Mdm2 inhibits the apoptotic function of p53 mainly by targeting it for degradation

The ability of Mdm2 to inhibit the activities of a C-terminal truncated p53 mutant, p53-Delta30, which can bind Mdm2 but is resistant to Mdm2-mediated protein degradation was investigated. The inhibitory function of an Mdm2 mutant, Mdm2-Delta(222-437), which can bind p53 but is defective in targeting p53 for degradation was also studied. We have demonstrated that targeting p53 for degradation is the most effective way for Mdm2 to inhibit the apoptotic function of p53. However, we have also shown that Mdm2 can inhibit the transactivation function of p53 without targeting it for degradation, although Mdm2 releases the transrepression ability of p53 mainly by targeting it for degradation. The ability of Mdm2 to inhibit the apoptotic function of p53 was linked to its ability to inhibit the transrepression but not the transactivation function of p53. Furthermore, we have demonstrated that the transrepression function of p53 was specific to p53-induced apoptosis and was not simply a result of cell death.     

MageA2 restrains cellular senescence by targeting the function of PMLIV/p53 axis at the PML-NBs

MAGE-A genes are a subfamily of the melanoma antigen genes (MAGEs), whose expression is restricted to tumor cells of different origin and normal tissues of the human germline. Although the specific function of individual MAGE-A proteins is being currently explored, compelling evidence suggest their involvement in the regulation of different pathways during tumor progression. We have previously reported that MageA2 binds histone deacetylase (HDAC)3 and represses p53-dependent apoptosis in response to chemotherapeutic drugs. The promyelocytic leukemia (PML) tumor suppressor is a regulator of p53 acetylation and function in cellular senescence. Here, we demonstrate that MageA2 interferes with p53 acetylation at PML-nuclear bodies (NBs) and with PMLIV-dependent activation of p53. Moreover, a fraction of MageA2 colocalizes with PML-NBs through direct association with PML, and decreases PMLIV sumoylation through an HDAC-dependent mechanism. This reduction in PML post-translational modification promotes defects in PML-NBs formation. Remarkably, we show that in human fibroblasts expressing RasV12 oncogene, MageA2 expression decreases cellular senescence and increases proliferation. These results correlate with a reduction in NBs number and an impaired p53 response. All these data suggest that MageA2, in addition to its anti-apoptotic effect, could have a novel role in the early progression to malignancy by interfering with PML/p53 function, thereby blocking the senescence program, a critical barrier against cell transformation.     

Therapeutic targeting of the p53 tumor suppressor gene

The p53 tumor suppressor gene is a logical target for cancer therapy. Several therapeutic strategies can be envisioned based upon recent advances concerning structure and function of the p53 protein, its interaction with cellular and viral proteins and its roles in repairing DNA, regulating cell division and promoting apoptosis.     

Skp2B attenuates p53 function by inhibiting prohibitin

The F‐box protein Skp2 and its isoform Skp2B are both overexpressed in breast cancers. Skp2 alters the activity of p53 by inhibiting its interaction with p300 and by promoting p300 degradation. Here, we report that Skp2B also attenuates the activity of p53; however, this effect is independent of p300, suggesting that another mechanism might be involved. Prohibitin, a protein reported to activate p53, was isolated in a two‐hybrid screen with the carboxy‐terminal domain unique to Skp2B. We observed that prohibitin is a new substrate of Skp2B and that the degradation of prohibitin is responsible for the attenuated activity of p53 in cells overexpressing Skp2B. Furthermore, we show that the activity of p53 is reduced in the mammary glands of Skp2B transgenic mice. This study indicates that both Skp2 and Skp2B attenuate p53 activity through different pathways, suggesting that amplification of the Skp2 locus represents a powerful mechanism to attenuate p53 function in cancer.     

Cancer-derived p53 mutants suppress p53-target gene expression--potential mechanism for gain of function of mutant p53

Tumour-derived p53 mutants are thought to have acquired ‘gain-of-function’ properties that contribute to oncogenicity. We have tested the hypothesis that p53 mutants suppress p53-target gene expression, leading to enhanced cellular growth. Silencing of mutant p53 expression in several human cell lines was found to lead to the upregulation of wild-type p53-target genes such as p21, gadd45, PERP and PTEN. The expression of these genes was also suppressed in H1299-based isogenic cell lines expressing various hot-spot p53 mutants, and silencing of mutant p53, but not TAp73, abrogated the suppression. Consistently, these hot-spot p53 mutants were able to suppress a variety of p53-target gene promoters. Analysis using the proto-type p21 promoter construct indicated that the p53-binding sites are dispensable for mutant p53-mediated suppression. However, treatment with the histone deacetylase inhibitor trichostatin-A resulted in relief of mutant p53-mediated suppression, suggesting that mutant p53 may induce hypo-acetylation of target gene promoters leading to the suppressive effects. Finally, we show that stable down-regulation of mutant p53 expression resulted in reduced cellular colony growth in human cancer cells, which was found to be due to the induction of apoptosis. Together, the results demonstrate another mechanism through which p53 mutants could promote cellular growth.     

Control of tumor suppressor p53 function by endoplasmic reticulum stress

Alterations in the homeostasis of the endoplasmic reticulum (ER) by various forms of stress can lead to the accumulation of unfolded proteins and protein aggregates that are detrimental to cell survival. Eukaryotic cells can adapt to ER stress by activating specific signaling pathways and mechanisms, whose primary purpose is to limit the accumulation of unfolded proteins in the ER. We recently reported a novel mechanism of cell adaptation to ER stress, which proceeds through the inhibition of the apoptotic function of the tumor suppressor p53 (Genes Dev 2004;18:261-277). We found that ER stress increases the cytoplasmic localization and enhances the destabilization of the tumor suppressor. This process requires the phosphorylation of p53 at serine 315 and serine 376, which is mediated by the activation of glycogen synthase kinase-3beta (GSK-3beta). ER stress also prevents p53 activation and p53-mediated apoptosis in response to DNA damage. These findings demonstrate that ER stress utilizes mechanisms that are distinct from other types of stress to modulate p53. In addition, they reveal that ER stress and nuclear DNA damage can induce inter-organellar cross-talk pathways targeting p53 with important implications for the treatment of tumors with dysfunctional ER.