recently gained attention due to its association with cardiovascular health, cancers, gastrointestinal disorders, and non-communicable diseases. One critical question is how the composition of the microbiota contributes to cardiovascular diseases (CVDs). Insightful reviews on the gut microbiota, its metabolites and the mechanisms that underlie its contribution to CVD are limited. Hence, the aim of this review was to describe linkages between the composition of the microbiota and CVD, CVD risk factors such as hypertension, diet, ageing, and sex differences. We have also highlighted potential therapies for improving the composition of the gut microbiota, which may result in better cardiovascular health.

Bile acids and salt-sensitive hypertension: a role of the gut-liver axis

Salt-sensitivity of blood pressure (SSBP) affects 50% of the hypertensive and 25% of the normotensive populations. Importantly, SSBP is associated with increased risk for mortality in both populations independent of blood pressure. Despite its deleterious effects, the pathogenesis of SSBP is not fully understood. Emerging evidence suggests a novel role of bile acids in salt-sensitive hypertension and that they may play a crucial role in regulating inflammation and fluid volume homeostasis. Mechanistic evidence implicates alterations in the gut microbiome, the epithelial sodium channel (ENaC), the farnesoid X receptor, and the G protein-coupled bile acid receptor TGR5 in bile acid-mediated effects on cardiovascular function. The mechanistic interplay between excess dietary sodium-induced alterations in the gut microbiome and immune cell activation, bile acid signaling, and whether such interplay may contribute to the etiology of SSBP is still yet to be defined. The main goal of this review is to discuss the potential role of bile acids in the pathogenesis of cardiovascular disease with a focus on salt-sensitive hypertension.

Nuclear Receptors and Transcription Factors in Obesity-Related Kidney Disease

Both obesity and chronic kidney disease are increasingly common causes of morbidity and mortality worldwide. Although obesity often co-exists with diabetes and hypertension, it has become clear over the past several decades that obesity is an independent cause of chronic kidney disease, termed obesity-related glomerulopathy. This review defines the attributes of obesity-related glomerulopathy and describes potential pharmacologic interventions. Interventions discussed include peroxisome proliferator-activated receptors, the farnesoid X receptor, the Takeda G-protein-coupled receptor 5, and the vitamin D receptor.