p53 mediates bcl-2 phosphorylation and apoptosis via activation of the Cdc42/JNK1 pathway

A member of the small G protein family, cdc42, was isolated from a screen undertaken to identify p53-inducible genes during apoptosis in primary baby rat kidney (BRK) cells transformed with E1A and a temperature-sensitive mutant p53 using a PCR-based subtractive hybridization method. Cdc42 is a GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. In response to external stimuli, Cdc42 is known to transduce signals to regulate the organization of the actin cytoskeleton, induce DNA synthesis in quiescent fibroblasts, and promote apoptosis in neuronal and immune cells. In this study, we have demonstrated that cdc42 mRNA and protein were up-regulated in the presence of wild-type p53 in BRK cells, followed by cytoplasmic to plasma membrane translocation of Cdc42. Overexpression of Cdc42 in the presence of a dominant-negative mutant p53 induced apoptosis rapidly, indicating that Cdc42 functions downstream of p53. Furthermore, stable expression of a dominant-negative mutant of Cdc42 partially inhibited p53-mediated apoptosis. The Bcl-2 family members Bcl-xL, and the adenovirus protein E1B 19K, inhibited Cdc42-mediated apoptosis, whereas Bcl-2 did not. We provide evidence that PAK1 and JNK1 may play a role downstream of Cdc42 to transduce its apoptotic signal. Cdc42/PAK1 activates JNK1-induced phosphorylation of Bcl-2, thereby inactivating its function, and that a phosphorylation resistant mutant (Bcl-2S70,87A,T56,74A) gains the ability to inhibit Cdc42- and p53-mediated apoptosis. Thus, one mechanism by which p53 promotes apoptosis is through activation of Cdc42 and inactivation of Bcl-2.     

p53AIP1 regulates the mitochondrial apoptotic pathway

We identified recently the p53AIP1 gene, a novel p53 target that mediates p53-dependent apoptosis. In the experiments reported here, ectopic expression of p53AIP1 induced down-regulation of mitochondrial DeltaPsim and release of cytochrome c from mitochondria in human cells. Immunoprecipitation and immunostaining experiments indicated interaction between p53AIP1 and bcl-2 proteins at mitochondria. Overexpression of bcl-2 blocked the down-regulation of mitochondrial DeltaPsim and the proapoptotic activity of p53AIP1. Our results implicate p53AIP1 as a pivotal mediator of the p53-dependent mitochondrial apoptotic pathway.     

Association of JNK1 with p21waf1 and p53: modulation of JNK1 activity

Exposure of mammalian cells to genotoxic stress results in activation of the c-jun amino-terminal kinase (JNK)-stress-activated protein kinase (SAPK) pathway and induction of DNA repair enzymes and cell cycle-regulatory proteins such as p53 and p21waf1. The p53 tumor suppressor protein transmits signals that activate p21waf1 gene expression. The p21waf1 protein then restricts cell-cycle progression, thereby allowing time for DNA repair to occur. In this study, we investigated the effects of modulation of the level of wild-type and mutant p53 protein on basal JNK1 activity in the A1-5 rat fibroblast cell line. This cell line contains a p53 gene coding for a temperature-sensitive p53 protein, which allows us to regulate the relative level of wild-type and mutant p53 protein produced in cells. Using the immune complex kinase assay to measure JNK1 activity, we demonstrated that cells expressing the wild-type-conformation p53 protein (when grown at 32.5 degrees C) exhibited a very low level of JNK1 activity. When cells were grown at 37 degrees C or 39 degrees C to express predominantly mutant p53 protein, basal level of JNK1 activity was significantly higher than at 32.5 degrees C. We also demonstrated protein-protein interactions between the p53, p21waf1, and JNK1 proteins in this cell line. Both wild-type p53 protein (expressed at 32.5 degrees C) and mutant p53(val135) protein (expressed at 37 degrees C and 39 degrees C) were present in immunocomplexes of JNK1 protein. Under conditions where wild-type p53 protein was present to induce p21waf1 expression (at 32.5 degrees C), a higher level of p21waf1 protein was also detected in the JNK1 immunocomplexes than in those at 37 degrees C and 39 degrees C. We next investigated the effect that co-association of p53 protein and p21waf1 protein would have on JNK1 activity. We measured basal levels of JNK1 activity in cells expressing wild-type p53 and p21waf1, or in p21waf1-null cells, and demonstrated that cells expressing both p53 and p21waf1 proteins exhibited an approximately threefold lower basal level of JNK1 activity when compared with p21waf1-null cells. To confirm that p21waf1 protein expression in cells resulted in reduced JNK1 activity, we transfected p21waf1-/- cells with a p21waf1 expression vector. We observed that JNK1 activity was inhibited after exogenous p21waf1 protein was expressed in these cells. Our results provide evidence for modulation of the JNK1 pathway by p53 and p21waf1 proteins and support the hypothesis that modulation of JNK1 activity occurred through protein-protein interactions between JNK1, p53, and p21waf1 proteins.     

Aplidin induces the mitochondrial apoptotic pathway via oxidative stress-mediated JNK and p38 activation and protein kinase C delta

Aplidin, a new antitumoural drug presently in phase II clinical trials, has shown both in vitro and in vivo activity against human cancer cells. Aplidin effectively inhibits cell viability by triggering a canonical apoptotic program resulting in alterations in cell morphology, caspase activation, and chromatin fragmentation. Pro-apoptotic concentrations of Aplidin induce early oxidative stress, which results in a rapid and persistent activation of both JNK and p38 MAPK and a biphasic activation of ERK. Inhibition of JNK and p38 MAPK blocks the apoptotic program induced by Aplidin demonstrating its central role in the integration of the cellular stress induced by the drug. JNK and p38 MAPK activation results in downstream cytochrome c release and activation of caspases -9 and -3 and PARP cleavage, demonstrating the mediation of the mitochondrial apoptotic pathway in this process. We also demonstrate that protein kinase C delta (PKC-delta) mediates the cytotoxic effect of Aplidin and that it is concomitantly processed and activated late in the apoptotic process by a caspase mediated mechanism. Remarkably, cells deficient in PKC-delta show enhanced survival upon drug treatment as compared to its wild type counterpart. PKC-delta thus appears as an important component necessary for full caspase cascade activation and execution of apoptosis, which most probably initiates a positive feedback loop further amplifying the apoptotic process.     

Cooperation between the ribosomal proteins L5 and L11 in the p53 pathway

MDM2 is a ubiquitin ligase that plays a key role in regulating the stability of the p53 tumor suppressor protein. Several proteins have been shown to activate the p53 pathway by interacting with and inhibiting the E3 function of MDM2, thereby leading to an accumulation of p53. These include the alternate reading frame (ARF) proteins and the ribosomal proteins L5 and L11. We found that when overexpressed alone, L11 is much less potent in inhibiting MDM2 than p14(ARF). However, L11 cooperates with L5, resulting in a robust inhibition of the E3 activity of MDM2, and a stabilization and activation of p53 approaching that achieved by p14(ARF). We further showed that the ability of L11 to bind the 5S rRNA is important for the cooperation with L5, and a mutant L11, which cannot bind the 5S rRNA, cannot cooperate with L5 in inhibiting MDM2.     

Pharmacologic activation of p53-dependent and p53-independent apoptotic pathways in Hodgkin/Reed-Sternberg cells.

The status of the p53 pathway in classical Hodgkin lymphoma (cHL) remains unclear, and a lack of proven TP53 mutations contrasts with often high expression levels of p53 protein. In this study, we demonstrate that pharmacologic activation of the p53 pathway with the murine double minute 2 (MDM2) antagonist nutlin-3 in Hodgkin lymphoma-derived cell lines leads to effective apoptosis induction and sensitizes the cells to other anticancer drugs. Cells with mutant p53 are resistant to nutlin-3, but sensitive to geldanamycin, a pharmacologic inhibitor of heat shock 90 kDa protein (HSP90), indicating that HSP90 inhibition can induce apoptosis in a p53-independent manner. Conversely, cells with defects in the HSP90/nuclear factor-kappa B pathway expressing wild-type p53 are more resistant to geldanamycin, but still sensitive to nutlin-3. Our results suggest that selective activation of p53 by MDM2 antagonists as a single agent or in combination with conventional chemotherapeutics and/or inhibitors of p53-independent survival pathways may offer effective treatment options for patients with cHL. Importantly, because nutlins and HSP90 inhibitors are non-genotoxic agents, their use might offer a means to reduce the genotoxic burden of current chemotherapeutic regimens.     

Lupulone triggers p38 MAPK-controlled activation of p53 and of the TRAIL receptor apoptotic pathway in human colon cancer-derived metastatic cells

We previously reported that the chemopreventive agent lupulone induces apoptosis through activation of the extrinsic pathway via TRAIL DR4/DR5 death receptors overcoming SW620 cell resistance to TRAIL. However, the underlying molecular mechanisms remain unknown. Since the mitogen-activated protein kinases (MAPKs), Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK) and p38 control fundamental cellular processes such as apoptosis, we determined the role of these MAPKs in lupulone-triggered apoptosis. We analyzed the effects of JNK, ERK and p38 MAPK inhibitors on lupulone-induced apoptosis by flow cytometry using specific antibodies and real-time RT-PCR. Our data showed that among the MAPKs, only p38 played a major role in lupulone-triggered apoptosis. In contrast to JNK and ERK inhibition, the specific inactivation of p38 inhibited the lupulone-triggered up-regulation of p53 and TRAIL-death receptor DR4/DR5 expression, and prevented DNA fragmentation. Lupulone treatment enhanced the expression of the anti-apoptotic Mcl-1 protein by 60% favoring the preservation of mitochondrial integrity. The inactivation of p38 initiated a 50% reduction in Mcl-1, Bcl-2 and Bax expression without changing the Mcl-1/Bax ratio suggesting that p38 was not involved in the protective effect of lupulone on mitochondria. Our data support the view that the lupulone-triggered enhanced expression of p38 plays a major role in the activation of p53 and of the TRAIL-death receptor apoptotic pathway in SW620 human colon cancer-derived metastatic cells.     

Cooperation between BRCA1 and p53 in repair of cyclobutane pyrimidine dimers

DNA repair defects can predispose to cancer development and progression. We previously showed that the breast and ovarian cancer susceptibility gene product BRCA1, through p53, upregulates expression of the XPE gene DDB2 encoding the nucleotide excision repair protein p 48. Both XPE and XPC are p53 target genes containing p53 response elements. To further explore the role of BRCA1 and p53 in repair of photoproducts, we eliminated wild type p53 from U2OS osteosarcoma cells and found that cyclobutane pyrimidine dimer (CPD) repair was markedly impaired following UV damage whereas repair of 6-4 photoproduct (6-4 PP) occurred efficiently. Overexpression of p53 in p53-null Calu-6 cells also enhanced CPD repair. In HCC1937 breast cancer cells, harboring mutant BRCA1 and p53 genes, repair of CPD was markedly impaired. Reintroduction of either p53 or BRCA1 using adenovirus vectors into HCC1937 alone had little effect on repair of CPD whereas the combination of p53 and BRCA1 resulted in efficient repair of CPD. Thus there appears to be a cooperative effect between p53 and BRCA1 that may involve induction of repair proteins, inhibition of p53-induced cell death by BRCA1 with altered p53 selectivity towards repair pathways and/or p53-independent effects of BRCA1 on CPD repair.     

Extinction of rac1 and Cdc42Hs signalling defines a novel p53-dependent apoptotic pathway

Apoptosis is a normal physiological process which eliminates cells that do not receive adequate extracellular signals. One of the pathways signalling apoptosis is controlled by the small GTPases of the Rho family, also involved in cell proliferation, differentiation and motility. Another major apoptosis signalling pathway involves the p53 tumour suppressor which is activated by a variety of stress and mediates growth arrest or apoptosis in normal cells. We show here that upon detachment from the extracellular matrix, fibroblasts undergo rapid apoptosis that can be rescued by constitutive activation of Rac1 and Cdc42Hs GTPases. Conversely, inhibition of Rac1 and Cdc42Hs efficiently triggers apoptosis in adherent cells. Interestingly, apoptosis is not observed in p53-/- cells either cultured in suspension or inhibited for Rac1 and Cdc42Hs activity. Moreover, Rac1 and Cdc42Hs extinction in normal cells activates endogenous p53. Using specific inhibitors of MAPK pathways, we demonstrate that, in our experimental system, p38 signals survival, while ERK activity is required for apoptosis. Our data constitute the first demonstration that Rac1 and Cdc42Hs control pathways that require simultaneous signalling through MAPK ERK and p53 to induce apoptosis.     

Suppression of ATAD2 inhibits hepatocellular carcinoma progression through activation of p53- and p38-mediated apoptotic signaling

The ATPase family, AAA domain containing 2 (ATAD2) is highly expressed in multiple cancers. We aim to understand the clinical and biological significance of ATAD2 over-expression in hepatocellular carcinoma (HCC), as a means to validate it as a therapeutic target in HCC. We demonstrated that ATAD2 was over-expressed in HCC patients, where high ATAD2 levels were significantly correlated with aggressive phenotypes such as high AFP levels, advanced tumor stages, and vascular invasion. Using RNA interference, suppression of ATAD2 in HCC cell lines decreased cell viability, migration, and invasion, and induced apoptosis in vitro. Furthermore, we identified p53 and p38 as key proteins that mediate apoptosis induced by ATAD2 suppression. In HCC cells, we demonstrated that ATAD2 directly interacted with MKK3/6, which prevented p38 activation and therefore inhibited p38-mediated apoptosis. In vivo, suppression of ATAD2 impaired the growth of HepG2 and Hep3B subcutaneous xenografts, accompanied by enhanced apoptosis and p-p53 and p-p38 levels. Our results validate that ATAD2 is an important negative regulator of apoptosis, and that neutralizing its activity has promising anti-tumor effects in HCC cells.     

Synergistic tumor suppression by coexpression of FUS1 and p53 is associated with down-regulation of murine double minute-2 and activation of the apoptotic protease-activating factor 1-dependent apoptotic pathway in human non-small cell lung cancer cells

FUS1 is a novel tumor suppressor gene identified in human chromosome 3p21.3 region. Loss of expression and deficiency of posttranslational modification of FUS1 protein have been found in a majority of human lung cancers. Restoration of wild-type FUS1 in 3p21.3-deficient human lung cancer cells exhibited a potent tumor suppression function in vitro and in vivo. In this study, we evaluated the combined effects of FUS1 and tumor suppressor p53 on antitumor activity and explored the molecular mechanisms of their mutual actions in human non-small cell lung cancer (NSCLC) cells. We found that coexpression of FUS1 and p53 by N-[1-(2,3-dioleoyloxyl)propyl]-NNN-trimethylammoniummethyl sulfate:cholesterol nanoparticle-mediated gene transfer significantly and synergistically inhibited NSCLC cell growth and induced apoptosis in vitro. We also found that a systemic treatment with a combination of FUS1 and p53 nanoparticles synergistically suppressed the development and growth of tumors in a human H322 lung cancer orthotopic mouse model. Furthermore, we showed that the observed synergistic tumor suppression by FUS1 and p53 concurred with the FUS1-mediated down-regulation of murine double minute-2 (MDM2) expression, the accumulation and stabilization of p53 protein, as well as the activation of the apoptotic protease-activating factor 1 (Apaf-1)-dependent apoptotic pathway in human NSCLC cells. Our results therefore provide new insights into the molecular mechanism of FUS1-mediated tumor suppression activity and imply that a molecular therapy combining two or more functionally synergistic tumor suppressors may constitute a novel and effective strategy for cancer treatment.     

Cooperation between p53 and p73 in cisplatin-induced apoptosis in ovarian carcinoma cells

The present study was designed to explore the role of p73 in response to cisplatin. In contrast to cisplatin-resistant ovarian carcinoma cells, pharmacological drug concentrations, which caused induction of p53, induced a marginal increase of p73 in sensitive cells. The effect was more marked at high concentrations, with no evidence of p21(WAF1) induction. This behaviour, associated with substantial apoptosis, suggests cell inability to activate DNA damage checkpoint following extensive stress. Although p73 appears to be implicated mainly in response to high stress conditions, the available results support a cooperation between p53 and p73 in cisplatin-induced apoptosis in sensitive cells.     

Protein phosphatase-2A is a target of epigallocatechin-3-gallate and modulates p53-Bak apoptotic pathway

(-)-Epigallocatechin-3-gallate (EGCG) is a well-known chemoprevention factor. Recent studies have revealed that EGCG triggers cancer cells undergoing apoptosis through p53-dependent pathway. How EGCG activates p53-dependent apoptosis is not fully understood. In the present study using JB6 cell as a model system, we have shown that EGCG can negatively regulate protein serine/threonine phosphatase-2A (PP-2A) to positively regulate p53-dependent apoptosis. First, EGCG at physiologic levels down-regulates PP-2A at the protein and enzyme activity levels. Second, EGCG induces apoptosis of JB6 cells, which is associated with hyperphosphorylation of p53 and up-regulation of the proapoptotic gene, Bak. DNA sequence analysis, gel mobility shifting, chromatin immunoprecipitation, and reporter gene activity assays revealed that p53 directly controls Bak in JB6 cells. Knockdown of p53 and Bak expression with RNAi substantially inhibits EGCG-induced apoptosis. Third, PP-2A directly interacts with p53 and dephosphorylates p53 at Ser-15 in vitro and in vivo. Fourth, overexpression of the catalytic subunit for PP-2A down-regulates p53 phosphorylation at Ser15, attenuates expression of the downstream proapoptotic gene, Bak, and antagonizes EGCG-induced apoptosis. Inhibition of PP-2A activity enhances p53 phosphorylation at Ser-15 and up-regulates Bak expression to promote EGCG-induced apoptosis. Finally, in the p53(-/-) H1299 and p53(+/+) H1080 cells, EGCG down-regulates PP-2A similarly but induces differential apoptosis. In summary, our results show that (a) PP-2A directly dephosphorylates p53 at Ser-15; (b) P53 directly controls Bak expression; and (c) EGCG negatively regulates PP-2A. Together, our results show that EGCG-mediated negative regulation of PP-2A is an important molecular event for the activation of p53-dependent apoptosis during its chemoprevention.