Smad ubiquitination regulatory factor-2 in the fibrotic kidney: regulation target specificity and functional implication

摘要

Smad ubiquitination regulatory factor-2 (Smurf2) is an E3 ubiqutin ligase that plays a pivotal role in regulating the TGF-β signaling via selectively targeting key components of Smad pathway for degradation. In this study, we have investigated the regulation of Smurf2 expression, its target specificity and the functional implication of its induction in the fibrotic kidney. Immunohistochemical staining revealed that Smurf2 was upregulated specifically in renal tubules of kidney biopsies from patients with various nephropathies. In vitro, Smurf2 mRNA and protein were induced in human proximal tubular epithelial cells (HKC-8) upon TGF-β1 stimulation. Ectopic expression of Smurf2 was sufficient to reduce the steady-state levels of Smad2, but not Smad1, Smad3, Smad4 and Smad7, in HKC-8 cells. Interestingly, Smurf2 was also able to down-regulate the Smad transcriptional corepressors SnoN, Ski and TGIF. Inhibition of the proteasomal pathway prevented Smurf2-mediated down-regulation of Smad2 and Smad corepressors. Functionally, over-expression of Smurf2 enhanced the transcription of TGF-β-responsive promoter and augmented the TGF-β1-mediated E-cadherin suppression, as well as fibronectin and type I collagen induction in HKC-8 cells. These results indicate that Smurf2 specifically targets both positive and negative Smad regulators for destruction in tubular epithelial cells, thereby providing a complex fine-tuning of the TGF-β signaling. It appears that dysregulation of Smurf2 could contribute to an aberrant TGF-β/Smad signaling in the pathogenesis of kidney fibrosis.