Wenwei Hu

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 alpha-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 alpha-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.

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 beta1 and beta2 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 signal-specific 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.