Stress responses and conditioning effects in mesothelial cells exposed to peritoneal dialysis fluid

Lithium preserves peritoneal membrane integrity by suppressing mesothelial cell αB-crystallin

Life-saving renal replacement therapy by peritoneal dialysis (PD) is limited in use and duration by progressive impairment of peritoneal membrane integrity and homeostasis. Preservation of peritoneal membrane integrity during chronic PD remains an urgent but long unmet medical need. PD therapy failure results from peritoneal fibrosis and angiogenesis caused by hypertonic PD fluid (PDF)-induced mesothelial cytotoxicity. However, the pathophysiological mechanisms involved are incompletely understood, limiting identification of therapeutic targets. We report that addition of lithium chloride (LiCl) to PDF is a translatable intervention to counteract PDF-induced mesothelial cell death, peritoneal membrane fibrosis, and angiogenesis. LiCl improved mesothelial cell survival in a dose-dependent manner. Combined transcriptomic and proteomic characterization of icodextrin-based PDF-induced mesothelial cell injury identified αB-crystallin as the mesothelial cell protein most consistently counter-regulated by LiCl. In vitro and in vivo overexpression of αB-crystallin triggered a fibrotic phenotype and PDF-like up-regulation of vascular endothelial growth factor (VEGF), CD31-positive cells, and TGF-β-independent activation of TGF-β-regulated targets. In contrast, αB-crystallin knockdown decreased VEGF expression and early mesothelial-to-mesenchymal transition. LiCl reduced VEGF release and counteracted fibrosis- and angiogenesis-associated processes. αB-crystallin in patient-derived mesothelial cells was specifically up-regulated in response to PDF and increased in peritoneal mesothelial cells from biopsies from pediatric patients undergoing PD, correlating with markers of angiogenesis and fibrosis. LiCl-supplemented PDF promoted morphological preservation of mesothelial cells and the submesothelial zone in a mouse model of chronic PD. Thus, repurposing LiCl as a cytoprotective PDF additive may offer a translatable therapeutic strategy to combat peritoneal membrane deterioration during PD therapy.

Peritoneal Dialysis Fluid Supplementation with Alanyl-Glutamine Attenuates Conventional Dialysis Fluid-Mediated Endothelial Cell Injury by Restoring Perturbed Cytoprotective Responses

Long-term clinical outcome of peritoneal dialysis (PD) depends on adequate removal of small solutes and water. The peritoneal endothelium represents the key barrier and peritoneal transport dysfunction is associated with vascular changes. Alanyl-glutamine (AlaGln) has been shown to counteract PD-induced deteriorations but the effect on vascular changes has not yet been elucidated. Using multiplexed proteomic and bioinformatic analyses we investigated the molecular mechanisms of vascular pathology in-vitro (primary human umbilical vein endothelial cells, HUVEC) and ex-vivo (arterioles of patients undergoing PD) following exposure to PD-fluid. An overlap of 1813 proteins (40%) of over 3100 proteins was identified in both sample types. PD-fluid treatment significantly altered 378 in endothelial cells and 192 in arterioles. The HUVEC proteome resembles the arteriolar proteome with expected sample specific differences of mainly immune system processes only present in arterioles and extracellular region proteins primarily found in HUVEC. AlaGln-addition to PD-fluid revealed 359 differentially abundant proteins and restored the molecular process landscape altered by PD fluid. This study provides evidence on validity and inherent limitations of studying endothelial pathomechanisms in-vitro compared to vascular ex-vivo findings. AlaGln could reduce PD-associated vasculopathy by reducing endothelial cellular damage, restoring perturbed abundances of pathologically important proteins and enriching protective processes.

Alanyl-Glutamine Restores Tight Junction Organization after Disruption by a Conventional Peritoneal Dialysis Fluid

Understanding and targeting the molecular basis of peritoneal solute and protein transport is essential to improve peritoneal dialysis (PD) efficacy and patient outcome. Supplementation of PD fluids (PDF) with alanyl-glutamine (AlaGln) increased small solute transport and reduced peritoneal protein loss in a recent clinical trial. Transepithelial resistance and 10 kDa and 70 kDa dextran transport were measured in primary human endothelial cells (HUVEC) exposed to conventional acidic, glucose degradation products (GDP) containing PDF (CPDF) and to low GDP containing PDF (LPDF) with and without AlaGln. Zonula occludens-1 (ZO-1) and claudin-5 were quantified by Western blot and immunofluorescence and in mice exposed to saline and CPDF for 7 weeks by digital imaging analyses. Spatial clustering of ZO-1 molecules was assessed by single molecule localization microscopy. AlaGln increased transepithelial resistance, and in CPDF exposed HUVEC decreased dextran transport rates and preserved claudin-5 and ZO-1 abundance. Endothelial clustering of membrane bound ZO-1 was higher in CPDF supplemented with AlaGln. In mice, arteriolar endothelial claudin-5 was reduced in CPDF, but restored with AlaGln, while mesothelial claudin-5 abundance was unchanged. AlaGln supplementation seals the peritoneal endothelial barrier, and when supplemented to conventional PD fluid increases claudin-5 and ZO-1 abundance and clustering of ZO-1 in the endothelial cell membrane.

Proteomic Research in Peritoneal Dialysis

Peritoneal dialysis (PD) is an established home care, cost-effective renal replacement therapy (RRT), which offers several advantages over the most used dialysis modality, hemodialysis. Despite its potential benefits, however, PD is an under-prescribed method of treating uremic patients. Infectious complications (primarily peritonitis) and bio-incompatibility of PD solutions are the main contributors to PD drop-out, due to their potential for altering the functional and anatomical integrity of the peritoneal membrane. To improve the clinical outcome of PD, there is a need for biomarkers to identify patients at risk of PD-related complications and to guide personalized interventions. Several recent studies have shown that proteomic investigation may be a powerful tool in the prediction, early diagnosis, prognostic assessment, and therapeutic monitoring of patients on PD. Indeed, analysis of the proteome present in PD effluent has uncovered several proteins involved in inflammation and pro-fibrotic insult, in encapsulating peritoneal sclerosis, or even in detecting early changes before any measurable modifications occur in the traditional clinical parameters used to evaluate PD efficacy. We here review the proteomic studies conducted thus far, addressing the potential use of such omics methodology in identifying potential new biomarkers of the peritoneal membrane welfare in relation to dialytic prescription and adequacy.

The Peritoneal Surface Proteome in a Model of Chronic Peritoneal Dialysis Reveals Mechanisms of Membrane Damage and Preservation

Peritoneal dialysis (PD) fluids are cytotoxic to the peritoneum. Recent studies have shown that alanyl-glutamine (AlaGln) modulates the cellular stress response, improves mesothelial cell survival, reduces submesothelial thickening in experimental models of PD, and in clinical studies improves PD effluent cell stress and immune responses. However, the mechanisms of AlaGln-mediated membrane protection are not yet fully understood. Here, we explore those mechanisms through application of a novel proteomics approach in a clinically relevant in vivo model in rats. Experimental PD was performed for 5 weeks using conventional single-chamber bag (SCB) or neutral dual-chamber bag (DCB), PD fluid (PDF), with or without AlaGln supplementation, via a surgically implanted catheter. Rats subjected to a single dwell without catheter implantation served as controls. The peritoneal surface proteome was directly harvested by detergent extraction and subjected to proteomic analysis by two-dimensional difference gel electrophoresis (2D-DiGE) with protein identification by mass spectrometry. An integrated bioinformatic approach was applied to identify proteins significantly affected by the treatments despite biological variation and interfering high abundance proteins. From 505 of 744 common spots on 59 gels, 222 unique proteins were identified. Using UniProt database information, proteins were assigned either as high abundance plasma proteins, or as cellular proteins. Statistical analysis employed an adapted workflow from RNA-sequencing, the trimmed mean of M-values (TMM) for normalization, and a mixed model for computational identification of significantly differentially abundant proteins. The most prominently enriched pathways after 5 weeks chronic treatment with SCB or DCB, PDFs belonged to clusters reflecting tissue damage and cell differentiation by cytoskeletal reorganization, immune responses, altered metabolism, and oxidative stress and redox homeostasis. Although the AlaGln effect was not as prominent, associated enriched pathways showed mostly regression to control or patterns opposite that of the PDF effect. Our study describes the novel peritoneal surface proteome through combined proteomic and bioinformatic analyses, and assesses changes elicited by chronic experimental PD. The biological processes so identified promise to link molecular mechanisms of membrane damage and protection in the in vivo rat model to pathomechanisms and cytoprotective effects observed in vitro and in clinical PD.

Preventing Peritoneal Dialysis-Associated Fibrosis by Therapeutic Blunting of Peritoneal Toll-Like Receptor Activity

Peritoneal dialysis (PD) is an essential daily life-saving treatment for end-stage renal failure. PD therapy is limited by peritoneal inflammation, which leads to peritoneal membrane failure as a result of progressive fibrosis. Peritoneal infections, with the concomitant acute inflammatory response and membrane fibrosis development, worsen PD patient outcomes. Patients who remain infection-free, however, also show evidence of inflammation-induced membrane damage and fibrosis, leading to PD cessation. In this case, uraemia, prolonged exposure to bio-incompatible PD solutions and surgical catheter insertion have been reported to induce sterile peritoneal inflammation and fibrosis as a result of cellular stress or tissue injury. Attempts to reduce inflammation (either infection-induced or sterile) and, thus, minimize fibrosis development in PD have been hampered because the immunological mechanisms underlying this PD-associated pathology remain to be fully defined. Toll-like receptors (TLRs) are central to mediating inflammatory responses by recognizing a wide variety of microorganisms and endogenous components released following cellular stress or generated as a consequence of extracellular matrix degradation during tissue injury. Given the close link between inflammation and fibrosis, recent investigations have evaluated the role that TLRs play in infection-induced and sterile peritoneal fibrosis development during PD. Here, we review the findings and discuss the potential of reducing peritoneal TLR activity by using a TLR inhibitor, soluble TLR2, as a therapeutic strategy to prevent PD-associated peritoneal fibrosis.

PPAR-γ agonist rosiglitazone ameliorates peritoneal deterioration in peritoneal dialysis rats with LPS-induced peritonitis through up-regulation of AQP-1 and ZO-1

Peritonitis is still a major cause of the death in peritoneal dialysis (PD) patients despite the significant decline of the peritonitis rates in recent years. The present study is designed to evaluate the therapeutic potential of peroxisome proliferator-activated receptor-γ agonist, rosiglitazone, on the structure and function of the peritoneum in a PD rat accompanied with peritonitis induced by lipopolysaccharide (LPS). Our data showed that the peritoneal membrane in the LPS-only group showed increased peritoneal thickness, vessel density, and hypercellularity compared with the PD-only group. Rosiglitazone administration significantly inhibited increase of the three indicators in PD rats with LPS treatment. In line with this, rosiglitazone improved function of the peritoneum in LPS-induced PD rats receiving rosiglitazone, which was reflected by decreased D/P urea and D/P albumin. Mechanistically, rosiglitazone-mediated improvements in the damaged structure and function of the peritoneum in PD rats with LPS treatment were associated with reduced inflammation and preserving mesothelial cell monolayer resulted from up-regulation of AQP-1 and ZO-1. Our findings thus suggest that peroxisome proliferator-activated receptor γ (PPAR-γ) activation might be a reasonable strategy to prevent and ameliorate peritoneal deterioration in PD patients, especially with peritonitis.

Effects of Alanyl-Glutamine Treatment on the Peritoneal Dialysis Effluent Proteome Reveal Pathomechanism-Associated Molecular Signatures

Peritoneal dialysis (PD) is a modality of renal replacement therapy in which the high volumes of available PD effluent (PDE) represents a rich source of biomarkers for monitoring disease and therapy. Although this information could help guide the management of PD patients, little is known about the potential of PDE to define pathomechanism-associated molecular signatures in PD.

We therefore subjected PDE to a high-performance multiplex proteomic analysis after depletion of highly-abundant plasma proteins and enrichment of low-abundance proteins. A combination of label-free and isobaric labeling strategies was applied to PDE samples from PD patients (n = 20) treated in an open-label, randomized, two-period, cross-over clinical trial with standard PD fluid or with a novel PD fluid supplemented with alanyl-glutamine (AlaGln).

With this workflow we identified 2506 unique proteins in the PDE proteome, greatly increasing coverage beyond the 171 previously-reported proteins. The proteins identified range from high abundance plasma proteins to low abundance cellular proteins, and are linked to larger numbers of biological processes and pathways, some of which are novel for PDE. Interestingly, proteins linked to membrane remodeling and fibrosis are overrepresented in PDE compared with plasma, whereas the proteins underrepresented in PDE suggest decreases in host defense, immune-competence and response to stress. Treatment with AlaGln-supplemented PD fluid is associated with reduced activity of membrane injury-associated mechanisms and with restoration of biological processes involved in stress responses and host defense.

Our study represents the first application of the PDE proteome in a randomized controlled prospective clinical trial of PD. This novel proteomic workflow allowed detection of low abundance biomarkers to define pathomechanism-associated molecular signatures in PD and their alterations by a novel therapeutic intervention.

Biomarker research to improve clinical outcomes of peritoneal dialysis: consensus of the European Training and Research in Peritoneal Dialysis (EuTRiPD) network

Peritoneal dialysis (PD) therapy substantially requires biomarkers as tools to identify patients who are at the highest risk for PD-related complications and to guide personalized interventions that may improve clinical outcome in the individual patient. In this consensus article, members of the European Training and Research in Peritoneal Dialysis Network (EuTRiPD) review the current status of biomarker research in PD and suggest a selection of biomarkers that can be relevant to the care of PD patients and that are directly accessible in PD effluents. Currently used biomarkers such as interleukin-6, interleukin-8, ex vivo-stimulated interleukin-6 release, cancer antigen-125, and advanced oxidation protein products that were collected through a Delphi procedure were first triaged for inclusion as surrogate endpoints in a clinical trial. Next, novel biomarkers were selected as promising candidates for proof-of-concept studies and were differentiated into inflammation signatures (including interleukin-17, M₁/M₂ macrophages, and regulatory T cell/T helper 17), mesothelial-to-mesenchymal transition signatures (including microRNA-21 and microRNA-31), and signatures for senescence and inadequate cellular stress responses. Finally, the need for defining pathogen-specific immune fingerprints and phenotype-associated molecular signatures utilizing effluents from the clinical cohorts of PD patients and "omics" technologies and bioinformatics-biostatistics in future joint-research efforts was expressed. Biomarker research in PD offers the potential to develop valuable tools for improving patient management. However, for all biomarkers discussed in this consensus article, the association of biological rationales with relevant clinical outcomes remains to be rigorously validated in adequately powered, prospective, independent clinical studies.

Addition of Alanyl-Glutamine to Dialysis Fluid Restores Peritoneal Cellular Stress Responses - A First-In-Man Trial

BACKGROUND

Peritonitis and ultrafiltration failure remain serious complications of chronic peritoneal dialysis (PD). Dysfunctional cellular stress responses aggravate peritoneal injury associated with PD fluid exposure, potentially due to peritoneal glutamine depletion. In this randomized cross-over phase I/II trial we investigated cytoprotective effects of alanyl-glutamine (AlaGln) addition to glucose-based PDF.

METHODS

In a prospective randomized cross-over design, 20 stable PD outpatients underwent paired peritoneal equilibration tests 4 weeks apart, using conventional acidic, single chamber 3.86% glucose PD fluid, with and without 8 mM supplemental AlaGln. Heat-shock protein 72 expression was assessed in peritoneal effluent cells as surrogate parameter of cellular stress responses, complemented by metabolomics and functional immunocompetence assays.

RESULTS

AlaGln restored peritoneal glutamine levels and increased the primary outcome heat-shock protein expression (effect 1.51-fold, CI 1.07-2.14; p = 0.022), without changes in peritoneal ultrafiltration, small solute transport, or biomarkers reflecting cell mass and inflammation. Further effects were glutamine-like metabolomic changes and increased ex-vivo LPS-stimulated cytokine release from healthy donor peripheral blood monocytes. In patients with a history of peritonitis (5 of 20), AlaGln supplementation decreased dialysate interleukin-8 levels. Supplemented PD fluid also attenuated inflammation and enhanced stimulated cytokine release in a mouse model of PD-associated peritonitis.

CONCLUSION

We conclude that AlaGln-supplemented, glucose-based PD fluid can restore peritoneal cellular stress responses with attenuation of sterile inflammation, and may improve peritoneal host-defense in the setting of PD.

Senescence-Associated Changes in Proteome and O-GlcNAcylation Pattern in Human Peritoneal Mesothelial Cells

INTRODUCTION

Senescence of peritoneal mesothelial cells represents a biological program defined by arrested cell growth and altered cell secretory phenotype with potential impact in peritoneal dialysis. This study aims to characterize cellular senescence at the level of global protein expression profiles and modification of proteins with O-linked N-acetylglucosamine (O-GlcNAcylation).

METHODS

A comparative proteomics analysis between young and senescent human peritoneal mesothelial cells (HPMC) was performed using two-dimensional gel electrophoresis. O-GlcNAc status was assessed by Western blot under normal conditions and after modulation with 6-diazo-5-oxo-L-norleucine (DON) to decrease O-GlcNAcylation or O-(2-acetamido-2-deoxy-D-glucopyranosylidene) amino N-phenyl carbamate (PUGNAc) to increase O-GlcNAcylation.

RESULTS

Comparison of protein pattern of senescent and young HPMC revealed 29 differentially abundant protein spots, 11 of which were identified to be actin (cytoplasmic 1 and 2), cytokeratin-7, cofilin-2, transgelin-2, Hsp60, Hsc70, proteasome β-subunits (type-2 and type-3), nucleoside diphosphate kinase A, and cytosolic 5'(3')-deoxyribonucleotidase. Although the global level of O-GlcNAcylation was comparable, senescent cells were not sensitive to modulation by PUGNAc.

DISCUSSION

This study identified changes of the proteome and altered dynamics of O-GlcNAc regulation in senescent mesothelial cells. Whereas changes in cytoskeleton-associated proteins likely reflect altered cell morphology, changes in chaperoning and housekeeping proteins may have functional impact on cellular stress response in peritoneal dialysis.

Injury-induced inflammation and inadequate HSP expression in mesothelial cells upon repeat exposure to dual-chamber bag peritoneal dialysis fluids

PURPOSE

Peritoneal dialysis fluids (PDFs) may induce inadequate heat-shock protein (HSP) expression and injury-related inflammation in exposed mesothelial cells. The aim of this study was to relate cellular injury to these cellular responses in mesothelial cells following repeated exposure to 3 commercial PDFs with different biocompatibility profiles.

METHODS

Primary cultures of human peritoneal mesothelial cells (HPMC) were exposed to a 1:2 mixture of cell culture medium and CAPD2 (single-chamber bag PDF; Fresenius, Bad Homburg, Germany), Physioneal (dual-chamber bag PDF; Baxter, Deerfield, IL, USA) or Balance (dual-chamber bag PDF, Fresenius) for up to 10 days exposure time (4 dwells). Supernatant was analyzed for LDH, IL-6, and IL-8, cells for HSP-72 expression, and protein content.

RESULTS

PDF exposure resulted in a biphasic pattern of cell damage switching from an earlier phase with increased injury by single-chamber PDF to a delayed phase with increased susceptibility to dual-chamber PDF. Sterile inflammation was related to LDH release over time and could be reproduced by exposure to necrotic cellular material. PDF exposure resulted in low HSP-72 expression in all tested PDFs.

CONCLUSIONS

Exposure to single-chamber as well as to dual-chamber bag PDFs induce increased vulnerability of mesothelial cells to repeated exposure of the same solution. These effects were delayed with dual-chamber PDFs. Injury-induced inflammation and impaired HSP expression upon PDF exposure might initiate a vicious cycle with progredient mesothelial cell damage upon repeated PDF exposure. Certainly, interventional studies and translation of these results into the in vivo system is needed.

Cross-omics comparison of stress responses in mesothelial cells exposed to heat- versus filter-sterilized peritoneal dialysis fluids

Recent research suggests that cytoprotective responses, such as expression of heat-shock proteins, might be inadequately induced in mesothelial cells by heat-sterilized peritoneal dialysis (PD) fluids. This study compares transcriptome data and multiple protein expression profiles for providing new insight into regulatory mechanisms. Two-dimensional difference gel electrophoresis (2D-DIGE) based proteomics and topic defined gene expression microarray-based transcriptomics techniques were used to evaluate stress responses in human omental peritoneal mesothelial cells in response to heat- or filter-sterilized PD fluids. Data from selected heat-shock proteins were validated by 2D western-blot analysis. Comparison of proteomics and transcriptomics data discriminated differentially regulated protein abundance into groups depending on correlating or noncorrelating transcripts. Inadequate abundance of several heat-shock proteins following exposure to heat-sterilized PD fluids is not reflected on the mRNA level indicating interference beyond transcriptional regulation. For the first time, this study describes evidence for posttranscriptional inadequacy of heat-shock protein expression by heat-sterilized PD fluids as a novel cytotoxic property. Cross-omics technologies introduce a novel way of understanding PDF bioincompatibility and searching for new interventions to reestablish adequate cytoprotective responses.

Janus kinase signaling activation mediates peritoneal inflammation and injury in vitro and in vivo in response to dialysate

Peritoneal membrane pathology limits long-term peritoneal dialysis (PD). Here, we tested whether JAK/STAT signaling is implicated and if its attenuation might be salutary. In cultured mesothelial cells, PD fluid activated, and the pan-JAK inhibitor P6 reduced, phospho-STAT1 and phospho-STAT3, periostin secretion, and cleaved caspase-3. Ex vivo, JAK was phosphorylated in PD effluent cells from long-term but not new PD patients. MCP-1 and periostin were increased in PD effluent in long term compared with new patients. In rats, twice daily, PD fluid infusion induced phospho-JAK, mesothelial cell hyperplasia, inflammation, fibrosis, and hypervascularity after 10 days of exposure to PD fluid. Concomitant instillation of a JAK1/2 inhibitor virtually completely attenuated these changes. Thus, our studies directly implicate JAK/STAT signaling in the mediation of peritoneal membrane pathology as a consequence of PD.

Dynamic O-linked N-acetylglucosamine modification of proteins affects stress responses and survival of mesothelial cells exposed to peritoneal dialysis fluids

The ability of cells to respond and survive stressful conditions is determined, in part, by the attachment of O-linked N-acetylglucosamine (O-GlcNAc) to proteins (O-GlcNAcylation), a post-translational modification dependent on glucose and glutamine. This study investigates the role of dynamic O-GlcNAcylation of mesothelial cell proteins in cell survival during exposure to glucose-based peritoneal dialysis fluid (PDF). Immortalized human mesothelial cells and primary mesothelial cells, cultured from human omentum or clinical effluent of PD patients, were assessed for O-GlcNAcylation under normal conditions or after exposure to PDF. The dynamic status of O-GlcNAcylation and effects on cellular survival were investigated by chemical modulation with 6-diazo-5-oxo-L-norleucine (DON) to decrease or O-(2-acetamido-2-deoxy-D-glucopyranosylidene)amino N-phenyl carbamate (PUGNAc) to increase O-GlcNAc levels. Viability was decreased by reducing O-GlcNAc levels by DON, which also led to suppressed expression of the cytoprotective heat shock protein 72. In contrast, increasing O-GlcNAc levels by PUGNAc or alanyl-glutamine led to significantly improved cell survival paralleled by higher heat shock protein 72 levels during PDF treatment. Addition of alanyl-glutamine increased O-GlcNAcylation and partly counteracted its inhibition by DON, also leading to improved cell survival. Immunofluorescent analysis of clinical samples showed that the O-GlcNAc signal primarily originates from mesothelial cells. In conclusion, this study identified O-GlcNAcylation in mesothelial cells as a potentially important molecular mechanism after exposure to PDF. Modulating O-GlcNAc levels by clinically feasible interventions might evolve as a novel therapeutic target for the preservation of peritoneal membrane integrity in PD.

A method to resolve the composition of heterogeneous affinity-purified protein complexes assembled around a common protein by chemical cross-linking, gel electrophoresis and mass spectrometry

Protein complexes form, dissociate and re-form in order to perform specific cellular functions. In this two-pronged protocol, noncovalent protein complexes are initially isolated by affinity purification for subsequent identification of the components by liquid chromatography high-resolution mass spectrometry (LC-MS) on a hybrid LTQ Orbitrap Velos. In the second prong of the approach, the affinity-purification strategy includes a chemical cross-linking step to 'freeze' a series of concurrently formed, heterogeneous protein subcomplex species that are visualized by gel electrophoresis. This branch of the methodology amalgamates standard and well-practiced laboratory methods to reveal compositional changes that occur in protein complex architecture. By using mouse N-terminally tagged streptavidin-binding peptide-hemagglutinin-TANK-binding kinase 1 (SH-TBK1), we chemically cross-linked the affinity-purified complex of SH-TBK1 with the homobifunctional lysine-specific reagent bis(sulfosuccinimidyl) suberate (BS(3)), and we separated the resultant protein complexes by denaturation and by silver-stained one- and two-dimensional SDS-PAGE. We observed a range of cross-linked TBK1 complexes of variable pI and M(r) and confirmed them by immunoblotting. LC-MS analysis of in situ-digested cross-linked proteins shows differences in the composition of the TBK1 subcomplexes. The protocol is inherently simple and can be readily extended to the investigation of a range of protein complexes. From cell lysis to data generation by LC-MS, the protocol takes approximately 2.5 to 5.5 d to perform.

Interleukin-1 receptor-mediated inflammation impairs the heat shock response of human mesothelial cells

Bioincompatibility of peritoneal dialysis fluids (PDF) limits their use in renal replacement therapy. PDF exposure harms mesothelial cells but induces heat shock proteins (HSP), which are essential for repair and cytoprotection. We searched for cellular pathways that impair the heat shock response in mesothelial cells after PDF-exposure. In a dose-response experiment, increasing PDF-exposure times resulted in rapidly increasing mesothelial cell damage but decreasing HSP expression, confirming impaired heat shock response. Using proteomics and bioinformatics, simultaneously activated apoptosis-related and inflammation-related pathways were identified as candidate mechanisms. Testing the role of sterile inflammation, addition of necrotic cell material to mesothelial cells increased, whereas addition of the interleukin-1 receptor (IL-1R) antagonist anakinra to PDF decreased release of inflammatory cytokines. Addition of anakinra during PDF exposure resulted in cytoprotection and increased chaperone expression. Thus, activation of the IL-1R plays a pivotal role in impairment of the heat shock response of mesothelial cells to PDF. Danger signals from injured cells lead to an elevated level of cytokine release associated with sterile inflammation, which reduces expression of HSP and other cytoprotective chaperones and exacerbates PDF damage. Blocking the IL-1R pathway might be useful in limiting damage during peritoneal dialysis.

Peritoneal dialysis fluid induces p38-dependent inflammation in human mesothelial cells

BACKGROUND

Noninfectious upregulation of proinflammatory pathways in mesothelial cells may represent an integral part of their stress response upon exposure to peritoneal dialysis fluids (PDF).

OBJECTIVE

The aim of this study was to evaluate the role of the stress-inducible mitogen-activated protein kinase (MAPK) p38 in regulation of inflammatory and stress responses in mesothelial cells following in vitro exposure to PDF.

MATERIALS AND METHODS

Human peritoneal mesothelial cells were exposed to Dianeal PD4 or Physioneal (Baxter AG, Vienna, Austria) containing 1.36% glucose and then allowed to recover. Phosphorylation of p38, induction of heat shock protein-70 (HSP70), release of lactate dehydrogenase (LDH), secretion of interleukin (IL)-8, gene transcription, and mRNA stability were assessed with and without the MAPK p38 inhibitor SB203580.

RESULTS

Exposure to Dianeal resulted in phosphorylation of p38 within 30 minutes (309% of control, p < 0.05) and increased IL-8 release (370% of control, p < 0.05), HSP70 expression (151% of control, p < 0.05), and LDH release (180% of control, p < 0.05). Exposure to Physioneal resulted in attenuated changes in IL-8, HSP70, and LDH. Addition of the p38 inhibitor SB203580 to Dianeal resulted in dampened IL-8 release (-55%; p < 0.05) and basal HSP70 expression, and unchanged LDH release. Effects of p38 on IL-8 were at transcriptional, posttranscriptional, and translational levels.

CONCLUSION

These data confirm concordant p38-dependent upregulation of IL-8 and HSP70 following exposure to bioincompatible PDF. The MAPK p38 pathway therefore links proinflammatory processes and the cellular stress response in human peritoneal mesothelial cells.

Peritoneal dialysis fluids can alter HSP expression in human peritoneal mesothelial cells

BACKGROUND

Acute exposure of mesothelial cells to peritoneal dialysis fluid (PDF) has been shown not only to result in injury but also to induce cytoprotective heat shock proteins (HSP). The aim of the present study was to evaluate the expression of HSP in a more chronic in vitro PDF exposure system, searching for a role of glucose degradation products (GDP).

METHODS

Human peritoneal mesothelial cells (HPMC) were chronically incubated in filter- or heat-sterilized PDF (mixed 1:1 with cell culture medium), or in control cell culture medium. After incubation periods of 1, 3 and 10 days, cell extract was assessed for Ezrin, Hsp27 and Hsp72, and supernatant for IL-6 and IL-8. After 24-h exposure to the GDP 3.4-di-deoxyglucosone-3-ene (3.4-DGE), HPMC were assessed for expression of Hsp27 and Hsp72, and for release of LDH, IL-6 and IL-8.

RESULTS

In vitro PDF exposure for more than 1 day resulted in reduced cell mass, lower expression of the epithelial marker Ezrin and depressed cellular levels of both HSP, associated with increased IL-6 and IL-8 release. These effects occurred earlier and stronger with heat-sterilized than with filter-sterilized PDF. Exposure of HPMC to 3.4-DGE resulted in suppression of HSP, and increased release of LDH, IL-6 and IL-8.

CONCLUSION

Our data show that GDP (dys)regulate the mesothelial cell stress response. This was associated with reduced cell mass, loss of the epithelial phenotype and sterile cellular inflammation following extended exposure to heat-sterilized PDF. Toxic effects of PDF might thus be extended to reduced mesothelial cell stress responses.

Proteome profile of peritoneal effluents in children on glucose- or icodextrin-based peritoneal dialysis

BACKGROUND

We compared the proteome profile of peritoneal effluents obtained with icodextrin (Ico) or glucose (Glu) in paediatric patients and defined the oxido-redox status of proteins.

METHODS

Sixteen patients underwent two 14-h daytime dwells performed on subsequent days with 7.5% Ico and 3.86% Glu solutions. Protein composition was analysed by two-dimensional electrophoresis and mass spectrometry; oxidized products were evaluated by cyanine labelling.

RESULTS

Peritoneal transport kinetics of β2-microglobulin and cystatin C was linear for both solutions, but was significantly higher with Ico than with Glu, suggesting a better efficiency for these molecules. There was a linear correlation between total protein removal during Ico and Glu dialysis in the same patient, suggesting that it is a function of peritoneal membrane characteristics. The ratio between proteins removed by Ico and by Glu solutions was higher at low removal rate. Image gel analysis revealed 1064 and 774 spots, respectively, in Ico and Glu solutions; 524 were common, and 314 were higher in Ico than Glu effluents. Analysis of protein oxido-redox status showed a greater amount of oxidized albumin in Ico dialysate that was correlated with lower serum levels.

CONCLUSIONS

Our results indicate a better efficiency of Ico in removing small proteins. Removal of big proteins and their oxidized isoforms reflects potentially opposite effects. The long-term clinical consequences of removing also potentially important molecules are to be defined.

HSP-mediated cytoprotection of mesothelial cells in experimental acute peritoneal dialysis

BACKGROUND

Low biocompatibility of peritoneal dialysis solution (PDS) injures mesothelial cells but also induces heat shock proteins (HSP), the main effectors of the cellular stress response. This study investigated whether overexpression of HSP upon pharmacologic induction results in cytoprotection of mesothelial cells in experimental PD.

METHODS

Stress response of mesothelial cells upon exposure to PDS was pharmacologically manipulated using glutamine as a co-inducer. In vitro, HSP-mediated cytoprotection was assessed by simultaneous measurements of HSP expression using Western blot analysis and viability testing using release of lactic dehydrogenase in cultured human mesothelial cells. In vivo, detachment of mesothelial cells from their peritoneal monolayer was assessed following exposure to PDS with and without the addition of glutamine in the acute rat model of PD.

RESULTS

In vitro, mesothelial cell viability following exposure to PDS was significantly improved upon pharmacologic co-induction of HSP expression by glutamine (226% +/- 29% vs 190% +/- 19%, p = 0.001). In vivo, mesothelial cell detachment during exposure to PDS was reduced upon pharmacologic induction of HSP expression by glutamine (93 +/- 39 vs 38 +/- 38 cells, p = 0.044), resulting in reduced peritoneal protein loss (75 +/- 7 vs 65 +/- 4 mg, p = 0.045).

CONCLUSION

Our results represent the first study of pharmacologic manipulation of HSP expression for cytoprotection of mesothelial cells following acute in vitro and in vivo exposure to PDS. PDS with added glutamine might represent a promising therapeutic approach against low biocompatibility of PDS but needs validation in a chronic PD model.