Hippuric Acid Promotes Renal Fibrosis by Disrupting Redox Homeostasis via Facilitation of NRF2–KEAP1–CUL3 Interactions in Chronic Kidney Disease

Abstract

Chronic kidney disease (CKD) is characterized by the accumulation of protein-bound uremic toxins (PBUTs), which play a pathophysiological role in renal fibrosis (a common pathological process resulting in CKD progression). Accumulation of the PBUT hippuric acid (HA) is positively correlated with disease progression in CKD patients, suggesting that HA may promote renal fibrosis. Oxidative stress is the most important factor affecting PBUTs nephrotoxicity. Herein, we assessed the ability of HA to promote kidney fibrosis by disrupting redox homeostasis. In HK-2 cells, HA increased fibrosis-related gene expression, extracellular matrix imbalance, and oxidative stress. Additionally, reactive oxygen species (ROS)-mediated TGFβ/SMAD signaling contributed to HA-induced fibrotic responses. HA disrupted antioxidant networks by decreasing the levels of nuclear factor erythroid 2-related factor 2 (NRF2), leading to ROS accumulation and fibrotic responses, as evidenced by NRF2 activation and knockdown. Moreover, NRF2 levels were reduced by NRF2 ubiquitination, which was regulated via increased interactions of Kelch-like ECH-associated protein 1 with Cullin 3 and NRF2. Finally, renal fibrosis and redox imbalance promoted by HA were confirmed in rats. Importantly, sulforaphane (NRF2 activator) reversed HA-promoted renal fibrosis. Thus, HA promotes renal fibrosis in CKD by disrupting NRF2-driven antioxidant system, indicating that NRF2 is a potential therapeutic target for CKD.

Uremic Toxins in the Progression of Chronic Kidney Disease and Cardiovascular Disease: Mechanisms and Therapeutic Targets

“…Hippuric acid has been implicated in promoting renal fibrosis and endothelial dysfunction by inducing oxidative stress. A recently published study demonstrated that hippuric acid contributes to the progression of renal fibrosis by disrupting redox homeostasis [ 78 ]. In this study, incubation of HK-2 cells with hippuric acid resulted in expression of fibrosis markers and induced extracellular matrix (ECM) imbalance, increased ROS and Nox4 expression, and activated TGF-β/Smad signaling.…”

“…In this study, incubation of HK-2 cells with hippuric acid resulted in expression of fibrosis markers and induced extracellular matrix (ECM) imbalance, increased ROS and Nox4 expression, and activated TGF-β/Smad signaling. Nuclear factor erythroid 2-related factor 2 (NRF2) is a transcription factor that regulates the expression of thiol molecules, antioxidants, and detoxifying enzymes [ 78 ]. The formation of a NRF2-KEAP1-CUL3 complex negatively regulates NRF2 by ubiquitinating it for degradation.…”

Eat Your Broccoli: Oxidative Stress, NRF2, and Sulforaphane in Chronic Kidney Disease

“…The beneficial effects of SFN have also been illustrated in vitro. In human kidney 2 (HK2) cell culture, hippuric acid-mediated fibrosis was accompanied by an alteration of the redox state due to disruption of the NRF-2-driven antioxidant system [ 123 ]. Treatment of the cells with SFN significantly suppressed hippuric acid-mediated ROS and H 2 O 2 production, and upregulated the antioxidant proteins NRF2, HO-1, and NQO1 [ 123 ].…”

“…In human kidney 2 (HK2) cell culture, hippuric acid-mediated fibrosis was accompanied by an alteration of the redox state due to disruption of the NRF-2-driven antioxidant system [ 123 ]. Treatment of the cells with SFN significantly suppressed hippuric acid-mediated ROS and H 2 O 2 production, and upregulated the antioxidant proteins NRF2, HO-1, and NQO1 [ 123 ]. SFN also decreased expression levels of the fibrosis-associated proteins collagen-I, alpha-smooth muscle actin, and vimentin [ 123 ].…”

“…Treatment of the cells with SFN significantly suppressed hippuric acid-mediated ROS and H 2 O 2 production, and upregulated the antioxidant proteins NRF2, HO-1, and NQO1 [ 123 ]. SFN also decreased expression levels of the fibrosis-associated proteins collagen-I, alpha-smooth muscle actin, and vimentin [ 123 ]. In an HK2 cell culture model of hyperglycemia, treatment with SFN prevented high glucose-induced epithelial-to-mesenchymal transition, likely via an NRF2-dependent mechanism [ 124 ].…”