Integrity of the N-terminal transcription domain of p53 is required for mutant p53 interference with drug-induced apoptosis

p53R172H and p53R245W Hotspot Mutations Drive Distinct Transcriptomes in Mouse Mammary Tumors Through a Convergent Transcriptional Mediator

Aggressive breast cancers harbor TP53 missense mutations. Tumor cells with TP53 missense mutations exhibit enhanced growth and survival through transcriptional rewiring. To delineate how TP53 mutations in breast cancer contribute to tumorigenesis and progression in vivo, we created a somatic mouse model driven by mammary epithelial cell-specific expression of Trp53 mutations. Mice developed primary mammary tumors reflecting the human molecular subtypes of luminal A, luminal B, HER2-enriched, and triple-negative breast cancer with metastases. Transcriptomic analyses comparing MaPR172H/- or MaPR245W/- mammary tumors to MaP-/- tumors revealed (1) differences in cancer-associated pathways activated in both p53 mutants and (2) Nr5a2 as a novel transcriptional mediator of distinct pathways in p53 mutants. Meta-analyses of human breast tumors corroborated these results. In vitro assays demonstrate mutant p53 upregulates specific target genes that are enriched for Nr5a2 response elements in their promoters. Co-immunoprecipitation studies revealed p53R172H and p53R245W interact with Nr5a2. These findings implicate NR5A2 as a novel mediator of mutant p53 transcriptional activity in breast cancer.

SIGNIFICANCE

Our findings implicate NR5A2 as a novel mediator of mutant p53 transcriptional activity in breast cancer. NR5A2 may be an important therapeutic target in hard-to-treat breast cancers such as endocrine-resistant tumors and metastatic triple-negative breast cancers harboring TP53 missense mutations.

Applications of Recombinant Adenovirus-p53 Gene Therapy for Cancers in the Clinic in China

Suppression of TP53 function is nearly ubiquitous in human cancers, and a significant fraction of cancers have mutations in the TP53 gene itself. Therefore, the wild-type TP53 gene has become an important target gene for transformation research of cancer gene therapy. In 2003, the first anti-tumor gene therapy drug rAd-p53 (recombinant human p53 adenovirus), trade name Gendicine™, was approved by the China Food and Drug Administration (CFDA) for treatment of head and neck squamous cell carcinoma (HNSCC) in combination with radiotherapy. The recombinant human TP53 gene is delivered into cancer cells by an adenovirus vector constructed to express the functional p53 protein. Although the only currently approved used of Gendicine is in combination with radiotherapy for treatment of HNSCC, clinical studies have been carried out for more than 20 other applications of Gendicine in treating cancer, including treatment of advanced lung cancer, advanced liver cancer, malignant gynecological tumors, and soft tissue sarcomas. Currently more than 30,000 patients have been treated with Gendicine. This review provides an overview of the clinical applications of Gendicine in China. We summarize a total of 48 studies with 2,561 patients with solid tumors, including 34 controlled clinical studies and 14 open clinical studies, i.e., clinical studies without a control group. There are 11 studies for head and neck cancer, 10 for liver cancer, 6 for malignant gynecological tumors, 4 for non-small cell lung cancer, 4 for soft tissue sarcoma, 4 for malignant effusion, 2 for gastrointestinal tumors, and 7 for other types of cancer. In all the reported clinical studies, the most common side effect was self-limited fever. Intratumoral injection and intra-arterial infusion were the most common routes of administration. Overall, Gendicine combined with chemotherapy, radiotherapy, or other conventional treatment regimens demonstrated significantly higher response rates compared to standard therapies alone. Some of the published studies also showed that Gendicine combination regimens demonstrated longer progression-free survival times than conventional treatments alone. To date, Gendicine has been clinically used in China for treatment of cancers other than HNSCC for more than ten years, mainly for patients with advanced or unresectable malignant tumors. However, the establishment of standard treatment regimens using TP53 gene therapy is still needed in order to advance its use in clinical practice.

Mutant TP53 interacts with BCAR1 to contribute to cancer cell invasion

BACKGROUND

Mutant TP53 interacts with other proteins to produce gain-of-function properties that contribute to cancer metastasis. However, the underlying mechanisms are still not fully understood.

METHODS

Using immunoprecipitation and proximity ligation assays, we evaluated breast cancer anti-estrogen resistance 1 (BCAR1) as a novel binding partner of TP53R273H, a TP53 mutant frequently found in human cancers. The biological functions of their binding were examined by the transwell invasion assay. Clinical outcome of patients was analysed based on TP53 status and BCAR1 expression using public database.

RESULTS

We discovered a novel interaction between TP53R273H and BCAR1. We found that BCAR1 translocates from the cytoplasm into the nucleus and binds to TP53R273H in a manner dependent on SRC family kinases (SFKs), which are known to enhance metastasis. The expression of full-length TP53R273H, but not the BCAR1 binding-deficient mutant TP53R273HΔ102-207, promoted cancer cell invasion. Furthermore, among the patients with mutant TP53, high BCAR1 expression was associated with a poorer prognosis.

CONCLUSIONS

The interaction between TP53R273H and BCAR1 plays an important role in enhancing cancer cell invasion. Thus, our study suggests a disruption of the TP53R273H-BCAR1 binding as a potential therapeutic approach for TP53R273H-harbouring cancer patients.

A Driver Never Works Alone-Interplay Networks of Mutant p53, MYC, RAS, and Other Universal Oncogenic Drivers in Human Cancer

The knowledge accumulating on the occurrence and mechanisms of the activation of oncogenes in human neoplasia necessitates an increasingly detailed understanding of their systemic interactions. None of the known oncogenic drivers work in isolation from the other oncogenic pathways. The cooperation between these pathways is an indispensable element of a multistep carcinogenesis, which apart from inactivation of tumor suppressors, always includes the activation of two or more proto-oncogenes. In this review we focus on representative examples of the interaction of major oncogenic drivers with one another. The drivers are selected according to the following criteria: (1) the highest frequency of known activation in human neoplasia (by mutations or otherwise), (2) activation in a wide range of neoplasia types (universality) and (3) as a part of a distinguishable pathway, (4) being a known cause of phenotypic addiction of neoplastic cells and thus a promising therapeutic target. Each of these universal oncogenic factors—mutant p53, KRAS and CMYC proteins, telomerase ribonucleoprotein, proteasome machinery, HSP molecular chaperones, NF-κB and WNT pathways, AP-1 and YAP/TAZ transcription factors and non-coding RNAs—has a vast network of molecular interrelations and common partners. Understanding this network allows for the hunt for novel therapeutic targets and protocols to counteract drug resistance in a clinical neoplasia treatment.

Inhibition of Obtusifolin on retinal pigment epithelial cell growth under hypoxia

AIM

To explore the effect of Obtusifolin on retinal pigment epithelial cell growth under hypoxia.

METHODS

In vitro chemical hypoxia model of ARPE-19 cells was established using cobalt chloride (CoCl₂). Cell viability was tested by cell counting kit-8 (CCK-8) assay. Western blot and real-time quantitative polymerase chain reaction were applied to detect proteins and mRNAs respectively. Flow cytometry was used to examine the cell cycle. Secretion of vascular endothelial growth factor (VEGF) was tested by using enzyme linked immunosorbent assay (ELISA).

RESULTS

Under the chemical hypoxia model established by CoCl₂, hypoxia inducible factor-1α (HIF-1α) mRNA and protein levels was up-regulated. Cell viability was increased and the proportion of S phase was higher. Obtusifolin could reduce cell viability under hypoxic conditions and arrest cells in G1 phase. Obtusifolin reduced the expression of Cyclin D1 and proliferating cell nuclear antigen (PCNA) in the hypoxic environment and increased the expression of p53 and p21. The levels of VEGF, VEGFR2 and eNOS proteins and mRNA were significantly increased under hypoxia while Obtusifolin inhibited the increasing.

CONCLUSION

Obtusifolin can inhibit cell growth under hypoxic conditions and down-regulate HIF-1/VEGF/eNOS secretions in ARPE-19 cells.

p53 Phosphomimetics Preserve Transient Secondary Structure but Reduce Binding to Mdm2 and MdmX

The disordered p53 transactivation domain (p53TAD) contains specific levels of transient helical secondary structure that are necessary for its binding to the negative regulators, mouse double minute 2 (Mdm2) and MdmX. The interactions of p53 with Mdm2 and MdmX are also modulated by posttranslational modifications (PTMs) of p53TAD including phosphorylation at S15, T18 and S20 that inhibits p53-Mdm2 binding. It is unclear whether the levels of transient secondary structure in p53TAD are changed by phosphorylation or other PTMs. We used phosphomimetic mutants to determine if adding a negative charge at positions 15 and 18 has any effect on the transient secondary structure of p53TAD and protein-protein binding. Using a combination of biophysical and structural methods, we investigated the effects of single and multisite phosphomimetics on the transient secondary structure of p53TAD and its interaction with Mdm2, MdmX, and the KIX domain. The phosphomimetics reduced Mdm2 and MdmX binding affinity by 3–5-fold, but resulted in minimal changes in transient secondary structure, suggesting that the destabilizing effect of phosphorylation on the p53TAD-Mdm2 interaction is primarily electrostatic. Phosphomimetics had no effect on the p53-KIX interaction, suggesting that increased binding of phosphorylated p53 to KIX may be influenced by decreased competition with its negative regulators.

Cytotoxicity of nimbolide towards multidrug-resistant tumor cells and hypersensitivity via cellular metabolic modulation

Nimbolide is considered a promising natural product in cancer prevention and treatment. However, it is not known yet, whether the different mechanisms of multidrug resistance (MDR) influence its anticancer activity. In this study, well-known MDR mechanisms (ABCB1, ABCG2, ABCB5, TP53, EGFR) were evaluated against nimbolide. The P-glycoprotein (ABCB1/MDR1)-overexpressing CEM/ADR5000 cell line displayed remarkable hypersensitivity to nimbolide, which was mediated through upregulation of the tumor suppressor, PTEN, and its downstream components resulted in significant downregulation in ABCB1/MDR1 mRNA and P-glycoprotein. In addition, nimbolide targeted essential cellular metabolic-regulating elements including HIF1α, FoxO1, MYC and reactive oxygen species. The expression of breast cancer resistance protein (BCRP) as well as epidermal growth factor receptor (EGFR) and mutant tumor suppressor TP53 did not correlate to nimbolide’s activity. Furthermore, this paper looked for other molecular determinants that might determine tumor cellular response towards nimbolide. COMPARE and hierarchical cluster analyses of transcriptome-wide microarray-based mRNA expressions of the NCI 60 cell line panel were performed, and a set of 40 genes from different functional groups was identified. The data suggested NF-κB as master regulator of nimbolide’s activity. Interestingly, HIF1α was determined by COMPARE analysis to mediate sensitivity to nimbolide, which would be of great benefit in targeted therapy.

Mutant p53 partners in crime

Mutant p53 proteins impart changes in cellular behavior and function through interactions with proteins that alter gene expression. The milieu of intracellular proteins available to interact with mutant p53 is context specific and changes with disease, cell type, and environmental conditions. Varying conformations of mutant p53 largely dictate protein–protein interactions as different point mutations within protein-coding regions greatly alter the extent and array of gain-of-function (GOF) activities. Given such variables, how can knowledge regarding p53 missense mutations be translated into predicting or altering biologic activity for therapy? How may knowledge regarding mutant p53 functions within certain disease contexts be harnessed to blunt or ablate mutant p53 GOF for therapy? In this article, we review known proteins that interact with mutant p53 and result in the activation of genes that contribute to p53 GOF with particular emphasis on context dependency and an evolving appreciation of GOF mechanisms.

Li-Fraumeni Syndrome Disease Model: A Platform to Develop Precision Cancer Therapy Targeting Oncogenic p53

Li-Fraumeni syndrome (LFS) is a rare hereditary autosomal dominant cancer disorder. Germline mutations in TP53, the gene encoding p53, are responsible for most cases of LFS. TP53 is also the most commonly mutated gene in human cancers. Because inhibition of mutant p53 is considered to be a promising therapeutic strategy to treat these diseases, LFS provides a perfect genetic model to study p53 mutation-associated malignancies as well as to screen potential compounds targeting oncogenic p53. In this review we briefly summarize the biology of LFS and current understanding of the oncogenic functions of mutant p53 in cancer development. We discuss the strengths and limitations of current LFS disease models, and touch on existing compounds targeting oncogenic p53 and in vitro clinical trials to develop new ones. Finally, we discuss how recently developed methodologies can be integrated into the LFS induced pluripotent stem cell (iPSC) platform to develop precision cancer therapy.

Mutant p53 Protein and the Hippo Transducers YAP and TAZ: A Critical Oncogenic Node in Human Cancers

p53 protein is a well-known tumor suppressor factor that regulates cellular homeostasis. As it has several and key functions exerted, p53 is known as “the guardian of the genome” and either loss of function or gain of function mutations in the TP53 coding protein sequence are involved in cancer onset and progression. The Hippo pathway is a key regulator of developmental and regenerative physiological processes but if deregulated can induce cell transformation and cancer progression. The p53 and Hippo pathways exert a plethora of fine-tuned functions that can apparently be in contrast with each other. In this review, we propose that the p53 status can affect the Hippo pathway function by switching its outputs from tumor suppressor to oncogenic activities. In detail, we discuss: (a) the oncogenic role of the protein complex mutant p53/YAP; (b) TAZ oncogenic activation mediated by mutant p53; (c) the therapeutic potential of targeting mutant p53 to impair YAP and TAZ oncogenic functions in human cancers.

Transcriptional Regulation by Wild-Type and Cancer-Related Mutant Forms of p53

TP53 missense mutations produce a mutant p53 protein that cannot activate the p53 tumor suppressive transcriptional response, which is the primary selective pressure for TP53 mutation. Specific codons of TP53, termed hotspot mutants, are mutated at elevated frequency. Hotspot forms of mutant p53 possess oncogenic properties in addition to being deficient in tumor suppression. Such p53 mutants accumulate to high levels in the cells they inhabit, causing transcriptional alterations that produce pro-oncogenic activities, such as increased pro-growth signaling, invasiveness, and metastases. These forms of mutant p53 very likely use features of wild-type p53, such as interactions with the transcriptional machinery, to produce oncogenic effects. In this review, we discuss commonalities between wild-type and mutant p53 proteins with an emphasis on transcriptional processes.

Oncogenic Mutant p53 Gain of Function Nourishes the Vicious Cycle of Tumor Development and Cancer Stem-Cell Formation

More than half of human tumors harbor an inactivated p53 tumor-suppressor gene. It is well accepted that mutant p53 shows an oncogenic gain-of-function (GOF) activity that facilitates the transformed phenotype of cancer cells. In addition, a growing body of evidence supports the notion that cancer stem cells comprise a seminal constituent in the initiation and progression of cancer development. Here, we elaborate on the mutant p53 oncogenic GOF leading toward the acquisition of a transformed phenotype, as well as placing mutant p53 as a major component in the establishment of cancer stem cell entity. Therefore, therapy targeted toward cancer stem cells harboring mutant p53 is expected to pave the way to eradicate tumor growth and recurrence.

Oncogenic Intra-p53 Family Member Interactions in Human Cancers

The p53 gene family members p53, p73, and p63 display several isoforms derived from the presence of internal promoters and alternative splicing events. They are structural homologs but hold peculiar functional properties. p53, p73, and p63 are tumor suppressor genes that promote differentiation, senescence, and apoptosis. p53, unlike p73 and p63, is frequently mutated in cancer often displaying oncogenic “gain of function” activities correlated with the induction of proliferation, invasion, chemoresistance, and genomic instability in cancer cells. These oncogenic functions are promoted either by the aberrant transcriptional cooperation of mutant p53 (mutp53) with transcription cofactors (e.g., NF-Y, E2F1, Vitamin D Receptor, Ets-1, NF-kB and YAP) or by the interaction with the p53 family members, p73 and p63, determining their functional inactivation. The instauration of these aberrant transcriptional networks leads to increased cell growth, low activation of DNA damage response pathways (DNA damage response and DNA double-strand breaks response), enhanced invasion, and high chemoresistance to different conventional chemotherapeutic treatments. Several studies have clearly shown that different cancers harboring mutant p53 proteins exhibit a poor prognosis when compared to those carrying wild-type p53 (wt-p53) protein. The interference of mutantp53/p73 and/or mutantp53/p63 interactions, thereby restoring p53, p73, and p63 tumor suppression functions, could be among the potential therapeutic strategies for the treatment of mutant p53 human cancers.

Mutant p53 and ETS2, a Tale of Reciprocity

TP53 is one of the most frequently inactivated tumor suppressor genes in human cancer. However, unlike other tumor suppressor genes whose expression is lost, TP53 is usually inactivated as a result of a single nucleotide change within the coding region. Typically, these single nucleotide mutations result in a codon change that creates an amino acid substitution. Thus, unlike other tumor suppressor genes whose expression is lost due to genetic or epigenetic changes, the p53 gene primarily suffers missense mutations, and therefore, the cells retain and express a mutant form of the p53 protein (mtp53). It is now well established that mtp53 contributes to tumor development through its gain-of-function (GOF) activities. These GOF activities can arise from novel protein–protein interactions that can either disable other tumor suppressors (e.g., p63 and p73) or enable oncogenes such as ETS2, an ETS family member. In this review, I will focus on the identification of the mtp53/ETS2 complex and outline the diverse activities that this transcriptional regulatory complex controls to promote cancer.

Comparison of effects of p53 null and gain-of-function mutations on salivary tumors in MMTV-Hras transgenic mice

p53 is an important tumor suppressor gene which is mutated in ~50% of all human cancers. Some of these mutants appear to have acquired novel functions beyond merely losing wild-type functions. To investigate these gain-of-function effects in vivo, we generated mice of three different genotypes: MMTV-Hras/p53(+/+), MMTV-Hras/p53(-/-), and MMTV-Hras/p53R172H/R172H. Salivary tumors from these mice were characterized with regard to age of tumor onset, tumor growth rates, cell cycle distribution, apoptotic levels, tumor histopathology, as well as response to doxorubicin treatment. Microarray analysis was also performed to profile gene expression. The MMTV-Hras/p53(-/-) and MMTV-Hras/p53R172H/R172H mice displayed similar properties with regard to age of tumor onset, tumor growth rates, tumor histopathology, and response to doxorubicin, while both groups were clearly distinct from the MMTV-Hras/p53(+/+) mice by these measurements. In addition, the gene expression profiles of the MMTV-Hras/p53(-/-) and MMTV-Hras/p53(R172H/R172H) tumors were tightly clustered, and clearly distinct from the profiles of the MMTV-Hras/p53(+/+) tumors. Only a small group of genes showing differential expression between the MMTV-Hras/p53(-/-) and MMTV-Hras/p53(R172H/R172H) tumors, that did not appear to be regulated by wild-type p53, were identified. Taken together, these results indicate that in this MMTV-Hras-driven salivary tumor model, the major effect of the p53 R172H mutant is due to the loss of wild-type p53 function, with little or no gain-of-function effect on tumorigenesis, which may be explained by the tissue- and tumor type-specific properties of this gain-of-function mutant of p53.

Pla2g16 phospholipase mediates gain-of-function activities of mutant p53

p53(R172H/+) mice inherit a p53 mutation found in Li-Fraumeni syndrome and develop metastatic tumors at much higher frequency than p53(+/-) mice. To explore the mutant p53 metastatic phenotype, we used expression arrays to compare primary osteosarcomas from p53(R172H/+) mice with metastasis to osteosarcomas from p53(+/-) mice lacking metastasis. For this study, 213 genes were differentially expressed with a P value <0.05. Of particular interest, Pla2g16, which encodes a phospholipase that catalyzes phosphatidic acid into lysophosphatidic acid and free fatty acid (both implicated in metastasis), was increased in p53(R172H/+) osteosarcomas. Functional analyses showed that Pla2g16 knockdown decreased migration and invasion in mutant p53-expressing cells, and vice versa: overexpression of Pla2g16 increased the invasion of p53-null cells. Furthermore, Pla2g16 levels were increased upon expression of mutant p53 in both mouse and human osteosarcoma cell lines, indicating that Pla2g16 is a downstream target of the mutant p53 protein. ChIP analysis revealed that several mutant p53 proteins bind the Pla2g16 promoter at E26 transformation-specific (ETS) binding motifs and knockdown of ETS2 suppressed mutant p53 induction of Pla2g16. Thus, our study identifies a phospholipase as a transcriptional target of mutant p53 that is required for metastasis.

Differential regulation of the REGγ-proteasome pathway by p53/TGF-β signalling and mutant p53 in cancer cells

Proteasome activity is frequently enhanced in cancer to accelerate metastasis and tumorigenesis. REGγ, a proteasome activator known to promote p53/p21/p16 degradation, is often overexpressed in cancer cells. Here we show that p53/TGF-β signalling inhibits the REGγ–20S proteasome pathway by repressing REGγ expression. Smad3 and p53 interact on the REGγ promoter via the p53RE/SBE region. Conversely, mutant p53 binds to the REGγ promoter and recruits p300. Importantly, mutant p53 prevents Smad3/N-CoR complex formation on the REGγ promoter, which enhances the activity of the REGγ–20S proteasome pathway and contributes to mutant p53 gain of function. Depletion of REGγ alters the cellular response to p53/TGF-β signalling in drug resistance, proliferation, cell cycle progression and proteasome activity. Moreover, p53 mutations show a positive correlation with REGγ expression in cancer samples. These findings suggest that targeting REGγ–20S proteasome for cancer therapy may be applicable to human tumours with abnormal p53/Smad protein status. Furthermore, this study demonstrates a link between p53/TGF-β signalling and the REGγ–20S proteasome pathway, and provides insight into the REGγ/p53 feedback loop.

REGγ is a proteasome activator and is frequently overexpressed in cancer cells. Here Ali et al. demonstrate that p53/TGF-β signalling inhibits REGγ expression, whereas p53 mutations increase REGγ transcription, identifying a gain of function for mutant p53.

Contribution of p53 to metastasis

The tumor suppressor p53 is lost or mutated in about half of all human cancers, and in those tumors in which it is wild-type, mechanisms exist to prevent its activation. p53 loss not only prevents incipient tumor cells from undergoing oncogene-induced senescence and apoptosis, but also perturbs cell-cycle checkpoints. This enables p53-deficient tumor cells with DNA damage to continue cycling, creating a permissive environment for the acquisition of additional mutations. Theoretically, this could contribute to the evolution of a cancer genome that is conducive to metastasis. Importantly, p53 loss also results in the disruption of pathways that inhibit metastasis, and transcriptionally defective TP53 mutants are known to gain additional functions that promote metastasis. Here, we review the evidence supporting a role for p53 loss or mutation in tumor metastasis, with an emphasis on breast cancer.

SIGNIFICANCE

The metastatic potential of tumor cells can be positively infl uenced by loss of p53 or expression of p53 gain-of-function mutants. Understanding the mechanisms by which p53 loss and mutation promote tumor metastasis is crucial to understanding the biology of tumor progression and how to appropriately apply targeted therapies.

The paradigm of mutant p53-expressing cancer stem cells and drug resistance

It is well accepted that expression of mutant p53 involves the gain of oncogenic-specific activities accentuating the malignant phenotype. Depending on the specific cancer type, mutant p53 can contribute to either the early or the late events of the multiphase process underlying the transformation of a normal cell into a cancerous one. This multifactorial system is evident in ~50% of human cancers. Mutant p53 was shown to interfere with a variety of cellular functions that lead to augmented cell survival, cellular plasticity, aberration of DNA repair machinery and other effects. All these effects culminate in the acquisition of drug resistance often seen in cancer cells. Interestingly, drug resistance has also been suggested to be associated with cancer stem cells (CSCs), which reside within growing tumors. The notion that p53 plays a regulatory role in the life of stem cells, coupled with the observations that p53 mutations may contribute to the evolvement of CSCs makes it challenging to speculate that drug resistance and cancer recurrence are mediated by CSCs expressing mutant p53.

Mutant p53 in cancer: new functions and therapeutic opportunities

Many different types of cancer show a high incidence of TP53 mutations, leading to the expression of mutant p53 proteins. There is growing evidence that these mutant p53s have both lost wild-type p53 tumor suppressor activity and gained functions that help to contribute to malignant progression. Understanding the functions of mutant p53 will help in the development of new therapeutic approaches that may be useful in a broad range of cancer types.

Gain-of-function mutant p53 downregulates miR-223 contributing to chemoresistance of cultured tumor cells

Mutant p53 proteins are expressed at high frequency in human tumors and are associated with poor clinical prognosis and resistance to chemotherapeutic treatments. Here we show that mutant p53 proteins downregulate micro-RNA (miR)-223 expression in breast and colon cancer cell lines. Mutant p53 binds the miR-223 promoter and reduces its transcriptional activity. This requires the transcriptional repressor ZEB-1. We found that miR-223 exogenous expression sensitizes breast and colon cancer cell lines expressing mutant p53 to treatment with DNA-damaging drugs. Among the putative miR-223 targets, we focused on stathmin-1 (STMN-1), an oncoprotein known to confer resistance to chemotherapeutic drugs associated with poor clinical prognosis. Mutant p53 silencing or miR-223 exogenous expression lowers the levels of STMN-1 and knockdown of STMN-1 by small interfering RNA increases cell death of mutant p53-expressing cell lines. On the basis of these findings, we propose that one of the pathways affected by mutant p53 to increase cellular resistance to chemotherapeutic agents involves miR-223 downregulation and the consequent upregulation of STMN-1.

p53 mutations in cancer

In the past fifteen years, it has become apparent that tumour-associated p53 mutations can provoke activities that are different to those resulting from simply loss of wild-type tumour-suppressing p53 function. Many of these mutant p53 proteins acquire oncogenic properties that enable them to promote invasion, metastasis, proliferation and cell survival. Here we highlight some of the emerging molecular mechanisms through which mutant p53 proteins can exert these oncogenic functions.

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.

MicroRNAs, wild-type and mutant p53: more questions than answers

The tumor suppressor p53 is a sequence-specific transcription factor that activates the expression of genes involved in apoptosis, cell cycle arrest and senescence. p53 can also inhibit gene expression and this effect is partly mediated by inducing several microRNAs (miRNAs). MiRNAs have emerged as a new class of regulators of the expression and function of eukaryotic genomes. Tumor suppressive or oncogenic functions have been attributed to some miRNAs. Recent studies have shown that p53 can alter the transcription of several miRNAs, and in some cases, it can also influence miRNA maturation. Conversely, miRNAs can also modulate the abundance and activity of p53 by direct or indirect mechanisms. Moreover, mutant p53 can actively repress the expression of some miRNAs that are activated by wild-type p53. In this review, we discuss recent evidences of this crosstalk between miRNAs and the p53 network and also highlight its implications in cancer.

Mutant p53 disrupts mammary tissue architecture via the mevalonate pathway

p53 is a frequent target for mutation in human tumors, and mutant p53 proteins can actively contribute to tumorigenesis. We employed a three-dimensional culture model in which nonmalignant breast epithelial cells form spheroids reminiscent of acinar structures found in vivo, whereas breast cancer cells display highly disorganized morphology. We found that mutant p53 depletion is sufficient to phenotypically revert breast cancer cells to a more acinar-like morphology. Genome-wide expression analysis identified the mevalonate pathway as significantly upregulated by mutant p53. Statins and sterol biosynthesis intermediates reveal that this pathway is both necessary and sufficient for the phenotypic effects of mutant p53 on breast tissue architecture. Mutant p53 associates with sterol gene promoters at least partly via SREBP transcription factors. Finally, p53 mutation correlates with highly expressed sterol biosynthesis genes in human breast tumors. These findings implicate the mevalonate pathway as a therapeutic target for tumors bearing mutations in p53.

Tumor suppressor and T-regulatory functions of Foxp3 are mediated through separate signaling pathways

Foxp3 is a nuclear transcription factor that is both a tumor suppressor factor and regulator of T-regulatory cell (Treg) function, and is a potential therapeutic target in both autoimmunity and cancer. In order to distinguish molecular pathways responsible for these separate Foxp3 functions, deletion mutants of Foxp3 proteins were transduced and analyzed for cytotoxic activity in human cancer cell lines Skov3, MDA-MB-231, MCF-7 and Jurkat. Human Foxp3 cDNA mutants were amplified and ligated to produce plasmids for direct cell transfection. Constructs were produced and confirmed by DNA sequencing. Lipofectamine 2000 was used for plasmid transfection. Foxp3 cells were then examined. The results of our experiments reveal retention of tumor suppressor function in the absence of NFAT binding and transcriptional activation required for Treg function. Our results have significant implications for the design of autoimmune and cancer therapies that target Foxp3 and Treg cells.

p53 and its mutants in tumor cell migration and invasion

In about half of all human cancers, the tumor suppressor p53 protein is either lost or mutated, frequently resulting in the expression of a transcriptionally inactive mutant p53 protein. Loss of p53 function is well known to influence cell cycle checkpoint controls and apoptosis. But it is now clear that p53 regulates other key stages of metastatic progression, such as cell migration and invasion. Moreover, recent data suggests that expression of mutant p53 is not the equivalent of p53 loss, and that mutant p53s can acquire new functions to drive cell migration, invasion, and metastasis, in part by interfering with p63 function.

Mutant p53 gain-of-function in cancer

In its wild-type form, p53 is a major tumor suppressor whose function is critical for protection against cancer. Many human tumors carry missense mutations in the TP53 gene, encoding p53. Typically, the affected tumor cells accumulate excessive amounts of the mutant p53 protein. Various lines of evidence indicate that, in addition to abrogating the tumor suppressor functions of wild-type p53, the common types of cancer-associated p53 mutations also endow the mutant protein with new activities that can contribute actively to various stages of tumor progression and to increased resistance to anticancer treatments. Collectively, these activities are referred to as mutant p53 gain-of-function. This article addresses the biological manifestations of mutant p53 gain-of-function, the underlying molecular mechanisms, and their possible clinical implications.

Mutant p53-induced up-regulation of mitogen-activated protein kinase kinase 3 contributes to gain of function

Mitogen-activated protein kinase kinase 3 (MAP2K3) is a member of the dual specificity kinase group. Growing evidence links MAP2K3 to invasion and tumor progression. Here, we identify MAP2K3 as a transcriptional target of endogenous gain-of-function p53 mutants R273H, R175H, and R280K. We show that MAP2K3 modulation occurred at the mRNA and protein levels and that endogenous mutant p53 proteins are capable of binding to and activate the MAP2K3 promoter. In addition, we found that the studied p53 mutants regulate MAP2K3 gene expression through the involvement of the transcriptional cofactors NF-Y and NF-kappaB. Finally, functional studies showed that endogenous MAP2K3 knockdown inhibits proliferation and survival of human tumor cells, whereas the ectopic expression of MAP2K3 can rescue the proliferative defect induced by mutant p53 knockdown. Taken together, our findings define a novel player through which mutant p53 exerts its gain-of-function activity in cancer cells.

Modulation of the vitamin D3 response by cancer-associated mutant p53

The p53 gene is mutated in many human tumors. Cells of such tumors often contain abundant mutant p53 (mutp53) protein, which may contribute actively to tumor progression via a gain-of-function mechanism. We applied ChIP-on-chip analysis and identified the vitamin D receptor (VDR) response element as overrepresented in promoter sequences bound by mutp53. We report that mutp53 can interact functionally and physically with VDR. Mutp53 is recruited to VDR-regulated genes and modulates their expression, augmenting the transactivation of some genes and relieving the repression of others. Furthermore, mutp53 increases the nuclear accumulation of VDR. Importantly, mutp53 converts vitamin D into an antiapoptotic agent. Thus, p53 status can determine the biological impact of vitamin D on tumor cells.

Characterization of functional domains necessary for mutant p53 gain of function

Tumor cells, including SW480 carcinoma cells that carry a mutant p53, are addicted to the mutant for their survival and resistance to growth suppression by chemotherapeutic agents. Here, we investigated whether various classes of p53 mutants share a common property and functional domains necessary for mutant p53 gain of function. To test this, we generated SW480 cell lines in which endogenous mutant R273H/P309S can be inducibly or stably knocked down, whereas a small interfering RNA-resistant mutant p53 along with a mutated functional domain can be inducibly or stably expressed. We found that both contact-site (R248W and R273H) and conformation (G245S and R249S) mutants are able to maintain the transformed phenotypes of SW480 cells conferred by endogenous mutant p53. We also found that activation domains 1-2 and the proline-rich domain are required for mutant p53 gain of function. Interestingly, we showed that the C-terminal basic domain, which is required for wild-type p53 activity, is an inhibitory domain for mutant p53. Furthermore, we showed that deletion of the basic domain enhances, whereas a mutation in activation domains 1-2 and deletion of the proline-rich domain abolish mutant p53 to regulate Gro1 and Id2, both of which are regulated by and mediate endogenous mutant p53 gain of function. These results indicate that both conformation and contact-site mutants share a property for cell transformation, and the domains critical for wild-type p53 tumor suppression are also required for mutant p53 tumor promotion. Thus, the inhibitory basic domain and the common property for p53 mutants can be explored for targeting tumors with mutant p53.

TAR1, a human anti-p53 single-chain antibody, restores tumor suppressor function to mutant p53 variants

The tumor suppressor gene p53 is mutated in more than half of human tumors. One important characteristic of p53 mutants is their accumulation in the nucleus of cancer cells. Thus, reactivation of mutant p53 proteins may trigger massive apoptosis in tumor cells. Pharmacologic methods are currently under development to induce mutant p53 proteins to resume their wild-type function. We have identified a human single-chain Fv fragment, designated as transcriptional transactivation and apoptosis restoring (TAR1), which specifically and with high affinity binds to mutant p53 and restores its wild-type active conformation. Binding of TAR1 to mutant p53 induced transcriptional transactivation of p53 target genes and down-regulation of mutant p53 transcriptional target genes. TAR1 treatment induced apoptosis in a variety of cell lines endogenously expressing p53 carrying different point mutations DNA contact or structural p53 mutants. Moreover, in an animal model of mice carrying human xenografts, TAR1 induced tumor regression with no apparent deleterious side effects. Thus, it may be considered as a potential candidate for anticancer treatment, targeting tumors with mutant p53.

Mutant p53 drives invasion by promoting integrin recycling

p53 is a tumor suppressor protein whose function is frequently lost in cancers through missense mutations within the Tp53 gene. This results in the expression of point-mutated p53 proteins that have both lost wild-type tumor suppressor activity and show gain of functions that contribute to transformation and metastasis. Here, we show that mutant p53 expression can promote invasion, loss of directionality of migration, and metastatic behavior. These activities of p53 reflect enhanced integrin and epidermal growth factor receptor (EGFR) trafficking, which depends on Rab-coupling protein (RCP) and results in constitutive activation of EGFR/integrin signaling. We provide evidence that mutant p53 promotes cell invasion via the inhibition of TAp63, and simultaneous loss of p53 and TAp63 recapitulates the phenotype of mutant p53 in cells. These findings open the possibility that blocking alpha5/beta1-integrin and/or the EGF receptor will have therapeutic benefit in mutant p53-expressing cancers.

p53 Amino-terminus region (1-125) stabilizes and restores heat denatured p53 wild phenotype

Background

The intrinsically disordered N-ter domain (NTD) of p53 encompasses approximately hundred amino acids that contain a transactivation domain (1–73) and a proline-rich domain (64–92) and is responsible for transactivation function and apoptosis. It also possesses an auto-inhibitory function as its removal results in remarkable reduction in dissociation of p53 from DNA.

Principal Findings/Methodology

In this report, we have discovered that p53-NTD spanning amino acid residues 1–125 (NTD125) interacted with WT p53 and stabilized its wild type conformation under physiological and elevated temperatures, both in vitro and in cellular systems. NTD125 prevented irreversible thermal aggregation of heat denatured p53, enhanced p21-5′-DBS binding and further restored DBS binding activity of heat-denatured p53, in vitro, in a dose-dependent manner. In vivo ELISA and immunoprecipitation analysis of NTD125-transfected cells revealed that NTD125 shifted equilibrium from p53 mutant to wild type under heat stress conditions. Further, NTD125 initiated nuclear translocation of cytoplasmic p53 in transcriptionally active state in order to activate p53 downstream genes such as p21, Bax, PUMA, Noxa and SUMO.

Conclusion/Significance

Here, we showed that a novel chaperone-like activity resides in p53-N-ter region. This study might have significance in understanding the role of p53-NTD in p53 stabilization, conformational activation and apoptosis under heat-stress conditions.

When mutants gain new powers: news from the mutant p53 field

Ample data indicate that mutant p53 proteins not only lose their tumour suppressive functions, but also gain new abilities that promote tumorigenesis. Moreover, recent studies have modified our view of mutant p53 proteins, portraying them not as inert mutants, but rather as regulated proteins that influence the cancer cell transcriptome and phenotype. This influence is clinically manifested as association of TP53 mutations with poor prognosis and drug resistance in a growing array of malignancies. Here, we review recent studies on mutant p53 regulation, gain-of-function mechanisms, transcriptional effects and prognostic association, with a focus on the clinical implications of these findings.

The execution of the transcriptional axis mutant p53, E2F1 and ID4 promotes tumor neo-angiogenesis

ID4 (inhibitor of DNA binding 4) is a member of a family of proteins that function as dominant-negative regulators of basic helix-loop-helix transcription factors. Growing evidence links ID proteins to cell proliferation, differentiation and tumorigenesis. Here we identify ID4 as a transcriptional target of gain-of-function p53 mutants R175H, R273H and R280K. Depletion of mutant p53 protein severely impairs ID4 expression in proliferating tumor cells. The protein complex mutant p53-E2F1 assembles on specific regions of the ID4 promoter and positively controls ID4 expression. The ID4 protein binds to and stabilizes mRNAs encoding pro-angiogenic factors IL8 and GRO-alpha. This results in the increase of the angiogenic potential of cancer cells expressing mutant p53. These findings highlight the transcriptional axis mutant p53, E2F1 and ID4 as a still undefined molecular mechanism contributing to tumor neo-angiogenesis.

Mutant p53 mediates survival of breast cancer cells

Background:

p53 is the most commonly mutated tumour-suppressor gene in human cancers. Unlike other tumour-suppressor genes, most p53 cancer mutations are missense mutations within the core domain, leading to the expression of a full-length mutant p53 protein. Accumulating evidence has indicated that p53 cancer mutants not only lose tumour suppression activity but also gain new oncogenic activities to promote tumourigenesis.

Methods:

The endogenous mutant p53 function in human breast cancer cells was studied using RNA interference (RNAi). Gene knockdown was confirmed by quantitative PCR and western blotting. Apoptosis was evaluated by morphological changes of cells, their PARP cleavage and annexin V staining.

Results:

We show that cancer-associated p53 missense mutants are required for the survival of breast cancer cells. Inhibition of endogenous mutant p53 by RNAi led to massive apoptosis in two mutant p53-expressing cell lines, T47D and MDA-MB-468, but not in the wild-type p53-expressing cells, MCF-7 and MCF-10A. Reconstitution of an RNAi-insensitive mutant p53 in MDA-MB-468 cells completely abolished the apoptotic effects after silencing of endogenous mutant p53, suggesting the specific survival effects of mutant p53. The apoptotic effect induced by mutant p53 ablation, however, is independent of p63 or p73 function.

Conclusion:

These findings provide clear evidence of a pro-survival ‘gain-of-function’ property of a subset of p53 cancer mutants in breast cancer cells.

Modulation of gene expression in U251 glioblastoma cells by binding of mutant p53 R273H to intronic and intergenic sequences

Missense point mutations in the TP53 gene are frequent genetic alterations in human tumor tissue and cell lines derived thereof. Mutant p53 (mutp53) proteins have lost sequence-specific DNA binding, but have retained the ability to interact in a structure-selective manner with non-B DNA and to act as regulators of transcription. To identify functional binding sites of mutp53, we established a small library of genomic sequences bound by p53^R273H in U251 human glioblastoma cells using chromatin immunoprecipitation (ChIP). Mutp53 binding to isolated DNA fragments confirmed the specificity of the ChIP. The mutp53 bound DNA sequences are rich in repetitive DNA elements, which are dispersed over non-coding DNA regions. Stable down-regulation of mutp53 expression strongly suggested that mutp53 binding to genomic DNA is functional. We identified the PPARGC1A and FRMD5 genes as p53^R273H targets regulated by binding to intronic and intra-genic sequences. We propose a model that attributes the oncogenic functions of mutp53 to its ability to interact with intronic and intergenic non-B DNA sequences and modulate gene transcription via re-organization of chromatin.

Mutant p53 targeting by the low molecular weight compound STIMA-1

Reactivation of mutant p53 in human tumor cells should induce cell death by apoptosis and thus eliminate the tumor. Several small molecules that reactivate mutant p53 have been identified. Here we show that STIMA-1, a low molecular weight compound with some structural similarities to the previously identified molecule CP-31398, can stimulate mutant p53 DNA binding in vitro and induce expression of p53 target proteins and trigger apoptosis in mutant p53-expressing human tumor cells. Human diploid fibroblasts are significantly more resistant to STIMA-1 than mutant or wild type p53-carrying tumor cells. STIMA-1 may provide new insights into possible mechanisms of mutant p53 reactivation and thus facilitate the development of novel anticancer drugs that target mutant p53-carrying tumors.

Modulatory effects of polyphenols on apoptosis induction: relevance for cancer prevention

Polyphenols, occurring in fruit and vegetables, wine, tea, extra virgin olive oil, chocolate and other cocoa products, have been demonstrated to have clear antioxidant properties in vitro, and many of their biological actions have been attributed to their intrinsic reducing capabilities. However, it has become clear that, in complex biological systems, polyphenols exhibit several additional properties which are yet poorly understood. Apoptosis is a genetically controlled and evolutionarily conserved form of cell death of critical importance for the normal embryonic development and for the maintenance of tissue homeostasis in the adult organism. The malfunction of the death machinery may play a primary role in various pathological processes, since too little or too much apoptosis can lead to proliferative or degenerative diseases, respectively. Cancer cells are characterized by a deregulated proliferation, and/or an inability to undergo programmed cell death. A large body of evidence indicates that polyphenols can exert chemopreventive effects towards different organ specific cancers, affecting the overall process of carcinogenesis by several mechanisms: inhibition of DNA synthesis, modulation of ROS production, regulation of cell cycle arrest, modulation of survival/proliferation pathways. In addition, polyphenols can directly influence different points of the apoptotic process, and/or the expression of regulatory proteins. Although the bulk of data has been obtained in in vitro systems, a number of clinical studies suggesting a preventive and therapeutic effectiveness of polyphenols in vivo is available. However, a deeper knowledge of the underlying mechanisms responsible for the modulation of apoptosis by polyphenols, and their real effectiveness, is necessary in order to propose them as potential chemopreventive and chemotherapeutic candidates for cancer treatment.

Hsp90 inhibition has opposing effects on wild-type and mutant p53 and induces p21 expression and cytotoxicity irrespective of p53/ATM status in chronic lymphocytic leukaemia cells

In chronic lymphocytic leukaemia (CLL), mutation/deletion of TP53 is strongly associated with early disease progression, resistance to chemotherapy and short patient survival. Consequently, there is a pressing need to develop novel treatment protocols for this high-risk patient group. The present study was performed to evaluate Hsp90 inhibition as a possible therapeutic approach for such patients. Primary CLL cells of defined ataxia telangiectasia mutated (ATM)/p53 status were incubated with the Hsp90 inhibitor geldanamycin (GA) and analysed by western blotting for the expression of p53, p21, MDM2 and Akt. GA downregulated overexpressed mutant p53 protein (an oncogene) and upregulated wild-type (wt) p53 (a tumour suppressor). The upregulation of wt p53 by GA was independent of ATM and was accompanied by downregulation of Akt and the active form of MDM2, indicating a possible mechanism. GA also produced a p53/ATM-independent increase in the levels of p21-a potent inducer of cell-cycle arrest. In-vitro cytotoxicity studies showed that GA killed cultured CLL cells in a dose- and time-dependent fashion irrespective of their p53/ATM status and more effectively than normal blood mononuclear cells. In summary, our findings reveal important consequences of inhibiting Hsp90 in CLL cells and strongly support the therapeutic evaluation of Hsp90 inhibitors in poor-prognosis patients with p53 defects.

Mutant p53 attenuates the SMAD-dependent transforming growth factor beta1 (TGF-beta1) signaling pathway by repressing the expression of TGF-beta receptor type II

Both transforming growth factor beta (TGF-beta) and p53 have been shown to control normal cell growth. Acquired mutations either in the TGF-beta signaling pathway or in the p53 protein were shown to induce malignant transformation. Recently, cross talk between wild-type p53 and the TGF-beta pathway was observed. The notion that mutant p53 interferes with the wild-type p53-induced pathway and acts by a "gain-of-function" mechanism prompted us to investigate the effect of mutant p53 on the TGF-beta-induced pathway. In this study, we show that cells expressing mutant p53 lost their sensitivity to TGF-beta1, as observed by less cell migration and a reduction in wound healing. We found that mutant p53 attenuates TGF-beta1 signaling. This was exhibited by a reduction in SMAD2/3 phosphorylation and an inhibition of both the formation of SMAD2/SMAD4 complexes and the translocation of SMAD4 to the cell nucleus. Furthermore, we found that mutant p53 attenuates the TGF-beta1-induced transcription activity of SMAD2/3 proteins. In searching for the mechanism that underlies this attenuation, we found that mutant p53 reduces the expression of TGF-beta receptor type II. These data provide important insights into the molecular mechanisms that underlie mutant p53 "gain of function" pertaining to the TGF-beta signaling pathway.

Mutant p53 proteins: between loss and gain of function

Cancer might result from both the aberrant activation of genes, whose physiological tuning is essential for the life of a normal cell, and the inactivation of tumor suppressor genes, whose main job is to preserve the integrity of cell genome. Among the latter, p53 is considered a key tumor suppressor gene that is inactivated mainly by missense mutations in half of human cancers. It is becoming increasingly clear that the resulting mutant p53 proteins gain oncogenic properties favoring the insurgence, the maintenance, and the spreading of malignant tumors. In this review, we mainly discuss the molecular mechanisms underlying gain of function of human tumor-derived p53 mutants, their impact on the chemoresistance and the prognosis of human tumors, with a special focus on head and neck cancers, and the perspectives of treating tumors through the manipulation of mutant p53 proteins.

Transcription regulation by mutant p53

In addition to the loss of wild-type p53 activity, a high percentage of tumor cells accumulate mutant p53 protein isoforms. Whereas the hallmark of the wild-type p53 is its tumor suppressor activities, tumor-associated mutant p53 proteins acquire novel functions enabling them to promote a large spectrum of cancer phenotypes. During the last years, it became clear that tumor-associated mutant p53 proteins are not only distinct from the wild-type p53, but they also represent a heterogeneous population of proteins with a variety of structure-function features. One of the major mechanisms underlying mutant p53 gain of function is the ability to regulate gene expression. Although a large number of specific target genes were identified, the molecular basis for this regulation is not fully elucidated. This review describes the present knowledge about the transcriptional activities of mutant p53 and the mechanisms that might underlie its target gene specificity.

The oncogenic roles of p53 mutants in mouse models

Tumors associated with p53 usually contain missense mutations in the p53 tumor suppressor gene rather than deletions of p53, suggesting a growth advantage for cells with missense mutations. The oncogenic roles of p53 mutants have been examined extensively in cell lines. Mouse models that inherit p53 mutations expressed at physiological levels have now been generated to examine the activities of mutant p53 upon tumorigenesis in vivo. Mice with p53 mutations develop tumor spectrums and metastatic phenotypes different from those of mice with a p53-null allele. Embryo fibroblasts with mutant p53 also show increased proliferative and transformation properties. One mechanism for this gain-of-function potential is the inhibition of function of the p53 family members p63 and p73.

p53 downregulates its activating vaccinia-related kinase 1, forming a new autoregulatory loop

The stable accumulation of p53 is detrimental to the cell because it blocks cell growth and division. Therefore, increases in p53 levels are tightly regulated, mainly by its transcriptional target, mdm2, that downregulates p53. Elucidation of new signaling pathways requires the characterization of the members and the nature of their connection. Vaccinia-related kinase 1 (VRK1) contributes to p53 stabilization by partly interfering with its mdm2-mediated degradation, among other mechanisms; therefore, it is likely that some form of autoregulation between VRK1 and p53 must occur. We report here the identification of an autoregulatory loop between p53 and its stabilizing VRK1. There is an inverse correlation between VRK1 and p53 levels in cell lines, and induction of p53 by UV light downregulates VRK1 in fibroblasts. As the amount of p53 protein increases, there is a downregulation of the VRK1 protein level independent of its promoter. This effect is indirect but requires a transcriptionally active p53. The three most common transcriptionally inactive mutations detected in hereditary (Li-Fraumeni syndrome) and sporadic human cancer, p53(R175H), p53(R248W), and p53(R273H), as well as p53(R280K), are unable to induce downregulation of VRK1 protein. The p53 isoforms Delta40p53 and p53beta, lacking the transactivation and oligomerization domains, respectively, do not downregulate VRK1. VRK1 downregulation induced by p53 is independent of mdm2 activity and proteasome-mediated degradation since it occurs in the presence of proteasome inhibitors and in mdm2-deficient cells. The degradation of VRK1 is sensitive to chloroquine, an inhibitor of the late endosome-lysosome transport, and to serine protease inhibitors of the lysosomal pathway.

Repression of the MSP/MST-1 gene contributes to the antiapoptotic gain of function of mutant p53

Tumor-associated mutant forms of p53 can exert an antiapoptotic gain of function activity, which confers a selective advantage upon tumor cells harboring such mutations. We report that mutant p53 suppresses the expression of the MSP (MST-1/HGFL) gene, encoding the ligand of the receptor tyrosine kinase RON, implicated in a variety of cellular responses. Mutant p53 associates with the MSP gene promoter and represses its transcriptional activity, leading to a decrease in mRNA levels and a subsequent decrease in the levels of secreted MSP protein. Forced downregulation of MSP expression in H1299 cells, derived from a large-cell lung carcinoma, confers increased resistance against etoposide-induced cell death. These antiapoptotic consequences of MSP downregulation seemingly conflict with the well-documented ability of the RON receptor to promote cell survival and tumor progression when aberrantly hyperactive. Yet, they are consistent with the fact that reduced MSP expression was observed in many types of human cancer, including large-cell lung carcinoma. Thus, repression of MSP gene expression by mutant p53 may contribute to oncogenesis in a cell type-specific manner.

Fate of UVB-induced p53 mutations in SKH-hr1 mouse skin after discontinuation of irradiation: relationship to skin cancer development

Chronic exposure to ultraviolet (UV) radiation causes skin cancer in humans and mice. We have previously shown that in hairless SKH-hr1 mice, UVB-induced p53 mutations arise very early, well before tumor development. In this study, we investigated whether discontinuation of UVB exposure before the onset of skin tumors results in the disappearance of p53 mutations in the skin of hairless SKH-hr1 mice. Irradiation of mice at a dose of 2.5 kJ/m2 three times a week for 8 weeks induced p53 mutations in the epidermal keratinocytes of 100% of the mice. UVB irradiation was discontinued after 8 weeks, but p53 mutations at most hotspot codons were still present even 22 weeks later. During that period, the percent of mice carrying p53(V154A/R155C), p53(H175H/H176Y), and p53R275C mutant alleles remained at or near 100%, whereas the percentage of mice with p53R270C mutation decreased by 45%. As expected, discontinuation of UVB after 8 weeks resulted in a delay in tumor development. A 100% of tumors carried p53(V154A/R155C) mutant alleles, 76% carried p53(H175H/H176Y) mutants, and 24 and 19% carried p53R270C and p53R275C mutants, respectively. These results suggest that different UVB-induced p53 mutants may provide different survival advantages to keratinocytes in the absence of further UVB exposure and that skin cancer development can be delayed but not prevented by avoidance of further exposure to UVB radiation.