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.

Altered senescence, apoptosis, and DNA damage response in a mutant p53 model of accelerated aging

The tumor suppressors p16(INK4a) and p53 have been implicated as contributors to age-associated stem cell decline. Key functions of p53 are the induction of cell cycle arrest, senescence, or apoptosis in response to DNA damage. Here, we examine senescence, apoptosis, and DNA damage responses in a mouse accelerated aging model that exhibits increased p53 activity, the p53(+/m) mouse. Aged tissues of p53(+/m) mice display higher percentages of senescent cells (as determined by senescence-associated beta-galactosidase staining and p16(INK4a) and p21 accumulation) compared to aged tissues from p53(+/+) mice. Surprisingly, despite having enhanced p53 activity, p53(+/m) lymphoid tissues exhibit reduced apoptotic activity in response to ionizing radiation compared to p53(+/+) tissues. Ionizing radiation treatment of p53(+/m) tissues also induces higher and prolonged levels of senescence markers p16(INK4a) and p21, suggesting that in p53(+/m) tissues the p53 stress response is enhanced and is shifted away from apoptosis toward senescence. One potential mechanism for accelerated aging in the p53(+/m) mouse is a failure to remove damaged or dysfunctional cells (including stem and progenitor cells) through apoptosis. The increased accumulation of dysfunctional and senescent cells may contribute to reduced tissue regeneration, tissue atrophy, and some of the accelerated aging phenotypes in p53(+/m) mice.

Timed somatic deletion of p53 in mice reveals age-associated differences in tumor progression

Inactivating mutations in the p53 tumor suppressor gene occur often in the progression of human cancers. p53 inhibits the outgrowth of nascent cancer cells through anti-proliferative actions (including induction of apoptosis or senescence). To test p53 tumor suppressor functions in a novel experimental context, we somatically deleted both p53 alleles in multiple tissues of mice at various ages. Mice homozygously deleted for p53 at 3 months of age showed a longer tumor latency compared to mice deleted for p53 at 6 and 12 months of age. These results are consistent with a model in which tissues accumulate oncogenically activated cells with age and these are held in check by wildtype p53. We also deleted p53 before, concurrent with, and after treatment of mice with ionizing radiation (IR). The absence or presence of p53 during IR treatment had no effect on radiation-induced lymphoma latency, confirming that the immediate p53 damage response was irrelevant for cancer prevention. Even the presence of wildtype p53 for up to four weeks post-IR provided no protection against early lymphoma incidence, indicating that long term maintenance of functional p53 is critical for preventing the emergence of a cancer. These experiments indicate that sustained p53 anti-oncogenic function acts as a final or near final line of defense preventing progression of oncogenically activated cells to malignant tumors.

MMP13, Birc2 (cIAP1), and Birc3 (cIAP2), amplified on chromosome 9, collaborate with p53 deficiency in mouse osteosarcoma progression

Osteosarcoma is the primary malignant cancer of bone and particularly affects adolescents and young adults, causing debilitation and sometimes death. As a model for human osteosarcoma, we have been studying p53(+/-) mice, which develop osteosarcoma at high frequency. To discover genes that cooperate with p53 deficiency in osteosarcoma formation, we have integrated array comparative genomic hybridization, microarray expression analyses in mouse and human osteosarcomas, and functional assays. In this study, we found seven frequent regions of copy number gain and loss in the mouse p53(+/-) osteosarcomas but have focused on a recurrent amplification event on mouse chromosome 9A1. This amplicon is syntenic with a similar chromosome 11q22 amplicon identified in several human tumor types. Three genes on this amplicon, the matrix metalloproteinase gene MMP13 and the antiapoptotic genes Birc2 (cIAP1) and Birc3 (cIAP2), show elevated expression in mouse and human osteosarcomas. We developed a functional assay using clonal osteosarcoma cell lines transduced with lentiviral short hairpin RNA vectors to show that down-regulation of MMP13, Birc2, or Birc3 resulted in reduced tumor growth when transplanted into immunodeficient recipient mice. These experiments revealed that high MMP13 expression enhances osteosarcoma cell survival and that Birc2 and Birc3 also enhance cell survival but only in osteosarcoma cells with the chromosome 9A1 amplicon. We conclude that the antiapoptotic genes Birc2 and Birc3 are potential oncogenic drivers in the chromosome 9A1 amplicon.

Notch signaling contributes to the pathogenesis of human osteosarcomas

Notch signaling plays an important role in developmental processes and adult tissue homeostasis. Altered Notch signaling has been associated with various diseases including cancer. While the importance of altered Notch signaling in cancers of hematopoietic and epithelial origins has been established, its role in tumors of mesenchymal origin is less clear. Here, we report that human osteosarcoma cell lines and primary human osteosarcoma tumor samples show significant up-regulation of Notch, its target genes and Osterix. Notch inhibition by gamma-secretase inhibitors or by using lentiviral mediated expression of dominant negative Mastermind-like protein (DN-MAML) decreases osteosarcoma cell proliferation in vitro. In vivo, established human tumor xenografts in nude mice show decreased tumor growth after chemical or genetic inhibition of Notch signaling. Finally, transcriptional profiling of osteosarcomas from p53 mutant mice confirmed up-regulation of Notch1 target genes Hes1, Hey1 and its ligand Dll4. Our data suggest that activation of Notch signaling contributes to the pathogenesis of human osteosarcomas and its inhibition may be a therapeutic approach for the treatment of this mesenchymal tumor.

The type 2C phosphatase Wip1: an oncogenic regulator of tumor suppressor and DNA damage response pathways

The Wild-type p53-induced phosphatase 1, Wip1 (or PPM1D), is unusual in that it is a serine/threonine phosphatase with oncogenic activity. A member of the type 2C phosphatases (PP2Cδ), Wip1 has been shown to be amplified and overexpressed in multiple human cancer types, including breast and ovarian carcinomas. In rodent primary fibroblast transformation assays, Wip1 cooperates with known oncogenes to induce transformed foci. The recent identification of target proteins that are dephosphorylated by Wip1 has provided mechanistic insights into its oncogenic functions. Wip1 acts as a homeostatic regulator of the DNA damage response by dephosphorylating proteins that are substrates of both ATM and ATR, important DNA damage sensor kinases. Wip1 also suppresses the activity of multiple tumor suppressors, including p53, ATM, p16^INK4a and ARF. We present evidence that the suppression of p53, p38 MAP kinase, and ATM/ATR signaling pathways by Wip1 are important components of its oncogenicity when it is amplified and overexpressed in human cancers.

Aging hematopoietic stem cells decline in function and exhibit epigenetic dysregulation

Age-related defects in stem cells can limit proper tissue maintenance and hence contribute to a shortened lifespan. Using highly purified hematopoietic stem cells from mice aged 2 to 21 mo, we demonstrate a deficit in function yet an increase in stem cell number with advancing age. Expression analysis of more than 14,000 genes identified 1,500 that were age-induced and 1,600 that were age-repressed. Genes associated with the stress response, inflammation, and protein aggregation dominated the up-regulated expression profile, while the down-regulated profile was marked by genes involved in the preservation of genomic integrity and chromatin remodeling. Many chromosomal regions showed coordinate loss of transcriptional regulation; an overall increase in transcriptional activity with age and inappropriate expression of genes normally regulated by epigenetic mechanisms was also observed. Hematopoietic stem cells from early-aging mice expressing a mutant p53 allele reveal that aging of stem cells can be uncoupled from aging at an organismal level. These studies show that hematopoietic stem cells are not protected from aging. Instead, loss of epigenetic regulation at the chromatin level may drive both functional attenuation of cells, as well as other manifestations of aging, including the increased propensity for neoplastic transformation.

Aging is marked by a decline in function of the entire organism. The effect of age on the regenerative capacity of adult stem cells, which should rejuvenate tissues throughout life, is poorly understood. Bone marrow stem cells, also known as hematopoietic stem cells (HSCs), continuously regenerate the cells that comprise the blood, including the immune system, which fails with age. Here, we show that older HSCs were less able to regenerate the blood system than young HSCs. Paradoxically, the HSC number increased concomitantly, leading to no major difference in overall blood production, even though the immune system did exhibit some defects. To determine why these changes occurred, we looked at global patterns of gene expression in young versus old HSC. Stem cells exhibited an elevated inflammatory response and a decline in factors, called chromatin regulators, that orchestrate DNA accessibility and gene expression. Additional evidence supports the idea that loss of overall gene regulation (epigenetic regulation) is a major event during aging. Whereas much of aging research is concentrated on accumulation of mutations in DNA rather than on global regulatory mechanisms, we speculate that these epigenetic changes could drive many of the manifestations of age. This view also may explain the increased incidence of cancer with age.

In highly purified hematopoietic stem cells from mice aged 2 to 21 months, gene expression analysis indicates a deficit in function yet an increase in stem cell number with advancing age.

The Wip1 phosphatase and Mdm2: cracking the "Wip" on p53 stability

The p53 tumor suppressor is essential in maintaining genomic integrity in response to cellular stresses. In response to DNA damage, p53 is activated and stabilized largely through post-translational modifications, including phosphorylation by DNA damage responsive kinases such as ATM and ATR. Activated p53 transactivates a battery of genes that can mediate either cell cycle arrest or apoptosis. In those instances where p53 facilitates cell cycle arrest, a means to return the cell to a pre-stress state with low p53 levels is important. The E3 ubiquitin ligase Mdm2 is one p53 transcriptional target that accumulates after damage and promotes p53 ubiquitination and degradation. Thus, p53 and Mdm2 form a critical negative feedback regulatory loop that helps to maintain appropriate p53 levels in the presence or absence of stress. We propose here that Wip1 (Wildtype p53-Induced Phosphatase 1), also known as PPM1D, plays an important role in the p53-Mdm2 autoregulatory loop. We have recently shown that Wip1, also a p53 target gene, dephosphorylates Mdm2 at Ser395 (an ATM target site), resulting in stabilization of Mdm2, enhanced Mdm2-p53 binding, and enhanced ubiquitination of p53 by Mdm2. Thus, Wip1 facilitates Mdm2-mediated degradation of p53. The p53 inhibitory role of Wip1 implicates it as a potential oncogene and indeed Wip1 is amplified and overexpressed in a number of human cancers. Wip1 may inhibit p53 signaling by multiple mechanisms, but our data suggests that its largest effects are due to dephosphorylation of Mdm2.

Analysis of cellular senescence in culture in vivo: the senescence-associated beta-galactosidase assay

Replicative senescence, a process first described almost 40 years ago, entails irreversible growth arrest with sustained metabolic functions, and it is also associated with increased resistance to apoptotic signals. Interest in this process has increased greatly over the last 10 years, as it has been demonstrated that senescence can function as a potent tumor suppressor mechanism. Although mounting evidence suggests that the senescent phenotype is associated with an extraordinarily complex array of gene expression patterns and interactions with the microenvironment, there is only one widely accepted marker for distinguishing such cells in vitro and in vivo. This marker is the senescence-associated expression of a pH 6 beta-galactosidase (SA-beta-gal). Here, a method for analyzing SA-beta-gal expression in cultured cells and in human and animal tissues is described, and important parameters to consider when performing such assays are also highlighted.

How does suppression of IGF-1 signaling by DNA damage affect aging and longevity?

Long-lived animals have evolved a robust set of defenses to maintain genomic integrity over their entire lifespan. The DNA damage response and DNA repair pathways are critical pillars of organismal defenses, minimizing somatic mutations in both post-mitotic and mitotic cells. These genomic maintenance systems not only prevent the premature emergence of cancers but may also maintain normal tissue function and organismal longevity. Genetic defects in a number of DNA repair and DNA damage response genes often leads to a dramatic increase in cancer incidence; in other cases, premature aging or progeroid syndromes may be induced. In this review, we discuss two recent reports of two nucleotide excision repair-deficient models that exhibit dramatic premature aging and shortened longevity. The DNA repair defects were also associated with a significant inhibition of the growth hormone/insulin-like growth factor 1 (GH/IGF-1) axis, an endocrine signaling pathway shown to influence aging and longevity in both vertebrates and invertebrates. Potential mechanisms of how DNA damage might affect IGF-1 signaling and aging are discussed, with a particular emphasis on the role of such signaling alterations in the adult tissue stem cell compartments.

Aging-associated truncated form of p53 interacts with wild-type p53 and alters p53 stability, localization, and activity

Evidence has accumulated that p53, a prototypical tumor suppressor, may also influence aspects of organismal aging. We have previously described a p53 mutant mouse model, the p53+/m mouse, which is cancer resistant yet exhibits reduced longevity and premature aging phenotypes. p53+/m mice express one full length p53 allele and one truncated p53 allele that is translated into a C-terminal fragment of p53 termed the M protein. The augmented cancer resistance and premature aging phenotypes in the p53+/m mice are consistent with a hyperactive p53 state. To determine how the M protein could increase p53 activity, we examined the M protein in various cellular contexts. Here, we show that embryo fibroblasts from p53+/m mice exhibit reduced proliferation and cell cycle progression compared to embryo fibroblasts from p53+/- mice (with equivalent wild-type p53 dosage). The M protein interacts with wild-type p53, increases its stability, and facilitates its nuclear localization in the absence of stress. Despite increasing p53 stability, the M protein does not disrupt p53-Mdm2 interactions and does not prevent p53 ubiquitination. These results suggest molecular mechanisms by which the M protein could influence the aging and cancer resistance phenotypes in the p53+/m mouse.

Altered mammary gland development in the p53+/m mouse, a model of accelerated aging

The tumor suppressor p53 is important for inhibiting the development of breast carcinomas. However, little is known about the effects of increased p53 activity on mammary gland development. Therefore, the effect of p53 dosage on mammary gland development was examined by utilizing the p53+/m mouse, a p53 mutant which exhibits increased wild-type p53 activity, increased tumor resistance, a shortened longevity, and a variety of accelerated aging phenotypes. Here we report that p53+/m virgin mice exhibit a defect in mammary gland ductal morphogenesis. Transplants of mammary epithelium into p53+/m recipient mice demonstrate decreased outgrowth of wild-type and p53+/m donor epithelium, suggesting systemic or stromal alterations in the p53+/m mouse. Supporting these data, p53+/m mice display decreased levels of serum IGF-1 and reduced IGF-1 signaling in virgin glands. The induction of pregnancy or treatment of p53+/m mice with estrogen, progesterone, estrogen and progesterone in combination, or IGF-1 stimulates ductal outgrowth, rescuing the p53+/m mammary phenotype. Serial mammary epithelium transplants demonstrate that p53+/m epithelium exhibits decreased transplant capabilities, suggesting early stem cell exhaustion. These data indicate that appropriate levels of p53 activity are important in regulating mammary gland ductal morphogenesis, in part through regulation of the IGF-1 pathway.

Medulloblastomas overexpress the p53-inactivating oncogene WIP1/PPM1D

Medulloblastoma is the most common malignant brain tumor of childhood. Despite numerous advances, clinical challenges range from recurrent and progressive disease to long-term toxicities in survivors. The lack of more effective, less toxic therapies results from our limited understanding of medulloblastoma growth. Although TP53 is the most commonly altered gene in cancers, it is rarely mutated in medulloblastoma. Accumulating evidence, however, indicates that TP53 pathways are disrupted in medulloblastoma. Wild-typep53-induced phosphatase 1 (WIP1 or PPM1D) encodes a negative regulator of p53. WIP1 amplification (17q22-q23) and its overexpression have been reported in diverse cancer types. We examined primary medulloblastoma specimens and cell lines, and detected WIP1 copy gain and amplification prevalent among but not exclusively in the tumors with 17q gain and isochromosome 17q (i17q), which are among the most common cytogenetic lesions in medulloblastoma. WIP1 RNA levels were significantly higher in the tumors with 17q gain or i17q. Immunoblots confirmed significant WIP1 protein in primary tumors, generally higher in those with 17q gain or i17q. Under basal growth conditions and in response to the chemotherapeutic agent, etoposide, WIP1 antagonized p53-mediated apoptosis in medulloblastoma cell lines. These results indicate that medulloblastoma express significant levels of WIP1 that modulate genotoxic responsiveness by negatively regulating p53.

Early development of histiocytic sarcomas in p53 knockout mice treated with N-bis(2-hydroxypropyl)nitrosamine

p53 knockout mice have been utilized for the functional analysis of p53 in carcinogenic processes and for the evaluation of the carcinogenic potential of chemicals. In this study, we established that p53 knockout mice have an elevated susceptibility to the induction of histiocytic sarcoma (HS) by N-bis(2-hydroxy-propyl)nitrosamine (BHP). p53 heterozygous (+/-) and wild-type (+/+) mice were treated with 20 or 200 ppm BHP in their drinking water for 15 weeks or with 20 ppm BHP for 40 weeks. An additional group of p53 nullizygous (-/-) mice were treated with 20 ppm BHP for 15 weeks. In a 15-week experiment, hepatic HSs were unexpectedly observed in BHP-treated p53 (-/-) mice (30.8%) but not in p53 (+/-) and (+/+) mice and untreated (-/-) mice, indicating that a complete loss of p53 dramatically accelerates the genesis of HS. In a 40-week experiment, HSs were significantly increased in female p53 (+/-) mice (37.5%) as compared with female (+/+) mice (5.0%). Additionally, PCR-SSCP and sequencing analysis revealed a high frequency of p53 gene mutations in HSs, demonstrating the involvement of p53 gene mutations in HS development. Our data add to the understanding of the carcinogenic susceptibility of p53 knockout mice, and may help to elucidate the pathogenesis of HS development.

The Wip1 Phosphatase acts as a gatekeeper in the p53-Mdm2 autoregulatory loop

The tumor suppressor p53 is a transcription factor that responds to cellular stresses by initiating cell cycle arrest or apoptosis. One transcriptional target of p53 is Mdm2, an E3 ubiquitin ligase that interacts with p53 to promote its proteasomal degradation in a negative feedback regulatory loop. Here we show that the wild-type p53-induced phosphatase 1 (Wip1), or PPM1D, downregulates p53 protein levels by stabilizing Mdm2 and facilitating its access to p53. Wip1 interacts with and dephosphorylates Mdm2 at serine 395, a site phosphorylated by the ATM kinase. Dephosphorylated Mdm2 has increased stability and affinity for p53, facilitating p53 ubiquitination and degradation. Thus, Wip1 acts as a gatekeeper in the Mdm2-p53 regulatory loop by stabilizing Mdm2 and promoting Mdm2-mediated proteolysis of p53.

Organ-dependent susceptibility of p53 knockout mice to 2-amino-3-methylimidazo[4,5-f]quinoline (IQ)

p53 knockout mice are now being frequently used to identify the carcinogenic potential of chemicals, thus it is important to precisely assess the susceptibility of the animals to various test chemicals. In the present study the susceptibility of p53 nullizygous((-/-)), heterozygous((+/-)), and wild-type((+/+)) mice to 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) was investigated. Mice of all three genotypes were first fed a diet containing 100 or 300 p.p.m. IQ for 15 weeks in a short-term experiment. p53((+/-)) and ((+/+)) mice were then treated with IQ for 40 weeks and maintained without further treatment for an additional 12 weeks in the long-term experiment. In the forestomach, the incidence of squamous cell hyperplasia was significantly higher in p53((-/-)) than in ((+/-)) and ((+/+)) mice at 15 weeks and higher in ((+/-)) mice than ((+/+)) mice with long-term IQ treatment, indicating an elevated susceptibility of p53 knockout mice. In contrast, in the liver, various hepatocellular lesions developed mainly in female mice with long-term IQ exposure but no significant differences were evident between p53 knockout and wild-type mice, indicating a lack of elevated susceptibility in the knockout animals. Furthermore, polymerase chain reaction-single strand conformation polymorphism and sequencing analysis revealed relatively high (13/30) and low (1/15) incidences of p53 mutations (exons 5-8) in squamous cell hyperplasia and hepatocellular tumors, respectively. These results clearly indicate that the susceptibility of p53 knockout mice is organ-dependent, coinciding to some extent with the likelihood of p53 gene alteration in the induced tumors.

Identification of novel amplification gene targets in mouse and human breast cancer at a syntenic cluster mapping to mouse ch8A1 and human ch13q34

Serial analysis of gene expression from aggressive mammary tumors derived from transplantable p53 null mouse mammary outgrowth lines revealed significant up-regulation of Tfdp1 (transcription factor Dp1), Lamp1 (lysosomal membrane glycoprotein 1) and Gas6 (growth arrest specific 6) transcripts. All of these genes belong to the same linkage cluster, mapping to mouse chromosome band 8A1. BAC-array comparative genomic hybridization and fluorescence in situ hybridization analyses revealed genomic amplification at mouse region ch8A1.1. The minimal region of amplification contained genes Cul4a, Lamp1, Tfdp1, and Gas6, highly overexpressed in the p53 null mammary outgrowth lines at preneoplastic stages, and in all its derived tumors. The same amplification was also observed in spontaneous p53 null mammary tumors. Interestingly, this region is homologous to human chromosome 13q34, and some of the same genes were previously observed amplified in human carcinomas. Thus, we further investigated the occurrence and frequency of gene amplification affecting genes mapping to ch13q34 in human breast cancer. TFDP1 showed the highest frequency of amplification affecting 31% of 74 breast carcinomas analyzed. Statistically significant positive correlation was observed for the amplification of CUL4A, LAMP1, TFDP1, and GAS6 genes (P < 0.001). Meta-analysis of publicly available gene expression data sets showed a strong association between the high expression of TFDP1 and decreased overall survival (P = 0.00004), relapse-free survival (P = 0.0119), and metastasis-free interval (P = 0.0064). In conclusion, our findings suggest that CUL4A, LAMP1, TFDP1, and GAS6 are targets for overexpression and amplification in breast cancers. Therefore, overexpression of these genes and, in particular, TFDP1 might be of relevance in the development and/or progression in a significant subset of human breast carcinomas.

Tumor suppressor dosage regulates stem cell dynamics during aging

The ability of tissues to maintain homeostasis is dependent in part on the function of adult tissue stem cells, which have the capability to self-renew and differentiate into multiple lineages. It has been hypothesized that the ability of stem cells to maintain tissue homeostasis declines functionally with age and that this decline may account for many of the biological phenotypes associated with aging. Recently, tumor suppressors such as p53 have been implicated in both aging and the regulation of stem cell dynamics. Our recent findings suggest that p53 may impact hematopoietic stem cell (HSC) dynamics during mammalian aging. Utilizing mouse models of varying levels of p53 dosage, we have shown that alteration of p53 activity affects stem cell number, proliferation, and functionality with age. Several other recent studies have implicated other tumor suppressors in potential age-related regulation of HSC dynamics as well. These data support a model in which aging is caused in part by a decline in tissue stem cell regenerative function, regulated in part by tumor suppressors.

The impact of altered p53 dosage on hematopoietic stem cell dynamics during aging

A temporal decline in tissue stem cell functionality may be a key component of mammalian aging. The tumor suppressor p53 has recently been implicated as a potential regulator of aging. We examined age-associated hematopoietic stem cell (HSC) dynamics in mice with varying p53 activities. Reduced p53 activity in p53+/- mice was associated with higher numbers of proliferating hematopoietic stem and progenitor cells in old age compared with aged wild-type (p53+/+) mice. We also assessed HSC dynamics in a p53 mutant mouse model (p53+/m) with higher apparent p53 activity than wild-type mice. The p53 hypermorphic (p53+/m) mice display phenotypes of premature aging. Many aged p53+/m organs exhibit reduced cellularity and atrophy, suggesting defects in stem-cell regenerative capacity. HSC numbers from old p53+/m mice fail to increase with age, unlike those of their p53+/+ and p53+/- counterparts. Moreover, transplantation of 500 HSCs from old p53+/m mice into lethally irradiated recipients resulted in reduced engraftment compared with old wild-type p53+/+ and p53+/- HSCs. Thus, alteration of p53 activity affects stem-cell numbers, proliferation potential, and hematopoiesis in older organisms, supporting a model in which aging is caused in part by a decline in tissue stem cell regenerative function.

Augmented cancer resistance and DNA damage response phenotypes in PPM1D null mice

The p53-induced serine/threonine phosphatase, protein phosphatase 1D magnesium-dependent, delta isoform (PPM1D) (or wild-type p53-induced phosphatase 1 (Wip1)), exhibits oncogenic activity in vitro and in vivo. It behaves as an oncogene in rodent fibroblast transformation assays and is amplified and overexpressed in several human tumor types. It may contribute to oncogenesis through functional inactivation of p53. Here, we show that the oncogenic function of PPM1D is associated with its phosphatase activity. While overexpressed PPM1D may be oncogenic, PPM1D null mice are resistant to spontaneous tumors over their entire lifespan. This cancer resistance may be based in part on an augmented stress response following DNA damage. PPM1D null mice treated with ionizing radiation display increased p53 protein levels and increased phosphorylation of p38 MAP kinase, p53, checkpoint kinase 1 (Chk1), and checkpoint kinase 2 (Chk2) in their tissues compared to their wild-type (WT) counterparts. Male PPM1D null mice show a modest reduction in longevity, reduced serum insulin-like growth factor 1 (IGF-1) levels, and reduced body weight compared to WT mice. The PPM1D null mouse phenotypes indicate that PPM1D has a homeostatic role in abrogating the DNA damage response and may regulate aspects of male longevity.

DeltaNp63alpha overexpression induces downregulation of Sirt1 and an accelerated aging phenotype in the mouse

p63 is highly expressed in the skin and appears to be an early marker of keratinocyte differentiation. To examine the role of p63 in vivo, we generated transgenic mice that overexpress deltaNp63alpha in the skin. These mice exhibited an accelerated aging phenotype in the skin characterized by striking wound healing defects, decreased skin thickness, decreased subcutaneous fat tissue, hair loss, and decreased cell proliferation. The accelerated skin aging was accompanied by a dramatic decrease in longevity of the mice. We found that aging in deltaNp63alpha transgenic mice and other mouse models correlated with levels of Sirt1, a mammalian SIR2 orthologue thought to extend the lifespan in lower species. Moreover, increased deltaNp63alpha expression induced cellular senescence that was rescued by Sirt1. Our data suggest that deltaNp63alpha levels may affect aging in mammals, at least in part, through regulation of Sirt1.

Oxidative and nitrative stress caused by subcutaneous implantation of a foreign body accelerates sarcoma development in Trp53+/- mice

Chronic inflammation is a recognized risk factor for human cancer at various sites because of persistent oxidative and nitrative tissue damage. Trp53+/- mice show the predisposition to tumor development, such as sarcomas and lymphomas, compared with Trp53+/+ mice. We investigated the effects of chronic inflammation, especially oxidative and nitrative stress, induced by subcutaneous implantation of a plastic plate (10 x 5 x 1 mm) as a foreign body on tumorigenesis in Trp53+/- and Trp53+/+ mice. The plastic plates were implanted at the age of about 11 weeks. Thirty out of 38 Trp53+/- mice (79%) developed sarcomas around the implant (mean time of tumor appearance was 45.8 +/- 12.0 weeks of age), whereas only one of 10 Trp53+/+ mice with an implant (10%) developed a tumor, at 56 weeks. No sarcomas developed at a sham-operation site. Two of 10 Trp53+/- mice with no implant (20%) also developed three sarcomas spontaneously at 77, 81 and 84 weeks. Increased immunostaining for markers of oxidative and nitrative stress (8-oxo-7,8-dihydro-2'-deoxyguanosine, 8-nitroguanine and 3-nitrotyrosine) and expression of inducible nitric oxide synthase in tumor cells and inflammatory cells were detected in implant-induced sarcomas compared with spontaneous sarcomas in Trp53+/- mice. Furthermore, p53 loss of heterozygosity was observed in 26 out of 29 implant-induced sarcomas (90%). These results indicate that implanted foreign bodies significantly enhanced sarcoma development in Trp53+/- mice, and this may be associated with increased oxidaive and nitrative stress. Loss of the remaining wild-type p53 allele and loss of p53 function appears to be, at least in part, underlying molecular mechanisms during the development of sarcomas at the implantation site in Trp53+/- mice. Such implant-induced sarcoma development in Trp53+/- mice could be useful for studying molecular mechanisms and developing new strategies for chemoprevention in human carcinogenesis induced by chronic inflammation and/or foreign bodies.

Oncogenic function for the Dlg1 mammalian homolog of the Drosophila discs-large tumor suppressor

The fact that several different human virus oncoproteins, including adenovirus type 9 E4-ORF1, evolved to target the Dlg1 mammalian homolog of the membrane-associated Drosophila discs-large tumor suppressor has implicated this cellular factor in human cancer. Despite a general belief that such interactions function solely to inactivate this suspected human tumor suppressor protein, we demonstrate here that E4-ORF1 specifically requires endogenous Dlg1 to provoke oncogenic activation of phosphatidylinositol 3-kinase (PI3K) in cells. Based on our results, we propose a model wherein E4-ORF1 binding to Dlg1 triggers the resulting complex to translocate to the plasma membrane and, at this site, to promote Ras-mediated PI3K activation. These findings establish the first known function for Dlg1 in virus-mediated cellular transformation and also surprisingly expose a previously unrecognized oncogenic activity encoded by this suspected cellular tumor suppressor gene.

Dual roles for the phosphatase PPM1D in regulating progesterone receptor function

Although protein phosphatase magnesium-dependent 1 delta (PPM1D) was initially characterized as a p53-regulated phosphatase responsible for inactivation of p38 MAPK and consequent inactivation of p53, its overexpression and amplification in human breast cancers led us to assess its role in steroid hormone action. We found that PPM1D stimulated the activity of several nuclear receptors including the progesterone receptor (PR) and estrogen receptor. Although p38 MAPK inhibited PR activity, PPM1D stimulation of PR activity was greater than that achieved by a chemical inhibitor of p38 MAPK, SB202190. This suggests an additional novel function for PPM1D. Consistent with this, the transcriptional activity of endogenous PR in MCF-7 breast cancer cells was preferentially inhibited by small interfering RNA for PPM1D; SB202190 failed to reverse the inhibition. Although PPM1D phosphatase activity was required for stimulation of transcriptional activity, the activity of a PR phosphorylation site null mutant was enhanced by PPM1D, indicating that PR is not the direct target. Additional studies revealed that PPM1D enhanced the intrinsic activity of p160 coactivators such as steroid receptor coactivator-1 and promoted the interaction between PR and steroid receptor coactivator-1 in a mammalian two-hybrid assay. Neither activity was induced by SB202190. Although PPM1D stimulated PR activity in part through inhibition of p38 MAPK, its primary action is novel and independent of p38 MAPK. Thus, we speculate that PPM1D promotes breast tumor growth both by inhibiting p53 activity and by enhancing steroid hormone receptor action.