Augmented cytoplasmic Smad4 induces acceleration of TGF-β1 signaling in renal tubulointerstitial cells of hereditary nephrotic ICGN mice with chronic renal fibrosis; possible role for myofibroblastic differentiation

The Institute of Cancer Research (ICR)-derived glomerulonephritis (ICGN) mouse is a hereditary model animal for nephrotic syndrome with chronic renal tubulointerstitial fibrosis. In most fibrotic diseases, myofibroblastic differentiation is considered to play crucial roles in pathogenesis of fibrosis and is dominantly regulated by the transforming growth factor (TGF)-β1 signaling system. To reveal the pathogenic mechanism of chronic renal fibrosis in ICGN mice, we examined the expression and localization of TGF-β1 signal transducer proteins (TGF-β receptor-I and -II, Smad2/3 and Smad4) in kidney sections and in primarily cultured tubulointerstitial fibroblasts (TIFs). In kidneys of ICGN mice, many tubulointerstitial cells were differentiated to myofibroblastic cells and were α-smooth muscle actin (αSMA)-positive. The numbers of αSMA-positive TIFs prepared from kidneys of ICGN mice (ICGN-TIFs), but not those of ICR control mice (ICR-TIFs), increased during cell culture. No significant differences in production or activation of TGF-β1 between ICGN-TIFs and ICR-TIFs were seen by enzyme-linked immunosorbent assay. In vitro transcriptional reporter assay for TGF-β1 and Western immunoblotting for TGF-β1 signal transducers showed no notable differences in the expression levels of TGF-β receptor-I or -II or Smad2/3 between these TIFs. However, augumented cytoplasmic Smad4 protein in ICGN-TIFs, but not ICR-TIFs, seemed to cause hypersensitivity against TGF-β1, and the eventual nuclear localization of Smad2/3-Smad4 complex was increased in ICGN-TIFs. Thus, the abnormal cytoplasmic augmentation of Smad4 induces acceleration of TGF-β1 signaling in the renal tubulointerstitial cells of ICGN mice.