Transduction of Apaf-1 or caspase-9 induces apoptosis in A-172 cells that are resistant to p53-mediated apoptosis

p53 replacement gene therapy has been carried out clinically for cancers with p53 mutations; however, some cancers are resistant to p53 gene therapy. In this study, we transduced A-172 and U251 cells harboring p53 mutations with wild-type p53 using adenovirus vectors to induce wild-type p53 protein at similar expression levels. A-172 cells did not undergo apoptosis after p53 transduction, whereas U251 cells were markedly sensitive to p53-mediated apoptosis. A-172 cells showed higher endogenous expression of Bcl-X(L) than U251, and transduction of Bcl-X(L) repressed p53-mediated apoptosis in U251 cells, suggesting that high endogenous expression of Bcl-X(L) renders A-172 cells, at least in part, resistant to p53-mediated apoptosis. We transduced A-172 cells and U251 cells with the Apaf-1 or caspase-9 genes; both are downstream components of p53-mediated apoptosis. We found that A-172 cells were highly sensitive to Apaf-1- and caspase-9-mediated apoptosis. The results indicate that A-172 cells harboring mutant p53 were not susceptible to p53-mediated apoptosis, possibly due to high endogenous expression of Bcl-X(L). Transduction of Apaf-1 or caspase-9 would override the resistance mechanism of apoptosis in A-172 cells. These findings provide potentially a novel approach in killing cancers that are resistant to p53 replacement gene therapy.     

p53-independent pRB degradation contributes to a drug-induced apoptosis in AGS cells

The retinoblastoma （RB） tumor suppressor protein, pRB, plays an important role in the regulation of mammalian cell cycle. Furthermore, several lines of evidence suggest that pRB also involves in the regulation of apoptosis. In the present study, the degradation of pRB was observed in apoptotic gastric tumor cells treated with a new potent anti-tumor component, tripchlorolide （TC）. The inhibition of pRB degradation by a general cysteine protease inhibitor IDAM resulted in the reduction of the apoptotic cells. Furthermore, the survival of the gastric tumor cells under the TC treatment was enhanced by an over-expression of exogenous pRB. These results suggest that the pRB degradation of the gastric tumor cells under the TC treatment involves in the apoptotic progression. In addition, the same extent of TCinduced pRB-degradation was detected in the gastric tumor cells containing a p53 dominant-negative construct, indicat- ing that this kind of pRB degradation is p53-independent.     

Essential postmitochondrial function of p53 uncovered in DNA damage-induced apoptosis in neurons

In postmitotic sympathetic neurons, unlike most mitotic cells, death by apoptosis requires not only the release of cytochrome c from the mitochondria, but also an additional step to relieve X-linked inhibitor of apoptosis protein (XIAP)'s inhibition of caspases. Here, we examined the mechanism by which XIAP is inactivated following DNA damage and found that it is achieved by a mechanism completely different from that following apoptosis by nerve growth factor (NGF) deprivation. NGF deprivation relieves XIAP by selectively degrading it, whereas DNA damage overcomes XIAP via a p53-mediated induction of Apaf-1. Unlike wild-type neurons, p53-deficient neurons fail to overcome XIAP and remain resistant to cytochrome c after DNA damage. Restoring Apaf-1 induction in p53-deficient neurons is sufficient to overcome XIAP and sensitize cells to cytochrome c. Although a role for p53 in apoptosis upstream of cytochrome c release has been well established, this study uncovers an additional, essential role for p53 in regulating caspase activation downstream of mitochondria following DNA damage in neurons.     

Tauroursodeoxycholic acid modulates p53-mediated apoptosis in Alzheimer's disease mutant neuroblastoma cells

Early onset familial Alzheimer's disease (FAD) is linked to autosomal dominant mutations in the amyloid precursor protein (APP) and presenilin 1 and 2 (PS1 and PS2) genes. These are critical mediators of total amyloid beta-peptide (Abeta) production, inducing cell death through uncertain mechanisms. Tauroursodeoxycholic acid (TUDCA) modulates exogenous Abeta-induced apoptosis by interfering with E2F-1/p53/Bax. Here, we used mouse neuroblastoma cells that express either wild-type APP, APP with the Swedish mutation (APPswe), or double-mutated human APP and PS1 (APPswe/DeltaE9), all exhibiting increased Abeta production and aggregation. Cell viability was decreased in APPswe and APPswe/DeltaE9 but was partially reversed by z-VAD.fmk. Nuclear fragmentation and caspase 2, 6 and 8 activation were also readily detected. TUDCA reduced nuclear fragmentation as well as caspase 2 and 6, but not caspase 8 activities. p53 activity, and Bcl-2 and Bax changes, were also modulated by TUDCA. Overexpression of p53, but not mutant p53, in wild-type and mutant neuroblastoma cells was sufficient to induce apoptosis, which, in turn, was reduced by TUDCA. In addition, inhibition of the phosphatidylinositide 3'-OH kinase pathway reduced TUDCA protection against p53-induced apoptosis. In conclusion, FAD mutations are associated with the activation of classical apoptotic pathways. TUDCA reduces p53-induced apoptosis and modulates expression of Bcl-2 family.     

Negative cross-talk between p53 and the glucocorticoid receptor and its role in neuroblastoma cells

The tumour suppressor p53 and the glucocorticoid receptor (GR) respond to different types of stress. We found that dexamethasone-activated endogenous and exogenous GR inhibit p53-dependent functions, including transactivation, up- (Bax and p21(WAF1/CIP1)) and down- (Bcl2) regulation of endogenous genes, cell cycle arrest and apoptosis. GR forms a complex with p53 in vivo, resulting in cytoplasmic sequestration of both p53 and GR. In neuroblastoma (NB) cells, cytoplasmic retention and inactivation of wild-type p53 involves GR. p53 and GR form a complex that is dissociated by GR antagonists, resulting in accumulation of p53 in the nucleus, activation of p53-responsive genes, growth arrest and apoptosis. These results suggest that molecules that efficiently disrupt GR-p53 interactions would have a therapeutic potential for the treatment of neuroblastoma and perhaps other diseases in which p53 is sequestered by GR.     

TrkA induces apoptosis of neuroblastoma cells and does so via a p53-dependent mechanism

Neuroblastoma (NB) is the most frequent solid extracranial tumor in children. Its clinical prognosis correlates with the expression of members of the Trk neurotrophin receptor family, which includes TrkA and TrkB. TrkA expression is associated with favorable prognosis, whereas TrkB expression is associated with poor prognosis. Here we show that TrkA expression induces the apoptosis of NB cells and does so by modulating the levels or activities of a number of proteins involved in regulating cell survival and apoptosis, including p53, Bcl-2, and caspase-3. TrkA increased the expression of p53 target proteins and failed to induce apoptosis in cells where p53 was inactivated by mutation or via expression of dominant inhibitory p53 or E1B55K, indicating that TrkA mediates apoptosis, at least in part, through p53. Treatment with a caspase inhibitor or overexpression of Bcl-X(L) also prevented TrkA from inducing apoptosis. In contrast, elevated expression of TrkA in non-transformed sympathetic neurons resulted in the suppression of p53 levels and enhanced survival. These results identify apoptosis as a novel biological response of TrkA in NB cells and imply that TrkA is a good prognosis marker for NB due in part to its ability to mediate apoptosis when expressed at sufficient levels.     

Chemotherapy-induced apoptosis of S-type neuroblastoma cells requires caspase-9 and is augmented by CD95/Fas stimulation

Stromal or S-type tumor cells are a distinct lineage found in neuroblastoma tumors and have an important role in the biology of this disease. Anticancer agents induce apoptosis through death receptor- and mitochondria-initiated pathways. The object of this work was to determine the involvement of these pathways in the response to doxorubicin (Dox) and cisplatin (CDDP) in S-type neuroblastoma cells. Both drugs activated caspase-9 and caspase-3 but not caspase-8. Caspase-9-specific inhibition blocked S-type cell death induced by Dox. SH-EP1 cells transfected to express dominant negative mutant caspase-9, but not those expressing DN caspase-8, were resistant to Dox- and CDDP-induced apoptosis. The lack of caspase-8 involvement in chemotherapy-induced death was not the result of an intrinsic inability of these cells to activate this enzyme because when they were treated with tumor necrosis factor-related apoptosis-inducing ligand, caspase-8 was activated. We also found that both drugs up-regulated CD95/Fas expression but that CD95/Fas signaling was not necessary for cell killing. Experiments testing the response of chemotherapy-treated cells to agonists of the CD95/Fas receptor established that Dox and CDDP treatment sensitizes cells to CD95/Fas killing. Together, these results are consistent with a model in which caspase-9 is of central importance in the death mechanism utilized by these drugs in S-type cells. Although the death response is not dependent on CD95/Fas, concomitant stimulation of this receptor amplifies the death response in drug-treated cells.     

A role for endogenous and radiation-induced DNA double-strand breaks in p53-dependent apoptosis during cortical neurogenesis

Prenatal exposure to low-dose radiation increases the risk of microcephaly and/or mental retardation. Microcephaly is also associated with genetic mutations that affect the non-homologous end-joining pathway of DNA double-strand break repair. To examine the link between these two causal factors, we characterized the neural developmental effects of acute radiation exposure in mouse littermate embryos harboring mutations in the Ku70 and p53 genes. Both low-dose radiation exposure and Ku70 deficiency induced morphologically indistinguishable cortical neuronal apoptosis. Irradiated Ku70-deficient embryos displayed anatomical damage indicative of increased radiosensitivity in the developing cerebral cortex. Deleting the p53 gene not only rescued cortical neuronal apoptosis at all levels but also restored the in vitro growth of Ku70-deficient embryonic fibroblasts despite the presence of unrepaired DNA/chromosomal breaks. The results confirm the role of DNA double-strand breaks as a common causative agent of apoptosis in the developing cerebral cortex. Furthermore, the findings suggest a disease mechanism by which the presence of endogenous DNA double-strand breaks in the newly generated cortical neurons becomes radiomimetic when DNA end joining is defective. This in turn activates p53-dependent neuronal apoptosis and leads to microcephaly and mental retardation.     

Nitric oxide promotes p53 nuclear retention and sensitizes neuroblastoma cells to apoptosis by ionizing radiation

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 ATR-kinase 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.     

Involvement of caspase-2 and caspase-9 in endoplasmic reticulum stress-induced apoptosis: a role for the IAPs

Dysregulation of apoptosis is involved in a wide spectrum of disease ranging from proliferative to degenerative disorders. An emerging area of study in apoptosis is the critical contribution of the endoplasmic reticulum (ER) in both mitochondrial and ER specific apoptosis pathways. Here we show that brefeldin A and tunicamycin-mediated ER stress lead to caspase-dependent apoptosis involving caspase-2. Confocal microscopy and subcellular fractionation indicate that caspase-2 is localized to the ER, and following ER stress, the processing of caspase-2 and -9 is an early event preceding the activation of caspase-3 and -7 and the cleavage of the caspase substrate poly(ADP-ribose) polymerase (PARP). Inhibition and silencing of either caspase-2 or caspase-9 suppress ER stress-induced apoptosis, as demonstrated by annexin V binding. Similarly, transduction with an adenovirus encoding either Inhibitors of Apoptosis (IAP) protein HIAP1/c-IAP2 or HIAP2/c-IAP1 also suppresses ER stress-induced apoptosis. However, among HIAP1, HIAP2 and XIAP, only HIAP2 binds and inhibits caspase-2. Our results thus indicate a novel mechanism by which HIAP2 can regulate ER-initiated apoptosis by modulating the activity of caspase-2.