A high salt diet induces tubular damage associated with a pro-inflammatory and pro-fibrotic response in a hypertension-independent manner

Sodium as an Important Regulator of Immunometabolism

Salt sensitivity concerns blood pressure alterations after a change in salt intake (sodium chloride). The heart is a pump, and vessels are tubes; sodium can affect both. A high salt intake increases cardiac output, promotes vascular dysfunction and capillary rarefaction, and chronically leads to increased systemic vascular resistance. More recent findings suggest that sodium also acts as an important second messenger regulating energy metabolism and cellular functions. Besides endothelial cells and fibroblasts, sodium also affects innate and adaptive immunometabolism, immune cell function, and influences certain microbes and microbiota-derived metabolites. We propose the idea that the definition of salt sensitivity should be expanded beyond high blood pressure to cellular and molecular salt sensitivity.

The Association between Urinary Sodium-Potassium Ratio, Kidney Function, and Blood Pressure in a Cohort from the General Population

INTRODUCTION

Subclinical kidney dysfunction may contribute to salt-sensitive hypertension. We assessed the association between the urinary sodium-potassium ratio (Na/K ratio) and blood pressure (BP) in a general population cohort without diabetes, chronic kidney disease, cardiovascular disease, or treated hypertension. We investigated whether any such association was mediated by the kidney function markers measured glomerular filtration rate (mGFR), urinary albumin-creatinine ratio (ACR), and urinary epidermal growth factor-creatinine ratio (EGF-Cr).

METHODS

The Tromsø Study is a population-based study of inhabitants of the municipality of Tromsø, Northern Norway. Participants aged 50-62 years, without diabetes, chronic kidney disease, or cardiovascular disease, were invited to the substudy Renal Iohexol Clearance Survey in Tromsø 6 (RENIS-T6; 2007-09). For the present study, we excluded participants reporting the use of 1 or more antihypertensive agents, leaving 1,311 RENIS-T6 participants for a cross-sectional analysis. We measured office BP, 24-h ambulatory blood pressure (ABP), and mGFR using iohexol clearance. Na/K ratio, ACR, and EGF-Cr were measured in morning urine samples.

RESULTS

Urinary Na/K ratio was significantly associated with systolic office BP and ABP independently of cardiovascular risk factors and kidney function markers. A one-standard deviation unit increase in the Na/K ratio was associated with increased systolic ABP by 1.0 (0.3-1.6) mm Hg. Urinary Na/K ratio showed a stronger association with office BP than ABP. EGF-Cr, ACR, and mGFR did not mediate the relationship between urinary Na/K ratio and systolic BP.

CONCLUSIONS

In a representative sample of the middle-aged North-European population without diabetes, chronic kidney disease, cardiovascular disease, or treated hypertension, there was a consistent association between urinary Na/K ratio and BP. The association with BP was not mediated through kidney function measures, suggesting a relationship between a diet with high sodium and low potassium and higher BP regardless of kidney function.

Salt Behind the Scenes of Systemic Lupus Erythematosus and Rheumatoid Arthritis

PURPOSE OF REVIEW

Sodium is vital for human health. High salt intake is a global health problem and is associated with cardiovascular morbidity and mortality. Recent evidence suggests that both innate and adaptive immune systems are affected by sodium. In general, excess salt intake drives immune cells toward a pro-inflammatory phenotype. The incidence of autoimmune diseases, including systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), is steadily increasing. As excess salt induces a pro-inflammatory state, increased salt intake may have impacts on autoimmune diseases. The relationship between salt intake and autoimmune diseases is most widely studied in patients with SLE or RA. This review aimed to summarize the relationship between salt intake and SLE and RA.

RECENT FINDINGS

Most, but not all, of these studies showed that high salt intake might promote SLE by M1 macrophage shift, increase in Th17/Treg cell ratio, activation of dendritic and follicular helper T cells, and increased secretion of pro-inflammatory cytokines. In RA, apart from driving immune cells toward a pro-inflammatory state, high salt intake also influences cellular signaling pathways, including receptor activator of nuclear factor κB ligand (RANKL), Rho GTPases, and MAPK (mitogen-activated protein kinase). There is now sufficient evidence that excess salt intake may be related to the development and progression of SLE and RA, although there are still knowledge gaps. More studies are warranted to further highlight the relationship between excess salt intake, SLE, and RA. Salt intake may affect cell types and pro-inflammatory cytokines and signaling pathways associated with the development and progression of systemic lupus erythematosus and rheumatoid arthritis. Bcl-6 B-cell lymphoma, 6 Erk extracellular signal-regulated kinases, IFN-γ interferon-gamma, JNK c-Jun N-terminal kinase, IL-4 interleukin 4, IL-6 interleukin 6, MAPK mitogen-activated protein kinase, STAT signal transducer and activator of transcription, Tnf-α tumor necrosis factor, Treg T regulatory cell.

Emerging roles of proximal tubular endocytosis in renal fibrosis

Endocytosis is a crucial component of many pathological conditions. The proximal tubules are responsible for reabsorbing the majority of filtered water and glucose, as well as all the proteins filtered through the glomerular barrier via endocytosis, indicating an essential role in kidney diseases. Genetic mutations or acquired insults could affect the proximal tubule endocytosis processes, by disturbing or overstressing the endolysosomal system and subsequently activating different pathways, orchestrating renal fibrosis. This paper will review recent studies on proximal tubular endocytosis affected by other diseases and factors. Endocytosis plays a vital role in the development of renal fibrosis, and renal fibrosis could also, in turn, affect tubular endocytosis.

Rapamycin treatment induces tubular proteinuria: role of megalin-mediated protein reabsorption

Introduction: Rapamycin is an immunosuppressor that acts by inhibiting the serine/threonine kinase mechanistic target of rapamycin complex 1. Therapeutic use of rapamycin is limited by its adverse effects. Proteinuria is an important marker of kidney damage and a risk factor for kidney diseases progression and has been reported in patients and animal models treated with rapamycin. However, the mechanism underlying proteinuria induced by rapamycin is still an open matter. In this work, we investigated the effects of rapamycin on parameters of renal function and structure and on protein handling by proximal tubule epithelial cells (PTECs). Methods: Healthy BALB/c mice were treated with 1.5 mg/kg rapamycin by oral gavage for 1, 3, or 7 days. At the end of each treatment, the animals were kept in metabolic cages and renal function and structural parameters were analyzed. LLC-PK1 cell line was used as a model of PTECs to test specific effect of rapamycin. Results: Rapamycin treatment did not change parameters of glomerular structure and function. Conversely, there was a transient increase in 24-h proteinuria, urinary protein to creatinine ratio (UPCr), and albuminuria in the groups treated with rapamycin. In accordance with these findings, rapamycin treatment decreased albumin-fluorescein isothiocyanate uptake in the renal cortex. This effect was associated with reduced brush border expression and impaired subcellular distribution of megalin in PTECs. The effect of rapamycin seems to be specific for albumin endocytosis machinery because it did not modify renal sodium handling or (Na⁺+K⁺)ATPase activity in BALB/c mice and in the LLC-PK1 cell line. A positive Pearson correlation was found between megalin expression and albumin uptake while an inverse correlation was shown between albumin uptake and UPCr or 24-h proteinuria. Despite its effect on albumin handling in PTECs, rapamycin treatment did not induce tubular injury measured by interstitial space and collagen deposition. Conclusion: These findings suggest that proteinuria induced by rapamycin could have a tubular rather than a glomerular origin. This effect involves a specific change in protein endocytosis machinery. Our results open new perspectives on understanding the undesired effect of proteinuria generated by rapamycin.

Inhibition of ferroptosis ameliorates hypertensive nephropathy through p53/Nrf2/p21 pathway by Taohongsiwu decoction: Based on network pharmacology and experimental validation

ETHNOPHARMACOLOGICAL RELEVANCE

Hypertensive nephropathy (HN) is a complication of hypertension. Taohongsiwu decoction (THSWD) is used clinically but its application in the prevention and treatment of HN remains unelucidated.

AIM OF STUDY

This study aims to explore the potential targets and molecular mechanisms of THSWD in the treatment of HN.

MATERIALS AND METHODS

A network pharmacology approach was used to predict the components and targets of THSWD for treating HN. Animal experiments were performed to verify the network pharmacology findings.

RESULTS

205 targets were identified and regarded as potential targets of THSWD in HN treatment. Subsequently, we screened 17 hub genes and identified TP53 as the most critical one. KEGG enrichment analysis showed that p53 signaling pathway might play a significant role. In vivo experiments indicated that high-salt diets can lead to high blood pressure, kidney injury, inflammation, and fibrosis. Furthermore, the altered levels of biomarkers (Iron, malondialdehyde, catalase, ferritin, transferrin, Superoxide dismutase and Glutathione Peroxidase 4) provided evidence of ferroptosis. We found that the ferroptosis inhibitor ferrostatin-1 (Fer-1) and THSWD could significantly alleviate HN by suppressing ferroptosis. THSWD and Fer-1 treatment downregulated the protein and mRNA expression of p53, p21, RB, and CTNNB1, which were upregulated by high salt. Meanwhile, THSWD and Fer-1 reversed the downregulation of Nrf2 caused by high-salt diet.

CONCLUSIONS

Our results suggested that THSWD attenuate HN induced by a high-salt diet through inhibiting ferroptosis via the p53/Nrf2/p21 pathway.

Tff3-/- Knock-Out Mice with Altered Lipid Metabolism Exhibit a Lower Level of Inflammation following the Dietary Intake of Sodium Chloride for One Week

A high salt intake causes hemodynamic changes and promotes immune response through cell activation and cytokine production, leading to pro-inflammatory conditions. Transgenic Tff3^-/- knock-out mice (TFF3ko) (n = 20) and wild-type mice (WT) (n = 20) were each divided into the (1) low-salt (LS) group and (2) high-salt (HS) group. Ten-week-old animals were fed with standard rodent chow (0.4% NaCl) (LS) or food containing 4% NaCl (HS) for one week (7 days). Inflammatory parameters from the sera were measured by Luminex assay. The integrin expression and rates of T cell subsets of interest from the peripheral blood leukocytes (PBLs) and mesenteric lymph nodes (MLNs) were measured using flow cytometry. There was a significant increase in high-sensitivity C reactive protein (hsCRP) only in the WT mice following the HS diet, while there were no significant changes in the serum levels of IFN-γ, TNF-α, IL-2, IL-4, or IL-6 as a response to treatment in either study groups. The rates of CD4⁺CD25⁺ T cells from MLNs decreased, while CD3⁺γδTCR⁺ from peripheral blood increased following the HS diet only in TFF3ko. γδTCR expressing T cell rates decreased in WT following the HS diet. The CD49d/VLA-4 expression decreased in the peripheral blood leukocytes in both groups following the HS diet. CD11a/LFA-1 expression significantly increased only in the peripheral blood Ly6C^-CD11a^high monocytes in WT mice following salt loading. In conclusion, salt-loading in knock-out mice caused a lower level of inflammatory response compared with their control WT mice due to gene depletion.

Gold nanoparticles reduce tubule-interstitial injury and proteinuria in a murine model of subclinical acute kidney injury

Subclinical acute kidney injury (subAKI) is characterized by tubule-interstitial injury without significant changes in glomerular function. SubAKI is associated with the pathogenesis and progression of acute and chronic kidney diseases. Currently, therapeutic strategies to treat subAKI are limited. The use of gold nanoparticles (AuNPs) has shown promising benefits in different models of diseases. However, their possible effects on subAKI are still unknown. Here, we investigated the effects of AuNPs on a mouse model of subAKI. Animals with subAKI showed increased functional and histopathologic markers of tubular injury. There were no changes in glomerular function and structure. The animals with subAKI also presented an inflammatory profile demonstrated by activation of Th1 and Th17 cells in the renal cortex. This phenotype was associated with decreased megalin-mediated albumin endocytosis and expression of proximal tubular megalin. AuNP treatment prevented tubule-interstitial injury induced by subAKI. This effect was associated with a shift to an anti-inflammatory Th2 response. Furthermore, AuNP treatment preserved megalin-mediated albumin endocytosis in vivo and in vitro. AuNPs were not nephrotoxic in healthy mice. These results suggest that AuNPs have a protective effect in the tubule-interstitial injury observed in subAKI, highlighting a promising strategy as a future antiproteinuric treatment.

Angiotensin II type 2 receptor activation preserves megalin in the kidney and prevents proteinuria in high salt diet fed rats

Proteinuria is a risk factor for and consequence of kidney injury. Angiotensin II type 2 receptor (AT₂R) is an emerging reno-protective target and is anti-proteinuric under pathological conditions, including high salt-fed obese animals. However, the mechanisms remain unknown, particularly whether the anti-proteinuric activity of AT₂R is independent of its anti-hypertensive and anti-inflammatory effects. In the present study, obese Zucker rats were fed high sodium (4%) diet (HSD) for 48 h, a time in which blood pressure does not change. HSD caused proteinuria without affecting glomerular slit diaphragm proteins (nephrin and podocin), glomerular filtration rate, inflammatory and fibrotic markers (TNFα, IL-6, and TGF-β), ruling out glomerular injury, inflammation and fibrosis but indicating tubular mechanisms of proteinuria. At cellular and molecular levels, we observed a glycogen synthase kinase (GSK)-3β-mediated megalin phosphorylation, and its subsequent endocytosis and lysosomal degradation in HSD-fed rat kidneys. Megalin is a major proximal tubular endocytic protein transporter. The AT₂R agonist C21 (0.3 mg/kg/day, i.p.) administration prevented proteinuria and rescued megalin surface expression potentially by activating Akt-mediated phosphorylation and inactivation of GSK-3β in HSD-fed rat kidneys. Overall, AT₂R has a direct anti-proteinuric activity, potentially via megalin regulation, and is suggested as a novel target to limit kidney injury.

Inhibition of SGLT2 co-transporter by dapagliflozin ameliorates tubular proteinuria and tubule-interstitial injury at the early stage of diabetic kidney disease

Diabetic kidney disease (DKD) is characterized by progressive impairment of kidney function. It has been postulated that tubule-interstitial injury, associated with tubular albuminuria, precedes glomerular damage in the early stage of DKD. Here, we wanted to determine if the development of tubule-interstitial injury at the early stage of DKD implies modulation of megalin-mediated protein reabsorption in proximal tubule epithelial cells (PTECs) by SGLT2-dependent high glucose influx. Rats with streptozotocin (STZ)-induced diabetes were treated or not with dapagliflozin (DAPA) for 8 weeks. Four experimental groups were generated: (1) CONT, control; (2) DAPA, rats treated with DAPA; (3) STZ, diabetic rats; (4) STZ + DAPA, diabetic rats treated with DAPA. No changes in glomerular structure and function were observed. The STZ group presented proteinuria and albuminuria associated with an increase in the fractional excretion of proteins. A positive correlation between glycemia and proteinuria was found. These phenomena were linked to a decrease in luminal and total megalin expression and, consequently, in albumin reabsorption in PTECs. We also observed tubule-interstitial injury characterized by an increase in urinary tubular injury biomarkers and changes in tubular histomorphometry parameters. In addition, inverse correlations were found between cortical albumin uptake and tubule-interstitial injury or glycemia. All these modifications were attenuated in the STZ + DAPA group. These results suggest that SGLT2-dependent high glucose influx into PTECs promotes a harmful effect on the PTECs, leading to the development of tubular albuminuria and tubule-interstitial injury preceding glomerular damage. These results expand current knowledge on the renoprotective effects of gliflozins.

The Increased Risk of Hypertension Caused by Irrational Dietary Pattern May Be Associated with Th17 Cell in the Middle-Aged and Elderly Rural Residents of Beijing City, Northern China: A 1:1 Matched Case-Control Study

An irrational diet has been widely considered as one of the vital risk factors of hypertension. Previous studies have indicated that immune dysfunction may be involved in the pathogenic process of hypertension, while fewer studies have mentioned whether CD4+ T cells are involved in the association between dietary pattern and hypertension. This present 1:1 matched case-control study was conducted to analyze the association among dietary pattern, CD4+ T cells and hypertension. A total of 56 patients with diagnosed hypertension and 56 subjects without diagnosed hypertension in the rural area of Beijing City, northern China, were matched by age and gender, and then classified into a case group and a control group, respectively. Compared with the control group, higher frequencies of pro-inflammatory CD4+ T cells, such as Th1, Th1(IFN-γ), Th17(IL-17A), and Th1/17 (IFN-γ/IL-17A), were found in the case group (p < 0.05). A significantly higher level of circulating IL-17A was also found in the case group (7.4 pg/mL vs. 8.2 pg/mL, p < 0.05). Five dietary patterns were identified using exploratory factor analysis. An irrational dietary pattern, characterized by high-factor loadings of refined wheat (0.65), meat (0.78), poultry (0.76), and alcoholic beverage (0.73), was positively associated with SBP (β = 5.38, 95%CI = 0.73~10.03, p < 0.05) in the multiple linear regression model with the adjustment of potential covariates. The other dietary patterns showed no significant association with blood pressure. Furthermore, meat, processed meat, and animal viscera were positively correlated with the peripheral Th17 or Th1/17. In conclusion, the irrational dietary pattern characterized by refined wheat, meat, poultry, and alcoholic beverage, was positively correlated with blood pressure, and may increase the risk of hypertension in the rural area of Beijing, northern China. Th17, a subset of the CD4+ T helper cells, may be involved in the association between irrational dietary pattern and hypertension.

TMBIM6 promotes diabetic tubular epithelial cell survival and albumin endocytosis by inhibiting the endoplasmic reticulum stress sensor, IRE1α

AIM

Reduced albumin reabsorption in proximal tubular epithelial cells (PTECs), resulting from decreased megalin plasma membrane (PM) localization due to prolonged endoplasmic reticulum (ER) stress, potentially contributes to albuminuria in early diabetic kidney disease (DKD). To examine this possibility, we investigated the cytoprotective effect of TMBIM6 in promoting diabetic PTEC survival and albumin endocytosis by attenuating ER stress with an IRE1α inhibitor, KIRA6.

METHODS AND RESULTS

Renal TMBIM6 distribution and expression were determined by immunohistochemistry, western blotting, and qPCR, whereas tubular injury was evaluated in db/db mice. High-glucose (HG)-treated HK-2 cells were either treated with KIRA6 or transduced with a lentiviral vector for TMBIM6 overexpression. ER stress was measured by western blotting and ER-Tracker Red staining, whereas apoptosis was determined by performing TUNEL assays. Megalin expression was measured by immunofluorescence, and albumin endocytosis was evaluated after incubating cells with FITC-labeled albumin. Tubular injury and TMBIM6 downregulation occurred in db/db mouse renal cortical tissues. Both KIRA6 treatment and TMBIM6 overexpression inhibited ER stress by decreasing the levels of phosphorylated IRE1α, XBP1s, GRP78, and CHOP, and stabilizing ER expansion in HG-treated HK-2 cells. TUNEL assays performed with KIRA6-treated or TMBIM6-overexpressing cells showed a significant decrease in apoptosis, consistent with the significant downregulation of BAX and upregulation of BCL-2, as measured by immunoblotting. Both KIRA6 and TMBIM6 overexpression promoted megalin PM localization and restored albumin endocytosis in HG-treated HK-2 cells.

CONCLUSION

TMBIM6 promoted diabetic PTEC survival and albumin endocytosis by negatively regulating the IRE1α branch of ER stress.

Surface megalin expression is a target to the inhibitory effect of bradykinin on the renal albumin endocytosis

Megalin-mediated albumin endocytosis plays a critical role in albumin reabsorption in proximal tubule (PT) epithelial cells (PTECs). Some studies have pointed out the modulatory effect of bradykinin (BK) on urinary protein excretion, but its role in PT protein endocytosis has not yet been determined. Here, we studied the possible correlation between BK and albumin endocytosis in PT. Using LLC-PK1 cells, a model of PTECs, we showed that BK specifically inhibited megalin-mediated albumin endocytosis. This inhibitory effect of BK was mediated by B2 receptor (B2R) because it was abolished by HOE140, an antagonist of B2R, but it was not affected by Lys-des-Arg⁹-BK, an antagonist of B1. BK induced the stall of megalin in EEA1⁺ endosomes, but not in LAMP1⁺ lysosomes, leading to a decrease in surface megalin expression. In addition, we showed that BK, through B2R, activated calphostin C-sensitive protein kinase C, which mediated its effect on the surface megalin expression and albumin endocytosis. These results reveal an important modulatory mechanism of PT albumin endocytosis by BK, which opens new possibilities to understanding the effect of BK on urinary albumin excretion.

Renal tubular cell necroptosis: A novel mechanism of kidney damage in trichloroethylene hypersensitivity syndrome mice

Trichloroethylene (TCE) hypersensitivity syndrome (THS), called occupational medicamentosa-like dermatitis due to TCE (OMDT) in China, is a fatal occupational disorder caused by TCE exposure. Visceral damage, including kidney injury, is one of the major complications. Necroptosis is a regulated cell death form linked to local inflammatory response. This study aimed to investigate whether renal cell necroptosis was involved in TCE-induced kidney injury. A Balb/c mouse model of TCE sensitization was utilized to study mechanisms of modulation of TCE-induced renal necroptosis. Renal histology (using light and transmission electron microscopy) and renal tubular impairment indexes, including α1-microglobulin (α1-MG), and β2-microglobulin (β2-MG), were evaluated. In addition, tissue expression of necroptosis-related proteins, including tumor necrosis factor (TNF)-α, TNF receptor 1 (TNFR1), receptor-interacting protein kinase 3 (RIPK3), p-RIK3, mixed lineage kinase domain-like protein (MLKL), and p-MLKL, were also evaluated. The study here confirmed TCE sensitization caused damage to renal tubules and renal tubular epithelial cell (RTEC) necroptosis. In mice treated with R7050 (a specific TNFα antagonist), it was also seen that inhibition of TNFα expression could effectively inhibit RTEC necroptosis and improve renal function in the TCE-sensitized mice. Taken together, these results help to define a novel mechanism by which RTEC necroptosis plays a key role in TCE-induced kidney damage.

Megalin-mediated albumin endocytosis in renal proximal tubules is involved in the antiproteinuric effect of angiotensin II type 1 receptor blocker in a subclinical acute kidney injury animal model

BACKGROUND

Tubule-interstitial injury (TII) is one of the mechanisms involved in the progression of renal diseases with progressive proteinuria. Angiotensin II (Ang II) type 1 receptor blockers (ARBs) have been successfully used to treat renal diseases. However, the mechanism correlating treatment with ARBs and proteinuria is not completely understood. The hypothesis that the anti-proteinuric effect of losartan is associated with the modulation of albumin endocytosis in PT epithelial cells (PTECs) was assessed.

METHODS

We used a subclinical acute kidney injury animal model (subAKI) and LLC-PK1 cells, a model of PTECs.

RESULTS

In subAKI, PT albumin overload induced TII development, measured by: (1) increase in urinary lactate dehydrogenase and γ-glutamyltranspeptidase activity; (2) proteinuria associated with impairment in megalin-mediated albumin reabsorption; (3) increase in luminal and interstitial space in tubular cortical segments. These effects were avoided by treating the animals with losartan, an ARB. Using LLC-PK1 cells, we observed that: (1) 20 mg/mL albumin increased the secretion of Ang II and decreased megalin-mediated albumin endocytosis; (2) the effects of Ang II and albumin were abolished by 10^-8 M losartan; (3) MEK/ERK pathway is the molecular mechanism underlying the Ang II-mediated inhibitory effect of albumin on PT albumin endocytosis.

CONCLUSION

Our results show that PT megalin-mediated albumin endocytosis is a possible target during the treatment of renal diseases patients with ARB.

GENERAL SIGNIFICANCE

The findings obtained in the present work represents a step forward to the current knowledge on about the role of ARBs in the treatment of renal disease.