Down-regulation of Smad7 expression by ubiquitin-dependent degradation contributes to renal fibrosis in obstructive nephropathy in mice

Abstract

Overexpression of transforming growth factor ␤ (TGF-␤) has been shown to play pathogenic roles in progression of renal fibrosis, and the severity of tubulointerstitial fibrosis correlates better with renal function than the severity of glomerulosclerosis. Smad proteins are signaling transducers downstream from TGF-␤ receptors. Three families of Smad proteins have been identified: receptorregulated Smad2 and Smad3, common partner Smad4, and inhibitory Smad7 (part of a negative-feedback loop). We investigated Smad-mediated TGF-␤ signaling pathway and regulatory mechanisms of inhibitory Smad7 in unilateral ureteral obstruction (UUO) kidneys in mice, a model of progressive tubulointerstitial fibrosis. Compared with sham-operated kidneys, the level of Smad7 protein, but not mRNA, decreased progressively in UUO kidneys, whereas immunoreactivity for nuclear phosphorylated Smad2 and Smad3 and renal fibrosis were inversely increased. Furthermore, we demonstrated that both the degradation and ubiquitination activity of Smad7 protein were increased markedly in UUO kidneys compared with sham-operated ones. We also found that both Smurf1 and Smurf2 (Smad ubiquitination regulatory factors), which are E3 ubiquitin ligases for Smad7, were increased and that they interacted with Smad7 in UUO kidneys. Our results suggest that the reduction of Smad7 protein resulting from enhanced ubiquitin-dependent degradation plays a pathogenic role in progression of tubulointerstitial fibrosis.transforming growth factor ␤ ͉ Smad proteins ͉ tubulointerstitial fibrosis

Low-dose paclitaxel ameliorates renal fibrosis in rat UUO model by inhibition of TGF-β/Smad activity

“…Recent reports indicate that Smad7 is selectively decreased, whereas phosphorylation of Smad2 and Smad3 is increased in UUO model. 12 Furthermore, it has been previously shown in several different cell lines that microtubules serve as negative regulator for TGF-b/Smad signaling by forming a complex with endogenous Smad2, Smad3 and Smad4, thus sequestering the R-Smads away from the TGF-b receptor. 11 Therefore, it is conceivable that stabilization of microtubules by low-dose paclitaxel can dampen the exacerbated TGF-b signaling as reported in TGF-b-induced inhibition of myogenesis in C2C12 myoblasts.…”

“…11 Also, there are certain literature reports that indicate aberrant ECM deposition in unilateral ureteral obstruction (UUO) model may be due to selective decrease of inhibitory Smad7 and increase of R-Smad3. 12 The aim of this study was to assess if microtubule stabilization with low-dose paclitaxel (Taxol) could inhibit TGF-b/Smad signaling and ameliorate renal fibrosis in the UUO model.…”

Regulating the stability of TGFβ receptors and Smads

“…The high Smad7 levels in pro-inflammatory immune cells from such patients are due to defective ubiquitination and degradation of Smad7 npg that correlates with its enhanced acetylation by p300. In a mouse model of kidney fibrosis the inverse scenario prevails, namely Smad7 levels are very low while Smurf1 and Smurf2 levels and ubiquitination activities are high, which possibly plays causative roles in the progression of renal fibrosis caused by ureteral obstruction [115]. More recent evidence points to Arkadia as the primary mediator of Smad7 downregulation in the fibrotic kidney, a mechanism that also leads to the process of epithelialmesenchymal transition of kidney epithelial cells to contractile myofibroblast-like cells [116,117].…”

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

“…[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.…”

“…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.…”