The gain of function of p53 mutant p53S in promoting tumorigenesis by cross-talking with H-RasV12

The dual role of the DREAM/G2M pathway in non-tumorigenic immortalization of senescent cells

Anti-aging and tumorigenesis share common genes and pathways, and thus targeting these genes as part of anti-aging interventions carries the risk of tumorigenesis. It is essential to understand the gene signatures that balance tumorigenesis and aging. To achieve this goal, we analyzed RNA-sequencing data from three non-tumorigenic immortalized cell lines that spontaneously escaped from senescence. By single sample gene set enrichment assay (ssGSEA) and GSEA analysis, we found that both cell growth signaling (E2F targets, MYC targets) and tumor surveillance mechanisms (DNA repair, G2M checkpoint, mitotic spindle) were up-regulated in all three cell lines, suggesting that these genes are potential signatures for non-tumorigenic immortalization. Further analysis revealed that the 182 commonly up-regulated genes in these three cell lines overlapped with the DREAM/G2M pathway, which is known to be the upstream regulator of E2F, Myc targets, DNA repair, G2M checkpoint and mitotic spindle pathways in its cell cycle activation or inhibitory form. By western blotting, quantitative PCR and co-immunoprecipitation, we verified that both forms of the DREAM pathway are up-regulated in all three cell lines; this pathway facilitates control of cell cycle progression, supporting a new mechanism for non-tumorigenic immortalization. Thus, we propose that the DREAM/G2M pathway plays important dual roles with respect to preventing tumorigenesis in the process of immortalization. Our data might serve as the basis for the identification of new signature pathways or gene biomarkers for non-tumorigenic immortalization, and may aid in the discovery of new targets for tumor-free anti-aging drug screening.

Modified iPOND revealed the role of mutant p53 in promoting helicase function and telomere maintenance

The G-rich DNA, such as telomere, tends to form G-quadruplex (G4) structure, which slows down the replication fork progression, induces replication stress, and becomes the chromosome fragile sites. Here we described a molecular strategy that cells developed to overcome the DNA replication stress via DNA helicase regulation. The p53N236S (p53S) mutation has been found in the Werner syndrome mouse embryo fibroblast (MEFs) escaped from senescence, could be the driving force for cell escaping senescence. We revealed that the p53S could transcriptionally up-regulate DNA helicases expression, including Wrn, Blm, Timeless, Ddx, Mcm, Gins, Fanc, as well as telomere specific proteins Terf1, Pot1, through which p53S promoted the unwinding of G4 structures, and protected the cells from DNA replication stress induced by G4 stabilizer. By modified iPOND (isolation of proteins on nascent DNA) assay and telomere assay, we demonstrated that the p53S could promote the recruitment of those helicases to the DNA replication forks, facilitated the maintenance of telomere, and prevent the telomere dysfunction induced by G4 stabilizer. Interestingly, we did not observe the function of promoting G4 resolving and facilitating telomere lengthening in the cells with Li-Fraumeni Syndrome mutation-p53R172H (p53H), which suggests that this is the specific gain of function for p53S. Together our data suggest that the p53S could gain the new function of releasing the replication stress via regulating the helicase function and G4 structure, which benefits telomere lengthening. This strategy could be applied to the treatment of diseases caused by telomere replication stress.

Mutant p53 leads to low-grade IFN-I-induced inflammation and impairs cGAS-STING signalling in mice

Type I interferons (IFN-Is) are a class of proinflammatory cytokines produced in response to viruses and environmental stimulations, resulting in chronic inflammation and even carcinogenesis. However, the connection between IFN-I and p53 mutation is poorly understood. Here, we investigated IFN-I status in the context of mutant p53 (p53N236S , p53S). We observed significant cytosolic double-stranded DNA (dsDNA) derived from nuclear heterochromatin in p53S cells, along with an increased expression of IFN-stimulated genes. Further study revealed that p53S promoted cyclic GMP-AMP synthase (cGAS) and IFN-regulatory factor 9 (IRF9) expression, thus activating the IFN-I pathway. However, p53S/S mice were more susceptible to herpes simplex virus 1 infection, and the cGAS-stimulator of IFN genes (STING) pathway showed a decline trend in p53S cells in response to poly(dA:dT) accompanied with decreased IFN-β and IFN-stimulated genes, whereas the IRF9 increased in response to IFN-β stimulation. Our results illustrated the p53S mutation leads to low-grade IFN-I-induced inflammation via consistent low activation of the cGAS-STING-IFN-I axis, and STAT1-IRF9 pathway, therefore, impairs the protective cGAS-STING signalling and IFN-I response encountered with exogenous DNA attack. These results suggested the dual molecular mechanisms of p53S mutation in inflammation regulation. Our results could be helping in further understanding of mutant p53 function in chronic inflammation and provide information for developing new therapeutic strategies for chronic inflammatory diseases or cancer.

Gain-of-Function p53N236S Mutation Drives the Bypassing of HRasV12-Induced Cellular Senescence via PGC-1α

One of the key steps in tumorigenic transformation is immortalization in which cells bypass cancer-initiating barriers such as senescence. Senescence can be triggered by either telomere erosion or oncogenic stress (oncogene-induced senescence, OIS) and undergo p53- or Rb-dependent cell cycle arrest. The tumor suppressor p53 is mutated in 50% of human cancers. In this study, we generated p53N236S (p53S) mutant knock-in mice and observed that p53S heterozygous mouse embryonic fibroblasts (p53^S/+) escaped HRas^V12-induced senescence after subculture in vitro and formed tumors after subcutaneous injection into severe combined immune deficiency (SCID) mice. We found that p53S increased the level and nuclear translocation of PGC-1α in late-stage p53^S/++Ras cells (LS cells, which bypassed the OIS). The increase in PGC-1α promoted the biosynthesis and function of mitochondria in LS cells by inhibiting senescence-associated reactive oxygen species (ROS) and ROS-induced autophagy. In addition, p53S regulated the interaction between PGC-1α and PPARγ and promoted lipid synthesis, which may indicate an auxiliary pathway for facilitating cell escape from aging. Our results illuminate the mechanisms underlying p53S mutant-regulated senescence bypass and demonstrate the role played by PGC-1α in this process.

p53N236S Activates Autophagy in Response to Hypoxic Stress Induced by DFO

Hypoxia can lead to stabilization of the tumor suppressor gene p53 and cell death. However, p53 mutations could promote cell survival in a hypoxic environment. In this study, we found that p53N236S (p53N239S in humans, hereinafter referred to as p53S) mutant mouse embryonic fibroblasts (MEFs) resistant to deferoxamine (DFO) mimic a hypoxic environment. Further, Western blot and flow cytometry showed reduced apoptosis in p53S/S cells compared to WT after DFO treatment, suggesting an antiapoptosis function of p53S mutation in response to hypoxia-mimetic DFO. Instead, p53S/S cells underwent autophagy in response to hypoxia stress presumably through inhibition of the AKT/mTOR pathway, and this process was coupled with nuclear translocation of p53S protein. To understand the relationship between autophagy and apoptosis in p53S/S cells in response to hypoxia, the autophagic inhibitor 3-MA was used to treat both WT and p53S/S cells after DFO exposure. Both apoptotic signaling and cell death were enhanced by autophagy inhibition in p53S/S cells. In addition, the mitochondrial membrane potential (MMP) and the ROS level results indicated that p53S might initiate mitophagy to clear up damaged mitochondria in response to hypoxic stress, thus increasing the proportion of intact mitochondria and maintaining cell survival. In conclusion, the p53S mutant activates autophagy instead of inducing an apoptotic process in response to hypoxia stress to protect cells from death.

The Distinct Function of p21Waf1/Cip1 With p16Ink4a in Modulating Aging Phenotypes of Werner Syndrome by Affecting Tissue Homeostasis

Human Werner syndrome (WS) is an autosomal recessive progeria disease. A mouse model of WS manifests the disease through telomere dysfunction-induced aging phenotypes, which might result from cell cycle control and cellular senescence. Both p21^Waf1/Cip1 (p21, encoded by the Cdkn1a gene) and p16^Ink4a (p16, encoded by the Ink4a gene) are cell cycle inhibitors and are involved in regulating two key pathways of cellular senescence. To test the effect of p21 and p16 deficiencies in WS, we crossed WS mice (DKO) with p21^–/– or p16^–/– mice to construct triple knockout (p21-TKO or p16-TKO) mice. By studying the survival curve, bone density, regenerative tissue (testis), and stem cell capacity (intestine), we surprisingly found that p21-TKO mice displayed accelerated premature aging compared with DKO mice, while p16-TKO mice showed attenuation of the aging phenotypes. The incidence of apoptosis and cellular senescence were upregulated in p21-TKO mice tissue and downregulated in p16-TKO mice. Surprisingly, cellular proliferation in p21-TKO mice tissue was also upregulated, and the p21-TKO mice did not show telomere shortening compared with age-matched DKO mice, although p16-TKO mice displayed obvious enhancement of telomere lengthening. Consistent with these phenotypes, the SIRT1-PGC1 pathway was upregulated in p16-TKO but downregulated in p21-TKO compared with DKO mouse embryo fibroblasts (MEFs). However, the DNA damage response pathway was highly activated in p21-TKO, but rescued in p16-TKO, compared with DKO MEFs. These data suggest that p21 protected the stem cell reservoir by regulating cellular proliferation and turnover at a proper rate and that p21 loss in WS activated fairly severe DNA damage responses (DDR), which might cause an abnormal increase in tissue homeostasis. On the other hand, p16 promoted cellular senescence by inhibiting cellular proliferation, and p16 deficiency released this barrier signal without causing severe DDR.

Spectrum of TP53 Mutations in BRCA1/2 Associated High-Grade Serous Ovarian Cancer

Objective: Mutations in TP53 lead to loss of function (LOF) or gain of function (GOF) of the corresponding protein p53 and produce a different effect on the tumor. Our goal was to determine the spectrum of somatic TP53 variants in BRCA1/2 associated high-grade serous ovarian cancer (HGSOC).

Methods: The population under study comprised of HGSOCs with pathogenic variants in BRCA1 (n = 78) or BRCA2 (n = 21). Only chemo-naive and platinum-sensitive patients were included in this study. The case group of the IARC database (n = 1249) with HGSOC not stratified by BRCA status was used as a reference. A custom NGS panel was used for sequencing TP53 and mutational hot-spots of other genes, and p53 expression was evaluated by immunohistochemistry for 68 cases of HGSOCs.

Results: Somatic TP53 variants (95) or inhibition of wild-type p53 expression (3) were observed in 98 cases. The sample with normal p53 had CDKNA1 variants. The frequency of truncating variants was significantly higher than in the reference cohort (30.3 vs. 21.0%, p = 0.01). Most of the samples (41/68) demonstrated low (or absent) expression of p53, and 17 samples overexpressed p53. LOH was typical for TP53 nonsense variants (14/15). In total, 68/95 samples were LOH positive and showed LOH in all tumorous cells, thus indicating the driver effect of TP53 mutations. Three specimens had KRAS, BAX, APC, and CTNNB1 subclones variants.

Conclusion: High frequency of TP53 truncating variants, the low expression of mutant p53, and low incidence of oncogene mutations show potential GOF properties of p53 to be poorly represented in BRCA1/2 associated HGSOC.

p53 Mutant p53N236S Induces Neural Tube Defects in Female Embryos

The p53 is one of the most important tumor suppressors through surveillance of DNA damages and abnormal proliferation signals, and activation the cell cycle arrest and apoptosis in response to stress. However, the mutation of p53 is known to be oncogenic by both loss of function in inhibiting cell cycle progress and gain of function in promoting abnormal proliferation. In the present study, we have established a knock in mouse model containing an Asn-to-Ser substitution at p53 amino acid 236 by homologous recombination (p53N236S). Other than tumorigenesis phenotype, we found that p53^S/S mice displayed female-specific phenotype of open neural tube in brain (exencephaly) and spinal cord (spina bifida). The occurrence rate for embryonic exencephaly is 68.5% in female p53^S/S mice, which is much more than that of in p53^-/- mice (37.1%) in the same genetic background. Further study found that p53^N236S mutation increased neuronal proliferation and decreased neuronal differentiation and apoptosis. To rescue the phenotype, we inhibited cell proliferation by crossing Wrn^-/- mice with p53^S/S mice. The occurrence of NTDs in p53^S/S Wrn^-/- mice was 35.2%, thus suggesting that the inhibition of cell proliferation through a Wrn defect partially rescued the exencephaly phenotype in p53^S/S mice. We also report that p53S decreased expression of UTX at mRNA and protein level via increasing Xist transcript, result in high female-specific H3K27me3 expression and repressed Mash1 transcription, which facilitating abnormal proliferation, differentiation, and apoptosis, result in the mis-regulation of neurodevelopment and neural tube defects (NTDs).

Loss of p21 promoted tumorigenesis in the background of telomere dysfunctions induced by TRF2 and Wrn deficiency

Werner syndrome (WS) is a rare autosomal recessive progeria disease with genetic instability/cancer predisposition, thus a good model in understanding aging related carcinogenesis. Telomere dysfunction induced cellular senescence is essential in the manifestation of the WS phenotype. Our previous data has shown that p21 (encoded by Cdkn1a gene) could induce cellular senescence and suppress cellular growth of ALT (alternative lengthening of telomere) tumors derived from WS, suggested that p21 might play a key role in maintaining senescence of WS cells.

To confirm the role of p21 in suppressing telomere dysfunction induced tumorigenesis, we overexpressed dominant negative protein TRF2^ΔBΔM in p21^-/- mouse embryonic fibroblasts (MEFs). To further stress the cell, we crossed Wrn^-/-mice with p21^-/- mice to obtained p21^-/-Wrn^-/- MEFs, and overexpressed TRF2^ΔBΔM in these MEFs to induce telomere dysfunction similar to that in WS cells. Our data showed that, in the context of p21^-/-TRF2^ΔBΔM, loss of p21 function rescued cellular senescence, and induced p53 mutation, but did not induce tumorigenesis. However, in the set of p21^-/-Wrn^-/-TRF2^ΔBΔM, loss of p21 function induced p53 mutation and tumorigenesis.

To further verify the role of p21 in suppressing telomere dysfunction related tumorigenesis, we knocked down p21 in non-tumorigenic immortalized cells derived from WS MEFs (mTerc^-/-Wrn^-/-), and found that loss of p21 could induce ALT tumorigenesis, which displayed typical smear pattern of telomere length and arc-shaped telomeric DNA. In another hand, recovering telomerase activity in these MEFs could also induce tumorigenesis without affecting p21 expression level. Together our data suggested that p21 controlled cell cycle regulation played an essential role in suppressing telomere dysfunction-related tumorigenesis. These data also suggested that the genetic context is essential in determining the role of p21 in cancer prevention. Therefore, targeting p21 in the treatment of human degenerative diseases would require a personalized genetic background screen.

Ocular squamous cell carcinoma in Holstein cows from the South of Brazil

Aim

The aim of this study was to investigate 10 cases of bovine ocular squamous cell carcinoma (OSCC) diagnosed in Holstein or Holstein-crosses cows.

Materials and Methods

The investigation was performed exclusively in OSCC cases diagnosed in the State of Paraná and Santa Catarina. A combination of two previously existing histopathological classifications systems was used. The tissue samples were tested for immunoexpression of p53 and p16 and polymerase chain reaction (PCR) for bovine herpesvirus and papillomavirus.

Results

A positive correlation between number of mitotic figures and tissue invasion was found. Anaplasia parameters did not correlate well with tumor invasion of deeper tissues and mitotic counts. Six of 10 OSCC cases were in animals with heavily pigmented eyes. Immunoexpression of p53 and p16 was observed in 3 cases each. Bovine herpesvirus and papillomavirus were not detected by PCR.

Conclusions

Our results indicate that OSCC occurrence is most likely multifactorial with genetic, phenotypic, and environmental influences contributing to the pathogenesis of the disease.

Gain of function in the mouse model of a recurrent mutation p53N236S promotes the formation of double minute chromosomes and the oncogenic potential of p19ARF

The mutation p53^N236S (p53S) has been identified as one of the recurrent mutations in human cancers by TCGA database. Our in vitro data revealed the oncogenic gain of function of p53S. To understand the function of p53S in vivo, we generated the p53S knock-in mouse. The p53^S/S mice manifested highly invasive lymphomas and metastatic sarcomas with dramatically increased double minute chromosomes. The survival curve, the incidence of tumors and the tumor spectrum of p53^S/S mice is very similar to the p53^R172H mouse model. The p53^S/+ mice showed delayed onset of tumorigenesis and a high metastasis rate (40%) and low loss of heterozygosity rate (2/16). The activation of CDKN2A pathway in p53^S/S MEF and tumors, and the accumulation of p19^ARF protein in tumor tissues suggested p19^ARF might contribute to the accumulation of mutant p53S protein in the tumor and promote tumorigenesis. The high expression of p19^ARF correlated with mutant p53 accumulation and tumor progression, suggesting a dual role of p19^ARF in tumor promotion or suppression that might depend on the p53 mutation status in tumor cells. The oncogenic gain of function of this recurrent mutation p53S prompts the reconsideration of p53 mutations function that occurs at a low frequency.

Conditional knockout mouse models of cancer

In 2007, three scientists, Drs. Mario R. Capecchi, Martin J. Evans, and Oliver Smithies, received the Nobel Prize in Physiology or Medicine for their contributions of introducing specific gene modifications into mice. This technology, commonly referred to as gene targeting or knockout, has proven to be a powerful means for precisely manipulating the mammalian genome and has generated great impacts on virtually all phases of mammalian biology and basic biomedical research. Of note, germline mutations of many genes, especially tumor suppressors, often result in lethality during embryonic development or at developmental stages before tumor formation. This obstacle has been effectively overcome by the use of conditional knockout technology in conjunction with Cre-LoxP- or Flp-Frt-mediated temporal and/or spatial systems to generate genetic switches for precise DNA recombination. Currently, numerous conditional knockout mouse models have been successfully generated and applied in studying tumor initiation, progression, and metastasis. This review summarizes some conditional mutant mouse models that are widely used in cancer research and our understanding of the possible mechanisms underlying tumorigenesis.

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.

The role of focal adhesion kinase in the regulation of cellular mechanical properties

The regulation of mechanical properties is necessary for cell invasion into connective tissue or intra- and extravasation through the endothelium of blood or lymph vessels. Cell invasion is important for the regulation of many healthy processes such as immune response reactions and wound healing. In addition, cell invasion plays a role in disease-related processes such as tumor metastasis and autoimmune responses. Until now the role of focal adhesion kinase (FAK) in regulating mechanical properties of cells and its impact on cell invasion efficiency is still not well known. Thus, this review focuses on mechanical properties regulated by FAK in comparison to the mechano-regulating protein vinculin. Moreover, it points out the connection between cancer cell invasion and metastasis and FAK by showing that FAK regulates cellular mechanical properties required for cellular motility. Furthermore, it sheds light on the indirect interaction of FAK with vinculin by binding to paxillin, which then impairs the binding of paxillin to vinculin. In addition, this review emphasizes whether FAK fulfills regulatory functions similar to vinculin. In particular, it discusses the differences and the similarities between FAK and vinculin in regulating the biomechanical properties of cells. Finally, this paper highlights that both focal adhesion proteins, vinculin and FAK, synergize their functions to regulate the mechanical properties of cells such as stiffness and contractile forces. Subsequently, these mechanical properties determine cellular invasiveness into tissues and provide a source sink for future drug developments to inhibit excessive cell invasion and hence, metastases formation.

Triterpenes from the aerial parts of Cimicifuga yunnanensis and their antiproliferative effects on p53(N236S) mouse embryonic fibroblasts

Nine new triterpene derivatives, yunnanterpenes A-F (1-6), 15,16-seco-cimiterpenes A and B (7, 8), and cimilactone C (9), and 15 known analogues (10-24) were isolated from the aerial parts of Cimicifuga yunnanensis. The new structures were established using a combination of MS, NMR, and single-crystal X-ray diffraction techniques. WT MEFs (wild-type mouse embryonic fibroblasts) and tumorigenic cell lines p53(-/-)+H-RasV12 and p53(-/-)+p53(N236S)+H-RasV12 were used for evaluating active structures, targeting p53(N236S) (corresponding to p53(N239S) in humans) mutation. Compound 5 showed nonselective activities against these cell lines, with IC50 values of 5.8, 8.6, and 6.0 μM, respectively. Compound 4 exhibited greater selectivity against the p53(-/-)+p53(N236S)+H-RasV12 cells (IC50 5.5 μM) than against the WT MEFs cells (IC50 14.3 μM).

[The cooperation between p53 and Ras in tumorigenesis]

Inactivation of the tumor suppressor gene and activation of the oncogene cooperate to promote the multistep process of tumorigenesis. p53 is considered to be the most important tumor suppressor gene. p53 is usually found as a result of somatic missense mutation in approximately 50% of human cancers. Ras is found to be one of the most frequently mutated oncogenes in human tumors with the reported frequencies ranging from 30% to 90%. It has been found in many studies synergistic effect between tumor suppressor p53 and oncogene Ras occurs during the multistep process of tumorigenesis. According to the current reports, the cooperative effect between p53 and Ras can be divided into three types: the regulation of p53 for Ras function, , the regulation of Ras for p53, and the cooperation between p53 and Ras to control critical genes that are closely related to tumorigenesis. Understanding their synergistic effects will not only help us further disclose mechanism of tumorigenesis caused by p53 inactivation and Ras activation, but also facilitate personalized treatments and pharmacological target selection for cancer therapy. Therefore, we reviewed the recent progress of synergistic effect be-tween p53 and Ras and its role in tumorigenesis.

Two mechanisms underlying the loss of p16(Ink4a) function are associated with distinct tumorigenic consequences for WS MEFs escaping from senescence

Werner syndrome (WS) mouse embryonic fibroblasts (MEFs) can spontaneously escape from senescence and become immortalized, either tumorigenic or non-tumorigenic. Our data revealed a single p53(N236S) point mutation in the tumorigenic cell lines, which was correlated with the down-regulation of p21(Waf1/Cip1). p16(Ink4a) expression was significantly decreased in all immortalized cell lines. Bisulfate sequencing indicated that the p16(Ink4a) gene was methylated in the tumorigenic cells. Exogenous overexpression of p21(Waf1/Cip1) demethylated p16(Ink4a) and restored its expression, which induced cell growth arrest and senescence. While in non-tumorigenic immortalized cells, the Ink4a loci and adjacent genomic DNA were found to be deleted. These data suggest that the loss of p16(Ink4a) function by either genomic DNA deletion or methylation have been adopted by senescent WS MEFs escaping from senescence, with distinct tumorigenic consequences. The fact that cells that had escaped senescence via the spontaneous biallelic deletion of the Ink4a loci could not form tumors suggests that the functional loss of p16(Ink4a)per se might not be sufficient for tumorigenesis; most likely, it is a byproduct and passenger mutation. The mutations in factors regulating p16(Ink4a) methylation might be the driver mutation. These findings shed light on the strategy of anti-aging by regulating p16(Ink4a) expression.