The Mdm2 and Mdm4 oncoproteins are key negative regulators of the p53 tumor suppressor. However, their physiological contributions to the regulation of p53 stability and activity remain highly controversial. Here, we combined a p53 knock-in allele, in which p53 is silenced by a transcriptional stop element flanked by loxP sites, with the mdm2-and mdm4-null alleles. This approach allows Cre-mediated conditional p53 expression in tissues in vivo and cells in vitro lacking Mdm2, Mdm4, or both. Using this strategy, we show that Mdm2 and Mdm4 are essential in a nonredundant manner for preventing p53 activity in the same cell type, irrespective of the proliferation͞differentiation status of the cells. Although Mdm2 prevents accumulation of the p53 protein, Mdm4 contributes to the overall inhibition of p53 activity independent of Mdm2. We propose a model in which Mdm2 is critical for the regulation of p53 levels and Mdm4 is critical for the fine-tuning of p53 transcriptional activity, both proteins acting synergistically to keep p53 in check.cre͞lox

Biochemical studies have suggested conflicting roles for the E3 ubiquitin ligase constitutive photomorphogenesis protein 1 (Cop1; also known as Rfwd2) in tumorigenesis, providing evidence for both the oncoprotein c-Jun and the tumor suppressor p53 as its targets. Here we present what we believe to be the first in vivo investigation of the role of Cop1 in cancer etiology. Using an innovative genetic approach to generate an allelic series of Cop1, we found that Cop1 hypomorphic mice spontaneously developed malignancy at a high frequency in the first year of life and were highly susceptible to radiation-induced lymphomagenesis. Further analysis revealed that c-Jun was a key physiological target for Cop1 and that Cop1 constitutively kept c-Jun at low levels in vivo and thereby modulated c-Jun/AP-1 transcriptional activity. Importantly, Cop1 deficiency stimulated cell proliferation in a c-Jun-dependent manner. Focal deletions of COP1 were observed at significant frequency across several cancer types, and COP1 loss was determined to be one of the mechanisms leading to c-Jun upregulation in human cancer. We therefore conclude that Cop1 is a tumor suppressor that functions, at least in part, by antagonizing c-Jun oncogenic activity. In the absence of evidence for a genetic interaction between Cop1 and p53, our data strongly argue against the use of Cop1-inhibitory drugs for cancer therapy. IntroductionThe turnover and the activity of several key oncoproteins and tumor suppressors are controlled by the ubiquitin proteasome system (UPS). The UPS comprises two discrete steps: the covalent attachment of multiple ubiquitin molecules to the protein substrate and degradation of the polyubiquitylated protein by the 26S proteasome complex. The first step is mediated by at least three enzymes: the ubiquitin-activating enzyme E1, the ubiquitin-conjugating enzyme E2, and the ubiquitin ligase E3. Few reports have linked E1 and E2 to cancer development; in contrast, cumulative evidence indicates alterations in the activity of some E3 ligases in the etiology of human malignancies (1). Because E3 ligases are the specific recognition element of the system and are themselves potentially "drugable'" enzymes, they can serve as potential cancer targets as well as cancer biomarkers.A direct molecular link between deregulation of E3 ligase activity and cancer is illustrated by the well-described oncogenic activity of the RING finger-containing protein Mdm2. In physiological conditions, Mdm2 directly binds the p53 tumor suppressor protein and via its RING finger domain acts as an E3 ligase to promote p53 ubiquitylation and proteasomal degradation (2). In tumors, p53 function is altered either by inactivating mutations of the TP53 gene itself or as the result of aberrant expression and/or func-

In response to DNA damage, p53 activates a G1 cell cycle checkpoint, in part through induction of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). Here we report the identification of a new direct p53 target, Ptprv (or ESP), encoding a transmembrane tyrosine phosphatase. Ptprv transcription is dramatically and preferentially increased in cultured cells undergoing p53-dependent cell cycle arrest, but not in cells undergoing p53-mediated apoptosis. This observation was further confirmed in vivo using a Ptprv null-reporter mouse line. A p53-responsive element is present in the Ptprv promoter and p53 is recruited to this site in vivo. Importantly, while p53-dependent apoptosis is intact in mice lacking Ptprv, Ptprv-null fibroblasts and epithelial cells of the small intestine are defective in G1 checkpoint control. Thus, Ptprv is a new direct p53 target and a key mediator of p53-induced cell cycle arrest. Finally, Ptprv loss enhances the formation of epidermal papillomas after exposure to chemical carcinogens, suggesting that Ptprv acts to suppress tumor formation in vivo.

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