title: Efficient TGF-β/SMAD signaling in human melanoma cells associated with high c-SKI/SnoN expression.
abstract: BACKGROUND
  SKI and SnoN proteins have been shown to inhibit TGF-β signaling, acting both as transcriptional co-repressors in the cell nucleus, and as sequestrators of SMAD proteins in the cytoplasm. TGF-β, on the other hand, induces rapid, proteasome-mediated, degradation of both proteins. How elevated SKI and SnoN protein levels co-exist with active autocrine TGF-β signaling in cancer cells is yet to be understood.
  
  
  RESULTS
  In this study, we found elevated SKI and SnoN protein levels in a panel of melanoma cell lines, as compared to normal melanocytes. There was no correlation between SKI protein content and the capacity of melanoma cells to invade Matrigel™, to form subcutaneous tumors, or to metastasize to bone after intracardiac inoculation into nude mice. Nor did we find a correlation between SKI expression and histopathological staging of human melanoma. TGF-β induced a rapid and dose-dependent degradation of SKI protein, associated with SMAD3/4 specific transcriptional response and induction of pro-metastatic target genes, partially prevented by pharmacologic blockade of proteasome activity. SKI knockdown in 1205Lu melanoma cells did not alter their invasive capacity or transcriptional responses to TGF-β, and did not allow p21 expression in response to TGF-β or reveal any growth inhibitory activity of TGF-β.
  
  
  CONCLUSIONS
  Despite high expression in melanoma cells, the role of SKI in melanoma remains elusive: SKI does not efficiently interfere with the pro-oncogenic activities of TGF-β, unless stabilized by proteasome blockade. Its highly labile nature makes it an unlikely target for therapeutic intervention.
authors.affiliation: Institut Curie, University Center, Building 110, Orsay, France.
authors.forename: Delphine
authors.initials: D
authors.lastname: Javelaud
authors.forename: Leon
authors.initials: L
authors.lastname: van Kempen
authors.forename: Vasileia I
authors.initials: VI
authors.lastname: Alexaki
authors.forename: Erwan
authors.initials: E
authors.lastname: Le Scolan
authors.forename: Kunxin
authors.initials: K
authors.lastname: Luo
authors.forename: Alain
authors.initials: A
authors.lastname: Mauviel
mesh_terms: D000818:Animals; D002460:Cell Line; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D055785:Gene Knockdown Techniques; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism*; D007976:Leupeptins / Q000494:pharmacology; D008545:Melanoma / Q000378:metabolism / Q000473:pathology*; D051379:Mice; D008819:Mice, Nude; D009361:Neoplasm Invasiveness; D009362:Neoplasm Metastasis; D009368:Neoplasm Transplantation; D061988:Proteasome Inhibitors; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference; D012878:Skin Neoplasms / Q000378:metabolism / Q000473:pathology*; D051785:Smad Proteins / Q000378:metabolism*; D015533:Transcriptional Activation; D016212:Transforming Growth Factor beta / Q000378:metabolism*; D015854:Up-Regulation
chemical_list: C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D004268:DNA-Binding Proteins; D047908:Intracellular Signaling Peptides and Proteins; D007976:Leupeptins; D061988:Proteasome Inhibitors; D011518:Proto-Oncogene Proteins; C060615:SKIL protein, human; D051785:Smad Proteins; D016212:Transforming Growth Factor beta; C060614:SKI protein, human; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde