Downregulation of SnoN Expression in Obstructive Nephropathy Is Mediated by an Enhanced Ubiquitin-Dependent Degradation

Abstract

Smad transcriptional co-repressor SnoN acts as an antagonist that tightly controls the trans-activation of TGF-␤/Smad target genes. SnoN protein is reduced progressively in the fibrotic kidney after obstructive injury, suggesting that the loss of Smad antagonist is a critical event that leads to an uncontrolled fibrogenic signaling. However, the mechanism underlying SnoN downregulation remains unknown. This study investigated the regulation and mechanism of renal SnoN expression in vivo. Whereas SnoN protein was markedly diminished, its mRNA levels remained relatively constant in the obstructed kidney after ureteral ligation. An increased ubiquitination and proteasome-dependent degradation of SnoN was found in obstructed kidney, compared with sham controls. Smad ubiquitination regulatory factor-2, an E3 ubiquitin ligase, was induced and formed a complex with SnoN in vivo. In vitro, TGF-␤1 promoted SnoN protein degradation, which was mediated by ubiquitination and a proteasome-dependent mechanism. SnoN constitutively interacted with another Smad co-repressor, Ski, and they formed ternary complex with Smad2/3 upon TGF-␤1 stimulation. However, ectopic expression of Ski did not alter the degradation rate of SnoN. Blockage of SnoN degradation by proteasome inhibitor abolished TGF-␤1-mediated ␣-smooth muscle actin and fibronectin induction, suggesting that SnoN degradation could be necessary for TGF-␤1 to exert its fibrogenic action. Furthermore, knockdown of Smad ubiquitination regulatory factor-2 expression by small interfering RNA strategy led to an increase in SnoN abundance and inhibited the TGF-␤1-mediated gene transcription. These results indicate that downregulation of SnoN expression in the obstructed kidney is mediated by an enhanced ubiquitin-dependent degradation. Preservation of SnoN by inhibiting its degradation may be a novel strategy for targeting hyperactive Smad signaling in renal fibrotic diseases.

TGFβ modulates cell-to-cell communication in early epithelial-to-mesenchymal transition

“…The hyperactive SMAD-signalling observed in certain types of renal disease reflects aberrant levels of both SMAD co-repressors and their subsequent regulators [47]. The inhibitory SMADs (SMAD6 and SMAD7) inhibit receptorregulated SMAD phosphorylation by blocking their access to TBRI, and/or by promoting degradation of the receptor complexes.…”

The role of TGF-β and epithelial-to mesenchymal transition in diabetic nephropathy

“…Indeed, a safeguard mechanism exists in the form of inhibitory Smads and transcriptional co-repressors [53]. However, the level of hyperactive Smad signalling observed in certain types of renal disease reflects abberant levels of both Smad corepressors and their subsequent regulators [54][55]. The inhibitory Smads (Smad6 and Smad7) inhibit The co-repressors SnoN (Ski-related novel gene, non Alu-containing), Ski (Sloan-Kettering…”

Effects of Arkadia on airway remodeling through enhancing TGF-β signaling in allergic rats

“…Arkadia and Smurf2, two E3 ubiqutin ligases, have a pivotal role in regulating TGF-b signaling through selectively targeting key components of the Smad pathway for degradation. 11,12,[20][21][22][23] Smurf2 is known to induce the degradation of TGF-b receptors through interaction with Smad7, which enhances the inhibitory effect of Smad7 on TGF-b signaling. 19,22 In addition, Lin et al 25 reported that Smurf2-regulated TGF-b signaling by potently reducing the transcriptional activity of Smad2.…”

“…3 In the process, negative regulators such as Smad7, SnoN and Ski can downregulate TGF-b signaling through multiple mechanisms. 3,4 Accumulative evidence indicates that the expression of Smad7, SnoN and Ski proteins in some fibrotic models is obviously lower than that in the normal controls, [5][6][7][8][9][10][11][12][13] and also shows that upregulation of the expression of Smad7, SnoN or Ski proteins reverses or blocks fibrogenesis. [14][15][16][17] Nakao et al 18 reported that Smad7 immunoreactivity, which was present mainly in bronchial epithelial cells, was markedly decreased in asthmatic patients, compared with that of control subjects.…”

“…[11][12][13][19][20][21][22][23] Whereas Fukasawa et al 6,13 reported that Smurf2 mediated the degradation of Smad7, SnoN and Ski in a mouse model of renal tubulointersitial fibrosis, Liu et al 7,24 have shown that the reduction of Smad7 in a rat model of renal tubulointersitial fibrosis and normal human renal tubular epithelial cells may be mainly attributed to Arkadia, but not to Smurf2. Thus, how Arkadia and Smurf2 affect Smad7, SnoN and Ski in organ fibrosis remains to be clarified.…”