No abstract

Nitric oxide (NO) is a potent activator of the p53 tumor suppressor protein. However, the mechanisms underlying p53 activation by NO have not been fully elucidated. We previously reported that a rapid downregulation of Mdm2 by NO may contribute to the early phase of p53 activation. Here we show that NO promotes p53 nuclear retention and inhibits Mdm2-mediated p53 nuclear export. NO induces phosphorylation of p53 on serine 15, which does not require ATM but rather appears to depend on the ATM-related ATR kinase. An ATRkinase dead mutant or caffeine, which blocks the kinase activity of ATR, effectively abolishes the ability of NO to cause p53 nuclear retention, concomitant with its inhibition of p53 serine 15 phosphorylation. Of note, NO enhances markedly the ability of low-dose ionizing radiation to elicit apoptotic killing of neuroblastoma cells expressing cytoplasmic wild-type p53. These findings imply that, through augmenting p53 nuclear retention, NO can sensitize tumor cells to p53-dependent apoptosis. Thus, NO donors may potentially increase the efficacy of radiotherapy for treatment of certain types of cancer.

Tumor-associated mutants of the p53 tumor suppressor protein exert biological activities compatible with an oncogenic gain of function. To explore the underlying molecular mechanism, we performed microarray analysis, comparing p53-null cells to mutant p53-expressing cells. One of the genes up-regulated in the presence of mutant p53 was EGR1, a transcription factor implicated in growth control, apoptosis, and cancer. EGR1 induction by various types of stress is markedly augmented in cells expressing mutant p53. Moreover, chromatin immunoprecipitation analysis indicates that mutant p53 is physically associated with the EGR1 promoter. Functional assays indicate that induction of EGR1 by mutant p53 contributes to enhanced transformed properties and resistance to apoptosis. We propose that EGR1 is a significant contributor to mutant p53 gain of function.

Tumor-associated mutant forms of p53 can exert an antiapoptotic gain of function activity, which confers a selective advantage upon tumor cells harboring such mutations. We report that mutant p53 suppresses the expression of the MSP (MST-1/HGFL) gene, encoding the ligand of the receptor tyrosine kinase RON, implicated in a variety of cellular responses. Mutant p53 associates with the MSP gene promoter and represses its transcriptional activity, leading to a decrease in mRNA levels and a subsequent decrease in the levels of secreted MSP protein. Forced downregulation of MSP expression in H1299 cells, derived from a large-cell lung carcinoma, confers increased resistance against etoposide-induced cell death. These antiapoptotic consequences of MSP downregulation seemingly conflict with the well-documented ability of the RON receptor to promote cell survival and tumor progression when aberrantly hyperactive. Yet, they are consistent with the fact that reduced MSP expression was observed in many types of human cancer, including large-cell lung carcinoma. Thus, repression of MSP gene expression by mutant p53 may contribute to oncogenesis in a cell type-specific manner.

The p53 tumor suppressor plays a key role in the natural protection against cancer. Activation of p53 by DNAdamaging agents can contribute to successful elimination of cancer cells via chemotherapy-induced apoptosis. The phosphatidylinositol-3 kinase (PI3K) pathway, triggered in normal cells upon exposure to growth factors, regulates a cascade of proliferation and survival signals. The PI3K pathway is abnormally active in many cancers, thus making it an attractive target for inactivation in an attempt to achieve better cancer therapy. We report here that exposure to LY294002, a potent PI3K inhibitor, aborts the activation of p53 by several drugs commonly used in cancer chemotherapy. Concomitantly, LY294002 attenuates p53-dependent, chemotherapy-induced apoptosis of cancer cells. These findings invoke an unexpected positive role for PI3K in p53 activation by anticancer agents, and suggest that the efficacy of PI3K inhibitors in cancer therapy may be greatly affected by the tumor p53 status.