P53-mediated Repression Research Articles

Down regulation of bcl‐2 by p53 in nasopharyngeal carcinoma and lack of detection of its specific t(14;18) chromosomal translocation in fixed tissues

High levels of bcl-2 protein have been found in a wide variety of human cancers. Since p53 gene inactivation occurs in over half of human cancers, it is possible that loss of p53-mediated repression of bcl-2 gene expression accounts, at least in part, for the frequent abnormalities in bcl-2 protein production seen in tumours. By using immunohistochemical methods, we have analysed thirty-three nasopharyngeal carcinomas for p53 and bcl-2 expression. We found an inverse correlation between the expression of these two proteins (P < 0.001). Moreover, we utilized universal oligonucleotide primers of a region 5' to the bcl-2 MBR and at the 3' end of JH segments to initiate a DNA polymerase chain reaction that amplified these bcl-2-JH junctures. Of the twelve nasopharyngeal carcinomas expressing bcl-2, none showed a t(14;18) chromosome translocation. These findings may indicate potential mechanisms by which bcl-2 regulates apoptosis.

P53-dependent and -independent transactivation by the E6 protein of human papillomavirus type 16

The mechanism by which the E6 protein of human papillomavirus type 16 (HPV-16) transactivates heterologous virus promoters has not been established. In this study, the involvement of p53-mediated transcriptional repression in transactivation by the HPV-16 E6 protein was examined using several virus promoters. HPV-16 E6 transactivated the TATA box-containing simian virus 40 early promoter and the Rous sarcoma virus long terminal repeat in p53-containing cells but not in p53-deficient cells. In contrast, the adenovirus E2 promoter was transactivated both in p53-containing and p53-deficient cells. These results indicate that the transactivation activity of the HPV-16 E6 protein is mediated by p53-dependent and promoter-specific p53-independent pathways.

Modulation of p53-mediated transcriptional repression and apoptosis by the adenovirus E1B 19K protein.

BRK cell lines that stably express adenovirus E1A and a murine temperature-sensitive p53 undergo apoptosis when p53 assumes the wild-type conformation. Expression of the E1B 19,000-molecular-weight (19K) protein rescues cells from this p53-mediated apoptosis and diverts cells to a growth-arrested state. As p53 likely functions as a tumor suppressor by regulating transcription, the ability of the E1B 19K protein to regulate p53-mediated transactivation and transcriptional repression was investigated. In promoter-reporter assays the E1B 19K did not block p53-mediated transactivation but did alleviate p53-mediated transcriptional repression. E1B 19K expression permitted efficient transcriptional activation of the p21/WAF-1/cip-1 mRNA by p53, consistent with maintenance of the growth arrest function of p53. The E1B 19K protein is thereby unique among DNA virus-transforming proteins that target p53 for inactivation in that it selectively modulates the transcriptional properties of p53. The E1B 19K protein also rescued cells from apoptosis induced by inhibitors of transcription and protein synthesis. This suggests that cell death may result from the inhibition of expression of survival factors which function to maintain cell viability. p53 may induce apoptosis through generalized transcriptional repression. In turn, the E1B 19K protein may prevent p53-mediated apoptosis by alleviating p53-mediated transcriptional repression.

Two domains of p53 interact with the TATA-binding protein, and the adenovirus 13S E1A protein disrupts the association, relieving p53-mediated transcriptional repression.

The tumor suppressor gene product p53 can activate and repress transcription. Both transcriptional activation and repression are thought to involve the direct interaction of p53 with the basal transcriptional machinery. Previous work has demonstrated an in vitro interaction between p53 and the TATA-binding protein that requires amino acids 20 to 57 of p53 and amino acids 220 to 271 of the TATA-binding protein. The present results show that a 75-amino-acid segment from the carboxy terminus of p53 also can bind to the TATA-binding protein in vitro, and this interaction requires amino acids 217 to 268 of the TATA-binding protein, essentially the same domain that is required for interaction with the amino-terminal domain of p53. A carboxy-terminal segment of p53 can mediate repression when bound to DNA as a GAL4-p53 fusion protein. The amino- and carboxy-terminal p53 interactions occur within the domain on the TATA-binding protein to which the adenovirus 13S E1A oncoprotein has previously been shown to bind. The 13S E1A oncoprotein can dissociate the complex formed between the carboxy-terminal domain of p53 and the TATA-binding protein and relieve p53-mediated transcriptional repression. These results demonstrate that two independent domains of p53 can potentially interact with the TATA-binding protein, and they define a mechanism--relief of repression--by which the 13S E1A oncoprotein can activate transcription through the TATA motif.

Relief of p53-mediated transcriptional repression by the adenovirus E1B 19-kDa protein or the cellular Bcl-2 protein.

The p53 tumor suppressor gene product is a transcriptional regulatory protein. It activates transcription from promoters that contain a p53 DNA binding site but represses many promoters that lack its binding site. High-level expression of wild-type p53 can induce apoptosis in certain cell types, and this activity can be blocked by the adenovirus E1B 19-kDa oncoprotein or by the cellular Bcl-2 oncoprotein. Here we report that p53-mediated repression of promoters that lack a p53 binding site is abrogated by the E1B 19-kDa protein or Bcl-2 oncoprotein. In contrast, transcriptional activation by p53 still occurs in the presence of either protein. The fact that two oncoproteins capable of preventing p53-mediated apoptosis also block transcriptional repression by p53 raises the possibility that p53 might induce apoptosis, at least in part, by repressing transcription.

Human papillomavirus E6 proteins bind p53 in vivo and abrogate p53-mediated repression of transcription.

The transforming proteins of DNA tumor viruses SV40, adenovirus and human papillomaviruses (HPV) bind the retinoblastoma and p53 cell cycle regulatory proteins. While the binding of SV40 large T antigen and the adenovirus E1B 55 kDa protein results in the stabilization of the p53 protein, the binding of HPV16 and 18 E6 results in enhanced degradation in vitro. To explore the effect of viral proteins on p53 stability in vivo, we have examined cell lines immortalized in tissue culture by HPV18 E6 and E7 or SV40 large T antigen, as well as cell lines derived from cervical neoplasias. The half-life of the p53 protein in non-transformed human foreskin keratinocytes in culture was found to be approximately 3 h while in cell lines immortalized by E6 and E7, p53 protein half-lives ranged from 2.8 h to less than 1 h. Since equivalent levels of E6 were found in these cells, the range in p53 levels observed was not a result of variability in amounts of E6. In keratinocyte lines immortalized by E7 alone, the p53 half-life was found to be similar to that in non-transformed cells; however, it decreased to approximately 1 h following supertransfection of an E6 gene. These observations are consistent with an interaction of E6 and p53 in vivo resulting in reductions in the stability of p53 ranging between 2- and 4-fold. We also observed that the expression of various TATA containing promoters was repressed in transient assays by co-transfection with plasmids expressing the wild-type p53 gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Wild-type p53 can down-modulate the activity of various promoters.

The wild-type (wt) p53 protein is the product of a tumor suppressor gene that is a frequent target for inactivation in many types of tumors. The nuclear localization of the protein, as well as additional features, suggest that it may be involved in the regulation of gene expression. To explore this possibility, the effects of overproduced wt p53 were investigated in a number of systems. Induction of growth arrest via the antiproliferative effect of wt p53 greatly impaired the ability of cells to exhibit an increase in c-fos mRNA upon serum stimulation. Experiments in which cells were cotransfected with p53 expression plasmids together with a reporter gene linked to various promoters revealed that wt p53 could effectively reduce transcription from a series of promoters derived from serum-inducible genes, but not from a major histocompatibility complex gene. The p53-mediated repression of c-fos gene expression occurred even in the presence of cycloheximide. Kinetic studies indicate that the effect of wt p53 is rapid, rather than representing a secondary consequence of growth arrest. These findings support a role for p53 in transcriptional regulation, perhaps by reducing the expression of genes that are needed for ongoing cell proliferation.