G(1) cell cycle arrest and apoptosis induction by nuclear Smad4/Dpc4: Phenotypes reversed by a tumorigenic mutation

The tumor suppressor Smad4/Dpc4 is a transcription activator that binds spe cific DNA sequences end whose nuclear localization is induced after exposur e to type beta transforming growth factor-like cytokines. We explored an in ducible system in which Smad4 protein is activated by translocation to the nucleus when cell lines that stably express wild-type or mutant Smad4 prote ins fused to a murine estrogen receptor domain are treated with 4-hydroxyta moxifen. This induced Smad4-mediated transcriptional activation and a decre ase in growth rate, attributable to a cell cycle arrest at the G(1) phase a nd an induction of apoptosis. A tumor-derived mutation (Arg-100 --> Thr) af fecting a residue critical for DNA-binding demonstrated an "oncogenic" phen otype, having decreases in both the G(1) fraction and apoptosis and, conseq uently, an augmentation of population growth. This model should be useful i n the exploration and control of components that lie further downstream in the Smad4 tumor-suppressor pathway.