Extra Domain A-Spliced Fibronectin Variant Contributes to Tubulointerstitial Fibrosis in CKD

Background: Chronic kidney disease (CKD) is characterized by progressive tubulo-interstitial fibrosis and tubular atrophy, leading to kidney failure. There is no effective treatment for kidney fibrosis. Fibronectin (native FN) is a large glycoprotein present in all tissues, and its accumulation in the tubulo-interstitium is a feature in progressive tubulo-interstitial fibrosis. Also, the extra domain A-spliced variant of FN isoform (EDA-FN) is enriched in fibrotic lesions. This study investigated the role of EDA-FN in the pathogenesis of tubulo-interstitial fibrosis in CKD. Methods: CKD was induced in wild-type (WT) and EDA-FN knockout (KO) mice by feeding with casein-based chow containing 0.2% adenine for 12 weeks, and mice fed casein-based chow served as controls. Mice were sacrificed and kidneys were harvested to assess histopathological changes and expression of mediators of inflammation and fibrosis. In vitro studies were performed on proximal renal tubular epithelial cells (PTEC). Results: Compared with WT casein-fed mice, WT CKD mice showed proteinuria, tubular atrophy, increased EDA-FN expression, and increased gene and protein expression of native FN, collagen type I and III, and α-smooth muscle actin (p < 0.05, for all). EDA-FN KO CKD mice showed comparable proteinuria as WT CKD mice, but less severe kidney histopathological features with reduced immune cell infiltration and decreased expression of fibrosis mediators. Under physiological condition, EDA-FN was not expressed by PTEC. Exposure to TGF-β1, IL-1β, but not IL-6, IL-8 or MCP-1, induced EDA-FN in PTEC, which was mediated in part through PI3K and p38 MAPK phosphorylation. Incubation of PTEC with EDA-peptide increased native FN, collagen I and SNAIL expression and IL-6, IL-8 and MCP-1 secretion. Conclusion: Our data showed that murine adenine-induced CKD had increased tubulo-interstitial EDA-FN expression, which participated in the pathogenesis of chronic tubulo-interstitial inflammation and fibrosis. Funding: Government Support – Non-U.S.