Screening for genes that reprogram cancer cells for the tumor reversion switch identified TCTP (encoding translationally controlled tumor protein) as a crucial regulator of apoptosis. Here we report a negative feedback loop between P53 and TCTP. TCTP promotes P53 degradation by competing with NUMB for binding to P53-MDM2-containing complexes. TCTP inhibits MDM2 auto-ubiquitination and promotes MDM2-mediated ubiquitination and degradation of P53. Notably, Tctp haploinsufficient mice are sensitized to P53-dependent apoptosis. In addition, P53 directly represses TCTP transcription. In 508 breast cancers, high-TCTP status associates with poorly differentiated, aggressive G3-grade tumors, predicting poor prognosis (P < 0.0005). Tctp knockdown in primary mammary tumor cells from ErbB2 transgenic mice results in increased P53 expression and a decreased number of stem-like cancer cells. The pharmacological compounds sertraline and thioridazine increase the amount of P53 by neutralizing TCTP's action on the MDM2-P53 axis. This study links TCTP and P53 in a previously unidentified regulatory circuitry that may underlie the relevance of TCTP in cancer.

Aging is a complex process that results in loss of the ability to reattain homeostasis following stress, leading, thereby, to increased risk of morbidity and mortality. Many factors contribute to aging, such as the time-dependent accumulation of macromolecular damage, including DNA damage. The integrity of the nuclear genome is essential for cellular, tissue, and organismal health. DNA damage is a constant threat because nucleic acids are chemically unstable under physiological conditions and vulnerable to attack by endogenous and environmental factors. To combat this, all organisms possess highly conserved mechanisms to detect and repair DNA damage. Persistent DNA damage (genotoxic stress) triggers signaling cascades that drive cells into apoptosis or senescence to avoid replicating a damaged genome. The drawback is that these cancer avoidance mechanisms promote aging. Here, we review evidence that DNA damage plays a causal role in aging. We also provide evidence that genotoxic stress is linked to other cellular processes implicated as drivers of aging, including mitochondrial and metabolic dysfunction, altered proteostasis and inflammation. These links between damage to the genetic code and other pillars of aging support the notion that DNA damage could be the root of aging.

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-

Unrepaired DNA double-strand breaks (DSBs) cause genetic instability that leads to malignant transformation or cell death. Cells respond to DSBs with the ordered recruitment of signaling and repair proteins to the sites of DNA lesions. Coordinated protein SUMOylation and ubiquitylation have crucial roles in regulating the dynamic assembly of protein complexes at these sites. However, how SUMOylation influences protein ubiquitylation at DSBs is poorly understood. We show herein that Rnf4, an E3 ubiquitin ligase that targets SUMO-modified proteins, accumulates in DSB repair foci and is required for both homologous recombination (HR) and non-homologous end joining repair. To establish a link between Rnf4 and the DNA damage response (DDR) in vivo, we generated an Rnf4 allelic series in mice. We show that Rnf4-deficiency causes persistent ionizing radiationinduced DNA damage and signaling, and that Rnf4-deficient cells and mice exhibit increased sensitivity to genotoxic stress. Mechanistically, we show that Rnf4 targets SUMOylated MDC1 and SUMOylated BRCA1, and is required for the loading of Rad51, an enzyme required for HR repair, onto sites of DNA damage. Similarly to inactivating mutations in other key regulators of HR repair, Rnf4 deficiency leads to age-dependent impairment in spermatogenesis. These findings identify Rnf4 as a critical component of the DDR in vivo and support the possibility that Rnf4 controls protein localization at DNA damage sites by integrating SUMOylation and ubiquitylation events.