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Fos-icking for control of angiogenesis: increasing the longevity of peritoneal dialysis

2013; Elsevier BV; Volume: 84; Issue: 6 Linguagem: Inglês

10.1038/ki.2013.306

1523-1755

Claudine S. Bonder, Lisa M. Ebert,

Acute Kidney Injury Research

Recurring peritonitis reduces the effectiveness of peritoneal dialysis by increasing fibrosis and angiogenesis, ultimately causing ultrafiltration failure (UFF). Identifying the processes underlying UFF will provide new hope for patients with chronic kidney disease. Catar and colleagues demonstrate that transforming growth factor-β, tumor necrosis factor-α, and interleukin-1 synergize to significantly increase the production and release of vascular endothelial growth factor by mesothelial cells, which, if untreated, will promote peritoneal angiogenesis, leading to UFF. Recurring peritonitis reduces the effectiveness of peritoneal dialysis by increasing fibrosis and angiogenesis, ultimately causing ultrafiltration failure (UFF). Identifying the processes underlying UFF will provide new hope for patients with chronic kidney disease. Catar and colleagues demonstrate that transforming growth factor-β, tumor necrosis factor-α, and interleukin-1 synergize to significantly increase the production and release of vascular endothelial growth factor by mesothelial cells, which, if untreated, will promote peritoneal angiogenesis, leading to UFF. Chronic kidney disease is a significant and growing public-health problem supported by the increasing incidence of obesity, diabetes, and hypertension, and increased aging population.1.Sarnak M.J. Levey A.S. Schoolwerth A.C. et al.Kidney disease as a risk factor for development of cardiovascular disease: a statement from the American Heart Association Councils on Kidney in Cardiovascular Disease, High Blood Pressure Research, Clinical Cardiology, and Epidemiology and Prevention.Circulation. 2003; 108: 2154-2169Crossref PubMed Scopus (2874) Google Scholar Peritoneal dialysis (PD) has been a mainstay alternative to kidney replacement therapy for chronic kidney disease patients for more than half a century.2.Price J.D. Ashby K.M. Reeve C.E. Results of 12 years' treatment of chronic renal failure by dialysis and transplantation.Can Med Assoc J. 1978; 118: 263-266PubMed Google Scholar However, despite the many benefits of PD, long-term treatment often leads to complications caused by the PD catheter and by the high concentration of glucose, hyperosmolarity, acidity, and presence of glucose degradation products in the dialysis fluid, which result in peritonitis. Morphological alterations in the peritoneum also occur at this time, including detachment of the mesothelial layer, increased submesothelial extracellular matrix deposits, angiogenesis, and fibrosis.3.Garcia-Lopez E. Lindholm B. Davies S. An update on peritoneal dialysis solutions.Nat Rev Nephrol. 2012; 8: 224-233Crossref PubMed Scopus (59) Google Scholar Peritonitis has been identified as a significant contributor to ultrafiltration failure (UFF); a Japanese study of biopsy specimens from 80 PD patients with no preexisting history of peritonitis demonstrated that the duration of PD was positively correlated with peritoneal thickness and vasculopathy.4.Honda K. Hamada C. Nakayama M. et al.Impact of uremia, diabetes, and peritoneal dialysis itself on the pathogenesis of peritoneal sclerosis: a quantitative study of peritoneal membrane morphology.Clin J Am Soc Nephrol. 2008; 3: 720-728Crossref PubMed Scopus (115) Google Scholar Taken together, the inflammation associated with PD is of significant concern, as it is the driving force behind UFF. A better understanding of the mechanisms underpinning peritonitis will provide new hope for the increasing number of patients with chronic kidney disease requiring long-term PD. In this context, the study by Catar and colleagues5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar (this issue) describes for the first time that the transcription factor subunit c-Fos regulates the production of vascular endothelial growth factor (VEGF) in response to transforming growth factor-β1 (TGF-β1) and tumor necrosis factor-α (TNF-α) during peritoneal inflammation. In their study, Catar et al. show that human peritoneal mesothelial cells (HPMCs) release only trace amounts of VEGF under quiescent conditions and that this is significantly upregulated in response to TGF-β1. Moreover, the release of VEGF was augmented by the addition of TNF-α, and the combined effect of TGF-β1 and TNF-α persisted for 24h even if the mediators were removed. VEGF is a major angiogenic factor that, when bound to VEGF receptor-2 (VEGFR-2), initiates a tyrosine kinase signaling cascade that stimulates the production of factors that stimulate vessel permeability (endothelial nitric oxide synthase (eNOS), producing nitric oxide), proliferation/survival (fibroblast growth factor (FGF)), migration (cellular adhesion molecules such as ICAM and VCAM), and finally differentiation into mature blood vessels. PD is intimately linked to increased fibrosis and vascular density of the peritoneal mesothelium, which ultimately results in UFF. In the study by Catar and colleagues, luciferase assays were used to investigate the activation of the promoter region of VEGF in response to TGF-β1, without and with TNF-α.5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar TGF-β potently activated the VEGF promoter, and this effect was significantly enhanced by TNF-α. Deletion analysis identified that positions –1339 to –839 decreased the ability of the VEGF promoter to respond to TGF-β1 and TNF-α, and in silico analysis predicted the binding of several transcription factors, one of which was confirmed to be c-Fos. Electrophoretic mobility shift assays (EMSAs) then validated the involvement of c-Fos in VEGF gene regulation by TGF-β1 and TNF-α and not other transcription factors—AP-4, Ets-2, c-Jun, and Sp1. Elegant knock-down assays using small interfering RNAs in HPMCs confirmed that VEGF was released in a concentration-dependent manner in response to TGF-β1 and TNF-α. This was further confirmed with the use of a pharmacological inhibitor of AP-1, the functional transcription factor complex formed by the association of c-Fos with c-Jun. As inflammatory peritoneal effluents contain high levels of TGF-β1, TNF-α, and interleukin-1β (IL-1β),6.Lai K.N. Lai K.B. Lam C.W. et al.Changes of cytokine profiles during peritonitis in patients on continuous ambulatory peritoneal dialysis.Am J Kidney Dis. 2000; 35: 644-652Abstract Full Text Full Text PDF PubMed Scopus (136) Google Scholar and as TNF-α and IL-1β share many properties, Catar and colleagues examined whether IL-1β exerted a similar effect on TGF-β1-induced VEGF production.5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar Firstly, they confirmed previous reports by showing that effluent drained from a patient during an episode of bacterial peritonitis contained detectable levels of TGF-β1, TNF-α, and IL-1β. Moreover, they went on to show that, like TNF-α, IL-1β also demonstrated an ability to synergize with TGF-β1 to increase VEGF production by HPMCs. EMSAs with a consensus sequence oligonucleotide for c-Fos showed increased binding of nuclear extracts from cells exposed to TGF-β1 plus IL-1β compared with either cytokine alone. In support, a pharmacological inhibitor of AP-1 blocked HPMCs' release of VEGF induced by TGF-β1 plus IL-1β. Catar and colleagues5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar have described, for the first time, that the transcription factor c-Fos, via AP-1, controls TGF-β1-induced mesothelial-cell VEGF synthesis and release in an in vitro model (Figure 1); and, moreover, that a combination of proinflammatory mediators increased during peritonitis—TNF-α and IL-1β—synergize to exert a more profound and prolonged effect on this pathway. AP-1 is known to be involved in the TNF-α receptor signaling pathway, enabling TNF-α to influence the expression of many genes.7.Baud V. Karin M. Signal transduction by tumor necrosis factor and its relatives.Trends Cell Biol. 2001; 11: 372-377Abstract Full Text Full Text PDF PubMed Scopus (1373) Google Scholar The AP-1 complex is composed of heterodimers of members of the Fos family (c-Fos, FosB, Fra-1 and Fra-2) and the Jun family (c-Jun, JunB, and JunD) or Jun–Jun homodimers.8.Vesely P.W. Staber P.B. Hoefler G. et al.Translational regulation mechanisms of AP-1 proteins.Mutat Res. 2009; 682: 7-12Crossref PubMed Scopus (171) Google Scholar The activation of AP-1 is mediated by at least two regulatory events. First, some AP-1 proteins (for instance, c-Jun) are encoded by immediate early genes that are transcriptionally induced following treatment. Second, AP-1 activity can also be regulated by post-translational modification, including phosphorylation by MAPKs. TNF-α regulates AP-1 activity, in part, by increasing the expression of members of the Fos and Jun families. The identification by Catar and colleagues of increased c-Fos but not c-Jun in this study5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar suggests that for increased VEGF production by HPMCs, c-Fos is critical and may be an important therapeutic target. Future directions in light of the study by Catar et al.5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar should include translation into in vivo models. The inclusion of the AP-1 inhibitor in in vivo models would also pave the way for potential clinical application. Furthermore, it would be relevant to investigate the pathway of c-Fos induction by the various cytokines, and especially the synergistic interaction between TGF-β1 and TNF-α/IL-1β. How is it that TNF-α and IL-1β have an effect only if TGF-β1 is also present? Further clarification is also required to address how the cytokines are affecting VEGF production via c-Fos if the partner subunit (for instance, c-Jun) is not induced. Is c-Jun present in excess, such that c-Fos levels are rate-limiting? Or is maybe another member of the Jun family induced? Also of interest are the generation of VEGF and its mode of action. Catar et al. show that mesothelial cells produce VEGF in vitro and that patients with peritonitis have elevated levels of VEGF in their dialysates.5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar Whether mesothelial cells are the only source of VEGF in these patients is unclear, as endothelial cells themselves may be an additional source. Furthermore, for VEGF to have its proangiogenic effect, it must not accumulate in the peritoneum but rather be secreted basolaterally to act on the local vasculature underlying the mesothelium. In future studies, it will be important to fully investigate these and other unanswered questions. Clearly, identifying common elements that inhibit treatment of vasculopathy of the peritoneum will go a long way toward supporting chronic kidney disease patients. To this end, the results of Catar and colleagues5.Catar R. Witowski J. Wagner P. et al.The proto-oncogene c-Fos transcriptionally regulates VEGF production during peritoneal inflammation.Kidney Int. 2013; 84: 1119-1128Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar may have important clinical relevance for peritoneal longevity and thus PD effectiveness and, ultimately, patient survival. C.S.B. is a South Australian Cardiovascular Research Development Program Fellow, and L.M.E. holds a Florey Fellowship from the Royal Adelaide Hospital and SA Pathology.