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G1 cell cycle arrest and apoptosis induction by nuclear Smad4/Dpc4: phenotypes reversed by a tumorigenic mutation
J L Dai1, R K Bansal, S E Kern
1Departments of Oncology and Pathology, The Johns Hopkins Medical Institutions, Baltimore, MD 21205, USA.
Abstract:
The tumor suppressor Smad4/Dpc4 is a transcription activator that binds specific DNA sequences and whose nuclear localization is induced after exposure to type beta transforming growth factor-like cytokines. We explored an inducible system in which Smad4 protein is activated by translocation to the nucleus when cell lines that stably express wild-type or mutant Smad4 proteins fused to a murine estrogen receptor domain are treated with 4-hydroxytamoxifen. This induced Smad4-mediated transcriptional activation and a decrease in growth rate, attributable to a cell cycle arrest at the G1 phase and an induction of apoptosis. A tumor-derived mutation (Arg-100 --> Thr) affecting a residue critical for DNA-binding demonstrated an "oncogenic" phenotype, having decreases in both the G1 fraction and apoptosis and, consequently, an augmentation of population growth. This model should be useful in the exploration and control of components that lie further downstream in the Smad4 tumor-suppressor pathway.
Insights
The tumor suppressor Smad4 protein, when activated, halts cell growth and induces apoptosis. A specific mutation in Smad4 can reverse this tumor-suppressive function, promoting cancer growth.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- Smad4 (also known as Dpc4) is a crucial tumor suppressor protein.
- Smad4 acts as a transcription activator, regulating gene expression.
- Nuclear localization of Smad4 is induced by TGF-beta-like cytokines.
Purpose of the Study:
- To explore an inducible system for Smad4 activation using a fusion protein with a murine estrogen receptor domain.
- To investigate the effects of Smad4 activation on cell cycle and apoptosis.
- To analyze the oncogenic potential of a specific Smad4 mutation.
Main Methods:
- Utilized cell lines stably expressing wild-type or mutant Smad4 fused to a murine estrogen receptor domain.
- Activated Smad4 by treating cells with 4-hydroxytamoxifen, inducing nuclear translocation.
- Assessed cell growth rate, cell cycle progression (G1 phase), and apoptosis.
Main Results:
- Induced Smad4-mediated transcriptional activation.
- Observed a decrease in cell growth rate due to G1 cell cycle arrest and induced apoptosis.
- A tumor-derived mutation (Arg-100 --> Thr) in a DNA-binding residue conferred an oncogenic phenotype, reducing G1 arrest and apoptosis, thereby increasing population growth.
Conclusions:
- The inducible Smad4 system effectively demonstrates its tumor-suppressive functions.
- The identified Smad4 mutation highlights its critical role in tumor suppression and suggests oncogenic potential.
- This model provides a valuable tool for studying downstream components of the Smad4 tumor suppressor pathway.