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Epidemiological studies strongly suggest that chronic psychological stress promotes tumorigenesis. However, its direct link in vivo and the underlying mechanisms that cause thi5 remain unclear. This study provides direct evidence that chronic stress promotes tumorigenesis in vivo; chronic restraint, a well-established mouse model to induce chronic stress, greatly promotes ionizing radiation (IR)-induced tumorigenesis in pS3+/- mice. The tumor suppressor protein p53 plays a central role in tumor prevention. Loss or attenuation of p53 function contriubutes greatly to tumorigenesis. We found that chronic restraint decreases the levels and function of p53 in mice, and furthermore, promotes the growth of human xenograft tumors in a largely p53-dependent manner. Our results show that glucocorticoids elevated during chronic restraint mediate the effect of chronic restraint on p53 through the induction of serumand glucocorticoid-induced protein kinase (SOK1), which in turn increases MDM2 activity and decreases p53 function. Taken together, this study demonstrates that chronic stress promotes tumorigenesis in mice, and the attenuation of p53 function is an important part of the underlying mechanism, which can be mediated by glucocortcoids elevated during chronic restraint . [ABSTRACT FROM AUTHOR]

Regulation of energy metabolism is a novel function of p53 in tumor suppression. Parkin (PARK2), a Parkinson disease-associated gene, is a potential tumor suppressor whose expression is frequently diminished in tumors. Here Parkin was identified as a p53 target gene that is an important mediator of p53's function in regulating energy metabolism. The human and mouse Parkin genes contain functional p53 responsive elements, and p53 increases the transcription of Parkin in both humans and mice. Parkin contributes to the function of p53 in glucose metabolism; Parkin deficiency activates glycolysis and reduces mitochondrial respiration, leading to the Warburg effect. Restoration of Parkin expression reverses the Warburg effect in cells. Thus, Parkin deficiency is a novel mechanism for the Warburg effect in tumors. Parkin also contributes to the function of p53 in antioxidant defense. Furthermore, Parkin deficiency sensitizes mice to γ-irradiation-induced tumorigenesis, which provides further direct evidence to support a role of Parkin in tumor suppression. Our results suggest that as a novel component in the p53 pathway, Parkin contributes to the functions of p53 in regulating energy metabolism, especially the Warburg effect, and antioxidant defense, and thus the function of p53 in tumor suppression. [ABSTRACT FROM AUTHOR]

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. Further- more, 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., H2O2)-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. [ABSTRACT FROM AUTHOR]