Overexpression of the tumor suppressor gene Smad4/DPC4 induces p21(waf1) expression and growth inhibition in human carcinoma cells

The Smad4/DPC4 protein functions as a key transcription factor in transforming growth factor β (TGF-β) signaling pathways. However, the downstream target genes regulated by Smad4/DPC4 have not been identified until now. We previously demonstrated that the loss of TGF-β-induced p21(waf1) expression and growth inhibition correlates with inactivation of the Smad4/DPC4 gene. Now we show that transient overexpression of Smad4/DPC4 can induce p21(waf1) expression, specific Smad4 DNA binding activity, SBE4- luc reporter gene activity, and subsequent growth inhibition in Smad4/DPC4- null cells and other carcinoma cells in the presence or absence of TGF-β. Taken together, these data show that p21(waf1) is one of the Smad4/DPC4- regulated downstream target genes and suggest that overexpression of the Smad4/DPC4 gene can bypass TGF-β receptor activation and reestablish one of the key regulatory controls of cell proliferation.