Opposing actions of renal tubular- and myeloid-derived porcupine in obstruction-induced kidney fibrosis

DKK3 promotes oxidative stress injury and fibrosis in HK-2 cells by activating NOX4 via β-catenin/TCF4 signaling

Oxidative stress and fibrosis may accelerate the progression of chronic kidney disease (CKD). DKK3 is related to regulating renal fibrosis and CKD. However, the molecular mechanism of DKK3 in regulating oxidative stress and fibrosis during CKD development has not been clarified, which deserves to be investigated. Human proximal tubule epithelial cells (HK-2 cells) were treated with H2O2 to establish a cell model of renal fibrosis. The mRNA and protein expressions were analyzed using qRT-PCR and western blot, respectively. Cell viability and apoptosis were evaluated using MTT assay and flow cytometry, respectively. ROS production was estimated using DCFH-DA. The interactions among TCF4, β-catenin and NOX4 were validated using luciferase activity assay, ChIP and Co-IP. Herein, our results revealed that DKK3 was highly expressed in HK-2 cells treated with H2O2. DKK3 depletion increased H2O2-treated HK-2 cell viability and reduced cell apoptosis, oxidative stress, and fibrosis. Mechanically, DKK3 promoted formation of the β-catenin/TCF4 complex, and activated NOX4 transcription. Upregulation of NOX4 or TCF4 weakened the inhibitory effect of DKK3 knockdown on oxidative stress and fibrosis in H2O2-stimulated HK-2 cells. All our results suggested that DKK3 accelerated oxidative stress and fibrosis through promoting β-catenin/TCF4 complex-mediated activation of NOX4 transcription, which could lead to novel molecules and therapeutic targets for CKD.

Pathway from Acute Kidney Injury to Chronic Kidney Disease: Molecules Involved in Renal Fibrosis

Acute kidney injury (AKI) is one of the main conditions responsible for chronic kidney disease (CKD), including end-stage renal disease (ESRD) as a long-term complication. Besides short-term complications, such as electrolyte and acid-base disorders, fluid overload, bleeding complications or immune dysfunctions, AKI can develop chronic injuries and subsequent CKD through renal fibrosis pathways. Kidney fibrosis is a pathological process defined by excessive extracellular matrix (ECM) deposition, evidenced in chronic kidney injuries with maladaptive architecture restoration. So far, cited maladaptive kidney processes responsible for AKI to CKD transition were epithelial, endothelial, pericyte, macrophage and fibroblast transition to myofibroblasts. These are responsible for smooth muscle actin (SMA) synthesis and abnormal renal architecture. Recently, AKI progress to CKD or ESRD gained a lot of interest, with impressive progression in discovering the mechanisms involved in renal fibrosis, including cellular and molecular pathways. Risk factors mentioned in AKI progression to CKD are frequency and severity of kidney injury, chronic diseases such as uncontrolled hypertension, diabetes mellitus, obesity and unmodifiable risk factors (i.e., genetics, older age or gender). To provide a better understanding of AKI transition to CKD, we have selected relevant and updated information regarding the risk factors responsible for AKIs unfavorable long-term evolution and mechanisms incriminated in the progression to a chronic state, along with possible therapeutic approaches in preventing or delaying CKD from AKI.

The role of Evi/Wntless in exporting Wnt proteins

Intercellular communication by Wnt proteins governs many essential processes during development, tissue homeostasis and disease in all metazoans. Many context-dependent effects are initiated in the Wnt-producing cells and depend on the export of lipidated Wnt proteins. Although much focus has been on understanding intracellular Wnt signal transduction, the cellular machinery responsible for Wnt secretion became better understood only recently. After lipid modification by the acyl-transferase Porcupine, Wnt proteins bind their dedicated cargo protein Evi/Wntless for transport and secretion. Evi/Wntless and Porcupine are conserved transmembrane proteins, and their 3D structures were recently determined. In this Review, we summarise studies and structural data highlighting how Wnts are transported from the ER to the plasma membrane, and the role of SNX3-retromer during the recycling of its cargo receptor Evi/Wntless. We also describe the regulation of Wnt export through a post-translational mechanism and review the importance of Wnt secretion for organ development and cancer, and as a future biomarker.

Ferroptosis and renal fibrosis: A new target for the future (Review)

Ferroptosis is a type of non-apoptotic controlled cell death triggered by oxidative stress and iron-dependent lipid peroxidation. Ferroptosis is regulated by signalling pathways that are associated with metabolism, including glutathione peroxidase 4 dysfunction, the cystine/glutamate antiporter system, lipid peroxidation and inadequate iron metabolism. Ferroptosis is associated with renal fibrosis; however, further research is required to understand the specific molecular mechanisms involved. The present review aimed to discuss the known molecular mechanisms of ferroptosis and outline the biological reactions that occur during renal fibrosis that may be associated with ferroptosis. Further investigation into the association between ferroptosis and renal fibrosis may lead to the development of novel treatment methods.

Liproxstatin-1 attenuates unilateral ureteral obstruction-induced renal fibrosis by inhibiting renal tubular epithelial cells ferroptosis

Renal fibrosis is a common pathological process that occurs with diverse etiologies in chronic kidney disease. However, its regulatory mechanisms have not yet been fully elucidated. Ferroptosis is a form of non-apoptotic regulated cell death driven by iron-dependent lipid peroxidation. It is currently unknown whether ferroptosis is initiated during unilateral ureteral obstruction (UUO)-induced renal fibrosis and its role has not been determined. In this study, we demonstrated that ureteral obstruction induced ferroptosis in renal tubular epithelial cells (TECs) in vivo. The ferroptosis inhibitor liproxstatin-1 (Lip-1) reduced iron deposition, cell death, lipid peroxidation, and inhibited the downregulation of GPX4 expression induced by UUO, ultimately inhibiting ferroptosis in TECs. We found that Lip-1 significantly attenuated UUO-induced morphological and pathological changes and collagen deposition of renal fibrosis in mice. In addition, Lip-1 attenuated the expression of profibrotic factors in the UUO model. In vitro, we used RSL3 treatment and knocked down of GPX4 level by RNAi in HK2 cells to induce ferroptosis. Our results indicated HK2 cells secreted various profibrotic factors during ferroptosis. Lip-1 was able to inhibit ferroptosis and thereby inhibit the secretion of the profibrotic factors during the process. Incubation of kidney fibroblasts with culture medium from RSL3-induced HK2 cells promoted fibroblast proliferation and activation, whereas Lip-1 impeded the profibrotic effects. Our study found that Lip-1 may relieve renal fibrosis by inhibiting ferroptosis in TECs. Mechanistically, Lip-1 could reduce the activation of surrounding fibroblasts by inhibiting the paracrine of profibrotic factors in HK2 cells. Lip-1 may potentially be used as a therapeutic approach for the treatment of UUO-induced renal fibrosis.

Autophagy attenuates renal fibrosis in obstructive nephropathy through inhibiting epithelialtomesenchymal transition

OBJECTIVES

To explore the relationship between autophagy and epithelial-to-mesenchymal transition (EMT), and to evaluate whether autophagy can affect the progression of renal fibrosis in obstructive nephropathy by regulating the EMT process.

METHODS

Unilateral ureteral obstruction (UUO) renal fibrosis model of rat was constructed, and the animals were divided into a sham group, an UUO group, an UUO+low-dose rapamycin group, and an UUO+high-dose rapamycin group. HE staining was used to observe the structure of the kidney, and Masson staining was used to observe renal interstitial collagen deposition. The expressions of E-cadherin, alpha-smooth muscle actin (α-SMA), Snail 1, and microtubule-associated protein-1 light chain 3II (LC3II) were detected by Western blotting, reflecting cellular EMT and autophagy. Transforming growth factor β1 (TGF-β1) induced-NRK52E cells model was constructed, and the cells were divided into a control group, a TGF-β1 group, and a TGF-β1+ Snail 1 siRNA group. To explore the effect of autophagy on EMT, the cells were also divided into a control group, a rapamycin group, and a Beclin 1 siRNA group. Western blotting was used to detect the expressions of E-cadherin, α-SMA, Snail 1, LC3II, collagen I, and fibronectin.

RESULTS

Compared with the sham group, the kidney damage in the UUO group was significantly worse; compared with the sham group, the collagen deposition in the kidney tissues in the UUO group was significantly increased, which were significantly reduced in the UUO+high-dose rapamycin group and the UUO+low-dose rapamycin group compared with the UUO group; compared with the sham group, the E-cadherin level in the kidney of the UUO group was significantly reduced, and the expression levels of α-SMA and LC3II were significantly increased (all P<0.05). Compared with the UUO group, the expression levels of E-cadherin and LC3II in the UUO+high-dose rapamycin group and the UUO+low-dose rapamycin group were significantly increased (P<0.01 and P<0.05, respectively), and the expression level of α-SMA was significantly decreased (P<0.01 and P<0.05, respectively). The expression levels of Snail 1, α-SMA, collagen I and fibronectin were significantly higher, and the E-cadherin level was significantly lower in the TGF-β1 group than those in the control group (all P<0.05). Compared with the TGF-β1 group, the expression of E-cadherin was increased significantly, and the expressions of α-SMA, collagen I and fibronectin were decreased significantly in the TGF-β1+Snail 1 siRNA group (all P<0.05). Compared with the control group, the expression levels of LC3II and E-cadherin were significantly elevated, and the expression levels of α-SMA and Snail 1 in the rapamycin group were significantly reduced (all P<0.05); the expression levels of LC3II and E-cadherin were significantly reduced, and the expression levels of α-SMA and Snail 1 were significantly increased in the Beclin 1 siRNA group (all P<0.05).

CONCLUSIONS

Autophagy plays an essential role in the regulation of EMT in obstructive nephropathy fibrosis. Autophagy can alleviate renal fibrosis by inhibiting EMT.

FoxM1 promotes Wnt/β-catenin pathway activation and renal fibrosis via transcriptionally regulating multi-Wnts expressions

The activation of Wnt/β‐catenin pathway plays a pivotal role in promoting renal fibrosis. The activation of Wnt/β‐catenin pathway relies on the binding of Wnts to Frizzled receptors on cell membrane. However, the factor regulating Wnts production remains unclear. Here, we demonstrated that transcriptional factor FoxM1 was significantly increased in obstructed kidneys and patients' kidneys with fibrosis. The up‐regulation of FoxM1 mainly distributed in tubular epithelial cells. Pharmacological inhibition of FoxM1 down‐regulated multi‐Wnts elevation in UUO mice and attenuated renal fibrosis. In cultured renal tubular epithelial cells, overexpression of FoxM1 promoted 8 Wnts expression, while knock‐down on FoxM1‐suppressed multi‐Wnts including Wnt1, Wnt2b and Wnt3 expression induced by Ang II. Chromatin immunoprecipitation PCR confirmed that FoxM1 bound to Wnt1, Wnt2b, Wnt3 promoters and luciferase assay further identified that the transcriptions of Wnt1, Wnt2b and Wnt3 were regulated by FoxM1. Thus, our findings show that multi‐Wnt family members were regulated by transcriptional factor FoxM1. FoxM1 might be a key switch for activating β‐catenin pathway and renal fibrosis. Therefore, FoxM1 might be a potential therapeutic target in manipulating renal fibrosis.

WNT-β-catenin signalling - a versatile player in kidney injury and repair

The WNT-β-catenin system is an evolutionary conserved signalling pathway that is of particular importance for morphogenesis and cell organization during embryogenesis. The system is usually suppressed in adulthood; however, it can be re-activated in organ injury and regeneration. WNT-deficient mice display severe kidney defects at birth. Transient WNT-β-catenin activation stimulates tissue regeneration after acute kidney injury, whereas sustained (uncontrolled) WNT-β-catenin signalling promotes kidney fibrosis in chronic kidney disease (CKD), podocyte injury and proteinuria, persistent tissue damage during acute kidney injury and cystic kidney diseases. Additionally, WNT-β-catenin signalling is involved in CKD-associated vascular calcification and mineral bone disease. The WNT-β-catenin pathway is tightly regulated, for example, by proteins of the Dickkopf (DKK) family. In particular, DKK3 is released by 'stressed' tubular epithelial cells; DKK3 drives kidney fibrosis and is associated with short-term risk of CKD progression and acute kidney injury. Thus, targeting the WNT-β-catenin pathway might represent a promising therapeutic strategy in kidney injury and associated complications.

Porcupine's dilemma in kidney fibrosis

Aberrant activation of Wnt/β-catenin signaling, an activity that is critically modulated by porcupine, a membrane-bound O-acyltransferase, is profibrotic. Previously, the Crowley laboratory demonstrated that oral administration of porcupine inhibitor attenuated experimental mouse kidney fibrosis. Now Lu et al., from that laboratory, using conditional knockout lineages, have shown that porcupine in the renal tubule accelerated fibrosis and Wnt generation, whereas myeloid cell porcupine counteracted the effect. Thus, porcupine has been attracting both positive and negative attention.