Targeting the transcription factor Nrf2 to ameliorate oxidative stress and inflammation in chronic kidney disease

Abstract

Oxidative stress and inflammation are mediators in the development and progression of chronic kidney disease (CKD) and its complications, and they are inseparably linked as each begets and amplifies the other. CKD-associated oxidative stress is due to increased production of reactive oxygen species (ROS) and diminished antioxidant capacity. The latter is largely caused by impaired activation of Nrf2, the transcription factor that regulates genes encoding antioxidant and detoxifying molecules. Protective effects of Nrf2 are evidenced by amelioration of oxidative stress, inflammation, and kidney disease in response to natural Nrf2 activators in animal models, while Nrf2 deletion amplifies these pathogenic pathways and leads to autoimmune nephritis. Given the role of impaired Nrf2 activity in CKD-induced oxidative stress and inflammation, interventions aimed at restoring Nrf2 may be effective in retarding CKD progression. Clinical trials of the potent Nrf2 activator bardoxolone methyl showed significant improvement in renal function in CKD patients with type 2 diabetes. However, due to unforeseen complications the BEACON trial, which was designed to investigate the effect of this drug on time to end-stage renal disease or cardiovascular death in patients with advanced CKD, was prematurely terminated. This article provides an overview of the role of impaired Nrf2 activity in the pathogenesis of CKD-associated oxidative stress and inflammation and the potential utility of targeting Nrf2 in the treatment of CKD.

Figure 1. Production and Metabolism of Reactive Oxygen Species

Normally over 95% of the oxygen consumed in the body is converted to water by acquisition of 2 electrons in a single step. However, for the remaining 5% this process occurs with the transfer of one electron at a time, leading to formation highly reactive and short-lived species, collectively referred to as ROS. The primary ROS produced in the body is superoxide which is formed from single electron reduction of molecular oxygen. The primary sources of superoxide include the mitochondria, endoplasmic reticulum, cyclooxygenase, lipoxygenase, uncoupled nitric oxide synthase (NOS), NAD(P)H oxidase, xanthine oxidase, and cytochrome P450. Antioxidants then act on ROS to generate less reactive species. For example, superoxide dismutase (SOD) converts superoxide into hydrogen peroxide, which is then reduced by catalase (CAT) into water and oxygen and by glutathione peroxidase (GPX) into water and oxidized glutathione. However in pathological states hydrogen peroxide serves as the substrate for formation of highly reactive and cytotoxic oxidants such as hydroxyl radical by catalytically-active iron (Fe²⁺) and hypochlorous acid by myeloperoxidase. An increase in ROS generation or decrease in antioxidant availability leads to oxidative stress and induction of the pro-inflammatory response, which contribute to disease pathogenesis.

Figure 2. Nrf2 inhibits reactive oxygen species and inflammatory pathways that lead to kidney dysfunction

Nrf2 is held in an inactive state bound to Keap1 in the cytoplasm. In response to oxidative stress signals from various sources, the Keap1-Nrf2 complex is disrupted, preventing degradation of Nrf2. As a result, newly synthesized Nrf2 translocates to the nucleus, where it activates the transcription of several antioxidant and detoxifying enzymes. Activation of Nrf2 also suppresses NFκB activity, thereby inhibiting inflammation.

Figure 3. Acute kidney injury leads to progressive renal disease and decline in anti-inflammatory HO-1 and IL-10

(A) Following the induction of unilateral ischemic injury, a progressive loss of renal mass occurs over a three week period, culminating in a 2/3rds reduction of renal weight (left panel). Progressive tubular injury during this period is underscored by a progressive increase in the mRNA for the biomarker protein NGAL. A role for inflammation in this process is indicated by a progressive increase in pro-inflammatory cytokines, with stepwise increases in TNF-α mRNA expression. Similar results were obtained for a pro-inflammatory chemokine, MCP-1, as well as for a pro-fibrotic cytokine, TGF-β1. (Data presented in modified form from studies presented in reference 117 and from additional unpublished data; RZ). BL, baseline values; 1 d, one day, 1 and 3 weeks (wks) post ischemia. The mRNA values were quantified by RT-PCR and were divided by simultaneously obtained GAPDH levels, used as a “housekeeping” gene. (B) Whereas progressive renal disease and inflammation were noted over 3 weeks post ischemia (as shown in Panel A), there was a relative failure of HO-1 expression. While HO-1 protein levels rose at 1 day post ischemia, the levels then fell over the ensuing 3 weeks. Even more dramatic reductions in anti-inflammatory IL-10 protein levels were observed. Thus, the falling anti-inflammatory protein levels, with rising expression of pro-inflammatory genes (as depicted in Panel A), appear to represent reciprocal changes that tip the balance towards a pro-inflammatory / injury promoting state. These results are from ref. 117 and additional unpublished data from one of the authors; RZ). The HO-1 and IL-10 values are presented after factoring by total protein in the tissue extract.