Dissecting p53 tumor suppressor function in vivo through the analysis of genetically modified mice

CircSCAP interacts with SF3A3 to inhibit the malignance of non-small cell lung cancer by activating p53 signaling

BACKGROUND

Circular RNA (circRNA) has been recently identified as a critical regulator during carcinogenesis. However, the biological function and potential underlying mechanisms of circRNAs in lung cancer remain to be further elucidated.

METHODS

Here, we first evaluated the differentially expressed circRNAs between tumor and the matched adjacent nontumor tissues (3 pairs) of lung cancer patients via circRNA microarray. The expression of top five dysregulated circRNAs were tested in lung cancer cell lines and the circSCAP with concordant alteration in microarray data and cell lines was selected for further investigation. Then we validated the expression level of circSCAP in tumor and corresponding adjacent tissues (161 pairs) from a lung cancer cohort by RT-PCR analysis followed by correlation and prognosis analysis between circSCAP and clinical characteristics. Non-small cell lung cancer (NSCLC) accounts for the majority of lung cancer diagnosis (about 80% in the cohort used in this study). Therefore, we focused the role of circSCAP in NSCLC in the present study. In vitro and in vivo assays were performed to study the biological function of circSCAP in NSCLC. Biotin-labeled RNA pulldown and RNA immunoprecipitation (RIP) assays were carried out to identify the proteins directly interacting with circSCAP. The molecular mechanism of circSCAP-driven tumor suppression was demonstrated by immunoblotting, immunoprecipitation and luciferase reporter assays. In vitro and in vivo rescue experiments were conducted to verify the role of the circSCAP/SF3A3/p53 signaling axis in NSCLC.

RESULTS

We screened the expression profiles of human circRNAs in lung cancer tissues and found that hsa_circ_0065214 (termed as circSCAP) was significantly decreased. Kaplan-Meier analysis showed that patients with low level of circSCAP had a significantly poor prognosis. Gain- and loss-of-function experiments suggested that circSCAP played an important role in NSCLC cell proliferation, cell migration and apoptosis. Mechanistically, circSCAP directly binds to the SF3A3 protein, facilitating the reduction of SF3A3 by promoting its ubiquitin-proteasome-mediated degradation, which enhances the expression of MDM4-S to finally activate its downstream p53 signaling.

CONCLUSION

These findings illustrate a novel circSCAP/SF3A3/p53 signaling axis involved in suppressing the malignance of NSCLC and provide a promising target for NSCLC prognosis prediction and treatment.

Decreased LIPF expression is correlated with DGKA and predicts poor outcome of gastric cancer

Gastric cancer (GC) is a common and deadly digestive tract tumor worldwide. Unfortunately, diagnosis of GC is usually confused and misleading because of atypical symptoms or incomplete complaints. Accordingly, exploring gene expression profile and identifying genes with analogical variance trend will bring new perspective into the diagnosis and treatment of GC. Herein, a RNA-Seq dataset from Caucasian GC and their matched non-cancerous samples [Gene Expression Omnibus (GEO): SRP049809] and datasets from four microarrays constituted with tumor and non-tumor tissues (GEO: GSE13911, GSE19826, GSE29272, GSE33335) were analyzed to explore the differentially expressed genes (DGEs). As a result, we identified a core set of 373 DGEs. Among these genes, we found that most downregulated genes were related to lipid-metabolic functions. Especially, the gastric lipase (LIPF) gene, which was connected with various lipid metabolism processes, was significantly decreased among all datasets. We then performed immunohistochemistry experiments using gastric tissue arrays to investigate the clinical effects, and the expression of a LIPF target gene, diacylglycerol kinase α (DGKA). Among the 90 samples of gastric adenocarcinoma, the LIPF and DGKA levels were both decreased in cancer tissues [LIPF, 59.1% (53/90); DGKA, 77.8% (70/90)] compared to normal tissues [LIPF, 94.4% (85/90); DGKA, 90% (81/90)]. The expression level of these two proteins in GC was associated with local invasion and disease stage. Cox regression identified high DGKA expression (HR, 0.49; 95% CI, 0.26–0.94; P=0.03) as a predictor of good prognosis and LNM status (HR, 4.63; 95% CI, 1.39–15.51; P=0.01) as a predictor of poor prognosis. Thus we speculated that LIPF-DGKA might serve as a potential possible biomarkers for diagnosis of GC, and their downregulation may bring new perspective into the investigation of GC prognosis.

Modeling the study of DNA damage responses in mice

Damaged DNA has a profound impact on mammalian health and overall survival. In addition to being the source of mutations that initiate cancer, the accumulation of toxic amounts of DNA damage can cause severe developmental diseases and accelerate aging. Therefore, understanding how cells respond to DNA damage has become one of the most intense areas of biomedical research in the recent years. However, whereas most mechanistic studies derive from in vitro or in cellulo work, the impact of a given mutation on a living organism is largely unpredictable. For instance, why BRCA1 mutations preferentially lead to breast cancer whereas mutations compromising mismatch repair drive colon cancer is still not understood. In this context, evaluating the specific physiological impact of mutations that compromise genome integrity has become crucial for a better dimensioning of our knowledge. We here describe the various technologies that can be used for modeling mutations in mice and provide a review of the genes and pathways that have been modeled so far in the context of DNA damage responses.

p53-dependent transcription and tumor suppression are not affected in Set7/9-deficient mice

Methylation of specific lysine residues in the C terminus of p53 is thought to govern p53-dependent transcription following genotoxic and oncogenic stress. In particular, Set7/9 (KMT7)-mediated monomethylation of human p53 at lysine 372 (p53K372me1) was suggested to be essential for p53 activation in human cell lines. This finding was confirmed in a Set7/9 knockout mouse model (Kurash et al., 2008). In an independent knockout mouse strain deficient in Set7/9, we have investigated its involvement in p53 regulation and find that cells from these mice are normal in their ability to induce p53-dependent transcription following genotoxic and oncogenic insults. Most importantly, we detect no impairment in canonical p53 functions in these mice, indicating that Set7/9-mediated methylation of p53 does not seem to represent a major regulatory event and does not appreciably control p53 activity in vivo.

p53 post-translational modification: deregulated in tumorigenesis

The p53 tumor suppressor protein has well-established roles in monitoring various types of stress signals by activating specific transcriptional targets that control cell cycle arrest and apoptosis, although some activities are also mediated in a transcription-independent manner. Here, we review the recent advances in our understanding of the wide spectrum of post-translational modifications that act as epigenetic-like codes for modulating specific functions of p53 in vivo and how deregulation of these modifications might contribute to tumorigenesis. We also discuss future research priorities to further understand p53 post-translational modifications and the interpretation of genetic data in appreciation of the increasing evidence that p53 regulates cellular metabolism, autophagy and many unconventional tumor suppressor activities.

Graded enhancement of p53 binding to CREB-binding protein (CBP) by multisite phosphorylation

The transcriptional activity of p53 is regulated by a cascade of posttranslational modifications. Although acetylation of p53 by CREB-binding protein (CBP)/p300 is known to be indispensable for p53 activation, the role of phosphorylation, and in particular multisite phosphorylation, in activation of CBP/p300-dependent p53 transcriptional pathways remains unclear. We investigated the role of single site and multiple site phosphorylation of the p53 transactivation domain in mediating its interaction with CBP and with the ubiquitin ligase HDM2. Phosphorylation at Thr18 functions as an on/off switch to regulate binding to the N-terminal domain of HDM2. In contrast, binding to CBP is modulated by the extent of p53 phosphorylation; addition of successive phosphoryl groups enhances the affinity for the TAZ1, TAZ2, and KIX domains of CBP in an additive manner. Activation of p53-dependent transcriptional pathways requires that p53 compete with numerous cellular transcription factors for binding to limiting amounts of CBP/p300. Multisite phosphorylation represents a mechanism for a graded p53 response, with each successive phosphorylation event resulting in increasingly efficient recruitment of CBP/p300 to p53-regulated transcriptional programs, in the face of competition from cellular transcription factors. Multisite phosphorylation thus acts as a rheostat to enhance binding to CBP/p300 and provides a plausible mechanistic explanation for the gradually increasing p53 response observed following prolonged or severe genotoxic stress.

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.

20 years studying p53 functions in genetically engineered mice

Cell and molecular biological studies of p53 functions over the past 30 years have been complemented in the past 20 years by studies that use genetically engineered mice. As expected, mice that have mutant Trp53 alleles usually develop cancers of various types more rapidly than their counterparts that have wild-type Trp53 genes. These mouse studies have been instrumental in providing important new insights into p53 tumour suppressor function. Such studies have been facilitated by the development of increasingly sophisticated genetic engineering approaches, which allow the more precise manipulation of p53 structure and function in a mammalian model.

Regulation by phosphorylation of the relative affinities of the N-terminal transactivation domains of p53 for p300 domains and Mdm2

The transcriptional activity of the tumor suppressor p53 requires direct binding between its transactivation domain (TAD, 1-57) and the transcriptional coactivator p300. We systematically assessed the role of TAD phosphorylation on binding of the p300 domains CH3, Taz1, Kix and IBiD. Thr18 phosphorylation increased the affinity up to 7-fold for CH3 and Taz1, with smaller increases from phosphorylation of Ser20, Ser15, Ser37, Ser33, Ser46 and Thr55. Binding of Kix and IBiD was less sensitive to phosphorylation. Strikingly, hepta-phosphorylation of all Ser and Thr residues increased binding 40- and 80-fold with CH3 and Taz1, respectively, but not with Kix or Ibid. Substitution of all phospho-sites with aspartates partially mimicked the effects of hepta-phosphorylation. Mdm2, the main negative regulator of p53, competes with p300 for binding TAD. Binding of Mdm2 to TAD was reduced significantly only on phosphorylation of Thr18 (7-fold) or by hepta-phosphorylation (24-fold). The relative affinities of Mdm2 and p300 for p53 TAD can thus be changed by up to three orders of magnitude by phosphorylation. Accordingly, phosphorylation of Thr18 and hepta-phosphorylation dramatically shifts the balance to favouring binding of p300 with p53 and is thus likely to be an important factor in its regulation.

When mutant p53 plays hide and seek: a new challenge for diagnosis and therapy?

p53 missense mutations observed in human cancers are often associated with an increased level of p53 protein in the tumour. Using mouse models, Terzian et al. recently showed that this accumulation of mutant p53 protein is not associated with specific properties of the protein itself but instead depends on the endogenous genetic background of the tumours and on two important genes, mouse double minute 2 (Mdm2) and the cyclin kinase inhibitor p16INK4a. Mice expressing mutant p53 in the absence of Mdm2 display more aggressive metastatic tumours. In light of these observations, targeting the MDM2-p53 interaction for therapy of human cancer could be more complicated than previously anticipated.

p53 regulates the self-renewal and differentiation of neural precursors

During embryo neurogenesis, neurons that originate from stem cells located in the forebrain subventricular zone (SVZ) continuously migrate to the olfactory bulb (OB). However, other authors describe the occurrence of resident stem cells in the OB. In the present work we report that the absence of tumor suppressor protein p53 increases the number of neurosphere-forming cells and the proliferation of stem cells derived from 13.5-day embryo OB. Interestingly, differentiation of p53 knockout-derived neurospheres was biased toward neuronal precursors, suggesting a role for p53 in the differentiation process. Moreover, we demonstrate the relevance of p53 in maintaining chromosomal stability in response to genotoxic insult. Finally, our data show that neurosphere stem cells are highly resistant to long-term epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) deprivation in a p53-independent fashion, and they preserve their differentiation potential. Thus, these data demonstrate that p53 controls the proliferation, chromosomal stability and differentiation pattern of embryonic mouse olfactory bulb stem cells.

Urodele p53 tolerates amino acid changes found in p53 variants linked to human cancer

Background

Urodele amphibians like the axolotl are unique among vertebrates in their ability to regenerate and their resistance to develop cancers. It is unknown whether these traits are linked at the molecular level.

Results

Blocking p53 signaling in axolotls using the p53 inhibitor, pifithrin-α, inhibited limb regeneration and the expression of p53 target genes such as Mdm2 and Gadd45, suggesting a link between tumor suppression and regeneration. To understand this relationship we cloned the p53 gene from axolotl. When comparing its sequence with p53 from other organisms, and more specifically human we observed multiple amino acids changes found in human tumors. Phylogenetic analysis of p53 protein sequences from various species is in general agreement with standard vertebrate phylogeny; however, both mice-like rodents and teleost fishes are fast evolving. This leads to long branch attraction resulting in an artefactual basal emergence of these groups in the phylogenetic tree. It is tempting to assume a correlation between certain life style traits (e.g. lifespan) and the evolutionary rate of the corresponding p53 sequences. Functional assays of the axolotl p53 in human or axolotl cells using p53 promoter reporters demonstrated a temperature sensitivity (ts), which was further confirmed by performing colony assays at 37°C. In addition, axolotl p53 was capable of efficient transactivation at the Hmd2 promoter but has moderate activity at the p21 promoter. Endogenous axolotl p53 was activated following UV irradiation (100 j/m²) or treatment with an alkylating agent as measured using serine 15 phosphorylation and the expression of the endogenous p53 target Gadd45.

Conclusion

Urodele p53 may play a role in regeneration and has evolved to contain multiple amino acid changes predicted to render the human protein defective in tumor suppression. Some of these mutations were probably selected to maintain p53 activity at low temperature. However, other significant changes in the axolotl proteins may play more subtle roles on p53 functions, including DNA binding and promoter specificity and could represent useful adaptations to ensure p53 activity and tumor suppression in animals able to regenerate or subject to large variations in oxygen levels or temperature.

Models and mechanisms in malignant melanoma

Human melanoma represents the fastest growing malignancy in the US. The etiology of melanoma is highly debated as is the role of ultraviolet (UV) radiation in the initiation and progression of melanoma. This article discusses data from UV exposure and its relationship to the development of melanoma from various models of melanoma as well as various genetic alterations seen in oncogenic transformation of melanocytes. Genetic alterations such as the p16(INK4a) deletion, melanocortin 1 receptor (MC1R), RAS, and v-raf murine sarcoma viral oncogene homolog B1 (BRAF) may be indicative of a predisposition to melanoma development. Historical research as well as current data on the significance of the hot spot mutation in BRAF is discussed in its relative potential to the activating mutation in RAS.

p53 alterations in human cancer: more questions than answers

The strongest and undisputed fact about p53 is the high frequency of p53 alterations in human cancer and that mutant p53 proteins constitute a complex family of several hundred proteins with heterogeneous properties. Beyond these observations, the p53 pathway and its regulation in a normal cell is like a desert trail, always moving with the wind of novel findings. The field is full of black boxes that are often ignored for sake of simplicity or because they do not fit with the current dominant view. Mutant p53 protein accumulation in tumours is the best example of a preconceived idea, as there is no experimental evidence to explain this observation. In this review, we will discuss several questions concerning the activity or selection of p53 mutations. The central domain of the p53 protein targeted by 80% of p53 mutations is associated with the DNA-binding activity of the p53 protein, but it is also the binding site for several proteins that play a key role in p53 regulation such as ASPP proteins or BclxL. The role of impaired DNA binding and/or protein interactions in tumour development has not been fully elucidated. Similarly, novel animal models carrying either missense p53 mutations or inducible p53 have provided abundant observations, some of which could challenge our view on p53 function as a tumour suppressor gene. Finally, the possible clinical applications of p53 will be discussed.

A single nucleotide polymorphism in the p53 pathway interacts with gender, environmental stresses and tumor genetics to influence cancer in humans

Cancer biology finds itself in a post-genomic era and the hopes of using inherited genetic variants to improve prevention and treatment strategies are widespread. One of the largest types of inherited genetic variation is the single nucleotide polymorphism (SNP), of which there are at least 4.5 million. The challenge now becomes how to discover which polymorphisms alter cancer in humans and how to begin to understand their mechanism of action. In this report, a series of recent publications will be reviewed that have studied a polymorphism in the p53 tumor suppressor pathway, MDM2 SNP309. These reports have lent insights into how germline genetic variants of the p53 pathway could interact with gender, environmental stresses and tumor genetics to affect cancer in humans. Importantly, these observations have also exposed potential nodes of intervention, which could prove valuable in both the prevention and treatment of this disease in humans.