TGF-beta1 is a profibrotic
cytokine that plays a central role in the onset and progression of
chronic renal diseases. The activity of
TGF-beta1 is tightly controlled by multiple mechanisms, in which antagonizing Smad-mediated gene transcription by
co-repressors is an important regulatory component. This study examined the expression of Smad transcriptional
co-repressors in the fibrotic kidney and investigated their potential functions in controlling
TGF-beta1 response. Western blot analysis demonstrated that the
protein levels of Smad transcriptional
co-repressors SnoN and Ski were progressively reduced in a time-dependent manner in the fibrotic kidney induced by unilateral
ureteral obstruction in mice, whereas renal Smad abundance was relatively unaltered. Consistently, SnoN and Ski staining was diminished in the nuclei of renal tubular epithelium and interstitium after obstructive injury. In vitro, knockdown of SnoN expression by RNA interference in tubular epithelial cells dramatically sensitized their responsiveness to
TGF-beta1 stimulation. Conversely, ectopic expression of exogenous SnoN or Ski after transfection conferred tubular epithelial cell resistance to TGF-beta1-induced epithelial to myofibroblast transition. Both SnoN and Ski could block Smad-mediated activation of TGF-beta1-responsive promoter and exhibited additive effect in abrogating the profibrotic actions of
TGF-beta1. These results indicate that as a result of loss of Smad transcriptional
co-repressors, the profibrotic
TGF-beta1 signaling in diseased kidney is markedly amplified in a magnitude much greater than previously thought. Therefore, new strategy aimed to increase Smad transcriptional
co-repressors expression may be effective in antagonizing
TGF-beta1 signaling and thereby blocking the progression of chronic renal
fibrosis.