Wt p53 impairs response to chemotherapy: make lemonade to spare normal cells

Cooperation of Nutlin-3a and a Wip1 inhibitor to induce p53 activity

Targeting the Mdm2 oncoprotein by drugs has the potential of re-establishing p53 function and tumor suppression. However, Mdm2-antagonizing drug candidates, e. g. Nutlin-3a, often fail to abolish cancer cell growth sustainably. To overcome these limitations, we inhibited Mdm2 and simultaneously a second negative regulator of p53, the phosphatase Wip1/PPM1D. When combining Nutlin-3a with the Wip1 inhibitor GSK2830371 in the treatment of p53-proficient but not p53-deficient cells, we observed enhanced phosphorylation (Ser 15) and acetylation (Lys 382) of p53, increased expression of p53 target gene products, and synergistic inhibition of cell proliferation. Surprisingly, when testing the two compounds individually, largely distinct sets of genes were induced, as revealed by deep sequencing analysis of RNA. In contrast, the combination of both drugs led to an expression signature that largely comprised that of Nutlin-3a alone. Moreover, the combination of drugs, or the combination of Nutlin-3a with Wip1-depletion by siRNA, activated p53-responsive genes to a greater extent than either of the compounds alone. Simultaneous inhibition of Mdm2 and Wip1 enhanced cell senescence and G2/M accumulation. Taken together, the inhibition of Wip1 might fortify p53-mediated tumor suppression by Mdm2 antagonists.

The imidazoacridinone C-1311 induces p53-dependent senescence or p53-independent apoptosis and sensitizes cancer cells to radiation

C-1311 is a small molecule, which has shown promise in a number of pre-clinical and clinical studies. However, the biological response to C-1311 exposure is complicated and has been reported to involve a number of cell fates. Here, we investigated the molecular signaling which determines the response to C-1311 in both cancer and non-cancer cell lines. For the first time we demonstrate that the tumor suppressor, p53 plays a key role in cell fate determination after C-1311 treatment. In the presence of wild-type p53, cells exposed to C-1311 entered senescence. In contrast, cells lines without functional p53 underwent mitotic catastrophe and apoptosis. C-1311 also induced autophagy in a non-p53-dependent manner. Cells in hypoxic conditions also responded to C-1311 in a p53-dependent manner, suggesting that our observations are physiologically relevant. Most importantly, we show that C-1311 can be effectively combined with radiation to improve the radiosensitivity of a panel of cancer cell lines. Together, our data suggest that C-1311 warrants further clinical testing in combination with radiotherapy for the treatment of solid tumors.

Silencing of CDK2, but not CDK1, separates mitogenic from anti-apoptotic signaling, sensitizing p53 defective cells for synthetic lethality

Small molecule inhibitors targeting CDK1/CDK2 have been clinically proven effective against a variety of tumors, albeit at the cost of profound off target toxicities. To separate potential therapeutic from toxic effects, we selectively knocked down CDK1 or CDK2 in p53 mutated HACAT cells by siRNA silencing. Using dynamic, cell cycle wide proteome arrays, we observed minor changes in overall abundance of proteins critically involved in cell cycle transition despite profound G₂/M or G₁/S arrest, respectively. Employing phospho site specific analyses, we identified uncoupled mitogenic, yet pro-apoptotic signaling from counter balancing anti-apoptotic activity in CDK2 disrupted cells. Moreover, a crucial role of CDK2 activity in early serum response was observed, extending well-established roles of CDKs outside their cell cycle regulating functions. In contrast, disruption of CDK1 only marginally affected phosphorylation events of crucial signaling nodes prior to G₂/S transition. The data presented here suggest that the temporal separation of pro- and anti-apoptotic pathways by selective inhibition of CDK2 disrupts coherent signaling modules and may synergize with anti-proliferative drugs, averting toxic side effects from CDK1 inhibition.

Mdm2 inhibition confers protection of p53-proficient cells from the cytotoxic effects of Wee1 inhibitors

Pharmacological inhibition of the cell cycle regulatory kinase Wee1 represents a promising strategy to eliminate cancer cells. Wee1 inhibitors cooperate with chemotherapeutics, e. g. nucleoside analogues, pushing malignant cells from S phase towards premature mitosis and death. However, considerable toxicities are observed in preclinical and clinical trials. A high proportion of tumor cells can be distinguished from all other cells of a patient's body by inactivating mutations in the tumor suppressor p53. Here we set out to develop an approach for the selective protection of p53-proficient cells against the cytotoxic effects of Wee1 inhibitors. We pretreated such cells with Nutlin-3a, a prototype inhibitor of the p53-antagonist Mdm2. The resulting transient cell cycle arrest effectively increased the survival of cells that were subsequently treated with combinations of the Wee1 inhibitor MK-1775 and/or the nucleoside analogue gemcitabine. In this constellation, Nutlin-3a reduced caspase activation and diminished the phosphorylation of Histone 2AX, an indicator of the DNA damage response. Both effects were strictly dependent on the presence of p53. Moreover, Nutlin pre-treatment reduced the fraction of cells that were undergoing premature mitosis in response to Wee1 inhibition. We conclude that the pre-activation of p53 through Mdm2 antagonists serves as a viable option to selectively protect p53-proficient cells against the cytotoxic effects of Wee1 inhibitors, especially when combined with a nucleoside analogue. Thus, Mdm2 antagonists might prove useful to avoid unwanted side effects of Wee1 inhibitors. On the other hand, when a tumor contains wild type p53, care should be taken not to induce its activity before applying Wee1 inhibitors.

Deregulation of the EGFR/PI3K/PTEN/Akt/mTORC1 pathway in breast cancer: possibilities for therapeutic intervention

The EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway plays prominent roles in malignant transformation, prevention of apoptosis, drug resistance and metastasis. The expression of this pathway is frequently altered in breast cancer due to mutations at or aberrant expression of: HER2, ERalpha, BRCA1, BRCA2, EGFR1, PIK3CA, PTEN, TP53, RB as well as other oncogenes and tumor suppressor genes. In some breast cancer cases, mutations at certain components of this pathway (e.g., PIK3CA) are associated with a better prognosis than breast cancers lacking these mutations. The expression of this pathway and upstream HER2 has been associated with breast cancer initiating cells (CICs) and in some cases resistance to treatment. The anti-diabetes drug metformin can suppress the growth of breast CICs and herceptin-resistant HER2+ cells. This review will discuss the importance of the EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway primarily in breast cancer but will also include relevant examples from other cancer types. The targeting of this pathway will be discussed as well as clinical trials with novel small molecule inhibitors. The targeting of the hormone receptor, HER2 and EGFR1 in breast cancer will be reviewed in association with suppression of the EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway.

Exploiting replicative stress to treat cancer

DNA replication in cancer cells is accompanied by stalling and collapse of the replication fork and signalling in response to DNA damage and/or premature mitosis; these processes are collectively known as 'replicative stress'. Progress is being made to increase our understanding of the mechanisms that govern replicative stress, thus providing ample opportunities to enhance replicative stress for therapeutic purposes. Rather than trying to halt cell cycle progression, cancer therapeutics could aim to increase replicative stress by further loosening the checkpoints that remain available to cancer cells and ultimately inducing the catastrophic failure of proliferative machineries. In this Review, we outline current and future approaches to achieve this, emphasizing the combination of conventional chemotherapy with targeted approaches.

Silencing of Dicer1 temporally separates pro- and anti-apoptotic signaling and confers susceptibility to chemotherapy in p53 mutated cells

miRNAs are critically implicated in the initiation process of and progression through cancerogenesis. The mechanisms, however, by which miRNAs interfere with the signalosomes of human cancer cells, are still obscure. We utilized the p53-mutated human keratinocyte cell line HACAT to investigate the biological significance and extent to which miRNAs regulate proliferation, cell growth, and apoptosis in transformed phenotypes. Silencing of the miRNA-processing enzyme Dicer1 resulted in cell cycle arrest at the G1/S border, along with restoration of CDK inhibitor p21(CIP)expression. Employing a cell cycle-wide phospho-proteomic approach, we detected neglectable changes in abundance and schedule of overall and cell cycle periodic protein expression despite cell cycle arrest of Dicer1-depleted cells. Instead, we found substantially delayed post-translational modifications of some, but not all, signaling nodes. Phospho-site-specific analyses revealed that pro-apoptotic information elicited by Myc, β-catenin, and other mitotic pathways early in G1 are absorbed and balanced by anti-apoptotic signaling from AKT and NFκB in Dicer1-competent cells. The absence of regulatory miRNAs, however, led to a substantial delay of anti-apoptotic signaling, leaving pro-apoptotic stress unbalanced in Dicer1-deprived cells. We here show that this temporal separation of pro- and anti-apoptotic signaling induced by inhibition of Dicer1 is synergistic and synthetic lethal to low-dose 5-FU chemotherapy in p53-mutated HACAT cells. The findings reported here contribute to the understanding of the complex interactions of miRNAs with the signalosom of transformed phenotypes and may help to design novel strategies to fight cancer.

ETV1 positively regulates transcription of tumor suppressor ARF

ETV1 (ETS variant 1) is a transcription factor from the ETS family and an oncogene in several types of human malignancies. Paradoxically, a predicted inactivating mutation in ETV1 was previously found in a clone of HT1080 cells with reduced activity of p53. We report that elevated expression of ETV1 makes p53-null tumor cells hypersensitive to restoration of said tumor suppressor. Furthermore, elevated levels of either wild-type ETV1 or its truncated derivative, dETV1, which mimics the product of an oncogenic rearrangement in certain tumors, results in increased expression of mRNA for p14ARF, a known activator of p53. Accordingly, expression of a luciferase reporter, which is driven by a putative ARF promoter, was elevated by concomitant expression of either ETV1 or dETV1. Our observations point to yet another example of a tumor suppressor gene being activated by a potentially oncogenic signal. A better understanding of the mechanisms that allow a cell to bypass such safeguards is needed in order to predict and prevent the development of an oncogene-tolerant state during cancer evolution.

Molecular dynamics of the full-length p53 monomer

The p53 protein is frequently mutated in a very large proportion of human tumors, where it seems to acquire gain-of-function activity that facilitates tumor onset and progression. A possible mechanism is the ability of mutant p53 proteins to physically interact with other proteins, including members of the same family, namely p63 and p73, inactivating their function. Assuming that this interaction might occurs at the level of the monomer, to investigate the molecular basis for this interaction, here, we sample the structural flexibility of the wild-type p53 monomeric protein. The results show a strong stability up to 850 ns in the DNA binding domain, with major flexibility in the N-terminal transactivations domains (TAD1 and TAD2) as well as in the C-terminal region (tetramerization domain). Several stable hydrogen bonds have been detected between N-terminal or C-terminal and DNA binding domain, and also between N-terminal and C-terminal. Essential dynamics analysis highlights strongly correlated movements involving TAD1 and the proline-rich region in the N-terminal domain, the tetramerization region in the C-terminal domain; Lys120 in the DNA binding region. The herein presented model is a starting point for further investigation of the whole protein tetramer as well as of its mutants.

Recent discoveries in the cycling, growing and aging of the p53 field

The P53 gene and it product p53 protein is the most studied tumor suppressor, which was considered as oncogene for two decades until 1990. More than 60 thousand papers on the topic of p53 has been abstracted in Pubmed. What yet could be discovered about its role in cell death, growth arrest and apoptosis, as well as a mediator of the therapeutic effect of anticancer drugs. Still during recent few years even more amazing discoveries have been done. Here we review such topics as suppression of epigenetic silencing of a large number of non-coding RNAs, role of p53 in suppression of the senescence phenotype, inhibition of oncogenic metabolism, protection of normal cells from chemotherapy and even tumor suppression without apoptosis and cell cycle arrest.

Battle of the eternal rivals: restoring functional p53 and inhibiting Polo-like kinase 1 as cancer therapy

Polo-like kinase 1, a pivotal regulator of mitosis and cytokinesis, is highly expressed in a broad spectrum of tumors and its expression correlates often with poor prognosis, suggesting its potential as a therapeutic target. p53, the guardian of the genome, is the most important tumor suppressor. In this review, we address the intertwined relationship of these two key molecules by fighting each other as eternal rivals in many signaling pathways. p53 represses the promoter of Polo-like kinase 1, whereas Polo-like kinase 1 inhibits p53 and its family members p63 and p73 in cancer cells lacking functional p53. Plk1 inhibitors target all rapidly dividing cells irrespective of tumor cells or non-transformed normal but proliferating cells. Upon treatment with Plk1 inhibitors, p53 in tumor cells is activated and induces strong apoptosis, whereas tumor cells with inactive p53 arrest in mitosis with DNA damage. Thus, inactive p53 is not associated with a susceptible cytotoxicity of Polo-like kinase 1 inhibition and could rather foster the induction of polyploidy/aneuploidy in surviving cells. In addition, compared to the mono-treatment, combination of Polo-like kinase 1 inhibition with anti-mitotic or DNA damaging agents boosts more severe mitotic defects, effectually triggers apoptosis and strongly inhibits proliferation of cancer cells with functional p53. In this regard, restoration of p53 in tumor cells with loss or mutation of p53 will reinforce the cytotoxicity of combined Polo-like kinase 1 therapy and provide a proficient strategy for combating relapse and metastasis of cancer.

High cytidine deaminase expression in the liver provides sanctuary for cancer cells from decitabine treatment effects

We document for the first time that sanctuary in an organ which expresses high levels of the enzyme cytidine deaminase (CDA) is a mechanism of cancer cell resistance to cytidine analogues. This mechanism could explain why historically, cytidine analogues have not been successful chemotherapeutics against hepatotropic cancers, despite efficacy in vitro. Importantly, this mechanism of resistance can be readily reversed, without increasing toxicity to sensitive organs, by combining cytidine analogue with an inhibitor of cytidine deaminase (tetrahydrouridine). Specifically, CDA rapidly metabolizes cytidine analogues into inactive uridine counterparts. Hence, to determine if sheltering/protection of cancer cells in organs which express high levels of CDA (e.g., liver) is a mechanism of resistance, we utilized a murine xenotransplant model of myeloid cancer that is sensitive to epigenetic therapeutic effects of the cytidine analogue decitabine in vitro and hepato-tropic in vivo. Treatment of tumor-bearing mice with decitabine (subcutaneous 0.2mg/kg 2X/week) doubled median survival and significantly decreased extra-hepatic tumor burden, but hepatic tumor burden remained substantial, to which the animals eventually succumbed. Combining a clinically-relevant inhibitor of CDA (tetrahydrouridine) with a lower dose of decitabine (subcutaneous 0.1mg/kg 2X/week) markedly decreased liver tumor burden without blood count or bone marrow evidence of myelotoxicity, and with further improvement in survival. In conclusion, sanctuary in a CDA-rich organ is a mechanism by which otherwise susceptible cancer cells can resist the effects of decitabine epigenetic therapy. This protection can be reversed without increasing myelotoxicity by combining tetrahydrouridine with a lower dose of decitabine.

Wild-type and mutant p53 mediate cisplatin resistance through interaction and inhibition of active caspase-9

The p53 gene has been implicated in many cancers due to its frequent mutations as well as mutations in other genes whose proteins directly affect p53's functions. In addition, high expression of p53 [wild-type (WT) or mutant] has been found in the cytoplasm of many tumor cells, and studies have associated these observations with more aggressive tumors and poor prognosis. Cytoplasmic mis-localization of p53 subsequently reduced its transcriptional activity and this loss-of-function (LOF) was used to explain the lack of response to chemotherapeutic agents. However, this hypothesis seemed inadequate in explaining the apparent selection for tumor cells with high levels of p53 protein, a phenomenon that suggests a gain-of-function (GOF) of these mis-localized p53 proteins. In this study, we explored whether the direct involvement of p53 in the apoptotic response is via regulation of the caspase pathway in the cytoplasm. We demonstrate that p53, when present at high levels in the cytoplasm, has an inhibitory effect on caspase-9. Concurrently, knockdown of endogenous p53 caused an increase in the activity of caspase-9. p53 was found to interact with the p35 fragment of caspase-9, and this interaction inhibits the caspase-9 activity. In a p53-null background, the high-level expression of both exogenous WT and mutant p53 increased the resistance of these cells to cisplatin, and the data showed a correlation between high p53 expression and caspase-9 inhibition. These results suggest the inhibition of caspase-9 as a potential mechanism in evading apoptosis in tumors with high-level p53 expression that is cytoplasmically localized.