P53 Gene Dosage Research Articles

Transient enhancement of p53 activity protects from radiation-induced gastrointestinal toxicity

Gastrointestinal (GI) syndrome is a serious side effect and dose-limiting toxicity observed in patients undergoing lower-abdominal radiotherapy. Previous mouse studies show that p53 gene dosage determines susceptibility to GI syndrome development. However, the translational relevance of p53 activity has not been addressed. Here, we used a knock-in mouse in which the p53-Mdm2 negative feedback loop is genetically disrupted. These mice retain biallelic p53 and thus, normal basal p53 levels and activity. However, due to the lack of p53-mediated Mdm2 transcription, irradiated Mdm2P2/P2 mice exhibit enhanced acute p53 activity, which protects them from GI failure. Intestinal crypt cells residing in the +4 and higher positions exhibit decreased apoptosis, increased p21 expression, and hyperproliferation to reinstate intestinal integrity. Correspondingly, pharmacological augmentation of p53 activity in wild-type mice with an Mdm2 inhibitor protects against GI toxicity without affecting therapeutic outcome. Our results suggest that transient disruption of the p53-Mdm2 interaction to enhance p53 activity could be a viable prophylactic strategy for alleviating GI syndrome in patients undergoing radiotherapy.

Obesity, Independent of p53 Gene Dosage, Promotes Mammary Tumor Progression and Upregulates the p53 Regulator MicroRNA-504

Obesity, prevalent in >35% of US women, is an established risk and progression factor for postmenopausal breast cancer, and strategies to break the obesity-breast cancer link are urgently needed. Approximately 30% of breast cancers carry p53 tumor suppressor gene alterations; however, the effects of obesity on breast cancer progression in relation to p53 gene dosage are unclear. Using murine models of postmenopausal breast cancer, we characterized the interactive effects of diet-induced obesity (DIO) and p53 gene dosage on mammary tumor growth and associated p53-related regulatory mechanisms. Ovariectomized C57BL/6 mice were randomly assigned to receive a DIO or control diet, and (at 10 weeks) orthotopic injection of MMTV-Wnt-1 p53+/− or MMTV-Wnt-1 p53+/+ mammary tumor cells (n = 20 mice per diet and genotype group). DIO and control diets produced distinct phenotypes (mean percent body fat at 10 weeks: 57% and 39%, respectively, P<0.001). Regardless of phenotype, time to first palpable tumor was 57% less for Wnt-1 p53+/− than Wnt-1 p53+/+ tumors. Regardless of tumoral p53 genotype, DIO (relative to control) increased tumor burden, tumor cell proliferation (Ki-67), severity of tumor pathology, local tissue invasion, epithelial-to-mesenchymal transition (EMT) programming, and tumoral microRNA-504 (a negative regulator of p53) expression; and suppressed p53, p21, and estrogen receptor-alpha protein expression. These findings in murine models of postmenopausal breast cancer suggest that obesity may augment procancer effects related to p53 gene alterations. Furthermore, microRNA-504, an obesity-responsive negative regulator of p53 and putative EMT regulator, may represent a novel molecular target for breaking the obesity-breast cancer link.

Mitochondrial Dysfunction in Pten Haplo-Insufficient Mice with Social Deficits and Repetitive Behavior: Interplay between Pten and p53

Etiology of aberrant social behavior consistently points to a strong polygenetic component involved in fundamental developmental pathways, with the potential of being enhanced by defects in bioenergetics. To this end, the occurrence of social deficits and mitochondrial outcomes were evaluated in conditional Pten (Phosphatase and tensin homolog) haplo-insufficient mice, in which only one allele was selectively knocked-out in neural tissues. Pten mutations have been linked to Alzheimer's disease and syndromic autism spectrum disorders, among others. By 4–6 weeks of age, Pten insufficiency resulted in the increase of several mitochondrial Complex activities (II–III, IV and V) not accompanied by increases in mitochondrial mass, consistent with an activation of the PI3K/Akt pathway, of which Pten is a negative modulator. At 8–13 weeks of age, Pten haplo-insufficient mice did not show significant behavioral abnormalities or changes in mitochondrial outcomes, but by 20–29 weeks, they displayed aberrant social behavior (social avoidance, failure to recognize familiar mouse, and repetitive self-grooming), macrocephaly, increased oxidative stress, decreased cytochrome c oxidase (CCO) activity (50%) and increased mtDNA deletions in cerebellum and hippocampus. Mitochondrial dysfunction was the result of a downregulation of p53-signaling pathway evaluated by lower protein expression of p21 (65% of controls) and the CCO chaperone SCO2 (47% of controls), two p53-downstream targets. This mechanism was confirmed in Pten-deficient striatal neurons and, HCT 116 cells with different p53 gene dosage. These results suggest a unique pathogenic mechanism of the Pten-p53 axis in mice with aberrant social behavior: loss of Pten (via p53) impairs mitochondrial function elicited by an early defective assembly of CCO and later enhanced by the accumulation of mtDNA deletions. Consistent with our results, (i) SCO2 deficiency and/or CCO activity defects have been reported in patients with learning disabilities including autism and (ii) mutated proteins in ASD have been found associated with p53-signaling pathways.

The molecular and cellular basis of variable craniofacial phenotypes and their genetic rescue in Twisted gastrulation mutant mice

The severity of numerous developmental abnormalities can vary widely despite shared genetic causes. Mice deficient in Twisted gastrulation ( Twsg1 −/− ) display such phenotypic variation, developing a wide range of craniofacial malformations on an isogenic C57BL/6 strain background. To examine the molecular basis for this reduced penetrance and variable expressivity, we used exon microarrays to analyze gene expression in mandibular arches from several distinct, morphologically defined classes of Twsg1 −/− and wild type (WT) embryos. Hierarchical clustering analysis of transcript levels identified numerous differentially expressed genes, clearly distinguishing severely affected and unaffected Twsg1 −/− mutants from WT embryos. Several genes that play well-known roles in craniofacial development were upregulated in unaffected Twsg1 −/− mutant embryos, suggesting that they may compensate for the loss of TWSG1. Imprinted genes were overrepresented among genes that were differentially expressed particularly between affected and unaffected mutants. The most severely affected embryos demonstrated increased p53 signaling and increased expression of its target, Trp53inp1. The frequency of craniofacial defects significantly decreased with a reduction of p53 gene dosage from 44% in Twsg1 −/− p53 +/+ pups (N = 675) to 30% in Twsg1 −/− p53 +/− (N = 47, p = 0.04) and 15% in Twsg1 −/− p53 −/− littermates (N = 39, p = 0.001). In summary, these results demonstrate that phenotypic variability in Twsg1 −/− mice is associated with differential expression of certain developmentally regulated genes, and that craniofacial defects can be partially rescued by reduced p53 levels. We postulate that variable responses to stress may contribute to variable craniofacial phenotypes by triggering differential expression of genes and variable cellular apoptosis.

Abstract 5084: Treadmill exercise modulates AMP-activated protein kinase and downstream signaling in mammary gland of p53+/−, but not p53+/+ mice

Abstract Epidemiological data show that exercise is generally protective against breast cancer. Inhibition of mammalian target of rapamyocin (mTOR) through activation of AMP-activated protein kinase (AMPK) has been shown to be responsive to exercise. Decreased activation of ribosomal protein S6, a marker for mTOR inhibition, is associated with decreased growth and proliferation. Recently, this energy-sensing pathway has been shown to be inhibited by increased production of reactive oxygen species (ROS) leading to AMPK activation. ROS levels are maintained by p53-inducible anti-oxidant enzymes. Therefore, we hypothesized that energy sensing pathways that mediate the protective effects of exercise in breast cancer are modulated by p53 gene dosage. To test this hypothesis, we fed ovariectomized mice a high-fat diet (60% kcal) ad libitum for 10 weeks to model post-menopausal obesity. Mice were then randomized to a sedentary or exercise regimen (n=15/group). Exercised mice ran on a variable speed treadmill (5% incline, 20 m/min, 40 min/day, 5 days/week for 22 weeks). Changes in percent body fat were measured every 2 weeks using quantitative magnetic resonance. Immunoblot analysis was performed to measure phosphorylation of AMPK (Thr 172) and ribosomal protein S6 (Ser 240/244) relative to total levels in the mammary fat pad (MFP). Also, qRT-PCR was performed to measure mRNA expression of the p53-inducible anti-oxidant enzyme sestrin 1. At the end of the diet-induced obesity regimen, there was no effect of p53 genotype on percent body fat (54.4 ± 0.7 in p53+/+ versus 55.9 ± 1.1 in p53+/−). After 22 weeks of exercise, the EX mice displayed signficantly less percent body fat than sedentary controls, with no effect of genotype (44.4 ± 0.6 versus 47.8 ± 0.8 % respectively, p&amp;lt;0.05). Immunoblot analysis revealed that in p53+/− mice, but not p53+/+ mice, exercise increased AMPK phosphorylation. This increased AMPK phosphorylation was associated with decreased phosphorylation of ribosomal protein S6, a marker of mTOR pathway activation (n=3/group). The observed increase in AMPK phosphorylation in p53+/− exercised mice relative to sedentary p53+/− was associated with decreased mRNA levels of sestrin 1 (n=4-6/group), indicating that the p53+/− mice may have higher levels of ROS in the MFP. In conclusion, exercise may increase AMPK activation in MFP of p53+/− mice relative to sedentary p53+/− mice through an ROS-mediated mechanism. Current analysis is focused on understanding how exercise affects AMPK activation in the mammary fat pad of MMTV-Wnt-1 transgenic mice heterozygous for p53, and how this impacts mammary tumorigenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5084.

Sensitivity to cadmium chloride‐induced forelimb ectrodactyly is independent of the p53 gene‐dosage in the C57BL/6J mouse

The p53 pathway plays an important role in the regulation of apoptosis, osteoblast differentiation, skeletal development, and teratogenic sensitivity. The administration of cadmium chloride (CdCl(2)) on gestational day 9 in susceptible mouse strains causes postaxial forelimb ectrodactyly in a percentage of fetuses through unknown mechanisms. In this study, the hypothesis that the p53 gene dosage might affect the incidence or severity of CdCl(2)-induced forelimb ectrodactyly was examined. Heterozygous p53-null female mice, on the C57BL/6J background known to be sensitive to CdCl(2)-induced forelimb ectrodactyly, were mated with heterozygous males and then treated with a single intraperitoneal (ip) dose of CdCl(2) (4 mg x kg(-1)) at embryonic day (ED) 9. Embryos and fetuses, genotyped using DNA isolated from the yolk sacs, were collected at ED10 and examined for the pattern of cell death in the limb buds or collected at ED18 and examined for limb malformations. In the wild type and heterozygous p53 embryonic limb buds, CdCl(2)-induced apoptosis involved mesenchymal cells as well as the apical ectodermal ridge (AER), whereas CdCl(2)-induced apoptosis was restricted mainly to the AER in the homozygous p53-null limb buds. No difference in the incidence or severity of forelimb ectrodactyly in the embryos of different p53 genotypes was observed. Despite the fact that CdCl(2) induced both p53-dependent (in the mesenchyme) and p53-independent (in the AER) cell death in the developing limb bud, CdCl(2)-induced ectrodactyly was independent of the p53 gene dosage at the studied time point.

New Roles for Copper Metabolism in Cell Proliferation, Signaling, and Disease

Over the past 2 decades, the molecular mechanisms by which cells acquire, distribute, and utilize copper have been under intense investigation. Significant progress has been made in the identification of genes encoding copper homeostasis proteins and in fundamental aspects of their structure, function, and mechanisms of action in copper balance. A number of more comprehensive reviews of the field with respect to the genetics, structure, function, and physiology of copper metabolism have recently appeared elsewhere (1–4). Here, we review general mechanisms for eukaryotic copper metabolism at the cellular level in the context of recent discoveries in the field, identifying potential new functions for copper and copper metabolism proteins in cell signaling, gene expression, tumor cell metastasis, and resistance to anti-neoplastic drugs.

Abstract PL05-01: Mechanistic targets for preventing obesity-related cancers

Abstract PL05-01 The prevalence of obesity, an established risk factor for many cancers (1), has risen steadily for the past several decades in the US and many other countries (2). Unfortunately, the mechanisms underlying the obesity and cancer connection are not well understood, and new targets and strategies for offsetting the impact of obesity on cancer risk are urgently needed. Several hormones and growth factors have been implicated as potential mediators of the energy metabolism -carcinogenesis relationship. These include insulin-like growth factor-1 (IGF-1), insulin, several adipokines, and inflammatory/ oxidative stress factors. Calorie restriction (CR), the most commonly recommended dietary strategy for preventing or reversing obesity, has been shown to inhibit spontaneous, transplanted and chemically induced tumors in a variety of animal models (3). In contrast, diet-induced obesity enhances tumorigenesis in many of these same models (3-6). We have also shown in a series of transgenic model systems and microarray studies that the insulin and insulin-like growth factor signaling pathways appear central to many of the anti-cancer effects of CR, as well as the pro-cancer effects of obesity (3). Using AZIP/F1 transgenic mice, which lack white adipose tissue but have high levels of insulin, IGF-1 and inflammatory markers, we have reported that elevated IGF-1 and insulin, as well as increased inflammatory factors (which typically accompany obesity), independent of the adipose tissue per se, appear to be the important dietary targets for disrupting the obesity-cancer link (4). In addition, biochemical and molecular analyses of tissues from lean, overweight and obese mice, as well as IGF-1 deficient mice, established that the Akt and mTOR pathways provide an important target for disrupting the obesity-cancer link via dietary energy balance (5,6). The other side of the energy balance equation is energy expenditure, which includes the energetics of growth, metabolism, thermoregulation and physical activity (the only truly modulatable aspect of energy expenditure). Epidemiologic studies have established that increased physical activity is associated with reduced risk of colorectal cancer and postmenopausal breast cancer (7). In addition, there is limited but encouraging epidemiologic data suggesting that increased physical activity may be associated with reduced risk of several other cancers as well (7). Physical activity also exerts anticancer effects in several of our transgenic models, but apparently through very different mechanisms than CR (7). For example, the protective effects of exercise appear to be dependent on p53 gene dosage and independent of IGF-1, based on studies in Apc min mice and Wnt-1 transgenic mice (7,8). A better understanding of the mechanisms underlying the energy balance-cancer link will facilitate the development of novel prevention and treatment strategies for offsetting the effects of obesity on cancer. Citation Information: Cancer Prev Res 2008;1(7 Suppl):PL05-01.

Increased p53 gene dosage reduces neointimal thickening induced by mechanical injury but has no effect on native atherosclerosis

The tumor suppressor p53 regulates cell proliferation and apoptosis, two key processes in the pathogenesis of occlusive vascular disease. Here, we examined the consequences of heightening p53 function on neointimal lesion formation in the setting of atherosclerosis and mechanical injury. For this study we employed immunohistopathological characterization of neointimal lesions in atherosclerosis-prone apolipoprotein E-null mice with normal p53 gene dosage (apoE-KO) and carrying a p53 transgene (Super-p53/apoE-KO). We also carried out molecular studies in macrophages and smooth muscle cells (SMCs) obtained from these mice. The p53 transgene conferred p53 gain-of-function in cultured cells and mice. In vitro, survival of irradiated Super-p53 macrophages and femoral SMCs was reduced, but only Super-p53 SMCs exhibited attenuated proliferation. In vivo, whereas the size of spontaneously formed and diet-induced aortic atheromas was indistinguishable in apoE-KO and Super-p53/apoE-KO mice, the latter exhibited attenuated neointimal thickening in mechanically injured femoral artery. In both models, neither apoptosis nor cell proliferation were affected by additional p53 gene dosage when examined in established neointimal lesions. However, at 2 days after mechanical injury when neointimal lesions were not yet formed, cell proliferation was significantly attenuated within medial SMCs of Super-p53/apoE-KO mice. Heightening p53 function has differential effects on in vitro proliferation of macrophages (unaffected) versus SMCs (reduced), and on native atherosclerosis (unaffected) versus mechanically induced neointimal thickening (reduced) in apoE-KO mice. The protective effect of p53 in mechanically injured femoral artery coincided with limited medial SMC proliferation at early time points preceding neointima formation, but neither medial nor neointimal cell proliferation was affected in vessels with established occlusive lesions. These findings corroborate p53 gain-of-function as a promising therapeutic strategy to limit post-angioplasty restenosis but not native atherosclerosis.

High Throughput Quantitative Analysis of Serum Proteins Using Glycopeptide Capture and Liquid Chromatography Mass Spectrometry

It is expected that the composition of the serum proteome can provide valuable information about the state of the human body in health and disease and that this information can be extracted via quantitative proteomic measurements. Suitable proteomic techniques need to be sensitive, reproducible, and robust to detect potential biomarkers below the level of highly expressed proteins, generate data sets that are comparable between experiments and laboratories, and have high throughput to support statistical studies. Here we report a method for high throughput quantitative analysis of serum proteins. It consists of the selective isolation of peptides that are N-linked glycosylated in the intact protein, the analysis of these now deglycosylated peptides by liquid chromatography electrospray ionization mass spectrometry, and the comparative analysis of the resulting patterns. By focusing selectively on a few formerly N-linked glycopeptides per serum protein, the complexity of the analyte sample is significantly reduced and the sensitivity and throughput of serum proteome analysis are increased compared with the analysis of total tryptic peptides from unfractionated samples. We provide data that document the performance of the method and show that sera from untreated normal mice and genetically identical mice with carcinogen-induced skin cancer can be unambiguously discriminated using unsupervised clustering of the resulting peptide patterns. We further identify, by tandem mass spectrometry, some of the peptides that were consistently elevated in cancer mice compared with their control littermates.

A novel pathological role of p53 in kidney development revealed by gene-environment interactions.

Gene-environment interactions are implicated in congenital human disorders. Accordingly, there is a pressing need to develop animal models of human disease, which are the product of defined gene-environment interactions. Previously, our laboratory demonstrated that gestational salt stress of bradykinin B(2) receptor (B(2)R)-null mice induces renal dysgenesis and early death of the offspring. In contrast, salt-stressed B(2)R +/+ or +/- littermates have normal development. The present study investigates the mechanisms underlying the susceptibility of B(2)R-null mice to renal dysgenesis. Proteomic and conventional Western blot screens identified E-cadherin among the differentially repressed proteins in B(2)R-/- kidneys, whereas the checkpoint kinase Chk1 and its substrate P-Ser(20) p53 were induced. We tested the hypothesis that p53 mediates repression of E-cadherin gene expression and is causally linked to the renal dysgenesis. Genetic crosses between B(2)R -/- and p53+/- mice revealed that germline reduction of p53 gene dosage rescues B(2)R-/- mice from renal dysgenesis and restores kidney E-cadherin gene expression. Furthermore, gamma-irradiation induces repression of E-cadherin gene expression in p53+/+ but not -/- cells. In transient transfection assays, p53 repressed human E-cadherin promoter-driven reporter activity, whereas a mutant p53, which cannot bind DNA, did not. Functional promoter analysis indicated the presence of a p53-responsive element in exon 1, which partially mediates p53-induced repression. Chromatin immunoprecipitation assays revealed that p53 inhibits histone acetylation of the E-cadherin promoter. Treatment with a histone deacetylase inhibitor reversed both p53-mediated promoter repression and deacetylation. In conclusion, this study demonstrates that gene-environment interactions cooperate to induce congenital defects through p53 activation.

Calorie Restriction Reduces Ulcerative Dermatitis and Infection-Related Mortality in p53-Deficient and Wild-Type Mice

In rodents calorie restriction (CR) reduces cancer incidence, improves health by delaying age-related declines in physiologic measures, and extends both median and maximal life span. The mechanisms underlying the various beneficial effects of CR remain undefined. In this study, heterozygous p53-deficient (p53(+/-)) mice (in which the inactivation of one allele of the p53 tumor suppressor gene increases susceptibility to spontaneous and carcinogen-induced tumor development) and wild-type (WT) litter mates were subjected to a two-stage skin carcinogenesis protocol with 7,12-dimethylbenz[a]anthracene and 12-O-tetradecanoylphorbol-13-acetate. Instead of skin carcinomas, however, the chemical treatment protocol caused ulcerous skin lesions, and 89% of mice fed ad libitum died from infection/septicemia. When WT mice were restricted to 60% of the average calorie intake of the respective ad libitum group, however, only 33% developed such lesions, and the CR mice survived twice as long on average as the ad libitum mice. CR also extended life span in p53(+/-) mice, but 50% of p53(+/-) mice subjected to CR still developed skin ulcers and mean life span was shorter than that seen in WT mice. Differences in response to CR between WT and p53(+/-) mice may be due to the reduction in p53 gene dosage, dissimilarity in the application of the CR treatment, or both. These results suggest that some of the beneficial effects of CR may need full expression of p53 for complete realization.

Chromosome 17 abnormalities and inactivation of the p53 gene in chronic myeloid leukemia and their prognostic significance.

We have reviewed all the relevant studies on the loss of the short arm of chromosome 17 (17p) and inactivation of the p53 gene in chronic myelogenous leukemia (CML) in an attempt to clarify their roles in the progression of CML. Loss of a 17p (hemizygous 17p) and p53 inactivation emerged as the disease progressed and were closely associated with each other. About half of the cases with loss of a 17p, however, did not show p53 inactivation. In these cases loss of a 17p preceded p53 inactivation, which suggested that either reduction of the p53 gene dosage or inactivation of another tumor-suppressor gene on 17p might contribute to the disease progression. Both loss of a 17p and p53 inactivation may serve as poor prognostic factors but the prognostic significance of the former only emerged when metaphase cells with loss of a 17p were dominant amongst the total cell population analyzed.

Reduction of p53 gene dosage does not increase initiation or promotion but enhances malignant progression of chemically induced skin tumors

The availability of p53 knockout mice generated by gene targeting has enabled us to investigate the functional role of the p53 tumor suppressor gene in initiation, promotion, and progression of carcinogenesis in vivo, using mouse skin as a model system. The number, size, and growth rate of benign papillomas were not increased in the p53 heterozygous mice in comparison with wild type. The p53 null mice showed a reduced yield of papillomas, but these underwent much more rapid malignant progression, with some poorly differentiated carcinomas developing after only 10 weeks of promotion. Progression rate was also greater in heterozygous than in wild-type mice and was associated with loss of the remaining wild-type allele. Most tumors from all groups had activating mutations in the H- ras gene. Absence of p53, therefore, does not augment the frequency of initiation or the rate of promotion but greatly enhances malignant progression.