|Here we want to describe in brief selected papers with special impact on apoptosis or cancer research, just to keep ourselves and you up to date about some major advances in this rapidly developing field. This selection cannot be complete and the choice of the discussed papers naturally will be subjective.|
|You are invited to contribute by describing a paper.|
|For suggestions please contact the ApoNews team.|
|KEYWORDS||p53; ASPP; promoter specificity, cancer, tumorigenesis|
|REFERENCE||ASPP Proteins specifically stimulate the Apoptotic Function of p53. Samuels-Lev et al., 2001, Mol. Cell, 8: 781-94|
|BACKGROUND||p53 is one of the most frequently mutated genes found in human cancer. In tumors with wild-type p53 other tumor suppressor genes ( Rb, p16, BRCA1, or BRCA2) can be impaired allowing the tumor cells to bypass the tumor suppression function of p53. But a lack of co-stimulators or overexpression of inhibitors of p53 could also account for the loss of the tumor suppressor function of wild-type p53. Indeed, a new category of tumor-derived p53 mutants has been identified that are defective in inducing apoptosis but are as active as wild-type p53 in inducing G1 arrest. They are less able to transctivate promoters of pro-apoptotic genes such as Bax, PIG-3, and IGF-BP3 but competent to transactivate promoters of other p53 target genes such as mdm2mdm2 and p21. However, how the promoter specificity of p53 is regulated remained unknown.|
In a genome database search, ASPP1 and ASPP2 were identified as proteins with homology to the previously isolated p53 binding protein 53BP2. Indeed, 53BP2 is a C-terminal fragment of the full-length ASPP2.
2. ASPP1 and ASPP2 were shown to physically interact with p53 as demonstrated by IP (antibodies against ASPP1 and ASPP2 were raised).
3. In addition to p53, ASPP2 also interacts with Bcl-2.
4. Complex formation between p53 and ASPP2, but not ASPP1, was increased following UV irradiation.
5. Coexpression of p53 with ASPP1 or ASPP2 resulted in an increased induction of apoptosis by p53 but not in increased cell cycle arrest. ASPPs did not enhance apoptosis induced by Bax.
6. p53 wild-type U2OS cells and MCF-7 cells expressing ASPP1 and ASPP2 antisense cDNA were more resistant to cisplatin-induced apoptosis.
7. Using U2OS cells inducible for ASPP2, it was shown that in ASPP2 overexpressing cells the DNA binding activity of p53 was increased for the Bax promoter but not for the p21 promoter. In transient reporter assays it was shown, that p53 in concert with ASPP1 or ASPP2 significantly enhanced transactivation function of p53 on the Bax promoter (about 20-fold) but not on a mutant Bax promoter and also not on promoters such as mdm2, cyclinG, p21 (only about 3-fold). In contrast, ASPPs did not at all influence transcriptional activity of E2F1 on several known promoter reporters suggesting that ASPPs are specific stimulators of p53.
8. ASPPs did not stimulate the Bax promoter binding activities of tumor-derived p53 mutants 181L and 181C, and those mutants were shown to interact less with especially ASPP1. This might explain why the two p53 mutants are defective in inducing apoptosis, and why mutation of this site of p53 may have been selected in some human tumors.
9. In several human breast carcinomas with wild-type p53, ASPP expression levels were detected to be downregulated (ASPP1 in 60% of all tested samples and ASPP2 in 23%).
|CONCLUSIONS||ASPP1 and ASPP2 constitute a novel family of p53 binding proteins which specifically enhance the apoptotic function of p53 by stimulating the DNA binding and transactivation function of p53 on the promoters of pro-apoptotic genes in vivo. The high percentage of wild-type p53-expressing breast tumors with altered expression patterns of ASPPs suggests an important tumor suppressor function for ASPP family members in the development of human cancer.|
|posted||Nov 22, 2001 by Andreas|
|KEYWORDS||PARP-1 cleavage; DNA damage; apoptosis-necrosis, ATP depletion|
|REFERENCE||Gain of function of Poly(ADP-ribose)Polymerase-1 upon Cleavage by Apoptotic Proteases: Implications in Apoptosis. Amours et al., 2001, J. Cell Science, 114(20): 3771-77|
|BACKGROUND||see PARP in the Apoptopedia.|
The activities of PARP-1 and its proteolytical cleavage fragments were investigated
biochemically in a purified in vitro system.
2. Cleavage of PARP-1 by caspases causes its enzymatic inactivation
3. The N-terminal 24 kDa fragment of PARP-1 bearing the DNA binding domain competitively inhibits the enzymatic activity of full-length PARP-1 and also inhibits DNA base excision repair in a cell-free system.
4. The C-terminal 89 kDa cleavage fragment containing the enzymatic domain of PARP-1 did not elicit any significant effect concerning PARP-1 activity.
|CONCLUSIONS||The data suggest that the 24 kDa apoptotic fragment of PARP-1 acquires a
dominant-negative activity repressing the enzymatic activity and DNA repair mediated by
still intact PARP-1.|
This is important since DNA damage induced by DNA damaging agents or by apoptotic DNA fragmentation would result in PARP-1 activation and depletion of NAD+. Depletion of NAD+ would result in a decrease of the ATP pool of the cell. A lack of ATP can cause a switch of the mode of cell-death from apoptotic to necrotic. Thus, inactivation of PARP-1 during apoptosis induction by the mechanisms described in this paper prevents the depletion of NAD+ and thus promotes apoptosis by preventing DNA repair-induced survival and by blocking energy depletion-induced necrosis.
|posted||Nov 4, 2001 by Andreas|