The uraemic milieu, the ubiquitin–proteasome pathway and the endothelial cell: a pathway to dysfunction

Chronic kidney disease (CKD) is one of the most common chronic diseases and often leads to end-stage kidney disease (ESKD) requiring dialysis and transplantation. Cardiovascular disease (CVD) is the major cause of morbidity and mortality in CKD and ESKD (Collins et al. 2011). In fact, more CKD patients die from CVD than progress to ESKD. Much of the vascular disease seen in ESKD is caused by atherosclerosis and is a consequence of many interconnected risk factors. These may be traditional risk factors, such as hypertension, dyslipidaemia, diabetes and smoking, or non-traditional factors, including oxidative stress, inflammation and endothelial dysfunction, which lead to arterial stiffness and atherosclerosis. However, the exact mechanism whereby vascular damage occurs in CKD remains unclear. Oxidative stress and inflammation are now accepted pathophysiological processes seen in CVD and progression of CKD. However, uraemic toxins may specifically, as well as indirectly, contribute to endothelial dysfunction, arterial stiffness and atherosclerosis (Feng et al. 2011). Recent evidence from a study conducted in a population of CKD patients without comorbidities, such as diabetes and hypertension, suggests that in CKD, stages 4–5, with higher degrees of uraemia and close to dialysis, exhibit endothelial dysfunction, whereas the earlier stages (2– 3) do not (Lilitkarntakul et al. 2011). This is in contrast to previous studies that included patients with comorbidities that showed the presence of endothelial dysfunction even in the earlier CKD stages (2–3). In this issue of Experimental Physiology, Feng et al. (2011) provide new information from in vitro experimental studies on endothelial cells exposed to uraemic serum. They explore potential mechanisms involving the ubiquitin–proteasome pathway (UPP), whereby uraemic toxins cause endothelial cell dysfunction. This pathway regulates cell homeostasis by clearing proteins and regulating signal transduction, proliferation and apoptosis. They had previously demonstrated that inflammation and atherosclerosis in the aortic wall were inhibited by the proteasome inhibitor, MG132, in uraemic rabbits (Feng et al. 2010). In the present experiments, they cultured rabbit aortic endothelial cells (RAECs) in the presence of 10% uraemic serum. The proliferative activity of RAECs was significantly increased when they were exposed to uraemic serum, which was attenuated by MG132. In addition, uraemic serum activated nuclear factor-κB in RAECs, which was inhibited by MG132. Also, tumour necrosis factor-α protein expression was increased in RAECs by uraemic serum, which was also inhibited by MG132. Finally, they showed that biomarkers of nitric oxide (NO) production were decreased after exposure to uraemic serum, which were again inhibited by MG132. Nitric oxide is considered to be a potent biomarker of endothelial function. This novel series of experiments clearly demonstrates uraemic activation of the UPP with resultant activation of inflammation. This provides at least one potential mechanism explaining uraemia-induced endothelial dysfunction assessed in vitro. Defining such mechanisms is important because it may lead to the development of new, targeted therapies, which may reduce the burden of vascular disease in patients with CKD. Therapies tested so far in uraemic patients on haemodialysis have endeavoured to reduce mortality and cardiovascular events. These have included large multicentre randomized controlled trials, which have all been unsuccessful. The only positive studies in this area used antioxidant therapies, such as N-acetyl cysteine and vitamin E. However, these trials were limited by study design and sample size, have not been translated into practice and hence require validation. Other new evidence from studies using a nephron reduction model in vivo have identified that bone marrow derived stromal cells (mesenchymal stem cells; MSCs), which may differentiate into endothelial cells, become functionally incompetent in the presence of uraemia (Noh et al. 2011). The investigators showed that there was decreased expression of vascular endothelial growth factor, vascular endothelial growth factor receptor and stromal cell derived factor-1α in uraemic animals. In addition, there was decreased angiogenesis in MSCs in the uraemic environment. In the present study reported in this issue of Experimental Physiology, Feng et al. (2011) demonstrated the role of the UPP on endothelial cells in uraemia but did not determine which molecules within the uraemic serum were responsible for activation of the UPP. One of the next challenges will be the identification of the individual uraemic toxins responsible for vascular damage. This would have the potential to lead to the design of specific targeted therapies, much needed in patients with uraemic kidney disease. In addition, the exact mechanism by which uraemic serum activates the UPP needs further elucidation. Future research should also investigate earlier stages of CKD with lower levels of uraemic toxins, using in vivo studies with animal CKD models, such as nephron reduction with five-sixth nephrectomy or the oral administration of adenine. This may determine the threshold where endothelial dysfunction is initiated and therapies need to be applied.