Induction Of P53 Target Genes Research Articles

Regulation of p53 by hypoxia: dissociation of transcriptional repression and apoptosis from p53-dependent transactivation.

Hypoxic stress, like DNA damage, induces p53 protein accumulation and p53-dependent apoptosis in oncogenically transformed cells. Unlike DNA damage, hypoxia does not induce p53-dependent cell cycle arrest, suggesting that p53 activity is differentially regulated by these two stresses. Here we report that hypoxia induces p53 protein accumulation, but in contrast to DNA damage, hypoxia fails to induce endogenous downstream p53 effector mRNAs and proteins. Hypoxia does not inhibit the induction of p53 target genes by ionizing radiation, indicating that p53-dependent transactivation requires a DNA damage-inducible signal that is lacking under hypoxic treatment alone. At the molecular level, DNA damage induces the interaction of p53 with the transcriptional activator p300 as well as with the transcriptional corepressor mSin3A. In contrast, hypoxia primarily induces an interaction of p53 with mSin3A, but not with p300. Pretreatment of cells with an inhibitor of histone deacetylases that relieves transcriptional repression resulted in a significant reduction of p53-dependent transrepression and hypoxia-induced apoptosis. These results led us to propose a model in which different cellular pools of p53 can modulate transcriptional activity through interactions with transcriptional coactivators or corepressors. Genotoxic stress induces both kinds of interactions, whereas stresses that lack a DNA damage component as exemplified by hypoxia primarily induce interaction with corepressors. However, inhibition of either type of interaction can result in diminished apoptotic activity.

Twist is a potential oncogene that inhibits apoptosis.

Oncogene activation increases susceptibility to apoptosis. Thus, tumorigenesis must depend, in part, on compensating mutations that protect from programmed cell death. A functional screen for cDNAs that could counteract the proapoptotic effects of the myc oncogene identified two related bHLH family members, Twist and Dermo1. Both of these proteins inhibited oncogene- and p53-dependent cell death. Twist expression bypassed p53-induced growth arrest. These effects correlated with an ability of Twist to interfere with activation of a p53-dependent reporter and to impair induction of p53 target genes in response to DNA damage. An underlying explanation for this observation may be provided by the ability of Twist to reduce expression of the ARF tumor suppressor. Thus, Twist may affect p53 indirectly through modulation of the ARF/MDM2/p53 pathway. Consistent with a role as a potential oncoprotein, Twist expression promoted colony formation of E1A/ras-transformed mouse embryo fibroblasts (MEFs) in soft agar. Furthermore, Twist was inappropriately expressed in 50% of rhabdomyosarcomas, a tumor that arises from skeletal muscle precursors that fail to differentiate. Twist is known to block myogenic differentiation. Thus, Twist may play multiple roles in the formation of rhabdomyosarcomas, halting terminal differentiation, inhibiting apoptosis, and interfering with the p53 tumor-suppressor pathway.

P53-Dependent and -independent responses to cisplatin in mouse testicular teratocarcinoma cells.

Testicular cancers respond favorably to chemotherapy with the platinum-containing drug cis-diamminedichloroplatinum(II) (cisplatin). One factor that could explain the efficacy of cisplatin is the low frequency of p53 mutations observed in this tumor type. The present study examines the p53-mediated responses in murine testicular teratocarcinoma cells exposed to the drug. Cisplatin treatment of teratocarcinoma cells with a wild-type p53 gene resulted in accumulation of the p53 protein through posttranscriptional mechanisms; induction of p53-target genes was also observed. Drug treatment resulted in rapid apoptosis in p53-wild-type cells but not in p53(-/-) teratocarcinoma cells. In the latter cells, cisplatin exposure caused prolonged cell cycle arrest accompanied by induction of the p21 gene. Clonogenic assays demonstrated that the p53 mutation did not confer resistance to cisplatin. These experiments suggest that cisplatin inhibits cellular proliferation of testicular teratocarcinoma cells by two possible mechanisms, p53-dependent apoptosis and p53-independent cell cycle arrest.

Differential Activation of p53 Targets in Cells Treated with Ultraviolet Radiation That Undergo Both Apoptosis and Growth Arrest

The ability of p53 to act as a tumor suppressor is tightly correlated with its ability to function as a transcriptional activator at the G1/S-phase cell cycle checkpoint. Previous overexpression studies have indicated simultaneous induction of p53 target genes, despite opposing cellular functions of their protein products. To delineate the response of endoansactivation function to DNA damage in a normal cell, we irradiated early-passage rat embryo fibroblasts with 10 or 50 J/m2 of ultraviolet light (mostly UV-C). We investigated the induction of p53 targets and the response of the cells over 48 h. In this system, northern analysis revealed differential regulation of the p53 targets p21WAFI/CIPI, Mdm2, Ccng (also known as cyclin G) and Bax in accordance with their proposed functions in the cell. The growth suppressor p21WAFI/CIPI was activated initially (within 6 h) after exposure to 10 J/m2, but not after 50 J/m2, in a p53-dependent manner. Both Ccng and Mdm2 were activated later than p21 (12-24 h) after exposure to 10 J/m2. Expression of Bax was increased after exposure to both 10 J/m2 (24 h after UV exposure) and 50 J/m2 (6 h after UV exposure), which correlated well with the apoptosis seen in cells exposed to either dose. These fibroblasts also exhibited a temporary cell cycle arrest (< 8 h) at 10 J/m2. Thus we have investigated the physiological response of the p53 pathway in normal cells and identified a temporal order for induction of p53 targets. We demonstrate that both apoptosis and cell cycle arrest occur simultaneously when cells are treated with UV radiation, indicating that the amount of DNA damage is not the sole determinant of the cellular response.