Das Tumorsuppressorprotein p53 - Rolle bei der Induktion von Apoptose und bei der Replikation von Adenoviren

The tumor suppressor p53 is a central regulator of proliferation and cell growth. It has become the subject of intensive study ever since it became clear that slightly more than 50% of human cancers contain mutations in this gene. Several types of DNA-damage can activate wildtype-p53. The result is a rapid increase in the level of p53 and activation of p53 as a transcriptional factor. p53 induces two major pathways, cell cycle arrest and/or apoptosis. p53 is also a target of viral oncoproteins. The adenoviral oncoproteins E1B-55kD and E4orf6 inactivate and destabilize p53, thereby contributing to malignant transformation. However, it is unclear whether the elimination of p53 contributes to the efficiency of viral replication. Furthermore, it is controversial whether adenoviruses with a deletion in the E1B55kD-coding region (dl1520/ ONYX 015) might selectively replicate in cells with a mutation or deletion of the p53 gene and, therefore, represent a tool in cancer therapy. A p53 protein lacking the proline-rich region (p53delta62-91) induces many p53-responsive genes but not PIG3. In parallel, this mutant induces growth arrest but not apoptosis. Some tumor-derived p53-mutants, especially p53M264I, specifically failed to induce the PIG3 promoter and apoptosis, thus resembling p53delta62-91. These results suggest that the proline-rich domain of p53 affects the ability of the central domain to bind DNA. Furthermore, an assay was established to identify other p53-responsive genes showing the same activation pattern as PIG3. Therefore, p53 (-/-) H1299 cells were tranduced by adenoviral vectors to express p53 or p53delta62-91. Differentially activated p53-responsive genes were compared by cDNA-arrays. A p53-responsive gene was identified (Ephrin-B4-Receptor) that showed the same pattern of activation as PIG3. Therefore, this gene represents a candidate mediator of p53 induced apoptosis. To address the role of p53 in virus replication, a p53-mutant has been constructed (p53mt24-28). This mutation leaves p53 transcriptionally active but confers complete resistance to inhibition and degradation by adenoviral oncoproteins. This mutant was expressed by an adenoviral vector. Surprisingly, even strong overexpression of p53 or p53mt24-28 allowed the virus to replicate as efficiently as in the absence of p53 proteins, both in tumor cells and in primary cells. Thus, active p53 does not inhibit the growth of adenovirus. Instead of deleting the p53-antagonist E1B-55kD, alternative strategies should be used to improve the utility of adenoviruses in cancer therapy.The tumor suppressor p53 is a central regulator of proliferation and cell growth. It has become the subject of intensive study ever since it became clear that slightly more than 50% of human cancers contain mutations in this gene. Several types of DNA-damage can activate wildtype-p53. The result is a rapid increase in the level of p53 and activation of p53 as a transcriptional factor. p53 induces two major pathways, cell cycle arrest and/or apoptosis. p53 is also a target of viral oncoproteins. The adenoviral oncoproteins E1B-55kD and E4orf6 inactivate and destabilize p53, thereby contributing to malignant transformation. However, it is unclear whether the elimination of p53 contributes to the efficiency of viral replication. Furthermore, it is controversial whether adenoviruses with a deletion in the E1B55kD-coding region (dl1520/ ONYX 015) might selectively replicate in cells with a mutation or deletion of the p53 gene and, therefore, represent a tool in cancer therapy. A p53 protein lacking the proline-rich region (p53delta62-91) induces many p53-responsive genes but not PIG3. In parallel, this mutant induces growth arrest but not apoptosis. Some tumor-derived p53-mutants, especially p53M264I, specifically failed to induce the PIG3 promoter and apoptosis, thus resembling p53delta62-91. These results suggest that the proline-rich domain of p53 affects the ability of the central domain to bind DNA. Furthermore, an assay was established to identify other p53-responsive genes showing the same activation pattern as PIG3. Therefore, p53 (-/-) H1299 cells were tranduced by adenoviral vectors to express p53 or p53delta62-91. Differentially activated p53-responsive genes were compared by cDNA-arrays. A p53-responsive gene was identified (Ephrin-B4-Receptor) that showed the same pattern of activation as PIG3. Therefore, this gene represents a candidate mediator of p53 induced apoptosis. To address the role of p53 in virus replication, a p53-mutant has been constructed (p53mt24-28). This mutation leaves p53 transcriptionally active but confers complete resistance to inhibition and degradation by adenoviral oncoproteins. This mutant was expressed by an adenoviral vector. Surprisingly, even strong overexpression of p53 or p53mt24-28 allowed the virus to replicate as efficiently as in the absence of p53 proteins, both in tumor cells and in primary cells. Thus, active p53 does not inhibit the growth of adenovirus. Instead of deleting the p53-antagonist E1B-55kD, alternative strategies should be used to improve the utility of adenoviruses in cancer therapy.