Angiogenic Role of Mesothelium-Derived Chemokine CXCL1 During Unfavorable Peritoneal Tissue Remodeling in Patients Receiving Peritoneal Dialysis as Renal Replacement Therapy

Peritoneal dialysis (PD) is a valuable ‘home treatment’ option, even more so during the ongoing Coronavirus pandemic. However, the long-term use of PD is limited by unfavourable tissue remodelling in the peritoneal membrane, which is associated with inflammation-induced angiogenesis. This appears to be driven primarily through vascular endothelial growth factor (VEGF), while the involvement of other angiogenic signaling pathways is still poorly understood. Here, we have identified the crucial contribution of mesothelial cell-derived angiogenic CXC chemokine ligand 1 (CXCL1) to peritoneal angiogenesis in PD. CXCL1 expression and peritoneal microvessel density were analysed in biopsies obtained by the International Peritoneal Biobank (NCT01893710 at www.clinicaltrials.gov), comparing 13 children with end-stage kidney disease before initiating PD to 43 children on chronic PD. The angiogenic potential of mesothelial cell-derived CXCL1 was assessed in vitro by measuring endothelial tube formation of human microvascular endothelial cells (HMECs) treated with conditioned medium from human peritoneal mesothelial cells (HPMCs) stimulated to release CXCL1 by treatment with either recombinant IL-17 or PD effluent. We found that the capillary density in the human peritoneum correlated with local CXCL1 expression. Both CXCL1 expression and microvessel density were higher in PD patients than in the age-matched patients prior to initiation of PD. Exposure of HMECs to recombinant CXCL1 or conditioned medium from IL-17-stimulated HPMCs resulted in increased endothelial tube formation, while selective inhibition of mesothelial CXCL1 production by specific antibodies or through silencing of relevant transcription factors abolished the proangiogenic effect of HPMC-conditioned medium. In conclusion, peritoneal mesothelium-derived CXCL1 promotes endothelial tube formation in vitro and associates with peritoneal microvessel density in uremic patients undergoing PD, thus providing novel targets for therapeutic intervention to prolong PD therapy.

Non-HLA antibodies targeting angiotensin II type 1 receptors and endothelin-1 type A receptors impair endothelial repair via a β2-arrestin link to the mTOR pathway

Functional non-HLA antibodies (antibodies to non-human leukocyte antigens) targeting the G protein-coupled receptors angiotensin II type 1 receptor (AT1R) and endothelin-1 type A receptor (ETAR) are implicated in the pathogenesis of transplant vasculopathy. While ERK signaling (a regulator of cell growth) may represent a general cellular response to agonist stimulation, the molecular link between receptor stimulation and development of vascular obliteration has not been fully established. Here we hypothesize involvement of the versatile adaptor proteins, β-arrestins, and the major regulator of cell growth, PI3K/mTOR signaling, in impaired endothelial repair. To test this, human microvascular endothelial cells were treated with AT1R/ETAR antibodies isolated from patients with kidney transplant vasculopathy. These antibodies activated both mTOR complexes via AT1R and ETAR in a PI3K-dependent and ERK-independent manner. The mTOR inhibitor, rapamycin, completely abolished activation of mTORC1 and mTORC2 after long-term treatment with receptor antibodies. Imaging studies revealed that β2- but not β1-arrestin was recruited to ETAR in response to ET1 and patient antibodies but not with antibodies isolated from healthy individuals. Silencing of β2-arrestin by siRNA transfection significantly reduced ERK1/2 and mTORC2 activation. Non-HLA antibodies impaired endothelial repair by AT1R- and ETAR-induced mTORC2 signaling. Thus, we provide evidence that functional AT1R/ETAR antibodies induce ERK1/2 and mTOR signaling involving β2-arrestin in human microvascular endothelium. Hence, our data may provide a translational rational for mTOR inhibitors in combination with receptor blockers in patients with non-HLA receptor recognizing antibodies.

Increased gene expression of the cardiac endothelin system in obese mice

Obesity is a well-known risk factor of atherosclerosis and heart failure. In the human heart, a local endothelin system containing prepro-endothelin-1, endothelin-converting enzyme-1, and endothelin receptors A and B has been described. The endothelin system is activated in heart failure; however, the impact of obesity on the cardiac endothelin system is unknown. In this study, 18-week-old male C57BL/6 mice fed either a control diet or a high-fat diet for 10 weeks were analyzed. High-fat diet significantly increased the body weight of the animals and augmented low-density lipoprotein, high-density lipoprotein, and cholesterol plasma levels, compared to control. The animal groups showed no significant differences in left ventricular size or function (heart rate, ejection fraction, fractional shortening, left ventricular posterior wall thickness, cardiac output) after control or high-fat diet. We did not observe signs of cardiac hypertrophy or changes in markers of cardiac fibrosis in these heart samples. The cardiac expression of prepro-endothelin-1 mRNA, endothelin-converting enzyme-1 mRNA, and protein and endothelin receptors A and B mRNA was increased in 18-week-old obese C57BL/6 mice compared to animals with normal weight (p<0.05 vs. control). Furthermore, endothelin-1 plasma levels showed an increasing trend. In conclusion, an increased expression of genes of the endothelin system was observed in the hearts of 18-week-old mice after high-fat diet, possibly contributing to later cardiovascular complications of obesity.

Low-density lipoproteins induce the renin-angiotensin system and their receptors in human endothelial cells

Increased levels of low-density lipoproteins are well-established risk factors of endothelial dysfunction and the metabolic syndrome. In this study, we evaluated the effect of native low-density lipoprotein (nLDL) and oxidized LDL (oxLDL) on the expression of genes of the renin-angiotensin system (angiotensin-converting enzyme, ACE; angiotensin II type 1 receptor, AT(1)) and their receptors (low-density lipoprotein receptor: LDLR; lectin-like oxLDL receptor: LOX-1; toll-like receptor 4: TLR4) in primary cultures of human umbilical vein endothelial cells. ACE and AT(1) expressions were significantly increased after stimulation with nLDL and oxLDL. OxLDL receptor LOX-1 showed a maximum induction after 7 hours. Increased LOX-1 protein expression in response to oxLDL could be blocked by a LOX-1-specific antibody. TLR4 expression was increased by nLDL and oxLDL as well. We conclude that LDL and oxLDL can activate the renin-angiotensin system and their receptors LDLR, LOX-1, and TLR4 in human endothelial cells. These data suggest a novel link between hypercholesterolemia and hypertension in patients with the metabolic syndrome.