The tumor suppressor p53 is inactivated by multiple mechanisms that include mutations of the p53 gene itself and increased levels of the p53 inhibitors MDM2 and MDM4. Mice lacking Mdm2 or Mdm4 exhibit embryo-lethal phenotypes that are completely rescued by concomitant deletion of p53. Here we show that Mdm2 and Mdm4 haploinsufficiency leads to increased p53 activity, exhibited as increased sensitivity to DNA damage and decreased transformation potential. Moreover, in in vivo tumor development, E-myc Mdm4 ؉/؊ mice show a delayed onset of B-cell lymphomas compared to E-myc mice. Additionally, Mdm2 ؉/؊ Mdm4 ؉/؊ doubleheterozygous mice are not viable and exhibit defects in hematopoiesis and cerebellar development. The defects in Mdm2 ؉/؊ Mdm4 ؉/؊ mice are corrected by deletion of a single p53 allele. These findings highlight the exquisite sensitivity of p53 to Mdm2 and Mdm4 levels and suggest that some cell types may be more sensitive to therapeutic drugs that inhibit the Mdm-p53 interaction.The tumor suppressor p53 is inactivated in most cancers by mutations or deletions of the p53 gene (10). Alternatively, increased levels of two p53 inhibitors, MDM2 and MDM4, common events in many cancers, also dampen p53 activity (5, 23). Two new p53 inhibitors have recently been identified, but their role in tumorigenesis remains unknown (6,14). Other mechanisms of inactivating the p53 pathway include deletions of Arf, which encodes an Mdm2-interacting protein (22). Arf loss results in increased availability of Mdm2 to inhibit p53. Thus, a likely scenario is that inactivation of the p53 pathway by any number of means is an essential step in tumorigenesis.While data showing high levels of MDM2 and MDM4 in human tumors are simply correlative, the essential role of Mdm2 and Mdm4 in the regulation of p53 activity in vivo is clear from mouse models. Mice lacking Mdm2 or Mdm4 exhibit embryo-lethal phenotypes that are completely rescued by concomitant deletion of p53 (13,20,21). Moreover, mice lacking Mdm2 and p53 have the same tumor phenotype as p53-null mice, suggesting that the only pivotal role of MDM2 in vivo is the negative regulation of p53 (16). Additionally, mice carrying a hypomorphic allele of Mdm2 that expressed ϳ30% of Mdm2 levels had increased p53 activity (17). As a result, these mice were small, lymphopenic, and radiosensitive. Importantly, these phenotypes were rescued by the deletion of p53 (17). Mice with decreased levels of Mdm2 also showed a delay in tumor onset in a p53-dependent manner, emphasizing again the importance of p53 and Mdm2 gene dosage in normal cellular survival and tumor onset (18).More recently, in humans, a single nucleotide polymorphism (SNP) in the MDM2 promoter that leads to increased Mdm2 levels and attenuation of p53 activity was discovered (3). The MDM2 SNP is associated with accelerated tumorigenesis in soft tissue sarcomas and in patients with Li-Fraumeni syndrome (3). These SNP data suggest that even small differences in Mdm2 levels may modify cancer risk (4).To determine directly whether...

The transcription factor p53 is a tumor suppressor. As such, the P53 gene is frequently altered in human cancers. However, over 80% of the P53 mutations found in human cancers are missense mutations that lead to expression of mutant proteins that not only lack p53 transcriptional activity but exhibit new functions as well. Recent studies show that restoration of p53 expression leads to tumor regression in mice carrying p53 deletions. However, the therapeutic efficacy of restoring p53 expression in tumors containing p53 missense mutations has not been evaluated. Here we demonstrate that restoring wild-type p53 expression halted tumor growth in mice inheriting a p53 R172H missense mutation that is equivalent to a P53 missense mutation detected in approximately 6% of human cancers. However, it did not lead to tumor regression, as was observed in mice lacking p53. We further showed that the dominant-negative effect of the mutant p53 encoded by p53 R172H dampened the activity of the restored wild-type p53. We therefore conclude that in a mutant p53 background, p53 restoration has the therapeutic potential to suppress tumor progression. Our findings support using p53 restoration as a strategy to treat human cancers with P53 missense mutations and provide direction for optimizing p53 restoration in cancer therapy. IntroductionThe tumor suppressor p53 is a transcription factor. Upon activation by signals, such as DNA damage, oncogenic stimuli, and hypoxia, wild-type p53 activates the transcription of genes involved in apoptosis, cell cycle arrest, differentiation, and senescence (1, 2). These potent antitumor activities prevent cells with aberrant growth signals from proliferating. Approximately, half of human cancers have P53 gene alterations that result in loss of p53 activity. While a few of these alterations are P53-null mutations, over 80% are P53 missense mutations that lead to expression of mutant p53 proteins (3, 4). Many p53 missense mutants lack p53 transcriptional activity and show gain-of-function activities.In particular, the arginine-to-histidine mutation at codon 175 of the P53 gene (corresponding to p53 R172H in mice) occurs in about 6% of human cancers (5). The p53 R172H mutation has gain-of-function properties, manifested as a tumor metastasis phenotype in p53 R172H heterozygous mice that is lacking in p53 +/-mice (6, 7). Another property of the p53R172H mutant is its dominant-negative effect that silences wild-type p53 under some circumstances (8). Thus, for example, in response to γ-irradiation, mutant p53R172H inactivates wild-type p53 activities (9). Additionally, mutant p53 binds and suppresses the activities of the related proteins, p63 and p73 (7). However, the p53 R172H heterozygous mice that express equal amounts of wild-type and mutant p53 have survival curves identical to those of p53 +/-mice, indicating that

Viral pneumonias cause profound worldwide morbidity, necessitating novel strategies to prevent and treat these potentially lethal infections. Stimulation of intrinsic lung defenses via inhalation of synergistically acting Toll-like receptor (TLR) agonists protects mice broadly against pneumonia, including otherwise-lethal viral infections, providing a potential opportunity to mitigate infectious threats. As intact lung epithelial TLR signaling is required for the inducible resistance and as these cells are the principal targets of many respiratory viruses, the capacity of lung epithelial cells to be therapeutically manipulated to function as autonomous antiviral effectors was investigated. Our work revealed that mouse and human lung epithelial cells could be stimulated to generate robust antiviral responses that both reduce viral burden and enhance survival of isolated cells and intact animals. The antiviral protection required concurrent induction of epithelial reactive oxygen species (ROS) from both mitochondrial and dual oxidase sources, although neither type I interferon enrichment nor type I interferon signaling was required for the inducible protection. Taken together, these findings establish the sufficiency of lung epithelial cells to generate therapeutically inducible antiviral responses, reveal novel antiviral roles for ROS, provide mechanistic insights into inducible resistance, and may provide an opportunity to protect patients from viral pneumonia during periods of peak vulnerability.

High levels of the critical p53 inhibitor Mdm4 is common in tumors that retain a wild-type p53 allele, suggesting that Mdm4 overexpression is an important mechanism for p53 inactivation during tumorigenesis. To test this hypothesis in vivo, we generated transgenic mice with widespread expression of Mdm4. Two independent lines of transgenic mice, Mdm4Tg1 and Mdm4 Tg15, developed spontaneous tumors, the most prevalent of which were sarcomas. To determine whether overexpression of Mdm4 also cooperated with p53 heterozygosity to induce tumorigenesis, we generated Mdm4Tg1 p53 +/− mice. These mice had significantly accelerated tumorigenesis and a distinct tumor spectrum with more carcinomas and significantly fewer lymphomas than p53 +/− or Mdm4 Tg1 mice. Importantly, the remaining wild-type p53 allele was retained in most Mdm4Tg1 p53 +/− tumors. Mdm4 is thus a bona fide oncogene in vivo and cooperates with p53 heterozygosity to drive tumorigenesis. These Mdm4 mice will be invaluable for in vivo drug studies of Mdm4 inhibitors. CancerRes; 70(18); 7148-54. ©2010 AACR.

Summary Extensive reprogramming of cellular energy metabolism is a hallmark of cancer. Despite its importance, the molecular mechanism controlling this tumour metabolic shift remains not fully understood. Here we show that 14-3-3σ regulates cancer metabolic reprogramming and protects cells from tumourigenic transformation. 14-3-3σ opposes tumour-promoting metabolic programs by enhancing c-Myc poly-ubiquitination and subsequent degradation. 14-3-3σ demonstrates the suppressive impact on cancer glycolysis, glutaminolysis, mitochondrial biogenesis and other major metabolic processes of tumours. Importantly, 14-3-3σ expression levels predict overall and recurrence-free survival rates, tumour glucose uptake and metabolic gene expression in breast cancer patients. Thus, these results highlight that 14-3-3σ is an important regulator of tumour metabolism, and loss of 14-3-3σ expression is critical for cancer metabolic reprogramming. We anticipate that pharmacologically elevating the function of 14-3-3σ in tumours could be a promising direction for targeted anti-cancer metabolism therapy development in future.