Resistance to DNA-damaging agents is discordant from experimental metastatic capacity in MEF ras-transformants-expressing gain of function MTp53

Addiction of lung cancer cells to GOF p53 is promoted by up-regulation of epidermal growth factor receptor through multiple contacts with p53 transactivation domain and promoter

Human lung cancers harboring gain-of-function (GOF) p53 alleles express higher levels of the epidermal growth factor receptor (EGFR). We demonstrate that a number of GOF p53 alleles directly upregulate EGFR. Knock-down of p53 in lung cancer cells lowers EGFR expression and reduces tumorigenicity and other GOF p53 properties. However, addiction of lung cancer cells to GOF p53 can be compensated by overexpressing EGFR, suggesting that EGFR plays a critical role in addiction. Chromatin immunoprecipitation (ChIP) using lung cancer cells expressing GOF p53 alleles showed that GOF p53 localized to the EGFR promoter. The sequence where GOF p53 is found to interact by ChIP seq can act as a GOF p53 response element. The presence of GOF p53 on the EGFR promoter increased histone H3 acetylation, indicating a mechanism whereby GOF p53 enhances chromatin opening for improved access to transcription factors (TFs). ChIP and ChIP-re-ChIP with p53, Sp1 and CBP histone acetylase (HAT) antibodies revealed docking of GOF p53 on Sp1, leading to increased binding of Sp1 and CBP to the EGFR promoter. Up-regulation of EGFR can occur via GOF p53 contact at other novel sites in the EGFR promoter even when TAD-I is inactivated; these sites are used by both intact and TAD-I mutated GOF p53 and might reflect redundancy in GOF p53 mechanisms for EGFR transactivation. Thus, the oncogenic action of GOF p53 in lung cancer is highly dependent on transactivation of the EGFR promoter via a novel transcriptional mechanism involving coordinated interactions of TFs, HATs and GOF p53.

The Contrived Mutant p53 Oncogene - Beyond Loss of Functions

Mutations in p53 are almost synonymous with cancer – be it susceptibility to the disease or response to treatment – and therefore, are a critical determinant of overall survival. As most of these mutations occur in the DNA-binding domain of p53, many of the clinical correlations with mutant p53 have been initially relegated to the loss of its transcription-dependent activities as a tumor suppressor. However, significant efforts over the last two decades have led to the vast knowledge on the potential functions of the mutated p53 protein, which have been attributed to the physical presence of the mutant protein rather than the loss of its wild-type (WT) functions. Beyond the inhibitory effects of mutant p53 on the remaining WT protein that leads to the dominant-negative effect in the heterozygous state, mutant p53’s presence has also been significantly attributed to novel gain-of-functions that lead to addiction of cancer cells to its presence for survival, as well as for their ability to invade and metastasize, elevating it to a contrived oncogene that drives the cancer cells forward. This review will summarize the functional consequences of the presence of mutant p53 protein on cellular and organismal physiology.

Nondisruptive p53 mutations are associated with shorter survival in patients with advanced non-small cell lung cancer

PURPOSE

TP53 mutations in early-stage non-small cell lung cancer (NSCLC) may be associated with worse survival but their prognostic role in advanced NSCLC is controversial. In addition, it remains unclear whether mutated patients represent a clinically homogeneous group.

EXPERIMENTAL DESIGN

We retrospectively examined TP53 mutations and outcome in a training cohort of 318 patients with stage IIIB-IV NSCLC: 125 epidermal growth factor receptor (EGFR) wild-type (wt) and 193 EGFR mutated (mut). An independent validation cohort of 64 EGFR-mut patients was subsequently analyzed. Mutations were classified as "disruptive" and "nondisruptive" according to their predicted degree of disturbance of the p53 protein structure and function.

RESULTS

In the training cohort, TP53 mutations were found in 43 of the 125 EGFR-wt patients (34.4%). Of these, 28 had nondisruptive TP53 mutations and a median overall survival (OS) of 8.5 months, compared with 15.6 months for the remaining 97 patients (P=0.003). In the EGFR-mut group, TP53 mutations were found in 50 of the 193 patients (25.9%). The OS for the 26 patients with TP53 nondisruptive mutations was 17.8 months versus 28.4 months for the remaining 167 patients (P=0.04). In the validation cohort, the 11 patients with nondisruptive TP53 mutations had a median OS of 18.1 months compared with 37.8 months for the 53 remaining patients (P=0.006). In multivariate analyses, nondisruptive TP53 mutations had an independent, significant association with a shorter OS.

CONCLUSIONS

Nondisruptive mutations in the TP53 gene are an independent prognostic factor of shorter survival in advanced NSCLC.

Mutant p53 and the response to chemotherapy and radiation

In addition to playing roles in the genesis and progression of cancer, mutant p53 also appears to play a significant role in the response to cancer therapy. In response to chemotherapy and radiation, two mainstays of cancer treatment, most cancer cells harboring p53 mutations show a reduced sensitivity compared to cells lacking p53 or those with wild type p53. However, there are also many instances where mutant p53 has shown no effect or enhances cellular sensitivity to chemotherapy and radiation. Similar to the in vitro cellular studies, the majority of clinical studies show a correlation between the presence of mutant p53 in patient tumors and adverse outcomes following treatment with chemotherapy agents or radiation in comparison to tumors with wild-type p53. However, it still remains unclear whether the presence of mutant p53 in tumors can serve as a reliable prognostic factor and aid in treatment planning. Thus, as genomic analysis of patient tumors becomes more cost effective, the role of mutant p53 in tumor responses from cancer therapy ultimately needs to be addressed. This chapter will discuss current mechanisms of how p53 mutations affect cellular responses to chemotherapy and radiation and discuss patient outcomes based on p53 status.

p53: its mutations and their impact on transcription

p53 is a tumor suppressor protein whose key function is to maintain the integrity of the cell. Mutations in p53 have been found in up to 50 % of all human cancers and cause an increase in oncogenic phenotypes such as proliferation and tumorigenicity. Both wild-type and mutant p53 have been shown to transactivate their target genes, either through directly binding to DNA, or indirectly through protein-protein interactions. This review discusses possible mechanisms behind both wild-type and mutant p53-mediated transactivation and touches on the concept of addiction to mutant p53 of cancer cells and how that may be used for future therapies.

RUNX Family Participates in the Regulation of p53-Dependent DNA Damage Response

A proper DNA damage response (DDR), which monitors and maintains the genomic integrity, has been considered to be a critical barrier against genetic alterations to prevent tumor initiation and progression. The representative tumor suppressor p53 plays an important role in the regulation of DNA damage response. When cells receive DNA damage, p53 is quickly activated and induces cell cycle arrest and/or apoptotic cell death through transactivating its target genes implicated in the promotion of cell cycle arrest and/or apoptotic cell death such as p21^WAF1, BAX, and PUMA. Accumulating evidence strongly suggests that DNA damage-mediated activation as well as induction of p53 is regulated by posttranslational modifications and also by protein-protein interaction. Loss of p53 activity confers growth advantage and ensures survival in cancer cells by inhibiting apoptotic response required for tumor suppression. RUNX family, which is composed of RUNX1, RUNX2, and RUNX3, is a sequence-specific transcription factor and is closely involved in a variety of cellular processes including development, differentiation, and/or tumorigenesis. In this review, we describe a background of p53 and a functional collaboration between p53 and RUNX family in response to DNA damage.

Gain-of-Function Activity of Mutant p53 in Lung Cancer through Up-Regulation of Receptor Protein Tyrosine Kinase Axl

p53 mutations are present in up to 70% of lung cancer. Cancer cells with p53 mutations, in general, grow more aggressively than those with wild-type p53 or no p53. Expression of tumor-derived mutant p53 in cells leads to up-regulated expression of genes that may affect cell growth and oncogenesis. In our study of this aggressive phenotype, we have investigated the receptor protein tyrosine kinase Axl, which is up-regulated by p53 mutants at both RNA and protein levels in H1299 lung cancer cells expressing mutants p53-R175H, -R273H, and -D281G. Knockdown of endogenous mutant p53 levels in human lung cancer cells H1048 (p53-R273C) and H1437 (p53-R267P) led to a reduction in the level of Axl as well. This effect on Axl expression is refractory to the mutations at positions 22 and 23 of p53, suggesting that p53's transactivation domain may not play a critical role in the up-regulation of Axl gene expression. Chromatin immunoprecipitation (ChIP) assays carried out with acetylated histone antibodies demonstrated induced histone acetylation on the Axl promoter region by mutant p53. Direct mutant p53 nucleation on the Axl promoter was demonstrated by ChIP assays using antibodies against p53. The Axl promoter has a p53/p63 binding site, which however is not required for mutant p53-mediated transactivation. Knockdown of Axl by Axl-specific RNAi caused a reduction of gain-of-function (GOF) activities, reducing the cell growth rate and motility rate in lung cancer cells expressing mutant p53. This indicates that for lung cancer cell lines with mutant p53, GOF activities are mediated in part through Axl.

Mutant p53: one name, many proteins

There is now strong evidence that mutation not only abrogates p53 tumor-suppressive functions, but in some instances can also endow mutant proteins with novel activities. Such neomorphic p53 proteins are capable of dramatically altering tumor cell behavior, primarily through their interactions with other cellular proteins and regulation of cancer cell transcriptional programs. Different missense mutations in p53 may confer unique activities and thereby offer insight into the mutagenic events that drive tumor progression. Here we review mechanisms by which mutant p53 exerts its cellular effects, with a particular focus on the burgeoning mutant p53 transcriptome, and discuss the biological and clinical consequences of mutant p53 gain of function.

p53 mutants induce transcription of NF-κB2 in H1299 cells through CBP and STAT binding on the NF-κB2 promoter and gain of function activity

Cancer cells with p53 mutations, in general, grow more aggressively than those with wild-type p53 and show "gain of function" (GOF) phenotypes such as increased growth rate, enhanced resistance to chemotherapeutic drugs, increased cell motility and tumorigenicity; although the mechanism for this function remains unknown. In this communication we report that p53-mediated NF-κB2 up-regulation significantly contributes to the aggressive oncogenic behavior of cancer cells. Lowering the level of mutant p53 in a number of cancer cell lines resulted in a loss of GOF phenotypes directly implicating p53 mutants in the process. RNAi against NF-κB2 in naturally occurring cancer cell lines also lowers GOF activities. In H1299 cells expressing mutant p53, chromatin immunoprecipitation (ChIP) assays indicate that mutant p53 induces histone acetylation at specific sites on the regulatory regions of its target genes. ChIP assays using antibodies against transcription factors putatively capable of interacting with the NF-κB2 promoter show increased interaction of CBP and STAT2 in the presence of mutant p53. Thus, we propose that in H1299 cells, mutant p53 elevates expression of genes capable of enhancing cell proliferation, motility, and tumorigenicity by inducing acetylation of histones via recruitment of CBP and STAT2 on the promoters causing CBP-mediated histone acetylation.

Protein-protein interactions occur between p53 phosphoforms and ATM and 53BP1 at sites of exogenous DNA damage

We have previously shown that the Ser15-phosphorylated p53 phosphoform, p53(Ser15), can localize at sites of ionizing radiation-induced DNA damage. In this study, we hypothesized that the non-specific DNA binding domain (NSDBD) of the p53 carboxy-terminus (C-terminus) mediates chromatin anchoring at sites of DNA damage to interact with two key mediators of the DNA damage response (DDR): ATM and 53BP1. Exogenous YFP-p53 fusion constructs expressing C-terminus deletion mutants of p53 were transfected into p53-null H1299 cells and tracked by microscopy and biochemistry to determine relative chromatin-binding pre- and postirradiation. We observed that exogenous YFP-p53(WT) and YFP-p53(Δ367-393) associated with ATM(Ser1981) and 53BP1 in the nuclear, chromatin-bound fractions after DNA damage. Of interest, YFP-p53(Δ1-299) fusion proteins, which lack transcriptional trans-activation and the Ser15-residue, bound to ATM(Ser1981) but not to 53BP1. In support of these data, we used subnuclear UV-microbeam and immunoprecipitation analyses of irradiated normal human fibroblasts (HDFs) that confirmed an interaction between endogenous p53 and ATM or 53BP1. Based on these observations, we propose a model whereby a pre-existing pool of p53 responds immediately to radiation-induced DNA damage using the C-terminus to spatially facilitate protein-protein interactions and the DDR at sites of DNA damage.

Tumor senescence and radioresistant tumor-initiating cells (TICs): let sleeping dogs lie!

Preclinical data from cell lines and experimental tumors support the concept that breast cancer-derived tumor-initiating cells (TICs) are relatively resistant to ionizing radiation and chemotherapy. This could be a major determinant of tumor recurrence following treatment. Increased clonogenic survival is observed in CD24^-/low/CD44⁺ TICs derived from mammosphere cultures and is associated with (a) reduced production of reactive oxygen species, (b) attenuated activation of γH2AX and CHK2-p53 DNA damage signaling pathways, (c) reduced propensity for ionizing radiation-induced apoptosis, and (d) altered DNA double-strand or DNA single-strand break repair. However, recent data have shed further light on TIC radioresistance as irradiated TICs are resistant to tumor cell senescence following DNA damage. Taken together, the cumulative data support a model in which DNA damage signaling and repair pathways are altered in TICs and lead to an altered mode of cell death with unique consequences for long-term clonogen survival. The study of TIC senescence lays the foundation for future experiments in isogenic models designed to directly test the capacity for senescence and local control (that is, not solely local regression) and spontaneous metastases following treatment in vivo. The study also supports the targeting of tumor cell senescence pathways to increase TIC clonogen kill if the targeting also maintains the therapeutic ratio.

Responsiveness of stromal fibroblasts to IFN-gamma blocks tumor growth via angiostasis

The importance of stromal cells for tumor is akin to soil for seed. However, the interaction among these cells is far from understood. In this study, we show that stromal fibroblasts exist not only during tumor progression but also during regression stage, together with immune effector cells. Coinjection of stromal fibroblasts with tumor cells often promotes tumor growth. However, the presence of IFN-gamma significantly impairs the ability of these cells to promote tumor growth due to a reduced angiogenesis. The mechanism relies mainly on the IFN-gamma-mediated down-regulation of vascular endothelial growth factor production by fibroblasts. The results reveal a novel link between immune cells and nonbone marrow-derived stromal cells, and define stromal fibroblasts as the main targets of IFN-gamma in tumor immunity.

Impaired p53 binding to importin: a novel mechanism of cytoplasmic sequestration identified in oxaliplatin-resistant cells

Previous studies have described one nuclear localization signal (NLSI) in p53 and speculated on two additional sites termed NLSII and NLSIII. Drug-resistant KB cells selected with cisplatin or oxaliplatin were found to have increased p53 levels and in oxaliplatin-selected cells, a larger p53 predominantly in the cytoplasm. In oxaliplatin-selected cells a single nucleotide deletion in the sequence-encoding amino acid 382, part of NLSIII, resulted in a frame shift and a 420 amino acid protein (p53(420)). We investigated explanations for the cytoplasmic sequestration of p53(420) while assessing the role, if any, of NLSII and NLSIII in p53 nuclear import. We found that neither NLSII nor NLSIII are essential for p53 nuclear localization. Furthermore, we confirmed p53(420) is able to tetramerize, transactivate a p21 promoter, bind dynein and that the reduced nuclear accumulation is not a consequence of increased p53 nuclear export. However, the association of p53(420) with importin-beta, essential for nuclear import, was significantly impaired. We conclude that despite sequence similarity to consensus NLSs neither NLSII nor NLSIII have roles in p53 nuclear transport. We also identified impaired association with importin as a novel mechanism of p53 cytoplasmic sequestration that impairs nuclear transport rendering cells functionally deficient in p53.

Evidence for the direct binding of phosphorylated p53 to sites of DNA breaks in vivo

Despite a clear link between ataxia-telangiectasia mutated (ATM)-dependent phosphorylation of p53 and cell cycle checkpoint control, the intracellular biology and subcellular localization of p53 phosphoforms during the initial sensing of DNA damage is poorly understood. Using G0-G1 confluent primary human diploid fibroblast cultures, we show that endogenous p53, phosphorylated at Ser15 (p53Ser15), accumulates as discrete, dose-dependent and chromatin-bound foci within 30 minutes following induction of DNA breaks or DNA base damage. This biologically distinct subpool of p53Ser15 is ATM dependent and resistant to 26S-proteasomal degradation. p53Ser15 colocalizes and coimmunoprecipitates with gamma-H2AX with kinetics similar to that of biochemical DNA double-strand break (DNA-dsb) rejoining. Subnuclear microbeam irradiation studies confirm p53Ser15 is recruited to sites of DNA damage containing gamma-H2AX, ATM(Ser1981), and DNA-PKcs(Thr2609) in vivo. Furthermore, studies using isogenic human and murine cells, which express Ser15 or Ser18 phosphomutant proteins, respectively, show defective nuclear foci formation, decreased induction of p21WAF, decreased gamma-H2AX association, and altered DNA-dsb kinetics following DNA damage. Our results suggest a unique biology for this p53 phosphoform in the initial steps of DNA damage signaling and implicates ATM-p53 chromatin-based interactions as mediators of cell cycle checkpoint control and DNA repair to prevent carcinogenesis.

No anti-apoptotic effects of single copies of mutant p53 genes in drug-treated tumor cells

Some mutant forms of the p53 tumor suppressor have been documented to exert novel oncogenic functions including the increase of tumorigenicity, metastatic potential, genomic instability and therapy resistance of tumor cells. The latter has been suggested to be caused, primarily, by inhibition of apoptosis and, in part, through the activation of genes by mutant p53 whose products can counteract drug activities. Recently described in this context was the dUTPase, which may confer resistance to fluoropyrimidine drugs such as 5-fluorouracil (5-FU). We report here findings that call in question the existence of a direct anti-apoptotic effect of mutant p53. Wild-type p53-negative human fibroblasts, and Saos-2, H1299 and HCT116 tumor cells, treated with adriamycin, etoposide, cisplatin or 5-FU, failed to show apoptosis resistance when retrovirally bulk-infected to express the p53 mutants 175H or 273H at levels observed in naturally mutant p53-producing tumor cells. Furthermore, dUTPase gene expression was not stimulated by mutant p53, but instead by cellular events that involve DNA synthesis. We interpret the combined available data to suggest that much of the anti-apoptotic effect of mutant p53 is indirect and secondary to DNA-damaging and/or repair-interfering effects of these proteins.

Differential Response to Radiation of TP53-Inactivated Cells by Overexpression of Dominant-Negative Mutant TP53 or HPVE6

The inactivation of TP53 by transfection of a dominant- negative mutated TP53 (MP53.13 cells) was compared with inactivation of TP53 by transfection with the HPV E6 gene (RC10.1 cells) with respect to PLD repair, G(1)-phase arrest, and induction of color junctions. Functional G(1) arrest was demonstrated in parental (RKO) cells with wild-type TP53, while in RC10.1 cells the G(1) arrest was eliminated. In MP53.13 cells an intermediate G(1) arrest was found. Functionality of endogenous TP53 was confirmed in RKO and MP53.13 cells by accumulation of TP53 protein and its downstream target CDKN1A (p21). Radiation survival of MP53.13 cells was higher than that of RKO cells, and PLD repair was found in RKO cells and MP53.13 cells but not in RC10.1 cells. Both with and without irradiation, the number of color junctions was 50 to 80% higher in MP53.13 cells than in RKO and RC10.1 cells. In the MP53.13 cells, the genetic instability appears to lead to more aberrations and to radioresistance. In spite of the presence of an excess of mutated TP53, wild- type TP53 functions appear to be affected only partly or not at all.