Fibrosis is the histological manifestation of a progressive usually irreversible process causing chronic and end stage kidney disease. Genome-wide transcriptome studies of a large cohort (n=95) of normal and fibrotic human kidney tubule samples followed by systems and network analyses identified inflammation and metabolism as top dysregulated pathways in diseased kidneys. In particular, we found that humans and mouse models with tubulointerstitial fibrosis had lower expression of key enzymes and regulators of fatty acid oxidation (FAO) and increased intracellular lipid deposition. In vitro experiments indicated that inhibition of fatty acid oxidation in tubule epithelial cells caused ATP depletion, cell death, dedifferentiation and intracellular lipid deposition; a phenotype observed in fibrosis. Restoring fatty acid metabolism by genetic or pharmacological methods protected mice from tubulointerstitial fibrosis. Our results raise the possibility that correcting the metabolic defect may be useful for preventing and treating chronic kidney disease.
The kidney has a tremendous capacity to regenerate following injury, but factors that govern this response are still largely unknown. We isolated cells from mouse kidneys with high proliferative and multi-lineage differentiation capacity. These cells expressed high level of Sox9. In regenerating kidneys, Sox9 expression was induced early and 89% of proliferating cells were Sox9 positive. In vitro, Sox9 positive cells showed unlimited proliferation and multi-lineage differentiation capacity. Using an inducible Sox9 cre line and lineage tagging methods, we show that Sox9 positive cells can supply new daughter cells, contributing to the regeneration of proximal tubule, loop of Henle and distal tubule segments, but not to collecting duct and glomerular cells. Furthermore, inducible deletion of Sox9 resulted in reduced epithelial proliferation, more severe injury and fibrosis development. In summary, we demonstrate that in the kidney, Sox9 positive cells show progenitor-like properties in vitro, and contribute to epithelial regeneration following injury in vivo.
The c-Myc oncogene (MYC) drives malignant progression, but also induces robust anabolic and proliferative programs leading to intrinsic stress. The mechanisms enabling adaptation to MYC-induced stress are not fully understood. We have uncovered an essential role for the transcription factor ATF4 in survival following MYC activation. MYC upregulates ATF4 by activating GCN2 kinase through uncharged tRNAs. Subsequently, ATF4 co-occupies promoter regions of over 30 MYC target genes, primarily those regulating amino acid and protein synthesis, including 4E-BP1, a negative regulator of translation. 4E-BP1 is essential to balance protein synthesis, relieving MYC-induced proteotoxic stress. 4E-BP1 activity is negatively regulated by mTORC1-dependent phosphorylation and inhibition of mTORC1 signaling rescues ATF4 deficient cells from MYC-induced ER stress. Acute deletion of ATF4 significantly delays MYC-driven tumor progression and increases survival in mouse models. Our results establish ATF4 as a cellular rheostat of MYC-activity, ensuring enhanced translation rates are compatible with survival and tumor progression.
Kidney fibrosis is the histologic manifestation of CKD. Sustained activation of developmental pathways, such as Notch, in tubule epithelial cells has been shown to have a key role in fibrosis development. The molecular mechanism of Notch-induced fibrosis, however, remains poorly understood. Here, we show that, that expression of peroxisomal proliferation g-coactivator (PGC-1) and fatty acid oxidation-related genes are lower in mice expressing active Notch1 in tubular epithelial cells (Pax8-rtTA/) compared to littermate controls. Chromatin immunoprecipitation assays revealed that the Notch target gene directly binds to the regulatory region of PGC-1 Compared with transgenic animals, transgenic mice showed improvement of renal structural alterations (on histology) and molecular defect (expression of profibrotic genes). Overexpression of PGC-1 restored mitochondrial content and reversed the fatty acid oxidation defect induced by Notch overexpression in tubule cells. Furthermore, compared with mice, mice exhibited improvement in renal fatty acid oxidation gene expression and apoptosis. Our results show that metabolic dysregulation has a key role in kidney fibrosis induced by sustained activation of the Notch developmental pathway and can be ameliorated by PGC-1.
Renal tubule epithelial cells are high-energy demanding polarized epithelial cells. Liver kinase B1 (LKB1) is a key regulator of polarity, proliferation, and cell metabolism in epithelial cells, but the function of LKB1 in the kidney is unclear. Our unbiased gene expression studies of human control and CKD kidney samples identified lower expression of LKB1 and regulatory proteins in CKD. Mice with distal tubule epithelial-specific Lkb1 deletion (Ksp-Cre/Lkb1 flox/flox ) exhibited progressive kidney disease characterized by flattened dedifferentiated tubule epithelial cells, interstitial matrix accumulation, and dilated cystic-appearing tubules. Expression of epithelial polarity markers b-catenin and E-cadherin was not altered even at later stages. However, expression levels of key regulators of metabolism, AMP-activated protein kinase (Ampk), peroxisome proliferative activated receptor gamma coactivator 1-a (Ppargc1a), and Ppara, were significantly lower than those in controls and correlated with fibrosis development. Loss of Lkb1 in cultured epithelial cells resulted in energy depletion, apoptosis, less fatty acid oxidation and glycolysis, and a profibrotic phenotype. Treatment of Lkb1-deficient cells with an AMP-activated protein kinase (AMPK) agonist (A769662) or a peroxisome proliferative activated receptor alpha agonist (fenofibrate) restored the fatty oxidation defect and reduced apoptosis. In conclusion, we show that loss of LKB1 in renal tubular epithelial cells has an important role in kidney disease development by influencing intracellular metabolism. 27: 439-453, 201627: 439-453, . doi: 10.1681 Renal tubular epithelial cells (TECs) display strict apico-basal polarity. They allow a highly regulated uptake or excretion of substances at its apical surface, while keeping a closed, impenetrable surface through the formation of tight intercellular junctions in the basolateral membrane. The establishment and maintenance of TEC polarity is incompletely understood. The liver kinase B1 (LKB1) is an important regular of polarity. Early studies indicated that single intestinal epithelial cells polarize in a cell-autonomous fashion in response to LKB1 expression. 1 The LKB1 or STK11 gene encodes an evolutionarily conserved serine/threonine protein kinase. Following LKB1 expression, intestinal epithelial cells reorganized their cytoskeleton to form an apical brush border, demonstrating LKB1's critical role in establishing epithelial polarity. On the other hand, the effect of LKB1 on cell polarity appears to be cell type specific and deletion of LKB1 did not alter polarity of lung epithelial and pancreatic cells. 2 J Am Soc Nephrol
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