Angiogenesis in peritoneal dialysis

Selective therapeutic efficacy of tyrosine kinase inhibitor sorafenib on the restoration of methylglyoxal-induced peritoneal fibrosis

Peritoneal fibrosis, a common complication observed in long-term peritoneal dialysis patients, can gradually lead to ultrafiltration failure and the development of encapsulating peritoneal sclerosis. Although mechanisms of peritoneal fibrosis have been proposed, effective therapeutic options are unsatisfactory. Recently, several tyrosine kinase inhibitors have proven to be anti-fibrosis in rodent models. To assess the potential therapeutic effects of tyrosine kinase inhibitors on peritoneal fibrosis in the larger animal model, a novel porcine model of peritoneal fibrosis induced by 40 mM methylglyoxal in 2.5 % dialysate was established, and two different doses (20 mg/kg and 30 mg/kg) of sorafenib were given orally to evaluate their therapeutic efficacy in this study. Our results showed that sorafenib effectively reduced adhesions between peritoneal organs and significantly diminished the thickening of both the parietal and visceral peritoneum. Angiogenesis, vascular endothelial growth factor A production, myofibroblast infiltration, and decreased endothelial glycocalyx resulting from dialysate and methylglyoxal stimulations were also alleviated with sorafenib. However, therapeutic efficacy in ameliorating loss of mesothelial cells, restoring decreased ultrafiltration volume, and improving elevated small solutes transport rates was limited. In conclusion, this study demonstrated that sorafenib could potentially be used for peritoneal fibrosis treatment, but applying sorafenib alone might not be sufficient to fully rescue methylglyoxal-induced peritoneal defects.

Peritoneal transformation shortly after kidney transplantation in pediatric patients with preceding chronic peritoneal dialysis

BACKGROUND

The unphysiological composition of peritoneal dialysis (PD) fluids induces progressive peritoneal fibrosis, hypervascularization and vasculopathy. Information on these alterations after kidney transplantation (KTx) is scant.

METHODS

Parietal peritoneal tissues were obtained from 81 pediatric patients with chronic kidney disease stage 5 (CKD5), 72 children on PD with low glucose degradation product (GDP) PD fluids, and from 20 children 4-8 weeks after KTx and preceding low-GDP PD. Tissues were analyzed by digital histomorphometry and quantitative immunohistochemistry.

RESULTS

While chronic PD was associated with peritoneal hypervascularization, after KTx vascularization was comparable to CKD5 level. Submesothelial CD45 counts were 40% lower compared with PD, and in multivariable analyses independently associated with microvessel density. In contrast, peritoneal mesothelial denudation, submesothelial thickness and fibrin abundance, number of activated, submesothelial fibroblasts and of mesothelial-mesenchymal transitioned cells were similar after KTx. Diffuse peritoneal podoplanin positivity was present in 40% of the transplanted patients. In subgroups matched for age, PD vintage, dialytic glucose exposure and peritonitis incidence, submesothelial hypoxia-inducible factor 1-alpha abundance and angiopoietin 1/2 ratio were lower after KTx, reflecting vessel maturation, while arteriolar and microvessel p16 and cleaved Casp3 were higher. Submesothelial mast cell count and interleukin-6 were lower, whereas transforming growth factor-beta induced pSMAD2/3 was similar as compared with children on PD.

CONCLUSIONS

Peritoneal membrane damage induced with chronic administration of low-GDP PD fluids was less severe after KTx. While peritoneal microvessel density, primarily defining PD transport and ultrafiltration capacity, was normal after KTx and peritoneal inflammation less pronounced, diffuse podoplanin positivity and profibrotic activity were prevalent.

Thalidomide Suppresses Angiogenesis Through the Signal Transducer and Activator of Transcription 3/SP4 Signaling Pathway in the Peritoneal Membrane

Peritoneal angiogenesis is the key pathophysiological factor that limits peritoneal ultrafiltration during peritoneal dialysis (PD) in uremic patients. Thalidomide has been confirmed to inhibit angiogenesis by inhibiting the secretion of vascular endothelial growth factor (VEGF), but the exact mechanism by which thalidomide inhibits vascular proliferation during PD is still unclear. Here, the objective of the present study was to investigate whether the reduction in VEGF production by human peritoneal mesothelial cells (HPMCs) was controlled by thalidomide. Stimulation of HPMCs with IL-6 in combination with soluble IL-6 receptor (sIL-6R) promoted VEGF expression and secretion, but these effects were attenuated by thalidomide treatment through a transcriptional mechanism that involved signal transducer and activator of transcription 3 (STAT3) and SP4. Conditioned medium from HPMCs cultured with thalidomide inhibited angiogenic endothelial tube formation, which could be further blocked by silencing SP4 and promoted by overexpressing SP4. In vivo, induction of peritoneal angiogenesis in sham rats, sham+PD rats, 5/6 nephrectomy (5/6Nx) rats, 5/6Nx+PD rats, and 5/6Nx+PD rats intraperitoneally treated with thalidomide showed that thalidomide was involved in the control of several key endothelial-specific targets, including VEGFR2, VEGFR3, SP4, and STAT3 expression and new vessel formation, confirming the role of thalidomide and STAT3/SP4 signaling in these processes. Taken together, these findings identify a novel mechanism that links thalidomide, STAT3/SP4 signaling, and angiogenesis in the peritoneal membrane.

Effects of bicarbonate/lactate-buffered neutral peritoneal dialysis fluids on angiogenesis-related proteins in patients undergoing peritoneal dialysis

Background

In order to facilitate the safe and long-term delivery of peritoneal dialysis (PD), it is necessary to improve the biocompatibility of peritoneal dialysis fluids (PDFs). The novel bicarbonate/lactate-buffered neutral PDFs (B/L-PDFs) are expected to be improved biocompatible. This study evaluated the biocompatibility of B/L-PDFs by analysis on the profile of angiogenesis-related proteins in drained dialysate of patients undergoing PD.

Methods

Concentrations of 20 angiogenesis-related proteins in the dialysate were semi-quantitatively determined using a RayBio® Human Angiogenesis Antibody Array and were compared between B/L-PDFs and conventional lactate-buffered neutral PDFs (L-PDFs).

Results

The expression of growth-related oncogene (GRO α/β/γ), which belongs to the CXC chemokine family, decreased significantly after use of the B/L-PDFs compared to the L-PDFs (P = 0.03). The number of the proteins with lower level in the B/L-PDFs compared with L-PDFs was significantly negatively correlated with the PD duration (Spearman ρ = − 0.81, P = 0.004).

Conclusion

This study suggested that B/L-PDFs are more biocompatible than conventional PDFs.

Vascular endothelial growth factor-mediated peritoneal neoangiogenesis in peritoneal dialysis

Peritoneal dialysis (PD) is an important renal replacement therapy for patients with end-stage renal diseases, which is limited by peritoneal neoangiogenesis leading to ultrafiltration failure (UFF). Vascular endothelial growth factor (VEGF) and its receptors are key angiogenic factors involved in almost every step of peritoneal neoangiogenesis. Impaired mesothelial cells are the major sources of VEGF in the peritoneum. The expression of VEGF will be up-regulated in specific pathological conditions in PD patients, such as with non-biocompatible peritoneal dialysate, uremia and inflammation, and so on. Other working cells (i.e. vascular endothelial cells, macrophages and adipocytes) can also stimulate the secretion of VEGF. Meanwhile, hypoxia and activation of complement system further aggravate peritoneal injury and contribute to neoangiogenesis. There are several signalling pathways participating in VEGF-mediated peritoneal neoangiogenesis including tumour growth factor-β, Wnt/β-catenin, Notch and interleukin-6/signal transducer and activator of transcription 3. Moreover, VEGF is highly expressed in dialysate effluent of long-term PD patients and is associated with peritoneal transport function, which supports its role in the alteration of peritoneal structure and function. In this review, we systematically summarize the angiogenic effect of VEGF and evaluate it as a potential target for the prevention of peritoneal neoangiogenesis and UFF. Preservation of the peritoneal membrane using targeted therapy of VEGF-mediated peritoneal neoangiogenesis may increase the longevity of the PD modality for those who require life-long dialysis.

Protein kinase C beta deficiency increases glucose-mediated peritoneal damage via M1 macrophage polarization and up-regulation of mesothelial protein kinase C alpha

BACKGROUND

Peritoneal membrane (PM) damage during peritoneal dialysis (PD) is mediated largely by high glucose (HG)-induced pro-inflammatory and neo-angiogenic processes, resulting in PM fibrosis and ultrafiltration failure. We recently demonstrated a crucial role for protein kinase C (PKC) isoform α in mesothelial cells.

METHODS

In this study we investigate the role of PKCβ in PM damage in vitro using primary mouse peritoneal macrophages (MPMΦ), human macrophages (HMΦ) and immortalized mouse peritoneal mesothelial cells (MPMCs), as well as in vivo using a chronic PD mouse model.

RESULTS

We demonstrate that PKCβ is the predominant classical PKC isoform expressed in primary MPMΦ and its expression is up-regulated in vitro under HG conditions. After in vitro lipopolysaccharides stimulation PKCβ-/- MPMΦ demonstrates increased levels of interleukin 6 (IL-6), tumour necrosis factor α, and monocyte chemoattractant protein-1 and drastically decrease IL-10 release compared with wild-type (WT) cells. In vivo, catheter-delivered treatment with HG PD fluid for 5 weeks induces PKCβ up-regulation in omentum of WT mice and results in inflammatory response and PM damage characterized by fibrosis and neo-angiogenesis. In comparison to WT mice, all pathological changes are strongly aggravated in PKCβ-/- animals. Underlying molecular mechanisms involve a pro-inflammatory M1 polarization shift of MPMΦ and up-regulation of PKCα in MPMCs of PKCβ-/- mice. Finally, we demonstrate PKCβ involvement in HG-induced polarization processes in HMΦ.

CONCLUSIONS

PKCβ as the dominant PKC isoform in MPMΦ is up-regulated by HG PD fluid and exerts anti-inflammatory effects during PD through regulation of MPMΦ M1/M2 polarization and control of the dominant mesothelial PKC isoform α.

Tamoxifen and bone morphogenic protein-7 modulate fibrosis and inflammation in the peritoneal fibrosis model developed in uremic rats

Background

Peritoneal fibrosis (PF) represents a long-term complication of peritoneal dialysis (PD), affecting peritoneal membrane (PM) integrity and function. Understanding the mechanisms underlying PF development in an uremic environment aiming alternative therapeutic strategies for treating this process is of great interest. The aim of this study was to analyze the effects of tamoxifen (TAM) and recombinant BMP7 (rBMP7) in an experimental model of PF developed in uremic rats.

Methods

To mimic the clinical situation of patients on long-term PD, a combo model, characterized by the combination of PF and CKD with severe uremia, was developed in Wistar rats. PF was induced by intraperitoneal (IP) injections of chlorhexidine gluconate (CG), and CKD was induced by an adenine-rich diet. Uremia was confirmed by severe hypertension, increased blood urea nitrogen (BUN> 120 mg/dL) and serum creatinine levels (> 2 mg/dL). Uremic rats with PF were treated with TAM (10 mg/Kg by gavage) or BMP7 (30 μg/Kg, IP). Animals were followed up for 30 days.

Results

CG administration in uremic rats induced a striking increase in PM thickness, neoangiogenesis, demonstrated by increased capillary density, and failure of ultrafiltration capacity. These morphological and functional changes were blocked by TAM or rBMP7 treatment. In parallel, TAM and rBMP7 significantly ameliorated the PM fibrotic response by reducing α-SMA, extracellular matrix proteins and TGF-ß expression. TAM or rBMP7 administration significantly inhibited peritoneal Smad3 expression in uremic rats with PF, prevented Smad3 phosphorylation, and induced a remarkable up-regulation of Smad7, an intracellular inhibitor of TGFβ/Smad signaling, contributing to a negative modulation of profibrotic genes. Both treatments were also effective in reducing local inflammation, possibly by upregulating IκB-α expression in the PM of uremic rats with PF. In vitro experiments using primary peritoneal fibroblasts activated by TGF-ß confirmed the capacity of TAM or rBMP7 in blocking inflammatory mediators, such as IL-1ß expression.

Conclusions

In conclusion, these findings indicate important roles of TGF-ß/Smad signaling in PF aggravated by uremia, providing data regarding potential therapeutic approaches with TAM or rBMP7 to block this process.

Electronic supplementary material

The online version of this article (10.1186/s10020-019-0110-5) contains supplementary material, which is available to authorized users.

Longer-Period Effects of Bicarbonate/Lactate-Buffered Neutral Peritoneal Dialysis Fluid in Patients Undergoing Peritoneal Dialysis

High concentrations of lactate are considered to contribute to impairment of the peritoneal membrane. We investigated the longer-period effects of bicarbonate/lactate-buffered neutral peritoneal dialysis fluid (PDF) in patients undergoing PD for about 2 years. Patients undergoing PD were changed from a lactate-buffered neutral PDF to a bicarbonate/lactate-buffered neutral PDF. We then investigated the patients' clinical outcomes and peritoneal membrane functions as well as the surrogate markers in the drained dialysate. Fourteen patients undergoing PD were enrolled. Peritonitis was observed in one patient. No other adverse events were observed. Peritoneal function did not change as the ultrafiltration volume decreased. Fibrin degradation products and vascular endothelial growth factor in the drained dialysate decreased while the interleukin level increased. These results suggest that bicarbonate/lactate-buffered neutral PDF may have beneficial effects in terms of peritoneal preservation and can be safely used in patients undergoing PD.

Differences in peritoneal response after exposure to low-GDP bicarbonate/lactate-buffered dialysis solution compared to conventional dialysis solution in a uremic mouse model

Background

Long-term exposure of conventional peritoneal dialysis (PD) fluid is associated with structural membrane alterations and technique failure. Previously, it has been shown that infiltrating IL-17-secreting CD4+T cells and pro-fibrotic M2 macrophages play a critical role in the PD-induced pathogenesis. Although more biocompatible PD solutions are recognized to better preserve the peritoneal membrane integrity, the impact of these fluids on the composition of the peritoneal cell infiltrate is unknown.

Materials and methods

In a uremic PD mouse model, we compared the effects of daily instillation of standard lactate (LS) or bicarbonate/lactate-buffered solutions (BLS) and respective controls on peritoneal fibrosis, vascularisation, and inflammation.

Results

Daily exposure of LS fluid during a period of 8 weeks resulted in a peritoneal increase of αSMA and collagen accompanied with new vessel formation compared to the BLS group. Effluent from LS-treated mouse showed a higher percentage of CD4⁺ IL-17⁺ cell population while BLS exposure resulted in an increased macrophage population. Significantly enhanced inflammatory cytokines such as TGFβ1, TNFα, INFγ, and MIP-1β were detected in the effluent of BLS-exposed mice when compared to other groups. Further, immunohistochemistry of macrophage subset infiltrates in the BLS group confirmed a higher ratio of pro-inflammatory M1 macrophages over the pro-fibrotic M2 subset compared to LS.

Conclusion

Development of the peritoneal fibrosis and angiogenesis was prevented in the BLS-exposed mice, which may underlie its improved biocompatibility. Peritoneal recruitment of M1 macrophages and lower number of CD4⁺ IL-17⁺ cells might explain the peritoneal integrity preservation observed in BLS-exposed mouse.

Histone deacetylase 6 inhibition counteracts the epithelial-mesenchymal transition of peritoneal mesothelial cells and prevents peritoneal fibrosis

The role of histone deacetylase 6 (HDAC6) in peritoneal fibrosis remains unknown. In this study, we examined the effect of HDAC6 inhibition on the epithelial–mesenchymal transition (EMT) of peritoneal mesothelial cells and development of peritoneal fibrosis. Treatment with tubastatin A, a highly selective HDAC6 inhibitor, or silencing of HDAC6 with siRNA inhibited transforming growth factor β1-induced EMT, as evidenced by decreased expression of α-smooth muscle actin, collagen I and preserved expression of E-cadherin in cultured human peritoneal mesothelial cells. In a mouse model of peritoneal fibrosis induced by high glucose dialysate, administration of TA prevented thickening of the submesothelial region and decreased expression of collagen I and α-SMA. Mechanistically, tubastatin A treatment inhibited expression of TGF-β1 and phosphorylation of Smad-3, epidermal growth factor receptor, STAT3, and NF-κBp65. HDAC6 inhibition also suppressed production of multiple inflammatory cytokines/chemokines and reduced the infiltration of macrophages to the injured peritoneum. Moreover, tubastatin A was effective in inhibiting peritoneal increase of CD31(+) blood vessels and expression of vascular endothelial growth factor in the injured peritoneum. Collectively, these results suggest that HDAC6 inhibition can attenuate peritoneal fibrosis by inhibiting multiple pro-fibrotic signaling pathways, EMT, inflammation and blood vessel formation.

Targeting Src attenuates peritoneal fibrosis and inhibits the epithelial to mesenchymal transition

Src has been reported to mediate tissue fibrosis in several organs, but its role in peritoneal fibrosis remains unknown. In this study, we evaluated the therapeutic effect of KX2-391, a highly selective inhibitor of Src, on the development of peritoneal fibrosis in a rat model. Daily intraperitoneal injections of chlorhexidine gluconate induced peritoneal fibrosis, as indicated by thickening of the submesothelial area with an accumulation of collagen fibrils and activation of myofibroblasts. This was accompanied by time-dependent phosphorylation of Src at tyrosine 416. Administration of KX2-391 attenuated peritoneal fibrosis and abrogated increased phosphorylation of Src and multiple signaling molecules associated with tissue fibrosis, including epidermal growth factor receptor, Akt, Signal transducer and activator of transcription 3 and nuclear factor-κB in the injured peritoneum. KX2-391 also inhibited the production of proinflammatory cytokines and the infiltration of macrophages into the injured peritoneum. In cultured human peritoneal mesothelial cells, inhibition of Src by KX2-391 or siRNA resulted in decreased expression of α-smooth muscle actin (α-SMA), fibronectin and collagen I, the hallmarks of epithelial to mesenchymal transition. These results suggest that Src is a critical mediator of peritoneal fibrosis and the epithelial to mesenchymal transition. Thus, Src could be a potential therapeutic target in the treatment of peritoneal fibrosis.

Strategies for preventing peritoneal fibrosis in peritoneal dialysis patients: new insights based on peritoneal inflammation and angiogenesis

Peritoneal dialysis (PD) is an established form of renal replacement therapy. Long-term PD leads to morphologic and functional changes to the peritoneal membrane (PM), which is defined as peritoneal fibrosis, a known cause of loss of peritoneal ultrafiltration capacity. Inflammation and angiogenesis are key events during the pathogenesis of peritoneal fibrosis. This review discusses the pathophysiology of peritoneal fibrosis and recent research progress on key fibrogenic molecular mechanisms in peritoneal inflammation and angiogenesis, including Toll-like receptor ligand-mediated, NOD-like receptor protein 3/interleukin-1β, vascular endothelial growth factor, and angiopoietin-2/Tie2 signaling pathways. Furthermore, novel strategies targeting peritoneal inflammation and angiogenesis to preserve the PM are discussed in depth.

Targeting cannabinoid signaling for peritoneal dialysis-induced oxidative stress and fibrosis

Long-term exposure to bioincompatible peritoneal dialysis (PD) solutions frequently results in peritoneal fibrosis and ultrafiltration failure, which limits the life-long use of and leads to the cessation of PD therapy. Therefore, it is important to elucidate the pathogenesis of peritoneal fibrosis in order to design therapeutic strategies to prevent its occurrence. Peritoneal fibrosis is associated with a chronic inflammatory status as well as an elevated oxidative stress (OS) status. Beyond uremia per se, OS also results from chronic exposure to high glucose load, glucose degradation products, advanced glycation end products, and hypertonic stress. Therapy targeting the cannabinoid (CB) signaling pathway has been reported in several chronic inflammatory diseases with elevated OS. We recently reported that the intra-peritoneal administration of CB receptor ligands, including CB₁ receptor antagonists and CB₂ receptor agonists, ameliorated dialysis-related peritoneal fibrosis. As targeting the CB signaling pathway has been reported to be beneficial in attenuating the processes of several chronic inflammatory diseases, we reviewed the interaction among the cannabinoid system, inflammation, and OS, through which clinicians ultimately aim to prolong the peritoneal survival of PD patients.

Angiogenesis and Inflammation in Peritoneal Dialysis: The Role of Adipocytes

Chronic inflammation and angiogenesis are the most common complications in patients undergoing maintenance peritoneal dialysis (PD), resulting in progressive peritoneum remolding and, eventually, utrafiltration failure. Contributing to the deeper tissue under the peritoneal membrane, adipocytes play a neglected role in this process. Some adipokines act as inflammatory and angiogenic promoters, while others have the opposite effects. Adipokines, together with inflammatory factors and other cytokines, modulate inflammation and neovascularization in a coordinated fashion. This review will also emphasize cellular regulators and their crosstalk in long-term PD. Understanding the molecular mechanism, targeting changes in adipocytes and regulating adipokine secretion will help extend therapeutic methods for preventing inflammation and angiogenesis in PD.

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.

Rapamycin Protects from Type-I Peritoneal Membrane Failure Inhibiting the Angiogenesis, Lymphangiogenesis, and Endo-MT

Preservation of peritoneal membrane (PM) is essential for long-term survival in peritoneal dialysis (PD). Continuous presence of PD fluids (PDF) in the peritoneal cavity generates chronic inflammation and promotes changes of the PM, such as fibrosis, angiogenesis, and lymphangiogenesis. Mesothelial-to-mesenchymal transition (MMT) and endothelial-to-mesenchymal transition (Endo-MT) seem to play a central role in this pathogenesis. We speculated that Rapamycin, a potent immunosuppressor, could be beneficial by regulating blood and lymphatic vessels proliferation. We demonstrate that mice undergoing a combined PD and Rapamycin treatment (PDF + Rapa group) presented a reduced PM thickness and lower number of submesothelial blood and lymphatic vessels, as well as decreased MMT and Endo-MT, comparing with their counterparts exposed to PD alone (PDF group). Peritoneal water transport in the PDF + Rapa group remained at control level, whereas PD effluent levels of VEGF, TGF-β, and TNF-α were lower than in the PDF group. Moreover, the treatment of mesothelial cells with Rapamycin in vitro significantly decreased VEGF synthesis and selectively inhibited the VEGF-C and VEGF-D release when compared with control cells. Thus, Rapamycin has a protective effect on PM in PD through an antifibrotic and antiproliferative effect on blood and lymphatic vessels. Moreover, it inhibits Endo-MT and, at least partially, MMT.

Protective Effects of Paricalcitol on Peritoneal Remodeling during Peritoneal Dialysis

Peritoneal dialysis (PD) is associated with structural and functional alterations of the peritoneal membrane, consisting of fibrosis, angiogenesis, and loss of ultrafiltration capacity. Vitamin D receptor activation (VDRA) plays an important role in mineral metabolism and inflammation, but also antiangiogenic and antifibrotic properties have been reported. Therefore, the effects of active vitamin D treatment on peritoneal function and remodeling were investigated. Rats were either kept naïve to PDF exposure or daily exposed to 10 mL PDF and were treated for five or seven weeks with oral paricalcitol or vehicle control. Non-PDF-exposed rats showed no peritoneal changes upon paricalcitol treatment. Paricalcitol reduced endogenous calcitriol but did not affect mineral homeostasis. However, upon PDF exposure, loss of ultrafiltration capacity ensued which was fully rescued by paricalcitol treatment. Furthermore, PD-induced ECM thickening was significantly reduced and omental PD-induced angiogenesis was less pronounced upon paricalcitol treatment. No effect of paricalcitol treatment on total amount of peritoneal cells, peritoneal leukocyte composition, and epithelial to mesenchymal transition (EMT) was observed. Our data indicates that oral VDRA reduces tissue remodeling during chronic experimental PD and prevents loss of ultrafiltration capacity. Therefore, VDRA is potentially relevant in the prevention of treatment technique failure in PD patients.

Pharmacologic targets and peritoneal membrane remodeling

Peritoneal dialysis (PD) is associated with functional and structural changes of the peritoneal membrane, also known as peritoneal remodeling. The peritoneal membrane is affected by many endogenous and exogenous factors such as cytokines, PD fluids, and therapeutic interventions. Here, we present an overview of various studies that have investigated pharmacologic interventions aimed at regression of peritoneal damage and prolongation of PD treatment.

Suramin inhibits the development and progression of peritoneal fibrosis

Peritoneal fibrosis is one of the most serious complications in patients with peritoneal dialysis (PD) and is associated with the loss of peritoneal membrane ultrafiltration function. In this study, we investigated whether suramin, an inhibitor that blocks multiple growth factors by binding to their receptors, would prevent development of peritoneal fibrosis in a rat model. Rats were given a daily intraperitoneal injection of chlorhexidine gluconate (CG) for 3 weeks to induce peritoneal fibrosis. Administration of suramin at 5, 10, and 20 mg/kg dose-dependently attenuated peritoneal membrane thickening and expression of collagen I, fibronectin, and α-smooth muscle actin. Increased expression of transforming growth factor-β1 (TGF-β1) and phosphorylation of Smad3 was detected in fibrotic peritoneum and inhibited by suramin treatment. Suramin was also effective in blocking CG-induced phosphorylation of inhibitor of κB (IκB) and nuclear factor (NF)-κBp65, expression of several inflammatory cytokines, and infiltration of macrophages in the peritoneum. Moreover, suramin suppressed angiogenesis and expression of vascular endothelial growth factor, a molecule associated with angiogenesis in the injured peritoneum. Therefore, our results indicate that suramin treatment can effectively alleviate the development of peritoneal fibrosis by suppression of TGF-β1 signaling, inflammation, and angiogenesis, and suggest that suramin may have therapeutic potential for prevention of peritoneal fibrosis in PD patients.

Detrimental dermal wound healing: what can we learn from the oral mucosa?

Wounds in adults are frequently accompanied by scar formation. This scar can become fibrotic due to an imbalance between extracellular matrix (ECM) synthesis and ECM degradation. Oral mucosal wounds, however, heal in an accelerated fashion, displaying minimal scar formation. The exact mechanisms of scarless oral healing are yet to be revealed. This review highlights possible mechanisms involved in the difference between scar-forming dermal vs. scarless oral mucosal wound healing. Differences were found in expression of ECM components, such as procollagen I and tenascin-C. Oral wounds contained fewer immune mediators, blood vessels, and profibrotic mediators but had more bone marrow-derived cells, a higher reepithelialization rate, and faster proliferation of fibroblasts compared with dermal wounds. These results form a basis for further research that should be focused on the relations among ECM, immune cells, growth factors, and fibroblast phenotypes, as understanding scarless oral mucosal healing may ultimately lead to novel therapeutic strategies to prevent fibrotic scars.

Cannabinoid receptors as therapeutic targets for dialysis-induced peritoneal fibrosis

BACKGROUND

Long-term exposure to bioincompatible peritoneal dialysis solutions is frequently complicated with peritoneal fibrosis and ultrafiltration failure. As cannabinoid receptor (CBR) ligands have been reported to be beneficial to ameliorate the process of liver fibrosis, we strove to investigate their therapeutic potential to prevent peritoneal fibrosis.

METHODS

We used the rat model of peritoneal fibrosis induced by intraperitoneal injection of methylglyoxal and in vitro mesothelial cell culture to test the effects of CBR ligands, including the type 1 CBR (CB(1)R) antagonist and the type 2 CBR (CB(2)R) agonist.

RESULTS

In the methylglyoxal model, both intraperitoneal CB(1)R antagonist (AM281) and CB(2)R agonist (AM1241) treatment significantly ameliorated peritoneal fibrosis. In addition, CB(1)R antagonist was able to alleviate TGF-β(1)-induced dedifferentiation of mesothelial cells and to maintain epithelial integrity in vitro.

CONCLUSIONS

Intraperitoneal administration of CBR ligands (CB(1)R antagonist and CB(2)R agonist) offers a potential therapeutic strategy to reduce dialysis-induced peritoneal fibrosis and to prolong the peritoneal survival in peritoneal dialysis patients.

Bioincompatible impact of different peritoneal dialysis fluid components and therapeutic interventions as tested in a rat peritoneal dialysis model

Peritoneal dialysis (PD) is associated with functional and structural changes of the peritoneal membrane. In this paper, we describe the impact of different factors contributing to peritoneal incompatibility of PD fluid installation including presence of a catheter, volume loading, and the PD fluid components itself. These factors initiate recruitment and activation of peritoneal immune cells such as macrophages and mast cells, as well as activation of peritoneal cells as mesothelial cells in situ. We provide an overview of PD-associated changes as seen in our rat PD-exposure model. Since these changes are partly reversible, we finally discuss therapeutic strategies in the rat PD model with possible consequences of long-term PD in the relevant human setting.