DNA damage induces two distinct modes of cell death in ovarian carcinomas

Activation of p53 by cellular stress may lead to either cell cycle arrest or apoptotic cell death. Restrictions in a cell's ability to halt the cell cycle might, in turn, cause mitotic catastrophe, a delayed type of cell death with distinct morphological features. Here, we have investigated the contribution of p53 and caspase-2 to apoptotic cell death and mitotic catastrophe in cisplatin-treated ovarian carcinoma cell lines. We report that both functional p53 and caspase-2 were required for the apoptotic response, which was preceded by translocation of nuclear caspase-2 to the cytoplasm. In the absence of functional p53, cisplatin treatment resulted in caspase-2-independent mitotic catastrophe followed by necrosis. In these cells, apoptotic functions could be restored by transient expression of wt p53. Hence, p53 appeared to act as a switch between apoptosis and mitotic catastrophe followed by necrosis-like lysis in this experimental model. Further, we show that inhibition of Chk2, and/or 14-3-3sigma deficiency, sensitized cells to undergo mitotic catastrophe upon treatment with DNA-damaging agents. However, apoptotic cell death seemed to be the final outcome of this process. Thus, we hypothesize that the final mode of cell death triggered by DNA damage in ovarian carcinoma cells is determined by the profile of proteins involved in the regulation of the cell cycle, such as p53- and Chk2-related proteins.     

p53-Independent ceramide formation in human glioma cells during gamma-radiation-induced apoptosis

Although the p53 tumor-suppressor gene product plays a critical role in apoptotic cell death induced by DNA-damaging chemotherapeutic agents, human glioma cells with functional p53 were more resistant to gamma-radiation than those with mutant p53. U-87 MG cells with wild-type p53 were resistant to gamma-radiation. U87-W E6 cells that lost functional p53, by the expression of type 16 human papillomavirus E6 oncoprotein, became susceptible to radiation-induced apoptosis. The formation of ceramide by acid sphingomyelinase (A-SMase), but not by neutral sphingomyelinase, was associated with p53-independent apoptosis. SR33557 (2-isopropyl-1-(4-[3-N-methyl-N-(3,4-dimethoxybphenethyl)amino]propyloxy)benzene-sulfonyl) indolizine, an inhibitor of A-SMase, suppressed radiation-induced apoptotic cell death. In contrast, radiation-induced A-SMase activation was blocked in glioma cells with endogenous functional p53. The expression of acid ceramidase was induced by gamma-radiation, and was more evident in cells with functional p53. N-oleoylethanolamine, which is known to inhibit ceramidase activity, unexpectedly downregulated acid ceramidase and accelerated radiation-induced apoptosis in U87-W E6 cells. Moreover, cells with functional p53 could be sensitized to gamma-radiation by N-oleoylethanolamine, which suppressed radiation-induced acid ceramidase expression and then enhanced ceramide formation. Sensitization to gamma-radiation was also observed in U87-MG cells depleted of functional p53 by retroviral expression of small interfering RNA. These results indicate that ceramide may function as a mediator of p53-independent apoptosis in human glioma cells in response to gamma-radiation, and suggest that p53-dependent expression of acid ceramidase and blockage of A-SMase activation play pivotal roles in protection from gamma-radiation of cells with endogenous functional p53.     

p53 regulates a non-apoptotic death induced by ROS

DNA damage induced by reactive oxygen species and several chemotherapeutic agents promotes both p53 and poly (ADP-ribose) polymerase (PARP) activation. p53 activation is well known to regulate apoptotic cell death, whereas robust activation of PARP-1 has been shown to promote a necrotic cell death associated with energetic collapse. Here we identify a novel role for p53 in modulating PARP enzymatic activity to regulate necrotic cell death. In mouse embryonic fibroblasts, human colorectal and human breast cancer cell lines, loss of p53 function promotes resistance to necrotic, PARP-mediated cell death. We therefore demonstrate that p53 can regulate both necrotic and apoptotic cell death, mutations or deletions in this tumor-suppressor protein may be selected by cancer cells to provide not only their resistance to apoptosis but also to necrosis, and explain resistance to chemotherapy and radiation even when it kills via non-apoptotic mechanisms.     

Activation of Cdc2 contributes to apoptosis in HPV E6 expressing human keratinocytes in response to therapeutic agents

Infection with human papillomaviruses (HPV) is strongly associated with the development of cervical cancer. The HPV E6 oncogene induces apoptosis in cervical cancer precursor lesions but the mechanism is poorly understood. While it is expected that inactivation of p53 by E6 should lead to a reduction in apoptosis, E6 also sensitizes cells to apoptosis under some experimental conditions. Here, we demonstrate that expression of E6 in human keratinocytes rendered sensitization to chemotherapeutic agents. The cell death was shown to be by apoptosis involving caspase activation and the mitochondria pathway. To explore mechanisms involved in sensitization of E6 expressing cells to apoptosis, we used a proteomic approach to identify proteins differentially expressed in E6 expressing and control keratinocytes. Among nearly a thousand proteins examined, Cdc2 was demonstrated to be the most dramatically up-regulated protein in E6 expressing cells. p53 degradation appears to be important for the up-regulation of Cdc2 by E6. Using genetic, pharmacologic, and siRNA strategies, a role for Cdc2 in E6 expression-conferred apoptosis was demonstrated. Thus, these results have important therapeutic implications in enhancing the efficacy of chemotherapy.     

DNA damage induces down-regulation of UDP-glucose ceramide glucosyltransferase, increases ceramide levels and triggers apoptosis in p53-deficient cancer cells

DNA damaging agents typically induce an apoptotic cascade in which p53 plays a central role. However, absence of a p53-mediated response does not necessarily abrogate programmed cell death, due to the existence of p53-independent apoptotic pathways, such as those mediated by the pro-apoptotic molecule ceramide. We compared ceramide levels before and after DNA damage in human osteosarcoma (U2OS) and colon cancer (HCT116) cells that were either expressing or deficient in p53. When treated with mitomycin C, p53-deficient cells, but not p53-expressing cells, showed a marked increase in ceramide levels. Microarray analysis of genes involved in ceramide metabolism identified acid ceramidase (ASAH1, up-regulated), ceramide glucosyltransferase (UGCG, down-regulated), and galactosylceramidase (GALC, up-regulated) as the three genes most affected. Experiments employing pharmacological and siRNA agents revealed that inhibition of UGCG is sufficient to increase ceramide levels and induce cell death. When inhibition of UGCG and treatment with mitomycin C were combined, p53-deficient, but not p53-expressing cells, showed a significant increase in cell death, suggesting that the regulation of sphingolipid metabolism could be used to sensitize cells to chemotherapeutic drugs.     

Itch inhibition regulates chemosensitivity in vitro

Itch is a member of the HECT family of ubiquitin E3 ligases, and regulates the stability of several proteins involved in response to genotoxic stress. We have previously shown that p73 and p63, two members of the p53 family of tumour suppressors, are targets for Itch-mediated ubiquitylation and degradation. Here, we show that depletion of Itch by RNA interference augments apoptosis upon treatment with chemotherapeutic drugs. We also show that cells with no functional p53 are more sensitive to Itch depletion, highlighting the importance that changes in levels of Itch may play in majority of cancers, where p53 is absent or mutated. Furthermore, reintroduction of Itch in fibroblasts obtained from Itch deficient mice results in reduced cell death upon DNA damage. Overall our findings suggest that inhibition of Itch potentiates the effect of chemotherapeutic drugs revealing the pharmacological potentials of targeting Itch for cancer therapy.     

Release of cytochrome c from isolated mitochondria by etoposide

The efficacy of chemotherapeutic agents on tumor cells has been shown to be modulated by tumor suppressor gene p53 and its target genes such as Bcl-2 family members (Bax, Noxa, and PUMA). However, various chemotherapeutic agents can induce cell death in tumor cells that do not express the functional p53, suggesting that some chemotherapeutic agents may induce cell death in a p53-independent pathway. Here we showed that etoposide can induce the similar degree of cell death in p53-deficient HCT 116 cells, whereas 5'-FU-mediated cell death is strongly dependent on the existence of functional p53 in HCT 116 cells. Further, we provide the evidence that etoposide can induce the cytochrome c release from isolated mitochondria, and etoposide-induced cytochrome c release is not accompanied with the large amplitude swelling of mitochondria. These data suggest that etoposide can directly induce the mitochondrial dysfunction irrespective of p53 status, and it may, at least in part, account for the p53-independent pathway in cell death induced by chemotherapeutic agents.     

Conditionally replicating adenoviruses kill tumor cells via a basic apoptotic machinery-independent mechanism that resembles necrosis-like programmed cell death

Conditionally replicating adenoviruses (CRAds) represent a promising class of novel anticancer agents that are used for virotherapy. The E1ADelta24 mutation-based viruses, Ad5-Delta24 [CRAd(E3-); E3 region deleted] and infectivity-enhanced Ad5-Delta24RGD [CRAd(E3+)] have been shown to potently eradicate tumor cells. The presence of the E3 region in the latter virus is known to improve cell killing that can be attributed to the presence of the oncolysis-enhancing Ad death protein. The more precise mechanism by which CRAds kill tumor cells is unclear, and the role of the host cell apoptotic machinery in this process has been addressed only in a limited way. Here, we examine the role of several major apoptotic pathways in the CRAd-induced killing of non-small-cell lung cancer H460 cells. As expected, CRAd(E3+) was more potent than CRAd(E3-). No evidence for the involvement of the p53-Bax apoptotic pathway was found. Western blot analyses demonstrated strong suppression of p53 expression and unchanged Bax levels during viral replication, and stable overexpression of human papillomavirus type 16-E6 in H460 cells did not affect killing by both CRAds. CRAd activity was also not hampered by stable overexpression of anti-apoptotic Bcl2 or BclXL, and endogenous Bcl2/BclXL protein levels remained constant during the oncolytic cycle. Some evidence for caspase processing was obtained at late time points after infection; however, the inhibition of caspases by the X-linked inhibitor of apoptosis protein overexpression or cotreatment with zVAD-fmk did not inhibit CRAd-dependent cell death. Analyses of several apoptotic features revealed no evidence for nuclear fragmentation or DNA laddering, although phosphatidylserine externalization was detected. We conclude that despite the known apoptosis-modulating abilities of individual Ad proteins, Ad5-Delta24-based CRAds trigger necrosis-like cell death. In addition, we propose that deregulated apoptosis in cancer cells, a possible drug resistance mechanism, provides no barrier for CRAd efficacy.     

Different cell cycle mechanisms between UV-induced and X-ray-induced apoptosis in WiDr colorectal carcinoma cells

Although DNA-damaging agents such as ultraviolet (UV) and X-ray can induce apoptosis, the difference in the apoptotic mechanism is not clearly understood. In the present study, we investigated the effects of these two genotoxic agents on the induction of DNA damage and subsequent apoptotic cell death from the viewpoint of cell cycle regulation by using WiDr cells. Transient G1 arrest was observed after UV exposure, whereas G2 but not G1 arrest was induced after X-ray irradiation. UV-exposure could induce G1 arrest in both mutant-type (mt-p53) and wild-type p53 (wt-p53) cells, but obvious G1 arrest was not observed in the cells lacking in p53 expression. An increase in the DNA fragmentation was observed at S phase in UV-irradiated cells and at G2 phase in X-irradiated cells, respectively. UV-irradiated cells showed an increase production of p53 protein and accumulation of p21 protein. On the contrary, both p53 and p21 proteins remained at a low level in X-irradiated cells. Treatment with aphidicolin, an S phase blocking agent, prolonged cell cycle arrest and reduced the rate of apoptotic cell death in both UV-irradiated and X-irradiated cells. From these results, it is suggested that UV-induced apoptosis occurs mainly at S phase and is regulated by increased production of p53 and p21 proteins, while X-ray-induced apoptosis occurs after G2 blockade and may be independent of p53.     

Retinoic acid confers resistance to p53-dependent apoptosis in SH-SY5Y neuroblastoma cells by modulating nuclear import of p53.

Many cell lines derived from neuroblastoma (NB) carry the wild-type p53 gene with a p53-dependent apoptotic pathway that is responsive to DNA damaging agents. A recent study has demonstrated that retinoic acid (RA) pretreatment of NB cells promotes chemoresistance to apoptosis induced by chemotherapeutic agents. We examine here the possible contribution of the p53 pathway to the chemoresistance response associated with the RA treatment in NB cells. Upon treatment with RA (1-10 microM) for 4 days, the human NB cells, SH-SY5Y, developed resistance selectively to p53-dependent apoptotic stimuli including gamma-irradiation, etoposide, and 1-(5-isoquinolinyl sulfonyl)-2-methylpiperazine (H-7). Interestingly, RA affected the ability of H-7 to induce nuclear accumulation of the p53 protein without altering its effect on elevating the steady-state level of p53, suggesting that drug-induced up-regulation and nuclear accumulation of the wild-type p53 protein are separable processes. The modulation of nuclear import of p53 protein by RA may thus represent a potential mechanism by which certain tumor cells with the wild-type p53 gene develop resistance to chemotherapeutic agents.     

CDIP, a novel pro-apoptotic gene, regulates TNFalpha-mediated apoptosis in a p53-dependent manner

We have identified a novel pro-apoptotic p53 target gene named CDIP (Cell Death Involved p53-target). Inhibition of CDIP abrogates p53-mediated apoptotic responses, demonstrating that CDIP is an important p53 apoptotic effector. CDIP itself potently induces apoptosis that is associated with caspase-8 cleavage, implicating the extrinsic cell death pathway in apoptosis mediated by CDIP. siRNA-directed knockdown of caspase-8 results in a severe impairment of CDIP-dependent cell death. In investigating the potential involvement of extrinsic cell death pathway in CDIP-mediated apoptosis, we found that TNF-alpha expression tightly correlates with CDIP expression, and that inhibition of TNF-alpha signaling attenuates CDIP-dependent apoptosis. We also demonstrate that TNF-alpha is upregulated in response to p53 and p53 inducing genotoxic stress, in a CDIP-dependent manner. Consistently, knockdown of TNF-alpha impairs p53-mediated stress-induced apoptosis. Together, these findings support a novel p53 --> CDIP --> TNF-alpha apoptotic pathway that directs apoptosis after exposure of cells to genotoxic stress. Thus, CDIP provides a new link between p53-mediated intrinsic and death receptor-mediated extrinsic apoptotic signaling, providing a novel target for cancer therapeutics aimed at maximizing the p53 apoptotic response of cancer cells to drug therapy.     

ERKs/p53 signal transduction pathway is involved in doxorubicin-induced apoptosis in H9c2 cells and cardiomyocytes

The cardiotoxic effects of doxorubicin, a potent chemotherapeutic agent, have been linked to DNA damage, oxidative mitochondrial damage, and nuclear translocation of p53, but the exact molecular mechanisms causing p53 transactivation and doxorubicin-induced cardiomyopathy are not clear. The present study was carried out to determine whether extracellular signal-regulated kinases (ERKs), which are known to be activated by DNA damaging agents, are responsible for doxorubicin-induced p53 activation and oxidative mitochondrial damage in H9c2 cells. Cell death was measured by terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling, annexin V-fluorescein isothiocyanate, activation of caspase-9 and -3, and cleavage of poly(ADP-ribose) polymerase (PARP). We found that doxorubicin produced cell death in H9c2 cells in a time-dependent manner, beginning at 6 h, and these changes are associated decreased expression of Bcl-2, increases in Bax and p53 upregulated modulator of apoptosis-alpha expression, and collapse of mitochondria membrane potential. The changes in cell death and Bcl-2 family proteins, however, were preceded by earlier activation and nuclear translocation of ERKs, followed by increased phosphorylation at Ser15 and nuclear translocation of the phosphorylated p53. The functional importance of ERK1/2 and p53 in doxorubicin-induced toxicity was further demonstrated by the specific ERK inhibitor U-0126 and p53 inhibitor pifithrin (PFT)-alpha, which abrogated the changes in Bcl-2 family proteins and cell death produced by doxorubicin. U-0126 blocked the phosphorylation and nuclear translocation of both ERK1/2 and p53, whereas PFT-alpha blocked only the changes in p53. Doxorubicin and ERK inhibitors produced similar changes in ERK1/2-p53, PARP, and caspase-3 in neonatal rat cultured cardiomyocytes. Thus we conclude that ERK1/2 are functionally linked to p53 and that the ERK1/2-p53 cascade is the upstream signaling pathway responsible for doxorubicin-induced cardiac cell apoptosis. ERKs and p53 may be considered as novel therapeutic targets for the treatment of doxorubicin-induced cardiotoxicity.     

P53-dependent expression of the stress-induced protein (SIP)

The mouse stress-induced protein (SIP) mRNA is activated in the pancreas with acute pancreatitis and in several cell lines in response to various stress agents. The SIP gene is alternatively spliced, generating two proteins (SIP'8 and SIP27). Both proteins, located mainly in the nucleus, promote cell death when overexpressed in vitro. We show that induction by stress agents of the expression of SIP18 and SIP27 mRNAs, observed in human- and mouse-derived cell lines, is absent from cells with deleted, mutated or inactive p53, suggesting that regulation of SIP gene expression is dependent on p53. That hypothesis is consistent with the presence of a functional p53-response element within the promoter region of the mouse SIP gene and confirmed by the induction of SIP mRNA expression in mouse embryo fibroblasts upon activation of a p53-dependent pathway by transfection with rasV12 or rasV12/E1A. In conclusion, SIP being a proapoptotic gene induced through p53 activation could be a stress-induced gene with antitumour properties.