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-β1 (
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/β-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/β-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-β1:
transforming growth factor, beta 1; VIM:
vimentin.