Yuhan Zhao

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.

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.

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.

The regulation of MDM2 oncogene and its impact on human cancers.

Tumor suppressor p53 plays a central role in preventing tumor formation. The levels and activity of p53 is under tight regulation to ensure its proper function. Murine double minute 2 (MDM2), a p53 target gene, is an E3 ubiquitin ligase. MDM2 is a key negative regulator of p53 protein, and forms an auto-regulatory feedback loop with p53. MDM2 is an oncogene with both p53-dependent and p53-independent oncogenic activities, and often has increased expression levels in a variety of human cancers. MDM2 is highly regulated; the levels and function of MDM2 are regulated at the transcriptional, translational and post-translational levels. This review provides an overview of the regulation of MDM2. Dysregulation of MDM2 impacts significantly upon the p53 functions, and in turn the tumorigenesis. Considering the key role that MDM2 plays in human cancers, a better understanding of the regulation of MDM2 will help us to develop novel and more effective cancer therapeutic strategies to target MDM2 and activate p53 in cells.

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.

Metadherin Mediates Lipopolysaccharide-Induced Migration and Invasion of Breast Cancer Cells

Background Breast cancer is the most prevalent cancer in women worldwide and metastatic breast cancer has very poor prognosis. Inflammation has been implicated in migration and metastasis of breast cancer, although the exact molecular mechanism remains elusive. Principal Findings We show that the pro-inflammatory endotoxin Lipopolysaccharide (LPS) upregulates the expression of Metadherin (MTDH), a recently identified oncogene, in a number of breast cancer lines. Stable knockdown of MTDH by shRNA in human breast MDA-MB-231 cells abolishes LPS-induced cell migration and invasion as determined by several in vitro assays. In addition, knockdown of MTDH diminishes Nuclear Factor-kappa B (NF-κB) activation by LPS and inhibited LPS-induced IL-8 and MMP-9 production. Conclusions These results strongly suggest that MTDH is a pivotal molecule in inflammation-mediated tumor metastasis. Since NF-κB, IL-8 and MMP-9 play roles in LPS-induced invasion or metastasis, the mechanism of MTDH-promoted invasion and metastasis may be through the activation of NF-κB, IL-8 and MMP-9, also suggesting a role of MTDH in regulating both inflammatory responses and inflammation-associated tumor invasion. These findings indicate that MTDH is involved in inflammation-induced tumor progression, and support that MTDH targeting therapy may hold promising prospects in treating breast cancer.

53BP1 functions as a tumor suppressor in breast cancer via the inhibition of NF-κB through miR-146a

p53-binding protein-1 (53BP1) plays a critical role in cell cycle checkpoint and DNA repair activities. Recently, 53BP1 was recognized as a potential tumor suppressor gene. In this study, we investigated its tumor suppressor function in breast cancer. In clinical samples, we observed a lower level of 53BP1 expression in the cancer lesions than in the matched non-tumor tissues. Furthermore, the 53BP1 level showed a gradual decrease during the progression from precancerous to cancer lesion. Ectopic expression of 53BP1 can significantly inhibit cell proliferation and curb the invasiveness in breast cancer cell lines, whereas knockdown of 53BP1 by RNA interference had the opposite effects. Additionally, 53BP1 markedly inhibited xenograft formation and metastasis of breast cancer cells in nude mice. Both in vitro and in vivo studies revealed that the 53BP1 expression level was inversely correlated to the function of nuclear factor-kappaB (NF-κB), which contributes to the invasion and metastasis of breast cancer. Importantly, the inhibitory effect of 53BP1 on NF-κB activity was shown to be mediated by the upregulation of miR-146a. Together, our findings demonstrated that 53BP1 has a potent tumor suppressor activity in breast cancer, and it may serve as a novel target for breast cancer prevention and treatment.

Mutant p53 in Cancer: Accumulation, Gain-of-Function, and Therapy

Tumor suppressor p53 plays a central role in tumor suppression. p53 is the most frequently mutated gene in human cancer, and over half of human cancers contain p53 mutations. Majority of p53 mutations in cancer are missense mutations, leading to the expression of full-length mutant p53 (mutp53) protein. While the critical role of wild-type p53 in tumor suppression has been firmly established, mounting evidence has demonstrated that many tumor-associated mutp53 proteins not only lose the tumor-suppressive function of wild-type p53 but also gain new activities to promote tumorigenesis independently of wild-type p53, termed gain-of-function. Mutant p53 protein often accumulates to very high levels in tumors, contributing to malignant progression. Recently, mutp53 has become an attractive target for cancer therapy. Further understanding of the mechanisms underlying mutp53 protein accumulation and gain-of-function will accelerate the development of targeted therapies for human cancer harboring mutp53. In this review, we summarize the recent advances in the studies on mutp53 protein accumulation and gain-of-function and targeted therapies for mutp53 in human cancer.

Inhibition of metadherin sensitizes breast cancer cells to AZD6244

The development of systemic therapy drug resistance for breast cancer treatment is an ongoing problem, thus, so are the potential molecular mechanisms of it. AZD6244 is a novel ATP-uncompetitive inhibitor to MAP/ERK kinase (MEK) 1/2 which has been demonstrated to be potent, selective and safe in the clinical trials and previous studies. However, the precise role of resistance to AZD6244 is largely unknown. We and other groups have reported that the novel oncogene Metadherin (MTDH) is associated with multiple drug resistance, but there is no report about its role in treatment of AZD6244. Here we report that the resistance to AZD6244 can be reserved by downregulating MTDH in breast cancer cell lines. When the MTDH was downregulated, the breast cancer cells exhibited a significantly increased sensitivity to AZD6244 as measured by MTT assay. After treated with AZD6244 the MTDH-knockdown cells showed more apoptosis rate and growth inhibition. We also showed that knockdown of MTDH cannot only increase expression of FOXO3a but also activate it by promoting its translocation via MTDH/ERK1/2/FOXO3a pathway rather than MTDH/AKT/FOXO3a pathway. In conclusion knockdown MTDH can enhance the breast cancer cells sensitivity to AZD6244 via regulating the expression and activity of FOXO3a. These indicate us that MTDH is a candidate marker to predict the clinical efficacy of AZD6244 and targeting MTDH could overcome the resistance to AZD6244 in breast cancer cells.

Pontin, a new mutant p53-binding protein, promotes gain-of-function of mutant p53

Tumor-suppressor p53 is frequently mutated in human cancers. Many tumor-associated mutant p53 (mutp53) proteins gain new functions in promoting tumorigenesis, defined as gain-of-function (GOF). The mechanisms for mutp53 GOF are not well understood. Here, we report Pontin, a highly conserved AAA+ ATPase important for various cellular functions, as a new mutp53-binding protein. This Pontin–mutp53 interaction promotes mutp53 GOF in invasion, migration and anchorage-independent growth of tumor cells. The ATPase domain of Pontin is crucial for its promoting effect on mutp53 GOF; blocking the ATPase activity of Pontin by a Pontin-specific ATPase inhibitor or an ATPase-deficient dominant-negative Pontin expression vector greatly diminished mutp53 GOF. Pontin promotes mutp53 GOF through regulation of mutp53 transcriptional activity; knockdown of Pontin abolished the transcriptional regulation of mutp53 toward a group of genes. Furthermore, overexpression of Pontin in tumors is associated with the poor survival in cancer patients, especially those containing mutp53. Our results highlight an important role and mechanism for Pontin, a new mutp53 partner, in promoting mutp53 GOF in tumorigenesis.