Mechanisms of progression of chronic renal diseases, a major healthcare burden, are poorly understood. Angiotensin II (AngII), the major renin-angiotensin system effector, is known to be involved in renal deterioration, but the molecular pathways are still unknown. Here, we show that mice overexpressing a dominant negative isoform of epidermal growth factor receptor (EGFR) were protected from renal lesions during chronic AngII infusion. Transforming growth factor-alpha (TGF-alpha) and its sheddase, TACE (also known as ADAM17), were induced by AngII treatment, TACE was redistributed to apical membranes and EGFR was phosphorylated. AngII-induced lesions were substantially reduced in mice lacking TGF-alpha or in mice given a specific TACE inhibitor. Pharmacologic inhibition of AngII prevented TGF-alpha and TACE accumulation as well as renal lesions after nephron reduction. These findings indicate a crucial role for AngII-dependent EGFR transactivation in renal deterioration and identify in TACE inhibitors a new therapeutic strategy for preventing progression of chronic renal diseases.

Mechanisms of progression of chronic kidney disease (CKD), a major health care burden, are poorly understood. EGFR stimulates CKD progression, but the molecular networks that mediate its biological effects remain unknown. We recently showed that the severity of renal lesions after nephron reduction varied substantially among mouse strains and required activation of EGFR. Here, we utilized two mouse strains that react differently to nephron reduction -FVB/N mice, which develop severe renal lesions, and B6D2F1 mice, which are resistant to early deterioration -coupled with genome-wide expression to elucidate the molecular nature of CKD progression. Our results showed that lipocalin 2 (Lcn2, also known as neutrophil gelatinase-associated lipocalin [NGAL]), the most highly upregulated gene in the FVB/N strain, was not simply a marker of renal lesions, but an active player in disease progression. In fact, the severity of renal lesions was dramatically reduced in Lcn2 -/-mice. We discovered that Lcn2 expression increased upon EGFR activation and that Lcn2 mediated its mitogenic effect during renal deterioration. EGFR inhibition prevented Lcn2 upregulation and lesion development in mice expressing a dominant negative EGFR isoform, and hypoxia-inducible factor 1α (Hif-1α) was crucially required for EGFR-induced Lcn2 overexpression. Consistent with this, cell proliferation was dramatically reduced in Lcn2 -/-mice. These data are relevant to human CKD, as we found that LCN2 was increased particularly in patients who rapidly progressed to end-stage renal failure. Together our results uncover what we believe to be a novel function for Lcn2 and a critical pathway leading to progressive renal failure and cystogenesis.

In chronic kidney disease (CKD), loss of functional nephrons results in metabolic and mechanical stress in the remaining ones, resulting in further nephron loss. Here we show that Akt2 activation has an essential role in podocyte protection after nephron reduction. Glomerulosclerosis and albuminuria were substantially worsened in Akt2(-/-) but not in Akt1(-/-) mice as compared to wild-type mice. Specific deletion of Akt2 or its regulator Rictor in podocytes revealed that Akt2 has an intrinsic function in podocytes. Mechanistically, Akt2 triggers a compensatory program that involves mouse double minute 2 homolog (Mdm2), glycogen synthase kinase 3 (Gsk3) and Rac1. The defective activation of this pathway after nephron reduction leads to apoptosis and foot process effacement of the podocytes. We further show that AKT2 activation by mammalian target of rapamycin complex 2 (mTORC2) is also required for podocyte survival in human CKD. More notably, we elucidate the events underlying the adverse renal effect of sirolimus and provide a criterion for the rational use of this drug. Thus, our results disclose a new function of Akt2 and identify a potential therapeutic target for preserving glomerular function in CKD.

Autophagy is an adaptation mechanism that is vital for cellular homeostasis in response to various stress conditions. Previous reports indicate that there is a functional interaction between the primary cilium (PC) and autophagy. The PC, a microtubule-based structure present at the surface of numerous cell types, is a mechanical sensor. Here we show that autophagy induced by fluid flow regulates kidney epithelial cell volume in vitro and in vivo. PC ablation blocked autophagy induction and cell-volume regulation. In addition, inhibition of autophagy in ciliated cells impaired the flow-dependent regulation of cell volume. PC-dependent autophagy can be triggered either by mTOR inhibition or a mechanism dependent on the polycystin 2 channel. Only the LKB1-AMPK-mTOR signalling pathway was required for the flow-dependent regulation of cell volume by autophagy. These findings suggest that therapies regulating autophagy should be considered in developing treatments for PC-related diseases.

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The effects of flow-induced mechanical strains on the phenotype of proximal tubular cells were addressed in vivo and in vitro by subjecting LLC-PK(1) and mouse proximal tubular cells to different levels of flow. Laminar flow (1 ml/min) induced a reorganization of the actin cytoskeleton and significantly inhibited the expression of plasminogen activators [tissue-type (tPA) activity: 25% of control cells; tPA mRNA: 70% of control cells; urokinase (uPA) mRNA: 56% of control LLC-PK(1) cells]. In vivo, subtotal nephrectomy (Nx) decreased renal fibrinolytic activity and uPA mRNA content detectable in proximal tubules. Nx also induced a reinforcement of the apical domain of the actin cytoskeleton analyzed by immunofluorescence. These effects of flow on tPA and uPA mRNA were prevented in vitro when reorganization of the actin cytoskeleton was blocked by cytochalasin D and were associated, in vitro and in vivo, with an increase in shear stress-responsive element binding activity detected by an electrophoretic mobility shift assay in proximal cell nuclear extracts. These results demonstrate that tubular flow affects the phenotype of renal epithelial cells and suggest that flow-induced mechanical strains could be one determinant of tubulointerstitial lesions during the progression of renal diseases.

In chronic kidney disease (CKD), proteinuria results in severe tubulointerstitial lesions, which ultimately lead to end-stage renal disease. Here we identify 4-phenylbutyric acid (PBA), a chemical chaperone already used in humans, as a novel therapeutic strategy capable to counteract the toxic effect of proteinuria. Mechanistically, we show that albumin induces tubular unfolded protein response via cytosolic calcium rise, which leads to tubular apoptosis by Lipocalin 2 (LCN2) modulation through ATF4. Consistent with the key role of LCN2 in CKD progression, Lcn2 gene inactivation decreases ER stress-induced apoptosis, tubulointerstitial lesions and mortality in proteinuric mice. More importantly, the inhibition of this pathway by PBA protects kidneys from morphological and functional degradation in proteinuric mice. These results are relevant to human CKD, as LCN2 is increased in proteinuric patients. In conclusion, our study identifies a therapeutic strategy susceptible to improve the benefit of RAS inhibitors in proteinuria-induced CKD progression.

In CKD, tubular cells may be involved in the induction of interstitial fibrosis, which in turn, leads to loss of renal function. However, the molecular mechanisms that link tubular cells to the interstitial compartment are not clear. Activation of the Stat3 transcription factor has been reported in tubular cells after renal damage, and Stat3 has been implicated in CKD progression. Here, we combined an experimental model of nephron reduction in mice from different genetic backgrounds and genetically modified animals with in silico and in vitro experiments to determine whether the selective activation of Stat3 in tubular cells is involved in the development of interstitial fibrosis. Nephron reduction caused Stat3 phosphorylation in tubular cells of lesion-prone mice but not in resistant mice. Furthermore, specific deletion of Stat3 in tubular cells significantly reduced the extent of interstitial fibrosis, which correlated with reduced fibroblast proliferation and matrix synthesis, after nephron reduction. Mechanistically, in vitro tubular Stat3 activation triggered the expression of a specific subset of paracrine profibrotic factors, including Lcn2, Pdgfb, and Timp1. Together, our results provide a molecular link between tubular and interstitial cells during CKD progression and identify Stat3 as a central regulator of this link and a promising therapeutic target.