PGC-1<em>α</em> Protects from Notch-Induced Kidney Fibrosis Development

Seung Han

Mei Wu

Bo Nam

Jung Park

Supplementary Material

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^{Department of Internal Medicine, Institute of Kidney Disease Research, Yonsei University College of Medicine, Seoul, Korea;}

^{Renal Electrolyte and Hypertension Division, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania;}

^{Severance Biomedical Science Institute, Brain Korea 21 PLUS, Yonsei University College of Medicine, Seoul, Korea; and}

^{Department of Nephrology, The First Hospital of Jilin University, Changchun, China}

^{Corresponding author.}

S.H.H. and M.Y.W. contributed equally to this work.

Correspondence: Dr. Katalin Susztak, Perelman School of Medicine, University of Pennsylvania, 415 Curie Boulevard, 405 Clinical Research Building, Philadelphia, PA 19104. Email: ude.nnepu.dem.liam@katzsusk

Received 2017 Feb 4; Accepted 2017 Jun 6.

Abstract

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-1α) and fatty acid oxidation-related genes are lower in mice expressing active Notch1 in tubular epithelial cells (Pax8-rtTA/ICN1) compared to littermate controls. Chromatin immunoprecipitation assays revealed that the Notch target gene Hes1 directly binds to the regulatory region of PGC-1α. Compared with Pax8-rtTA/ICN1 transgenic animals, Pax8-rtTA/ICN1/Ppargc1a transgenic mice showed improvement of renal structural alterations (on histology) and molecular defect (expression of profibrotic genes). Overexpression of PGC-1α restored mitochondrial content and reversed the fatty acid oxidation defect induced by Notch overexpression in vitro in tubule cells. Furthermore, compared with Pax8-rtTA/ICN1 mice, Pax8-rtTA/ICN1/Ppargc1a 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-1α.

Keywords: PGC-1α, Notch, kidney fibrosis, fatty acid oxidation, mitochondria

Abstract

Chronic kidney disease (CKD) has become an important public health problem worldwide. It is diagnosed by either reduction of the eGFR, quantified as estimated glomerular filtration rate (eGFR)<60 ml/min per 1.73 m^{, or abnormal leakiness of the glomerulus to albumin as urine albumin-to-creatinine ratio >30 mg/g.}¹ Patients with CKD have at least three- to fivefold greater mortality rate compared with matched subjects without CKD.²–⁴

Interstitial fibrosis shows the strongest correlation with future functional decline. Kidney fibrosis is the final common pathway that is observed in all forms of CKD. Fibrosis represents a complex architectural change characterized by glomerulosclerosis; tubular atrophy; accumulation of myofibroblast, collagen, and inflammatory cells; and peritubular capillary loss.⁵

Renal tubular epithelial cells (RTECs) represent >90% of the kidney mass. RTECs are fundamental to maintain fluid and electrolyte balance, and they transport a large amount of water, electrolytes, and other small molecules from the primary filtrate. RTECs have high baseline metabolic needs. RTECs mostly rely on fatty acids as their primary fuel source and generate energy via mitochondrial oxidative phosphorylation. RTECs, therefore, have high levels of peroxisomal proliferator–activated receptor-α (PPARα), peroxisomal proliferator–γ coactivator-1α (PGC-1α), and a dense mitochondrial network to support their metabolic and functional needs.⁶,⁷

Patients and animal models of CKD are characterized by sustained expression of developmental genes, such as Wnt, Notch, and Hedgehog, in RTECs. Studies have shown the critical role and contribution of these pathways to kidney fibrosis development.⁸–¹¹ Notch is a well known master regulator of cell specification, differentiation, and tissue patterning. In mammals, there are four Notch receptors (Notch1 to -4) and two classes of canonical ligands: Jagged 1 and 2 and Delta-like ligand 1, 3, and 4. The canonical Notch signaling pathway is initiated when the ligand binds to Notch receptors, thus causing proteolytic cleavage on the extracellular face by an ADAM/TACE protease and the intracellular side of the plasma membrane by γ-secretase. After the cleavage, the Notch intracellular domain translocates to the nucleus and forms a complex with RBPj and Mastermind-like proteins, leading to transcription of Notch target genes, such as helix-loop-helix proteins of the Hes and Hey families. Notch signaling is a crucial regulator of kidney development, although it is expressed at low level in normal healthy adults.¹²

It is hypothesized that, in CKD kidneys, Notch is activated in tubule cells in response to injury and cell death, likely as part of an injury repair mechanism.⁹,¹⁰,¹³,¹⁴ Sustained and high Notch expression seems to be harmful. Tubule-specific expression of Notch induces severe epithelial dedifferentiation and interstitial fibrosis and death of the animals. Notch is not only sufficient but also, necessary for fibrosis development, because inhibition of Notch signaling attenuates tubulointersitial fibrosis in a mouse model of folic acid– and ureteral obstruction–induced fibrosis.¹⁰

The mechanism of Notch-induced fibrosis is poorly understood. Several pathways have been proposed to contribute to Notch-induced development of fibrosis, including dedifferentiation, partial epithelial-to-mesenchymal transition (EMT), and enhanced proliferation.¹²,¹⁵ The role of these pathways has not been substantiated by in vivo studies. We hypothesized that metabolic alteration mediates the Notch (development pathway)–induced kidney fibrosis development, and we particularly tested the role of PGC-1α.

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Acknowledgments

This work was supported by faculty research grant 6-2015-0119 from Yonsei University College of Medicine for 2015 and a research grant from Hanmi Pharmaceuticals, Co., Ltd., and work in the laboratory of K.S. is supported by National Institutes of Health grant DK076077. M.W. thanks the China Scholarship Council for fellowship support through grant 3022 (2015).

Acknowledgments

Footnotes

Published online ahead of print. Publication date available at www.jasn.org.

This article contains supplemental material online at http://jasn.asnjournals.org/lookup/suppl/doi:10.1681/ASN.2017020130/-/DCSupplemental.

Footnotes

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