Smad7 Inhibits Fibrotic Effect of TGF-β on Renal Tubular Epithelial Cells by Blocking Smad2 Activation

Abstract

ABSTRACT. It has been shown that transforming growth factor–β (TGF-β) is a potent mediator in renal fibrosis and that Smad proteins are critical intracellular mediators in TGF-β signaling. It is here reported that TGF-β mediates renal fibrogenesis in tubular epithelial cells (TEC) in association with the activation of Smad2 and that overexpression of Smad7 blocks this fibrotic process. Using a normal rat kidney tubular epithelial cell line (NRK52E), it was determined that TGF-β1 induces Smad2 phosphorylation and nuclear localization in both a dose- and time-dependent manner. The activation of Smad2 was evident at 5 min (20%), peaked at 15 to 30 min (85%), and declined to baseline levels by 2 h (5 to 10%). This was associated with de novo expression of collagens I, III, and IV and the transformation of TEC into a “myofibroblast” phenotype with de novo expression of α-smooth muscle actin (α-SMA) and with the loss of E-cadherin (>50%). To investigate a negative regulatory role of Smad7 in renal fibrosis, the Smad 7 gene was stably transfected and its expression was tightly controlled by doxycycline into NRK52E cells. Overexpression of Smad7 induced by doxycycline results in marked inhibition of TGF-β–induced Smad2 activation (90%↓) with the prevention of collagen synthesis and myofibroblast transformation. Thus, Smad2 activation occurs in the fibrogenic response of TEC to TGF-β, and this process is blocked by overexpression of Smad7. This indicates that Smad signaling is a key pathway of TGF-β–mediated renal fibrosis and suggests that treatments targeting the inactivation of Smad2 by overexpression of Smad7 may provide a new therapeutic strategy for renal fibrosis.

N-Acetyl-Seryl-Aspartyl-Lysyl-Proline Inhibits TGF-β–Mediated Plasminogen Activator Inhibitor-1 Expression via Inhibition of Smad Pathway in Human Mesangial Cells

“…This translocation began at 1 h after treatment, consistent with the result that more than 1 h of preincubation with Ac-SDKP was necessary for its inhibitory effect on Smad2 phosphorylation stimulated by TGF-␤1. In renal tublar epithelial cells, overexpression of Smad7 inhibited TGF-␤-mediated phosphorylation of Smad2, but the late expression of Smad7 induced by TGF-␤ could not overcome the TGF-␤-induced R-Smad activation (46). These data and our present results suggest that the cytoplasmic translocation of Smad7 by Ac-SDKP before TGF-␤ stimulation might be an important in the inhibitory effect of Ac-SDKP on TGF-␤/Smad signal transduction.…”

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

“…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. SMAD7 represents a general antagonist of TGF-β1 and bone morphogenic protein signalling, with reports showing that induction of SMAD7 blocks tubular EMT and the development of fibrotic lesions [48]. The role and regulation of SMAD signalling in regulation of GJ expression and GJIC in the proximal tubule remain to be confirmed; however, it is highly likely that these effects are SMADdependent and subject to regulation via endogenous inhibitors.…”

Recombinant human anti–transforming growth factor β1 antibody therapy in systemic sclerosis: A multicenter, randomized, placebo‐controlled phase I/II trial of CAT‐192

“…Thus, soluble TGF␤ receptors that block all active TGF␤ ligand and the pan-isoform-specific antibody 1D11 are both effective in blocking fibrosis, as is a polyclonal anti-TGF␤ and a recombinant latency-associated peptide (26,27). Other blocking strategies, such as overexpression of Smad7 or decorin, have also been tested in animal models (28).…”