Aberrant regulation and function of wild-type p53 in radioresistant melanoma cells.

Sporadic human tumors and the hereditary cancer predisposition syndrome Li-Fraumeni are frequently associated with mutations in the p53 tumor suppressor gene that compromise its ability to function as a DNA damage checkpoint. A subset of Li-Fraumeni patients with wild-type p53 alleles have mutations in chk2/hcds1, one of the genes signaling the presence of DNA damage to the p53 protein. This suggests that p53 may be kept inactive in human cancer by mutations targeting DNA damage signaling pathways. Melanoma cells are highly radioresistant, yet they express wild-type p53 protein, raising the possibility of defects in the pathways that activate p53 in response to DNA damage. We have described a chk2/hcds1-independent DNA damage signaling pathway that targets Ser-376 within the COOH terminus of p53 for dephosphorylation and leads to increased p53 functional activity. We now report that in several human melanoma cell lines that express wild-type p53, the phosphorylation state of Ser-376 was not regulated by DNA damage. In these cell lines, neither the endogenous wild-type p53 protein nor high levels of ectopic wild-type p53 led to cell cycle arrest or apoptosis. Thus, defective activation of p53 in response to DNA damage may underlie the radioresistance of human melanoma cells.     

Multifaceted p21 in carcinogenesis,stemness of tumor and tumor therapy

Cancer cells possess metabolic properties that are different from those of benign cells. p21, encoded by CDKN1A gene, also named p21^Cip1/WAF1, was first identified as a cyclin-dependent kinase regulator that suppresses cell cycle G1/S phase and retinoblastoma protein phosphorylation. CDKN1A (p21) acts as the downstream target gene of TP53 (p53), and its expression is induced by wild-type p53 and it is not associated with mutant p53. p21 has been characterized as a vital regulator that involves multiple cell functions, including G1/S cell cycle progression, cell growth, DNA damage, and cell stemness. In 1994, p21 was found as a tumor suppressor in brain, lung and colon cancer by targeting p53 and was associated with tumorigenesis and metastasis. Notably, p21 plays a significant role in tumor development through p53-dependent and p53-independent pathways. In addition, expression of p21 is closely related to the resting state or terminal differentiation of cells. p21 is also associated with cancer stem cells and acts as a biomarker for such cells. In cancer therapy, given the importance of p21 in regulating the G1/S and G2 check points, it is not surprising that p21 is implicated in response to many cancer treatments and p21 promotes the effect of oncolytic virotherapy.     

Gain of Function of p53 Cancer Mutants in Disrupting Critical DNA Damage Response Pathways

Loss of the tumor suppression activity of p53 is required for the progression of most human cancers. In this context, p53 gene is somatically mutated in about half of all human cancers; in the rest human cancers, p53 is mostly inactivated due to the disruption of pathways important for its activation. Most p53 cancer mutations are missense mutations within the core domain, leading to the expression of full-length mutant p53 protein. The expression of p53 mutants is usually correlated with the poor prognosis of the cancer patients. Accumulating evidence has indicated that p53 cancer mutants not only lose the tumor suppression activity of WT p53, but also gain novel oncogenic activities to promote tumorigenesis and drug resistance. Therefore, to improve current cancer therapy, it is critical to elucidate the gain-of-functions of p53 cancer mutants. By analyzing the humanized p53 mutant knock-in mouse models, we have identified a new gain of function of the common p53 cancer mutants in inducing genetic instability by disrupting ATM-mediated cellular responses to DNA double-stranded break (DSB) damage. Considering that some current cancer therapies such as radiotherapy kills the cancer cells by inducing DSBs in their genome DNA, our findings will have important implications on the treatment of human cancers that express common p53 mutants.     

The tumor suppressor p53: Cancer and aging

Aging, like many other biological processes, is subject to regulation by genes that reside in pathways that have been conserved during evolution. The insulin/ IGF-1 pathway, mTOR pathway and p53 pathway are among those conserved pathways that impact upon longevity and aging-related diseases such as cancer. Most cancers arise in the last quarter of life span with the frequency increasing exponentially with time, and mutation accumulation in critical genes (e.g. p53) in individual cells over a lifetime is thought to be the reason. Recently, we found that the efficiency of the p53 response to stress decline significantly with age in mice, and the time of onset of this decreased p53 response correlates with the life span of mice. Given the crucial role of the p53 in tumor prevention, this decline in p53 activity at older ages in animals could contribute to the observed dramatic increases in cancer frequency, and provides a plausible explanation for the correlation between tumorigenesis and aging in addition to the accumulation of DNA mutations over lifetime. We discuss here the coordination and communication between the p53 pathway and the IGF-1-mTOR pathways, and their possible impact on cancer and longevity.     

Combining p53 stabilizers with metformin induces synergistic apoptosis through regulation of energy metabolism in castration-resistant prostate cancer

Since altered energy metabolism is a hallmark of cancer, many drugs targeting metabolic pathways are in active clinical trials. The tumor suppressor p53 is often inactivated in cancer, either through downregulation of protein or loss-of-function mutations. As such, stabilization of p53 is considered as one promising approach to treat those cancers carrying wild type (WT) p53. Herein, SIRT1 inhibitor Tenovin-1 and polo-like kinase 1 (Plk1) inhibitor BI2536 were used to stabilize p53. We found that both Tennovin-1 and BI2536 increased the anti-neoplastic activity of metformin, an inhibitor of oxidative phosphorylation, in a p53 dependent manner. Since p53 has also been shown to regulate metabolic pathways, we further analyzed glycolysis and oxidative phosphorylation upon drug treatments. We showed that both Tennovin-1 and BI2536 rescued metformin-induced glycolysis and that both Tennovin-1 and BI2536 potentiated metformin-associated inhibition of oxidative phosphorylation. Of significance, castration-resistant prostate cancer (CRPC) C4-2 cells show a much more robust response to the combination treatment than the parental androgen-dependent prostate cancer LNCaP cells, indicating that targeting energy metabolism with metformin plus p53 stabilizers might be a valid approach to treat CRPC carrying WT p53.     

Role of WW domain proteins WWOX in development,prognosis, and treatment response of glioma

Glioblastoma multiforme (GBM) is the most aggressive and malignant brain tumor. Delicate microenvironment and lineage heterogeneity of GBM cells including infiltration, hypoxia, angiogenesis, and stemness make them highly resistant to current conventional therapies, with an average life expectancy for GBM patients of less than 15 months. Poor response to cytotoxic agents of GBM cells remains the major challenge of GBM treatment. Resistance of GBM to clinical treatment is a result of genomic alternation and deregulated signaling pathways, such as p53 mutation and apoptosis signaling blockage, providing cancer cells more opportunities for survival rather than cell death. WW domain-containing oxidoreductase (WWOX) is a tumor suppressor gene, commonly downregulated in various types of tumors, including GBM. It has been found that the reintroduction of WWOX induced p53-mutant GBM cells to undergo apoptosis, but not in p53 wild-type GBM cells, indicating WWOX is likely to reopen apoptosis pathways in a p53-independent manner in GBM. Identifying the crucial target modulated by WWOX deficiency provides a potential therapeutic target for GBM treatment. Here, we have reviewed the literatures about the role of WWOX in development, signaling pathway, prognosis, and treatment response in malignant glioma.     

Loss of putative tumor suppressor EI24/PIG8 confers resistance to etoposide

Expression of p53-target gene EI24/PIG8 is lost in invasive breast cancers, suggesting that EI24/PIG8 is a tumor suppressor that prevents tumor spreading, and partially mediates p53-attributed tumor suppressor activity. EI24/PIG8 also has pro-apoptotic activity indicating that loss of EI24/PIG8 may modulate sensitivity to chemotherapy. Here it is demonstrated that suppression of EI24/PIG8 in fibroblasts and breast cancer cells significantly inhibits the apoptotic response to etoposide treatment. These findings suggest that loss of EI24/PIG8 contributes significantly to resistance of cells to chemotherapeutic agents that function through p53, and identify the EI24/PIG8 status as a potentially new prognostic marker of chemotherapy responsiveness.