The antioxidant function of the p53 tumor suppressor.

It is widely accepted that the p53 tumor suppressor restricts abnormal cells by induction of growth arrest or by triggering apoptosis. Here we show that, in addition, p53 protects the genome from oxidation by reactive oxygen species (ROS), a major cause of DNA damage and genetic instability. In the absence of severe stresses, relatively low levels of p53 are sufficient for upregulation of several genes with antioxidant products, which is associated with a decrease in intracellular ROS. Downregulation of p53 results in excessive oxidation of DNA, increased mutation rate and karyotype instability, which are prevented by incubation with the antioxidant N-acetylcysteine (NAC). Dietary supplementation with NAC prevented frequent lymphomas characteristic of Trp53-knockout mice, and slowed the growth of lung cancer xenografts deficient in p53. Our results provide a new paradigm for a nonrestrictive tumor suppressor function of p53 and highlight the potential importance of antioxidants in the prophylaxis and treatment of cancer.

Transcription of mammalian messenger RNAs by a nuclear RNA polymerase of mitochondrial origin.

Transcription of eukaryotic genes is performed by three nuclear RNA polymerases, of which RNA polymerase II is thought to be solely responsible for the synthesis of messenger RNAs. Here we show that transcription of some mRNAs in humans and rodents is mediated by a previously unknown single-polypeptide nuclear RNA polymerase (spRNAP-IV). spRNAP-IV is expressed from an alternative transcript of the mitochondrial RNA polymerase gene (POLRMT). The spRNAP-IV lacks 262 amino-terminal amino acids of mitochondrial RNA polymerase, including the mitochondrial-targeting signal, and localizes to the nucleus. Transcription by spRNAP-IV is resistant to the RNA polymease II inhibitor alpha-amanitin but is sensitive to short interfering RNA specific for the POLRMT gene. The promoters for spRNAP-IV differ substantially from those used by RNA polymerase II, do not respond to transcriptional enhancers and contain a common functional sequence motif.

Homeostatic functions of the p53 tumor suppressor: regulation of energy metabolism and antioxidant defense.

The p53 tumor suppressor plays pivotal role in the organism by supervising strict compliance of individual cells to needs of the whole organisms. It has been widely accepted that p53 acts in response to stresses and abnormalities in cell physiology by mobilizing the repair processes or by removing the diseased cells through initiating the cell death programs. Recent studies, however, indicate that even under normal physiological conditions certain activities of p53 participate in homeostatic regulation of metabolic processes and that these activities are important for prevention of cancer. These novel functions of p53 help to align metabolic processes with the proliferation and energy status, to maintain optimal mode of glucose metabolism and to boost the energy efficient mitochondrial respiration in response to ATP deficiency. Additional activities of p53 in non-stressed cells tune up the antioxidant defense mechanisms reducing the probability of mutations caused by DNA oxidation under conditions of daily stresses. The deficiency in the p53-mediated regulation of glycolysis and mitochondrial respiration greatly accounts for the deficient respiration of the predominance of aerobic glycolysis in cancer cells (the Warburg effect), while the deficiency in the p53-modulated antioxidant defense mechanisms contributes to mutagenesis and additionally boosts the carcinogenesis process.

Efficient downregulation of multiple mRNA targets with a single shRNA-expressing lentiviral vector.

Gene silencing based on RNA interference is widely used in fundamental research and in practical applications. However, a commonly incomplete functional suppression represents a serious drawback of this technology. We describe a series of lentiviral vectors each containing a single or multiple shRNA-expression cassette(s) driven by a RNA-polymerase III specific promoter and localized within the 3'-LTR of the lentiviral DNA backbone. The vectors also contain an antibiotic-resistance gene that allows positive selection of recipient cells. The combined expression of three different shRNAs specific to a single mRNA was shown to improve dramatically the level of mRNA inhibition, while the use of three different RNA-polymerase III specific promoters avoids the loss of shRNA-expression cassettes through the homologous recombination. The vector system was used for successful simultaneous suppression of three related SESN1, SESN2 and SESN3 genes, which suggests its particular value for testing phenotypes of functionally redundant genes.

Androgen regulates apoptosis induced by TNFR family ligands via multiple signaling pathways in LNCaP.

It has been suggested in many studies that combined treatment with chemotherapeutic agents and apoptosis-inducing ligands belonging to TNFR family is a more effective strategy for cancer treatment. However, the role of androgen regulation of TNFR family-induced apoptosis in prostate cancer is poorly understood. In this study, we investigated the dose-dependent effects of androgen on TNF-alpha and TRAIL-mediated apoptosis in LNCaP. To investigate the interaction between the androgen receptor (AR) and the caspase-2 gene, chromatin immunoprecipitation analysis was used, and we are the first to identify that AR interacts in vivo with an androgen-responsive elements in intron 8 of caspase-2 gene. We have found that DHT inhibited apoptosis in dose-dependent manner. There is a direct, androgen-dependent correlation between the levels of activated Akt and caspase activation after treatment with TNF-alpha and TRAIL. We have also found that there are at least two different regulatory mechanisms of p53 expression by androgen: at the gene and protein levels. At the same time, the level of AR was found to be higher in LNCaP-si-p53 compared to LNCaP-mock cells. These data indicate that there is a mutual regulation of expression between p53 and AR. Our study suggests that androgen-dependent outcome of apoptotic treatment can occur, at least in part, via the caspase-2, Akt and p53-mediated pathways.

Novel gain of function activity of p53 mutants: activation of the dUTPase gene expression leading to resistance to 5-fluorouracil.

Mutated forms of p53 are often expressed in a variety of human tumors. In addition to loss of function of the p53 tumor suppressor, mutant p53s contribute to malignant process by acquisition of novel functions that enhance transformed properties of cells and resistance to anticancer therapy in vitro, and increase tumorigenecity, invasiveness and metastatic ability in vivo. Searching for genes that change expression in response to p53 gain of function mutants may give a clue to the mechanisms underlying their oncogenic effects. Recently by subtraction hybridization cloning we found that the dUTPase gene is transcriptionally upregulated in p53-null mouse fibroblasts expressing the exogenous human tumor-derived His175 p53 mutant. Here we show that conditional expression of His175 and Trp248 hot-spot p53 mutants in p53-negative mouse 10(1) fibroblasts and human SK-OV3 and H1299 tumor cells results in increase in dUTPase gene transcription, an important marker predicting the efficacy of cancer therapy with fluoropyrimidine drugs. Using tetracycline-regulated retroviral vectors for conditional expression of p53 mutants, we found that transcription of the dUTPase gene is increased within 24 h after tetracycline withdrawal, and the cells acquire higher resistance to 5-FU. Additional inactivation of the N-terminal transcription activation domain of mutant p53 (substitutions in amino-acid residues 22 and 23) results in abrogation of both induction of dUTPase transcripts and 5-FU resistance.

p53-dependent expression of CXCR5 chemokine receptor in MCF-7 breast cancer cells.

Elevated expression of chemokine receptors in tumors has been reported in many instances and is related to a number of survival advantages for tumor cells including abnormal activation of prosurvival intracellular pathways. In this work we demonstrated an inverse correlation between expression levels of p53 tumor suppressor and CXCR5 chemokine receptor in MCF-7 human breast cancer cell line. Lentiviral transduction of MCF-7 cells with p53 shRNA led to elevated CXCR5 at both mRNA and protein levels. Functional activity of CXCR5 in p53-knockdown MCF-7 cells was also increased as shown by activation of target gene expression and chemotaxis in response to B-lymphocyte chemoattractant CXCL13. Using deletion analysis and site-directed mutagenesis of the cxcr5 gene promoter and enhancer elements, we demonstrated that p53 appears to act upon cxcr5 promoter indirectly, by repressing the activity of NFκB transcription factors. Using chromatin immunoprecipitation and reporter gene analysis, we further demonstrated that p65/RelA was able to bind the cxcr5 promoter in p53-dependent manner and to directly transactivate it when overexpressed. Through the described mechanism, elevated CXCR5 expression may contribute to abnormal cell survival and migration in breast tumors that lack functional p53.