The tumor suppressor p53 gene is mutated in minimally half of all cancers. It is therefore reasonable to assume that naturally occurring polymorphic genetic variants in the p53 stress response pathway might determine an individual's susceptibility to cancer. A central node in the p53 pathway is the MDM2 protein, a direct negative regulator of p53. In this report, a single nucleotide polymorphism (SNP309) is found in the MDM2 promoter and is shown to increase the affinity of the transcriptional activator Sp1, resulting in higher levels of MDM2 RNA and protein and the subsequent attenuation of the p53 pathway. In humans, SNP309 is shown to associate with accelerated tumor formation in both hereditary and sporadic cancers. A model is proposed whereby SNP309 serves as a rate-limiting event in carcinogenesis.

Whereas cell cycle arrest, apoptosis, and senescence are traditionally thought of as the major functions of the tumor suppressor p53, recent studies revealed two unique functions for this protein: p53 regulates cellular energy metabolism and antioxidant defense mechanisms. Here, we identify glutaminase 2 (GLS2) as a previously uncharacterized p53 target gene to mediate these two functions of the p53 protein. GLS2 encodes a mitochondrial glutaminase catalyzing the hydrolysis of glutamine to glutamate. p53 increases the GLS2 expression under both nonstressed and stressed conditions. GLS2 regulates cellular energy metabolism by increasing production of glutamate and α-ketoglutarate, which in turn results in enhanced mitochondrial respiration and ATP generation. Furthermore, GLS2 regulates antioxidant defense function in cells by increasing reduced glutathione (GSH) levels and decreasing ROS levels, which in turn protects cells from oxidative stress (e.g., H 2 O 2 )-induced apoptosis. Consistent with these functions of GLS2, the activation of p53 increases the levels of glutamate and α-ketoglutarate, mitochondrial respiration rate, and GSH levels and decreases reactive oxygen species (ROS) levels in cells. Furthermore, GLS2 expression is lost or greatly decreased in hepatocellular carcinomas and the overexpression of GLS2 greatly reduced tumor cell colony formation. These results demonstrated that as a unique p53 target gene, GLS2 is a mediator of p53's role in energy metabolism and antioxidant defense, which can contribute to its role in tumor suppression.reactive oxygen species | oxidative phosphorylation p 53 mainly exerts its tumor suppression function through the transcriptional regulation of its target genes. In response to stress, p53 selectively regulates the expression of its target genes, which results in cell cycle arrest, apoptosis, or senescence (1, 2). Whereas these responses are traditionally thought of as the major functions of p53 in tumor prevention, recent studies revealed two unique functions for this protein: p53 regulates cellular energy metabolism and antioxidant defense mechanisms. Emerging evidence has shown that these two functions of p53 contribute greatly to p53's role in tumor suppression (3-5).The recent identification of SCO2 and TIGAR as two p53 target genes revealed a unique function of p53 in the regulation of energy metabolism and ATP generation pathways (3, 4). The SCO2 gene is a key regulator of the cytochrome c oxidase complex that is essential for mitochondrial respiration. TIGAR functions to lower fructose-2, 6,-bisphosphate levels and thus slows glycolysis and directs glucose to the pentose phosphate pathway. p53 induces SCO2 expression to enhance mitochondrial respiration and induces TIGAR expression to slow glycolysis. Loss of p53 results in decreased oxygen consumption and impaired mitochondrial respiration and promotes a switch to high glucose utilization in aerobic glycolysis in cells. In mice, p53 loss results in reduced endurance during physical exercise, suggesting a ...

The p53 pathway is composed of hundreds of genes and their products that respond to a wide variety of stress signals. These responses to stress include apoptosis, cellular senescence or cell cycle arrest. In addition the p53-regulated genes produce proteins that communicate these stress signals to adjacent cells, prevent and repair damaged DNA and create feedback loops that enhance or attenuate p53 activity and communicate with other signal transduction pathways. Many questions remain to be explored in our understanding of how this network of genes plays a role in protection from cancers, therapy and integrating the homeostatic mechanisms of stress management and fidelity in a cell and organism. The goal of this chapter is to elucidate some of those questions and suggest new directions for this area of research.

The insulin-like growth factor 1 (IGF-1)-AKT-mTOR pathways sense the availability of nutrients and mitogens and respond by signaling for cell growth and division. The p53 pathway senses a variety of stress signals which will reduce the fidelity of cell growth and division, and responds by initiating cell cycle arrest, senescence, or apoptosis. This study explores four p53-regulated gene products, the B1 and B2 subunits of the AMPK, which are shown for the first time to be regulated by the p53 protein, TSC2, PTEN, and IGF-BP3, each of which negatively regulates the IGF-1-AKT-mTOR pathways after stress. These gene products are shown to be expressed under p53 control in a cell type and tissue-specific fashion with the TSC2 and PTEN proteins being coordinately regulated in those tissues that use insulin-dependent energy metabolism (skeletal muscle, heart, white fat, liver, and kidney). In addition, these genes are regulated by p53 in a stress signalspecific fashion. The mTOR pathway also communicates with the p53 pathway. After glucose starvation of mouse embryo fibroblasts, AMPK phosphorylates the p53 protein but does not activate any of the p53 responses. Upon glucose starvation of E1A-transformed mouse embryo fibroblasts, a p53-mediated apoptosis ensues. Thus, there is a great deal of communication between the p53 pathway and the IGF-1-AKT and mTOR pathways. [Cancer Res 2007;67(7):3043-53]

Recent observations show that the single-cell response of p53 to ionizing radiation (IR) is ''digital'' in that it is the number of oscillations rather than the amplitude of p53 that shows dependence on the radiation dose. We present a model of this phenomenon. In our model, double-strand break (DSB) sites induced by IR interact with a limiting pool of DNA repair proteins, forming DSB-protein complexes at DNA damage foci. The persisting complexes are sensed by ataxia telangiectasia mutated (ATM), a protein kinase that activates p53 once it is phosphorylated by DNA damage. The ATM-sensing module switches on or off the downstream p53 oscillator, consisting of a feedback loop formed by p53 and its negative regulator, Mdm2. In agreement with experiments, our simulations show that by assuming stochasticity in the initial number of DSBs and the DNA repair process, p53 and Mdm2 exhibit a coordinated oscillatory dynamics upon IR stimulation in single cells, with a stochastic number of oscillations whose mean increases with IR dose. The damped oscillations previously observed in cell populations can be explained as the aggregate behavior of single cells.DNA damage response ͉ mathematical model of p53 ͉ p53 regulation ͉ p53 pathway C ells under stresses such as DNA damage, hypoxia, and aberrant oncogene signals trigger their internal self-defense machinery. One critical response is the activation of the tumor suppressor protein p53, which transcribes genes that induce cell cycle arrest, DNA repair, and apoptosis (1-4). A central node in the p53 network is the Mdm2 protein, the product of one of the p53 target genes and a negative regulator of p53. The negative feedback loop formed by p53 and Mdm2 can produce oscillatory dynamics. Indeed, damped oscillations of p53 and Mdm2 protein level have been observed upon ionizing radiation (IR)-induced DNA damage in cell populations (5). Intriguingly, recent in vivo fluorescence measurements in individual cells revealed that in response to IR, these two proteins exhibit a ''digital'' response that produces discrete pulses of p53 and Mdm2. The average height and duration of these pulses are fixed, whereas the mean number increases with the strength of DNA damage (6).Several models have been proposed (5,7,8) to explain the damped oscillations of p53 in cell populations. However, these modeling efforts did not explore sustained pulses as found in single-cell responses and did not attempt to characterize the signaling between DNA damage and the activation of the p53 oscillatory response.In this study, we present a model for the digital, undamped oscillatory p53 activity elicited by IR at the single-cell level consisting of three subsystems: a DNA damage repair module, an ataxia telangiectasia mutated (ATM) switch, and the p53-Mdm2 oscillator. We investigate the controlling role of ATM to set a threshold level of DNA damage during the radiation response, as suggested by growing biochemical evidence (9-12). Finally, by adding stochasticity to selected model parameters, we replicate the va...