Mary Ellen Perry

mdm2 Is Critical for Inhibition of p53 during Lymphopoiesis and the Response to Ionizing Irradiation

ABSTRACT The function of the p53 tumor suppressor protein must be highly regulated because p53 can cause cell death and prevent tumorigenesis. In cultured cells, the p90MDM2 protein blocks the transcriptional activation domain of p53 and also stimulates the degradation of p53. Here we provide the first conclusive demonstration that p90MDM2 constitutively regulates p53 activity in homeostatic tissues. Mice with a hypomorphic allele of mdm2 revealed a heretofore unknown role for mdm2 in lymphopoiesis and epithelial cell survival. Phenotypic analyses revealed that both the transcriptional activation and apoptotic functions of p53 were increased in these mice. However, the level of p53 protein was not coordinately increased, suggesting that p90MDM2 can inhibit the transcriptional activation and apoptotic functions of p53 in a manner independent of degradation. Cre-mediated deletion of mdm2 caused a greater accumulation of p53, demonstrating that p90MDM2 constitutively regulates both the activity and the level of p53 in homeostatic tissues. The observation that only a subset of tissues with activated p53 underwent apoptosis indicates that factors other than p90MDM2 determine the physiological consequences of p53 activation. Furthermore, reduction of mdm2 in vivo resulted in radiosensitivity, highlighting the importance of mdm2 as a potential target for adjuvant cancer therapies.

The p53 tumor suppressor protein does not regulate expression of its own inhibitor, MDM2, except under conditions of stress.

ABSTRACT MDM2 is an important regulator of the p53 tumor suppressor protein. MDM2 inhibits p53 by binding to it, physically blocking its ability to transactivate gene expression, and stimulating its degradation. In cultured cells, mdm2 expression can be regulated by p53. Hence, mdm2 and p53 can interact to form an autoregulatory loop in which p53 activates expression of its own inhibitor. The p53/MDM2 autoregulatory loop has been elucidated within cultured cells; however, regulation of mdm2 expression by p53 has not been demonstrated within intact tissues. Here, we examine the role of p53 in regulating mdm2 expression in vivo in order to test the hypothesis that the p53/MDM2 autoregulatory loop is the mechanism by which low levels of p53 are maintained. We demonstrate that basal expression of mdm2 in murine tissues is p53 independent, even in tissues that express functional p53. Transcription ofmdm2 is induced in a p53-dependent manner following gamma irradiation, indicating that p53 regulates mdm2 expression in vivo following a stimulus. The requirement for a stimulus to activate p53-dependent regulation of mdm2 expression in vivo appeared to differ from the situation in early-passage mouse embryo fibroblasts, where mdm2 expression is enhanced by the presence of p53. Analysis of mdm2 expression in intact and dispersed embryos revealed that establishment of mouse embryo fibroblasts in culture induces p53-dependent mdm2expression, suggesting that an unknown stimulus activates p53 function in cultured cells. Together, these results indicate that p53 does not regulate expression of its own inhibitor, except in response to stimuli.

The Regulation of the p53-mediated Stress Response by MDM2 and MDM4

Exquisite control of the activity of p53 is necessary for mammalian survival. Too much p53 is lethal, whereas too little permits tumorigenesis. MDM2 and MDM4 are structurally related proteins critical for the control of p53 activity during development, homeostasis, and the response to stress. These two essential proteins regulate both the activation of p53 in response to stress and the recovery of cells following resolution of the damage, yet both are oncogenic when overexpressed. Thus, multiple regulatory circuits ensure that their activities are fine-tuned to promote tumor-free survival. Numerous diverse stressors activate p53, and much research has gone into trying to find commonalities between them that would explain the mechanism by which p53 becomes active. It is now clear that although these diverse stressors activate p53 by different biochemical pathways, one common feature is the effort they direct, through a variety of means, toward disrupting the functions of both MDM2 and MDM4. This article provides an overview of the relationship between MDM2 and MDM4, features the various biochemical mechanisms by which p53 is activated through inhibition of their functions, and proposes some emerging areas for investigation of the p53-mediated stress response.

Multiple Murine Double Minute Gene 2 (MDM2) Proteins Are Induced by Ultraviolet Light

The mdm2 (murinedouble minute 2) oncogene encodes several proteins, the largest of which (p90) binds to and inactivates the p53 tumor suppressor protein. Multiple MDM2 proteins have been detected in tumors and in cell lines expressing high levels ofmdm2 mRNAs. Here we show that one of these proteins (p76) is expressed, along with p90, in wild-type andp53-null mouse embryo fibroblasts, indicating that it may have an important physiological role in normal cells. Expression of this protein is induced, as is that of p90, by UV light in a p53-dependent manner. The p76 protein is synthesized via translational initiation at AUG codon 50 and thus lacks the N terminus of p90 and does not bind p53. In cells, p90 and p76 can be synthesized from mdm2 mRNAs transcribed from both the P1 (constitutive) and P2 (p53-responsive) promoters. Site-directed mutagenesis reveals that these RNAs give rise to p76 via internal initiation of translation. In addition, mdm2 mRNAs lacking exon 3 give rise to p76 exclusively, and such mRNAs are induced by p53 in response to UV light. These data indicate that p76 may be an important product of the mdm2 gene and a downstream effector of p53.

The p53 Inhibitor MDM2 Facilitates Sonic Hedgehog-Mediated Tumorigenesis and Influences Cerebellar Foliation

Disruption of cerebellar granular neuronal precursor (GNP) maturation can result in defects in motor coordination and learning, or in medulloblastoma, the most common childhood brain tumor. The Sonic Hedgehog (Shh) pathway is important for GNP proliferation; however, the factors regulating the extent and timing of GNP proliferation, as well as GNP differentiation and migration are poorly understood. The p53 tumor suppressor has been shown to negatively regulate the activity of the Shh effector, Gli1, in neural stem cells; however, the contribution of p53 to the regulation of Shh signaling in GNPs during cerebellar development has not been determined. Here, we exploited a hypomorphic allele of Mdm2 (Mdm2puro), which encodes a critical negative regulator of p53, to alter the level of wild-type MDM2 and p53 in vivo. We report that mice with reduced levels of MDM2 and increased levels of p53 have small cerebella with shortened folia, reminiscent of deficient Shh signaling. Indeed, Shh signaling in Mdm2-deficient GNPs is attenuated, concomitant with decreased expression of the Shh transducers, Gli1 and Gli2. We also find that Shh stimulation of GNPs promotes MDM2 accumulation and enhances phosphorylation at serine 166, a modification known to increase MDM2-p53 binding. Significantly, loss of MDM2 in Ptch1+/− mice, a model for Shh-mediated human medulloblastoma, impedes cerebellar tumorigenesis. Together, these results place MDM2 at a major nexus between the p53 and Shh signaling pathways in GNPs, with key roles in cerebellar development, GNP survival, cerebellar foliation, and MB tumorigenesis.

Mdm2 Regulates p53 Independently of p19ARF in Homeostatic Tissues

ABSTRACT Tumor suppressor proteins must be exquisitely regulated since they can induce cell death while preventing cancer. For example, the p19ARF tumor suppressor (p14ARF in humans) appears to stimulate the apoptotic function of the p53 tumor suppressor to prevent lymphomagenesis and carcinogenesis induced by oncogene overexpression. Here we present a genetic approach to defining the role of p19ARF in regulating the apoptotic function of p53 in highly proliferating, homeostatic tissues. In contrast to our expectation, p19ARF did not activate the apoptotic function of p53 in lymphocytes or epithelial cells. These results demonstrate that the mechanisms that control p53 function during homeostasis differ from those that are critical for tumor suppression. Moreover, the Mdm2/p53/p19ARF pathway appears to exist only under very restricted conditions.

Murine double minute-2 expression is required for capillary maintenance and exercise-induced angiogenesis in skeletal muscle

Exercise‐induced angiogenesis is a key determinant of skeletal muscle function. Here, we investigated whether the E3 ubiquitin ligase murine double minute‐2 (Mdm2) exerts a proangiogenic function in exercised skeletal muscle. Mdm2 hypomorphic (Mdm2Puro/Δ7‐9) mice have a 60% reduction in Mdm2 expression compared with that in wild‐type animals. Capillary staining on muscle sections from Mdm2Puro/Δ7‐9 sedentary mice with a wild‐type or knockout background for p53 revealed that deficiency in Mdm2 resulted in 20% capillary regression independently of p53 status. In response to one bout of exercise, protein expression of the proangiogenic vascular endothelial growth factor‐A (VEGF‐A) was increased by 64% in muscle from wild‐type animals, and endothelial cell outgrowth from exercised muscle biopsy samples cultured in a 3‐dimensional collagen gel was enhanced by 37%. These proangiogenic responses to exercise were impaired in exercised Mdm2Puro/Δ7‐9 mice. Prolonged exercise training resulted in increased Mdm2 protein expression (+49%) and capillarization (+24%) in wild‐type muscles. However, exercise training‐induced angiogenesis was abolished in Mdm2Puro/Δ7‐9 mice. Finally, exercise training restored Mdm2, VEGF‐A, and capillarization levels in skeletal muscles from obese Zucker diabetic fatty rats compared with those in healthy animals. Our results define Mdm2 as a crucial regulator of capillary maintenance and exercise‐induced angiogenesis in skeletal muscle.—Roudier, E., Forn, P., Perry, M. E., Birot, O. Murine double minute‐2 expression is required for capillary maintenance and exercise‐induced angiogenesis in skeletal muscle. FASEB J. 26, 4530–4539 (2012). www.fasebj.org

Tumor Suppression by p53 without Accelerated Aging: Just Enough of a Good Thing?

The prevalence of mutations that inactivate the p53 tumor suppressor gene in human cancers reveals the importance of p53 in preventing cancer. Recent progress has generated increased enthusiasm for re-activating p53 in tumors with mutant p53 proteins as well as for increasing p53 function in tumors expressing wild-type p53 that is inhibited in trans. However, excessive p53 activity can be detrimental to the host, potentially limiting the utility of p53 activation as a therapeutic strategy. For example, uncontrolled p53 activity is lethal to the murine embryo, and p53 has been associated with increased aging in people and mice. Here we review the literature linking p53 to aging and discuss reports demonstrating that p53 can suppress tumor formation without accelerating aging. We raise the possibility that activation of p53 remains a promising strategy for cancer chemoprevention and therapy even if, under some circumstances, p53 might accelerate aging.

Defects in transcription coupled repair interfere with expression of p90 MDM2 in response to ultraviolet light

Ultraviolet (UV) irradiation transiently stabilizes p53 through a mechanism that may require a decrease in the activity of the ubiquitin ligase, p90MDM2. Conversely, the recovery of low levels of p53 following UV exposure may depend on an increase in p90MDM2. The level of p90MDM2 is increased by UV light following the p53-dependent induction of an internal mdm2 promoter, P2. If this induction of mdm2 were critical for the recovery of low levels of p53 following UV exposure, defects in mdm2s transcription would result in a prolonged increase in p53. Cells defective in transcription coupled repair (TCR) maintain high levels of p53 for a prolonged period following UV exposure. Such cells also have defects in general transcription after UV irradiation. We investigated whether TCR-deficient cells express diminished levels of mdm2 mRNA and p90MDM2 following UV exposure. We found that transcription of mdm2 was reduced in TCR-deficient cells. The uninducible mdm2 promoter, P1, was more sensitive to the inhibitory effects of UV irradiation than the P2 promoter. The decrease in transcription from the P1 promoter was sufficient to reduce the level of p90MDM2 and correlated with a prolonged increase in p53. Thus, p53-independent transcription of mdm2 appears critical to p53s regulation.

Highlights of the National Cancer Institute Workshop on Mitochondrial Function and Cancer

This workshop was stimulated by the desire of the National Cancer Institute to examine various aspects of mitochondrial function as they relate to tumorigenesis, apoptosis, and cancer therapy. Through endosymbiosis, a bacterial ancestor took its position in the eukaryotic cytoplasm and serves as the