The p53 pathway is perturbed in the majority of human cancers. Although this most frequently occurs through the direct mutation or deletion of p53 itself, there are a number of other alterations that can attenuate the pathway and contribute to tumorigenesis. For example, amplification of important negative regulators, MDM2 and MDM4, occurs in a number of cancers. In this work, we will review both the normal regulation of the p53 pathway and the different mechanisms of pathway inhibition in cancer, discuss these alterations in the context of the global genomic analyses that have been conducted across tumor types, and highlight the translational implications for cancer diagnosis and treatment.

p53 R172H/+ mice inherit a p53 mutation found in Li-Fraumeni syndrome and develop metastatic tumors at much higher frequency than p53 +/− mice. To explore the mutant p53 metastatic phenotype, we used expression arrays to compare primary osteosarcomas from p53 R172H/+ mice with metastasis to osteosarcomas from p53 +/− mice lacking metastasis. For this study, 213 genes were differentially expressed with a P value <0.05. Of particular interest, Pla2g16, which encodes a phospholipase that catalyzes phosphatidic acid into lysophosphatidic acid and free fatty acid (both implicated in metastasis), was increased in p53 R172H/+ osteosarcomas. Functional analyses showed that Pla2g16 knockdown decreased migration and invasion in mutant p53-expressing cells, and vice versa: overexpression of Pla2g16 increased the invasion of p53-null cells. Furthermore, Pla2g16 levels were increased upon expression of mutant p53 in both mouse and human osteosarcoma cell lines, indicating that Pla2g16 is a downstream target of the mutant p53 protein. ChIP analysis revealed that several mutant p53 proteins bind the Pla2g16 promoter at E26 transformationspecific (ETS) binding motifs and knockdown of ETS2 suppressed mutant p53 induction of Pla2g16. Thus, our study identifies a phospholipase as a transcriptional target of mutant p53 that is required for metastasis. mammary tumor | fatty acid metabolism T he p53 tumor suppressor pathway is inactivated in ∼50% of human cancers (http://p53.iarc.fr). Missense mutations in particular account for 80% of p53 alterations, suggesting that mutant p53 proteins provide additional advantages for tumor cell growth (1). Li-Fraumeni syndrome patients with p53 missense mutations have a higher cancer incidence and an earlier age of tumor onset than individuals with truncating or splicing mutations (2). p53 knockin mice show a gain-of-function (GOF) phenotype in vivo, with high metastatic capacity compared with mice inheriting a p53-null allele (3, 4). GOF activities of mutant p53 are mediated by suppression of the p53 family members, p63 and p73 (3-6). Other mechanisms of mutant p53 GOF include mutant-p53 complexes with Smad that fuel TGF-β-induced metastasis (7) and integrin recycling (8). Additionally, mutant p53 interacts with the vitamin D receptor and converts vitamin D into an antiapoptotic agent (9-14). More recently, mutant p53 was reported to form transcriptional complexes on promoters of genes encoding several enzymes of the Mevalonate pathway, which increases metastasis of breast cancer cells (9). These data suggest multiple pathways contribute to the GOF phenotypes of cells with mutant p53. Although mutant p53 lacks sequencespecific DNA binding activity, its interaction with other transcriptional factors or the components of basic transcriptional machinery allow it to modulate gene expression (15). ChIP-onchip and ChIP-sequencing techniques show that mutant p53 affects transcription of many genes (9, 13, 16, 17).In this study, expression array analyses identified gene differences between p53 R172H/+ m...

MYC regulates a myriad of genes controlling cell proliferation, metabolism, differentiation, and apoptosis. MYC also controls the expression of DNA double-strand break (DSB) repair genes and therefore may be a potential target for anticancer therapy to sensitize cancer cells to DNA damage or prevent genetic instability. In this report, we studied whether MYC binds to DSB repair gene promoters and modulates cell survival in response to DNA-damaging agents. Chromatin immunoprecipitation studies showed that MYC associates with several DSB repair gene promoters including Rad51, Rad51B, Rad51C, XRCC2, Rad50, BRCA1, BRCA2, DNA-PKcs, XRCC4, Ku70, and DNA ligase IV. Endogenous MYC protein expression was associated with increased RAD51 and KU70 protein expression of a panel of cancer cell lines of varying histopathology. Induction of MYC in G 0 -G 1 and S-G 2 -M cells resulted in upregulation of Rad51 gene expression. MYC knockdown using small interfering RNA (siRNA) led to decreased RAD51 expression but minimal effects on homologous recombination based on a flow cytometry direct repeat green fluorescent protein assay. siRNA to MYC resulted in tumor cell kill in DU145 and H1299 cell lines in a manner independent of apoptosis. However, MYC-dependent changes in DSB repair protein expression were not sufficient to sensitize cells to mitomycin C or ionizing radiation, two agents selectively toxic to DSB repair-deficient cells. Our results suggest that anti-MYC agents may target cells to prevent genetic instability but would not lead to differential radiosensitization or chemosensitization. Cancer Res; 70(21); 8748-59. ©2010 AACR.