Smad4 Dependency Defines Two Classes of Transforming Growth Factor β (TGF-β) Target Genes and Distinguishes TGF-β-Induced Epithelial-Mesenchymal Transition from Its Antiproliferative and Migratory Responses

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Abstract
In response to transforming growth factor β (TGF-β), Smad4 forms complexes with activated Smad2 and Smad3, which accumulate in the nucleus, where they both positively and negatively regulate TGF-β target genes. Mutation or deletion of Smad4 is found in about 50% of pancreatic tumors and in about 15% of colorectal tumors. As Smad4 is a central component of the TGF-β/Smad pathway, we have determined whether Smad4 is absolutely required for all TGF-β responses, to evaluate the effect of its loss during human tumor development. We have generated cell lines from the immortalized human keratinocyte cell line HaCaT or the pancreatic tumor cell line Colo-357, which stably express a tetracyline-inducible small interfering RNA targeted against Smad4. In response to tetracycline, Smad4 expression is effectively silenced. Large-scale microarray analysis identifies two populations of TGF-β target genes that are distinguished by their dependency on Smad4. Some genes absolutely require Smad4 for their regulation, while others do not. Functional analysis also indicates a differential Smad4 requirement for TGF-β-induced functions; TGF-β-induced cell cycle arrest and migration, but not epithelial-mesenchymal transition, are abolished after silencing of Smad4. Altogether our results suggest that loss of Smad4 might promote TGF-β-mediated tumorigenesis by abolishing tumor-suppressive functions of TGF-β while maintaining some tumor-promoting TGF-β responses.