Long Non-coding RNA Neat1, NLRP3 Inflammasome, and Acute Kidney Injury.

BACKGROUND: Acute kidney injury (AKI) is common in hospitalized patients and is associated with high mortality. Inflammation plays a key role in the pathophysiology of AKI. Long non-coding RNAs (lncRNAs) are increasingly recognized as regulators of the inflammatory and immune response, but its role in AKI remains unclear. METHODS: We explored the role of lncRNA Neat1 in (1) a cross-sectional and a longitudinal cohort of AKI in human; (2) three murine models of septic and aseptic AKI and (3) cultured C1.1 mouse kidney tubular cells. RESULTS: In human, hospitalized patients with AKI (n=66) demonstrated significantly increased lncRNA Neat1 levels in urinary sediment cells and buffy coat versus control participants (n=152) from a primary care clinic; and among 6 kidney transplant recipients, Neat1 levels were highest immediately after transplant surgery followed by a prompt decline to normal levels in parallel with recovery of kidney function. In mice with AKI induced by sepsis (via LPS injection or cecal ligation and puncture) and renal ischemia-reperfusion, kidney tubular Neat1 was increased versus sham-operated mice. Knockdown of Neat1 in the kidney using short hairpin RNA preserved kidney function, suppressed overexpression of the AKI biomarker NGAL, leukocyte infiltration and both intrarenal and systemic inflammatory cytokines IL-6, CCL-2 and IL-1ß. In LPS-treated C1.1 cells, Neat1 was overexpressed via TLR4/NF-κB signaling, and translocated from the cell nucleus into the cytoplasm where it promoted activation of NLRP3 inflammasomes via binding with the scaffold protein Rack1. Silencing Neat1 ameliorated LPS-induced cell inflammation, whereas its overexpression upregulated IL-6 and CCL-2 expression even without LPS stimulation. CONCLUSIONS: Our findings demonstrate a pathogenic role of Neat1 induction in human and mice during AKI with alleviation of kidney injury in 3 experimental models of septic and aseptic AKI after knockdown of Neat1. LPS/TLR4-induced Neat1 overexpression in tubular epithelial cells increases the inflammatory response by binding with the scaffold protein, Rack1, to activate NLRP3 inflammasomes.

The long noncoding RNA Meg3 mediates TLR4-induced inflammation in experimental obstructive nephropathy.

Kidney inflammation contributes to the progression of chronic kidney disease (CKD). Modulation of Toll-like receptor 4 (TLR4) signaling is a potential therapeutic strategy for this pathology, but the regulatory mechanisms of TLR4 signaling in kidney tubular inflammation remains unclear. Here, we demonstrated that tubule-specific deletion of TLR4 in mice conferred protection against obstruction-induced kidney injury, with reduction in inflammatory cytokine production, macrophage infiltration and kidney fibrosis. Transcriptome analysis revealed a marked down-regulation of long noncoding RNA (lncRNA) Meg3 in the obstructed kidney from tubule-specific TLR4 knockout mice compared with wild-type control. Meg3 was also induced by lipopolysaccharide in tubular epithelial cells via a p53-dependent signaling pathway. Silencing of Meg3 suppressed LPS-induced cytokine production of CCL-2 and CXCL-2 and the activation of p38 MAPK pathway in vitro and ameliorated kidney fibrosis in mice with obstructive nephropathy. Together, these findings identify a proinflammatory role of lncRNA Meg3 in CKD and suggest a novel regulatory pathway in TLR4-driven inflammatory responses in tubular epithelial cells.

Tubulovascular protection from protease-activated receptor-1 depletion during AKI-to-CKD transition.

BACKGROUND: Thromboembolic events are prevalent in chronic kidney disease (CKD) patients due to increased thrombin generation leading to a hypercoagulable state. We previously demonstrated that inhibition of protease-activated receptor-1 (PAR-1) by vorapaxar reduces kidney fibrosis. METHODS: We used an animal model of unilateral ischemia-reperfusion injury-induced CKD to explore the tubulovascular crosstalk mechanisms of PAR-1 in acute kidney injury (AKI)-to-CKD transition. RESULTS: During the early phase of AKI, PAR-1-deficient mice exhibited reduced kidney inflammation, vascular injury, and preserved endothelial integrity and capillary permeability. During the transition phase to CKD, PAR-1 deficiency preserved kidney function and diminished tubulointerstitial fibrosis via downregulated transforming growth factor-ß/Smad signaling. Maladaptive repair in the microvasculature after AKI further exacerbated focal hypoxia with capillary rarefaction, which was rescued by stabilization of hypoxia-inducible factor and increased tubular vascular endothelial growth factor A in PAR-1-deficient mice. Chronic inflammation was also prevented with reduced kidney infiltration by both M1- and M2-polarized macrophages. In thrombin-induced human dermal microvascular endothelial cells (HDMECs), PAR-1 mediated vascular injury through activation of NF-κB and ERK MAPK pathways. Gene silencing of PAR-1 exerted microvascular protection via a tubulovascular crosstalk mechanism during hypoxia in HDMECs. Finally, pharmacologic blockade of PAR-1 with vorapaxar improved kidney morphology, promoted vascular regenerative capacity, and reduced inflammation and fibrosis depending on the time of initiation. CONCLUSIONS: Our findings elucidate a detrimental role of PAR-1 in vascular dysfunction and profibrotic responses upon tissue injury during AKI-to-CKD transition and provide an attractive therapeutic strategy for post-injury repair in AKI.

Protective role of kallistatin in renal fibrosis via modulation of Wnt/ß-catenin signaling.

Kallistatin is a multiple functional serine protease inhibitor that protects against vascular injury, organ damage and tumor progression. Kallistatin treatment reduces inflammation and fibrosis in the progression of chronic kidney disease (CKD), but the molecular mechanisms underlying this protective process and whether kallistatin plays an endogenous role are incompletely understood. In the present study, we observed that renal kallistatin levels were significantly lower in patients with CKD. It was also positively correlated with estimated glomerular filtration rate (eGFR) and negatively correlated with serum creatinine level. Unilateral ureteral obstruction (UUO) in animals also led to down-regulation of kallistatin protein in the kidney, and depletion of endogenous kallistatin by antibody injection resulted in aggravated renal fibrosis, which was accompanied by enhanced Wnt/ß-catenin activation. Conversely, overexpression of kallistatin attenuated renal inflammation, interstitial fibroblast activation and tubular injury in UUO mice. The protective effect of kallistatin was due to the suppression of TGF-ß and ß-catenin signaling pathways and subsequent inhibition of epithelial-to-mesenchymal transition (EMT) in cultured tubular cells. In addition, kallistatin could inhibit TGF-ß-mediated fibroblast activation via modulation of Wnt4/ß-catenin signaling pathway. Therefore, endogenous kallistatin protects against renal fibrosis by modulating Wnt/ß-catenin-mediated EMT and fibroblast activation. Down-regulation of kallistatin in the progression of renal fibrosis underlies its potential as a valuable clinical biomarker and therapeutic target in CKD.

The PAR-1 antagonist vorapaxar ameliorates kidney injury and tubulointerstitial fibrosis.

Protease-activated receptor (PAR)-1 has emerged as a key profibrotic player in various organs including kidney. PAR-1 activation leads to deposition of extracellular matrix (ECM) proteins in the tubulointerstitium and induction of epithelial-mesenchymal transition (EMT) during renal fibrosis. We tested the anti-fibrotic potential of vorapaxar, a clinically approved PAR-1 antagonist for cardiovascular protection, in an experimental kidney fibrosis model of unilateral ureteral obstruction (UUO) and an AKI-to-chronic kidney disease (CKD) transition model of unilateral ischemia-reperfusion injury (UIRI), and dissected the underlying renoprotective mechanisms using rat tubular epithelial cells. PAR-1 is activated mostly in the renal tubules in both the UUO and UIRI models of renal fibrosis. Vorapaxar significantly reduced kidney injury and ameliorated morphologic changes in both models. Amelioration of kidney fibrosis was evident from down-regulation of fibronectin (Fn), collagen and α-smooth muscle actin (αSMA) in the injured kidney. Mechanistically, inhibition of PAR-1 inhibited MAPK ERK1/2 and transforming growth factor-ß (TGF-ß)-mediated Smad signaling, and suppressed oxidative stress, overexpression of pro-inflammatory cytokines and macrophage infiltration into the kidney. These beneficial effects were recapitulated in cultured tubular epithelial cells in which vorapaxar ameliorated thrombin- and hypoxia-induced TGF-ß expression and ECM accumulation. In addition, vorapaxar mitigated capillary loss and the expression of adhesion molecules on the vascular endothelium during AKI-to-CKD transition. The PAR-1 antagonist vorapaxar protects against kidney fibrosis during UUO and UIRI. Its efficacy in human CKD in addition to CV protection warrants further investigation.

Activated renal tubular Wnt/ß-catenin signaling triggers renal inflammation during overload proteinuria.

Imbalance of Wnt/ß-catenin signaling in renal cells is associated with renal dysfunction, yet the precise mechanism is poorly understood. Previously we observed activated Wnt/ß-catenin signaling in renal tubules during proteinuric nephropathy with an unknown net effect. Therefore, to identify the definitive role of tubular Wnt/ß-catenin, we generated a novel transgenic "Tubcat" mouse conditionally expressing stabilized ß-catenin specifically in renal tubules following tamoxifen administration. Four weeks after tamoxifen injection, uninephrectomized Tubcat mice displayed proteinuria and elevated blood urea nitrogen levels compared to non-transgenic mice, implying a detrimental effect of the activated signaling. This was associated with infiltration of the tubulointerstitium predominantly by M1 macrophages and overexpression of the inflammatory chemocytokines CCL-2 and RANTES. Induction of overload proteinuria by intraperitoneal injection of low-endotoxin bovine serum albumin following uninephrectomy for four weeks aggravated proteinuria and increased blood urea nitrogen levels to a significantly greater extent in Tubcat mice. Renal dysfunction correlated with the degree of M1 macrophage infiltration in the tubulointerstitium and renal cortical up-regulation of CCL-2, IL-17A, IL-1ß, CXCL1, and ICAM-1. There was overexpression of cortical TLR-4 and NLRP-3 in Tubcat mice, independent of bovine serum albumin injection. Finally, there was no fibrosis, activation of epithelial-mesenchymal transition or non-canonical Wnt pathways observed in the kidneys of Tubcat mice. Thus, conditional activation of renal tubular Wnt/ß-catenin signaling in a novel transgenic mouse model demonstrates that this pathway enhances intrarenal inflammation via the TLR-4/NLRP-3 inflammasome axis in overload proteinuria.

Tubular ß-catenin alleviates mitochondrial dysfunction and cell death in acute kidney injury.

Mitochondria take part in a network of intracellular processes that regulate homeostasis. Defects in mitochondrial function are key pathophysiological changes during AKI. Although Wnt/ß-catenin signaling mediates mitochondrial dysfunction in chronic kidney fibrosis, little is known of the influence of ß-catenin on mitochondrial function in AKI. To decipher this interaction, we generated an inducible mouse model of tubule-specific ß-catenin overexpression (TubCat), and a model of tubule-specific ß-catenin depletion (TubcatKO), and induced septic AKI in these mice with lipopolysaccharide (LPS) and aseptic AKI with bilateral ischemia-reperfusion. In both AKI models, tubular ß-catenin stabilization in TubCat animals significantly reduced BUN/serum creatinine, tubular damage (NGAL-positive tubules), apoptosis (TUNEL-positive cells) and necroptosis (phosphorylation of MLKL and RIP3) through activating AKT phosphorylation and p53 suppression; enhanced mitochondrial biogenesis (increased PGC-1α and NRF1) and restored mitochondrial mass (increased TIM23) to re-establish mitochondrial homeostasis (increased fusion markers OPA1, MFN2, and decreased fission protein DRP1) through the FOXO3/PGC-1α signaling cascade. Conversely, kidney function loss and histological damage, tubular cell death, and mitochondrial dysfunction were all aggravated in TubCatKO mice. Mechanistically, ß-catenin transfection maintained mitochondrial mass and activated PGC-1α via FOXO3 in LPS-exposed HK-2 cells. Collectively, these findings provide evidence that tubular ß-catenin mitigates cell death and restores mitochondrial homeostasis in AKI through the common mechanisms associated with activation of AKT/p53 and FOXO3/PGC-1α signaling pathways.

Tubule-specific deletion of LincRNA-p21ameliorates lipotoxic kidney injury.

Lipotoxicity has been implicated in the pathogenesis of obesity-related kidney damage and propagates chronic kidney injury like diabetic kidney disease; however, the underlying mechanisms have not yet been fully elucidated. To date, reduction of lipid acquisition and enhancement of lipid metabolism are the major, albeit non-specific, approaches to improve lipotoxic kidney damage. In the kidneys of high-fat diet (HFD)-fed mice and tubule cells cultured with palmitic acid (PA), we observed a dramatic upregulation of the long intergenic non-coding RNA-p21 (LincRNA-p21) through a p53-dependent mechanism. Kidney tubule cell-specific deletion of LincRNA-p21 attenuated oxidative stress, inflammation, apoptosis, and endoplasmic reticulum stress, leading to reduction of histological and functional kidney injury despite persistent obesity and hyperlipidemia. Mechanistically, HFD- or PA-initiated lipotoxicity suppressed the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mechanistic target of rapamycin (mTOR)/murine double minute 2 homolog (MDM2) signaling cascade to activate p53 and enhance the transcriptional activity of LincRNA-p21. Collectively, our findings suggest that the p53/LincRNA-p21 axis is the downstream effector in lipotoxic kidney injury and that targeting this axis particularly in the kidney tubule could be a novel therapeutic strategy.

Amelioration of Endoplasmic Reticulum Stress by Mesenchymal Stem Cells via Hepatocyte Growth Factor/c-Met Signaling in Obesity-Associated Kidney Injury.

Recent advances in the understanding of lipid metabolism suggest a critical role of endoplasmic reticulum (ER) stress in obesity-induced kidney injury. Hepatocyte growth factor (HGF) is a pleiotropic cytokine frequently featured in stem cell therapy with distinct renotropic benefits. This study aims to define the potential link between human induced pluripotent stem cell-derived mesenchymal stem cells (iPS-MSCs)/bone marrow-derived MSCs (BM-MSCs) and ER stress in lipotoxic kidney injury induced by palmitic acid (PA) in renal tubular cells and by high-fat diet (HFD) in mice. iPS-MSCs or BM-MSCs alleviated ER stress (by preventing induction of Bip, chop, and unfolded protein response), inflammation (Il6, Cxcl1, and Cxcl2), and apoptosis (Bax/Bcl2 and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling-positive cells) in renal cortex of animals exposed to HFD thus mitigating histologic damage and albuminuria, via activating HGF/c-Met paracrine signaling that resulted in enhanced HGF secretion in the glomerular compartment and c-Met expression in the tubules. Coculture experiments identified glomerular endothelial cells (GECs) to be the exclusive source of glomerular HGF when incubated with either iPS-MSCs or BM-MSCs in the presence of PA. Furthermore, both GEC-derived HGF and exogenous recombinant HGF attenuated PA-induced ER stress in cultured tubular cells, and this effect was abrogated by a neutralizing anti-HGF antibody. Taken together, this study is the first to demonstrate that MSCs ameliorate lipotoxic kidney injury via a novel microenvironment-dependent paracrine HGF/c-Met signaling mechanism to suppress ER stress and its downstream pro-inflammatory and pro-apoptotic consequences. Stem Cells Translational Medicine 2019;8:898&910.