Kidney diseases impart a vast burden on affected individuals and the overall health care system. Progressive loss of renal parenchymal cells and functional decline following injury are often observed. Notch-1 and -2 receptors are crucially involved in nephron development and contribute to inflammatory kidney diseases. We specifically determined the participation of receptor Notch-3 following tubulointerstitial injury and in inflammatory responses. Here we show by heat map analyses that Notch-3 transcripts are up-regulated in human kidney diseases. A similar response was corroborated with kidney cells following TGF-β exposure in vitro. The murine unilateral ureteral obstruction (UUO) model mirrors hallmarks of tubulointerstitial injury and damage. A subset of tubular and interstitial cells demonstrated up-regulated Notch-3 receptor expression in diseased animals. We hypothesized a relevance of Notch-3 receptors for the chemotactic response. To address this question, animals with genetic ablation of receptor Notch-3 were analysed following UUO. As a result, we found that Notch-3-deficient animals are protected from tubular injury and cell loss with significantly reduced interstitial collagen deposition. Monocytic cell infiltration was significantly reduced and retarded, likely due to abrogated chemokine synthesis. A cell model was set up that mimics enhanced receptor Notch-3 expression and activation. Here a pro-mitogenic response was seen with activated signalling in tubular cells and fibroblasts. In conclusion, Notch-3 receptor fulfils non-redundant roles in the inflamed kidney that may not be replaced by other Notch receptor family members. Thus, specific blockade of this receptor may be suitable as therapeutic option to delay progression of kidney disease.

In vitro studies identified Y-box–binding protein (YB)-1 as a key regulator of inflammatory mediators. In this study, we observed increased levels of secreted YB-1 in sera from sepsis patients. This led us to investigate the in vivo role of YB-1 in murine models of acute peritonitis following LPS injection, in sterile renal inflammation following unilateral ureteral obstruction, and in experimental pyelonephritis. LPS injection enhanced de novo secretion of YB-1 into the urine and the peritoneal fluid of LPS-treated mice. Furthermore, we could demonstrate a significant, transient upregulation and posttranslational modification (phosphorylation at serine 102) of YB-1 in renal and inflammatory cells. Increased renal cytoplasmic YB-1 amounts conferred enhanced expression of proinflammatory chemokines CCL2 and CCL5. Along these lines, heterozygous YB-1 knockout mice (YB-1+/d) that display 50% reduced YB-1 levels developed significantly lower responses to both LPS and sterile inflammation induced by unilateral ureteral obstruction. This included diminished immune cell numbers due to impaired migration propensities and reduced chemokine expression. YB-1+/d mice were protected from LPS-associated mortality (20% mortality on day 3 versus 80% in wild-type controls); however, immunosuppression in YB-1+/d animals resulted in 50% mortality. In conclusion, our findings identify YB-1 as a major, nonredundant mediator in both systemic and local inflammatory responses.

Background: Transcription factor YB-1 constitutes a key regulator in immune cell homeostasis. It has been demonstrated to be involved in monocyte/macrophage differentiation. However, the underlying mechanisms are poorly understood. Results: Protein phosphatase calcineurin (CN) regulates YB-1 activities on the CCL5 promoter during macrophage differentiation. Conclusion: Dephosphorylation of YB-1 by CN is crucial to counteract the overwhelming pro-inflammatory propensities of YB-1. Significance: Overshooting inflammation may be counteracted by dephosphorylation of YB-1.

The immunosuppressive calcineurin inhibitors (CNIs) cyclosporine A (CsA) and tacrolimus are widely used in transplant organ recipients, but in the kidney allograft, they may cause tubulointerstitial as well as mesangial fibrosis, with TGF-β believed to be a central inductor. In this study, we report that the cold-shock protein Y-box binding protein-1 (YB-1) is a TGF-β independent downstream effector in CsA- as well as in tacrolimus- but not in rapamycin-mediated activation of rat mesangial cells (rMCs). Intracellular content of YB-1 is several-fold increased in MCs following CNI treatment in vitro and in vivo in mice. This effect ensues in a time-dependent manner, and the operative concentration range encompasses therapeutically relevant doses for CNIs. The effect of CNI on cellular YB-1 content is abrogated by specific blockade of translation, whereas retarding the transcription remains ineffective. The activation of rMCs by CNIs is accomplished by generation of reactive oxygen species. In contrast to TGF-β–triggered reactive oxygen species generation, hydrogen peroxide especially could be identified as a potent inductor of YB-1 accumulation. In line with this, hindering TGF-β did not influence CNI-induced YB-1 upregulation, whereas ERK/Akt pathways are involved in CNI-mediated YB-1 expression. CsA-induced YB-1 accumulation results in mRNA stabilization and subsequent generation of collagen. Our results provide strong evidence for a CNI-dependent induction of YB-1 in MCs that contributes to renal fibrosis via regulation of its own and collagen translation.