Smad ubiquitination regulatory factor-2 in the fibrotic kidney: regulation, target specificity, and functional implication.
Smad ubiquitination regulatory factor-2 (Smurf2) is an E3 ubiqutin ligase that plays a pivotal role in regulating TGF-beta signaling via selectively targeting key components of the 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-beta1 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 downregulate the Smad transcriptional corepressors Ski, SnoN, and TG-interacting factor. Inhibition of the proteasomal pathway prevented Smurf2-mediated downregulation of Smad2 and Smad corepressors. Functionally, overexpression of Smurf2 enhanced the transcription of the TGF-beta-responsive promoter and augmented TGF-beta1-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 TGF-beta signaling. It appears that dysregulation of Smurf2 could contribute to an aberrant TGF-beta/Smad signaling in the pathogenesis of kidney fibrosis.
Tan
Ruoyun
R
Department of Medicine, The Second Affiliated Hospital, Nanjing Medical University, Nanjing, China.
He
Weichun
W
Lin
Xia
X
Kiss
Lawrence P
LP
Liu
Youhua
Y
D015153:Blotting, Western; D002478:Cells, Cultured; D002908:Chronic Disease; D005353:Fibronectins / Q000096:biosynthesis / Q000235:genetics; D005355:Fibrosis; D005786:Gene Expression Regulation / Q000502:physiology; D017930:Genes, Reporter / Q000235:genetics; D006801:Humans; D007150:Immunohistochemistry; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D007674:Kidney Diseases / Q000235:genetics* / Q000503:physiopathology*; D008156:Luciferases / Q000235:genetics; D010957:Plasmids / Q000235:genetics; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D012313:RNA / Q000096:biosynthesis / Q000235:genetics; D034741:RNA, Small Interfering / Q000494:pharmacology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D051785:Smad Proteins / Q000378:metabolism; D014162:Transfection; D016212:Transforming Growth Factor beta / Q000096:biosynthesis / Q000235:genetics; D053773:Transforming Growth Factor beta1 / Q000096:biosynthesis / Q000235:genetics; D044767:Ubiquitin-Protein Ligases / Q000096:biosynthesis* / Q000235:genetics / Q000502:physiology*
D005353:Fibronectins; D047908:Intracellular Signaling Peptides and Proteins; D011518:Proto-Oncogene Proteins; D034741:RNA, Small Interfering; C060615:SKIL protein, human; D051785:Smad Proteins; D016212:Transforming Growth Factor beta; D053773:Transforming Growth Factor beta1; D012313:RNA; D008156:Luciferases; C471641:SMURF2 protein, human; D044767:Ubiquitin-Protein Ligases