Vascular endothelial growth factor (VEGF) is implicated in the peritoneal membrane remodeling that limits ultrafiltration in patients on peritoneal dialysis (PD). Although the exact mechanism of VEGF induction in PD is unclear, VEGF concentrations in drained dialysate correlate with IL-6 levels, suggesting a link between these cytokines. Human peritoneal mesothelial cells (HPMCs), the main source of IL-6 and VEGF in the peritoneum, do not bear the cognate IL-6 receptor and are thus unable to respond to classic IL-6 receptor signaling. Here, we investigated whether VEGF release by HPMCs is controlled by IL-6 in combination with its soluble receptor (IL-6 trans-signaling). Although treatment with either IL-6 or soluble IL-6 receptor (sIL-6R) alone had no effect on VEGF production, stimulation of HPMCs with IL-6 in combination with sIL-6R promoted VEGF expression and secretion through a transcriptional mechanism involving STAT3 and SP4. Conditioned medium from HPMCs cultured with IL-6 and sIL-6R promoted angiogenic endothelial tube formation, which could be blocked by silencing SP4. , induction of peritoneal inflammation in wild-type and IL-6-deficient mice showed IL-6 involvement in the control of and expression and new vessel formation, confirming the role of IL-6 trans-signaling in these processes. Taken together, these findings identify a novel mechanism linking IL-6 trans-signaling and angiogenesis in the peritoneal membrane.

Uraemia and long-term peritoneal dialysis (PD) can lead to fibrotic thickening of the peritoneal membrane, which may limit its dialytic function. Peritoneal fibrosis is associated with the appearance of myofibroblasts and expansion of extracellular matrix. The extent of contribution of resident peritoneal fibroblasts to these changes is a matter of debate. Recent studies point to a significant heterogeneity and complexity of the peritoneal fibroblast population. Here, we review recent developments in peritoneal fibroblast biology and summarize the current knowledge on the involvement of peritoneal fibroblasts in peritoneal inflammation and fibrosis.

Background Epithelial-to-mesenchymal transition (EMT) of human peritoneal mesothelial cells (HPMCs) contributes to fibrotic thickening of the peritoneum that develops in patients on peritoneal dialysis (PD). The process is thought to be largely mediated by transforming growth factor-beta (TGF-β). As TGF-β has also been implicated in senescence of HPMCs, we have performed an exploratory study to examine if senescent HPMCs can undergo EMT. Methods Omentum-derived HPMCs were rendered senescent by repeated passages in culture. Features of EMT were assessed by immunostaining and quantitative polymerase chain reaction (qPCR) at various stages of the HPMC lifespan and after treatment with or without TGF-β. The motility of HPMCs was assessed in a scratch wound migration assay. Results Replicative senescence of HPMCs was associated with a gradual increase in the constitutive expression of EMT markers, including increased production of extracellular matrix proteins. However, senescent HPMCs also retained epithelial cell features such as cytokeratin, calretinin, and E-cadherin and showed decreased, rather than increased, motility. In contrast, exposure to TGF-β resulted in an up-regulation of mesenchymal markers and down-regulation of epithelial markers. Such effects of TGF-β occurred both in young and senescent cells, although they were less pronounced in senescence. Conclusions Senescence of HPMCs is associated with spontaneous development of several EMT features. At the same time, senescent HPMCs preserve epithelial cell-like characteristics and are less prone to develop a full EMT phenotype in response to TGF-β. These observations may support the concept of cellular senescence being antagonistically pleiotropic with regard to EMT.

Mesothelial cell (MC) senescence contributes to malignancy and tissue fibrosis. The role of telomere erosion in MC senescence remains controversial, with evidence for both telomere-dependent and telomere-independent mechanisms reported. Single telomere length analysis revealed considerable telomere length heterogeneity in freshly isolated human peritoneal MCs, reflecting a heterogeneous proliferative history and providing high-resolution evidence for telomere-dependent senescence. By contrast the attenuated replicative lifespan, lack of telomere erosion and induction of p16 expression in in vitro-aged cells was consistent with stress-induced senescence. Given the potential pathophysiological impact of senescence in mesothelial tissues, high-resolution MC telomere length analysis may provide clinically useful information.

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.