Cen Zhang

Glutaminase 2, a novel p53 target gene regulating energy metabolism and antioxidant function

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., 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.

Parkin, a p53 target gene, mediates the role of p53 in glucose metabolism and the Warburg effect

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 p53s 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.

Tumor suppressor p53 meets microRNAs

Tumor suppressor p53 plays a central role in tumor prevention. As a transcription factor, p53 mainly exerts its function through transcription regulation of its target genes to initiate various cellular responses. To maintain its proper function, p53 is tightly regulated by a wide variety of regulators in cells. Thus, p53, its regulators and regulated genes form a complex p53 network which is composed of hundreds of genes and their products. microRNAs (miRNAs) are a class of endogenously expressed, small non-coding RNA molecules which play a key role in regulation of gene expression at the post-transcriptional level. Recent studies have demonstrated that miRNAs interact with p53 and its network at multiple levels. p53 regulates the transcription expression and the maturation of a group of miRNAs. On the other hand, miRNAs can regulate the activity and function of p53 through direct repression of p53 or its regulators in cells. These findings have demonstrated that miRNAs are important components in the p53 network, and also added another layer of complexity to the p53 network.

Tumour-associated mutant p53 drives the Warburg effect

Tumor cells primarily utilize aerobic glycolysis for energy production, a phenomenon known as the Warburg effect. Its mechanism is not well-understood. The tumor suppressor gene p53 is frequently mutated in tumors. Many tumor-associated mutant p53 (mutp53) proteins not only lose tumor suppressive function, but also gain new oncogenic functions that are independent of wild type p53, defined as mutp53 gain-of-function (GOF). Here we show that tumor-associated mutp53 stimulates the Warburg effect in cultured cells and mutp53 knock-in mice as a new mutp53 GOF. Mutp53 stimulates the Warburg effect through promoting GLUT1 translocation to plasma membrane, which is mediated by the activated RhoA and its downstream effector ROCK. Inhibition of the RhoA/ROCK/GLUT1 signaling largely abolishes mutp53 GOF in stimulating the Warburg effect. Furthermore, inhibition of glycolysis in tumor cells greatly compromises mutp53 GOF in promoting tumorigenesis. Thus, our results reveal a new mutp53 GOF and a mechanism for controlling the Warburg effect.

Tumor suppressor p53 and its mutants in cancer metabolism

Tumor-suppressor p53 plays a key role in tumor prevention. As a transcription factor, p53 transcriptionally regulates its target genes to initiate different biological processes in response to stress, including apoptosis, cell cycle arrest or senescence, to exert its function in tumor suppression. Recent studies have revealed that metabolic regulation is a novel function of p53. Metabolic changes have been regarded as a hallmark of tumors and a key contributor to tumor development. p53 regulates many different aspects of metabolism, including glycolysis, mitochondrial oxidative phosphorylation, pentose phosphate pathway, fatty acid synthesis and oxidation, to maintain the homeostasis of cellular metabolism, which contributes to the role of p53 in tumor suppression. p53 is frequently mutated in human tumors. In addition to loss of tumor suppressive function, tumor-associated mutant p53 proteins often gain new tumorigenic activities, termed gain-of-function of mutant p53. Recent studies have shown that mutant p53 mediates metabolic changes in tumors as a novel gain-of-function to promote tumor development. Here we review the functions and mechanisms of wild-type and mutant p53 in metabolic regulation, and discuss their potential roles in tumorigenesis.

Chronic restraint stress attenuates p53 function and promotes tumorigenesis.

Epidemiological studies strongly suggest that chronic psychological stress promotes tumorigenesis. However, its direct link in vivo and the underlying mechanisms that cause this 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 p53+/− 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 serum- and glucocorticoid-induced protein kinase (SGK1), 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.

Regulation of female reproduction by p53 and its family members

Tumor suppressor p53 is crucial for embryonic implantation through transcriptional up‐regulation of uterine leukemia inhibitory factor (LIF). This article reports that p53 and estrogen receptor α were activated in endometrial tissues during implantation to coordinately regulate LIF production. By using human p53 knockin (Hupki) mice carrying a single nucleotide polymorphism (SNP) at codon 72 (arginine/proline), the arginine allele was demonstrated to produce higher uterine LIF levels during implantation than the proline allele. In humans, the diversity of haplotypes of the p53 gene has decreased during evolution, because the arginine allele, existing in only a subset of haplotypes, is under positive selection. This observation is consistent with previous results showing that the proline allele is enriched in patients undergoing in vitro fertilization (IVF). Studies with p63‐ and p73‐knockout mice have demonstrated the involvement of p63 and p73 in female reproduction and their roles in egg formation and apoptosis (p63) and spindle checkpoint (p73) in female mice. Here, the role of p63 and p73 in human reproduction was investigated. Selected alleles of SNPs in p63 and p73 genes were enriched in IVF patients. These findings demonstrate that the p53 family members are involved in several steps to regulate female reproduction in mice and humans.—Feng, Z., Zhang, C., Kang, H.‐J., Sun, Y., Wang, H., Naqvi, A., Frank, A. K., Rosenwaks, Z., Murphy, M. E., Levine, A. J., Hu, W. Regulation of female reproduction by p53 and its family members. FASEB J. 25, 2245–2255 (2011). www.fasebj.org

Spliced MDM2 isoforms promote mutant p53 accumulation and gain-of-function in tumorigenesis

Tumor suppressor p53 is frequently mutated in tumors. Mutant p53 (Mutp53) proteins often gain new activities in promoting tumorigenesis, defined as gain-of-function (GOF). Mutp53 often accumulates at high levels in tumors, which promotes mutp53 GOF in tumorigenesis. The mechanism of mutp53 accumulation is poorly understood. Here we find that MDM2 isoforms promote mutp53 accumulation in tumors. MDM2 isoform B (MDM2-B), the MDM2 isoform most frequently over-expressed in human tumors, interacts with full-length MDM2 to inhibit MDM2-mediated mutp53 degradation, promoting mutp53 accumulation and GOF in tumorigenesis. Furthermore, MDM2-B over-expression correlates with mutp53 accumulation in human tumors. In mutp53 knock-in mice, a MDM2 isoform similar to human MDM2-B is over-expressed in the majority of tumors, which promotes mutp53 accumulation and tumorigenesis. Thus, over-expression of MDM2 isoforms promotes mutp53 accumulation in tumors, contributing to mutp53 GOF in tumorigenesis. Furthermore, promoting mutp53 accumulation and GOF is an important mechanism by which MDM2 isoforms promote tumorigenesis.

E3 ubiquitin ligase TRIM32 negatively regulates tumor suppressor p53 to promote tumorigenesis.

Tumor suppressor p53 has a key role in maintaining genomic stability and preventing tumorigenesis through its regulation of cellular stress responses, including apoptosis, cell cycle arrest and senescence. To ensure its proper levels and functions in cells, p53 is tightly regulated mainly through post-translational modifications, such as ubiquitination. Here, we identified E3 ubiquitin ligase TRIM32 as a novel p53 target gene and negative regulator to regulate p53-mediated stress responses. In response to stress, such as DNA damage, p53 binds to the p53 responsive element in the promoter of the TRIM32 gene and transcriptionally induces the expression of TRIM32 in cells. In turn, TRIM32 interacts with p53 and promotes p53 degradation through ubiquitination. Thus, TRIM32 negatively regulates p53-mediated apoptosis, cell cycle arrest and senescence in response to stress. TRIM32 is frequently overexpressed in different types of human tumors. TRIM32 overexpression promotes cell oncogenic transformation and tumorigenesis in mice in a largely p53-dependent manner. Taken together, our results demonstrated that as a novel p53 target and a novel negative regulator for p53, TRIM32 has an important role in regulation of p53 and p53-mediated cellular stress responses. Furthermore, our results also revealed that impairing p53 function is a novel mechanism for TRIM32 in tumorigenesis.

LIF negatively regulates tumour-suppressor p53 through Stat3/ID1/MDM2 in colorectal cancers.

Leukemia inhibitory factor (LIF) has been recently identified as a p53 target gene, which mediates the role of p53 in maternal implantation under normal physiological conditions. Here, we report that LIF is a negative regulator of p53; LIF downregulates p53 protein levels and function in human colorectal cancer (CRC) cells. The downregulation of p53 by LIF is mediated by the activation of Stat3, which transcriptionally induces ID1. ID1 upregulates MDM2, a key negative regulator of p53, and promotes p53 protein degradation. LIF is overexpressed in a large percentage of CRCs. LIF overexpression promotes cellular resistance towards chemotherapeutic agents in cultured CRC cells and colorectal xenograft tumors in a largely p53-dependent manner. Overexpression of LIF is associated with a poor prognosis in CRC patients. Taken together, LIF is a novel negative regulator of p53, overexpression of LIF is an important mechanism for the attenuation of p53, which promotes chemoresistance in CRCs.