Tubular cells produce FGF2 via autophagy after acute kidney injury leading to fibroblast activation and renal fibrosis.

ABSTRACT

Following acute kidney injury (AKI), renal tubular cells may stimulate fibroblasts in a paracrine fashion leading to interstitial fibrosis, but the paracrine factors and their regulation under this condition remain elusive. Here we identify a macroautophagy/autophagy-dependent FGF2 (fibroblast growth factor 2) production in tubular cells. Upon induction, FGF2 acts as a key paracrine factor to activate fibroblasts for renal fibrosis. After ischemic AKI in mice, autophagy activation persisted for weeks in renal tubular cells. In inducible, renal tubule-specific atg7 (autophagy related 7) knockout (iRT-atg7-KO) mice, autophagy deficiency induced after AKI suppressed the pro-fibrotic phenotype in tubular cells and reduced fibrosis. Among the major cytokines, tubular autophagy deficiency in iRT-atg7-KO mice specifically diminished FGF2. Autophagy inhibition also attenuated FGF2 expression in TGFB1/TGF-ß1 (transforming growth factor, beta 1)-treated renal tubular cells. Consistent with a paracrine action, the culture medium of TGFB1-treated tubular cells stimulated renal fibroblasts, and this effect was suppressed by FGF2 neutralizing antibody and also by fgf2- or atg7-deletion in tubular cells. In human, compared with non-AKI, the renal biopsies from post-AKI patients had higher levels of autophagy and FGF2 in tubular cells, which showed significant correlations with renal fibrosis. These results indicate that persistent autophagy after AKI induces pro-fibrotic phenotype transformation in tubular cells leading to the expression and secretion of FGF2, which activates fibroblasts for renal fibrosis during maladaptive kidney repair.Abbreviations 3-MA 3-methyladnine; ACTA2/α-SMA actin alpha 2, smooth muscle, aorta; ACTB/ß-actin actin, beta; AKI acute kidney injury; ATG/Atg autophagy related; BUN blood urea nitrogen; CCN2/CTGF cellular communication network factor 2; CDKN2A/p16 cyclin dependent kinase inhibitor 2A; CKD chronic kidney disease; CM conditioned medium; COL1A1 collagen, type I, alpha 1; COL4A1 collagen, type IV, alpha 1; CQ chloroquine; ECM extracellular matrix; eGFR estimated glomerular filtration rate; ELISA enzyme-linked immunosorbent assay; FGF2 fibroblast growth factor 2; FN1 fibronectin 1; FOXO3 forkhead box O3; GAPDH glyceraldehyde-3-phosphate dehydrogenase; HAVCR1/KIM-1 hepatitis A virus cellular receptor 1; IHC immunohistochemistry; IRI ischemia-reperfusion injury; ISH in situ hybridization; LTL lotus tetragonolobus lectin; MAP1LC3B/LC3B microtubule-associated protein 1 light chain 3 beta; MTOR mechanistic target of rapamycin kinase; PDGFB platelet derived growth factor, B polypeptide; PPIB/cyclophilin B peptidylprolyl isomerase B; RT-qPCR real time-quantitative PCR; SA-GLB1/ß-gal senescence-associated galactosidase, beta 1; SASP senescence-associated secretory phenotype; sCr serum creatinine; SQSTM1/p62 sequestosome 1; TASCC TOR-autophagy spatial coupling compartment; TGFB1/TGF-ß1 transforming growth factor, beta 1; VIM vimentin.