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Differential activation of target cellular promoters by p53 mutants with impaired apoptotic function
R L Ludwig1, S Bates, K H Vousden
1ABL Basic Research Program, National Cancer Institute-Frederick Cancer Research and Developmental Center, Maryland 21702-1201, USA.
Abstract:
The p53 tumor suppressor protein is a sequence-specific transcriptional activator, a function which contributes to cell cycle arrest and apoptosis induced by p53 in appropriate cell types. Analysis of a series of p53 point mutants has revealed the potential for selective loss of the ability to transactivate some, but not all, cellular p53-responsive promoters. p53 175P and p53 181L are tumor-derived p53 point mutants which were previously characterized as transcriptionally active. Both mutants retained the ability to activate expression of the cyclin-dependent kinase inhibitor p2lcip1/waf1, and this activity correlated with the ability to induce a G1 cell cycle arrest. However, an extension of this survey to include other p53 targets showed that p53 175P was defective in the activation of p53-responsive sequences derived from the bax promoter and the insulin-like growth factor-binding protein 3 gene (IGF-BP3) promoter, while p53 181L showed loss of the ability to activate a promoter containing IGF-BP3 box B sequences. Failure to activate transcription was also reflected in the reduced ability of the mutants to bind the p53-responsive DNA sequences present in these promoters. These specific defects in transcriptional activation correlated with the impaired apoptotic function displayed by these mutants, and the results suggest that activation of cell cycle arrest genes by p53 can be separated from activation of genes with a role in mediating the p53 apoptotic response. The cellular response to p53 activation may therefore depend, at least in part, on which group of p53-responsive genes become transcriptionally activated.
Insights
The p53 tumor suppressor protein
Area of Science:
- Molecular Biology
- Cancer Biology
- Genetics
Background:
- The p53 protein acts as a tumor suppressor by regulating genes involved in cell cycle arrest and apoptosis.
- p53's transcriptional activity is crucial for its tumor-suppressive functions.
- Tumor-derived p53 mutants can exhibit altered transcriptional profiles.
Purpose of the Study:
- To investigate the functional consequences of specific p53 point mutations on transcriptional activation of diverse p53 target genes.
- To determine if p53's ability to induce cell cycle arrest can be functionally separated from its role in apoptosis.
- To analyze the impact of mutations on p53's DNA-binding activity.
Main Methods:
- Analysis of p53 point mutants (p53 175P and p53 181L) derived from tumors.
- Assessing the transactivation of p53-responsive promoters, including those from p21, BAX, and IGF-BP3.
- Evaluating the DNA-binding capabilities of p53 mutants to specific promoter sequences.
- Correlating transcriptional activity with cellular responses like cell cycle arrest and apoptosis.
Main Results:
- p53 mutants 175P and 181L retained the ability to activate the p21 promoter, inducing G1 cell cycle arrest.
- p53 175P showed defects in activating BAX and IGF-BP3 promoters, while p53 181L was deficient in activating IGF-BP3 promoter sequences.
- These transcriptional defects correlated with reduced DNA binding and impaired apoptotic function.
- The study demonstrates a functional separation between p53's role in cell cycle arrest and apoptosis induction.
Conclusions:
- Specific p53 mutations can selectively impair the transactivation of apoptosis-related genes while preserving cell cycle arrest functions.
- The cellular outcome of p53 activation depends on the specific set of target genes that are transcriptionally regulated.
- These findings highlight the complex, context-dependent nature of p53's tumor-suppressive activities.