**4. Inflammation and Oxidative Stress in CKD**

Chronic inflammation is a common comorbid condition in CKD. The increased production and reduced clearance of pro-inflammatory cytokines, oxidative stress, and acidosis contribute to the chronic inflammatory state but also to metabolic alterations and chronic and recurrent infections, especially in dialysis patients. Furthermore, metabolic alterations and intestinal dysbiosis create additional inflammatory stimulation with the involvement of the cells of the innate immune response system [95–97]. Among the inflammatory markers in CKD, IL-1, IL-6, TNF (Tumor Necrosis Factor)-α, C-reactive protein (CRP), adipokines, adhesion molecules, and the CD40 ligand are particularly important and associated with many complications (e.g., malnutrition, coronary calcification, atherosclerosis, atrial fibrillation, left ventricular hypertrophy, heart failure, insulin resistance, oxidative stress, endothelial dysfunction, mineral and bone diseases, anaemia, and erythropoietin resistance). In addition to being produced by lymphocytes, these pro-inflammatory factors are produced by visceral adipose tissue, which becomes dysfunctional during CKD by expressing a high level of pro-inflammatory cytokine mRNA [98]. Alongside the levels of these cytokines, there is also an increase in pro-inflammatory enzymes such as cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS), which are positively regulated by the activation of NF-κB in CKD [99,100]. Several studies have also demonstrated that uremic toxins, such as IS, are able to increase the levels of TNF-α and IL-6, causing an exacerbation of the inflammatory state and favouring oxidative stress [101,102]. Indeed, oxidative stress is also frequently observed in the early stages of chronic renal failure onwards and, in addition to being a non-traditional risk factor for all causes of mortality, tends to exacerbate during the course of the disease and can sometimes persist to a certain degree after kidney transplant [103,104]. Oxidative stress is responsible for several pathological conditions that are considered risk factors for CKD, such as diabetes, hypertension, and atherosclerosis, and is also responsible for the progression of kidney damage, which leads to renal ischemia, glomerular damage, cell death, and apoptosis, which also exacerbate the severe inflammatory processes already underway [105,106]. Oxidative stress is a condition of imbalance between the excessive production of oxidants and the reduced capacity of antioxidant systems, which leads to metabolic dysregulation and the oxidation of lipids, DNA, and proteins, as well as affecting the cellular activity and inhibiting the activity of cytoprotective enzymes [102,107]. Oxidative stress is linked to the production of highly reactive intermediates, reactive oxygen (ROS), and nitrogen (RNS) species, whose excessive generation is associated with impaired electron transport chains, reduced ATP synthesis, mitochondrial dysfunction, cell damage, apoptosis, and even damage to all of the cellular constituents [108]. Mitochondria are mainly responsible for the production of reactive molecules through the electron transport chain, especially ROS, which are also able to improve the inflammatory response. Indeed, during the pathogenesis of kidney disease, mitochondria in damaged cells become one of the main sources of excess ROS, which in

turn implements the activation of transcription factors NF-κB, AP-1, and p53, exacerbating the production of pro-inflammatory cytokines and chemokines such as IL-1β, IL-6, IL-8, IL-1, and TNF-α, monocyte chemoattractant protein-1 (MCP-1), interferon-invasive protein-10 (IP-10), molecules of adhesion such as selectin, ICAM, VCAM, ELAM, inflammatory enzymes, such as iNOS and COX-2, and further ROS/RNS [109–113]. The main markers of oxidative stress that have significantly elevated levels in the blood and/or circulating tissues in patients with CKD are malondialdehyde (MDA), a low-density oxidized lipoprotein, advanced glycation end products, and l'8-hydroxide-oxyguanosine. For example, the interaction between AGE and the RAGE receptor induces the activation of the MAP kinase transduction pathway, leading to the nuclear translocation of NF-κB and the activation of second messengers, resulting in an increase in cytokines, pro-inflammatory enzymes, and adhesion molecules [114–116]. Thus, as oxidative stress can further exacerbate inflammation, inflammation and oxidative stress are important mediators in the development and progression of kidney disease and associated complications, where one generates and amplifies the other, and the antioxidant systems are severely compromised [94]. In fact, this condition also depends on the reduced activation of antioxidant responses, such as the transcription factor Nrf2 (Erythroid-related nuclear factor 2), the main cellular defence factor that regulates the genes coding for antioxidant and detoxifying proteins and enzymes. Generally, Nrf2 is in a quiescent state sequestered by the cytosolic repressor Keap1 (Kelchlike ECH-associated protein 1), which also promotes its rapid proteasomal degradation; in contrast, under oxidative and electrophilic stress conditions, Nrf2 is released by Keap1, which in this case acts as an electrophilic sensor and, together with the small Maf protein (sMAF), binds to the antioxidant response element (ARE) in the promoter region of genes coding for phase II and antioxidant enzymes to counteract oxidative stress [117–119]. In addition, Nrf2 also directly suppresses the expression of pro-inflammatory NF-κB target genes by binding to their promoters and inhibiting their transcription [120]. However, in the course of CKD, the excessive production of ROS reduces the activation of Nrf2, and its deficiency increases the susceptibility to kidney damage. Indeed, several studies have shown that during CKD, Nrf2 has a renoprotective effect by controlling uremic inflammation and improving antioxidant defences, leading to a reduction in renal fibrosis, tubular damage, and renal hypoxia [121].
