Repression of p53-mediated transcription by MDM2: a dual mechanism

The mammalian transcriptional repressor RBP (CBF1) targets TFIID and TFIIA to prevent activated transcription

RBP is a cellular protein that functions as a transcriptional repressor in mammalian cells. RBP has elicited great interest lately because of its established roles in regulating gene expression, in Drosophila and mouse development, and as a component of the Notch signal transduction pathway. This report focuses on the mechanism by which RBP represses transcription and thereby regulates expression of a relatively simple, but natural, promoter. The results show that, irrespective of the close proximity between RBP and other transcription factors bound to the promoter, RBP does not occlude binding by these other transcription factors. Instead, RBP interacts with two transcriptional coactivators: dTAFII110, a subunit of TFIID, and TFIIA to repress transcription. The domain of dTAFII110 targeted by RBP is the same domain that interacts with TFIIA, but is disparate from the domain that interacts with Sp1. Repression can be thwarted when stable transcription preinitiation complexes are formed before RBP addition, suggesting that RBP interaction with TFIIA and TFIID perturbs optimal interactions between these coactivators. Consistent with this, interaction between RBP and TFIIA precludes interaction with dTAFII110. This is the first report of a repressor specifically targeting these two coactivators to subvert activated transcription.

Cellular responses to DNA damage

The exposure of cells to DNA damage inducers triggers a wide range of cellular responses including an alteration in gene expression, a delay in cell-cycle progression and the stimulation of DNA repair. In multicellular organisms, DNA damage can also activate programmed cell death. Recently, several signaling pathways that link DNA damage to gene expression and to the cell-cycle checkpoints have been identified. These pathways establish a framework for future studies of DNA damage responses.

Adenovirus E1B 55K represses p53 activation in vitro

Adenovirus E1B 55K protein cooperates with E1A gene products to induce cell transformation. E1B 55K mediates its effects by binding to and inhibiting the transcriptional activation and growth-suppression functions of the tumor suppressor p53. Previous studies in vivo have suggested that E1B 55K has an active role in repressing p53 transcriptional activation and that this repression function is directed to specific promoters through E1B 55K's interaction with DNA-bound p53. Flag-tagged E1B 55K (e55K) was expressed with the baculovirus expression system and immunopurified. Gel filtration, velocity sedimentation centrifugation, and glutaraldehyde cross-linking indicated that e55K is a dimer with a nonglobular conformation. e55K bound directly to purified p53, causing an approximately 10-fold increase in p53 affinity for tandem binding sites. Using in vitro transcription assays reconstituted with purified p53, e55K, and HeLa cell nuclear extracts, we found that e55K specifically repressed p53 activation. These results demonstrate that as postulated from earlier transient expression experiments, E1B 55K is a specific repressor of transcription from a promoter with bound p53. Since HeLa nuclear extracts contain little detectable histone protein, E1B 55K probably represses transcription through direct or indirect interactions with the RNA polymerase II transcription machinery.

ARF promotes MDM2 degradation and stabilizes p53: ARF-INK4a locus deletion impairs both the Rb and p53 tumor suppression pathways

The INK4a-ARF locus encodes two unrelated proteins that both function in tumor suppression. p16INK4 binds to and inhibits the activity of CDK4 and CDK6, and ARF arrests the cell cycle in a p53-dependent manner. We show here that ARF binds to MDM2 and promotes the rapid degradation of MDM2. This interaction is mediated by the exon 1beta-encoded N-terminal domain of ARF and a C-terminal region of MDM2. ARF-promoted MDM2 degradation is associated with MDM2 modification and concurrent p53 stabilization and accumulation. The functional consequence of ARF-regulated p53 levels via MDM2 proteolysis is evidenced by the ability of ectopically expressed ARF to restore a p53-imposed G1 cell cycle arrest that is otherwise abrogated by MDM2. Thus, deletion of the ARF-INK4a locus simultaneously impairs both the INK4a-cyclin D/CDK4-RB and the ARF-MDM2-p53 pathways.

The Ink4a tumor suppressor gene product, p19Arf, interacts with MDM2 and neutralizes MDM2's inhibition of p53

The INK4a gene encodes two distinct growth inhibitors--the cyclin-dependent kinase inhibitor p16Ink4a, which is a component of the Rb pathway, and the tumor suppressor p19Arf, which has been functionally linked to p53. Here we show that p19Arf potently suppresses oncogenic transformation in primary cells and that this function is abrogated when p53 is neutralized by viral oncoproteins and dominant-negative mutants but not by the p53 antagonist MDM2. This finding, coupled with the observations that p19Arf and MDM2 physically interact and that p19Rrf blocks MDM2-induced p53 degradation and transactivational silencing, suggests that p19Arf functions mechanistically to prevent MDM2's neutralization of p53. Together, our findings ascribe INK4a's potent tumor suppressor activity to the cooperative actions of its two protein products and their relation to the two central growth control pathways, Rb and p53.

BRCA1 regulates p53-dependent gene expression

Mutations in BRCA1 are present in 45% of families that segregate with susceptibility for breast cancer and in 80-90% of families with both breast and ovarian cancer. Here we report that BRCA1 stimulates artificial and genomic promoter constructs containing p53-responsive elements. This activity of BRCA1 depends on the presence of wild-type p53, which was shown by using mouse fibroblasts expressing temperature-sensitive forms of p53, or p53(+/+) and p53(-/-) fibroblasts obtained from p53 knockout mice. Furthermore, mutant forms of BRCA1 lacking the C-terminal second BRCA1 C-terminal (BRCT) domain showed reduced p53-mediated transcriptional activation. Finally, we found that BRCA1 coimmunoprecipitates with p53, in vitro and in vivo. These findings suggest a function of BRCA1 as a p53 coactivator.

The caspase-RB connection in cell death

Execution of the cell-death programme requires the activation of a family of cysteine proteases known as caspases. Specific cellular proteins are cleaved by caspases during apoptosis, including the retinoblastoma tumour-suppressor protein (RB1). A caspase-resistant RB1 can attenuate the death response to tumour necrosis factor alpha. The cleavage of RB1 during cell death, together with the increased cell death during embryonic development of Rb-knockout mice, suggests that RB1 degradation contributes to the activation of the cell-death pathway.

Li-Fraumeni syndrome and the role of the p53 tumor suppressor gene in cancer susceptibility

Mutation of the tumor suppressor gene p53 is a molecular genetic event frequently observed in human cancer, and inactivating missense mutations usually are accompanied by the resultant overexpression of mutant p53 protein. In gynecologic cancers, p53 is also often altered; the frequency varies depending on types of cancers and where they develop. Further, human papillomavirus oncoproteins that inactivate p53 and Rb proteins play important roles in the development of several gynecologic cancers. Individuals who are heterozygous for germline mutations of the p53 gene are strongly predisposed to a variety of cancers. The identification of these individuals may have profound value in the future when therapies or chemopreventive agents specific for the p53 alteration are available. The role of p53 tumor suppressor gene in gynecologic cancers and heritable cancer susceptibility syndromes including Li-Fraumeni and Lynch II syndromes is an active and important area of study.

CDF-1-mediated repression of cell cycle genes targets a specific subset of transactivators

The cdc25C , cyclin A and cdc2 genes are regulated during the cell cycle through two contiguous repressor binding sites, the CDE and CHR, located in the region of transcription initiation and interacting with a factor termed CDF-1. The target of this repression seems to be transcriptional activation of these promoters by transcription factors bound upstream. The majority of these factors falls into the class of glutamine-rich activators, suggesting that CDF-1-mediated repression might be activation domain specific. In the present study we have used chimeric promoter constructs to demonstrate that the cdc25C UAS, but not the core promoter, is crucial for repression. In addition, we show that only specific transcription factors and activation domains are responsive to CDE-CHR-mediated cell cycle regulation. These observations clearly indicate that CDF-1 interferes with activation of transcription by a specific subset of transactivators. The repressible activation domains belong to the same class of glutamine-rich activators, pointing to specific interactions of CDF-1 with components of the transcription machinery. In agreement with this conclusion we find that a simple inversion of the CDE-CHR module completely abrogates cell cycle-regulated repression.