Protecting the peritoneal membrane: factors beyond peritoneal dialysis solutions

Omega-3 polyunsaturated fatty acids protect peritoneal mesothelial cells from hyperglycolysis and mesothelial-mesenchymal transition through the FFAR4/CaMKKβ/AMPK/mTOR signaling pathway

Peritoneal fibrosis is a severe clinical complication associated with peritoneal dialysis (PD) and impacts its efficacy and patient outcomes. The process of mesothelial-mesenchymal transition (MMT) in peritoneal mesothelial cells plays a pivotal role in fibrogenesis, whereas metabolic reprogramming, characterized by excessive glycolysis, is essential in MMT development. No reliable therapies are available despite substantial progress made in understanding the mechanisms underlying peritoneal fibrosis. Protective effect of omega-3 polyunsaturated fatty acids (ω3 PUFAs) has been described in PD-induced peritoneal fibrosis, although the detailed mechanisms remain unknown. It is known that ω3 PUFAs bind to and activate the free fatty acid receptor 4 (FFAR4). However, the expression and role of FFAR4 in the peritoneum have not been investigated. Thus, we hypothesized that ω3 PUFAs would alleviate peritoneal fibrosis by inhibiting hyperglycolysis and MMT through FFAR4 activation. First, we determined FFAR4 expression in peritoneal mesothelium in humans and mice. FFAR4 expression was abnormally decreased in patients on PD and mice and HMrSV5 mesothelial cells exposed to PD fluid (PDF); this change was restored by the ω3 PUFAs (EPA and DHA). ω3 PUFAs significantly inhibited peritoneal hyperglycolysis, MMT, and fibrosis in PDF-treated mice and HMrSV5 mesothelial cells; these changes induced by ω3 PUFAs were blunted by treatment with the FFAR4 antagonist AH7614 and FFAR4 siRNA. Additionally, ω3 PUFAs induced FFAR4, Ca2+/calmodulin-dependent protein kinase kinase β (CaMKKβ), and AMPK and suppressed mTOR, leading to the inhibition of hyperglycolysis, demonstrating that the ω3 PUFAs-mediated FFAR4 activation ameliorated peritoneal fibrosis by inhibiting hyperglycolysis and MMT via CaMKKβ/AMPK/mTOR signaling. As natural FFAR4 agonists, ω3 PUFAs may be considered for the treatment of PD-associated peritoneal fibrosis.

IL-1β may be an indicator of peritoneal deterioration after healing of peritoneal dialysis-associated peritonitis

BACKGROUND

Peritoneal dialysis (PD) is an essential lifesaving treatment for end-stage renal disease. However, PD therapy is limited by peritoneal inflammation, which leads to peritoneal membrane failure because of progressive peritoneal deterioration. Peritonitis is the most common complication in patients undergoing PD. Thus, elucidating the mechanism of chronic peritoneal inflammation after PD-associated peritonitis is an urgent issue for patients undergoing PD. This first case report suggests that an increased interleukin-1β (IL-1β) expression in the peritoneal dialysate after healing of peritonitis can contribute to peritoneal deterioration.

CASE PRESENTATION

A 64-year-old woman was diagnosed with diabetes mellitus 10 years ago and had been started on PD for end-stage renal disease. One day, the patient developed PD-associated acute peritonitis and was admitted to our hospital for treatment. Thus, treatment with antimicrobial agents was initiated for PD-associated peritonitis. Dialysate turbidity gradually disappeared after treatment with antimicrobial agents, and the number of cells in the PD fluid decreased. After 2 weeks of antimicrobial therapy, peritonitis was clinically cured, and the patient was discharged. Thereafter, the patient did not develop peritonitis; however, residual renal function tended to decline, and peritoneal function also decreased in a relatively short period. We evaluated pro-inflammatory cytokine levels before and after PD-associated peritonitis; interestingly, the levels of IL-1β remained high in the PD fluid, even after remission of bacterial peritonitis. In addition, it correlated with decreased peritoneal function.

CONCLUSIONS

This case suggests that inflammasome-derived pro-inflammatory cytokines may contribute to chronic inflammation-induced peritoneal deterioration after PD-related peritonitis is cured.

Knockdown of AK142426 suppresses M2 macrophage polarization and inflammation in peritoneal fibrosis via binding to c-Jun

BACKGROUND

Peritoneal fibrosis is a common complication of peritoneal dialysis, which may lead to ultrafiltration failure and ultimately treatment discontinuation. LncRNAs participate in many biological processes during tumorigenesis. We investigated the role of AK142426 in peritoneal fibrosis.

METHODS

The AK142426 level in peritoneal dialysis (PD) fluid was detected by quantitative real-time-PCR assay. The M2 macrophage distribution was determined by flow cytometry. The inflammatory cytokines of TNF-α and TGF-β1 were measured by ELISA assay. The direct interaction between AK142426 and c-Jun was evaluated by RNA pull-down assay. In addition, the c-Jun and fibrosis related proteins were assessed by western blot analysis.

RESULTS

The PD-induced peritoneal fibrosis mouse model was successfully established. More importantly, PD treatment induced M2 macrophage polarization and the inflammation in PD fluid, which might be associated with exosome transmission. Fortunately, AK142426 was observed to be upregulated in PD fluid. Mechanically, knockdown of AK142426 suppressed M2 macrophage polarization and inflammation. Furthermore, AK142426 could upregulate c-Jun through binding c-Jun protein. In rescue experiments, overexpression of c-Jun could partially abolish the inhibitory effect of sh-AK142426 on the activation of M2 macrophages and inflammation. Consistently, knockdown of AK142426 alleviated peritoneal fibrosis in vivo.

CONCLUSIONS

This study demonstrated that knockdown of AK142426 suppressed M2 macrophage polarization and inflammation in peritoneal fibrosis via binding to c-Jun, suggesting that AK142426 might be a promising therapeutic target for patients of peritoneal fibrosis.

Activation of the inflammasome drives peritoneal deterioration in a mouse model of peritoneal fibrosis

During peritoneal dialysis (PD), the peritoneum is exposed to a bioincompatible dialysate, deteriorating the tissue and limiting the long-term effectiveness of PD. Peritoneal fibrosis is triggered by chronic inflammation induced by a variety of stimuli, including peritonitis. Exposure to PD fluid alters peritoneal macrophages phenotype. Inflammasome activation triggers chronic inflammation. First, it was determined whether inflammasome activation causes peritoneal deterioration. In the in vivo experiments, the increased expression of the inflammasome components, caspase-1 activity, and concomitant overproduction of IL-1β and IL-18 were observed in a mouse model of peritoneal fibrosis. ASC-positive and F4/80-positive cells colocalized in the subperitoneal mesothelial cell layer. These macrophages expressed high CD44 levels indicating that the CD44-positive macrophages contribute to developing peritoneal deterioration. Furthermore, intravital imaging of the peritoneal microvasculature demonstrated that the circulating CD44-positive leukocytes may contribute to peritoneal fibrosis. Bone marrow transplantation in ASC-deficient mice suppressed inflammasome activation, thereby attenuating peritoneal fibrosis in a high glucose-based PD solution-injected mouse model. Our results suggest inflammasome activation in CD44-positive macrophages may be involved in developing peritoneal fibrosis. The inflammasome-derived pro-inflammatory cytokines might therefore serve as new biomarkers for developing encapsulating peritoneal sclerosis.

Peritoneal equilibration testing: Your questions answered

The peritoneal equilibration test (PET), first described in 1987, is a semiquantitative assessment of peritoneal transfer characteristics in patients undergoing peritoneal dialysis. It is typically performed as a 4-h exchange using 2.27/2.5% dextrose dialysate with serial measurements of blood and dialysate creatinine, urea, and glucose concentrations. The percentage absorption of glucose and D/P creatinine ratio are used to determine peritoneal solute transfer rates. It is used to both help guide peritoneal dialysis prescriptions and to prognosticate. There are several derivative tests which have been described in the literature. In this review, we describe the original PET, the various iterations of the PET, the information gleaned, and the use in the setting of poor solute clearance and in the diagnosis of membrane dysfunction, and limitations of the PET.

Tissue Sodium Accumulation Induces Organ Inflammation and Injury in Chronic Kidney Disease

High salt intake is a primary cause of over-hydration in chronic kidney disease (CKD) patients. Inflammatory markers are predictors of CKD mortality; however, the pathogenesis of inflammation remains unclear. Sodium storage in tissues has recently emerged as an issue of concern. The binding of sodium to tissue glycosaminoglycans and its subsequent release regulates local tonicity. Many cell types express tonicity-responsive enhancer-binding protein (TonEBP), which is activated in a tonicity-dependent or tonicity-independent manner. Macrophage infiltration was observed in the heart, peritoneal wall, and para-aortic tissues in salt-loading subtotal nephrectomized mice, whereas macrophages were not prominent in tap water-loaded subtotal nephrectomized mice. TonEBP was increased in the heart and peritoneal wall, leading to the upregulation of inflammatory mediators associated with cardiac fibrosis and peritoneal membrane dysfunction, respectively. Reducing salt loading by a diuretic treatment or changing to tap water attenuated macrophage infiltration, TonEBP expression, and inflammatory marker expression. The role of TonEBP may be crucial during the cardiac fibrosis and peritoneal deterioration processes induced by sodium overload. Anti-interleukin-6 therapy improved cardiac inflammation and fibrosis and peritoneal membrane dysfunction. Further studies are necessary to establish a strategy to regulate organ dysfunction induced by TonEBP activation in CKD patients.

Proteomic Characterization of Peritoneal Extracellular Vesicles in a Mouse Model of Peritoneal Fibrosis

Peritoneal fibrosis progression is regarded as a significant cause of the loss of peritoneal function, markedly limiting the application of peritoneal dialysis (PD). However, the pathogenesis of peritoneal fibrosis remains to be elucidated. Tissue-derived extracellular vesicles (EVs) change their molecular cargos to adapt the environment alteration, mediating intercellular communications and play a significant role in organ fibrosis. Hence, we performed, for the first time, four-dimensional label-free quantitative liquid chromatography-tandem mass spectrometry proteomic analyses on EVs from normal peritoneal tissues and PD-induced fibrotic peritoneum in mice. We demonstrated the alterations of EV concentration and protein composition between normal control and PD groups. A total of 2339 proteins containing 967 differentially expressed proteins were identified. Notably, upregulated proteins in PD EVs were enriched in processes including response to wounding and leukocyte migration, which participated in the development of fibrosis. In addition, EV proteins of the PD group exhibited unique metabolic signature compared with those of the control group. The glycolysis-related proteins increased in PD EVs, while oxidative phosphorylation and fatty acid metabolism-related proteins decreased. We also evaluated the effect of cell-type specificity on EV proteins, suggesting that mesothelial cells mainly cause the alterations in the molecular composition of EVs. Our study provided a useful resource for further validation of the key regulator or therapeutic target of peritoneal fibrosis.

The prognosis and risk factors of baseline high peritoneal transporters on patients with peritoneal dialysis

The relationship between baseline high peritoneal solute transport rate (PSTR) and the prognosis of peritoneal dialysis (PD) patients remains unclear. The present study combined clinical data and basic experiments to investigate the impact of baseline PSTR and the underlying molecular mechanisms. A total of 204 incident CAPD patients from four PD centres in Shanghai between 1 January 2014 and 30 September 2020 were grouped based on a peritoneal equilibration test after the first month of dialysis. Analysed with multivariate Cox and logistic regression models, baseline high PSTR was a significant risk factor for technique failure (AHR 5.70; 95% CI 1.581 to 20.548 p = 0.008). Baseline hyperuricemia was an independent predictor of mortality (AHR 1.006 95%CI 1.003 to 1.008, p < 0.001) and baseline high PSTR (AOR 1.007; 95%CI 1.003 to 1.012; p = 0.020). Since uric acid was closely related to high PSTR and adverse prognosis, the in vitro experiments were performed to explore the underlying mechanisms of which uric acid affected peritoneum. We found hyperuricemia induced epithelial‐to‐mesenchymal transition (EMT) of cultured human peritoneal mesothelial cells by activating TGF‐β1/Smad3 signalling pathway and nuclear transcription factors. Conclusively, high baseline PSTR induced by hyperuricaemia through EMT was an important reason of poor outcomes in CAPD patients.

Adipose-derived mesenchymal stem cells attenuate dialysis-induced peritoneal fibrosis by modulating macrophage polarization via interleukin-6

Background

Life-long peritoneal dialysis (PD) as a renal replacement therapy is limited by peritoneal fibrosis. Previous studies showed immunomodulatory and antifibrotic effects of adipose-derived mesenchymal stem cells (ADSCs) on peritoneal fibrosis. However, the role of the peritoneal macrophage in this process remains uninvestigated.

Methods

We examined the therapeutic effects of ADSC and bone marrow-derived mesenchymal stem cells (BM-MSC) in the rat model of dialysis-induced peritoneal fibrosis using methylglyoxal. In addition, treatment of macrophages with the conditioned medium of ADSC and BM-MSC was performed individually to identify the beneficial component of the stem cell secretome.

Results

In the in vivo experiments, we found dialysis-induced rat peritoneal fibrosis was attenuated by both ADSC and BM-MSC. Interestingly, ADSC possessed a more prominent therapeutic effect than BM-MSC in ameliorating peritoneal membrane thickening while also upregulating epithelial cell markers in rat peritoneal tissues. The therapeutic effects of ADSC were positively associated with M2 macrophage polarization. In the in vitro experiments, we confirmed that interleukin-6 (IL-6) secreted by MSCs upon transforming growth factor-β1 stimulation promotes M2 macrophage polarization.

Conclusions

In dialysis-induced peritoneal fibrosis, MSCs are situated in an inflammatory environment of TGF-β1 and secrete IL-6 to polarize macrophages into the M2 phenotype. Our findings reveal a previously unidentified role of tissue macrophage in this antifibrotic process. ADSC has the advantage of abundance and accessibility, making the application values extremely promising.

Graphical abstract

In dialysis-induced peritoneal fibrosis, peritoneal mesothelial cells secrete transforming growth factor-β1 (TGF-β1) when exposed to methylglyoxal (MGO)-containing peritoneal dialysate. When situated in TGF-β1, the inflammatory environment induces mesenchymal stem cells to secrete interleukin-6 (IL-6), IL-6 polarizes macrophages into the M2 phenotype. The dominant peritoneal tissue M2 macrophages, marked by upregulated Arg-1 expression, account for the attenuation of MGO-induced dedifferentiation of peritoneal mesothelial cells to maintain epithelial integrity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02270-4.

Recent Understanding of Peritoneal Pathology in Peritoneal Dialysis Patients in Japan

The thin peritoneum covering the peritoneal cavity has been used as a dialysis membrane for peritoneal dialysis (PD) because it is highly vascularized and has a large body surface area. However, it has been reported that peritoneal membranes affected by peritonitis, as well as those exposed to the nonphysiological high glucose levels containing PD dialysate, may undergo histological and functional changes. Patients undergoing PD may experience encapsulating peritoneal sclerosis (EPS), which is a life-threatening serious complication of PD that can significantly impair activities of daily living. The incidence of EPS was 1.4-7.3% of maintenance PD patients in the 1980s. The incidence has improved to 1.0% after a neutral dialysate became the standard PD treatment in Japan. Furthermore, the pathogenesis of EPS is uncertain although its onset may be explained by the "two-hit theory," in which some factors leading to impairment had an additive effect on simple peritoneal sclerosis. The evaluation of histopathological findings has shown the impact of the neutral dialysate on peritoneal deterioration as well as its role in the development of functional changes. In the present report, we discuss the advances in the understanding of peritoneal deterioration based on histological and macroscopic evaluations of the peritoneum of patients undergoing PD. We also discuss the recent treatment for PD patients.

Loosening of the mesothelial barrier as an early therapeutic target to preserve peritoneal function in peritoneal dialysis

Phenotype transition of peritoneal mesothelial cells (MCs) including the epithelial-to-mesenchymal transition (EMT) is regarded as an early mechanism of peritoneal dysfunction and fibrosis in peritoneal dialysis (PD), producing proinflammatory and pro-fibrotic milieu in the intra-peritoneal cavity. Loosening of intercellular tight adhesion between adjacent MCs as an initial process of EMT creates the environment where mesothelium and submesothelial tissue are more vulnerable to the composition of bio-incompatible dialysates, reactive oxygen species, and inflammatory cytokines. In addition, down-regulation of epithelial cell markers such as E-cadherin facilitates de novo acquisition of mesenchymal phenotypes in MCs and production of extracellular matrices. Major mechanisms underlying the EMT of MCs include induction of oxidative stress, pro-inflammatory cytokines, endoplasmic reticulum stress and activation of the local renin-angiotensin system. Another mechanism of peritoneal EMT is mitigation of intrinsic defense mechanisms such as the peritoneal antioxidant system and anti-fibrotic peptide production in the peritoneal cavity. In addition to use of less bio-incompatible dialysates and optimum treatment of peritonitis in PD, therapies to prevent or alleviate peritoneal EMT have demonstrated a favorable effect on peritoneal function and structure, suggesting that EMT can be an early interventional target to preserve peritoneal integrity.

Inhibition of hyperglycolysis in mesothelial cells prevents peritoneal fibrosis

Progressive peritoneal fibrosis affects patients receiving peritoneal dialysis (PD) and has no reliable treatment. The mechanisms that initiate and sustain peritoneal fibrosis remain incompletely elucidated. To overcome these problems, we developed a strategy that prevents peritoneal fibrosis by suppressing PD-stimulated mesothelial-to-mesenchymal transition (MMT). We evaluated single-cell transcriptomes of mesothelial cells obtained from normal peritoneal biopsy and effluent from PD-treated patients. In cells undergoing MMT, we found cellular heterogeneity and intermediate transition states associated with up-regulation of enzymes involved in glycolysis. The expression of glycolytic enzymes was correlated with the development of MMT. Using gene expression profiling and metabolomics analyses, we confirmed that PD fluid induces metabolic reprogramming, characterized as hyperglycolysis, in mouse peritoneum. We found that transforming growth factor β1 (TGF-β1) can substitute for PD fluid to stimulate hyperglycolysis, suppressing mitochondrial respiration in mesothelial cells. Blockade of hyperglycolysis with 2-deoxyglucose (2-DG) inhibited TGF-β1-induced profibrotic cellular phenotype and peritoneal fibrosis in mice. We developed a triad of adeno-associated viruses that overexpressed microRNA-26a and microRNA-200a while inhibiting microRNA-21a to target hyperglycolysis and fibrotic signaling. Intraperitoneal injection of the viral triad inhibited the development of peritoneal fibrosis induced by PD fluid in mice. We conclude that hyperglycolysis is responsible for MMT and peritoneal fibrogenesis, and this aberrant metabolic state can be corrected by modulating microRNAs in the peritoneum. These results could provide a therapeutic strategy to combat peritoneal fibrosis.

Determinants of volume status in peritoneal dialysis: A longitudinal study

AIM

In peritoneal dialysis (PD), fluid overload is frequent and associated with cardiac dysfunction and mortality. As it is considered a modifiable risk factor, we wished to describe clinical determinants of fluid overload in a longitudinal cohort of PD outpatients.

METHODS

We consecutively included PD outpatients treated with continuous ambulatory PD (CAPD) or automated PD (APD) attending for their routine clinical visit at a single tertiary hospital. Extracellular water (ECW) to total body water (TBW) ratio was measured by multifrequency bioelectrical impedance. Peritoneal transport characteristics were measured with a standard peritoneal equilibration test. Patients had a second follow-up visit with the same measurements. Univariable and multivariable mixed linear regression models were conducted with ECW/TBW as the dependent variable.

RESULTS

A total of 155 patients were enrolled with a median follow-up time of 12 months. Median dialysis vintage was 13.5 ± 3.4 months. Overall mean value of ECW/TBW was 39.3% ± 1.1. In multivariable analysis, factors positively associated with ECW/TBW were: Age (P < .001), diabetes (P = .002), and SBP (P = .028). Factors negatively associated with ECW/TBW were: nPNA (P = .001), serum albumin (P < .001) and PTH (P = .014). None of the considered variable showed a significant interaction with time.

CONCLUSION

We confirm a high prevalence of fluid overload in PD patients and show that it is strongly associated with older age, diabetes, hypoalbuminemia and protein energy wasting. In contrast, when PD prescription is tailored to patient's individual characteristics, residual renal function, PD modality and peritoneal characteristics are not decisive in controlling volume status.

IL-6 trans-signaling drives a STAT3-dependent pathway that leads to structural alterations of the peritoneal membrane

IL-6 is a vital inflammatory factor in the peritoneal cavity of patients undergoing peritoneal dialysis (PD). The present study examined the effect of IL-6 trans-signaling on structural alterations of the peritoneal membrane. We investigated whether the epithelial-to-mesenchymal transition (EMT) process of human peritoneal mesothelial cells (HPMCs) and the production of proangiogenic factors were controlled by IL-6 trans-signaling. Its role in the peritoneal alterations was detected in a mouse model. The morphology of HPMCs and levels of cytokines in PD effluent were also explored. Stimulation of HPMCs with the IL-6 and soluble IL-6 receptor complex (IL-6/S) promoted the EMT process of HPMCs depending on the STAT3 pathway. In a coculture system of HPMCs and human umbilical vein endothelial cells, IL-6/S mediated the production of VEGF and angiopoietins so as to downregulate the expression of endothelial junction molecules and finally affect vascular permeability. Daily intraperitoneal injection of high glucose-based dialysis fluid induced peritoneal fibrosis, angiogenesis, and macrophage infiltration in a mouse model, accompanied by phosphorylation of STAT3. Blockade of IL-6 trans-signaling prevented these peritoneum alterations. The fibroblast-like appearance of HPMCs ex vivo was upregulated in patients undergoing prevalent PD accompanied by increasing levels of IL-6, VEGF, and angiopoietin-2 in the PD effluent. Taken together, these findings identified a critical link between IL-6 trans-signaling and structural alterations of the peritoneal membrane, and it might be a potential target for the treatment of patients undergoing PD who have developed peritoneal alterations.

Expression of XBP1s in peritoneal mesothelial cells is critical for inflammation-induced peritoneal fibrosis

Intraperitoneal inflammation is the most important determinant of peritoneal fibrosis in patients with long-term peritoneal dialysis (PD). Spliced x-box binding protein-1 (XBP1s), a major proximal effector of unfolded protein response (UPR) signaling, plays an indispensable role in inflammation. Our study demonstrated that the inflammatory factor interleukin-1β (IL-1β) dose- and time-dependently induced XBP1s upregulation and interleukin-6 (IL-6) secretion, as well as the expression of the fibrotic marker fibronectin. However, these effects were prevented by the IRE1 endonuclease inhibitor STF083010 since it time-dependently reduced IL-1β-induced Xbp1 mRNA splicing, XBP1s protein expression, inflammatory factor IL-6 secretion and the expression of the fibrotic marker fibronectin in human peritoneal mesothelial cells (HPMCs). The overexpression and knockdown of XBP1s in HPMCs had a similar effect on fibronectin expression. In a rat model of peritoneal inflammation, STF083010 significantly attenuated chlorhexidine digluconate-induced XBP1s and α-smooth muscle actin expression, as well as fibrotic tissue proliferation, in the peritoneum. Our results suggest that XBP1s is a strong pathogenic factor that mediates inflammation-induced peritoneal fibrosis in peritoneal dialysis.

ST2 blockade mitigates peritoneal fibrosis induced by TGF-β and high glucose

Peritoneal fibrosis (PF) is an intractable complication of peritoneal dialysis (PD) that leads to peritoneal membrane failure. This study investigated the role of suppression of tumorigenicity (ST)2 in PF using patient samples along with mouse and cell‐based models. Baseline dialysate soluble (s)ST2 level in patients measured 1 month after PD initiation was 2063.4 ± 2457.8 pg/mL; patients who switched to haemodialysis had elevated sST2 levels in peritoneal effluent (1576.2 ± 199.9 pg/mL, P = .03), which was associated with PD failure (P = .04). Baseline sST2 showed good performance in predicting PD failure (area under the receiver operating characteristic curve = 0.780, P = .001). In mice with chlorhexidine gluconate‐induced PF, ST2 was expressed in fibroblasts and mesothelial cells within submesothelial zones. In primary cultured human peritoneal mesothelial cells (HPMCs), transforming growth factor‐β treatment increased ST2, fibronectin, β‐galactosidase and Snail protein levels and decreased E‐cadherin level. Anti‐ST2 antibody administration reversed the up‐regulation of ST2 and fibronectin expression; it also reduced fibrosis induced by high glucose (100 mmol/L) in HPMCs. Thus, high ST2 level in dialysate is a marker for fibrosis and inflammation during peritoneal injury, and blocking ST2 may be an effective therapeutic strategy for renal preservation.

Repeated Bioimpedance Measurements Predict Prognosis of Peritoneal Dialysis Patients

BACKGROUND

Fluid overload is a major risk factor for mortality in patients undergoing peritoneal dialysis (PD). However, few studies have investigated the effect of chronic exposure to sustained fluid overload on long-term outcomes.

METHOD

A total of 284 prevalent PD patients were included in this prospective study. Repeated multifrequency body composition analysis was performed 12 months apart, and 1-year cumulative chronic fluid overload were used to predict all-cause mortality and the risk for transfer to hemodialysis (HD) during the ensuing 15.6 ± 9.1 months.

RESULTS

The prevalence of fluid overload was approximately 27%. Interestingly, a substantial number of hypervolemic patients at first test were persistently hypervolemic at their second test. With this, chronic fluid overload was observed in 18.3% (n = 52). Notably, most of chronic fluid overload patients had diabetes (86.5%), and it was accompanied by concomitant changes in peritoneal membrane characteristics, a higher progression rate to high transporter. The risk of transfer to HD increased 2.8 times in patients with chronic fluid overload than in those without. Also, it significantly increased the risk of mortality (p = 0.038). Surprisingly, subgroup analysis found that patients with euvolemic status at follow-up experienced no mortality despite being in a fluid overload state at baseline.

CONCLUSIONS

One-year chronic exposure to fluid overload is a strong independent risk factor for transfer to HD and death in prevalent PD patients. Although the fluid status of most PD patients is not easily changed over time, becoming euvolemic during the entire PD treatment period seems to be very important.

Mesoporous Silica Nanoparticles-Encapsulated Agarose and Heparin as Anticoagulant and Resisting Bacterial Adhesion Coating for Biomedical Silicone

Silicone catheter has been widely used in peritoneal dialysis. The research missions of improving blood compatibility and the ability of resisting bacterial adhesion of silicone catheter have been implemented for the biomedical requirements. However, most of modification methods of surface modification were only able to develop the blood-contacting biomaterials with good hemocompatibility. It is difficult for the biomaterials to resist bacterial adhesion. Here, agarose was selected to resist bacterial adhesion, and heparin was chosen to improve hemocompatibility of materials. Both of them were loaded into mesoporous silica nanoparticles (MSNs), which were successfully modified on the silicone film surface via electrostatic interaction. Structures of the mesoporous coatings were characterized in detail by dynamic light scattering, transmission electron microscopy, Brunauer-Emmett-Teller surface area, thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscope, and water contact angle. Platelet adhesion and aggregation, whole blood contact test, hemolysis and related morphology test of red blood cells, in vitro clotting time tests, and bacterial adhesion assay were performed to evaluate the anticoagulant effect and the ability of resisting bacterial adhesion of the modified silicone films. Results indicated that silicone films modified by MSNs had a good anticoagulant effect and could resist bacterial adhesion. The modified silicone films have potential as blood-contacting biomaterials that were attributed to their biomedical properties.

CD147 expression in peritoneal injury

BACKGROUND

Peritoneal injury is an important cause of technical failure of long-term peritoneal dialysis (PD). Encapsulating peritoneal sclerosis (EPS) is a severe complication of long-term PD with potentially life threatening consequences. CD147 is a glycoprotein with diverse functions including modulation of extracellular matrix via induction of matrix metalloproteinases, cell adhesion, and regulation of immune reactions. We hypothesized that CD 147 plays a role in the peritoneal cavity.

METHODS

In this retrospective study, we localized CD147 by immunohistochemistry in peritoneal biopsies from uremic patients not on PD (n = 8), on PD without signs of EPS (n = 7), and in biopsies in patients with the diagnosis of EPS (n = 7). Double immunofluorescence was used to co-localize α-smooth-muscle actin (α-SMA) and CD147 in selected biopsies from each group. Expression was scored semi-quantitatively.

RESULTS

In biopsies from uremic controls, CD147 was prominently expressed in mesothelial cells, focally between fat cells and by some perivascular cells. In patients on PD, a similar distribution was present (although mesothelium was rarely conserved), with some focal accentuation. In EPS, layers of fibroblastic cells were positive for CD147. EPS biopsies demonstrated a significantly higher score in a blinded evaluation, compared to uremic patients. Cells expressing CD147 were α-SMA positive myofibroblasts as demonstrated by double immunofluorescence. Mean CD147 scores did not differ between patients with different transporter status.

CONCLUSIONS

This is the first study demonstrating CD147 on a major part of fibroblastic cells in EPS. Future studies need to address the role of these cells in this severe complication of long-term PD.

Local amplifiers of IL-4Rα-mediated macrophage activation promote repair in lung and liver

The type 2 immune response controls helminth infection and maintains tissue homeostasis but can lead to allergy and fibrosis if not adequately regulated. We have discovered local tissue-specific amplifiers of type 2-mediated macrophage activation. In the lung, surfactant protein A (SP-A) enhanced interleukin-4 (IL-4)-dependent macrophage proliferation and activation, accelerating parasite clearance and reducing pulmonary injury after infection with a lung-migrating helminth. In the peritoneal cavity and liver, C1q enhancement of type 2 macrophage activation was required for liver repair after bacterial infection, but resulted in fibrosis after peritoneal dialysis. IL-4 drives production of these structurally related defense collagens, SP-A and C1q, and the expression of their receptor, myosin 18A. These findings reveal the existence within different tissues of an amplification system needed for local type 2 responses.

Selective Blocking of TNF Receptor 1 Attenuates Peritoneal Dialysis Fluid Induced Inflammation of the Peritoneum in Mice

Chronic inflammatory conditions during peritoneal dialysis (PD)-treatment lead to the impairment of peritoneal tissue integrity. The resulting structural and functional reorganization of the peritoneal membrane diminishes ultrafiltration rate and thereby enhances mortality by limiting dialysis effectiveness over time. Tumour necrosis factor (TNF) and its receptors TNFR1 and TNFR2 are key players during inflammatory processes. To date, the role of TNFR1 in peritoneal tissue damage during PD-treatment is completely undefined. In this study, we used an acute PD-mouse model to investigate the role of TNFR1 on structural and morphological changes of the peritoneal membrane. TNFR1-mediated TNF signalling in transgenic mice expressing human TNFR1 was specifically blocked by applying a monoclonal antibody (H398) highly selective for human TNFR1 prior to PD-treatment. Cancer antigen-125 (CA125) plasma concentrations were measured by enzyme-linked immunosorbent assay (ELISA). Western blot analyses were applied to determine TNFR2 protein concentrations. Histological staining of peritoneal tissue sections was performed to assess granulocytes within the peritoneal membrane as well as the content of hyaluronic acid and collagen. We show for the first time that the number of granulocytes within the peritoneal membrane is significantly reduced in mice pre-treated with H398. Moreover, we demonstrate that blocking of TNFR1 not only influences CA125 values but also hyaluronic acid and collagen contents of the peritoneal tissue in these mice. These results strongly suggest that TNFR1 inhibition attenuates peritoneal damage caused by peritoneal dialysis fluid (PDF) and therefore may represent a new therapeutic approach in the treatment of PD-related side effects.

The Role of Tyrosine Kinase Receptors in Peritoneal Fibrosis

Peritoneal dialysis (PD) is a modality for treatment of patients with end-stage renal disease (ESRD) that depends on the structural and functional integrity of the peritoneal membrane. However, long-term PD can lead to morphological and functional changes in the peritoneum; in particular, peritoneal fibrosis has become one of the most common complications that ultimately results in ultrafiltration failure (UFF) and discontinuation of PD. Several factors and mechanisms such as inflammation and overproduction of transforming growth factor-β1 have been implicated in the development of peritoneal fibrosis, but there is no effective therapy to prevent or delay this process. Recent studies have shown that activation of multiple receptor tyrosine kinases (RTKs) is associated with the development and progression of tissue fibrosis in various organs, and there are also reports indicating the involvement of some RTKs in peritoneal fibrosis. This review will describe the role and mechanisms of RTKs in peritoneal fibrosis and discuss the possibility of using them as therapeutic targets for prevention and treatment of this complication.

Inhibition of EGF Receptor Blocks the Development and Progression of Peritoneal Fibrosis

Inhibitors of EGF receptor (EGFR) have antifibrotic effects in several organs, but the effect of these inhibitors on the development of peritoneal fibrosis is unknown. Here, we explored the therapeutic effect of gefitinib, a specific inhibitor of EGFR, on the development and progression of peritoneal fibrosis in a rat model. Daily intraperitoneal injections of chlorhexidine gluconate induced peritoneal fibrosis, indicated by thickening of the submesothelial area with an accumulation of collagen fibrils and activation of myofibroblasts, accompanied by time-dependent phosphorylation of EGFR. Administration of gefitinib immediately after injury prevented the onset of peritoneal fibrosis and delayed administration after the onset of peritoneal fibrosis halted fibrosis progression. Gefitinib treatment abrogated the increased phosphorylation of EGFR, Smad3, signal transducer and activator of transcription 3, and NF-κB during peritoneal fibrosis; it also inhibited the accompanying overproduction of TGF-β1 and proinflammatory cytokines and the infiltration of macrophages to the injured peritoneum. Moreover, gefitinib significantly reduced the peritoneal increase of CD31-positive blood vessels and vascular EGF-positive cells after injury. Finally, gefitinib also attenuated high glucose-induced peritoneal fibrosis in rats and abrogated TGF-β1-induced phosphorylation of Smad3 and the epithelial-to-mesenchymal transition of cultured human peritoneal mesothelial cells. These results demonstrate that EGFR contributes to peritoneal fibrosis, inflammation, and angiogenesis, suggesting that EGFR inhibitors may have therapeutic potential in attenuating peritoneal fibrosis.

Transition of mesothelial cell to fibroblast in peritoneal dialysis: EMT, stem cell or bystander?

Long-term peritoneal dialysis (PD) can lead to fibrotic changes in the peritoneum, characterized by loss of mesothelial cells (MCs) and thickening of the submesothelial area with an accumulation of collagen and myofibroblasts. The origin of myofibroblasts is a central question in peritoneal fibrosis that remains unanswered at present. Numerous clinical and experimental studies have suggested that MCs, through epithelial-mesenchymal transition (EMT), contribute to the pool of peritoneal myofibroblasts. However, recent work has placed significant doubts on the paradigm of EMT in organ fibrogenesis (in the kidney particularly), highlighting the need to reconsider the role of EMT in the generation of myofibroblasts in peritoneal fibrosis. In particular, selective cell isolation and lineage-tracing experiments have suggested the existence of progenitor cells in the peritoneum, which are able to switch to fibroblast-like cells when stimulated by the local environment. These findings highlight the plastic nature of MCs and its contribution to peritoneal fibrogenesis. In this review, we summarize the key findings and caveats of EMT in organ fibrogenesis, with a focus on PD-related peritoneal fibrosis, and discuss the potential of peritoneal MCs as a source of myofibroblasts.

The PPARβ/δ agonist GW501516 attenuates peritonitis in peritoneal fibrosis via inhibition of TAK1-NFκB pathway in rats

Peritoneal fibrosis is a common consequence of long-term peritoneal dialysis (PD), and peritonitis is a factor in its onset. Agonist-bound peroxisome proliferator-activated receptors (PPARs) function as key regulators of energy metabolism and inflammation. Here, we examined the effects of PPARβ/δ agonist GW501516 on peritonitis in a rat peritoneal fibrosis model. Peritoneal fibrosis secondary to inflammation was induced into uremic rats by daily injection of Dianeal 4.25% PD solutions along with six doses of lipopolysaccharide before commencement of GW501516 treatment. Normal non-uremic rats served as control, and all rats were fed with a control diet or a GW501516-containing diet. Compared to control group, exposure to PD fluids caused peritoneal fibrosis that was accompanied by increased mRNA levels of monocyte chemoattractant protein-1, tumor necrotic factor-α, and interleukin-6 in the uremic rats, and these effects were prevented by GW501516 treatment. Moreover, GW501516 was found to attenuate glucose-stimulated inflammation in cultured rat peritoneal mesothelial cells via inhibition of transforming growth factor-β-activated kinase 1 (TAK1), and nuclear factor kappa B (NFκB) signaling pathway (TAK1-NFκB pathway), a main inflammation regulatory pathway. In conclusion, inhibition of TAK1-NFκB pathway with GW501516 may represent a novel therapeutic approach to ameliorate peritonitis-induced peritoneal fibrosis for patients on PD.

A microrna screen to identify regulators of peritoneal fibrosis in a rat model of peritoneal dialysis

Background

Peritoneal fibrosis is a common complication in patients treated with long-term peritoneal dialysis. The aim of this study was to identify the microRNAs (miRNAs) involved in regulation of peritoneal fibrosis in a rat model of peritoneal dialysis.

Methods

Twenty-four Sprague–Dawley (SD) rats were randomly allocated into three groups: (i) Control group (Cg, n = 8); (ii) Saline group (Sg, n = 8): daily intraperitoneal injection with 0.9% normal saline; (iii) Hypertonic dialysate group (HDg, n = 8): daily intraperitoneal injection with 4.25% peritoneal dialysis solution. Rats were sacrificed after four weeks for histological evaluation of peritoneal membrane and the expression of α-SMA and COL-1. A miRNA screen was performed using microarray analysis to identify differentially expressed miRNAs, which were then validated by real-time PCR.

Results

Compared with the control and the saline groups, hypertonic dialysate group showed impaired peritoneal function accompanied by a spectrum of morphological changes including thicker peritoneal membrane, higher collagen deposition, infiltration of mononuclear cells and neovascularization in the peritoneum. Increased mRNA and protein levels of α-SMA and COL-1 were observed in hypertonic dialysate group, indicating the progression of peritoneal fibrosis. The miRNA screen identified 8 significantly down-regulated miRNAs (miR-31, miR-93, miR-100, miR-152, miR-497, miR-192, miR-194 and miR-200b) and one highly up-regulated miRNA (miR-122) in the hypertonic dialysate group. The results were confirmed by real-time PCR.

Conclusions

Altered miRNA expression in peritoneum was found in the rat model of peritoneal fibrosis, indicating that these miRNAs may be associated with pathogenesis of peritoneal fibrosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12882-015-0039-z) contains supplementary material, which is available to authorized users.

Encapsulating peritoneal sclerosis-a rare but devastating peritoneal disease

Encapsulating peritoneal sclerosis (EPS) is a devastating but, fortunately, rare complication of long-term peritoneal dialysis. The disease is associated with extensive thickening and fibrosis of the peritoneum resulting in the formation of a fibrous cocoon encapsulating the bowel leading to intestinal obstruction. The incidence of EPS ranges between 0.7 and 3.3% and increases with duration of peritoneal dialysis therapy. Dialysis fluid is hyperosmotic, hyperglycemic, and acidic causing chronic injury and inflammation in the peritoneum with loss of mesothelium and extensive tissue fibrosis. The pathogenesis of EPS, however, still remains uncertain, although a widely accepted hypothesis is the "two-hit theory," where, the first hit is chronic peritoneal membrane injury from long standing peritoneal dialysis followed by a second hit such as an episode of peritonitis, genetic predisposition and/or acute cessation of peritoneal dialysis, leading to EPS. Recently, EPS has been reported in patients shortly after transplantation suggesting that this procedure may also act as a possible second insult. The process of epithelial-mesenchymal transition of mesothelial cells is proposed to play a central role in the development of peritoneal sclerosis, a common characteristic of patients on dialysis, however, its importance in EPS is less clear. There is no established treatment for EPS although evidence from small case studies suggests that corticosteroids and tamoxifen may be beneficial. Nutritional support is essential and surgical intervention (peritonectomy and enterolysis) is recommended in later stages to relieve bowel obstruction.

Platelet-derived growth factor receptor-β expression in human peritoneum

INTRODUCTION

Simple peritoneal fibrosis and encapsulating peritoneal sclerosis (EPS) are important lesions in the peritoneum of patients on peritoneal dialysis (PD). We have previously described a population of podoplanin-positive myofibroblasts in peritoneal biopsies from patients with EPS. Platelet-derived growth factor receptor-β (PDGFRβ) is a marker of pericytes, and PDGFs might be involved in the fibrotic response of the peritoneum. This study aimed to describe PDGFRβ in the human peritoneum.

METHODS

In this retrospective analysis, we localized PDGFRβ in peritoneal biopsies from patients with EPS (n = 6) and patients on PD without signs of EPS (n = 5), and compared them with normal peritoneum (n = 4) and peritoneum from uremic patients (n = 5). Consecutive sections were stained for smooth-muscle actin (SMA) and podoplanin. Slides were scored semiquantitatively by 2 observers blinded to the diagnosis.

RESULTS

PDGFRβ was expressed by cells of arterial walls in all biopsies. A prominent population of PDGFRβ-positive cells was present in the normal peritoneum, which were SMA negative on consecutive sections. In patients on PD, a high number of PDGFRβ were also positive for SMA. In EPS, the majority of podoplanin-positive cells were positive for PDGFRβ. In peritoneal biopsies from normal and uremic patients, the expression of SMA was mainly restricted to cells of arterial walls. Podoplanin expression was restricted to lymphatic vessels in normal peritoneum, in uremic patients, and in patients on PD without EPS.

CONCLUSIONS

As podoplanin-positive myofibroblasts express PDGFRβ, these cells might be related to pericytes (rather than other sources of fibroblasts). PDGFRβ might turn out to be a therapeutic target in EPS.

A pathogenetic role for endothelin-1 in peritoneal dialysis-associated fibrosis

In patients undergoing peritoneal dialysis (PD), chronic exposure to nonphysiologic PD fluids elicits low-grade peritoneal inflammation, leading to fibrosis and angiogenesis. Phenotype conversion of mesothelial cells into myofibroblasts, the so-called mesothelial-to-mesenchymal transition (MMT), significantly contributes to the peritoneal dysfunction related to PD. A number of factors have been described to induce MMT in vitro and in vivo, of which TGF-β1 is probably the most important. The vasoconstrictor peptide endothelin-1 (ET-1) is a transcriptional target of TGF-β1 and mediates excessive scarring and fibrosis in several tissues. This work studied the contribution of ET-1 to the development of peritoneal damage and failure in a mouse model of PD. ET-1 and its receptors were expressed in the peritoneal membrane and upregulated on PD fluid exposure. Administration of an ET receptor antagonist, either bosentan or macitentan, markedly attenuated PD-induced MMT, fibrosis, angiogenesis, and peritoneal functional decline. Adenovirus-mediated overexpression of ET-1 induced MMT in human mesothelial cells in vitro and promoted the early cellular events associated with peritoneal dysfunction in vivo. Notably, TGF-β1-blocking peptides prevented these actions of ET-1. Furthermore, a positive reciprocal relationship was observed between ET-1 expression and TGF-β1 expression in human mesothelial cells. These results strongly support a role for an ET-1/TGF-β1 axis as an inducer of MMT and subsequent peritoneal damage and fibrosis, and they highlight ET-1 as a potential therapeutic target in the treatment of PD-associated dysfunction.

Calcitriol decreases TGF-β1 and angiotensin II production and protects against chlorhexide digluconate-induced liver peritoneal fibrosis in rats

Peritoneal fibrosis is a major complication of peritoneal dialysis that can lead to ultrafiltration failure. This study investigates the protective effects of calcitriol on chlorhexidine digluconate-induced peritoneal fibrosis in rats. Peritoneal fibrosis was induced in Sprague-Dawley rats by daily administration of 0.5mL 0.1% chlorhexidine digluconate in normal saline via peritoneal dialysis for 1week. Rats received daily intravenous injections of calcitriol (low-dose, 10ng/kg; or high-dose, 100ng/kg) for 1week. After 7days, conventional 4.25% Dianeal (30mL) was administered via peritoneal dialysis over 4h. Peritoneal solute transport was calculated from the dialysate concentration relative to its concentration in the initial infused dialysis solution (D4/D0 glucose) for glucose, and the dialysate-to-plasma concentration ratio (D4/P4 urea) at 4h for urea. Rats were then sacrificed and the liver peritoneum was harvested for immunohistochemical analysis via microscopy. After dialysis, the D4/P4 Urea level was reduced; increases were observed in the D4/D0 glucose level and the levels of active transforming growth factor-β1 and angiotensin II in serum and dialysate; the liver peritoneum and muscle peritoneum was markedly thickened, and the expression of α-SMA, fibronectin, collagen, vascular endothelial growth factor, angiotensin II, transforming growth factor-β1, and phosphorylated Smad2/3 (P-Smad2/3)-positive cells in the liver peritoneum was elevated in the peritoneal fibrosis group compared with the vehicle group. Calcitriol decreased the serum and dialysate active transforming growth factor-β1 and angiotensin II level, decreased the thickness of the liver peritoneum and muscle peritoneum, and decreased the expression of α-SMA, fibronectin, collagen, vascular endothelial growth factor, angiotensin II, transforming growth factor-β1, and P-Smad2/3-positive cells in liver peritoneum cells. High-dose calcitriol exhibited better protective effects against peritoneal fibrosis than did the lower dose. Calcitriol protected against chlorhexidine digluconate-induced peritoneal fibrosis in rats by decreasing transforming growth factor-β1 and angiotensin II production.

Peritoneal dialysis--current status and future challenges

Peritoneal dialysis is now a well established, mature treatment modality for advanced chronic kidney disease. The medium term (at least 5 year) survival of patients on peritoneal dialysis is currently equivalent to that of those on haemodialysis, and is particularly good in patients who are new to renal replacement therapy and have less comorbidity. Nevertheless the modality needs to keep pace with the constantly evolving challenges associated with the provision and delivery of health care. These challenges, which are gradually converging at a global level, include ageing of the population, multimorbidity of patients, containment of cost, increasing self care and environmental issues. In this context, peritoneal dialysis faces particular challenges that include multiple barriers to the therapy and unsatisfactory and poorly defined technique survival as well as limitations relating to intrinsic aspects of the therapy, such as peritoneal membrane longevity and hypoalbuminaemia. To move the therapy forward and favourably influence health-care policy, the peritoneal dialysis community needs to integrate their research effort more effectively by undertaking clinically meaningful studies-with a strong focus on technique survival--that are supported by multidisciplinary expertise in patient-centred outcomes, study design and analysis.