title: Inability of transforming growth factor-beta to cause SnoN degradation leads to resistance to transforming growth factor-beta-induced growth arrest in esophageal cancer cells.
abstract: It is well established that loss of a growth inhibitory response to transforming growth factor-beta (TGF-beta) is a common feature of epithelial cancers including esophageal cancer. However, the molecular basis for the abrogation of this key homeostatic mechanism is poorly understood. In esophageal cancer cell lines that are resistant to TGF-beta-induced growth inhibition, TGF-beta also fails to decrease transcription of c-myc despite the presence of functional signaling components. Consequently, to gain a better understanding of the mechanisms leading to resistance to TGF-beta-induced growth arrest, the basis for the inability to decrease c-myc transcription was investigated. Regardless of sensitivity to TGF-beta-induced growth arrest, TGF-beta enhanced the ability of Smad3-protein complexes to bind c-myc regulatory elements. However, in a growth inhibition-resistant esophageal cancer cell line, the Smad3-protein complexes contained the SnoN oncoprotein. Furthermore, in esophageal cancer cell lines that are resistant to TGF-beta-induced growth arrest, TGF-beta does not cause degradation of SnoN. Analyses of the effect of modulating SnoN expression in both growth inhibition-sensitive and growth inhibition-resistant cell lines showed that degradation of SnoN is a prerequisite for both TGF-beta-induced repression of c-myc transcription and growth arrest. The data indicate that SnoN-Smad3 complexes do not cause repression of c-myc transcription but rather prevent functionality of active repressor complexes. Thus, these studies reveal a novel mechanism for resistance to TGF-beta-induced growth inhibition in esophageal cancer, namely the failure to degrade SnoN. In addition, they show that SnoN can block TGF-beta repression of gene transcription.
authors.affiliation: Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond 23298-0678, USA.
authors.forename: Jeffery S
authors.initials: JS
authors.lastname: Edmiston
authors.forename: W Andrew
authors.initials: WA
authors.lastname: Yeudall
authors.forename: Theodore D
authors.initials: TD
authors.lastname: Chung
authors.forename: Deborah A
authors.initials: DA
authors.lastname: Lebman
mesh_terms: D000230:Adenocarcinoma / Q000378:metabolism / Q000473:pathology; D002294:Carcinoma, Squamous Cell / Q000378:metabolism / Q000473:pathology; D048708:Cell Growth Processes / Q000187:drug effects / Q000502:physiology; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000378:metabolism; D004938:Esophageal Neoplasms / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D016259:Genes, myc; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000096:biosynthesis / Q000235:genetics; D051900:Smad3 Protein; D015534:Trans-Activators / Q000378:metabolism; D014158:Transcription, Genetic; D016212:Transforming Growth Factor beta / Q000037:antagonists & inhibitors / Q000494:pharmacology*
chemical_list: D004268:DNA-Binding Proteins; D047908:Intracellular Signaling Peptides and Proteins; C489427:MYC protein, human; D011518:Proto-Oncogene Proteins; D016271:Proto-Oncogene Proteins c-myc; C060615:SKIL protein, human; C497790:SMAD3 protein, human; D051900:Smad3 Protein; D015534:Trans-Activators; D016212:Transforming Growth Factor beta