Increased Smad4‐Mediated Acceleration of TGF‐β1 Signaling and Myofibroblastic Differentiation in Tubulointerstitial Cells in Kidneys of Hereditary Nephrotic Mice

Abstract

ICR‐derived glomerulonephritis (ICGN) mouse is a hereditary model animal for nephrotic syndrome with chronic renal tubulointerstitial fibrosis. In fibrotic disorder, myofibroblastic differentiation plays crucial roles in pathogenesis and is dominantly regulated by the transforming growth factor‐beta1 (TGF‐β1) signaling system. To reveal the relationships between the pathogenic mechanism of the renal fibrosis and TGF‐β1 signaling in ICGN mice, we examined the expression and localization of TGF‐β1 signal transducer proteins, TGF‐β receptor (TGFR)‐I, TGFR‐II, Smad2/3 and Smad4, in kidney sections and in primarily cultured renal tubulointerstitial fibroblasts (TIFs). In kidneys of ICGN mice, many TIFs were differentiated to myofibroblastic cells, confirmed as alpha‐smooth muscle actin (αSMA)‐positive cells, and though no significant increase in production and activation of TGF‐β1 were observed in ICGN‐TIFs as compared with control ICR‐TIFs. The rate of αSMA positive cells in ICGN‐TIFs increased during cell culture but not in those of ICR‐TIFs. In vitro transcriptional reporter assay for TGF‐β1 and Western blotting for TGF‐β1 signal transducers revealed that no notable differences in the expression levels of TGFR‐I, TGFR‐II or Smad2/3 were seen between ICR‐ and ICGN‐TIFs. However, augmented cytoplasmic Smad4 in ICGN‐TIFs was observed, and it may cause hypersensitivity against TGF‐β1. These results indicate that abnormal cytoplasmic augmentation of Smad4 induces acceleration of TGF‐β1 signaling and fibrogenic myofibroblastic differentiation in the renal tubulointerstitial cells of ICGN mice.